Study of cerebrovascular reserve capacity by magnetic resonance perfusion weighted imaging and photoacoustic imaging

Available online at www.sciencedirect.com

Magnetic Resonance Imaging 27 (2009) 155 – 162

Study of cerebrovascular reserve capacity by magnetic resonance perfusion weighted imaging and photoacoustic imaging☆ Quan Zhou a,b,⁎, Yang Dongc , Li Huanga , Shihua Yang b , Wenli Chend a

b

Medical Imaging Center, First Affiliated Hospital of Jinan University, Guangzhou 510630, China MOE Key Laboratory of Laser Life Science and Institute of Laser Life Science, South China Normal University, Guangzhou 510631, China c GD Hospital of Traditional Chinese Medicine, Guangzhou 510120, China d College of Life Science, South China Normal University, Guangzhou 510631, China Received 23 March 2008; revised 24 June 2008; accepted 1 July 2008

Abstract The aim of this work was to assess the feasibility of photoacoustic imaging (PAI) and MR imaging for evaluating the cerebrovascular reserve capacity (CVRC) in animal models. Wistar-Kyoto (WKY) rats and spontaneous hypertensive rats (SHR) were used for MRI. BALB/c mice were used for PAI. MR perfusion weighted imaging (PWI) was performed on a 1.5-T whole-body MR system before and after oral administration of acetazolamide (ACZ). The region of interest (ROI) was chosen in the bilateral frontal lobe for measuring regional cerebral blood flow (rCBF), regional cerebral blood volume (rCBV) and mean transit time (MTT). The vessel diameters of the superficial layer of the cortex were measured by PAI in the resting and ACZ-activated mice. The results showed that there was a statistical difference between the resting and ACZ-activated animals in vessel diameter, rCBV and rCBF values. The increments in rCBV and rCBF of WKY rats between resting and ACZ test states were significantly higher than that of SHR. The pathological findings of small arterial walls and lumen of the brain were also different between WKY and SHR rats. The diameters of blood vessels in the superficial layer of the brain measured by PAI were enlarged after the ACZ tolerance test. This result was also observed in the MRI CBV map, where the signal of the vessel in the superficial layer of the cortex became redder after the ACZ stimulation, suggesting the increase of blood flow. It can be concluded that MR PWI and PAI combined with the ACZ test might be useful in evaluating the CVRC and revealing the pathologic changes in cerebral vessels. © 2009 Elsevier Inc. All rights reserved. Keywords: Perfusion-weighted imaging (PWI); Cerebrovascular reserve capacity (CVRC); Photoacoustic imaging (PAI); Magnetic resonance imaging (MRI)

Abbreviations: ACZ, acetazolamide; CVRC, cerebrovascular reserve capacity; MRI, magnetic resonance imaging; MTT, mean transit time; PAI, Photoacoustic imaging; PWI, perfusion weighted imaging; rCBF, regional cerebral blood flow; rCBV, regional cerebral blood volume; ROI, region of interest; T1WI, T1-weighted imaging; T2WI, T2-weighted imaging; ΔrCBF, margin of regional cerebral blood flow between the resting and ACZactivated state. ☆ This research was supported by the National Natural Science Foundation of China (60378043; 30470494; 30627003), the Natural Science Foundation of Guangdong Province (06025211; 015012; 2004B10401011) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry, 2004-180). ⁎ Corresponding author. Medical Imaging Center, First Affiliated Hospital of Jinan University, Guangzhou 510630, China. Tel.: +86 20 85228497; fax: +86 20 38688416. E-mail addresses: [email protected] (Q. Zhou), [email protected] (Y. Dong), [email protected] (L. Huang), [email protected] (S. Yang), [email protected] (W. Chen). 0730-725X/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.mri.2008.07.002

1. Introduction Cerebrovascular diseases are the leading causes of death in industrial countries, and approximately 50% of survivors have a residual neurologic deficit and more than 25% require long-term care [1]. Cerebrovascular diseases occur when the blood vessels supplying the brain with oxygenated blood are damaged or their function compromised. Early diagnosis and treatment are the key to reducing mortality and increase curability. Therefore, a technique that would accurately reveal vessel damages would be of a great value. Cerebrovascular reserve capacity (CVRC) describes how far cerebral perfusion can be increased after stimulation [2]. CVRC is a useful index in evaluating the cerebral vascular reserve function and in assessing the

