High resolution polymer gel dosimetry for small beam irradiation using a 7T micro-MRI scanner

Application of high field magnetic resonance microimaging in polymer gel dosimetry

Purpose

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 × 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 × 7 ms) sequences at the spatial resolutions of 0.2–0.4 × 0.2–0.4 × 3 mm³ and 0.2–0.4 × 0.2–0.4 × 1 mm³ 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 ± 0.002 Gy⁻¹ s⁻¹ 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 × 0.2 × 3 mm³) and 2.0 Gy at day 4 after irradiation using a multislice sequence (0.2 × 0.2 × 1 mm³) 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 × 0.2 mm² to 0.4 × 0.4 mm² 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 × 0.2 × 3 mm³ (acquisition time: 15 min). Further studies are required to improve a temporal stability of the gel‐derived dose distribution.

A feasibility study of multislice X-ray CT imaging of gel dosimeters using the "zero scan" method

This study extends the ‘zero scan’ method for CT imaging of polymer gel dosimeters to include multislice acquisitions. Multislice CT images consisting of 24 slices of 1.2 mm thickness were acquired of an irradiated polymer gel dosimeter and processed with the zero scan technique. The results demonstrate that zero scan‐based gel readout can be successfully applied to generate a three‐dimensional image of the irradiated gel field. Compared to the raw CT images, the processed figures and cross‐gel profiles demonstrated reduced noise and clear visibility of the penumbral region. Moreover, these improved results further highlight the suitability of this method in volumetric reconstruction with reduced CT data acquisition per slice. This work shows that 3D volumes of irradiated polymer gel dosimeters can be acquired and processed with X‐ray CT.

PACS number: 87.57.Q‐, 87.57.nf, 87.55.‐x