Saturation power dependence of amide proton transfer image contrasts in human brain tumors and strokes at 3 T

Abstract

Amide proton transfer (APT) imaging is capable of detecting mobile cellular proteins and peptides in tumor and monitoring pH effects in stroke, through the saturation transfer between irradiated amide protons and water protons. In this work, four healthy subjects, eight brain tumor patients (four with high-grade glioma, one with lung cancer metastasis, and three with meningioma), and four stroke patients (average 4.3 6 2.5 days after the onset of the stroke) were scanned at 3 T, using different radiofrequency saturation powers. The APT effect was quantified using the magnetization transfer ratio (MTR) asymmetry at 3.5 ppm with respect to the water resonance. At a saturation power of 2 mT, the measured APT-MRI signal of the normal brain tissue was almost zero, due to the contamination of the negative conventional magnetization transfer ratio asymmetry. This irradiation power caused an optimal hyperintense APT-MRI signal in the tumor and an optimal hypointense signal in the stroke, compared to the normal brain tissue. The results suggest that the saturation power of 2 mT is ideal for APT imaging of these two pathologies at 3 T with the existing clinical hardware. Magn Reson Med 66:1033-1041,

Chemical Exchange Saturation Transfer MR Imaging: Preliminary Results for Differentiation of Malignant and Benign Thoracic Lesions

“…APT-weighted CEST MR imaging resulted in a strong correlation of APT effect with pH levels because proton exchange reduced strongly during the pH level reduction, owing to ischemia (25)(26)(27)(28)(33)(34)(35)44,45). Therefore, the diminished CEST effect in the ischemic area was mainly due to a change in pH level.…”

“…In tumors, on the other hand, the intracellular exchangeable proton content became higher than that in normal tissue, while the intracellular pH level remained approximately unchanged, so that the CEST effect increased (23,27). Additionally, the effect of CEST resulted in a significant difference between glioma and normal tissue, as well as the development of edema or an ischemic area, including radiation necrosis (23,27,31,(44)(45)(46)(47). Moreover, the effect of CEST produced a significant difference between malignant and benign with high MTR asym (at 3.5 ppm).…”

Measurement of Lactate Content and Amide Proton Transfer Values in the Basal Ganglia of a Neonatal Piglet Hypoxic-Ischemic Brain Injury Model Using MRI

“…An off-resonance continuous-wave radiofrequency saturation pulse was used. APT single-section imaging was performed, and the saturation time, at 2 T excitation power, was 500 ms (the maximum time available for this body coil [22][23][24] ). The acquisition mode was TSE, and the factor was 38.…”

Depiction of Acute Stroke Using 3-Tesla Clinical Amide Proton Transfer Imaging: Saturation Time Optimization Using an in vivo Rat Stroke Model, and a Preliminary Study in Human

“…Saturation times of 3 and 4 s gave better contrast between ischemic regions and the NAB, as compared to 5 s. This may come from a coexisting solid macromolecular MT effect (9,13) that is more pronounced at a higher RF power, or possible intra-and inter-molecular nuclear Overhauser effects (10). Similar results were obtained by Sun et al (9), who showed that the optimal saturation power is achieved at around 1 μT, and APT contrast decreased at higher power fields.…”

“…The study showed that the optimal RF power is approximately 1 μT on 3T, which sufficiently labels the exchangeable protons with minimal spillover effects. On the other hand, Zhao et al (10) showed that a 2 μT amplitude had better APT difference compared to 1 or 3 μT, but the study was based on a duration of 500 ms. We adopted an amplitude of 1.2 μT, in order to maximize the pH-weighted effect using 3T TSE imaging.…”