Correlated spectroscopic imaging of calf muscle in three spatial dimensions using group sparse reconstruction of undersampled single and multichannel data

Abstract

Purpose To implement a 5D (3 spatial + 2 spectral) correlated spectroscopic imaging sequence for application to human calf. Theory and Methods Nonuniform sampling was applied across the two phase encoded dimensions and the indirect spectral dimension of an echo planar correlated spectroscopic imaging sequence. Reconstruction was applied that minimized the group sparse mixed ℓ2,1-norm of the data. Multichannel data was compressed using a sensitivity map-based approach with a spatially-dependent transform matrix and utilized the self sparsity of the individual coil images to simplify the reconstruction. Results Single channel data with 8× and 16× undersampling are shown in the calf of a diabetic patient. A 15 channel scan with 12× undersampling of a healthy volunteer was reconstructed using 5 virtual channels and compared to a fully sampled single slice scan. Group sparse reconstruction faithfully reconstructs the lipid cross peaks much better than ℓ1 minimization. Conclusion COSY spectra can be acquired over a 3D spatial volume with scan time under 15 minutes using echo planar readout with highly undersampled data and group sparse reconstruction.

Sparse Reconstruction Techniques in Magnetic Resonance Imaging

“…Multispectral imaging approaches for imaging around metal can be accelerated using sparse reconstruction techniques, without loss of image quality, in clinically relevant settings such as patients with implanted spinal fixation hardware 168 . Spectroscopic imaging of areas such as the calf 169 may also be feasible with the increases in imaging speed afforded by sparse reconstruction techniques.…”

Fast data acquisition techniques in magnetic resonance spectroscopic imaging

“…One must also take into account the additional time required for reconstruction, post‐processing, and display of metabolite maps. The computation time for the more advanced reconstruction routines can range from 6 min 30 s for CS reconstructions to between 10 min/coil and over 4 h in total for the non‐uniform undersampling (NUS) 4D/5D echo‐planar correlated spectroscopic imaging (EP‐COSI) and echo‐planar J ‐resolved spectroscopic imaging (EP‐JRESI) datasets . Furthermore, it is important to note that the time gained from high‐speed MRSI techniques may be offset by the need for higher numbers of averages in situations where conventional phase‐encoded MRSI is itself SNR limited at the desired spatial resolution.…”

“…Fast MRSI will also aid in higher volume coverage, and incorporation of specialized spectral editing techniques for mapping metabolites such as lactate and gamma‐amino‐butyric acid (GABA) that typically require more averaging to obtain the desired SNR. Other techniques such as J ‐resolved spectroscopy and correlation spectroscopy (COSY) could also be incorporated into current clinical protocols to obtain high‐resolution spectroscopy data . Thus, either the acceleration offered by high‐speed techniques can be employed to decrease the scan time, leading to reduced motion sensitivity and patient discomfort, or the time‐saving can be traded for higher SNR/resolution and/or for imaging a larger volume of interest.…”

“…The computation time for the more advanced reconstruction routines can range from 6 min 30 s for CS reconstructions 129 to between 10 min/coil and over 4 h in total for the non-uniform undersampling (NUS) 4D/5D echo-planar correlated spectroscopic imaging (EP-COSI) and echo-planar J-resolved spectroscopic imaging (EP-JRESI) datasets. 48,49,83,133,174 Furthermore, it is important to note that the time gained from high-speed MRSI techniques may be offset by the need for higher numbers of averages in situations where conventional phase-encoded MRSI is itself SNR limited at the desired spatial resolution. The SNR penalty in conventional MRSI is often large to begin with, and this tends to be further aggravated when employing accelerated MRSI techniques, leading to difficulty in imaging certain anatomical regions (eg the prostate without the use of an endorectal coil).…”

“…Other techniques such as J-resolved spectroscopy and correlation spectroscopy (COSY) could also be incorporated into current clinical protocols to obtain high-resolution spectroscopy data. 48,49,131,174 Thus, either the acceleration offered by high-speed techniques can be employed to decrease the scan time, leading to reduced motion sensitivity and patient discomfort, or the time-saving can be traded for higher SNR/resolution and/or for imaging a larger volume of interest. The reconstruction and quantification of sparsely sampled spectroscopic data on the scanner immediately following data acquisition will also help in improving the practical utility of fast MRSI techniques in current imaging protocols.…”