High spatial resolution in vivo 2D 1H magnetic resonance spectroscopic imaging of human muscles with a band-selective technique

Magnetic Resonance Imaging 19 (2001) 1091–1096

High spatial resolution in vivo 2D 1H magnetic resonance spectroscopic imaging of human muscles with a band-selective technique Jiani Hua,*, Quan Jiangb, Yang Xiac, Chunsong Zuod a

Department of Radiology, Wayne State University, Detroit, MI 48201, USA Department of Neurology, Henry Ford Health Science Center, Detroit, MI 48202, USA c Department of Physics, Oakland University, Rochester, MI 48309, USA d Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02215, USA b

Received 14 April 2001; accepted 5 August 2001

Abstract This report demonstrates a 2D 1H magnetic resonance spectroscopic imaging (MRSI) technique that can address some technical difficulties often encountered in MRS studies of human muscles. A preliminary application of this whole-slice technique in human skeletal muscles demonstrates clearly noticeable differences in 1H metabolite spectra between different human muscles. This observation illustrates the importance of multi-voxel and high spatial resolution in a heterogeneous environment. This technique is robust, can be easily implemented on a commercial MR scanner, and should prove useful for investigators in both basic and clinical 1H MRS studies. © 2001 Elsevier Science Inc. All rights reserved. Keywords: High spatial resolution; Multi-voxel; Proton MRS; Band-selective technique; Muscle

1. Introduction Despite significant progress achieved in the past decade, in vivo 1H magnetic resonance spectroscopy (MRS) of organs other than the brain still faces a number of technical challenges. In human muscles, for example, lipid as well as tissue water can be massive, particularly for the whole-slice metabolite imaging of a human limb. The signals from both lipids and water, therefore, have to be sufficiently suppressed for the signals from metabolites to be observed without distortion. Due in part to these difficulties, in vivo 1 H MRS studies of muscles (human or animal) have been conducted largely using single-voxel techniques [1–13], with only a few exceptions [14,15]. While the shimming and data-process are relatively easy in the single-voxel techniques, these techniques have poor sampling efficiency and have difficulty accommodating irregularly shaped anatomy and/or pathology. Magnetic resonance spectroscopic imaging (MRSI) techniques can reduce these technical limitations by utilizing a large volume of interest (VOI) which * Corresponding author. Tel.: ⫹1-313-745-1389; fax: ⫹1-313-9667297. E-mail address: [email protected] (J. Hu).

is then partitioned into smaller, adjacent individual voxels [16,17]. Consequently, it is unnecessary to have a priori knowledge of the precise location and size of the anatomy/ pathology of interest. Spectra from multiple adjacent voxels can be used to observe local heterogeneity while other voxels, far from a pathologic site, can serve as internal controls. An important and unique property of MRSI is the ability to arbitrarily shift the voxels in the localization grid during post-processing. This makes the placement of a VOI in a pathologic or desired site more flexible and less timeconsuming. Originally developed for high resolution NMR [18,19], band-selective techniques can offer excellent frequency suppression for in vivo 1H spectroscopy while being insensitive to homonuclear multiple-quantum and polarization transfer effects [20 –22]. There are two slightly different approaches in the selection of the desired signals [23]. One approach uses a band-selective pulse to excite the undesired signals while leaving the desired signals undisturbed [22]. The other approach, which is used in this study, uses the band-selective pulse to excite the desired signals while suppressing the undesired ones [20,21]. In both cases, the undesired signals are suppressed by gradient pulses while the desired signals are refocused. The shape of the band-

0730-725X/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S 0 7 3 0 - 7 2 5 X ( 0 1 ) 0 0 4 3 8 - 6

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Fig. 1. The 2-part pulse sequence used in this study: the outer volume presaturation (OVP) and band-selective spin-echo pulse sequence.

selective pulse has been found to be critically important for the success and robustness of the frequency-suppression technique. In this paper, we provide a quantitative evaluation of the suppression efficiency of the band-selective technique. This evaluation may be used as a tool in selecting and optimizing the frequency-selective pulse for a given situation. In addition, we have successfully implemented the band-selective technique on a clinical scanner and obtained in vivo 2D high spatial resolution MRSI of human muscle proton metabolites from a whole slice of calf. Because of the lack of high spatial resolution multi-voxel 1H spectra from different human muscles for comparison in the literature, heterogeneous characteristics of proton metabolites in muscles have not been widely recognized. For example, the 1H spectra pattern of different muscles has not been systemically studied, and the total creatine has often been used as an internal reference even though the total creatine may not distributed uniformly across all muscles [9]. We demonstrate that there is a clear heterogeneity in the characteristics of 1H-metabolite spectra in human muscles.