156

Q. Zhou et al. / Magnetic Resonance Imaging 27 (2009) 155–162

subclinical conditions of the cerebrovascular diseases before their onset. It is of great clinical relevance to measure hemodynamic parameters such as the diameter of cerebral vessels, regional cerebral blood flow (rCBF), regional cerebral blood volume (rCBV) and mean transit time (MTT). MTT is the mean time required by tracer particles to pass through the capillary bed and can be calculated as rCBV/rCBF [3]. Single-photon emission computed tomography (SPECT) and positron emission tomography (PET) are mature techniques for measuring the resting and acetazolamide (ACZ)-activated cerebral blood flow without sampling blood [4,5]. The ACZ dilates the cerebral vessels by increasing the blood CO2 level and induces a local extracellular acidosis in the brain that triggers an increase in rCBF [4]. However, they have the disadvantage of potential radiation damage and inadequate resolution for the functional mapping of the cortex. MR perfusion weighted imaging (MR PWI) with the wellestablished ACZ tolerance test has been used for assessing brain function [6]. Photoacoustic imaging (PAI) is a noninvasive and nonionizing imaging modality based on different absorptions of laser light by biological tissues. PAI can successfully visualize mouse brain microvasculature and physiological changes [7,8]. Coupled with the ACZ tolerance test, MRI and PAI might be useful means for assessing CVRC. In this study, the values of rCBV, rCBF and vessel diameter were measured by MR PWI and PAI in conjunction with the ACZ tolerance test in rat and mice models. The aim of this work was to assess and compare the effectiveness and feasibility of MRI and PAI for the evaluation of CVRC. It is well known that hypertension is the most common cause of cerebrovascular diseases. Patients with hypertension are often accompanied with a decrease in CVRC [9]. Wistar-Kyoto (WKY) rats and spontaneous hypertensive rats (SHR — derived from WKY rats) are commonly used small animal models for studying hypertension and brain function [10]. In this study, they were used as normal controls and hypertension models for the assessment of the effectiveness of noninvasive MR PWI for the evaluation of CVRC. To explore the feasibility of noninvasive PAI for evaluation of CVRC, BALB/c mice were used because their size fits with our PAI system although they were not suitable for our MR system.

2.2. Photoacoustic imaging protocol The photoacoustic system for animal brain imaging is shown in Fig. 1, where a laboratory coordinate system (x, y, z) is also depicted. An Nd:YAG laser (532 nm; LS2134, LOTIS TII, Belarus) was employed to provide laser pulses with a pulse width of 10 ns and a repetition rate of 15 Hz. The wavelength was selected for the high absorption of blood. The laser beam was homogenized by a ground glass and expanded by a concave lens. The beam (diameter=1.3 mm) was then irradiated onto the mouse brain. The incident energy density on the brain surface was controlled at 8 mJ/cm2, which was less than the maximal permissible exposure set by the American National Standards Institute [11]. A hydrophone (diameter=1.0 mm; Precision Acoustics, Ltd., Dorchest, UK) with a sensitivity of 850 nV/Pa was driven by a computer-controlled step motor to capture signals. Water was used to couple the hydrophone and the mouse head. The hydrophone could scan circularly around the mouse head in the horizontal plane (x–y plane). The photoacoustic signals acquired from the hydrophone were amplified and subsequently recorded by a digital oscilloscope (TDS3032, Tektronix, USA) at a sampling rate of 500 million samples per second. At each angle, the photoacoustic scans were repeated 64 times. The typical scanning radius was 4 cm and the scanning step was 1.8°. A circular scan of 360° with 200 steps took 15 min. The optical absorption distribution was extracted by a modified filtered back-projection algorithm [12]. Photoacoustic signals were processed by a computer and then the imaging of blood vessels in the superficial layer of the mouse brain was reconstructed. The diameters of blood vessels in mouse brain were measured and recorded. Before experiment, the fur on the mouse head was shaven and chemically depilated. The mice were anesthetized by sodium pentobarbital (40 mg/kg) during experiment. A temperature controller was used to keep the temperature of the coupling water at 37°C. Then the anesthetized mouse was placed in a home-made stereotaxic holder. The mouse head was protruded into the water tank through a hole at the bottom of the tank and underneath a piece of clear

