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Time-Resolved Response of Cerebral Stiffness to Hypercapnia in Humans
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https://doi.org/10.1016/j.ultrasmedbio.2019.12.019
Review Article TIME-RESOLVED RESPONSE OF CEREBRAL STIFFNESS TO HYPERCAPNIA IN HUMANS € ,y FELIX SCHRANK,y JUDITH BERGS,y TAGEDPBERNHARD KREFT,* HEIKO TZSCHATZSCH z € KASPAR-JOSCHE STREITBERGER, STEPHAN WALDCHEN ,x STEFAN HETZER,{ JU¨RGEN BRAUN,* and INGOLF SACKyAGEDNTE
* Institute of Medical Informatics, ChariteUniversit€atsmedizin Berlin, Berlin, Germany; y Department of Radiology, ChariteUniversit€atsmedizin Berlin, Berlin, Germany; z Department of Neurology, ChariteUniversit€atsmedizin Berlin, Berlin, Germany; x Department of Mathematics, Technical University Berlin, Berlin, Germany; and { Berlin Center for Advanced Neuroimaging (BCAN), Berlin, Germany (Received 26 July 2019; revised 20 December 2019; in final from 20 December 2019)
Abstract—Cerebral blood flow, cerebral stiffness (CS) and intracranial pressure are tightly linked variables of cerebrovascular reactivity and cerebral autoregulation. Transtemporal ultrasound time-harmonic elastography was used for rapid measurement of CS changes in 10 volunteers before, during and after administration of a gas mixture of 95% O2 and 5% CO2 (carbogen). Within the first 2.2 § 2.0 min of carbogen breathing, shear wave speed determined as a surrogate parameter of CS increased from 1.57 § 0.04 to 1.66 § 0.05 m/s (p < 0.01) in synchrony with end-tidal CO2 while post-hypercapnic CS recovery was delayed by 2.7 § 1.4 min in relation to end-tidal CO2. Our results indicate that CS is highly sensitive to changes in CO2 levels of inhaled air. Possible mechanisms underlying the observed CS changes might be associated with cerebrovascular reactivity, cerebral blood flow adaptation and intracranial regulation, all of which are potentially relevant for future diagnostic applications of transtemporal time-harmonic elastography in a wide spectrum of neurologic diseases. (E-mail: [email protected]) © 2019 World Federation for Ultrasound in Medicine & Biology. All rights reserved. Key Words: Time-harmonic elastography, Transtemporal ultrasound, Brain stiffness, Hypercapnia, Intracranial pressure, Vasodilatation.
and increased intracranial pressure (ICP) (Rich et al. 1953; Giller et al. 1993; Fierstra et al. 2013). Long-term CVR (on the order of minutes) can be studied by perfusion-sensitive magnetic resonance imaging (MRI) combined with a hypercapnia challenge (Chen and Pike 2010; Hare et al. 2013). In the clinical context, dynamic CVR (on the order of seconds) can be measured after carbogen breathing using transcranial Doppler ultrasound, which is sensitive to pulsatile flow in larger intracranial vessels (Willie et al. 2011; McDonnell et al. 2013). However, CVR regulates CBF across the full hierarchy of arteries from large cerebral arteries to downstream arterioles, all of which contribute to the brain’s effective-medium soft tissue properties (Parker 2017) but are incompletely sensed by medical imaging technologies including transcranial Doppler ultrasound and MRI (Tzsch€atzsch 2017; Zwanenburg and van Osch 2017). It has been reported that in several tissues including the liver (Millonig et al. 2010; Rotemberg et al.
INTRODUCTION Cerebrovascular reactivity (CVR) comprises the brain’s ability to autoregulate acute changes in the partial pressure of arterial CO2 by altering cerebral blood flow (CBF) (Brian 1998; Ainslie and Duffin 2009). A loss of CVR has been associated with cerebrovascular stenoocclusive disease (Markus and Cullinane 2001), reductions in cognitive performance (Balucani et al. 2012) and an increased risk of stroke (Silvestrini et al. 2013). CVR can be challenged by breathing CO2-enriched air or a gas mixture of 95% O2 with 5% CO2, which is available as “carbogen” for medical applications (Yang et al. 2014). Breathing air with an elevated CO2 level leads to hypercapnia, a condition of cerebral vasodilation
Address correspondence to: Ingolf Sack, Department of Radiology, ChariteUniversit€atsmedizin Berlin, Chariteplatz 1, 10117 Berlin, Germany. E-mail: [email protected]
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2012; Ipek-Ugay et al. 2016), kidney (Gennisson et al. 2012; Marticorena Garcia et al. 2016) and myocardium (Vejdani-Jahromi et al. 2017), effective-medium stiffness increases because of an increased blood supply, possibly as an effect of pre-stretched vessel walls (Bilston 2002). Similarly, CVR, which is associated with the distensibility of vessel walls in the brain, might influence cerebral stiffness (CS). In fact, perfusion-sensitive MRI and magnetic resonance elastography (MRE) have revealed that CS changes in response to regional variations in CBF in deep gray matter as well as to steadystate hypercapnia (Hetzer et al. 2018, 2019). Other neurologic processes affecting CS include aging (Sack et al. 2009, 2011) or diseases such as Alzheimer’s (Murphy et al. 2015; Gerischer et al. 2018), Parkinson (Lipp et al. 2013, 2018) and multiple sclerosis (Wuerfel et al. 2010; Streitberger et al. 2012; Fehlner et al. 2016). All of these conditions can affect cerebral circulation and CVR. The relationship between CS and ICP has been investigated by MRE of in vivo swine brain (Arani et al. 2018) and healthy volunteers subjected to transjugular compression (Hatt et al. 2015), as well as by transtemporal ultrasound elastography during the Valsalva maneuver (Tzschatzsch et al. 2018). The latter method, cerebral time-harmonic elastography (THE), uniquely allows rapid measurement of short-term CS variations with a temporal resolution of 10 s and appears particularly useful for studying the effect of carbogen on CS in a time-resolved manner. The aim of this study is twofold: (i) CS measurement by a novel transtemporal ultrasound technique still awaits validation for imaging investigation of brain physiology (Ertl et al. 2019; Tzschatzsch 2019). Therefore, first-time quantification of hypercapnia by cerebral THE can provide insight into the sensitivity of transtemporal elastography to cerebrovascular mechanical changes. (ii) Carbogen is administered as a vasoreactive stimulus either diagnostically (Hare et al. 2013; McDonnell et al. 2013) or therapeutically (Bernier et al. 1999; Yang et al. 2014). Further insight into physiologic effects of carbogen on the brain’s biophysical properties including stiffness is required for the interpretation and analysis of CVR-related imaging markers in future applications. METHODS Study cohort The study protocol conformed to the guidelines of the Declaration of Helsinki and was approved by the institutional review board of ChariteUniversit€atsmedizin Berlin. Informed written consent was obtained from all study participants. None of the study participants had any history of neurologic or pulmonary disease including psychiatric disorders and/or a history of major head injury. A
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Table 1. Patients and physiologic parameters Subject No.
Sex
Age (y)
Heart rate* (bpm)
Systolic BP* (mm Hg)
Diastolic BP* (mm Hg)
1 2 3 4 5 6 7 8 9 10 Group*
M F F F M M M M F F
34 41 27 27 28 22 28 31 20 28 29 (6)
48 (2) 70 (4) 70 (5) 66 (4) 65 (3) 54 (3) 72 (5) 58 (3) 68 (3) 84 (13) 66 (10)
125 (3) 107 (2) 123 (5) 120 (4) 123 (4) 149 (4) 120 (8) 111 (5) 104 (4) 114 (9) 119 (12)
73 (2) 68 (2) 69 (2) 71 (2) 73 (3) 67 (3) 69 (3) 73 (5) 63 (4) 73 (6) 70 (3)
* Mean (standard deviation).
total of 10 healthy volunteers (5 females, mean age: 29 § 6 y, age range: 2041 y) were investigated. Information on age, sex, heart rate and blood pressure of every study participant is summarized in Table 1. Cerebral THE Figure 1 illustrates our experimental setup including the system, breathing apparatus and physiologic monitoring system. The system consisted of three main components: vibration bed, B-mode ultrasound scanner and elastography computer. A special patient bed (THED Research, GAMPT, Merseburg, Germany) modified with an amplifier and bass shaker was used to produce audible sound waves underneath the volunteer’s head. The waveform fed into the amplifier consisted of six superimposed frequencies of 27, 33, 39, 44, 50 and 56 Hz with identical amplitudes. A standard B-mode clinical ultrasound scanner (SonixMDP, UltraSonix, Scottsdale, AZ, USA) equipped with a phased-array transducer (SA4-2/50) of 2-MHz frequency was used for ultrasound data acquisition. Ultrasound raw B-mode data were acquired over 1 s with a frame rate of 80 Hz, yielding 80 time-resolved frames of raw radiofrequency (RF) data for a single scan. Raw data were transferred to the elastography computer in real time for further processing. THE data processing First, an analytical signal S was generated based on the Hilbert transform of RF data along each line of sight. Axial displacement di was recovered from the phase shift between adjacent frames of S along the time axis (i, i + 1): di / arg Si Siþ1 ð1Þ The resulting time-resolved displacement field was then decomposed into complex Fourier components at vibration frequency (f = 27, 33, 39, 44, 50 and 56 Hz). Because of the low frame rate of 80 Hz, all higher vibration frequencies with a frequency above frame rate/2
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Fig. 1. Experimental setup consisting of the vibration unit mounted underneath the head of the bed, marked in red (1), standard ultrasound scanner (2), elastography computer (3), probe holder (head set) and breathing mask (4), gas bottle for carbogen supply (5) and patient monitoring system (6). (a) Enlarged head set for fixing the ultrasound probe. (b) Enlarged screen of the patient monitor with patient data. Heart rate, blood pressure, oxygen saturation and etCO2 were monitored.
