Multi-echo length and offset VARied saturation (MeLOVARS) method for improved CEST imaging.

PURPOSE

The aim of this study was to develop a technique for rapid collection of chemical exchange saturation transfer images with the saturation varied to modulate signal loss transfer and enhance contrast.

METHODS

Multi-echo Length and Offset VARied Saturation (MeLOVARS) divides the saturation pulse of length Tsat into N = 3-8 submodules, each consisting of a saturation pulse with length of Tsat /N (∼0.3-1 s), one or more low flip-angle gradient-echo readout(s) and a flip back pulse. This results in N readouts with increasing saturation time from Tsat /N to Tsat without extra scan time.

RESULTS

For phantoms, eight images with Tsat incremented every 0.5 s from 0.5-4 s were collected simultaneously using MeLOVARS, which allows rapid determination of exchange rates for agent protons. For live mice bearing glioblastomas, the Z-spectra for five different Tsat values from 0.5 to 2.5 s were acquired in a time normally used for one Tsat . With the additional Tsat -dependence information, LOVARS phase maps were produced with a more clearly defined tumor boundary and an estimated 4.3-fold enhanced contrast-to-noise ratio (CNR). We also show that enhancing CNR is achievable by simply averaging the collected images or transforming them using the principal component analysis.

CONCLUSIONS

MeLOVARS enables collection of multiple saturation-time-weighted images without extra time, producing a LOVARS phase map with increased CNR.