Low SAR 31 P (multi-echo) spectroscopic imaging using an integrated whole-body transmit coil at 7T.

Phosphorus (31 P) MRSI provides opportunities to monitor potential biomarkers. However, current applications of 31 P MRS are generally restricted to relatively small volumes as small coils are used. Conventional surface coils require high energy adiabatic RF pulses to achieve flip angle homogeneity, leading to high specific absorption rates (SARs), and occupy space within the MRI bore. A birdcage coil behind the bore cover can potentially reduce the SAR constraints massively by use of conventional amplitude modulated pulses without sacrificing patient space. Here, we demonstrate that the integrated 31 P birdcage coil setup with a high power RF amplifier at 7 T allows for low flip angle excitations with short repetition time (TR ) for fast 3D chemical shift imaging (CSI) and 3D T1 -weighted CSI as well as high flip angle multi-refocusing pulses, enabling multi-echo CSI that can measure metabolite T2 , over a large field of view in the body. B1+ calibration showed a variation of only 30% in maximum B1 in four volunteers. High signal-to-noise ratio (SNR) MRSI was obtained in the gluteal muscle using two fast in vivo 3D spectroscopic imaging protocols, with low and high flip angles, and with multi-echo MRSI without exceeding SAR levels. In addition, full liver MRSI was achieved within SAR constraints. The integrated 31 P body coil allowed for fast spectroscopic imaging and successful implementation of the multi-echo method in the body at 7 T. Moreover, no additional enclosing hardware was needed for 31 P excitation, paving the way to include larger subjects and more space for receiver arrays. The increase in possible number of RF excitations per scan time, due to the improved B1+ homogeneity and low SAR, allows SNR to be exchanged for spatial resolution in CSI and/or T1 weighting by simply manipulating TR and/or flip angle to detect and quantify ratios from different molecular species.

(1) H MRS in the human spinal cord at 7 T using a dielectric waveguide transmitter, RF shimming and a high density receive array.

Multimodal MRI is the state of the art method for clinical diagnostics and therapy monitoring of the spinal cord, with MRS being an emerging modality that has the potential to detect relevant changes of the spinal cord tissue at an earlier stage and to enhance specificity. Methodological challenges related to the small dimensions and deep location of the human spinal cord inside the human body, field fluctuations due to respiratory motion, susceptibility differences to adjacent tissue such as vertebras and pulsatile flow of the cerebrospinal fluid hinder the clinical application of (1) H MRS to the human spinal cord. Complementary to previous studies that partly addressed these problems, this work aims at enhancing the signal-to-noise ratio (SNR) of (1) H MRS in the human spinal cord. To this end a flexible tight fit high density receiver array and ultra-high field strength (7 T) were combined. A dielectric waveguide and dipole antenna transmission coil allowed for dual channel RF shimming, focusing the RF field in the spinal cord, and an inner-volume saturated semi-LASER sequence was used for robust localization in the presence of B1 (+) inhomogeneity. Herein we report the first 7 T spinal cord (1) H MR spectra, which were obtained in seven independent measurements of 128 averages each in three healthy volunteers. The spectra exhibit high quality (full width at half maximum 0.09 ppm, SNR 7.6) and absence of artifacts and allow for reliable quantification of N-acetyl aspartate (NAA) (NAA/Cr (creatine) 1.31 ± 0.20; Cramér-Rao lower bound (CRLB) 5), total choline containing compounds (Cho) (Cho/Cr 0.32 ± 0.07; CRLB 7), Cr (CRLB 5) and myo-inositol (mI) (mI/Cr 1.08 ± 0.22; CRLB 6) in 7.5 min in the human cervical spinal cord. Thus metabolic information from the spinal cord can be obtained in clinically feasible scan times at 7 T, and its benefit for clinical decision making in spinal cord disorders will be investigated in the future using the presented methodology. Copyright © 2016 John Wiley & Sons, Ltd.

(31)P magnetic resonance spectroscopy of the breast and the influence of the menstrual cycle.

Phosphorus metabolite ratios are potential biomarkers in breast cancer diagnosis and treatment monitoring. Our purpose was to investigate the metabolite ratios phosphomonoester to phosphodiester, phosphoethanolamine (PE) to glycerophosphoethanolamine (GPE), and phosphocholine (PC) to glycerophosphocholine (GPC) in glandular breast tissue, and the potential effect of the menstrual cycle, using (31)P magnetic resonance spectroscopy (MRS) at 7T. Seven women with regular menstrual cycles each underwent four examinations using a 3D (31)P multi-echo magnetic resonance spectroscopic imaging sequence. Peak integrals were assessed using IDL and JMRUI software. First, T2 relaxation times were calculated using multi-echo data pooled across subjects and time points. Subsequent, metabolite ratios were calculated for each phase of the menstrual cycle using the calculated T2 values to account for when combining the free induction decay and all five echoes. The metabolite ratios were calculated both on group level and individually. T2 decay fits resulted in a T2 relaxation time for PE of 154 ms (95 % CI 144-164), for PC of 173 ms (95 % CI 148-205), for Pi of 188 ms (95 % CI 182-193), for GPE of 48 ms (95 % CI 44-53), and for GPC of 23 ms (95 % CI 21-26). The metabolite ratios analyzed on group level showed negligible variation throughout the menstrual cycle. Individual results did show an apparent intra-individual variation; however, not significant due to the measurements' uncertainty. To conclude, phospholipids in glandular tissue as measured with (31)P MRS at 7 T are not significantly affected by the menstrual cycle.

Adiabatic multi-echo ³¹P spectroscopic imaging (AMESING) at 7 T for the measurement of transverse relaxation times and regaining of sensitivity in tissues with short T2 values.

An adiabatic multi-echo spectroscopic imaging (AMESING) sequence, used for (31) P MRSI, with spherical k-space sampling and compensated phase-encoding gradients, was implemented on a whole-body 7-T MR system. One free induction decay (FID) and up to five symmetric echoes can be acquired with this sequence. In tissues with low T2 and high T2 , this can theoretically lead to a potential maximum signal-to-noise ratio (SNR) increase of almost a factor of three, compared with a conventional FID acquisition with Ernst-angle excitation. However, with T2 values being, in practice, ≤400 ms, a maximum enhancement of approximately two compared with low flip Ernst-angle excitation should be feasible. The multi-echo sequence enables the determination of localized T2 values, and was validated with (31) P three-dimensional MRSI on the calf muscle and breast of a healthy volunteer, and subsequently applied in a patient with breast cancer. The T2 values of phosphocreatine, phosphodiesters (PDE) and inorganic phosphate in calf muscle were 193 ± 5 ms, 375 ± 44 ms and 96 ± 10 ms, respectively, and the apparent T2 value of γ-ATP was 25 ± 6 ms. A T2 value of 136 ± 15 ms for inorganic phosphate was measured in glandular breast tissue of a healthy volunteer. The T2 values of phosphomonoesters (PME) and PDE in breast cancer tissue (ductulolobular carcinoma) ranged between 170 and 210 ms, and the PME to PDE ratios were calculated to be phosphoethanolamine/glycerophosphoethanolamine = 2.7, phosphocholine/glycerophosphocholine = 1.8 and PME/PDE = 2.3. Considering the relatively short T2 values of the metabolites in breast tissue at 7 T, the echo spacing can be short without compromising spectral resolution, whilst maximizing the sensitivity.