Reproducibility of human cardiac phosphorus MRS (³¹P‐MRS) at 7 T

Abstract

Purpose We test the reproducibility of human cardiac phosphorus MRS ( 31 P‐MRS) at ultra‐high field strength (7 T) for the first time. The primary motivation of this work was to assess the reproducibility of a ‘rapid’ 6½ min 31 P three‐dimensional chemical shift imaging (3D‐CSI) sequence, which if sufficiently reproducible would allow the study of stress‐response processes. We compare this with an established 28 min protocol, designed to record high‐quality spectra in a clinically feasible scan time. Finally, we use this opportunity to compare the effect of per‐subject B 0 shimming on data quality and reproducibility in the 6½ min protocol. Methods 10 healthy subjects were scanned on two occasions: one to test the 28 min 3D‐CSI protocol, and one to test the 6½ min protocol. Spectra were fitted using the OXSA MATLAB toolbox. The phosphocreatine to adenosine triphosphate concentration ratio (PCr/ATP) from each scan was analysed for intra‐ and intersubject variability. The impact of different strategies for voxel selection was assessed. Results There were no significant differences between repeated measurements in the same subject. For the 28 min protocol, PCr/ATP in the midseptal voxel across all scans was 1.91 ± 0.36 (mean ± intersubject SD). For the 6½ min protocol, PCr/ATP in the midseptal voxel was 1.76 ± 0.40. The coefficients of reproducibility (CRs) were 0.49 (28 min) and 0.67 (6½ min). Per‐subject B 0 shimming improved the fitted PCr/ATP precision (for 6½ min scans), but had negligible effect on the CR (0.67 versus 0.66). Conclusions Both 7 T protocols show improved reproducibility compared with a previous 3 T study by Tyler et al. Our results will enable informed power calculations and protocol selection for future clinical research studies.

Quantifying the effect of dobutamine stress on myocardial Pi and pH in healthy volunteers: A 31P MRS study at 7T

“…The reasonable length of the scan meant that Pi estimation was also straightforward during stress. Although SNR, primarily due to low concentrations and the heart’s unfavourable geometry, limited our ability to measure Pi in 100% of scans, we report a technique whose repeatability competes with the slower CSI approach to Pi estimation 32 and the computation of PCr/ATP both at 3T 48 and 7T 39 . The protocol is feasible for use in patients, which will be further investigated, and can be delivered during dobutamine infusion at 7T; indeed, our study is the first to deliver vasoactive stress during MR examinations at this field strength.…”

“…The nominal voxel size was 25 × 15 × 33 mm 3 . Excitation was with a shaped pulse centred at +300 Hz relative to PCr 39 . The CSI orientation was cardiac short axis in‐plane rotated so the phase‐encoding direction with the highest resolution was in the anteroposterior direction to minimize skeletal muscle contamination.…”

“…Using a 10 cm 1 H transmit/receive loop (Rapid Biomedical, Rimpar, Germany), localizers were acquired with 3-lead ECG gating in the main cardiac orientations, 37 and subject-specific B 0shimming was performed ensuring coverage of the whole heart. 39 Localizers were acquired at the end of expiration, making contamination of the voxel of interest by subdiaphragmatic structures during our free-breathing MRS protocols highly unlikely. With the participant asked to remain as still as possible, the table was moved out, and the 1 H coil was then removed and swapped for a 16-element array 31 P coil (Rapid Biomedical, Rimpar, Germany).…”

“…Excitation was with a shaped pulse centred at +300 Hz relative to PCr. 39 The CSI orientation was cardiac short axis in-plane rotated so the phase-encoding direction with the highest resolution was in the anteroposterior direction to minimize skeletal muscle contamination. Next, ECG-triggered, free-breathing STEAM-31 P-MRS spectra were acquired with shot-to-shot interleaving of acquisitions centered: 1) on PCr (0 ppm, T M = 7 ms) ( Figure 3A, PCr interleaf), and 2) on Pi (4.8 ppm, T M = 60 ms) ( Figure 3B, Pi interleaf).…”

A Flexible Array for Cardiac 31P MR Spectroscopy at 7 T

“…However, for applications where large anatomical inter-subject variability is expected, flexible RF coils are favorable to account for this heterogeneity in anatomy. Due to the longer wavelength for 31 P it is still possible to use single loop coils for 7 T 31 P MRS on the torso, which is the RF coil of choice in most published studies [13,20,21]. More elaborate designs include a 2-element Tx/Rx overlap array [9] and combinations of a Tx volume coil with Rx arrays [22][23][24].…”