2. Materials and methods 2.1. Experimental animals Specific pathogen-free animals were purchased from the agent of Charles River Laboratories, Inc. (Beijing, China). WKY (12 weeks old, n=10, body weight: 350±20 g) and SHR (20 weeks old, n=10, body weight: 350±15 g) were used for MR imaging. BALB/c mice (4 weeks old, n=10, body weight: 30±5 g) were used for PAI.

Fig. 1. Photoacoustic imaging system for noninvasive imaging of mouse brain.

Q. Zhou et al. / Magnetic Resonance Imaging 27 (2009) 155–162

membrane. A thin layer of ultrasonic coupling gel was applied to the surface of the mouse head to couple the head and the flexible membrane at the bottom of the water tank. After the experiment, all the mice were sacrificed and openskull photographs of the brain cortex were taken. 2.3. MR Imaging protocol MRI scans were performed on a clinical 1.5-T wholebody MR system (GE Signa HD MR) with a conventional gradient system and 4-in. surface coil. Before perfusion, T1-weighted fluid attenuation inversion recovery (T1Flair) [200 ms repetition time (TR), 17 ms echo-time (TE)] and fast spin echo T2-weighted MR images (TR: 2000.0 ms, TE: 85.0 ms) were acquired to establish baselines. Perfusion weighted MR imaging (TR: 1500.0 ms, TE: 37.14 ms) were performed after a bolus injection (1 ml/s) of Gd-DTPA (1.6 mmol/kg body weight) through the iliac communis vein. Contrast medium MRI used T1WI (TR: 200 ms, TE: 17.0 ms). Images were acquired with a 128×128 raw matrix and interpolated to 256×256 after data acquisition. A field-of-view (FOV) of 8 mm and a slice thickness of 4 mm were used to obtain a high spatial resolution. Scan time was 61 s with no interscan delay. Images were transferred to a workstation for data processing. The region of interest (ROI) was chosen in the bilateral frontal lobe, which was within the vascular zone of the middle cerebral artery (MCA). The MCA could also be observed by our PAI system. Oval ROIs of 10 pixels were drawn and automatically repeated on all the images of the time series in the same position of both sides of the brain (Fig. 2) [13]. The rCBV, rCBF and MTT values and all the indexes of CVRC were measured in a workstation (GE Advantage Workstation: AW4.2_07). Hemodynamic parameters could be determined from the changes in the perfusion curve. This perfusion curve in Fig. 2C shows the signal intensity time series. All variables (rCBV, MTT and rCBF) were calculated by numerical

157

integration. MTT was expressed in seconds, while rCBV and rCBF were expressed in arbitrary units. Statistical analyses (paired-samples t test and one-way ANOVA) were performed by using the software SPSS 13.0 for Windows. P values less than .05 were considered as statistically significant. 2.4. Acetazolamide tolerance test First, MR PWI or PAI was performed before the ACZ stimulation to obtain baselines. Then, an identical MR PWI or PAI examination was performed 2.5 h after the oral administration of ACZ (4 mg/kg body weight). The pre- and post-ACZ measurements (i.e., rCBV, rCBF and MTT in the ROI of the bilateral frontal lobe and the diameter of cerebral vessels in the superficial layer of the mouse cortex) were used for the assessment of CVRC. 2.5. The pathological examination of the rat brain After the ACZ challenge experiment and MRI PWI scans, SHR and WKY rats were sacrificed and their brains were examined after hematoxylin–eosin (H&E), elastin van Gieson and Masson's trichrome staining. The arteriolar wall and lumen in the same ROI were examined under a light microscopy (magnification=×400). The number of smooth muscle per unit area (FOV under light microscopy), the diameter of arteriolar lumen (μm) and the diameter of whole small artery (μm) were measured. The number of capillary vessels and arteriolar in the same brain ROI of WKY and SHR rats was measured under a light microscopy (magnification=×100). 3. Results 3.1. Photoacoustic imaging of the mouse brain Fig. 3 shows the PAI of a mouse brain obtained noninvasively through the intact skin and skull. These

Fig. 2. MR imaging of axial scan T2WI (A), CBV map (B) and perfusion curve (C) in WSK rats show the choice of ROI in both sides of the frontal lobe and the perfusion curve.