2. Materials and methods 2.1. Pulse sequence Fig. 1 shows the pulse sequence utilized in this study. It consists of two parts: the outer volume pre-saturation (OVP) and the band-selective spin-echo sequence. The OVP was determined by applying slice-excitation pulses to select the area outside the VOI, and can be turned on or off as needed [24,25]. Each OVP excitation consists of a 2.56-ms sinc pulse (with a bandwidth of 3400 Hz) and a 2-mT/m gradient pulse in the pre-saturation slice direction, corresponding to a thickness of 40 mm. The band-selective spin-echo sequence consists of a slice-selective sinc pulse to define a VOI for imaging, followed by a frequency-selective (FS) 180° refocusing pulse with equally strong gradients on each

side of the FS pulse. The duration of the slice sinc pulse was 5.12 ms. The FS refocusing pulse was a 38.4 ms reburp pulse with a bandwidth of 90 Hz. The reburp pulse is a numerically optimized excitation pulse with a uniform excitation profile [26]. The spoiling gradients (G and G1) were (12 mT/m)*(6 ms) and (10 mT/m)*(5 ms), respectively. The undesired water and lipid signals are suppressed in two ways: (1) passively, by virtue of their position outside the frequency range of the 180° FS pulse and (2) actively, by the gradients that destroy the phase coherence of any residual undesired signals due to the imperfections of the FS pulse. This “double” suppression mechanism ensures efficient suppression of the undesired signal with a properly designed FS pulse. It is well known that the effective amplitude of magnetization for a spin-echo sequence after the rf pulse ␣1 and ␣2 can be expressed as M0sin(␣1)sin2(␣2) [27]. For our pulse sequence, because ␣1 ⫽ 90°, the dependence between the effective amplitude of the magnetization and the flip angle of the frequency-selective pulse (␣) can be written as M0sin2(␣) ⫽ M0[1 ⫺ cos (␣)]/2. Thus, the suppression factor (SF) of the band-selective sequence can be determined by the following equation: SF ⫽ 2/[1 ⫺ cos(␣)]

(1)

where ␣ is the flip angle of the frequency-selective pulse at a given frequency (or over a given frequency range). 2.2. Experimental procedure All studies were performed in a Siemens 1.5 Tesla whole-body clinical imager (VisionPlus). A circularly polarized knee coil was used for both imaging and spectroscopy of human muscles at rest. A spherical one-liter phantom filled with doped water (5 mM Cu(NO3)2) was used to obtain an experimental suppression factor to compare with the theoretical calculation by Eq (1). After automated shimming, the amplitudes of the water signal were obtained at different frequencies for the calculation of the suppression factor. For the human study (9 subjects, ages ranging from 23 to 69), the following procedure was used. After global shimming, standard spin-echo images were obtained to define the slice for the MRSI studies. Subsequently, a 1H spectrum from the whole slice of human calf muscle was obtained to check the performance of the global shimming and the suppression of the undesired signal before the MRSI acquisition. The MRSI spectra were then acquired. Typical measurement parameters were: field of view (FOV) ⫽ 300 ⫻ 300 mm2; phasing-encoding steps ⫽ 24 ⫻ 24, TR ⫽ 1200 ms, TE ⫽ 60 to 240 ms, slice thickness ⫽ 10 to 15 mm, and signal averages ⫽ 2. Data were processed using Siemens’ package for in-line post-processing. Data processing consisted of zero-filling to 32 ⫻ 32 in the k space, line broadening (Gaussian function with 256 ms), standard fast Fourier transformations, and spectrum phasing. No baseline correction was applied.

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terized by a ratio of TMA/tCr equal to or greater than one, while other muscles, such as tibialis posterior or gastrocnemius, have a spectra characterized by a ratio of TMA/tCr less than one. Table 1 lists the ratio of TMA/tCr amplitudes in gastrocnemius and soleus at a TE of 160 ms. The tprobability from the Student t-test analysis was less than 0.0001, demonstrating that the difference between the two muscle groups is highly significant.

4. Discussion

Fig. 2. Comparison between the experimental results (■) and theoretical (–) response curve of the reburp frequency-selective pulse.