(here: 44, 50 and 56 Hz) were above the Nyquist limit and appeared in aliased spectral positions (here: 36, 30 and 24 Hz). Therefore, complex-valued Fourier components were taken from spectral positions of 24, 27, 30, 33, 36 and 39 Hz, corresponding to the unaliased mechanical vibration frequencies of f = 56, 27, 50, 33, 44 and 39 Hz, respectively. Henceforth, this way of stroboscopic sampling of higher vibration frequencies by lower frame rates is referred to as controlled aliasing. The lateral phase shift caused by line-based image acquisition was corrected as explained in Tzschatzsch et al. (2016b). After this phase correction and noise suppression by a spatial bandpass filter, every complex wave image at vibration frequency was decomposed into unidirectional plane waves as further specified in Tzschatzsch et al. (2016a). In the next step, the reconstruction of wavenumbers k for every shear wave direction and every vibration frequency was derived from complex shear waves u employing the phase gradient method: k ¼ k r argðuÞ k
ð2Þ
Shear wave speed (SWS, in m/s), as a surrogate of CS, was then calculated by inversion of wavenumbers k for every wave direction and driving frequency. Finally, all SWS images were combined into a compound SWS map by amplitude-weighted averaging as explained in Tzschatzsch et al. (2016a). The weighted averaging for SWS compounding limits the effect of noise and low shear wave amplitudes (e.g., from wave directions that do not reflect true shear wave propagation) on SWS maps. Cerebral THE in hypercapnia The ultrasound transducer was positioned at the right temples of the volunteers based on the B-mode display of anatomic landmarks including the brainstem, basal cisterns, aqueduct and meninges, as illustrated in Figure 2. The region between these landmarks was identified as the temporal lobe parenchyma, in which SWS values were averaged. Identical to Tzschatzsch et al. (2018), SWS values larger than 1.2 m/s were considered trustworthy, while all values below that threshold were
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Fig. 2. Screenshots of ultrasound imaging and elastography setup. (a) Standard transtemporal B-mode image with 14-cm penetration depth; white dashed lines indicate the 8-cm penetration depth of our elastography method. (b) B-Mode image reconstructed from motion data. (c) Shear wave speed (SWS) map overlaid on the B-mode image shown in (b). Meninges (M), temporal parenchyma (TP), cerebellum (CB) and the hypo-echogenic brainstem (BS) surrounded by hyperechogenic basal cisterns (BC) and aqueduct (AQ) are highlighted by black and white lines.
neglected to minimize the effect of noise on our data. In our prior work, this threshold was empirically derived from the response of the reconstruction pipeline to pure Gaussian noise and tested in healthy volunteers over a wide age range (Tzschatzsch et al. 2018). However, it should be noted that this threshold still has to be validated in patients. To prevent displacement of the transducer relative to the temples during repeat measurements we used a custom-designed mounting device for the ultrasound probe, as illustrated in Figure 1. Volunteers were in a supine position on the vibration bed wearing a breathing mask throughout the experiment. Carbogen (Linde, Pullach, Germany), a gas mixture of 5% CO2 and 95% O2, was administered via the breathing mask by opening connections to a gas bottle and flow reservoir. Physiologic parameters of heart rate, blood oxygenation level (SpO2) and end-tidal CO2 (end-tidal CO2) were continuously recorded in 10-s increments using a mobile clinical patient monitoring system (Tesla Guard, Mammendorf, Germany), while blood pressure was recorded every 2 min. The experiment was subdivided into three phases, each 7 min in length: (i) baseline, (ii) carbogen breathing and (iii) recovery. Between the phases, the pipelines of the breathing gas were switched to administer carbogen to induce hypercapnia or normal air for recovery, resulting in a 30-s gap without data acquisition. A total of 129 SWS values (43 values for each phase) were acquired within 22 min.
Data analysis SWS maps were computed for each THE frame acquired with 10-s increments. Regions of interest (ROIs) in the temporal lobe parenchyma were manually drawn for spatially averaging SWS values to obtain the function of SWS over time for every volunteer. The times of hypercapnia-induced increases and decreases in SWS and etCO2, t i and t d, were determined by fitting the time-resolved parameter functions f(t) with the following sigmodal model function: f ðt Þ ¼ a þ
b 1 þ
eC1 ðtti Þ
þ eC2 ðttd Þ
ð3Þ
In this function, a and b represent baseline values and baseline values minus plateau values (D increase), while C1 and C2 denote the increasing and decreasing slopes of the function, respectively.
Statistics To determine the statistical significance of changes between time-averaged SWS values in each of the individual measurement intervals of baseline, hypercapnia and recovery, 25 SWS values from the second half of each interval were aggregated for every volunteer. A paired t-test was performed with the aggregated data. Furthermore, a paired t-test was used to test for statistically significant differences between time t parameters of SWS and etCO2. As ti and t d indicate the time
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responses of effective-medium stiffness including vessels to hypercapnia, we consider these parameters significant with respect to CVR. All p values <0.05 were considered to indicate statistical significance.
In Figure 3 are representative elastograms (SWS maps) and ROIs acquired in one volunteer during the baseline (a), carbogen challenge (b) and recovery (c) phases. There is a clearly perceivable increase in SWS in the temporal lobe ROI (temporal lobe tissue) during carbogen breathing (Fig. 3b) with a subsequent return to baseline values after the end of carbogen breathing (Fig. 3c). In Figure 4 are boxplots of SWS averaged over the second halves of the baseline, hypercapnia and recovery phases. Statistically significant differences were observed between SWS during hypercapnia and SWS values acquired before and after carbogen breathing (p = 0.001) but not between the two normal-breathing phases. Mean SWS values and standard deviations for every volunteer, as well as group mean SWS values and standard deviations, are summarized in Table 2. SWS values increased in all volunteers after breathing carbogen, with a mean increase of 0.09 § 0.06 m/s (range: 0.020.2 m/s). Figure 5 illustrates time-resolved group mean values and standard deviations of SWS (blue graph) and etCO2 (red graph). The baseline SWS of 1.57 § 0.04 m/s increased to a plateau value of 1.66 § 0.05 m/s (relative SWS change: 5.9 § 4.0 %, p = 0.001) during early hypercapnia at ti = 2.2 § 2.0 min. From that plateau, SWS decreased to normal values (1.59 § 0.03 m/s, p = 0.003) in the early recovery phase at td = 3.6 § 1.5 min. Baseline etCO2 was 32 § 3 mm Hg, which is within the normal range of 3035 mm Hg (Haubrich et al. 2013). From this normal range, etCO2 increased during early hypercapnia at ti = 1.9 § 1.3 min to a plateau value of 42 § 2 mm Hg (relative etCO2 change: 29.1 § 8.6 %, p = 0.00006), from where it returned to normal without
**
**
1.70 1.65 SWS in m/s
RESULTS
5
1.60
1.55 1.50
1.45
Baseline
Hypercapnia
Recovery
Fig. 4. Statistical illustration of the data. Group values corresponding to pooled data analysis, in which data were averaged over time during the second halves (late phases) of baseline, hypercapnia and recovery. Gray lines connect shear wave speed (SWS) values in each volunteer. Red symbols indicate outliers. Table 2. Mean shear wave speed values and standard deviations in each of the three phases analyzed: Baseline, hypercapnia and recovery Subject No.
Baseline* (m/s)
Hypercapnia* (m/s)
Recovery* (m/s)
1 2 3 4 5 6 7 8 9 10 Group*
1.61 (0.01) 1.55 (0.03) 1.61 (0.03) 1.56 (0.02) 1.60 (0.02) 1.58 (0.03) 1.54 (0.03) 1.56 (0.03) 1.49 (0.04) 1.60 (0.02) 1.57 (0.04)
1.72 (0.03) 1.57 (0.04) 1.64 (0.03) 1.62(0.04) 1.66 (0.04) 1.63 (0.02) 1.64 (0.03) 1.75 (0.05) 1.69 (0.04) 1.70 (0.03) 1.66 (0.05)
1.59 (0.02) 1.60 (0.03) 1.65 (0.03) 1.58 (0.02) 1.60 (0.03) 1.54 (0.05) 1.57 (0.03) 1.63 (0.06) 1.59 (0.06) 1.59 (0.03) 1.59 (0.03)
* Mean (standard deviation).
Fig. 3. Representative shear wave speed (SWS) maps of a volunteer taken during the second halves (late phases) of (a) baseline, (b) hypercapnia and (c) recovery. Regions of interest are demarcated by white lines.