158

Q. Zhou et al. / Magnetic Resonance Imaging 27 (2009) 155–162

Fig. 3. Photoacoustic imaging of cerebral hemodynamic changes in response to the ACZ stimulation. Noninvasive photoacoustic image of the vessels in the superficial layer of the mouse cortex acquired under both resting (A) and ACZ test state (B). The gross photograph (C) of the mouse brain was acquired after experiment (tolerance test).

images had a very high resolution and the vasculatures in the superficial layer of the cortex were clearly visible (Fig. 3A and B). The PAI was in a good agreement with the gross image (Fig. 3C). It was obvious that PAI could clearly show the structures of the superficial layer of the mouse brain. Comparing PAI obtained with and without the ACZ stimulation, it could be concluded that the diameters of blood vessels were enlarged after the ACZ tolerance test. The average size of vessels increased 120–140 μm based on the full widths at half maximum of the photoacoustic signal profiles. This experiment demonstrated the potential of PAI for cerebral functional hemodynamic imaging. 3.2. MR Imaging of the rat brain MR perfusion weight images were obtained before and after the ACZ stimulation with a 1.5-T whole-body MR system using a high-resolution surface coil, thin slice, small FOV and large matrix. The absolute values of rCBV, rCBF and MTT of WKY and SHR rats under the resting and ACZ test states are shown in Table 1. Calculated relative values (i.e., mean±S.D.) of (rCBV1−rCBV2)/ rCBV 1 , (rCBF 2 −rCBF 1 )/rCBF 1 and (MTT 2 −MTT 1 )/ MTT1 were 0.74±0.05, 0.84±0.16 and −0.04±0.02 in WKY rats (12 weeks old), and 0.26±0.11, 0.20±0.15 and 0.06±0.04 in SHR rats (20 weeks old), respectively. This means that, due to the ACZ stimulation, the mean values of rCBF and rCBV of WKY rats increased 74% and 84%, respectively. The results were also shown in perfusion curve and in MRI CBV map of WSK rats in Figs. 2B,C and 4, respectively. The MRI imaging could be used to determine the mean values of rCBF and rCBV in both hemispheres of a normal rat which had normal blood flow and normal CVRC. Those values increased significantly after the ACZ test. The signal changes from the vessels

located in the superficial layer of the rat cortex suggested the increase of blood flow after the ACZ stimulation. In contrast, the mean values of rCBF and rCBV of SHR rats increased only 26% and 20%, respectively. The mean values of the MTT of WKY rats decreased 4% after the ACZ stimulation, but increased 6% in SHR rats. Statistical analysis indicated that the values of rCBV and rCBF had statistically significant differences between resting and ACZ test states (Pb.05). Their relative increased values of rCBV and rCBF under the resting and ACZ test state between WKY and SHR rats were also statistically different (Pb.05). The values of MTT between the resting and ACZ-activated SHR rats had no statistical difference. After the ACZ tolerance test, rCBF and rCBV were significantly increased in WKY rats. These results suggested that MR PWI combined with the ACZ tolerance test could be used to evaluate the CVRC by measuring rCBF and rCBV. Although those values also increased in SHR, the incremental values were relatively small since the CVRC had already been decreased in SHR rats prior to the ACZ test. 3.3. The pathological images of the rat brain The changes in the small arterial wall and lumen of the brain of 12-week-old WKY and 20-week-old SHR

Table 1 Absolute values of cerebral hemodynamic parameters at the resting and ACZ test state (mean±S.D., n=10) Group

Experiment state

rCBV

rCBF

MTT

WKY (12 weeks old) SHR (20 weeks old)

Resting state ACZ test Resting state ACZ test

237.91±18.96 412.89±23.39 235.65±15.79 296.42±24.21

0.25±0.02 935.1±21.7 0.46±0.02 899.3±17.3 0.24±0.02 985.8±58.8 0.29±0.03 1041.6±66.6

Q. Zhou et al. / Magnetic Resonance Imaging 27 (2009) 155–162

159

Fig. 5. The small arterial wall and lumen of the brain of 12-week-old WKY (A) and 20-week-old SHR (B) rats under light microscopy (H&E staining, ×400).

