A fast multislice sequence for 3D MRI‐CEST pH imaging

Abstract

Purpose: Chemical exchange saturation transfer MRI can provide accurate pH images, but the slow scan time (due to long saturation periods and multiple offsets sampling) reduce both the volume coverage and spatial resolution capability, hence the possibility to interrogate the heterogeneity in tumors and organs. To overcome these limitations, we propose a fast multislice CEST-MRI sequence with high pH accuracy and spatial resolution. Methods: The sequence first uses a long saturation pulse to induce the steady-state CEST contrast and a second short saturation pulse repeated after each image acquisition to compensate for signal losses based on an uneven irradiation scheme combined with a single-shot rapid acquisition with refocusing echoes readout. Sequence sensitivity and accuracy in measuring pH was optimized by simulation and assessed by in vitro studies in pH-varying phantoms. In vivo validation was performed in two applications by acquiring multislice pH images covering the whole tumors and kidneys after iopamidol injection. Results: Simulated and in vivo data showed comparable contrast efficiency and pH responsiveness by reducing saturation time. The experimental data from a homogeneous, pH-varying, iopamidol-containing phantom show that the sequence produced a uniform CEST contrast across slices and accurate values across slices in less than 10 minutes. In vivo measurements allowed us to quantify the 3D pH gradients of tumors and kidneys, with pH ranges comparable with the literature. Conclusion: The proposed fast multislice CEST-MRI sequence allows volumetric acquisitions with good pH sensitivity, accuracy, and spatial resolution for several in vivo pH imaging applications.

Average saturation efficiency filter (ASEF) for CEST imaging

“…For example, the combination of a long primary and several short secondary saturation pulses where 1 secondary saturation is added to each subsequent imaging slice to compensate for T 1 recovery. 10,60 With ASEF, both the primary and the secondary saturation can be selected as high or low DC pulse trains with different pulse durations, respectively. Similarly, ASEF can also be incorporated into 3D imaging sequences.…”

Chemical Exchange Saturation Transfer for Lactate-Weighted Imaging at 3 T MRI: Comprehensive In Silico, In Vitro, In Situ, and In Vivo Evaluations

“…In addition, we optimized the LATEST imaging protocol for 3 T MRI scanners, the field strength commonly used in clinical practice. However, compared to high-field MRI systems with 7 T and more, the optimized imaging protocol must be considered less sensitive; in particular, higher field strengths allow for volumetric multi-slice approaches due to the higher SNR, whereas our measurements record only one slice [ 54 ]. Furthermore, the in vitro experiments showed that pH changes in a physiological range of 7.0–7.3 can be neglected compared to the change in lactate levels.…”

Evaluation of a similarity anisotropic diffusion denoising approach for improving in vivo CEST‐MRI tumor pH imaging

“…In contrast, our proposed approach combines both an efficient noise‐reduction algorithm and the redundancy of spatial information inside the image to improve the CEST contrast quantification. Finally, we think that the proposed approach could potentially also be widely exploited for the denoising of other CEST‐MRI applications, both for pH imaging as well as for GlucoCEST or APT imaging, 71‐73 after optimizing and evaluating the proposed denoising method for each CEST modality. Moreover, we want to emphasize that, before any application, either in a human whole‐body scanner or in conjunction with parallel imaging, further investigations concerning the influence of B 0 and B 1 inhomogeneities and non‐Rician noise distribution are required to verify the applicability of the approach.…”