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60
1.75
45
SWS in m/s
1.65 1.60
30
1.55
etCO2 in mmHg
1.70
1.50 τi 1.45 0
2
4
6
8
τd
10 12 14 16 18 20 Time in min
15 22
Fig. 5. Group mean shear wave speed (SWS) values (blue graph) and end-tidal CO2 (etCO2, red graph) with mean fit curves shown as dotted lines. Baseline mean values are demarcated by dashed lines, and standard deviations for SWS and etCO2 are represented by light gray and light red areas. Vertical lines indicate the transition to the next phase. The horizontal arrows indicate time points of increase (t i) and decrease (t d) in SWS and etCO2. Although t is nearly the same for both parameters during hypercapnia, it significantly differs during recovery.
significant delay at td = 0.9 § 0.4 min after stopping the gas supply. Although SWS and etCO2 increased with similar ti (p = 0.48), etCO2 returned to normal values significantly earlier than SWS (p < 0.001). Physiologic parameters monitored in this study (SpO2, heart rate, breathing frequency and blood pressure) did not change because of hypercapnia (see Supplementary Figure S1, online only). DISCUSSION To our knowledge, this is the first time-resolved quantification of CS changes in volunteers exposed to elevated levels of CO2 in breathing gas. Although changes in hypercapnia-induced CBF have been extensively studied and are exploited to measure cerebral autoregulation and CVR (Brian 1998; Ainslie and Duffin 2009), our data suggest that CS also is influenced by effects related to the regulation of vascular pressure and blood perfusion in the brain. Consequently, THE-measured CS holds promise as a new imaging marker for cerebral autoregulation and CVR that is affected by many neurovascular diseases (Markus and Cullinane 2001; Balucani et al. 2012; Silvestrini et al. 2013). It has been proposed that the mechanism underlying CBF changes in hypercapnia is enhanced washout of CO2 from the brain tissue for attenuating a further rise in partial pressure (PCO2) (Ainslie and Duffin 2009). As hypercapnia is associated with an elevated ICP (Brian 1998; Gelb et al. 2008), the THE-measured CS might reflect, at least partially, ICP changes, which would be
consistent with previous observations in volunteers performing the Valsalva maneuver (Tzschatzsch et al. 2018). Further speculating on a possible link between CS and ICP, we can compare our results with those of Van Hulst et al. (2002), who reported an ICP increase in pigs, because of hypercapnia, of approximately 7 % for each 10 mm Hg increase in PCO2, which is in the range of CS changes measured by THE (approximately 6 %). In addition, Arani et al. (2018) reported a linear increase in CS in the range of 0.6 %/mm Hg ICP (at 60-Hz vibration frequency). Combining this with the rate of change of 0.7 %/mm Hg reported by van Hulst et al. (2002), we can predict the increase in SWS per 1 mm Hg increase in ICP to be on the order of 0.75 %. Although this is similar to our value (0.59 % per 1 mm Hg), interspecies differences as well as lower excitation frequencies (mean THE frequency: 41.5 Hz, MRE: 60 Hz) might explain the slightly lower value found in our study. In principle, stiffness is a marker of a material’s intrinsic properties and can be compared across systems and modalities such as MRI and ultrasound. Our previous work in MRE revealed a hypercapnia-induced increase in brain stiffness of 3.3 § 1.9 %, which overlaps with the range of values observed in the present study (5.9 § 4.0 %) (Hetzer et al. 2018). Notably, voluntary abdominal muscle contraction during breath hold (Valsalva maneuver) increases CS by about 10.8 § 2.5 % (Tzschatzsch et al. 2018), suggesting that ICP is approximately twice as effectively increased by a Valsalva maneuver as by carbogen. Baseline values in Tzschatzsch et al. (2018) (1.56 § 0.08 m/s) are similar to our present values (1.57 § 0.04
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m/s), illustrating the high consistency of CS values measured by transtemporal THE. Previous studies reported CBF to rise with only a short delay of 34 § 10 s after a PCO2 increase (Ogoh et al. 2009). Although our method did not resolve such short delays, our observations suggest an instantaneous response of CS to the increase in etCO2 in hypercapnia. The observed delayed recovery of SWS relative to etCO2 is an interesting finding that may point to either delayed vasoconstrictive responses of brain vessels compared with vasodilation or a delayed gasblood exchange with normal air in the lungs after 7 min of carbogen breathing. A limitation of our study is that we cannot provide a mechanistic validation of the effects of CBF or ICP on CS. We assume that CS is coupled to CBF and ICP by the nonlinear elastic properties of expanding vessel walls, which are embedded in the effective-medium soft-solid brain tissue below the scale of image resolution (Parker 2015; Arani et al. 2018). In addition, we measured a wide range of SWS changes, possibly a result of different CO2 uptakes, among all patients. In fact, it is known that the partial pressure of CO2 in the arterial blood (PaCO2) differs from that in brain tissue (PtCO2), as well as from etCO2 (Hare et al. 2013). Otherwise, it has been shown that CBF is similarly affected by PaCO2 and etCO2 (Young et al. 1991), indicating the feasibility of etCO2 recordings as a ground truth of hypercapnia-induced CS changes in the brain. Furthermore, the way in which ICP and cerebral perfusion are coupled into our CS values remains to be investigated by measuring CS changes after manipulation of cerebral hemodynamics with a protocol similar to that used in the present study. It would be interesting to see if hyperventilation (hypocapnia) or hypoxia (O2 deprivation of the brain) influence CS in a way similar to that seen here. Ultimately, cerebral THE should be tested in patients with elevated ICP before and after decompression treatment such as by lumbar puncture. The development of non-invasive state-of-the-art ultrasound elastography techniques toward quantification of CS and related tissue pressure is clinically desirable for many neurologic applications (Hirsch et al. 2017). In summary, we used a novel transtemporal THE technique to quantify CS changes induced by carbogen breathing in healthy volunteers. Our data reveal an increase in CS caused by hypercapnia of approximately 6 %, which is consistent with published data on ICP increases and suggests that THE-measured CS is sensitive to cerebral vasodilation. Transtemporal THE might be useful for CS-related studies of brain physiology as well as for the non-invasive quantification of CVR in clinical applications. Acknowledgments—Support of the German Research Foundation (GRK2260 BIOQIC and SFB1340 Matrix in Vision) is gratefully acknowledged. The authors declare no conflict of interest. Conflict of interest disclosure—The authors have no conflicts of interest to disclose
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SUPPLEMENTARY MATERIALS Supplementary material associated with this article can be found in the online version at doi:10.1016/j.ultra smedbio.2019.12.019.