3. Results Fig. 2 shows the comparison between the experimental results and the theoretical calculation for the excitation profiles of the reburp frequency-selective pulse. The experimental results were obtained by offsetting the carrier frequency with a well-shimmed water signal (a full-width at half-height (FWHH) of 4 Hz). The theoretical profile was obtained by substituting the flip angle of the reburp pulse into Eq (1), over the frequency range of interest. As illustrated, the agreement is excellent. Fig. 3a is a localized proton spectrum from the middle calf of a healthy volunteer using the band-selective scheme. The resonance at 3.2 ppm is from N-(CH3)3 of the compound class of trimethylammonium (TMA) which includes carnitine and cholines. The resonance at 3.0 ppm is from N-CH3 of creatine/phosphocreatine (tCr). The resonance at 3.9 ppm is from the methylene protons of total creatine (tCr2) or possibly from the methylene protons of phosphocreatine [8]. The resonances around 2.5 ppm and 1.4 ppm are from ␣-/␤-methylene (␣/␤ lipid) and the residual long chain methylene lipid protons, respectively. The resonances around 4.7 ppm are from residual water (H2O). The spectral features are assigned according to previous reports in the literature [9,11,28]. Fig. 3a clearly shows that both water and lipid were sufficiently suppressed. Fig. 3b is an axial MRI of the middle calf of a healthy volunteer. Fig. 3c is the 2.5–3.5 ppm region of the corresponding 2D 1H MRSI spectra from the whole slice without using OVP. These spectra were acquired at an echo-time (TE) of 160 ms. Other experimental parameters were: FOV ⫽ 300 ⫻ 300 mm2, slice thickness ⫽ 15 mm, TR ⫽ 1200 ms, signal average ⫽ 2, and phase-encoding steps ⫽ 24 ⫻ 24 (which is zero-filled to 32 ⫻ 32 during postprocessing). The high spatial resolution and quality 2D MRSI spectra in Fig. 3c clearly demonstrate that different muscles can give distinctively different 1H spectral patterns at a TE of 160 ms. For example, soleus spectra are charac-

Massive water and lipid signals impose technical difficulties for 1H MRSI of many tissues/organs other than the brain. This is especially true for a whole-slice 1H MRSI of human muscle, where lipid can be massive (as shown in Fig. 3a, where the strongest signal in the water/lipid suppressed spectra is the ␣-/␤-methylene lipid) and the air-tissue interface can be worse (compared to cases where the VOI is inside the muscle). The results in this report demonstrate that the band-selective technique can sufficiently address these technical difficulties. Unlike the commonly used CHESS techniques, which excite the undesired signals and then use the spoiling gradients to destroy them, the bandselective technique excites the desired signals while suppressing the undesired ones by the “double” suppression mechanism. Therefore, its suppression efficiency is insensitive to the B0 inhomogeneity as long as the desired signals (e.g., TMA and tCr around 3.0 –3.2 ppm in muscles) and the undesired signals (e.g., water and long-chain signal of lipid around 4.7 and 1.3 ppm, respectively) do not overlap. This makes the band-selective technique with a properly designed frequency-selective pulse an excellent tool for studies of proton metabolites in most tissues other than brain. As indicated by Eq (1), the excitation profile of the frequencyselective pulse directly determines the suppression efficiency of the band-selective scheme. For example, if a suppression factor of more than 10,000 is required at a given frequency (e.g., lipid or water resonance frequency, or a range of frequencies), the flip angle for the frequencyselective pulse at this frequency (or a range of frequencies) must be less than 1.14°. Ideally, the frequency-selective pulse should have a uniform 180° excitation profile over the frequency range of interest and a uniform 0° excitation profile outside the frequency range of interest. Ultimately, the trade-off between excitation profile and pulse length will determine the optimal pulse shape. Compared to a binomial pulse, the reburp pulse has a much better excitation profile [26]. An improved homogeneous excitation profile will be less sensitive to magnetic field inhomogeneity, as long as the frequency dispersion is within the uniform bandwidth of the frequency-selective pulse. Thus, a quantitative estimation of suppression efficiency for a frequency-selective pulse in a band-selective spin-echo scheme can be helpful in guiding the selection and/or design of a suitable frequency-selective

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J. Hu et al. / Magnetic Resonance Imaging 19 (2001) 1091–1096 Table 1 The ratio of TMA/tCr amplitude in gastrocnemius and soleus at a TE of 160 ms* Study (Sex, Age)

Gastrocnemius

Soleus

1 (Female, 24) 2 (Male, 23) 3 (Male, 28) 4 (Male, 35) 5 (Female, 32) 6 (Male, 46) 7 (Male, 69) 8 (Male, 47) 9 (Male, 48) Average

0.30 0.69 0.42 0.59 0.49 0.40 0.40 0.48 0.27 0.45 ⫾ 0.15**

1.33 1.27 0.84 0.92 1.27 1.23 1.46 1.28 1.06 1.20 ⫾ 0.20**

* Results are from nine healthy adults. ** The t-probability from the Student t-test analysis (unpaired data with unequal variance) was less than 0.0001.

