Original contributionComparison of single voxel brain MRS AT 3 T and 7 T using 32-channel head coils
Introduction
Magnetic resonance spectroscopy (MRS) is believed to be one of the MR techniques most likely to benefit from the use of high B0 magnetic field strengths, because of the expected improvements in both signal-to-noise ratio (SNR) and spectral resolution [1], [2]. Theory suggests that SNR increases linearly with field strength in biological samples, as does chemical shift dispersion [3]. However, spectral linewidths in brain tissue (measured in Hz) are also known to increase with field strength [2], so resolution improvements may not be as great as expected from simple theory [4], [5]. Nevertheless, the overall improved spectral quality at higher field strengths such as 7 T has been shown to result in lower uncertainty values of metabolite concentrations [2], [6] and also to allow the estimation of various compounds that are either undetectable or require specialized pulse sequences [7], [8] for accurate quantification at lower field strengths. In particular, previous studies using surface coils have shown that the SNR, spectral resolution and measurement precision in the occipital lobe are all superior at 7 T compared to lower field strengths (either 3 T or 4 T) [2], [6]. However, to our knowledge, there has only been one comparison MRS study [9] between field strengths that used multi-channel receiver head coils, which compared measurements made in the parietal white matter at 3 T and 7 T using an 8-channel head coil. The current study compared measurements made in multiple-brain regions using 32-channel head coils at 3 T and 7 T. This comparison is of particular importance since 32-channel coils are now being increasingly used for both research and clinical state-of-the-art neuroimaging studies.
Higher field strengths are also associated with technical challenges such as inhomogeneities in the B0 and B1 fields, longer T1 and shorter T2 relaxation times, higher radiofrequency power deposition (specific absorption rate (SAR)), and increased chemical shift displacement errors (CSDE). Some of these issues are inherent physical properties to the biochemical under investigation; whereas, others may be addressed by development of new techniques either in pulse sequence design or scanner hardware. Inhomogeneity in the B1 transmit field causes deviations of the RF pulse flip angles from their ideal values, which may lead to decreased SNR. Also, conventional amplitude-modulated RF pulses have relatively small frequency bandwidths, leading to large CSDE artifacts at high fields such as 7 T. Both of these problems may be at least partially addressed by the use of frequency-modulated adiabatic RF pulses [10], which provide uniform flip angle rotations independent of B1 as long as the B1 levels are high enough to fulfill the adiabatic condition [11], [12], [13]. Adiabatic pulses usually have high bandwidths, determined by the frequency sweep of the RF pulse. Usage of either fully [10], [14] or partially [15], [16] adiabatic pulse sequences have been proposed for in vivo MRS. The partially adiabatic semi-LASER sequence [15], [16] has become popular over the last few years, since it involves fewer RF pulses, and hence lower SAR, than fully adiabatic sequences such as SADLOVE [12] or LASER [14]. A recent study at 7 T has shown a more than two-fold increase in SNR when using sLASER as opposed to the non-adiabatic STEAM sequence [17].
In this study, a systemic comparison was performed of brain MRS at 3 and 7 T, using the sLASER sequence for volume selection and 32-channel receive head coils at both field strengths. Experimental parameters and pulse sequences were closely matched between systems, and the same subjects were scanned at both field strengths. The use of a volume transmit coil paired with the 32-channel receive array allowed for measurements from several brain locations.
Section snippets
Methods
After obtaining written consent under local IRB approval, 4 healthy subjects (age 35 ± 7 years, 2 males) were scanned at both 3 T and 7 T the same day using ‘Achieva’ scanners (Philips Healthcare LLC, Cleveland, OH) equipped with 32-channel receive head coils (7 T: Nova Medical, Wilmington, MA, 3 T: Invivo, Gainesville, FL). On the 7 T system, a quadrature head coil was used to transmit RF pulses, while on the 3 T the scanner body coil was used (nominal maximum B1 = 15 μT at 7 T and 13.5 μT at 3 T).
Results
Representative spectra acquired from the ACC, CSO and DLPFC at 3 T and 7 T are shown in Fig. 2. Good quality spectra were acquired from all subjects from all three regions at both field strengths. As expected, the appearance of spectra, for example the 4CH2 resonances of Glu and Gln at 2.3–2.4 ppm are better resolved at 7 T, and the Glu peak at 3 T displays more multiplet structure compared to 7 T, most likely because of the wider spread of the outer peaks (in ppm) of the 2.3 and 2.4 ppm resonances of
Discussion
This study used a closely matched methodology between field strengths, including pulse sequence to compare the MRS measurements made at 3 T and 7 T more precisely. Consistent with prior studies of the occipital lobe using surface coils [2], [6] and one study of parietal white matter using an 8-channel head coil [9], this study confirms the improved SNR and generally lower metabolite CRLBs at 7 T compared to 3 T, across three brain regions using 32-channel receiver coil arrays. Improvements in SNR
Acknowledgments
This work was funded in part by NIH P41EB015909 and R01MH096263.
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