T1ρ‐based dynamic glucose‐enhanced (DGEρ) MRI at 3 T: method development and early clinical experience in the human brain

Abstract

Purpose: The aim of this study was to translate the T 1 ρ-based dynamic glucoseenhanced (DGEρ) experiment from ultrahigh magnetic field strengths to a clinical field strength of 3 T. Although the protocol would seem to be as simple as gadoliniumenhanced imaging, several obstacles had to be addressed, including signal-to-noise ratio (SNR), robustness of contrast, and postprocessing, especially motion correction. Methods: Spin-lock based presaturation and a 3D gradient-echo snapshot readout were optimized for 3 T with regard to robustness, chemical exchange saturation transfer effect strength, and SNR. Postprocessing steps, including dynamic B 0 and motion correction, were analyzed and optimized in 7 healthy volunteers. The final protocol, including glucose injection, was applied to 3 glioblastoma patients. Results: With appropriate postprocessing, motion-related artifacts could be drastically reduced, and an SNR of approximately 90 could be achieved for a single dynamic measurement. In 2 patients with blood-brain barrier breakdown, a significant glucose uptake could be observed with a DGEρ effect strength in the range of 0.4% of the water signal. Thorough analysis of possible residual motion revealed that the statistical evidence can decrease when tested against pseudo effects attributed to uncorrected motion. Conclusion: DGEρ imaging was optimized for clinical field strengths of 3 T, and a robust protocol was established for broader application. Early experience shows that DGEρ seems possible at 3 T and could not only be attributed to motion artifacts.Observed DGEρ maps showed unique patterns, partly matching with the T 1 -ce tumor ring enhancement. However, effect sizes are small and careful clinical application is necessary.

Dynamic glucose‐enhanced (DGE) MRI in the human brain at 7 T with reduced motion‐induced artifacts based on quantitative R1ρ mapping

“…A prerequisite for a reliable retrospective image coregistration was the acquisition of 3D volumetric image data, which was realized by translating the snapshot‐CEST approach to 7 T. The 3D snapshot‐CEST image readout was already successfully applied for volumetric coregistration of human DGE images at 3 T . In contrast, the initial DGE‐MRI examinations in humans were only coregistered in plane due to a limited 2D image readout .…”

“…Under the assumption that no subject motion occurs in between the two consecutive images (i.e., time gap of 7 s), this ratio normalizes the spatially varying sensitivity of the coil (Figure B). In addition, the quantitative R 1ρ approach compensates for the slab selection profile of the 3D image readout, which is also dependent on the position of the head within the coil . Acquisition of only two different R 1ρ ‐weighted images for calculation of the R 1ρ maps (Equation ) was sufficient to yield accurate and reliable maps (cf.…”

“…This enabled fast ΔR 1ρ mapping, which was stable against motion‐induced artifacts at only the cost of a decrease in temporal resolution of a factor of 2 compared to the previously used DGE ρ approach . Alternatively, interleaved normalization scans can be acquired during the time course of the examination to handle the issue of an erroneous normalization in the presence of subject motion as recently proposed by Herz et al…”

“…In contrast to the on‐resonant CESL approach utilized in this study at 7 T, the recent DGE‐MRI approach at 3 T by Herz et al used off‐resonant SL pulses in order to increase the contrast‐to‐noise ratio. This is particularly important at lower magnetic field strengths (e.g., 3 T) where the exchange weighting of glucose (i.e., the exchange‐dependent relaxation rate in the rotating frame R ex of glucose contributing to R 1ρ ) is considerably reduced .…”

“…DGE‐MRI after administration of glucose or glucose analogs (e.g., 2‐deoxy‐D‐glucose or L‐glucose) has been shown to provide valuable insights into tissue glucose concentration changes in small animal models of cancer and Alzheimer’s disease . Whether glucose in blood vessels, extracellular glucose, or intracellular phosphorylated glucose derivatives are the driving source for the observed DGE contrast remains to be investigated in depth . Furthermore, the applicability for examinations in humans has already been verified in independent pilot studies based on glucoCEST and glucoCESL .…”