Fig. 4. Parametric rCBV map of MRI PWI of WSK (12 weeks old) in control group under both resting (A) and ACZ test state (B). The color from blue to red means the blood flow from lower to higher. The gray imaging in both images is the same. The blood flow is normal in both hemispheres; the rCBV increased obviously (74%) after the ACZ test. The signal of the vessel in the superficial layer of the mouse cortex becomes redder after the ACZ stimulation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

rats under a light microscopy are shown in Table 2 and Fig. 5. The pathological findings in the small arterial wall and lumen of the brain were different between WKY and Table 2 Size of the small arterial wall and lumen of the brain of 12-week-old WKY and 20-week-old SHR rats (mean±S.D., n=10) Arteriolar wall and lumen in brain (×400) Number of Diameter of smooth arteriolar muscle cells lumen (μm)

WKY 23±2 SHR 28±3

Diameter of whole small artery (μm)

133.17±15.21 165.24±18.67 77.45±11.12 135.05±16.33

Number of capillary vessels in ROI (×200)

Arteriolar number in ROI (×100)

78±8 69±5

8±1 7±1

SHR rats. The SHR rat had a thickened arteriolar wall and shrunken arteriolar lumen. The 20-week-old SHR rat was in the intermediate stage of hypertension and the cerebral arteriole was remolded. This might suggest the pathological mechanism of CVRC decrease.

4. Discussion Cerebrovascular reserve reflects the capacity of the brain to maintain adequate blood flow in the case of decreased perfusion pressure which can assess the subclinical conditions of the cerebrovascular disease before their onset. A reliable measurement of this parameter is of great clinical relevance. Perfusion imaging, combined with a physiologic or pharmacologic challenge, is a direct method of measuring cerebrovascular reserve [3,4]. Clinically, consecutive SPECT and PET with ACZ tolerance test are mature techniques, and PET has become the gold standard for the quantitative assessment of cerebrovascular disorders and of the pathophysiology of stroke. SPECT and PET rely on a family of tracer methods for quantitatively measuring physiological changes in blood volume, blood flow and regional oxygen extraction. However, they are not widely

160

Q. Zhou et al. / Magnetic Resonance Imaging 27 (2009) 155–162

available to most clinicians not only because of the limited number of PET facilities, but also of their inadequate resolution [14]. MRI and PAI are useful modalities for the evaluation of the cerebrovascular function due to their highresolution, noninvasive and nonionizing features. Similar to SPECT and PET examinations, the ACZ tolerance test can also be used in conjunction with MRI and PAI. The MR method described in this article may offer a practical means to assess the hemodynamic state of the brain tissue with a spatial resolution higher than that of established radioactive tracer techniques. PWI detects changes in the homogeneity of local magnetic fields during the passage of a compact bolus of gadolinium, which causes a drop in signal intensity, providing information on the pattern of brain perfusion in the microvascular bed. For each ROI, signal intensity time series are generated. Upon arrival of the gadolinium bolus in the tissue, these time series exhibit a change in signal intensity due to the change of the apparent spin–spin relaxation rate (e.g., T2*). The signal intensity is proportional to the concentration of the contrast agent in the tissue. Thus, the signal intensity time series could be converted into concentration time series. All variables, such as rCBF, rCBV, MTT, ΔrCBF and ΔrCBV, can be measured by MR PWI under the resting and ACZ tolerance test and calculated by the numerical integration. In this study, MTT was expressed in seconds, while rCBV and rCBF were expressed in arbitrary units. The average rCBV, rCBF, MTT and regional cerebrovascular reserve (ΔrCBF and ΔrCBV) values can provide the important information about the cerebral tissue in the ROIs. Their examinations can reveal whether the brain tissue is potentially at risk of infarction. In this work, after ACZ stimulation the mean values of rCBF and rCBV increased 74% and 84% in WKY rats, respectively. These results suggest that the normal WKY rats have enough CVRC, whereas the mean values of rCBF and rCBV in SHR rats increased only 26% and 20%, respectively. Similar results are also reported in other studies [6,13,15,16], which further confirms the feasibility of the MR method for the assessment of CVRC. It is well known that high blood pressure is the most important independent risk factor for stroke and other vascular diseases. The incidence of stroke in people with hypertension is six times higher than that in people with normal blood pressure. CVRC in hypertensive diseases is even lower than that in control patients [9]. The brain anatomy of rats and mice is similar to that of the human brain. So rats and mice have been chosen as experimental animal models in this study. WKY and SHR are cognate rats, which have very good comparability. Many studies have used them to study hypertension and brain [10]. In this study, WKY was used as a normal control since their blood flow and the CVRC were normal. Their mean values of rCBF and rCBV in both hemispheres increased significantly after the ACZ test. However, those values in SHR did not increase significantly. MR PWI was used to detect those physiological and pathological changes.