REFERENCES Ainslie PN, Duffin J. Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: Mechanisms of regulation, measurement, and interpretation. Am J Physiol Regul Integr Comp Physiol 2009;296:R1473–R1495. Arani A, Min HK, Fattahi N, Wetjen NM, Trzasko JD, Manduca A, Jack CR, Jr, Lee KH, Ehman RL, Huston J, III. Acute pressure changes in the brain are correlated with MR elastography stiffness measurements: Initial feasibility in an in vivo large animal model. Magn Reson Med 2018;79:1043–1051. Balucani C, Viticchi G, Falsetti L, Silvestrini M. Cerebral hemodynamics and cognitive performance in bilateral asymptomatic carotid stenosis. Neurology 2012;79:1788–1795. Bernier J, Denekamp J, Rojas A, Trovo M, Horiot JC, Hamers H, Antognoni P, Dahl O, Richaud P, Kaanders J, van Glabbeke M, Pierart M. ARCON: Accelerated radiotherapy with carbogen and nicotinamide in non small cell lung cancer: A phase I/II study by the EORTC. Radiother Oncol 1999;52:149–156. Bilston LE. The effect of perfusion on soft tissue mechanical propertiesa: Computational model. Comput Methods Biomech Biomed Eng 2002;5:283–290. Brian JE, Jr. Carbon dioxide and the cerebral circulation. Anesthesiology 1998;88:1365–1386. Chen JJ, Pike GB. MRI measurement of the BOLD-specific flowvolume relationship during hypercapnia and hypocapnia in humans. Neuroimage 2010;53:383–391. Ertl M, Raasch N, Hammel G, Harter K, Lang C. Transtemporal investigation of brain parenchyma elasticity using 2-D shear wave elastography: Validation appreciated!. Ultrasound Med Biol 2019;45:1344. Fehlner A, Behrens JR, Streitberger KJ, Papazoglou S, Braun J, Bellmann-Strobl J, Ruprecht K, Paul F, Wurfel J, Sack I. Higher-resolution MR elastography reveals early mechanical signatures of neuroinflammation in patients with clinically isolated syndrome. J Magn Reson Imaging 2016;44:51–58. Fierstra J, Sobczyk O, Battisti-Charbonney A, Mandell DM, Poublanc J, Crawley AP, Mikulis DJ, Duffin J, Fisher JA. Measuring cerebrovascular reactivity: What stimulus to use? J Physiol 2013;591: 5809–5821. Gelb AW, Craen RA, Rao GS, Reddy KR, Megyesi J, Mohanty B, Dash HH, Choi KC, Chan MT. Does hyperventilation improve operating condition during supratentorial craniotomy? A multicenter randomized crossover trial. Anesth Analg 2008;106:585–594 table of contents. Gennisson JL, Grenier N, Combe C, Tanter M. Supersonic shear wave elastography of in vivo pig kidney: Influence of blood pressure, urinary pressure and tissue anisotropy. Ultrasound Med Biol 2012;38:1559–1567. Gerischer LM, Fehlner A, Kobe T, Prehn K, Antonenko D, Grittner U, Braun J, Sack I, Floel A. Combining viscoelasticity, diffusivity and volume of the hippocampus for the diagnosis of Alzheimer’s disease based on magnetic resonance imaging. Neuroimage Clin 2018;18:485–493. Giller CA, Bowman G, Dyer H, Mootz L, Krippner W. Cerebral arterial diameters during changes in blood pressure and carbon dioxide during craniotomy. Neurosurgery 1993;32:737–741 discussion 4142. Hare HV, Germuska M, Kelly ME, Bulte DP. Comparison of CO2 in air versus carbogen for the measurement of cerebrovascular reactivity with magnetic resonance imaging. J Cereb Blood Flow Metab 2013;33:1799–1805. Hatt A, Cheng S, Tan K, Sinkus R, Bilston LE. MR elastography can be used to measure brain stiffness changes as a result of altered
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cranial venous drainage during jugular compression. AJNR Am J Neuroradiol 2015;36:1971–1977. Haubrich C, Steiner LA, Diehl RR, Kasprowicz M, Smielewski P, Pickard JD, Czosnyka M. Doppler flow velocity and intra-cranial pressure: Responses to short-term mild hypocapnia help to assess the pressurevolume relationship after head injury. Ultrasound Med Biol 2013;39:1521–1526. Hetzer S, Birr P, Fehlner A, Hirsch S, Dittmann F, Barnhill E, Braun J, Sack I. Perfusion alters stiffness of deep gray matter. J Cereb Blood Flow Metab 2018;38:116–125. Hetzer S, Dittmann F, Bormann K, Hirsch S, Lipp A, Wang DJ, Braun J, Sack I. Hypercapnia increases brain viscoelasticity. J Cereb Blood Flow Metab 2019;39:2445–2455. Hirsch S, Braun J, Sack I. Magnetic resonance elastography: Physical background and medical applications. Weinheim: Wiley-VCH; 2017. Ipek-Ugay S, Tzsch€atzsch Hudert C, Marticorena Garcia S, Fischer T, Braun J, Althoff C, Sack I. Time harmonic elastography reveals sensitivity of liver stiffness to water ingestion. Ultrasound Med Biol 2016;42:1289–1294. Lipp A, Trbojevic R, Paul F, Fehlner A, Hirsch S, Scheel M, Noack C, Braun J, Sack I. Cerebral magnetic resonance elastography in supranuclear palsy and idiopathic Parkinson’s disease. Neuroimage Clin 2013;3:381–387. Lipp A, Skowronek C, Fehlner A, Streitberger KJ, Braun J, Sack I. Progressive supranuclear palsy and idiopathic Parkinson’s disease are associated with local reduction of in vivo brain viscoelasticity. Eur Radiol 2018;28:3347–3354. Markus H, Cullinane M. Severely impaired cerebrovascular reactivity predicts stroke and TIA risk in patients with carotid artery stenosis and occlusion. Brain 2001;124:457–467. Marticorena Garcia SR, Fischer T, Durr M, Gultekin E, Braun J, Sack I, Guo J. Multifrequency magnetic resonance elastography for the assessment of renal allograft function. Invest Radiol 2016;51: 591–595. McDonnell MN, Berry NM, Cutting MA, Keage HA, Buckley JD, Howe PR. Transcranial Doppler ultrasound to assess cerebrovascular reactivity: Reliability, reproducibility and effect of posture. PeerJ 2013;1:e65. Millonig G, Friedrich S, Adolf S, Fonouni H, Golriz M, Mehrabi A, Stiefel P, Poschl G, Buchler MW, Seitz HK, Mueller S. Liver stiffness is directly influenced by central venous pressure. J Hepatol 2010;52:206–210. Murphy MC, Jones DT, Jack CR, Jr, Glaser KJ, Senjem ML, Manduca A, Felmlee JP, Carter RE, Ehman RL, Huston J, III. Regional brain stiffness changes across the Alzheimer’s disease spectrum. Neuroimage Clin 2015;10:283–290. Ogoh S, Ainslie PN, Miyamoto T. Onset responses of ventilation and cerebral blood flow to hypercapnia in humans: Rest and exercise. J Appl Physiol (1985) 2009;106:880–886. Parker KJ. Experimental evaluations of the microchannel flow model. Phys Med Biol 2015;60:4227–4242. Parker KJ. Are rapid changes in brain elasticity possible?. Phys Med Biol 2017;62:7425–7439. Rich M, Scheinberg P, Belle MS. Relationship between cerebrospinal fluid pressure changes and cerebral blood flow. Circ Res 1953;1: 389–395. Rotemberg V, Palmeri M, Nightingale R, Rouze N, Nightingale K. The impact of hepatic pressurization on liver shear wave speed
Volume 00, Number 00, 2019 estimates in constrained versus unconstrained conditions. Phys Med Biol 2012;57:329–341. Sack I, Beierbach B, Wuerfel J, Klatt D, Hamhaber U, Papazoglou S, Martus P, Braun J. The impact of aging and gender on brain viscoelasticity. Neuroimage 2009;46:652–657. Sack I, Streitberger KJ, Krefting D, Paul F, Braun J. The influence of physiological aging and atrophy on brain viscoelastic properties in humans. PlosOne 2011;6:e23451. Silvestrini M, Viticchi G, Falsetti L, Balucani C. Cerebral hemodynamics and cognitive performance in bilateral asymptomatic carotid stenosis. Author response. Neurology 2013;80:2080–2081. Streitberger KJ, Sack I, Krefting D, Pfuller C, Braun J, Paul F, Wuerfel J. Brain viscoelasticity alteration in chronic-progressive multiple sclerosis. PLoS One 2012;7:e29888. Tzsch€atzsch H. Methods and approaches in ultrasound elastography. In: Sack I, Sch€affter T, (eds). Quantification of biophysical parameters in medical imaging. Berlin/New York: Springer; 2017. p. 281–302. Tzschatzsch H, Guo J, Dittmann F, Hirsch S, Barnhill E, Johrens K, Braun J, Sack I. Tomoelastography by multifrequency wave number recovery from time-harmonic propagating shear waves. Med Image Anal 2016;30:1–10. Tzschatzsch H, Nguyen Trong M, Scheuermann T, Ipek-Ugay S, Fischer T, Schultz M, Braun J, Sack I. Two-dimensional time-harmonic elastography of the human liver and spleen. Ultrasound Med Biol 2016;42:2562–2571. Tzschatzsch H, Kreft B, Schrank F, Bergs J, Braun J, Sack I. In vivo time-harmonic ultrasound elastography of the human brain detects acute cerebral stiffness changes induced by intracranial pressure variations. Sci Rep 2018;8:17888. Tzschatzsch H, Kreft B, Braun J, Sack I. Transtemporal Investigation of Brain Parenchyma Elasticity Using 2-D Shear Wave Elastography: Trustworthy? Ultrasound Med Biol 2019;45:1344–1345. van Hulst RA, Hasan D, Lachmann B. Intracranial pressure, brain PCO2, PO2, and pH during hypo- and hyperventilation at constant mean airway pressure in pigs. Intensive Care Med 2002;28:68–73. Vejdani-Jahromi M, Freedman J, Nagle M, Kim YJ, Trahey GE, Wolf PD. Quantifying myocardial contractility changes using ultrasound-based shear wave elastography. J Am Soc Echocardiogr 2017;30:90–96. Willie CK, Colino FL, Bailey DM, Tzeng YC, Binsted G, Jones LW, Haykowsky MJ, Bellapart J, Ogoh S, Smith KJ, Smirl JD, Day TA, Lucas SJ, Eller LK, Ainslie PN. Utility of transcranial Doppler ultrasound for the integrative assessment of cerebrovascular function. J Neurosci Methods 2011;196:221–237. Wuerfel J, Paul F, Beierbach B, Hamhaber U, Klatt D, Papazoglou S, Zipp F, Martus P, Braun J, Sack I. MR-elastography reveals degradation of tissue integrity in multiple sclerosis. Neuroimage 2010;49:2520–2525. Yang XF, Shi XY, Ju J, Zhang WN, Liu YJ, Li XY, Zou LP. 5% CO2 inhalation suppresses hyperventilation-induced absence seizures in children. Epilepsy Res 2014;108:345–348. Young WL, Prohovnik I, Ornstein E, Ostapkovich N, Matteo RS. Cerebral blood flow reactivity to changes in carbon dioxide calculated using end-tidal versus arterial tensions. J Cereb Blood Flow Metab 1991;11:1031–1035. Zwanenburg JJM, van Osch MJP. Targeting cerebral small vessel disease with MRI. Stroke 2017;48:3175–3182.
