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

Abstract

Phosphorus (31P) MRSI provides opportunities to monitor potential biomarkers. However, current applications of 31P 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 31P birdcage coil setup with a high power RF amplifier at 7 T allows for low flip angle excitations with short repetition time (T R) for fast 3D chemical shift imaging (CSI) and 3D T 1‐weighted CSI as well as high flip angle multi‐refocusing pulses, enabling multi‐echo CSI that can measure metabolite T 2, over a large field of view in the body. B 1 + calibration showed a variation of only 30% in maximum B 1 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 31P 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 31P 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 B 1 + homogeneity and low SAR, allows SNR to be exchanged for spatial resolution in CSI and/or T 1 weighting by simply manipulating T R and/or flip angle to detect and quantify ratios from different molecular species.

PCA denoising and Wiener deconvolution of 31P 3D CSI data to enhance effective SNR and improve point spread function

“…Exciting developments in CSI acquisition 13 and hardware 8,16,17 on ultrahigh field MRI scanners 14,15 are enabling faster CSI measurements with an increasing FOV and higher spatial resolution. Therefore, CSI is getting closer to conventional MR imaging methods and processing methods used in the imaging field also become applicable to CSI, as was demonstrated in this study.…”

“…Data acquisition was according to the institutional guidelines and with approval from the local ethics committee. For this, we used a 7 Tesla Philips scanner equipped with a 31 P bore coil for transmitting and a 16-channel 31 P receive array 8,16,17 with 8 1 H Tx/Rx antennas for proton imaging. 46 On top of the middle of the left leg, a falcon tube containing polyphosphoric acid (20 ppm, 0.97 M) was placed.…”

“…Phosphorus ( 31 P) MRS allows to noninvasively measure phosphorus-containing metabolites and membrane phospholipid metabolites and derivatives. 1,2 In recent years, 31 P chemical shift imaging (CSI) has been shown useful in a variety of applications and organs such as the brain, [3][4][5] heart, 6,7 liver, 8,9 muscle, 10,11 and lung. 12 However, in vivo concentrations of observable phosphorus-containing metabolites are in the millimolar range, and 31 P has a low gyromagnetic ratio and thus low intrinsic sensitivity.…”

“…13 Higher field strengths (7T) can be used to mitigate the low SNR in 31 P MRS acquisitions. 14,15 In combination with an integrated 31 P bore coil, 8,16,17 providing a homogeneous B + 1 field, and multi-channel phased array receive coils, 8,17 this allows optimal sensitivity throughout the organ(s) of interest. Moreover, short TRs and Ernst-angle excitation can be used to maximize the number of samples for a given acquisition time and thus also enabling acquisitions with a large FOV.…”

7 Tesla and Beyond

“…The development of 31 P volume coils 150 – 152 or dedicated 31 P coils for specific body regions 153 – 155 for 7 T 31 P MR applications enables valuable insights into body regions beyond the head, with increased SNR and spectroscopic resolution. A first human cardiac 31 P spectrum at 7 T 156 showed that SNR of every peak increases compared with 3 T, for example, the SNR of PCr increased by factor of 2.8.…”