The results showed that the CVRC were lower in SHR than in the control group. To reveal their pathological mechanism, histopathological studies of the brain of 12week-old WKY and 20-week-old SHR rats were performed when the SHR and WKY rats were sacrificed after finishing the MRI PWI ACZ challenge experiments. The 20-week-old SHR rat was in the intermediate stage of hypertensive disease and its cerebral arteriole had already remolded. CVRC reflects the cerebral vasomotor reactivity and cerebral vasodilatory capacity. With this capacity, our body can make an autoregulation to ensure enough cerebral blood supply. The physiological conditions of the cerebral blood vessels play important roles in maintaining their functions. In this study, the pathological findings showed that the SHR rat had a thickened arteriolar wall and shrunken arteriolar lumen, which might be the pathological mechanism of CVRC decrease in SHR. Due to the highresolution and multiparameter imaging of MRI, MR PWI combined with the ACZ tolerance test can be used to evaluate the CVRC, reveal the pathologic changes in cerebral vessels to some degrees, provide qualitative and semi-quantitative information on the cerebral perfusion and evaluate the cerebrovascular function. However, because its high resolution depends on the intensity of magnetic field, the size of surface coil and so on, MRI cannot show fine structure, such as the vasculature in the superficial layer of the mouse cortex, but it may still have a potential in evaluating cerebrovascular diseases. To explore feasibility of noninvasive PAI for evaluation of CVRC, BALB/c mice were used to test PAI. Photoacoustic imaging is a noninvasive, nonionizing imaging modality. It is based on the generation of acoustic waves by pulsed light being absorbed by tissue chromophores such as hemoglobin's in the blood. The amplitude of the generated ultrasound is dependent on the amount of absorbed light, being determined by the local energy fluence and the optical absorption coefficient of the target. Photoacoustic imaging is able to accurately localize the microvascular system because of the large optical absorption differences between blood and dermis. Photoacoustic imaging combines the advantages of both ultrasound imaging and optical imaging, and it can provide high ultrasonic resolution and high optical contrast tissue imaging [12,17,18]. The resulting image of the absorber distribution can be used for the quantification of blood vessel size or the vessel density, which is not only important for screening of tissue, but also for monitoring of treatment effects. Brain functional detection is a very promising application of PAI. Changes in optical signals can be associated with the brain's physiological and pathological status since they can reflect changes in blood volume, oxygen consumption and cellular swelling. This association enables optical contrast to be used to assess the physiological and functional properties of the brain. Based on intrinsic optical absorptions in biological tissues, the photoacoustic technique can provide a noninvasive method

