A hybrid radiation detector for simultaneous spatial and temporal dosimetry

In this feasibility study an organic plastic scintillator is calibrated against ionisation chamber measurements and then embedded in a polymer gel dosimeter to obtain a quasi-4D experimental measurement of a radiation field. This hybrid dosimeter was irradiated with a linear accelerator, with temporal measurements of the dose rate being acquired by the scintillator and spatial measurements acquired with the gel dosimeter. The detectors employed in this work are radiologically equivalent; and we show that neither detector perturbs the intensity of the radiation field of the other. By employing these detectors in concert, spatial and temporal variations in the radiation intensity can now be detected and gel dosimeters can be calibrated for absolute dose from a single irradiation

Towards real-time 4D radiation dosimetry on an MRI-Linac

4D radiation dosimetry using a highly radiation-sensitive polymer gel dosimeter with real-time quantitative magnetic resonance imaging (MRI) readout is presented as a technique to acquire the accumulated radiation dose distribution during image-guided radiotherapy on an MRI-Linac. Optimized T 2-weighted Turbo-Spin-Echo (TSE) scans are converted into quantitative ΔR 2 maps and subsequently to radiation dose maps. The concept of temporal uncertainty is introduced as a metric of effective temporal resolution. A mathematical framework is presented to optimize the echo time of the TSE sequence in terms of dose resolution, and the trade-off between temporal resolution and dose resolution is discussed. The current temporal uncertainty achieved with the MAGAT gel dosimeter on a 1 T MRI-Linac is 3.8 s which is an order of magnitude better than what has been achieved until now. The potential of real-time 4D radiation dosimetry in a theragnostic MRI-Linac is demonstrated for two scenarios: an irradiation with three coplanar beams on a head phantom and a dynamic arc treatment on a cylindrical gel phantom using a rotating couch. The dose maps acquired on the MRI-Linac are compared with a treatment plan and with dose maps acquired on a clinical 3 T MRI scanner. 3D gamma map evaluations for the different modalities are provided. While the presented method demonstrates the potential of gel dosimetry for tracking the dose delivery during radiotherapy in 4D, a shortcoming of the MAGAT gel dosimeter is a retarded dose response. The effect of non-ideal radiofrequency pulses resulting from limitations in the specific absorption rate or B1-field inhomogeneity on the TSE acquired ΔR 2 values is analysed experimentally and by use of computational modelling with a Bloch simulator

Polymer gel dosimetry of an electron beam in the presence of a magnetic field

The effect of a strong external magnetic field on 4 MeV electron beam was measured with polymer gel dosimetry. The measured entrance dose distribution was compared with a calculated fluence map. The magnetic field was created by use of two permanent Neodymium (NdFeB) magnets that were positioned perpendicular to the electron beam. The magnetic field between the magnets was measured with Hall sensors. Based on the magnetic field measurement and the law of Biot-Savart, the magnetic field distribution was extrapolated. Electron trajectories were calculated using a relativistic Lorentz force operator. Although the simplified computational model that was applied, the shape and position of the calculated entrance fluence map are found to be in good agreement with the measured dose distribution in the first layer of the phantom. In combination with the development of low density polymer gel dosimeters, these preliminary results show the potential of 3D gel dosimetry in MRI-linac applications.5 page(s

R₂-dispersion simulation of foam microstructure

The spin-spin relaxation rate R₂ (=1/T₂) in hydrogel foams measured by use of a multi spin echo sequence is found to be dependent on the echo time spacing. This property, referred to as R₂-dispersion, originates from both surface relaxation and molecular self-diffusion of water within internal field gradients that result from magnetic susceptibility differences between the gel and air phase. In hydrogel foams, correlations between the average air bubble size and R₂-values are found (S. Baete and Y. De Deene, Proc. Intl. Soc. Mag. Reson. Med. (15) 37, 2007.). Random walk diffusion is simulated to correlate the R₂-dispersion with the foam microstructure (i.e. the mean air bubble radius and standard deviation of the air bubble radius) and foam composition properties (i.e. magnetic susceptibilities, diffusion coefficient and surface relaxivity). Simulations of R₂-dispersion are in agreement with NMR measurements of a hydrogel foam. By correlating the R₂-dispersion parameters and microstructure properties a semi-empirical relationship is obtained that enables the mean air bubble size to be derived from measured R₂-dispersion curves. The R₂-derived mean air bubble size of a hydrogel foam is in agreement with the bubble size measured with X-ray micro-CT. This illustrates the applicability of ¹H R₂-dispersion measurements for the macroscopic determination of the size of air bubbles in hydrogel foams and alveoli in lung tissue.4 page(s

On the reliability of 3D gel dosimetry

Gel dosimetry has a unique role to play in safeguarding conformal radiotherapy treatments as it covers the whole treatment chain and provides the radiation oncologist with the integrated dose distribution in 3D. A major obstacle that has hindered the wider dissemination of polymer gel dosimetry in radiotherapy centres is the lack of confidence in the reliability of the measured dose. Discrepancies in dose response of small versus large polymer gel dosimeters have been reported and although several hypothesis for these discrepancies have been postulated, the actual contribution of these error sources to the overall inaccuracy of the dose maps has not been determined. Several gel dosimetry research groups have chosen to use an internal calibration of gel dosimeters. In this study, the inter-and intra-batch reproducibility of the current state-of-the-art 3D gel dosimeters has been assessed. It is demonstrated that with a carefully designed scanning set-up, the overall accuracy that can be obtained with an independent calibration is well within 5% of all pixels.30 page(s

Comparison of optical CT imaging versus NMR imaging for nPAG gel dosimetry

Radiotherapy has evolved towards a more accurate technique of delivering a prescribed dose to the target volume. As a consequence, steep dose gradients in three dimensions occur near the edges of the target volume. Polymer gel dosimeters are able to record the integrated absorbed dose in three dimensions (3D). To read out the dose information of a 3D gel dosimeter we need a dedicated imaging technique. The aim of this study was to compare magnetic resonance imaging (MRI), an extensively studied imaging modality, with a more user friendly technique of optical computed tomography (optical CT). An optical CT scanner was constructed on site which is able to measure 3D polymer gels. The first challenge involved the elimination of all imaging artifacts. Different components were characterized such as laser stability, laser spot size, detector stability and refractive index matching fluid (RIMF). Some imaging artifacts were compensated by post processing techniques. Finally polymer gel phantoms were fabricated and irradiated with square beams. The dose distribution measured with optical CT was compared with the dose distribution measured with MRI. In order to make a fair comparison of precision between both imaging modalities, the intrinsic dosimetric precision on readout (IPD) was calculated. Laser stability and detector stability were accepted to be sufficient. RIMF was optimized using an automated procedure. Most artifacts were dealt with on a hardware level. The IPD value shows that optical CT scanning has a five times better intrinsic dose precision than MRI. In this study it is shown that optical CT is able to produce high quality images, but further quantitative evaluation is required to valorize the dosimetric value.4 page(s