Application of chemical exchange saturation transfer (CEST) MRI for endogenous contrast at 7 Tesla

Abstract

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) indirectly images exchangeable solute protons resonating at frequencies different than bulk water. These solute protons are selectively saturated using low bandwidth RF irradiation and saturation is transferred to bulk water protons via chemical exchange, resulting in an attenuation of the measured water proton signal. CEST MRI is an advanced MRI technique with wide application potential due to the ability to examine complex molecular contributions. CEST MRI at high field (7 Tesla [7 T]) will improve the overall results due to increase in signal, T1 relaxation time, and chemical shift dispersion. Increased field strength translates to enhanced quantification of the metabolite of interest, allowing more fundamental studies on underlying pathophysiology. CEST contrast is affected by several tissue properties, such as the concentrations of exchange partners and their rate of proton exchange, whose effects have been examined and explored in this review. We have highlighted the background of CEST MRI, typical implementation strategy, and complications at 7 T.

Keywords: 7 T; CEST; MRI.

Figure 1

Simulated CEST effect demonstrating the effects of changes in saturation power (B₁) for a sample containing bulk water and amide protons in slow/intermediate exchange.

Figure 2

Pulse sequence diagrams for the continuous wave and pulse-train saturation methods to acquire CEST MRI data. B₁ represents the pulse amplitude, t_sat is the saturation time, t_d is the delay between pulses, and dur is the duration of each constituent pulse of the pulse-train saturation.

Figure 3

Shifting maps used to center z-spectra calculated using the A) ΔB₀ map, B) higher-order polynomial fit, C) Lorentizan fit, and D) WASSR.

Figure 4

A) CEST_asym maps calculated for the amide protons (APT_asym) using A) point subtraction at ± 3.5 ppm and B) integration of asymmetry from -4 to -3 ppm.

Figure 5

Results of CEST MRI at 7T on healthy control and MS patient. A) Z-spectra arising from healthy white matter, MS patient white matter, and MS lesion, solid lines, left y-axis. CEST asymmetry is also shown, dashed lines, right y-axis. B) Anatomical image of healthy subject with the calculated APT asymmetry map shown in panel C. D) Anatomical image of MS patient with calculated APT asymmetry map found in panel E.

Figure 6

Results from GlycoCEST of skeletal muscle at 7T. A) T₁-weighted anatomical image, B) reference image for glycogen resonance (1.0 ppm), C) normalized image for glycogen resonance (-1.0 ppm), D) shift map calculated from polynomial fit with color scale in Hz, and E) asymmetry map for glycogen (1.0 ppm).