Q. Zhou et al. / Magnetic Resonance Imaging 27 (2009) 155–162

to image functional hemodynamics and monitor regional oxygen consumption in brains with satisfactory spatial resolution. Compared to existing optical imaging techniques (e.g., confocal microscopy, optical coherence tomography, fluorescence imaging, etc.), PAI overcomes the overwhelming scattering of light in biological tissues by utilizing ultrasonic wave to translate the signal. Therefore, as the depth increases, PAI can provide higher spatial resolution compared to conventional optical techniques [19,20]. However, photoacoustic signal can neither reach the deep layer of the mouse brain nor penetrate through the skull in WKY rats (1–2 mm) because of its poor penetrability. Unlike MRI, PAI can only measure the diameter of the cerebral vessels. The study of CVRC by PAI in this work is only in the initial stage. Because of the limit of present PAI systems, only small animals such as mice can be used. Nonetheless, the results showed that the diameters of blood vessels in the superficial layer of the mouse brain were enlarged after the ACZ tolerance test, which was in good agreement with the finding on MRI CBV map; the signal of the vessel in the superficial layer of the rat cortex becomes redder after the ACZ stimulation, which suggests the increase of blood flow. Although the potential systematic errors arising from the comparison of rat and mouse data may exist, the results in this study can still provide the same qualitative information and semi-quantitative information on CVRC to some degrees. The vessel diameter measurements conducted by PAI might approximate the associated rCBV changes obtained by MR PWI. Therefore, PAI's rCBV estimations might be comparable to MR's rCBV measurements. Although we were not able to use the same animal models for PAI and MRI examinations due to the physical limitations of the two systems used in this study (i.e., PAI for mouse and MRI for rat only), it can be concluded that MR PWI and PAI combined with the ACZ test are the effective and feasible means in evaluating the CVRC. The preliminary results show that PAI is comparable to MR PWI. Our initial study explored the feasibility of PAI in small animals, but this technique can be potentially applied to human organs, such as the brain. In order to fulfill this goal, electromagnetic sources with longer wavelengths, such as NIR light and radiofrequency microwave, will be adopted [21–23]. Employing ultrasonic detector(s) with a wide detection bandwidth, PAI is potentially capable of visualizing anatomical information and functional activities in human brain cortex. Because of the high sensitivity of PAI to both blood oxygenation and blood flow, this technique will contribute to the management of many life-threatening illnesses, including severe traumatic brain injury, ischemia, sepsis and shock [8,24,25]. In addition, compact PAI systems for human brain can be developed at a low cost, which enable bedside continuous imaging and earlier initiation of monitor-

161

ing in emergency vehicles. Before reaching this goal, further research and validation of PAI in large animals are necessary. 5. Conclusions This study has demonstrated the effectiveness of the MRI PWI for the noninvasive monitoring of small animal brain CVRC. This observation indicates that the use of a combination of the ACZ tolerance test and MR PWI may offer an alternative to PET for monitoring CVRC by measuring the values of rCBV, rCBF and MTT, which could reflect the pathologic changes in cerebral vessels to some degrees. Comparing the photoacoustic images of mouse brain vascular between the resting and ACZ-activated states, it is concluded that the diameters of blood vessels were enlarged with drug stimulation, which suggests that there is an increase in CBF because of the ACZ action on the brain. It is feasible to use PAI for evaluating the CVRC of small animals. Although this is only the preliminary analysis, which needs further improvement and validation, this study demonstrates the potential of PAI for functional cerebral hemodynamic imaging. References [1] Bravata DM, Ho SY, Brass LM, Concato J, Scinto J, Meehan TP. Long-term mortality in cerebrovascular disease. Stroke 2003;34: 699. [2] Stolland M, Hamann GF. Cerebrovascular reserve capacity. Nervenarzt 2002;73:711. [3] Barbier EL, Lamalle L, De'corps M. Methodology of brain perfusion imaging. J Magn Reson Imaging 2001;13:496. [4] Bonte FJ, Devous MD, Reisch JS. The effect of acetazolamide on regional cerebral blood flow in normal human subjects as measured by SPECT. Invest Radiol 1998;23:564. [5] Nariai T, Suzuki R, Hirakawa K, Maehara T, Ishii K, Senda M. Vascular reserve in chronic cerebral ischemia measured by the acetazolamide challenge test: comparison with positron emission tomography. Am J Neuroradiol 1995;16:563. [6] Petrella JR, Provenzale JM. MR perfusion imaging of the brain: techniques and applications. Am J Roentgenol 2002;1:207. [7] Zeng Y, Xing D, Wang Y, Yin B, Chen Q. Photoacoustic and ultrasonic coimage with a linear transducer array. Opt Lett 2004;29: 1760. [8] Wang X, Pang Y, Ku G, Xie X, Stoica G, Wang LV. Noninvasive laserinduced photoacoustic tomography for structural and functional in vivo imaging of the brain. Nat Biotechnol 2003;21:803. [9] Baumbach GL, Heistad DD. Remodeling of cerebral arterioles in chronic hypertension. Hypertension 1981;13:968. [10] Danker JF, Duong TQ. Quantitative regional cerebral blood flow MRI of animal model of attention-deficit/hyperactivity disorder. Brain Res 2007;1150:217–24. [11] American National Standards Institute. American National Standard for the Safe Use of Lasers. Washington, DC: American National Standards Institute; 2000. Standard Z 136.1-2000. [12] Wang Y, Xing D, Zeng Y, Chen Q. Photoacoustic imaging with deconvolution algorithm. Phys Med Biol 2004;49:3117. [13] Apruzzese A, Silvestrini M, Floris R, Vernieri F, Bozzao A, Hagberg G, et al. Cerebral hemodynamics in asymptomatic patients with

