A 4-element dual-tuned 1H/31P coil array for MR imaging and spectroscopy of the human heart at 3 Tesla
Adrianus J. Bakermans1, Bart-Jan van den Berg2, Gustav J. Strijkers3, Maarten J. Versluis4, Dennis W.J. Klomp2,5, Aart J. Nederveen1, and Jeroen A.L. Jeneson1,6

1Department of Radiology, Academic Medical Center, Amsterdam, Netherlands, 2MR Coils B.V., Drunen, Netherlands, 3Department of Biomedical Engineering and Physics, Academic Medical Center, Amsterdam, Netherlands, 4Philips Healthcare Benelux, Eindhoven, Netherlands, 5Department of Radiology, University Medical Center Utrecht, Utrecht, Netherlands, 6Neuroimaging Center, University Medical Center Groningen, Groningen, Netherlands

Synopsis

Maximizing the signal-to-noise ratio (SNR) in in vivo cardiac phosphorus-31 magnetic resonance spectroscopy (31P-MRS) remains a major challenge in investigations of human myocardial energy metabolism. Here, we demonstrate that with a dual-tuned 1H/31P coil array with 4 elements, it is feasible to achieve a higher SNR and a more homogeneous receive sensitivity distribution over the region of interest for cardiac 31P-MRS at 3 Tesla than with a standard linear single-turn 31P surface coil. Furthermore, the 4 channels available for 1H signal reception allow for cardiac cine 1H-MRI with parallel imaging.

Background

Maximizing the signal-to-noise ratio (SNR) in in vivo cardiac phosphorus-31 magnetic resonance spectroscopy (31P-MRS) remains a major challenge in investigations of human myocardial energy metabolism. Typically, past studies used a 31P transmit/receive surface coil positioned over the chest close to the heart. Although a surface coil offers superior SNR compared to a volume coil, it suffers from flip angle variations due to B1 inhomogeneity and limited penetration depth. This compromises the spatial localization of 31P-MRS signal acquisition from the heart, and hampers the quantification of metabolite concentrations. Here, we present a novel coil design consisting of dual-tuned 1H/31P coil array with 4 elements in a geometric alignment tailored to achieve a larger and more homogeneous field of view for in vivo 31P-MRS compared to a standard 31P transmit/receive surface coil. Moreover, the elements are arranged in quadrature to maximize SNR for 31P-MRS, while detuning of the 1H resonance was integrated to facilitate imaging and B0 shimming.

Methods

Coil design: The proposed coil system consists of 4 dual-tuned (1H @ 127.8 MHz; 31P @ 51.8 MHz) ∅ 15-cm elements in an anterior-posterior 2×2 combination (Figure 1) with active detuning at the 1H frequency. Quadrature RF transmission and signal reception for 31P-MRS was routed via two X-nuclei channels. Four channels were used for 1H signal reception. The setup was interfaced to a Philips Ingenia 3.0 Tesla MR system (R5.1.8; Philips Healthcare, Best, The Netherlands). The coil’s performance was compared with a standard linear single-turn 31P transmit/receive surface coil (∅ 14 cm, Philips).

Phantom studies: Two 9×9 gridded cryogenic storage boxes were used as a spacer between the anterior and posterior elements. To achieve a coil load similar to the human thorax, 500-mL 0.9% NaCl bags were placed on either side of the spacer, directly adjacent to the coil elements (Figure 1). A 3-mL vial filled with 15M phosphoric acid (H3PO4) was positioned in the center of the field of view and used for 31P B1 calibrations. Coil reference B1 and drive scale were adjusted such that a block pulse flip angle sweep yielded maximal signal intensity at the intended 90° excitation angle and zero signal at the intended 180° excitation angle. Next, the receive sensitivity distribution was measured by relocating the 3-mL 15M H3PO4 vial to different positions within the grids, and acquiring signal with an adiabatic pulse-acquire sequence.

In vivo 1H-MRI and 31P-MRS studies: In vivo cardiac data were acquired in a healthy male volunteer (29 y/76 kg/1.76 m) positioned supine in the MR scanner. Acquisition of a left-ventricular short-axis cine 1H-MRI series was performed with a balanced turbo field echo (BTFE) sequence (voxel size: 2.0×1.6 mm; matrix: 176×196; slice thickness: 8 mm; TR/TE: 2.8/1.41 ms; number of averages: 2; SENSE reduction factor: 1.5; 30 heart phases; retrospective ECG synchronization; single breath hold). Cardiac-triggered 3D-CSI was performed to acquire localized 31P-MR spectra of the heart (voxel size: 40×40×40 mm; matrix: 8×8×8; block pulse; flip angle: 45°; TR: 1 heart beat; number of averages: 1; 2048 data points; bandwidth 3000 Hz). Acquisition time was approximately 8 min 30 s. For comparison, the same 3D-CSI sequence and settings were used to obtain 31P-MR spectra from the same subject with the standard linear single-turn 31P surface coil.

Results

Figure 2 shows 31P receive sensitivity maps comparing the spatial distribution of SNR between the 4-element 1H/31P coil array (A) and the linear single-turn 31P surface coil (B). In the region relevant for cardiac 31P-MRS (boxed areas in Figures 1-2), SNR was 24% higher (p = 0.0002, paired t-test) and more homogeneous for the 4-element coil system (32.2 ± 4.8, mean ± SD) than for the linear single-turn surface coil (26.0 ± 9.5). This translated into a higher SNR for in vivo 31P-MRS of a 40×40×40 mm cardiac voxel with the 4-element 1H/31P coil compared to the linear single-turn 31P surface coil (Figure 3). In addition, cardiac cine 1H-MRI with the 4-element 1H/31P coil yielded images at a spatial and temporal resolution that will be sufficient for the quantification of cardiac function (Figures 4-5).

Discussion

We demonstrated that with a dual-tuned 1H/31P coil array with 4 elements, it is feasible to achieve a higher SNR and a more homogeneous receive sensitivity in the region of interest for cardiac 31P-MRS than with a standard linear single-turn surface coil. Furthermore, the 4 channels available for 1H signal reception allow for cardiac cine 1H-MRI with parallel imaging. Combined, the current setup allows for assessments of heart function with 1H-MRI and myocardial energy metabolism with 31P-MRS during a single examination.

Acknowledgements

This work was supported by the National Institutes of Health (A.J.B., G.J.S., A.J.N., and J.A.L.J.; subcontract to NIH grant HL072011).

References

No reference found.

Figures

Figure 1: The 4-element dual-tuned 1H/31P coil system in an anterior-posterior 2×2 combination. The dotted rectangle indicates the region of interest for cardiac 31P-MRS. A 3-mL vial filled with 15M phosphoric acid was used to probe the receive sensitivity distribution.

Figure 2: (A) 31P receive sensitivity map for the 1H/31P coil system. The dotted rectangle indicates the region of interest for cardiac 31P-MRS. (B) 31P receive sensitivity map for the linear single-turn 31P surface coil.

Figure 3: In vivo human cardiac 31P-MR spectra obtained from a single 40×40×40 mm voxel within a 3D-CSI dataset acquired with the two-channel 1H/31P coil (top, SNR PCr = 12.5) vs. the linear single-turn 31P surface coil (bottom, SNR PCr = 7.1).

Figure 4: End-diastolic (left) and end-systolic (right) short-axis 1H-MR images of the healthy human heart acquired with the four-channel 1H/31P coil.

Figure 5: Cine 1H-MRI series (movie).



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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