Fast high‐resolution 3D segmented echo planar imaging for dose mapping using a superheated emulsion chamber

Abstract

The superheated emulsion chamber (SEC) consists of superheated droplets of halocarbons in an aqueous gel. The gel resides in a pressure chamber. Brachytherapy sources can be implanted in the SEC for radiation dosimetry studies. Upon irradiation by ionizing radiation, the metastable droplets vaporize to form microbubbles. MRI can be used to determine the distribution of bubbles following irradiation of the SEC. In order to generate sufficient statistical accuracy in the determination of dose distributions around brachytherapy sources, it is necessary to use hundreds of irradiation cycles. Susceptibility-weighted images provide contrast between the gel and the vapor microbubbles. This article describes a 3D, blipped, double-sampled, segmented echo-planar imaging technique for rapidly imaging the SEC at 650 m isotropic 3D resolution in about 2 min. This method was used with a pressure cycling SEC to acquire hundreds of images in several hours. Results are presented showing the 2D dose distribution generated by an Implantation of radioactive sources, or brachytherapy, is frequently performed for the treatment of cancer and other conditions. The optimal use of these sources requires accurate benchmark dosimetry to fully characterize effects such as source anisotropy (1) and source-to-source shielding (2). Dosimetry close to implantable radiation sources has also become more important because of the recent successful application of brachytherapy in prevention of restenosis after coronary angioplasty or other interventions for peripheral vessel disease (3). The current dosimetry techniques employ 1D dosimeters, such as thermoluminescent dosimeters (TLDs) in a grid, or 2D dosimeters such as radiochromic films in phantoms. Using these devices to generate 2D or 3D dose distributions is a laborintensive process which can take months of effort.There is increasing interest in using MRI in conjunction with polymer gels for performing 3D radiation dosimetry (4 -8). We have previously described a reusable superheated emulsion chamber (SEC) for radiation dosimetry (9). Here we describe a new MR imaging technique applied to the SEC which allows for more rapid data acquisition. The SEC contains an emulsion of uniformly distributed, superheated, chloropentafluoroethane (C 2 ClF 5 ) droplets, suspended at a density of ϳ1 ϫ 10 4 droplets per cm 3 in a glycerin-based gel. The droplets have a physical density of 1.29 g/cm 3 and are nearly neutrally buoyant in the glycerol based-gel, which has a density of 1.25 g/cm 3 . The emulsion droplets have a boiling point of -39.1°C at atmospheric pressure and remain contained by the smooth walls of the gel in a metastable superheated liquid state. Gamma ray interactions in the gel create showers of secondary electrons. The secondary electrons have sufficient linear energy transfer at the end of their travel to initiate vaporization of the superheated droplets. The 40 -50 m diameter droplets then completely vaporize into an ϳ300 m bubble. The gel fixes the bubble in a stable position. MR imaging ...

Visualization of air bubbles by magnetic resonance imaging

“…In some applications, the air bubbles themselves can provide information about other processes such as brachytherapy dosage [12] or fluid pressure [13]. During the course of the several studies summarized here, it has become increasingly obvious that it is often extremely difficult to eliminate air bubbles from systems that have complex internal structures through which water is pumped, and it may be that air bubbles are more prevalent than may be expected.…”