UCEPR: Ultrafast localized CEST-spectroscopy with PRESS in phantoms and in vivo

Abstract

Purpose Chemical Exchange Saturation Transfer (CEST) is a contrast mechanism enhancing low-concentration molecules through saturation transfer from their exchangeable protons to bulk water. Often many scans are acquired to form a Z-spectrum, making the CEST method time-consuming. Here, an ultrafast localized CEST-spectroscopy with PRESS (UCEPR) is proposed to obtain the entire Z-spectrum of a voxel using only two scans, significantly accelerating CEST. Methods The approach combines ultrafast non-localized CEST spectroscopy with localization using PRESS. A field gradient is applied concurrently with the saturation pulse producing simultaneous saturation of all Z-spectrum frequencies that are also spatially encoded. A readout gradient during data acquisition resolves the spatial dependence of the CEST responses into frequency. UCEPR was tested on a 3T scanner both in phantoms and in vivo. Results In phantoms, a fast Z-spectroscopy acquisition of multiple pH-variant iopamidol samples was achieved with 4–7-fold acceleration as compared to the conventional CEST methods. In vivo, amide proton transfer (APT) in white matter of healthy human brain was measured rapidly in 48 s and with high frequency resolution (≤ 0.2 ppm). Conclusions Compared to conventional CEST methods, UCEPR has the advantage of rapidly acquiring high-resolution Z-spectra. Potential in vivo applications include ultra-fast localized Z-spectroscopy, quantitative, or dynamic CEST studies.

Clinical applications of chemical exchange saturation transfer (CEST) MRI

“…The ultrafast CEST MRI method can rapidly perform CEST MRI studies, although this method requires more development for clinical use. Ultrafast CEST MRI applies a magnetic field gradient across the tissue that spatially spreads the MR frequency of the water and the CEST biomolecule or agent, and then applies frequency‐selective saturation that generates a spatially encoded CEST spectrum . This method is “ultrafast,” because the entire CEST spectrum is generated with only one RF saturation pulse.…”

“…Ultrafast CEST MRI applies a magnetic field gradient across the tissue that spatially spreads the MR frequency of the water and the CEST biomolecule or agent, and then applies frequency-selective saturation that generates a spatially encoded CEST spectrum. 29,30 This method is "ultrafast," because the entire CEST spectrum is generated with only one RF saturation pulse. However, the tissue must be homogenous across the space that is encoded by the magnetic field gradients, which has currently limited clinical applications of this technique to imaging small brain FIGURE 1: CEST MRI.…”

CEST, ASL, and magnetization transfer contrast: How similar pulse sequences detect different phenomena

“…Similar to the CASL method, CW‐CEST also is not commonly used on clinical scanners. To reach maximum CEST signal, the length of the saturation pulse in CW‐CEST would be best set to approximately 2 to 3 s for low saturation powers . Unfortunately, a much shorter saturation time (<1 s) has been mandated for use on clinical scanners due to RF amplifier limitations .…”

Screening CEST contrast agents using ultrafast CEST imaging

“…Compared with the conventional Z-spectrum acquisition of one frequency per repetition time TR, this method accelerates Z-spectrum acquisition greatly and was therefore named ultrafast Z-spectroscopy (UFZ)[4]. A similar method named gradient-encoded Z-spectroscopy was developed by Döpfert et al [5] UFZ was also implemented for the acquisition of in vivo spectroscopy [6] and ultrafast HyperCEST Z-spectra [7, 8]. One particular advantage of the UFZ in the context of HyperCEST is that the Z-spectrum becomes less dependent of shot-noise originating, for example, from varying polarization levels in individual experiments.…”