Three-dimensional detection, dosimetry and imaging of an energy

Details Three-dimensional detection, dosimetry and imaging of an energy Three-dimensional detection, dosimetry and imaging of an energy

1995-03-31

1997-05-27

Arana, Louis M.

Electricity: measuring and testing

Particle precession resonance

324307, G01V 300

Patent

active

056335849

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to a method of forming a radiant energy-generated, permanent three-dimensional polymeric image in a gel and to an article of manufacture comprising a gel adapted for use as a dosimeter, e.g., of ionizing radiation.
There are numerous applications for a dosimetric technique that can accurately measure radiation dose distributions in three dimensions with high spatial resolution. For example, in radiation therapy there are several treatment regimes for which it would be valuable to characterize the distribution of radiation with millimeter resolution in tissue-equivalent, homogeneous and heterogeneous media. Towards this end, the use of aqueous gels containing Fricke dosimeter solution in combination with magnetic resonance imaging (MRI) techniques was suggested. See Gore, J. C., et al., Phys. Med. Biol. 29:1189-1197; 1984; Abstract. Res. Imaging 2:244; 1984.
It was shown that the radiation-induced oxidation of ferrous ions to ferric in solution could be detected by a change in the water proton spin relaxation times, T1 and T2, and the changes were large enough to be mapped with high spatial resolution by magnetic resonance imaging (MRI) when the Fricke solution was dispersed in a gelatin or agarose gel. Gore, J. C., et al., Mag. Res. Imaging, supra; Schulz, R. J., et al., Phys. Med. Biol. 35:1611-1622; 1990.
The conversion of ferrous to ferric ions promotes proton relaxation because the ferric ions produce stronger paramagnetic dipole-dipole interactions with the water protons. Also, the water relaxation rates are proportional to radiation dose over a wide range of doses. This work promoted several groups to investigate this approach to radiation dosimetry. Day, J. J. Phys. Med. 35:1605-1609; 1990; Olsson, L. E., Ph.D. Thesis, Lund University, Malmo, Sweden, 1991.
Their results verify the potential value of such an imaging technique. For example, the practical value and unique ability of this technique to map the dose distributions of the complex irradiations used in stereotactic radiosurgery have recently been demonstrated. Olsson, L. E., supra; Schultz, R. J., et al., "Assessment of the accuracy of stereotactic radiosurgery using Fricke-infused gels and MRI" (Unpub., submitted to Med. Phys.). However, there is a major disadvantage that is intrinsic to the Fricke gel medium, viz., ferric ions are able to diffuse quite freely through the gel after irradiation, with a point spread function whose width is about 2 mm after one hour. Schulz et al., supra; Olsson, L. E., supra. This leads to a gradual blurring of the radiation dose pattern and a consequent loss of spatial resolution, and places constraints on the minimum dose rate that can be applied, on the time between irradiation and imaging, and on a number of images that can be recorded before diffusion introduces errors in the subsequent calculation of relaxation times. There are ways to correct for this diffusion but the complexities of this approach present a major limitation to the practicality of the technique. A gel in which diffusion of, the radiation-induced, NMR-sensitive species is insignificant over relatively long periods of time would be highly advantageous, especially when weak radioactive brachytherapy sources are to be studied.
Other dosimetry systems employing gels or polymerizable resins are described in the patent literature. U.S. Pat. No. 4,430,258 discloses a method of producing liquid equivalent solid gamma ray calibration standards comprising the steps of adding a first organic solvent to a calibrated aqueous solution of radioactive material, either in the form of a single isotope or mixed isotopes, to form a first solution; mixing the first solution with a polymerizable resin dissolved in a second organic solvent together with a hardening catalyst to form a second solution; and curing the second solution. With isotope standards requiring highly acidic conditions, a stabilizing agent is added to the first solvent to complex with metal and hydrogen ions so as to prevent plate-out.
U.S

REFERENCES:
patent: 4532218 (1985-07-01), Strom
patent: 4790919 (1988-12-01), Baylor, Jr.
patent: 5321357 (1994-06-01), Maryanski et al.

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