https://doi.org/10.1016/j.ultrasmedbio.2019.12.019
Review Article TIME-RESOLVED RESPONSE OF CEREBRAL STIFFNESS TO HYPERCAPNIA IN HUMANS € ,y FELIX SCHRANK,y JUDITH BERGS,y TAGEDPBERNHARD KREFT,* HEIKO TZSCHATZSCH z € KASPAR-JOSCHE STREITBERGER, STEPHAN WALDCHEN ,x STEFAN HETZER,{ JU¨RGEN BRAUN,* and INGOLF SACKyAGEDNTE
* Institute of Medical Informatics, ChariteUniversit€atsmedizin Berlin, Berlin, Germany; y Department of Radiology, ChariteUniversit€atsmedizin Berlin, Berlin, Germany; z Department of Neurology, ChariteUniversit€atsmedizin Berlin, Berlin, Germany; x Department of Mathematics, Technical University Berlin, Berlin, Germany; and { Berlin Center for Advanced Neuroimaging (BCAN), Berlin, Germany (Received 26 July 2019; revised 20 December 2019; in final from 20 December 2019)
Abstract—Cerebral blood flow, cerebral stiffness (CS) and intracranial pressure are tightly linked variables of cerebrovascular reactivity and cerebral autoregulation. Transtemporal ultrasound time-harmonic elastography was used for rapid measurement of CS changes in 10 volunteers before, during and after administration of a gas mixture of 95% O2 and 5% CO2 (carbogen). Within the first 2.2 § 2.0 min of carbogen breathing, shear wave speed determined as a surrogate parameter of CS increased from 1.57 § 0.04 to 1.66 § 0.05 m/s (p < 0.01) in synchrony with end-tidal CO2 while post-hypercapnic CS recovery was delayed by 2.7 § 1.4 min in relation to end-tidal CO2. Our results indicate that CS is highly sensitive to changes in CO2 levels of inhaled air. Possible mechanisms underlying the observed CS changes might be associated with cerebrovascular reactivity, cerebral blood flow adaptation and intracranial regulation, all of which are potentially relevant for future diagnostic applications of transtemporal time-harmonic elastography in a wide spectrum of neurologic diseases. (E-mail: [email protected]) © 2019 World Federation for Ultrasound in Medicine & Biology. All rights reserved. Key Words: Time-harmonic elastography, Transtemporal ultrasound, Brain stiffness, Hypercapnia, Intracranial pressure, Vasodilatation.
and increased intracranial pressure (ICP) (Rich et al. 1953; Giller et al. 1993; Fierstra et al. 2013). Long-term CVR (on the order of minutes) can be studied by perfusion-sensitive magnetic resonance imaging (MRI) combined with a hypercapnia challenge (Chen and Pike 2010; Hare et al. 2013). In the clinical context, dynamic CVR (on the order of seconds) can be measured after carbogen breathing using transcranial Doppler ultrasound, which is sensitive to pulsatile flow in larger intracranial vessels (Willie et al. 2011; McDonnell et al. 2013). However, CVR regulates CBF across the full hierarchy of arteries from large cerebral arteries to downstream arterioles, all of which contribute to the brain’s effective-medium soft tissue properties (Parker 2017) but are incompletely sensed by medical imaging technologies including transcranial Doppler ultrasound and MRI (Tzsch€atzsch 2017; Zwanenburg and van Osch 2017). It has been reported that in several tissues including the liver (Millonig et al. 2010; Rotemberg et al.
INTRODUCTION Cerebrovascular reactivity (CVR) comprises the brain’s ability to autoregulate acute changes in the partial pressure of arterial CO2 by altering cerebral blood flow (CBF) (Brian 1998; Ainslie and Duffin 2009). A loss of CVR has been associated with cerebrovascular stenoocclusive disease (Markus and Cullinane 2001), reductions in cognitive performance (Balucani et al. 2012) and an increased risk of stroke (Silvestrini et al. 2013). CVR can be challenged by breathing CO2-enriched air or a gas mixture of 95% O2 with 5% CO2, which is available as “carbogen” for medical applications (Yang et al. 2014). Breathing air with an elevated CO2 level leads to hypercapnia, a condition of cerebral vasodilation
Address correspondence to: Ingolf Sack, Department of Radiology, ChariteUniversit€atsmedizin Berlin, Chariteplatz 1, 10117 Berlin, Germany. E-mail: [email protected]
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2012; Ipek-Ugay et al. 2016), kidney (Gennisson et al. 2012; Marticorena Garcia et al. 2016) and myocardium (Vejdani-Jahromi et al. 2017), effective-medium stiffness increases because of an increased blood supply, possibly as an effect of pre-stretched vessel walls (Bilston 2002). Similarly, CVR, which is associated with the distensibility of vessel walls in the brain, might influence cerebral stiffness (CS). In fact, perfusion-sensitive MRI and magnetic resonance elastography (MRE) have revealed that CS changes in response to regional variations in CBF in deep gray matter as well as to steadystate hypercapnia (Hetzer et al. 2018, 2019). Other neurologic processes affecting CS include aging (Sack et al. 2009, 2011) or diseases such as Alzheimer’s (Murphy et al. 2015; Gerischer et al. 2018), Parkinson (Lipp et al. 2013, 2018) and multiple sclerosis (Wuerfel et al. 2010; Streitberger et al. 2012; Fehlner et al. 2016). All of these conditions can affect cerebral circulation and CVR. The relationship between CS and ICP has been investigated by MRE of in vivo swine brain (Arani et al. 2018) and healthy volunteers subjected to transjugular compression (Hatt et al. 2015), as well as by transtemporal ultrasound elastography during the Valsalva maneuver (Tzschatzsch et al. 2018). The latter method, cerebral time-harmonic elastography (THE), uniquely allows rapid measurement of short-term CS variations with a temporal resolution of 10 s and appears particularly useful for studying the effect of carbogen on CS in a time-resolved manner. The aim of this study is twofold: (i) CS measurement by a novel transtemporal ultrasound technique still awaits validation for imaging investigation of brain physiology (Ertl et al. 2019; Tzschatzsch 2019). Therefore, first-time quantification of hypercapnia by cerebral THE can provide insight into the sensitivity of transtemporal elastography to cerebrovascular mechanical changes. (ii) Carbogen is administered as a vasoreactive stimulus either diagnostically (Hare et al. 2013; McDonnell et al. 2013) or therapeutically (Bernier et al. 1999; Yang et al. 2014). Further insight into physiologic effects of carbogen on the brain’s biophysical properties including stiffness is required for the interpretation and analysis of CVR-related imaging markers in future applications. METHODS Study cohort The study protocol conformed to the guidelines of the Declaration of Helsinki and was approved by the institutional review board of ChariteUniversit€atsmedizin Berlin. Informed written consent was obtained from all study participants. None of the study participants had any history of neurologic or pulmonary disease including psychiatric disorders and/or a history of major head injury. A
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Table 1. Patients and physiologic parameters Subject No.
Sex
Age (y)
Heart rate* (bpm)
Systolic BP* (mm Hg)
Diastolic BP* (mm Hg)
1 2 3 4 5 6 7 8 9 10 Group*
M F F F M M M M F F
34 41 27 27 28 22 28 31 20 28 29 (6)
48 (2) 70 (4) 70 (5) 66 (4) 65 (3) 54 (3) 72 (5) 58 (3) 68 (3) 84 (13) 66 (10)
125 (3) 107 (2) 123 (5) 120 (4) 123 (4) 149 (4) 120 (8) 111 (5) 104 (4) 114 (9) 119 (12)
73 (2) 68 (2) 69 (2) 71 (2) 73 (3) 67 (3) 69 (3) 73 (5) 63 (4) 73 (6) 70 (3)
* Mean (standard deviation).
total of 10 healthy volunteers (5 females, mean age: 29 § 6 y, age range: 2041 y) were investigated. Information on age, sex, heart rate and blood pressure of every study participant is summarized in Table 1. Cerebral THE Figure 1 illustrates our experimental setup including the system, breathing apparatus and physiologic monitoring system. The system consisted of three main components: vibration bed, B-mode ultrasound scanner and elastography computer. A special patient bed (THED Research, GAMPT, Merseburg, Germany) modified with an amplifier and bass shaker was used to produce audible sound waves underneath the volunteer’s head. The waveform fed into the amplifier consisted of six superimposed frequencies of 27, 33, 39, 44, 50 and 56 Hz with identical amplitudes. A standard B-mode clinical ultrasound scanner (SonixMDP, UltraSonix, Scottsdale, AZ, USA) equipped with a phased-array transducer (SA4-2/50) of 2-MHz frequency was used for ultrasound data acquisition. Ultrasound raw B-mode data were acquired over 1 s with a frame rate of 80 Hz, yielding 80 time-resolved frames of raw radiofrequency (RF) data for a single scan. Raw data were transferred to the elastography computer in real time for further processing. THE data processing First, an analytical signal S was generated based on the Hilbert transform of RF data along each line of sight. Axial displacement di was recovered from the phase shift between adjacent frames of S along the time axis (i, i + 1): di / arg Si Siþ1 ð1Þ The resulting time-resolved displacement field was then decomposed into complex Fourier components at vibration frequency (f = 27, 33, 39, 44, 50 and 56 Hz). Because of the low frame rate of 80 Hz, all higher vibration frequencies with a frequency above frame rate/2
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Fig. 1. Experimental setup consisting of the vibration unit mounted underneath the head of the bed, marked in red (1), standard ultrasound scanner (2), elastography computer (3), probe holder (head set) and breathing mask (4), gas bottle for carbogen supply (5) and patient monitoring system (6). (a) Enlarged head set for fixing the ultrasound probe. (b) Enlarged screen of the patient monitor with patient data. Heart rate, blood pressure, oxygen saturation and etCO2 were monitored.
