Chemical exchange saturation transfer (CEST) MR technique for in-vivo liver imaging at 3.0 tesla

Abstract

Purpose: To evaluate Chemical Exchange Saturation Transfer (CEST) MRI for liver imaging at 3.0-T.

Materials and methods: Images were acquired at offsets (n = 41, increment = 0.25 ppm) from -5 to 5 ppm using a TSE sequence with a continuous rectangular saturation pulse. Amide proton transfer-weighted (APTw) and GlycoCEST signals were quantified as the asymmetric magnetization transfer ratio (MTRasym) at 3.5 ppm and the total MTRasym integrated from 0.5 to 1.5 ppm, respectively, from the corrected Z-spectrum. Reproducibility was assessed for rats and humans. Eight rats were devoid of chow for 24 hours and scanned before and after fasting. Eleven rats were scanned before and after one-time CCl4 intoxication.

Results: For reproducibility, rat liver APTw and GlycoCEST measurements had 95 % limits of agreement of -1.49 % to 1.28 % and -0.317 % to 0.345 %. Human liver APTw and GlycoCEST measurements had 95 % limits of agreement of -0.842 % to 0.899 % and -0.344 % to 0.164 %. After 24 hours, fasting rat liver APTw and GlycoCEST signals decreased from 2.38 ± 0.86 % to 0.67 ± 1.12 % and from 0.34 ± 0.26 % to -0.18 ± 0.37 % respectively (p < 0.05). After CCl4 intoxication rat liver APTw and GlycoCEST signals decreased from 2.46 ± 0.48 % to 1.10 ± 0.77 %, and from 0.34 ± 0.23 % to -0.16 ± 0.51 % respectively (p < 0.05).

Conclusion: CEST liver imaging at 3.0-T showed high sensitivity for fasting as well as CCl4 intoxication.

Key points: • CEST MRI of in-vivo liver was demonstrated at clinical 3 T field strength. • After 24-hour fasting, rat liver APTw and GlycoCEST signals decreased significantly. • After CCl4 intoxication both rat liver APTw and GlycoCEST signals decreased significantly. • Good scan-rescan reproducibility of liver CEST MRI was shown in healthy volunteers.

Keywords: Amide proton transfer (APT); Chemical exchange saturation transfer (CEST); Glycogen; Liver; Magnetic resonance imaging.

Fig. 1

A&B. An example of placement of ROIs on a rat liver parenchyma region of T2 weighted image (A) and CEST image (B); C: An example of placement of ROIs on a human subject liver parenchyma region of CEST image. Artifacts and blood vessels were excluded.

Fig. 2

Bland-Altman plots for APTw (A) and GlycoCEST (B) imaging of 16 rats. The inter-scan differences of MTR_asym values are plotted against the average MTR_asym values of two scans for each rat.

Fig. 3

Typical APTw image and GlycoCEST image (A) of a rat liver before and after 24-hour fasting with the corresponding ROI-averaged Z-spectrum and MTR_asym spectrum (B). In this rat, the APTw MTR_asym was 3.67% and the GlycoCEST MTR_asym was 0.3% before 24-hour fasting. After 24-hour fasting, the APTw MTR_asym was 1.41% and the GlycoCEST MTR_asym was 0.04%. C: Bar plot of rat liver MTR_asym value of APTw and GlycoCEST before and after 24-hour fasting (n=8).

Fig. 4

Typical APTw image and GlycoCEST image (A) of a rat liver before and after CCl4 injection with the corresponding ROI-averaged Z-spectrum and MTR_asym spectrum (B). In this rat, the baseline APTw MTR_asym was 2.66% and the GlycoCEST MTR_asym was 0.56%. After 48-hour CCl4 intoxication, the APTw MTR_asym was 1.91% and the GlycoCEST MTR_asym was -0.05%. C: Bar plot of rat liver MTR_asym value of APTw and GlycoCEST before and after 48-hour CCl4 injection (n = 11).

Fig. 5

Liver APTw image (A), GlycoCEST image (B), ΔB₀ map (C) and R² map (D) in a healthy 24-year-old female volunteer.

Fig. 6

Bland-Altman plots for the liver APTw (A) and the liver GlycoCEST (B) of 8 healthy volunteers. The inter-scan differences of MTR_asym values are plotted against the average MTR_asym values of two scans for each subject.