Prospective acceleration of parallel RF transmission-based 3D chemical exchange saturation transfer imaging with compressed sensing

Abstract

Purpose: To develop prospectively accelerated 3D CEST imaging using compressed sensing (CS), combined with a saturation scheme based on time-interleaved parallel transmission.

Methods: A variable density pseudo-random sampling pattern with a centric elliptical k-space ordering was used for CS acceleration in 3D. Retrospective CS studies were performed with CEST phantoms to test the reconstruction scheme. Prospectively CS-accelerated 3D-CEST images were acquired in 10 healthy volunteers and 6 brain tumor patients with an acceleration factor (R_CS ) of 4 and compared with conventional SENSE reconstructed images. Amide proton transfer weighted (APTw) signals under varied RF saturation powers were compared with varied acceleration factors.

Results: The APTw signals obtained from the CS with acceleration factor of 4 were well-preserved as compared with the reference image (SENSE R = 2) both in retrospective phantom and prospective healthy volunteer studies. In the patient study, the APTw signals were significantly higher in the tumor region (gadolinium [Gd]-enhancing tumor core) than in the normal tissue (p < .001). There was no significant APTw difference between the CS-accelerated images and the reference image. The scan time of CS-accelerated 3D APTw imaging was dramatically reduced to 2:10 minutes (in-plane spatial resolution of 1.8 $\times$ 1.8 mm² ; 15 slices with 4-mm slice thickness) as compared with SENSE (4:07 minutes).

Conclusion: Compressed sensing acceleration was successfully extended to 3D-CEST imaging without compromising CEST image quality and quantification. The CS-based CEST imaging can easily be integrated into clinical protocols and would be beneficial for a wide range of applications.

Keywords: APT; CEST; brain tumor; compressed sensing; parallel RF transmission.

Figure 1.

(A) A variable density k-space undersampling scheme with the centric elliptical k-space ordering for 3D acquisition. The central region is densely sampled and the outer region is progressively undersampled by the acceleration factor (e.g., R_CS = 4). The colormap represents echo ordering acquisition (start of acquisition and end of acquisition). Comparison of (B) the MTR_asym(2.5ppm) maps and (C) Z-spectra and MTR asymmetry spectra (insets) reconstructed with SENSE with R_SENSE = 2 and CS with R_CS = 4 (maps) and 6 (maps and spectra). Phantom contained NH₄Cl and 1% agarose (ROI I: pH 4.5, 0.5 M NH₄Cl + 1% agarose + PBS, ROI II: pH 5.0, 0.5 M NH₄Cl + 1% agarose + PBS, ROI III: pH 4.6, 1 M NH₄Cl + 1% agarose + PBS, ROI IV: pH 7.0, 1% agarose + PBS).

Figure 2.

A comparison of APTw images from a healthy volunteer reconstructed with SENSE and CS using prospective acceleration factors of 2 and 4, respectively. Only odd numbered slices are shown. The APTw images were obtained from RF saturation powers of 1.5 and 2 μT, and an RF saturation duration of 2 sec.

Figure 3.

(A) Correlation and (B) Bland-Altman plots of the APTw signal at RF saturation powers of 1.5 and 2 μT, reconstructed from the reference standard (SENSE = 2) and CS factor 4 for ten healthy volunteers. In the Bland-Altman plot, the mean bias is indicated by the solid black line and the 97% limits of agreement are indicated by dotted black lines.

Figure 4.

APTw images obtained for a patient with a glioblastoma using a RF saturation power of 2 μT and saturation duration of 2 sec, reconstructed by (A) SENSE (R_SENSE = 2) and (B) CS (R_CS = 4). (C) Average APTw signal intensities with the two acceleration schemes from normal tissue and glioma tissue (from five patients with a glioblastoma). Error bars depict standard deviations. (D) Post-contrast T₁w images.

Figure 5.

Conventional MR images and APTw images for a representative patient with an anaplastic oligodendroglioma. Five out of a total of fifteen APTw image slices are shown together with corresponding FLAIR, T₂w, T₁w, and Gd-T₁w images. The grade III lesion was slightly enhanced on the post-contrast images and also showed an intermediate APTw signal.