INTRODUCTION

High Spectral and Spatial resolution (HiSS) MRI has primarily been applied to breast imaging, with high lesion characterization performance and potential to serve as a powerful tool for non-contrast enhanced breast cancer screening,^1-3 anand it has also recently been applied in prostate.⁴ Successful implementation of compressed sensing (CS) k-space under-sampling would allow for longer echo trains necessary for spectral/spatial imaging without a comparable increase in imaging time. This would produce stronger T2* weighting and/or higher spectral resolution in HiSS MRI, potentially increasing sensitivity and diagnostic performance. Additionally, implementation of CS would increase utility of HiSS MRI in applications where SENSE acceleration is constrained by coil geometry. Here, we evaluate the effect of CS acceleration on effective image resolution, noise levels, and artifact levels at acceleration factors of up to R = 10.

METHODS

A two-bottle phantom was prepared from agar gel and vegetable oil, such that a water-fat boundary was created perpendicular to the readout and phase encoding directions (Figure 1), and each bottle was positioned in one of the volumes of a dedicated 15-channel breast coil. Imaging was conducted on a 3T Philips dStream Ingenia scanner. An axial image through the center of the phantom was acquired using the High Spectral and Spatial (HiSS) MRI sequence, which is based on a 2D echo-planar spectroscopic imaging (EPSI) sequence (FOV 256 x 384 x 3 mm³; spatial resolution 0.8 x 0.8 x 3 mm³; TR/TE/ΔTE 1000/122/1.89 ms, flip angle 90°; echo train length 127; spectral resolution 4.17 Hz), with with SENSE acceleration factors of 1, 2, 3, 4, 6, 8, and 10.⁴ At SENSE acceleration factor of 1, HiSS was done with full k-space coverage on a Cartesian grid and the data for each coil element were exported individually for CS post-processing. All acquisitions were repeated five times to allow for evaluation of variability and noise levels.



k-space variable-density random under-sampling of HiSS MRI data was simulated by using k-space under-sampling masks, constant for all echoes. Full k-space information and complex gradient echo images at 127 individual TEs were then reconstructed for acceleration factors R of 2, 3, 4, 6, 8 and 10 by using a distributed multi-sensor implementation⁵ of CS⁶ including sparsifying operators in space. A Fourier transform in the temporal direction yielded the proton spectrum in each voxel, and water resonance peak height (WPH) images were constructed.¹

To quantify the effect of the CS algorithm on effective image resolution, HiSS MRI WPH signal profiles across the lateral (orthogonal to phase encoding direction) and anterior (orthogonal to readout direction) oil/agar boundary of the breast phantom were extracted at a representative location, to approximate a sharp edge (Figure 1). The spatial derivatives of the edge profiles were fit to a Gaussian function whose Fourier transform provided the modulation transfer function (MTF). Equivalent image resolution was calculated for each acceleration factor by determining k values at which the MTF equals that of R = 1 at k = 0.625 1/mm (corresponding to the 0.8 mm in-plane resolution), as illustrated in Figure 2. This analysis was repeated on SENSE-accelerated HiSS WPH images.

To evaluate noise and artifact levels, two 50 x 50 regions of interest (ROIs), placed symmetrically relative to the coil geometry, in either agar or oil regions, were considered. To evaluate noise, ROI were placed in the agar area and images from acquisitions 2-5 were normalized to the corresponding images from acquisition 1, removing spatial gradients. The noise was quantified as $$$1/\sqrt{2}$$$ of the standard deviation of the resulting image intensity ratio in the agar area, averaged over acquisitions 2-5. The geometric artifact level could be quantified in areas of low signal and was calculated as the mean signal intensity in ROIs placed in the oil area, averaged over acquisitions 1-5.

RESULTS

SENSE acceleration did not degrade spatial resolution of HiSS MRI, which was nominally 0.8 in-plane and measured as 0.78 ± 0.02 mm in the readout direction, and 0.79 ± 0.01 mm in the phase encoding direction (Figure 3). Under CS reconstructions, it was 0.81 ± 0.02 mm in the readout direction, and 0.80 mm, 0.91 mm, 1.00 mm, 1.08 mm, 1.55 mm, 1.94 mm, and 2.16 mm in the phase encoding direction, for R = 1, 2, 3, 4, 6, 8, and 10, respectively (Figure 4). The noise level remained approximately constant with R for CS-accelerated imaging, while SENSE acceleration significantly amplified noise, approximately 6-fold for R = 10 (Figure 5a). Similarly, the geometric artifact level was approximately constant with R under CS acceleration, and increased up to about 3.5-fold for SENSE acceleration factor of 10 (Figure 5b).

DISCUSSION AND CONCLUSION

Minimal to acceptable blurring in HiSS MRI WPH images due to CS implementation can be expected with acceleration factors up to R = 4, without amplification of either noise or artifact levels. SENSE acceleration preserved the spatial resolution robustly, but greatly amplified noise and suffered from geometric artifacts that were evident in fat-suppressed areas. Therefore, CS is a promising acceleration strategy for HiSS MRI in noise- or geometrically-constrained applications and for acquisitions with a low number of coil elements, such as in prostate imaging.

Acknowledgements

This work was supported by NIH R01 CA167785.