Application of high field magnetic resonance microimaging in polymer gel dosimetry

Abstract

The purpose of this work was to examine the suitability of VIPAR nd polymer gel-9.4 T magnetic resonance microimaging system for high spatial resolution dose distribution measurements. Methods: The VIPAR nd samples (3 cm in outside diameter and 12 cm in height) were exposed to ionizing radiation by using a linear accelerator (Varian TrueBeam, USA; 6 MV x-ray beam). In the calibration stage, nine gel dosimeter vials were irradiated in a water phantom homogenously to the doses from 1.5 to 30 Gy in order to obtain R2-dose relation. In the verification stage, two gel dosimeter vials were irradiated in the half beam penumbra area of 10 9 10 cm radiation field using the amount of monitor units appropriate to deliver 20 Gy at the field center. The gels were imaged on a vertical 9.4 T magnetic resonance (MR) microimaging scanner using single slice and multislice (9 slices) multiecho (90 9 7 ms) sequences at the spatial resolutions of 0.2-0.4 9 0.2-0.4 9 3 mm 3 and 0.2-0.4 9 0.2-0.4 9 1 mm 3 respectively. The gels were subjected to microimaging during the period of two weeks after irradiation. The reference data consisted of the dose profiles measured using the diode dosimetry, radiochromic film, ionization chamber, and the water phantom system. Results: The VIPAR nd-9.4 T MR microimaging system was characterized by the dose sensitivity of 0.067 AE 0.002 Gy À1 s À1 at day 3 after irradiation. The dose resolution at 10 Gy (at P = 95%) was equal to 0.42 Gy at day 3 after irradiation using a single slice sequence (0.2 9 0.2 9 3 mm 3) and 2.0 Gy at day 4 after irradiation using a multislice sequence (0.2 9 0.2 9 1 mm 3) for one signal acquisition (measurement time: 15 min). These values were improved by~1.4-fold when using four signal acquisitions in the single slice sequence, and by~2.78-fold for 12 signal acquisitions in the multislice sequence. Furthermore, decreasing the in-plane resolution from 0.2 9 0.2 mm 2 to 0.4 9 0.4 mm 2 resulted in a dose resolution of 0.3 Gy and 1 Gy at 10 Gy (at P = 95%) for one signal acquisition in the single slice and multislice sequences respectively (measurement time: 7.5 min). As reveals from the gamma index analysis the dose distributions measured at days 3-4 postirradiation using both VIPAR nd verification phantoms agree with the data obtained using a silicon diode, assuming 1 mm/5% criterion. A good interphantom reproducibility of the polymer gel dosimetry was proved by monitoring of two phantoms up to 10 days after irradiation. However, the agreement between the dose distributions measured using the diode and polymer gel started to get worse from day 5 after irradiation. Conclusion: The VIPAR nd-9.4T MR microimaging system allows to obtain dose resolution of 0.42 Gy at 10 Gy (at P = 95%) for a spatial resolution of 0.2 9 0.2 9 3 mm 3 (acquisition time:

Hydrogels for Three-Dimensional Ionizing-Radiation Dosimetry

“…Mapping of the magnetization transfer (MT) has also been investigated [ 185 ] and was recently advanced for scanning low-density gel dosimeters [ 58 ]. Furthermore, investigations on polyacrilamide gels for dosimetric purposes were performed with high field magnetic resonance microimaging [ 186 ]. Recently, a new MRI approach, based on detecting nuclear Overhauser enhancement (NOE) mediated saturation transfer effects, was adopted on MAGIC gels for 3D-dose reconstruction [ 187 ].…”

Laser Light Trapping Phenomenon in a 3D Radiotherapy Polymer Gel Dosimeter

“…The dosimeter belongs to the family of 3D radiation therapy dosimeters containing N -vinylpyrrolidone (NVP). A summary of all NVP dosimeters and their recent applications is described elsewhere [ 16 , 17 , 18 , 19 , 20 ]. The results of the VIC-T [ 16 ] provided an optimal new composition of the gel dosimeter containing NVP, demonstrated the broad dose-response characteristics and thermal stability of the VIC-T, and indicated chemical (THPC-induced) and physical cross-linking of gelatine occurring in parallel after the manufacture of the dosimeter.…”