Department of Radiation Oncology, Dana-Farber Cancer Institute and Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, USA.
Department of Physics and Applied Physics, University of Massachusetts Lowell, Lowell, Massachusetts, USA.
Med Phys. 2023 Jan;50(1):495-505. doi: 10.1002/mp.16011. Epub 2022 Oct 26.
Paramagnetic species such as O and free radicals can enhance T and T 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 O and free radicals during water radiolysis, it may be possible for radiation to induce MR signal change.
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.
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 T change, and analytical MR signal equations were used to calculate the associated signal change.
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 T 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 T change.
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.
顺磁性物质（如 O 和自由基）可增强 T 和 T 弛豫时间。如果弛豫时间的变化足够大，则磁共振图像中会产生对比度。由于已知辐射能够在水辐射分解过程中改变 O 和自由基的浓度，因此辐射可能会引起 MR 信号变化。
我们首次报道了在水模型中 X 射线诱导的 MR 信号变化，并研究了与氧和自由基浓度辐射化学变化相关的潜在顺磁弛豫增强机制。
在使用水抑制反转恢复双回波序列对 0.35 T MR-直线加速器进行 80 Gy 剂量输送之前、期间和之后，获取水和 10 mM 香豆素模型的图像。对这些条件下的辐射化学模拟进行了计算，以计算氧和自由基浓度的变化。然后应用已发表的弛豫率值来计算导致的 T 变化，并使用分析性 MR 信号方程来计算相关的信号变化。
与辐照前的参考图像相比，辐照期间和之后拍摄的水模型图像几乎没有变化，而香豆素模型图像在辐照区域显示出较小的信号丢失，对比度噪声比（CNR）为 1.0-2.5。辐射化学模拟发现水中氧耗竭为-11 µM，香豆素中氧耗竭为-31 µM，分别导致 T 延长 24 ms 和 68 ms，模拟 CNR 分别为 1.0 和 2.8。这一变化与观察到的方向和幅度一致。发现稳态超氧化物、羟基、过氧自由基和氢自由基浓度导致的 T 变化小于 1 ms。
观察到的辐射诱导的 MR 信号变化主要由氧耗竭机制主导。自由基在稳态条件下被认为是次要的次要作用。未来的应用可能包括体内 FLASH 治疗验证，但需要比本研究中使用的 MR 序列具有更好的信噪比和更高的时间分辨率。