(here: 44, 50 and 56 Hz) were above the Nyquist limit and appeared in aliased spectral positions (here: 36, 30 and 24 Hz). Therefore, complex-valued Fourier components were taken from spectral positions of 24, 27, 30, 33, 36 and 39 Hz, corresponding to the unaliased mechanical vibration frequencies of f = 56, 27, 50, 33, 44 and 39 Hz, respectively. Henceforth, this way of stroboscopic sampling of higher vibration frequencies by lower frame rates is referred to as controlled aliasing. The lateral phase shift caused by line-based image acquisition was corrected as explained in Tzschatzsch et al. (2016b). After this phase correction and noise suppression by a spatial bandpass filter, every complex wave image at vibration frequency was decomposed into unidirectional plane waves as further specified in Tzschatzsch et al. (2016a). In the next step, the reconstruction of wavenumbers k for every shear wave direction and every vibration frequency was derived from complex shear waves u employing the phase gradient method: k ¼ k r argðuÞ k
ð2Þ
Shear wave speed (SWS, in m/s), as a surrogate of CS, was then calculated by inversion of wavenumbers k for every wave direction and driving frequency. Finally, all SWS images were combined into a compound SWS map by amplitude-weighted averaging as explained in Tzschatzsch et al. (2016a). The weighted averaging for SWS compounding limits the effect of noise and low shear wave amplitudes (e.g., from wave directions that do not reflect true shear wave propagation) on SWS maps. Cerebral THE in hypercapnia The ultrasound transducer was positioned at the right temples of the volunteers based on the B-mode display of anatomic landmarks including the brainstem, basal cisterns, aqueduct and meninges, as illustrated in Figure 2. The region between these landmarks was identified as the temporal lobe parenchyma, in which SWS values were averaged. Identical to Tzschatzsch et al. (2018), SWS values larger than 1.2 m/s were considered trustworthy, while all values below that threshold were
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Fig. 2. Screenshots of ultrasound imaging and elastography setup. (a) Standard transtemporal B-mode image with 14-cm penetration depth; white dashed lines indicate the 8-cm penetration depth of our elastography method. (b) B-Mode image reconstructed from motion data. (c) Shear wave speed (SWS) map overlaid on the B-mode image shown in (b). Meninges (M), temporal parenchyma (TP), cerebellum (CB) and the hypo-echogenic brainstem (BS) surrounded by hyperechogenic basal cisterns (BC) and aqueduct (AQ) are highlighted by black and white lines.
neglected to minimize the effect of noise on our data. In our prior work, this threshold was empirically derived from the response of the reconstruction pipeline to pure Gaussian noise and tested in healthy volunteers over a wide age range (Tzschatzsch et al. 2018). However, it should be noted that this threshold still has to be validated in patients. To prevent displacement of the transducer relative to the temples during repeat measurements we used a custom-designed mounting device for the ultrasound probe, as illustrated in Figure 1. Volunteers were in a supine position on the vibration bed wearing a breathing mask throughout the experiment. Carbogen (Linde, Pullach, Germany), a gas mixture of 5% CO2 and 95% O2, was administered via the breathing mask by opening connections to a gas bottle and flow reservoir. Physiologic parameters of heart rate, blood oxygenation level (SpO2) and end-tidal CO2 (end-tidal CO2) were continuously recorded in 10-s increments using a mobile clinical patient monitoring system (Tesla Guard, Mammendorf, Germany), while blood pressure was recorded every 2 min. The experiment was subdivided into three phases, each 7 min in length: (i) baseline, (ii) carbogen breathing and (iii) recovery. Between the phases, the pipelines of the breathing gas were switched to administer carbogen to induce hypercapnia or normal air for recovery, resulting in a 30-s gap without data acquisition. A total of 129 SWS values (43 values for each phase) were acquired within 22 min.
Data analysis SWS maps were computed for each THE frame acquired with 10-s increments. Regions of interest (ROIs) in the temporal lobe parenchyma were manually drawn for spatially averaging SWS values to obtain the function of SWS over time for every volunteer. The times of hypercapnia-induced increases and decreases in SWS and etCO2, t i and t d, were determined by fitting the time-resolved parameter functions f(t) with the following sigmodal model function: f ðt Þ ¼ a þ
b 1 þ
eC1 ðtti Þ
þ eC2 ðttd Þ
ð3Þ
In this function, a and b represent baseline values and baseline values minus plateau values (D increase), while C1 and C2 denote the increasing and decreasing slopes of the function, respectively.
Statistics To determine the statistical significance of changes between time-averaged SWS values in each of the individual measurement intervals of baseline, hypercapnia and recovery, 25 SWS values from the second half of each interval were aggregated for every volunteer. A paired t-test was performed with the aggregated data. Furthermore, a paired t-test was used to test for statistically significant differences between time t parameters of SWS and etCO2. As ti and t d indicate the time
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responses of effective-medium stiffness including vessels to hypercapnia, we consider these parameters significant with respect to CVR. All p values <0.05 were considered to indicate statistical significance.
In Figure 3 are representative elastograms (SWS maps) and ROIs acquired in one volunteer during the baseline (a), carbogen challenge (b) and recovery (c) phases. There is a clearly perceivable increase in SWS in the temporal lobe ROI (temporal lobe tissue) during carbogen breathing (Fig. 3b) with a subsequent return to baseline values after the end of carbogen breathing (Fig. 3c). In Figure 4 are boxplots of SWS averaged over the second halves of the baseline, hypercapnia and recovery phases. Statistically significant differences were observed between SWS during hypercapnia and SWS values acquired before and after carbogen breathing (p = 0.001) but not between the two normal-breathing phases. Mean SWS values and standard deviations for every volunteer, as well as group mean SWS values and standard deviations, are summarized in Table 2. SWS values increased in all volunteers after breathing carbogen, with a mean increase of 0.09 § 0.06 m/s (range: 0.020.2 m/s). Figure 5 illustrates time-resolved group mean values and standard deviations of SWS (blue graph) and etCO2 (red graph). The baseline SWS of 1.57 § 0.04 m/s increased to a plateau value of 1.66 § 0.05 m/s (relative SWS change: 5.9 § 4.0 %, p = 0.001) during early hypercapnia at ti = 2.2 § 2.0 min. From that plateau, SWS decreased to normal values (1.59 § 0.03 m/s, p = 0.003) in the early recovery phase at td = 3.6 § 1.5 min. Baseline etCO2 was 32 § 3 mm Hg, which is within the normal range of 3035 mm Hg (Haubrich et al. 2013). From this normal range, etCO2 increased during early hypercapnia at ti = 1.9 § 1.3 min to a plateau value of 42 § 2 mm Hg (relative etCO2 change: 29.1 § 8.6 %, p = 0.00006), from where it returned to normal without
**
**
1.70 1.65 SWS in m/s
RESULTS
5
1.60
1.55 1.50
1.45
Baseline
Hypercapnia
Recovery
Fig. 4. Statistical illustration of the data. Group values corresponding to pooled data analysis, in which data were averaged over time during the second halves (late phases) of baseline, hypercapnia and recovery. Gray lines connect shear wave speed (SWS) values in each volunteer. Red symbols indicate outliers. Table 2. Mean shear wave speed values and standard deviations in each of the three phases analyzed: Baseline, hypercapnia and recovery Subject No.
Baseline* (m/s)
Hypercapnia* (m/s)
Recovery* (m/s)
1 2 3 4 5 6 7 8 9 10 Group*
1.61 (0.01) 1.55 (0.03) 1.61 (0.03) 1.56 (0.02) 1.60 (0.02) 1.58 (0.03) 1.54 (0.03) 1.56 (0.03) 1.49 (0.04) 1.60 (0.02) 1.57 (0.04)
1.72 (0.03) 1.57 (0.04) 1.64 (0.03) 1.62(0.04) 1.66 (0.04) 1.63 (0.02) 1.64 (0.03) 1.75 (0.05) 1.69 (0.04) 1.70 (0.03) 1.66 (0.05)
1.59 (0.02) 1.60 (0.03) 1.65 (0.03) 1.58 (0.02) 1.60 (0.03) 1.54 (0.05) 1.57 (0.03) 1.63 (0.06) 1.59 (0.06) 1.59 (0.03) 1.59 (0.03)
* Mean (standard deviation).
Fig. 3. Representative shear wave speed (SWS) maps of a volunteer taken during the second halves (late phases) of (a) baseline, (b) hypercapnia and (c) recovery. Regions of interest are demarcated by white lines.
