MRI of radiation chemistry: First images and investigation of potential mechanisms

Abstract

Background Paramagnetic species such as O2 and free radicals can enhance T1 and T2 relaxation times. If the change in relaxation time is sufficiently large, the contrast will be generated in magnetic resonance images. Since radiation is known to be capable of altering the concentration of O2 and free radicals during water radiolysis, it may be possible for radiation to induce MR signal change. Purpose We present the first reported instance of x‐ray‐induced MR signal changes in water phantoms and investigate potential paramagnetic relaxation enhancement mechanisms associated with radiation chemistry changes in oxygen and free radical concentrations. Methods Images of water and 10 mM coumarin phantoms were acquired on a 0.35 T MR‐linac before, during, and after a dose delivery of 80 Gy using an inversion‐recovery dual‐echo sequence with water nullified. Radiation chemistry simulations of these conditions were performed to calculate changes in oxygen and free radical concentrations. Published relaxivity values were then applied to calculate the resulting T1 change, and analytical MR signal equations were used to calculate the associated signal change. Results Compared to pre‐irradiation reference images, water phantom images taken during and after irradiation showed little to no change, while coumarin phantom images showed a small signal loss in the irradiated region with a contrast‐to‐noise ratio (CNR) of 1.0–2.5. Radiation chemistry simulations found oxygen depletion of −11 µM in water and −31 µM in coumarin, resulting in a T1 lengthening of 24 ms and 68 ms respectively, and a simulated CNR of 1.0 and 2.8 respectively. This change was consistent with observations in both direction and magnitude. Steady‐state superoxide, hydroxyl, hydroperoxyl, and hydrogen radical concentrations were found to contribute less than 1 ms of T1 change. Conclusion Observed radiation‐induced MR signal changes were dominated by an oxygen depletion mechanism. Free radicals were concluded to play a minor secondary role under steady‐state conditions. Future applications may include in vivo FLASH treatment verification but would require an MR sequence with a better signal‐to‐noise ratio and higher temporal resolution than the one used in this study.

Investigation of contrast mechanisms for MRI phase signal‐based proton beam visualization in water phantoms

“…In the second experiment, no changes in phase difference contrast were observed under variation of TE. Consequently, because of the established proportionality of susceptibility‐induced phase difference shifts to TE, 31 the emerging phase difference contrast cannot be explained by locally altered magnetic susceptibility secondary to radiochemical changes in radical 13 or oxygen concentrations 14 . Furthermore, this observation confirms that the previously excluded influence of a beam‐induced perturbation of the magnetic field, which would also act throughout TE, does not cause the observed phase difference contrast.…”

“…Second, the measurement of a susceptibility‐related shift in resonance frequency induced by the radiochemical formation of paramagnetic radicals was suggested 13 . Closely related to this, photon beam‐induced hypointense MRI signatures recently observed on an MR‐Linac, a hybrid device combining MRI and a clinical electron linear accelerator to deliver photon beams, were ascribed to the radiochemical depletion of paramagnetic oxygen 14 . Moreover, the beam‐induced bulk convection observed in a previous magnitude signal‐based beam visualization study 9 could also be visualizable as phase contrast 15 …”

“…13 Closely related to this, photon beam-induced hypointense MRI signatures recently observed on an MR-Linac, a hybrid device combining MRI and a clinical electron linear accelerator to deliver photon beams, were ascribed to the radiochemical depletion of paramagnetic oxygen. 14 Moreover, the beam-induced bulk convection observed in a previous magnitude signal-based beam visualization study 9 could also be visualizable as phase contrast. 15 The purpose of the present study was, therefore, to assess the feasibility of MRI phase signal-based proton beam visualization and to test its ability to reflect energy-dependent changes in beam range.…”

“…29,30 Moreover, the photon beam of an MR-Linac has recently been visualized during the irradiation of oxygen-enhanced liquid water phantoms based on oxygen depletion. 14 All these MRI methods rely on chemical reactions with radicals formed as a consequence of the radiation hydrolysis of water, leading to detectable changes in the MR relaxation times. Although these achievements have been made in the field of MRI-based photon beam detection, they are indicative of the potential of in-beam MRI for visualization of proton and heavier particle beams, as substantial overlap in the underlying mechanisms can be expected.…”

“…With the increasing clinical availability of MR‐Linac devices, MRI has already demonstrated its capabilities for the on‐line verification of volumetric photon beam dose distributions in polymer and Fricke‐type gel phantoms 29,30 . Moreover, the photon beam of an MR‐Linac has recently been visualized during the irradiation of oxygen‐enhanced liquid water phantoms based on oxygen depletion 14 . All these MRI methods rely on chemical reactions with radicals formed as a consequence of the radiation hydrolysis of water, leading to detectable changes in the MR relaxation times.…”