Human cardiac 31P magnetic resonance spectroscopy at 3T with a receive array: is single-loop or dual-loop RF transmission superior?
Journal of Cardiovascular Magnetic Resonance volume 17, Article number: P248 (2015)
Human cardiac phosphorous MR spectroscopy (31P-MRS) provides direct insight into cardiac energetics by measuring concentrations of ATP, ADP, phosphocreatine (PCr) and other species. Yet at 3T, excitation flip angles in the inferior segments of the myocardium have been limited to be much less than the "Ernst angle" needed to maximise spectral SNR. This has made it impossible so far to acquire spectroscopic images that cover the whole heart, which in turn has limited 31P-MRS to the study of diffuse rather than focal disease. In this study, we test whether splitting the RF transmission between anterior and posterior coils improves spectral quality across the myocardium compared to transmission from the anterior side alone.
Data were acquired using a 3T Trio MRI scanner (Siemens) and two state-of-the-art 8-element cardiac receive array coils (Rapid Biomedical). Each comprised four anterior and four posterior 20x6 cm2 receive elements. One coil had only a 30x29 cm2 anterior transmit loop and the other had 30x29 cm2 transmit loops in both anterior and posterior pieces.
Normal volunteers (7 males, 23-41y, 65-82kg, 1.70-1.93m) were recruited with ethical approval. Cardiac 31P spectra were acquired with an ECG-triggered, 3D-UTE-CSI pulse sequence over a 35x35x35 cm3 FOV of 22x22x10 voxels, with 2 averages at k=0 and WSVD coil combination in a total of ~24min for a 70bpm subject. Using subject-specific B1 maps, the excitation voltage was set each time to give a 30° flip angle in the interventricular septum.
Spectra in each voxel were fitted with a custom Matlab implementation of AMARES, with prior knowledge specifying 11 Lorentzian peaks (α,β,γ-ATP multiplet components, PCr, PDE and 2x 2,3-DPG), with fixed amplitude ratios and scalar couplings for the multiplets. Spectral SNR was defined as peak height / baseline SD after application of a matched filter.
Across the phantom (Fig. 1), we observed B1+ variation of 50% for single-loop transmit, but only 10% with dual-loop transmit. The receive SNR at 10cm (the depth of the septum) differed by <10% between the coils (because they have identical receive circuitry).
In-vivo, both coils performed similarly in the anterior segments, but the dual-transmit coil had a significantly improved (P = 0.021, 0.001, 0.048) spectral SNR in the inferior cardiac segments relative to the single-loop coil. Even so, the SNR in the inferior cardiac segments obtained with this 24min protocol was still below that desirable for clinical studies with modest numbers of subjects at this point.
The first human cardiac 31P-MRS study at 7T saw a 2.8x SNR increase compared to 3T using the same coil geometry at both fields. Together with our findings here, this suggests that a dual-loop transmit coil at 7T will be able to achieve whole-heart coverage with clinically-acceptable SNR and total scan duration.
Dual-loop transmit significantly improves the SNR in 31P-MRS of the inferior segments of the heart compared to single-loop transmit at 3T.
Funded by a Sir Henry Dale Fellowship from the Wellcome Trust and the Royal Society [Grant Number 098436/Z/12/Z], the British Heart Foundation Centre of Research Excellence (CoRE) award (Edinburgh) and the Centre for Cardiovascular Sciences (Edinburgh).
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Rodgers, C.T., Li, M., MacNaught, G. et al. Human cardiac 31P magnetic resonance spectroscopy at 3T with a receive array: is single-loop or dual-loop RF transmission superior?. J Cardiovasc Magn Reson 17 (Suppl 1), P248 (2015). https://doi.org/10.1186/1532-429X-17-S1-P248