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Sensitivity of cardiac ³¹P MRSI in prone and supine positions using an integrated ³¹P whole-body transmit coil at 7T

Mark Gosselink¹, Kathy Verkerk¹, Lieke van den Wildenberg¹, Ayhan Gursan¹, Dimitri Welting¹, Martijn Froeling¹, Dennis W.J. Klomp¹, and Jeanine J. Prompers¹
¹Center for image Sciences, University Medical Center Utrecht, Utrecht, Netherlands

Synopsis

Keywords: Myocardium, Spectroscopy, RF Coils, Metabolism

Motivation: Cardiac ³¹P MRS is limited by the low sensitivity of commonly used transmit-receive surface coils at larger distances from the coil. Therefore, with transmit-receive surface coils, data is often acquired in prone position, which brings the heart closer to the coil.

Goal(s): Determine the sensitivity of cardiac 3D ³¹P MRSI in prone and supine positions using an integrated ³¹P whole-body birdcage transmit coil at 7T.

Approach: A male and female subject were scanned in prone and supine position.

Results: 3D ³¹P MRSI data were obtained with good sensitivity throughout the whole heart, regardless of the position of the subject.

Impact: With an integrated ³¹P whole-body transmit coil at 7T, cardiac ³¹P MRS can be performed in the more comfortable supine position, with good sensitivity also in the posterior cardiac wall.

Introduction

Metabolic stress can impair myocardial energy balance, affecting cardiac performance and contributing to cardiomyopathy. Phosphorus-31 magnetic resonance spectroscopy (³¹P MRS) allows non-invasive assessment of cardiac energy status by detecting adenosine triphosphate (ATP) and phosphocreatine (PCr).¹ While clinical field strengths of 1.5T and 3T are preferred for myocardial structure and function assessment, precise quantification of cardiac PCr and ATP by ³¹P MRS benefits from the increased signal-to-noise ratio (SNR) and spectral resolution at 7T.^2,3 Moreover, clinical applications of cardiac ³¹P MRS are limited by the low sensitivity of commonly used transmit-receive surface radiofrequency coils at larger distances from the coil.⁴ To address the latter, cardiac ³¹P MRS studies are often performed in prone position, which brings the heart closer to the coil but which can be less comfortable.
Here we show the application of an integrated ³¹P whole-body birdcage transmit coil, with homogeneous excitation over a large field-of-view (FOV),⁵ in combination with a 16-channel ³¹P body receive array for cardiac ³¹P MRSI at a field strength of 7T. The aim of the study was to compare the sensitivity of cardiac 3D ³¹P MRSI in prone and supine positions using this setup.

Methods

Measurements were performed on a 7T whole-body MR system (Philips Healthcare, Best, the Netherlands), equipped with a shielded ³¹P whole-body birdcage transmit coil with a diameter of 60cm and a length of 40cm, embedded in the outside of the patient tube.⁴ ³¹P signals were received with a local body array, containing 16 ³¹P loop coils and 8 fractionated ¹H dipole antennas.^6,7
A ³¹P B₁⁺ map was acquired on a body size phantom containing 3g/L NaCl and 43mM KH₂PO₄, using the actual flip angle imaging (AFI) method⁸ (3D gradient echo: FOV=480×300×300mm³, resolution=20×20×20mm³, TE=1.75ms, TR1=50ms, TR2=950ms, nominal flip angle=60°, reference B₁⁺=10µT, NSA=64). B₁⁺ maps were reconstructed using QMRITools.⁹
Two healthy volunteers (one male with BMI 21.0kg/m², one female with BMI 24.6kg/m²) were scanned in prone and supine position. Image-based B₀ shimming was performed during a breath hold and transversal and coronal anatomical images were acquired with a T₁-FFE sequence. ³¹P spectra were acquired with a 3D FID-MRSI sequence with Hamming weighted k-space sampling without respiratory gating or cardiac triggering, using a block pulse for excitation (carrier frequency set to PCr, B₁⁺=10μT). The following parameters were used: FOV=500(LR)×280(AP)×360(FH)mm³, nominal resolution=20mm isotropic, TR=60ms, acquisition delay=0.50ms, BW=5000Hz, NSA=20, acquisition time=22:37min. ³¹P MRSI data were reconstructed in MATLAB (The MathWorks Inc., Natick, MA) using whitened singular value decomposition (WSVD)¹⁰ and PCA-based denoising.¹¹ SNR was calculated based on the intensity of the α-ATP signal in the spectral domain by dividing the signal intensity of the α-ATP peak by the standard deviation of the noise (10-20 ppm). Nine metabolite signals were fitted with AMARES¹² using OXSA¹³, from which PCr/γ-ATP ratios (not corrected for saturation or blood contamination) were determined.

