High-resolution polymer gel dosimetry by parameter selective MR-microimaging on a whole body scanner at 3T.

High dose variations across small spatial distances, as present in brachytherapeutic applications or radiosurgery and especially gamma-knife therapy, are difficult to quantify by standard dosimetry. We demonstrate the possibility to obtain planar spatial resolutions for dose imaging at pixel sizes below 200 microm within multislice parameter selective MR imaging on polymer gels. The sensitivity of the transversal and longitudinal relaxation time as well as diffusivity on dose is shown. High spatial resolution is achieved by parameter selective microimaging of polymer gels on a high-field (3 T) whole-body MR system equipped with a dedicated strong gradient system and a small probe head matched to the sample size. In addition to the spin-spin relaxation rate R2 = 1/T2 we investigate the sensitivity of the longitudinal relaxation rate R1 = 1/T1 and the diffusivity Dapp in acrylic polymer gels on irradiation up to dose levels of about 20 Gy. Dose images are obtained after calibration of the corresponding MR parameters by known dose levels of gamma irradiation. Also the MR-parameter T1 may be used for dose imaging. The impact of all of the three parameters T1, T2, and diffusivity on obtained signal intensities in irradiated regions has to be taken into account in nonoptimized pulse sequences. Further, very high spatial resolution imposes several restrictions on the evaluation of R2, which have to be considered for quantitative dosimetry. These restrictions are discussed in detail. We also demonstrate the importance of such a high spatial resolution in case of a set of differently sized gamma-knife stereotactic irradiation schemes. Gel dosimetry based on MR parameter selective microimaging represents a potent alternative for the detection of dose distributions characterized by steep dose gradients, typical in brachytherapeutic and radiosurgical applications.