Fast Chemical Exchange Saturation Transfer Research Articles

Acquisition sequences and reconstruction methods for fast chemical exchange saturation transfer imaging.

Chemical exchange saturation transfer (CEST) imaging is an emerging molecular magnetic resonance imaging (MRI) technique that has been developed and employed in numerous diseases. Based on the unique saturation transfer principle, a family of CEST-detectable biomolecules in vivo have been found capable of providing valuable diagnostic information. However, CEST MRI needs a relatively long scan time due to the common long saturation labeling module and typical acquisition of multiple frequency offsets and signal averages, limiting its widespread clinical applications. So far, a plethora of imaging schemes and techniques has been developed to accelerate CEST MRI. In this review, the key acquisition and reconstruction methods for fast CEST imaging are summarized from a practical and systematic point of view. The first acquisition sequence section describes the major development of saturation schemes, readout patterns, ultrafast z-spectroscopy, and saturation-editing techniques for rapid CEST imaging. The second reconstruction method section lists the important advances of parallel imaging, compressed sensing, sparsity in the z-spectrum, and algorithms beyond the Fourier transform for speeding up CEST MRI.

Fast chemical exchange saturation transfer imaging based on PROPELLER acquisition and deep neural network reconstruction.

To develop a method for fast chemical exchange saturation transfer (CEST) imaging. The periodically rotated overlapping parallel lines enhanced reconstruction (PROPELLER) sampling scheme was introduced to shorten the acquisition time. Deep neural network was employed to reconstruct CEST contrast images. Numerical simulation and experiments on a creatine phantom, hen egg, and in vivo tumor rat brain were performed to test the feasibility of this method. The results from numerical simulation and experiments show that there is no significant difference between reference images and CEST-PROPELLER reconstructed images under an acceleration factor of 8. Although the deep neural network is trained entirely on synthesized data, it works well on reconstructing experimental data. The proof of concept study demonstrates that the combination of the PROPELLER sampling scheme and the deep neural network enables considerable acceleration of saturated image acquisition and may find applications in CEST MRI.

NIMG-60. IDH MUTANT GLIOMAS WITH 1p/19q CO-DELETION ARE LESS ACIDIC THAN NON-CO-DELETED GLIOMAS AS MEASURED WITH PH-WEIGHTED AMINE CEST-MRI AND AMINO ACID PET

Abstract 1p/19q co-deleted gliomas are known to have slower growth rates and are more sensitive to chemotherapy and radiotherapy. This may be partially explained by the lower tumor acidosis compared to non-co-deleted gliomas, as extracellular acidosis is one of the driving forces toward tumor invasion and resistance to treatments. Amine CEST-EPI is a fast chemical exchange saturation transfer (CEST) imaging technique sensitive to decreased extracellular pH, transverse relaxation rate, and amino acid concentration. In the current study, we demonstrated that 1p/19q co-deleted gliomas are less acidic than non-co-deleted gliomas, using a combination of pH-sensitive amineCEST-EPI, T2 relaxometry, and 18F-FDOPA (18[F] fluorodopa) amino acid PET. 70 histologically-confirmed glioma patients (World Health Organization WHO grade II, N=35; grade III, N=35) received amine CEST-EPI scans. Among them, 16 patients received 18F-FDOPA PET scan and 45 patients received T2 relaxometry quantification. Mann-Whitney u-test is performed to evaluate the differences. Median MTRasym at 3ppm (magnetization transfer ratio asymmetry at amine proton resonance frequency) within T2 hyperintense lesions was significantly lower in 1p/19q co-deleted gliomas compared to non-co-deleted ones (co-deleted 1.19±0.31%; non-co-deleted 1.66±0.45%; p< 0.0001). The significantly lower MTRasym persists when comparing within grade II (p=0.003), grade III (p=0.031), IDH1 mutated gliomas (p=0.002), and gliomas exhibiting classical oligodendroglial histology (p=0.0007). The ROC analysis shows that the prediction of 1p/19q status using MTRasym has area under the curve (AUC) of 0.80 (sensitivity 75.6%. specificity 72.7%). Median FDOPA and T2 in T2-hyperintense lesions were not different between 1p/19q co-deleted and non-co-deleted tumors (FDOPA p=0.84; T2 p=0.63). Results suggest 1p/19q co-deleted gliomas have notably lower acidity compared with non-co-deleted gliomas, as indicated by lower MTRasym and no differences in amino acid concentration or transverse relaxation rate. Further, data indicate the 1p/19q co-deleted gliomas may have distinct metabolic characteristics and tumor microenvironment that can be measured using pH-sensitive amineCEST-MRI at 3T.

