On the comparison of two NIPAM gel dosimeters with high resolution 3D MRI sequences

Abstract

Polymer gel dosimeters allow the measurement of a dose distribution in 3 dimensions. Irradiation induces polymerization reactions in the long chains of the gel molecules, which is characterized by a change in the transverse relaxation time T2. Gel dosimetry is mainly advantageous for its high resolution, but can also be expensive. The main goal of this project is to compare two normoxic polymer gel recipes made with NIPAM as monomer. The analysis is based on the quality of the images obtained for a fixed MRI scan time and a comparative cost of analysis of each 1L phantom. The ΔT2 maps are obtained by scanning the gels before and after irradiation with 3D sequences and the DESPOT2 technique is used to reconstruct the T2 maps. The phantoms were irradiated with 8 photon beams of 6 MV with a fixed 250 MU, 4 at two opposite sides of the jar (AP-PA configuration), to obtain 4 dose distributions of 4.7 Gy, 8.9 Gy, 13.2 Gy and 17.3 Gy. The SNR is then determined in the ΔT2 maps in function of the dose step and the concentration of NIPAM in the phantom. The first gel phantom is made with 15% NIPAM and has a total cost of $1561.65, including products and MRI scan costs. The SNR obtained for the 4 dose steps are 10.02, 31.18, 45.12 and 37.54. The contrast between the T2 before and after the irradiation in the 4 regions of dose are 0.45, 0.66, 0.78 and 0.85. The second phantom contained 5% NIPAM, cost 1186.85$ and the SNR obtained are 16.80, 27.08, 30.11 and 26.17 and the ΔT2 are of 0.61, 0.75, 0.79 and 0.77 for the 4 dose steps respectively. The comparison of the two recipes has shown that the increase of NIPAM concentration does not allow a significant increase in image quality for MRI. The recipe with 5% of NIPAM has a lower dynamic range but a better sensitivity at lower dose and is less expensive than the one with 15% of NIPAM. The results also show that by increasing the NIPAM concentration 3-fold, the cost of a 1L phantom increases by 30%.