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60
1.75
45
SWS in m/s
1.65 1.60
30
1.55
etCO2 in mmHg
1.70
1.50 τi 1.45 0
2
4
6
8
τd
10 12 14 16 18 20 Time in min
15 22
Fig. 5. Group mean shear wave speed (SWS) values (blue graph) and end-tidal CO2 (etCO2, red graph) with mean fit curves shown as dotted lines. Baseline mean values are demarcated by dashed lines, and standard deviations for SWS and etCO2 are represented by light gray and light red areas. Vertical lines indicate the transition to the next phase. The horizontal arrows indicate time points of increase (t i) and decrease (t d) in SWS and etCO2. Although t is nearly the same for both parameters during hypercapnia, it significantly differs during recovery.
significant delay at td = 0.9 § 0.4 min after stopping the gas supply. Although SWS and etCO2 increased with similar ti (p = 0.48), etCO2 returned to normal values significantly earlier than SWS (p < 0.001). Physiologic parameters monitored in this study (SpO2, heart rate, breathing frequency and blood pressure) did not change because of hypercapnia (see Supplementary Figure S1, online only). DISCUSSION To our knowledge, this is the first time-resolved quantification of CS changes in volunteers exposed to elevated levels of CO2 in breathing gas. Although changes in hypercapnia-induced CBF have been extensively studied and are exploited to measure cerebral autoregulation and CVR (Brian 1998; Ainslie and Duffin 2009), our data suggest that CS also is influenced by effects related to the regulation of vascular pressure and blood perfusion in the brain. Consequently, THE-measured CS holds promise as a new imaging marker for cerebral autoregulation and CVR that is affected by many neurovascular diseases (Markus and Cullinane 2001; Balucani et al. 2012; Silvestrini et al. 2013). It has been proposed that the mechanism underlying CBF changes in hypercapnia is enhanced washout of CO2 from the brain tissue for attenuating a further rise in partial pressure (PCO2) (Ainslie and Duffin 2009). As hypercapnia is associated with an elevated ICP (Brian 1998; Gelb et al. 2008), the THE-measured CS might reflect, at least partially, ICP changes, which would be
consistent with previous observations in volunteers performing the Valsalva maneuver (Tzschatzsch et al. 2018). Further speculating on a possible link between CS and ICP, we can compare our results with those of Van Hulst et al. (2002), who reported an ICP increase in pigs, because of hypercapnia, of approximately 7 % for each 10 mm Hg increase in PCO2, which is in the range of CS changes measured by THE (approximately 6 %). In addition, Arani et al. (2018) reported a linear increase in CS in the range of 0.6 %/mm Hg ICP (at 60-Hz vibration frequency). Combining this with the rate of change of 0.7 %/mm Hg reported by van Hulst et al. (2002), we can predict the increase in SWS per 1 mm Hg increase in ICP to be on the order of 0.75 %. Although this is similar to our value (0.59 % per 1 mm Hg), interspecies differences as well as lower excitation frequencies (mean THE frequency: 41.5 Hz, MRE: 60 Hz) might explain the slightly lower value found in our study. In principle, stiffness is a marker of a material’s intrinsic properties and can be compared across systems and modalities such as MRI and ultrasound. Our previous work in MRE revealed a hypercapnia-induced increase in brain stiffness of 3.3 § 1.9 %, which overlaps with the range of values observed in the present study (5.9 § 4.0 %) (Hetzer et al. 2018). Notably, voluntary abdominal muscle contraction during breath hold (Valsalva maneuver) increases CS by about 10.8 § 2.5 % (Tzschatzsch et al. 2018), suggesting that ICP is approximately twice as effectively increased by a Valsalva maneuver as by carbogen. Baseline values in Tzschatzsch et al. (2018) (1.56 § 0.08 m/s) are similar to our present values (1.57 § 0.04
ARTICLE IN PRESS Cerebral stiffness response to hypercapnia in humans B. KREFT et al.
m/s), illustrating the high consistency of CS values measured by transtemporal THE. Previous studies reported CBF to rise with only a short delay of 34 § 10 s after a PCO2 increase (Ogoh et al. 2009). Although our method did not resolve such short delays, our observations suggest an instantaneous response of CS to the increase in etCO2 in hypercapnia. The observed delayed recovery of SWS relative to etCO2 is an interesting finding that may point to either delayed vasoconstrictive responses of brain vessels compared with vasodilation or a delayed gasblood exchange with normal air in the lungs after 7 min of carbogen breathing. A limitation of our study is that we cannot provide a mechanistic validation of the effects of CBF or ICP on CS. We assume that CS is coupled to CBF and ICP by the nonlinear elastic properties of expanding vessel walls, which are embedded in the effective-medium soft-solid brain tissue below the scale of image resolution (Parker 2015; Arani et al. 2018). In addition, we measured a wide range of SWS changes, possibly a result of different CO2 uptakes, among all patients. In fact, it is known that the partial pressure of CO2 in the arterial blood (PaCO2) differs from that in brain tissue (PtCO2), as well as from etCO2 (Hare et al. 2013). Otherwise, it has been shown that CBF is similarly affected by PaCO2 and etCO2 (Young et al. 1991), indicating the feasibility of etCO2 recordings as a ground truth of hypercapnia-induced CS changes in the brain. Furthermore, the way in which ICP and cerebral perfusion are coupled into our CS values remains to be investigated by measuring CS changes after manipulation of cerebral hemodynamics with a protocol similar to that used in the present study. It would be interesting to see if hyperventilation (hypocapnia) or hypoxia (O2 deprivation of the brain) influence CS in a way similar to that seen here. Ultimately, cerebral THE should be tested in patients with elevated ICP before and after decompression treatment such as by lumbar puncture. The development of non-invasive state-of-the-art ultrasound elastography techniques toward quantification of CS and related tissue pressure is clinically desirable for many neurologic applications (Hirsch et al. 2017). In summary, we used a novel transtemporal THE technique to quantify CS changes induced by carbogen breathing in healthy volunteers. Our data reveal an increase in CS caused by hypercapnia of approximately 6 %, which is consistent with published data on ICP increases and suggests that THE-measured CS is sensitive to cerebral vasodilation. Transtemporal THE might be useful for CS-related studies of brain physiology as well as for the non-invasive quantification of CVR in clinical applications. Acknowledgments—Support of the German Research Foundation (GRK2260 BIOQIC and SFB1340 Matrix in Vision) is gratefully acknowledged. The authors declare no conflict of interest. Conflict of interest disclosure—The authors have no conflicts of interest to disclose
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SUPPLEMENTARY MATERIALS Supplementary material associated with this article can be found in the online version at doi:10.1016/j.ultra smedbio.2019.12.019.
