Noise in polymer gel measurements using MRI

With the development of conformal radiotherapy, particularly intensity modu lated radiation therapy (IMRT), there is a clear need for multidimensional dosimeters. A commercial polymerizing gel, BANG-2(R) gel (MGS Research, Inc ., Guilford, CT), has recently been developed that shows potential as a mul ti-dimensional dosimeter. This study investigates and characterizes the noi se and magnetic resonance (MR) artifacts from imaging BANG-2 gels. Seven cy lindrical vials (4 cm diam, 20 cm length) were irradiated end on in a water bath and read using MRI (B-0 = 1.5 T, TE=20 ms/100 ms, TR=3000 ms). The ge l calibration compared the measured depth-dose distributions in water again st the change in solvent-proton R-2 relaxivity of the gel. A larger vial (1 3 cm diam, 14 cm length) was also irradiated to test the calibration accura cy in a vial of sufficient volume for dose distribution measurements. The c alibration curve proved accurate to within 1.3% in determining the depth do se measured by the larger vial. An investigation of the voxel-to-voxel (IXI X 3 mm(3)) noise and sensitivity response curve showed that the voxel-to-vo xel variation dominated the dose measurement uncertainty. The voxel-to-voxe l standard deviation ranged from 0.2 Gy for the unirradiated gel to 0.7 Gy at 20 Gy. Slice-to-slice R-2 magnitude deviations were also observed corres ponding to 0.2 Gy. These variations limited the overall accuracy of the gel dose measurements and warrant an investigation of more accurate MR readout sequences. (C) 2000 American Association of Physicists in Medicine. [S0094 -2405(00)02508-6].