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High resolution polymer gel dosimetry by parameter selective MR‐microimaging on a whole body scanner at 3 T
Author(s) -
Berg A.,
Ertl A.,
Moser E.
Publication year - 2001
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1358304
Subject(s) - dosimetry , image resolution , dosimeter , materials science , thermal diffusivity , imaging phantom , calibration , optics , sensitivity (control systems) , relaxation (psychology) , radiosurgery , medical imaging , scanner , nuclear magnetic resonance , biomedical engineering , nuclear medicine , physics , radiation therapy , medicine , engineering , quantum mechanics , electronic engineering , radiology
High dose variations across small spatial distances, as present in brachytherapeutic applications or radiosurgery and especially γ‐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 μm 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 R 2 = 1 / T 2 we investigate the sensitivity of the longitudinal relaxation rate R 1 = 1 / T 1 and the diffusivity D appin 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 γ irradiation. Also the MR‐parameter T 1 may be used for dose imaging. The impact of all of the three parameters T 1 ,T 2 , 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 R 2 , 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 γ‐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.