REFERENCES Ainslie PN, Duffin J. Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: Mechanisms of regulation, measurement, and interpretation. Am J Physiol Regul Integr Comp Physiol 2009;296:R1473–R1495. Arani A, Min HK, Fattahi N, Wetjen NM, Trzasko JD, Manduca A, Jack CR, Jr, Lee KH, Ehman RL, Huston J, III. Acute pressure changes in the brain are correlated with MR elastography stiffness measurements: Initial feasibility in an in vivo large animal model. Magn Reson Med 2018;79:1043–1051. Balucani C, Viticchi G, Falsetti L, Silvestrini M. Cerebral hemodynamics and cognitive performance in bilateral asymptomatic carotid stenosis. Neurology 2012;79:1788–1795. Bernier J, Denekamp J, Rojas A, Trovo M, Horiot JC, Hamers H, Antognoni P, Dahl O, Richaud P, Kaanders J, van Glabbeke M, Pierart M. ARCON: Accelerated radiotherapy with carbogen and nicotinamide in non small cell lung cancer: A phase I/II study by the EORTC. Radiother Oncol 1999;52:149–156. Bilston LE. The effect of perfusion on soft tissue mechanical propertiesa: Computational model. Comput Methods Biomech Biomed Eng 2002;5:283–290. Brian JE, Jr. Carbon dioxide and the cerebral circulation. Anesthesiology 1998;88:1365–1386. Chen JJ, Pike GB. MRI measurement of the BOLD-specific flowvolume relationship during hypercapnia and hypocapnia in humans. Neuroimage 2010;53:383–391. Ertl M, Raasch N, Hammel G, Harter K, Lang C. Transtemporal investigation of brain parenchyma elasticity using 2-D shear wave elastography: Validation appreciated!. Ultrasound Med Biol 2019;45:1344. Fehlner A, Behrens JR, Streitberger KJ, Papazoglou S, Braun J, Bellmann-Strobl J, Ruprecht K, Paul F, Wurfel J, Sack I. Higher-resolution MR elastography reveals early mechanical signatures of neuroinflammation in patients with clinically isolated syndrome. J Magn Reson Imaging 2016;44:51–58. Fierstra J, Sobczyk O, Battisti-Charbonney A, Mandell DM, Poublanc J, Crawley AP, Mikulis DJ, Duffin J, Fisher JA. Measuring cerebrovascular reactivity: What stimulus to use? J Physiol 2013;591: 5809–5821. Gelb AW, Craen RA, Rao GS, Reddy KR, Megyesi J, Mohanty B, Dash HH, Choi KC, Chan MT. Does hyperventilation improve operating condition during supratentorial craniotomy? A multicenter randomized crossover trial. Anesth Analg 2008;106:585–594 table of contents. Gennisson JL, Grenier N, Combe C, Tanter M. Supersonic shear wave elastography of in vivo pig kidney: Influence of blood pressure, urinary pressure and tissue anisotropy. Ultrasound Med Biol 2012;38:1559–1567. Gerischer LM, Fehlner A, Kobe T, Prehn K, Antonenko D, Grittner U, Braun J, Sack I, Floel A. Combining viscoelasticity, diffusivity and volume of the hippocampus for the diagnosis of Alzheimer’s disease based on magnetic resonance imaging. Neuroimage Clin 2018;18:485–493. Giller CA, Bowman G, Dyer H, Mootz L, Krippner W. Cerebral arterial diameters during changes in blood pressure and carbon dioxide during craniotomy. Neurosurgery 1993;32:737–741 discussion 4142. Hare HV, Germuska M, Kelly ME, Bulte DP. Comparison of CO2 in air versus carbogen for the measurement of cerebrovascular reactivity with magnetic resonance imaging. J Cereb Blood Flow Metab 2013;33:1799–1805. Hatt A, Cheng S, Tan K, Sinkus R, Bilston LE. MR elastography can be used to measure brain stiffness changes as a result of altered
ARTICLE IN PRESS 8
Ultrasound in Medicine & Biology
cranial venous drainage during jugular compression. AJNR Am J Neuroradiol 2015;36:1971–1977. Haubrich C, Steiner LA, Diehl RR, Kasprowicz M, Smielewski P, Pickard JD, Czosnyka M. Doppler flow velocity and intra-cranial pressure: Responses to short-term mild hypocapnia help to assess the pressurevolume relationship after head injury. Ultrasound Med Biol 2013;39:1521–1526. Hetzer S, Birr P, Fehlner A, Hirsch S, Dittmann F, Barnhill E, Braun J, Sack I. Perfusion alters stiffness of deep gray matter. J Cereb Blood Flow Metab 2018;38:116–125. Hetzer S, Dittmann F, Bormann K, Hirsch S, Lipp A, Wang DJ, Braun J, Sack I. Hypercapnia increases brain viscoelasticity. J Cereb Blood Flow Metab 2019;39:2445–2455. Hirsch S, Braun J, Sack I. Magnetic resonance elastography: Physical background and medical applications. Weinheim: Wiley-VCH; 2017. Ipek-Ugay S, Tzsch€atzsch Hudert C, Marticorena Garcia S, Fischer T, Braun J, Althoff C, Sack I. Time harmonic elastography reveals sensitivity of liver stiffness to water ingestion. Ultrasound Med Biol 2016;42:1289–1294. Lipp A, Trbojevic R, Paul F, Fehlner A, Hirsch S, Scheel M, Noack C, Braun J, Sack I. Cerebral magnetic resonance elastography in supranuclear palsy and idiopathic Parkinson’s disease. Neuroimage Clin 2013;3:381–387. Lipp A, Skowronek C, Fehlner A, Streitberger KJ, Braun J, Sack I. Progressive supranuclear palsy and idiopathic Parkinson’s disease are associated with local reduction of in vivo brain viscoelasticity. Eur Radiol 2018;28:3347–3354. Markus H, Cullinane M. Severely impaired cerebrovascular reactivity predicts stroke and TIA risk in patients with carotid artery stenosis and occlusion. Brain 2001;124:457–467. Marticorena Garcia SR, Fischer T, Durr M, Gultekin E, Braun J, Sack I, Guo J. Multifrequency magnetic resonance elastography for the assessment of renal allograft function. Invest Radiol 2016;51: 591–595. McDonnell MN, Berry NM, Cutting MA, Keage HA, Buckley JD, Howe PR. Transcranial Doppler ultrasound to assess cerebrovascular reactivity: Reliability, reproducibility and effect of posture. PeerJ 2013;1:e65. Millonig G, Friedrich S, Adolf S, Fonouni H, Golriz M, Mehrabi A, Stiefel P, Poschl G, Buchler MW, Seitz HK, Mueller S. Liver stiffness is directly influenced by central venous pressure. J Hepatol 2010;52:206–210. Murphy MC, Jones DT, Jack CR, Jr, Glaser KJ, Senjem ML, Manduca A, Felmlee JP, Carter RE, Ehman RL, Huston J, III. Regional brain stiffness changes across the Alzheimer’s disease spectrum. Neuroimage Clin 2015;10:283–290. Ogoh S, Ainslie PN, Miyamoto T. Onset responses of ventilation and cerebral blood flow to hypercapnia in humans: Rest and exercise. J Appl Physiol (1985) 2009;106:880–886. Parker KJ. Experimental evaluations of the microchannel flow model. Phys Med Biol 2015;60:4227–4242. Parker KJ. Are rapid changes in brain elasticity possible?. Phys Med Biol 2017;62:7425–7439. Rich M, Scheinberg P, Belle MS. Relationship between cerebrospinal fluid pressure changes and cerebral blood flow. Circ Res 1953;1: 389–395. Rotemberg V, Palmeri M, Nightingale R, Rouze N, Nightingale K. The impact of hepatic pressurization on liver shear wave speed
Volume 00, Number 00, 2019 estimates in constrained versus unconstrained conditions. Phys Med Biol 2012;57:329–341. Sack I, Beierbach B, Wuerfel J, Klatt D, Hamhaber U, Papazoglou S, Martus P, Braun J. The impact of aging and gender on brain viscoelasticity. Neuroimage 2009;46:652–657. Sack I, Streitberger KJ, Krefting D, Paul F, Braun J. The influence of physiological aging and atrophy on brain viscoelastic properties in humans. PlosOne 2011;6:e23451. Silvestrini M, Viticchi G, Falsetti L, Balucani C. Cerebral hemodynamics and cognitive performance in bilateral asymptomatic carotid stenosis. Author response. Neurology 2013;80:2080–2081. Streitberger KJ, Sack I, Krefting D, Pfuller C, Braun J, Paul F, Wuerfel J. Brain viscoelasticity alteration in chronic-progressive multiple sclerosis. PLoS One 2012;7:e29888. Tzsch€atzsch H. Methods and approaches in ultrasound elastography. In: Sack I, Sch€affter T, (eds). Quantification of biophysical parameters in medical imaging. Berlin/New York: Springer; 2017. p. 281–302. Tzschatzsch H, Guo J, Dittmann F, Hirsch S, Barnhill E, Johrens K, Braun J, Sack I. Tomoelastography by multifrequency wave number recovery from time-harmonic propagating shear waves. Med Image Anal 2016;30:1–10. Tzschatzsch H, Nguyen Trong M, Scheuermann T, Ipek-Ugay S, Fischer T, Schultz M, Braun J, Sack I. Two-dimensional time-harmonic elastography of the human liver and spleen. Ultrasound Med Biol 2016;42:2562–2571. Tzschatzsch H, Kreft B, Schrank F, Bergs J, Braun J, Sack I. In vivo time-harmonic ultrasound elastography of the human brain detects acute cerebral stiffness changes induced by intracranial pressure variations. Sci Rep 2018;8:17888. Tzschatzsch H, Kreft B, Braun J, Sack I. Transtemporal Investigation of Brain Parenchyma Elasticity Using 2-D Shear Wave Elastography: Trustworthy? Ultrasound Med Biol 2019;45:1344–1345. van Hulst RA, Hasan D, Lachmann B. Intracranial pressure, brain PCO2, PO2, and pH during hypo- and hyperventilation at constant mean airway pressure in pigs. Intensive Care Med 2002;28:68–73. Vejdani-Jahromi M, Freedman J, Nagle M, Kim YJ, Trahey GE, Wolf PD. Quantifying myocardial contractility changes using ultrasound-based shear wave elastography. J Am Soc Echocardiogr 2017;30:90–96. Willie CK, Colino FL, Bailey DM, Tzeng YC, Binsted G, Jones LW, Haykowsky MJ, Bellapart J, Ogoh S, Smith KJ, Smirl JD, Day TA, Lucas SJ, Eller LK, Ainslie PN. Utility of transcranial Doppler ultrasound for the integrative assessment of cerebrovascular function. J Neurosci Methods 2011;196:221–237. Wuerfel J, Paul F, Beierbach B, Hamhaber U, Klatt D, Papazoglou S, Zipp F, Martus P, Braun J, Sack I. MR-elastography reveals degradation of tissue integrity in multiple sclerosis. Neuroimage 2010;49:2520–2525. Yang XF, Shi XY, Ju J, Zhang WN, Liu YJ, Li XY, Zou LP. 5% CO2 inhalation suppresses hyperventilation-induced absence seizures in children. Epilepsy Res 2014;108:345–348. Young WL, Prohovnik I, Ornstein E, Ostapkovich N, Matteo RS. Cerebral blood flow reactivity to changes in carbon dioxide calculated using end-tidal versus arterial tensions. J Cereb Blood Flow Metab 1991;11:1031–1035. Zwanenburg JJM, van Osch MJP. Targeting cerebral small vessel disease with MRI. Stroke 2017;48:3175–3182.