Magnetic resonance imaging of microbubbles in a superheated emulsion chamber for brachytherapy dosimetry

This paper describes development of magnetic resonance imaging (MRI) techni ques for three-dimensional (3D) imaging of a position-sensitive detector fo r brachytherapy dosimetry. The detector is a 0.5 1 chamber containing an em ulsion of halocarbon-115 droplets in a tissue-equivalent glycerin-based gel . The halocarbon droplets are highly superheated and expand into vapor micr obubbles upon irradiation. Brachytherapy sources can be inserted into the s uperheated emulsion chamber to create distributions of bubbles. Three-dimen sional MRI of the chamber is then performed. A 3D gradient-echo technique w as optimized for spatial resolution and contrast between bubbles and gel. S usceptibility gradients at the interfaces between bubbles and gel are explo ited to enhance contrast so microscopic bubbles can be imaged using relativ ely large voxel sizes. Three-dimensional gradient-echo images are obtained with an isotropic resolution of 300 mu m over a 77 mmx77 mmx9.6 mm field-of -view in an imaging time of 14 min. A post-processing technique was develop ed to semi-automatically segment the bubbles from the images and to assess dose distributions based on the measured bubble densities. Relative dose di stributions are computed from MR images for a I-125 brachytherapy source an d the results compare favorably to relative radial dose distributions calcu lated as recommended by Task Group 43 of the American Association of Physic ists in Medicine. (C) 1998 American Association of Physicists in Medicine. [S0094-2405(98)00112-6].