Fig. 3. Typical proton spectra of the middle calf of a normal volunteer using a band-selective echo filter pulse sequence with an echo-time of 160 ms; (a) A proton spectrum from the middle calf of a healthy volunteer; (b) An axial MRI of the middle calf of a healthy volunteer; (c) The corresponding 2D 1H spectra. The resonances, labeled as TMA and tCr from left to right in one voxel are N-(CH3)3 of the compound class of betains such as carnitine and cholines (3.2 ppm, TMA) and N-CH3 of total creatine (3.0 ppm, tCr), respectively.

pulse for a given situation. As demonstrated in Fig. 2, Eq (1) gives a good agreement between the theoretical and experimental results. One limitation of the band-selective technique is that the echo-time is restricted by the duration of the frequencyselective pulse. The shortest TE that we have developed for whole-slice 1H MRSI of muscle metabolite with a reburp pulse is 60 ms. The spectral quality, particularly those voxels near the edge, can be deteriorated by air-tissue interface at a short TE unless excellent OVP is employed. As shown in Fig. 3c, there are contamination signals in the voxels near the edge because here OVP was not used. The contamination signals can be reduced with a long TE, utilizing the nature of line broadening by air-tissue interface. A TE of 160 ms was chosen here due in part to this consideration. The other reason, which is more important, is that this relatively long TE offers a good contrast between 1H spectra of different muscle groups (see Fig. 3c). Other drawbacks of the band-selective technique are also associated with the excitation profile of the frequency-selective pulse. For example, simultaneous observation of lipid signal or other metabolites outside the frequency range of the frequency-selective pulse cannot be obtained. If the excitation profile of the frequency-selective pulse is not uniform over the frequency range of interest, the signal amplitude can also be artificially distorted unless uniform magnetic field homogeneity over the volume of interest can be achieved. As given in Eq (1), if the flip angle of the FS pulse is 30% off the desired 180° for a desired signal frequency (i.e., 126° or 234°), its amplitude will be distorted by 21%. However, the signal distortion can be acceptable if the variation in the flip angle is moderate. For example, if the flip angle varies within 10% over the range of the frequency of interest (i.e., 162° ⬍ ␣ ⬍ 198°), the amplitude will be distorted by less than 2.5%. If necessary, the distortion can be corrected during post processing. The procedure for correction should include measurement of the B0 map

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and implementation of a post-processing correction software package using the information from the B0 map, the excitation profile, and Eq (1). The B0 map will provide information on the frequency offset at each location; the excitation profile and Eq (1) will tell how much correction is needed. Because of the technical difficulties associated with water and lipid suppression, a 2D 1H MRSI study of muscle metabolites from a whole slice of human limb has not been reported previously. In fact, only a handful 2D 1H MRSI studies of muscle metabolites with the volume of interest inside muscles have been published [14,15]. In this report, the in vivo results from a whole slice of calf with high spatial resolution clearly demonstrates the spectral heterogeneity of different muscles (Fig. 3c and Table 1). Unlike the spectra of other muscles where tCr peaks are much higher than TMA, soleus spectra have comparable signals for both tCr and TMA, which make them stand out even to the naked eye. This spectral heterogeneity is consistent with differences in physicochemical characteristics of different muscles [28]. Soleus is a well-known representative of type I dominated muscle, while the other calf muscles are typically mixed type I/II muscles [29]. Thus, it is possible that this spectral heterogeneity is the reflection of underlying differences in biochemical characteristics of different muscles. Other possible mechanisms include differences in muscle fiber orientations. Dipolar coupling effects due to anisotropic motional averaging have been shown for in vivo 1 H MRS of skeletal muscle [30], and might contribute to these observations. Understanding the mechanism behind the observation is important and could offer insight into the underlying biophysical characteristics of human muscle. Nevertheless, this study clearly demonstrates that different muscle can give different spectra. It is worth noting that high spatial resolution and multi-voxel spectra may be needed to accurately characterize heterogeneous 1H spectra of muscles. Unlike single-voxel techniques, MRSI provides spectra of different muscles from the same experiment for comparison; thus, it is more suited to study spectral heterogeneity. In conclusion, we have demonstrated the potential of a pulse scheme to perform in vivo high spatial resolution 2D 1 H MRSI of human muscles. Our results clearly demonstrate that different muscles can have distinctively different proton spectral patterns of metabolites. This observation demonstrates the importance of high spatial resolution and multi-voxel for in vivo 1H MRS in a heterogeneous environment. The technique is robust, can be implemented with relative ease on a standard clinical scanner, and should prove to be a useful tool for basic and clinical studies of proton metabolites.

Acknowledgments JH is supported in part by NIH grant (R21 CA81532), QJ is supported in part by NIH grant (R01 NS38292), YX is

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