Results

Figure 1 shows B₁⁺ maps of the ³¹P whole-body birdcage transmit coil. Over the whole phantom, the variation of B₁⁺ (coefficient of variation=SD/mean) was 21%.
Figures 2 and 3 show ³¹P MRS spectra from three voxels in the myocardial wall, at different depths, from the datasets recorded in prone and supine position, in the male and female volunteer, respectively. Spectral quality was similar for prone and supine positions, but SNR decreased slightly when going from the most anterior (red) to the most posterior (yellow) voxel for both positions, for both subjects. The slight SNR drop in the anterior-posterior direction (factor 1.2-1.4) is also apparent from the cardiac α-ATP SNR maps (Figure 4) and reflects the sensitivity profile of the ³¹P receive array.
Cardiac PCr/ATP ratio maps are shown in Figure 5. PCr/ATP ratios were noticeably higher in the most anterior row, closest to the chest muscles. Average PCr/ATP ratios in the male subject were 1.52±0.23 and 1.37±0.31 in prone and supine positions, respectively, and in the female subject 1.36±0.29 and 1.56±0.29, respectively.

Discussion and Conclusion

Using an integrated ³¹P whole-body birdcage transmit coil in combination with a 16-channel ³¹P body receive array at 7T, cardiac 3D ³¹P MRSI data were obtained with good sensitivity throughout the whole heart, regardless of the position of the subject, i.e., prone or supine. The setup allows for easy patient positioning and large FOV scanning, similar to ¹H body imaging at 1.5T or 3T, and enabling simultaneous acquisition of e.g. heart and liver ³¹P MRSI data. In conclusion, with an integrated ³¹P whole-body transmit coil at 7T, cardiac ³¹P MRS can be performed in the more comfortable supine position, with good sensitivity also in the posterior cardiac wall.

Acknowledgements

No acknowledgement found.

References

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2. Bakermans AJ, Bazil JN, Nederveen AJ, Strijkers GJ, Boekholdt SM, Beard DA, Jeneson JAL. Human cardiac 31P-MR spectroscopy at 3 Tesla cannot detect failing myocardial energy homeostasis during exercise. Front Physiol. 2017;8:939.

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5. van den Wildenberg L, Gursan A, Seelen LWF, van der Velden TA, Gosselink MWJM, Froeling M, van der Kemp WJM, Klomp DWJ, Prompers JJ. In vivo phosphorus magnetic resonance spectroscopic imaging of the whole human liver at 7 T using a phosphorus whole-body transmit coil and 16-channel receive array: Repeatability and effects of principal component analysis-based denoising. NMR Biomed. 2023;36(5):e4877.

6. van Houtum QQ, Mohamed Hoesein FFAA, Verhoeff JJJC, van Rossum PPSN, van Lindert AASR, van der Velden TTA, van der Kemp WWJM, Klomp DDWJ, Arteaga de Castro CCS. Feasibility of 31 P spectroscopic imaging at 7 T in lung carcinoma patients. NMR Biomed. 2021;34(5):e4204.

7. Raaijmakers AJ, Italiaander M, Voogt IJ, Luijten PR, Hoogduin JM, Klomp DW, van den Berg CA. The fractionated dipole antenna: A new antenna for body imaging at 7 Tesla. Magn Reson Med. 2016;75(3):1366-1374.

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Figures

Figure 1. Actual flip angle imaging (AFI) B₁⁺ maps of the ³¹P whole-body birdcage transmit coil measured on a body size phantom (dimensions 40cm×20cm×39cm; volume 25.6 L). Data are shown for five central transversal slices. In addition, a histographic B₁⁺ distribution is shown for the whole phantom. The reference B₁⁺ was 10 μT. The color bar indicates the percentage of this reference B₁⁺.

Figure 2. Cardiac 3D ³¹P MRSI data of a male subject in (A) prone and (B) supine position. The transverse anatomical image is overlaid with the ³¹P MRSI grid and ³¹P MR spectra are shown for three voxels (red, orange and yellow) in the myocardial wall at different depths. ³¹P MR spectra were apodized with 20 Hz and zero-filled to 512 points for visualization purposes. Peak assignments: 2,3-DPG, 2,3-diphosphoglycerate; Pi, inorganic phosphate; PDE, phosphodiesters; PCr, phosphocreatine; ATP, adenosine triphosphate; NAD, nicotinamide adenine dinucleotide.

Figure 3. Cardiac 3D ³¹P MRSI data of a female subject in (A) prone and (B) supine position. The transverse anatomical image is overlaid with the ³¹P MRSI grid and ³¹P MR spectra are shown for three voxels (red, orange and yellow) in the myocardial wall at different depths. ³¹P MR spectra were apodized with 20 Hz and zero-filled to 512 points for visualization purposes. Peak assignments: 2,3-DPG, 2,3-diphosphoglycerate; Pi, inorganic phosphate; PDE, phosphodiesters; PCr, phosphocreatine; ATP, adenosine triphosphate; NAD, nicotinamide adenine dinucleotide.

Figure 4. Cardiac α-ATP SNR maps for the male and female subjects in prone and supine position. The SNR was calculated by dividing the signal intensity of the α-ATP peak by the standard deviation of the noise (10-20 ppm).

Figure 5. Cardiac PCr/ATP ratio maps for the male and female subjects in prone and supine position. Average PCr/ATP ratios in the male subject were 1.52±0.23 and 1.37±0.31 in prone and supine positions, respectively. Average cardiac PCr/ATP ratios in the female subject were 1.36±0.29 and 1.56±0.29 in prone and supine positions, respectively.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)

1786

DOI: https://doi.org/10.58530/2024/1786