No abstract

MAGIC gel dosimeter with 3T MRI scanner as a read-out makes it an ideal tool for small field dosimetry.

Purpose: To evaluate brain synchronization and connectivity differences in post‐surgery/per‐irradiation patients and healthy controls with two different MEG approaches: ECD (equivalent current dipole) and SAM (synthetic aperture magnetometry). Methods: An 8 min resting state scan was collected from 5 post‐surgery/pre‐irradiation patients and 5 sex/age‐matched control subjects. Scans were collected on a CTF MEG 275‐sensors system. All subjects were scanned in upright, open‐eyes position in a magnetically shielded room. MRI whole brain volume was divided into 4mm voxels allowing sensor signal space to be mapped into brain space. Measured signals were filtered into Δ, o/, α and β bandwidths and localized by ECD and SAM. SAM is a type of beamforming, a signal processing technique for sensor array data; the method works like a digital filter by focusing on each voxel while attenuating the majority of signal from all other voxels. Resultant SAM waveforms were evaluated to reject muscle and/or non‐physiological artifacts. The ECD method was applied to the same data and bandwidths to localize sources, its orientations and strengths. Statistical analysis evaluated connectivity and synchronization using dPLI (direct phase lag index) and IC (imaginary coherence). Resultant SAM and ECD maps were compared for differences and similarities. Results: Currently the project is collecting data. The full per‐irradiation data is expected to be completed by June 2013. The study goal is to establish the best method/approach to evaluate resting state MEG data. Conclusion: We expect 3 possible outcomes of the study: no difference, subtle difference, statistically significant difference in resting state analysis between these two methods. The two first outcomes are easy to address by choosing the most time‐efficient method. The last case will require further investigation of both methods to establish the ground of the difference and the most appropriate method analysis for the resting state. Partial support by Radiation Oncology and Neurology Departments at Wake Forest Baptist Health System and by Dalton McMichael Fund in Cancer Research. No conflict of interest is reported.

Purpose: Dose distribution characteristics for small radiation fields can be very difficult to determine. In this work a MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) 3D polymer gel is combined with 7T micro‐MR imaging for high resolution measurements of the small field (<1cm) 3D dose distributions. Methods: MAGIC(Gelatin 9%; Methacrylic acid 4%; CuSO4 0.1 mM; Ascorbic ascid 2mM; Glucose 22%) 3D gel phantoms were irradiated with very small 6MV x‐ray beams (5x5 mm2 and 10 × 10 mm2 square fields; 2mm diameter round field). Gel dose measurements were performed in Bruker 7T mirco‐MRI and GE Signa 3T scanners and with simultaneously‐obtained radiochromic films. T2 maps were acquired using a 10‐echo‐Multi‐Spin Multi‐Echo (MSME) pulse sequence on both MR scanners. Normalized 3D dose maps were calculated in Matlab. Results: Dose distributions determined from 30 minute scans for the 5×5 mm2 and 10×10 mm2 square fields on the 7T MR unit were superior to 3T MR unit in spatial resolution (7T: 0.156mm × 0.156mm × 1mm voxel; 3T: 0.254mm × 0.254mm × 2mm voxel). For the very small field size (2mm diameter), the MAGIC gel with 7T MRI provided even better quality dose distribution images (1.5hour scan; spatial resolution 79um ×79um × 1mm and; 12 hour scan: spatial resolution 38um × 38um × 1mm), 3T MRI was not able to read accurate dose profile due to a low SNR in the same 1.5hour scan time. Conclusions: This study indicates that the MAGIC polymer gel with 7T micro‐MRI for 3D dose readout can potentially be used for dosimetric characterization of very small radiation beams, including measurements for micro‐beams (field size ∼ 100um). Techniques and limitations for MAGIC gel dosimetry via high field MR imaging for dosimetric assessment are detailed in this work.