Fast 3D chemical exchange saturation transfer imaging with variably-accelerated sensitivity encoding (vSENSE).

To extend the variably-accelerated sensitivity encoding (vSENSE) method from 2D to 3D for fast chemical exchange saturation transfer (CEST) imaging, and prospectively implement it for clinical MRI. The CEST scans were acquired from 7 normal volunteers and 15 brain tumor patients using a 3T clinical scanner. The 2D and 3D "artifact suppression" (AS) vSENSE algorithms were applied to generate sensitivity maps from a first scan acquired with conventional SENSE-accelerated 2D and 3D CEST data. The AS sensitivity maps were then applied to reconstruct the other CEST frames at higher acceleration factors. Both retrospective and prospective acceleration in phase-encoding and slice-encoding dimensions were implemented. Applying the 2D AS vSENSE algorithm to a 2-fold undersampled 3.5-ppm CEST frame halved the scan time of conventional SENSE, while generating essentially identical reconstruction errors (p ≈ 1.0). The 3D AS vSENSE algorithm permitted prospective acceleration by up to 8-fold, in total, from phase-encoding and slice-encoding directions for individual source CEST images, and an overall speed-up in scan time of 5-fold. The resulting vSENSE-accelerated amide proton transfer-weighted images agreed with conventional 2-fold-accelerated SENSE CEST results in brain tumor patients and healthy volunteers. Importantly, the vSENSE method eliminated unfolding artifacts in the slice-encoding direction that compromised conventional SENSE CEST scans. The vSENSE method can be extended to 3D CEST imaging to provide higher acceleration factors than conventional SENSE without compromising accuracy.

A fast chemical exchange saturation transfer imaging scheme based on single-shot spatiotemporal encoding.

To design a new approach that can not only keep the spatial and temporal resolution but also have better built-in immunity to magnetic field inhomogeneity and chemical shift effects than the single-shot echo planar imaging (EPI) for chemical exchange saturation transfer (CEST) MRI. The single-shot spatiotemporally encoded (SPEN) MRI sequence was combined with a continuous wave saturation pulse for fast CEST MRI (CEST-SPEN MRI). The resulting images were super-resolved reconstructed by a hybrid method that solves the l1 norm minimization together with total variation (TV) regularization. Partial Lorentzian fitting was used to analyze the subsequent Z-spectra. Experimental results of a creatine phantom and in vivo tumor rat brains show that CEST-SPEN MRI has good capability in providing CEST-based and NOE-based contrast images. Compared with CEST-EPI, CEST-SPEN MRI has better immunity to magnetic field inhomogeneity and provides better contrast images within identical acquisition time, especially under an identical inhomogeneous field. Magn Reson Med 77:1786-1796, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Variable delay multi-pulse train for fast chemical exchange saturation transfer and relayed-nuclear overhauser enhancement MRI

Chemical exchange saturation transfer (CEST) imaging is a new MRI technology allowing the detection of low concentration endogenous cellular proteins and metabolites indirectly through their exchangeable protons. A new technique, variable delay multi-pulse CEST (VDMP-CEST), is proposed to eliminate the need for recording full Z-spectra and performing asymmetry analysis to obtain CEST contrast. The VDMP-CEST scheme involves acquiring images with two (or more) delays between radiofrequency saturation pulses in pulsed CEST, producing a series of CEST images sensitive to the speed of saturation transfer. Subtracting two images or fitting a time series produces CEST and relayed-nuclear Overhauser enhancement CEST maps without effects of direct water saturation and, when using low radiofrequency power, minimal magnetization transfer contrast interference. When applied to several model systems (bovine serum albumin, crosslinked bovine serum albumin, l-glutamic acid) and in vivo on healthy rat brain, VDMP-CEST showed sensitivity to slow to intermediate range magnetization transfer processes (rate < 100-150 Hz), such as amide proton transfer and relayed nuclear Overhauser enhancement-CEST. Images for these contrasts could be acquired in short scan times by using a single radiofrequency frequency. VDMP-CEST provides an approach to detect CEST effect by sensitizing saturation experiments to slower exchange processes without interference of direct water saturation and without need to acquire Z-spectra and perform asymmetry analysis.