CEST contrast (particularly the amine peak amplitude) performed especially well in distinguishing areas of apoptosis and/or necrosis from actively progressing tumor, as validated by histology.

Phantom experiments in agar and ammonium chloride were performed to evaluate a three-pool model of magnetization transfer and chemical exchange saturation transfer (CEST) in a pulsed saturation transfer experiment. The utility of the pulsed CEST method was demonstrated by varying the pH of the phantoms and observing the effect upon the CEST spectra both with and without the solid agar (the magnetization transfer pool), while fitting the spectra to the Bloch equation model with exchange. Pulsed CEST could be used to robustly quantify parameters related to CEST, including the exchange rate constant describing proton exchange with free water and the concentration of exchanging protons. Furthermore, the exchange rate constant and the CEST pool offset frequency of the ammonium chloride remained unchanged in the presence of a magnetization transfer pool. The logarithm of the fitted exchange rate constant was linearly related to pH: this relationship was maintained in the presence of magnetization transfer. Magn Reson Med 67:979-990,

Stereotactic radiosurgery (SRS) is a common treatment used in patients with brain metastases and is associated with high rates of local control, however, at the risk of radiation necrosis. It is difficult to differentiate radiation necrosis from tumor progression using conventional MRI, making it a major diagnostic dilemma for practitioners. This prospective study investigated whether chemical exchange saturation transfer (CEST) was able to differentiate these two conditions. Sixteen patients with brain metastases who had been previously treated with SRS were included. Average time between SRS and evaluation was 12.6 months. Lesion type was determined by pathology in 9 patients and the other 7 were clinically followed. CEST imaging was performed on a 3T Philips scanner and the following CEST metrics were measured: amide proton transfer (APT), magnetization transfer (MT), magnetization transfer ratio (MTR), and area under the curve for CEST peaks corresponding to amide and nuclear Overhauser effect (NOE). Five lesions were classified as progressing tumor and 11 were classified as radiation necrosis (using histopathologic confirmation and radiographic follow-up). The best separation was obtained by NOE (NOE = 8.9 ± 0.9%, NOE = 12.6 ± 1.6%, < 0.0001) and Amide (Amide = 8.2 ± 1.0%, Amide = 12.0 ± 1.9%, < 0.0001). MT (MT = 4.7 ± 1.0%, MT = 6.7 ± 1.7%, = 0.009) and NOE (NOE = 4.3 ± 2.0% Hz, NOE = 7.2 ± 1.9% Hz, = 0.019) provided statistically significant separation but with higher values. CEST was capable of differentiating radiation necrosis from tumor progression in brain metastases. Both NOE and Amide provided statistically significant separation of the two cohorts. However, APT was unable to differentiate the two groups. .

CEST metrics, in particular, the NOE peak amplitude, can predict volume changes 1 month post-SRS. Magn Reson Med 78:1110-1120, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Quantitative MR Study Group, this article provides an overview of considerations for the development, validation, qualification, and dissemination of quantitative MR (qMR) methods. This process is framed in terms of two central technical performance properties, i.e., bias and precision. Although qMR is confounded by undesired effects, methods with low bias and high precision can be iteratively developed and validated. For illustration, two distinct qMR methods are discussed throughout the manuscript: quantification of liver proton-density fat fraction, and cardiac T 1 . These examples demonstrate the expansion of qMR methods from research centers toward widespread clinical dissemination. The | 1185 WEINGÄRTNER et al.