3D gradient echo snapshot CEST MRI with low power saturation for human studies at 3T

Abstract

Purpose For clinical implementation, a chemical exchange saturation transfer (CEST) imaging sequence must be fast, with high signal‐to‐noise ratio (SNR), 3D coverage, and produce robust contrast. However, spectrally selective CEST contrast requires dense sampling of the Z‐spectrum, which increases scan duration. This article proposes a compromise: using a 3D snapshot gradient echo (GRE) readout with optimized CEST presaturation, sampling, and postprocessing, highly resolved Z‐spectroscopy at 3T is made possible with 3D coverage at almost no extra time cost. Methods A 3D snapshot CEST sequence was optimized for low‐power CEST MRI at 3T. Pulsed saturation was optimized for saturation power and saturation duration. Spectral sampling and postprocessing (B 0 correction, denoising) was optimized for spectrally selective Lorentzian CEST effect extraction. Reproducibility was demonstrated in 3 healthy volunteers and feasibility was shown in 1 tumor patient. Results Low‐power saturation was achieved by a train of 80 pulses of duration t p = 20 ms (total saturation time t sat = 3.2 seconds at 50% duty cycle) with B 1 = 0.6 μT at 54 irradiation frequency offsets. With the 3D snapshot CEST sequence, a 180 × 220 × 54 mm field of view was acquired in 7 seconds per offset. Spectrally selective CEST effects at +3.5 and –3.5 ppm were quantified using multi‐Lorentzian fitting. Reproducibility was high with an intersubject coefficient of variation below 10% in CEST contrasts. Amide and nuclear overhauser effect CEST effects showed similar correlations in tumor and necrosis as show in previous ultra‐high field work. Conclusion A sophisticated CEST tool ready for clinical application was developed and tested for feasibility.

Structure or Exchange? On the Feasibility of Chemical Exchange Detection with Balanced Steady‐State Free Precession in Tissue – An In Vitro Study

“…Fig. b, similar as reported in literature for in vivo white matter structures in the human brain . The CEST effects in the homogenate with marginal dilution (homogenate 1) and in the corticospinal tract of intact pig brain hemispheres correspond well to each other.…”

Adaptive denoising for chemical exchange saturation transfer MR imaging

“…Experiments are often performed at ultra–high field strengths to take advantage of the inherent high signal–to–noise ratio (SNR). Still, both at clinical field strengths and at ultra–high field, these spectrally selective CEST effects are small and difficult to detect reliably. This is true not only for multi–Lorentzian fitting approaches, which can have dozens of free parameters and depend on initial and boundary conditions, but also for simpler approaches like the three–point method, where noise in the baseline points can lead to noise in the contrast.…”

DeepCEST 3T: Robust MRI parameter determination and uncertainty quantification with neural networks—application to CEST imaging of the human brain at 3T

“…This is important to note, because the current Lorentzian fitting is also known to be a simplified model, not including the super‐Lorentzian line shape and only few of the known CEST pools . This was recently shown to fit the data well at low saturation powers …”