162

[14]

[15]

[16]

[17]

[18]

Q. Zhou et al. / Magnetic Resonance Imaging 27 (2009) 155–162 internal carotid artery occlusion: a dynamic susceptibility contrast MR and transcranial Doppler study. Am J Neuroradiol 2001;22: 1062. Meyer JS, Shinohara T, Imai A. Imaging local cerebral blood flow by xenon-enhanced computed tomography technical optimization procedures. Neuroradiology 1998;30:283. Warach S, Li W, Ronthal M, Edelman RR. Acute cerebral ischemia: evaluation with dynamic contrast-enhanced MR imaging and MR angiography. Radiology 1992;182:41. Gückel FJ, Brix G, Schmiedek P, Piepgras A, Becker G, Köpke J, et al. Cerebrovascular reserve capacity in patients with occlusive cerebrovascular disease: assessment with dynamic susceptibility contrastenhanced MR imaging and the acetazolamide stimulation test. Radiology 1996;201:405. Yang D, Xing D, Gu H, Tan Y, Zeng L. Fast multielement phasecontrolled photoacoustic imaging based on limited-field-filtered backprojection algorithm. Appl Phys 2005;87:194101. Yang D, Xing D, Tan Y, Gu H, Yang S. Integrative prototype B-scan photoacoustic tomography system based on a novel hybridized scanning head. Appl Phys 2006;88:174101.

[19] Yin B, Xing D, Wang Y, Zeng Y, Tan Y, Chen Q. Fast photoacoustic imaging system based on 320-element linear transducer array. Phys Med Biol 2004;49:1339. [20] Yao Y, Xing D, Ueda KI, Chen Q. Technique for measurement of photoacoustic waves in situ with ultrasound probe beam. J Appl Phys 2003;94:1278. [21] Ku G, Wang LH. Scanning thermoacoustic tomography in biological tissue. Med Phys 2000;27:1195. [22] Xu M, Ku G, Wang LH. Microwave-induced thermoacoustic tomography using multi-sector scanning. Med Phys 2001;28:1958. [23] Feng D, Xu Y, Ku G, Wang LH. Microwave-induced thermoacoustic tomography: reconstruction by synthetic aperture. Med Phys 2001;28: 2427. [24] Yang S, Xing D, Zhou Q, Xiang L, Lao Y. Functional imaging of cerebrovascular activities using high resolution photoacoustic tomography. Med Phys 2007;34:3294. [25] Yang S, Xing D, Lao Y, Yang D, Zeng L, Xiang L, et al. Noninvasive monitoring of traumatic brain injury and posttraumatic rehabilitation with laser-induced photoacoustic imaging. Appl Phys Lett 2007;90: 24392.