Optical CT reconstruction of 3D dose distributions using the ferrous–benzoic–xylenol (FBX) gel dosimeter

Abstract

In recent years, magnetic-resonance imaging of gelatin doped with the Fricke solution has been applied to the direct measurement of three-dimensional (3D) radiation dose distributions. However, the 3D dose distribution can also be imaged more economically and efficiently using the method of optical absorption computed tomography. This is accomplished by first preparing a gelatin matrix containing a radiochromic dye and mapping the radiation-induced local change in the optical absorption coefficient. Ferrous-Benzoic-Xylenol (FBX) was the dye of choice for this investigation. The complex formed by Fe3+ and xylenol orange exhibits a linear change in optical attenuation (cm-1) with radiation dose in the range between 0 and 1000 cGy, and the local concentration of this complex can be probed using a green laser light (lambda = 543.5 nm). An optical computed tomography (CT) scanner was constructed analogous to a first-generation x-ray CT scanner, using a He-Ne laser, photodiodes, and rotation-translation stages controlled by a personal computer. The optical CT scanner itself can reconstruct attenuation coefficients to a baseline accuracy of < 2% while yielding dose images accurate to within 5% when other uncertainties are taken into account. Optical tomography is complicated by the reflection and refraction of light rays in the phantom materials, producing a blind spot in the transmission profiles which, results in a significant dose artifact in the reconstructed images. In this report we develop corrections used to reduce this artifact and yield accurate dosimetric maps. We also report the chemical reaction kinetics, the dose sensitivity and spatial resolution (< 1 mm3) obtained by optical absorption computed tomography. The article concludes with sample dose distributions produced by "cross-field" 6 MV x-ray beams, including a radiosurgery example.

Fricke gel dosimeter with improved sensitivity for low‐dose‐level measurements

“…Any system that is to be considered a suitable dosimeter has to fulfill acceptable characteristics of reproducibility and independence with the radiation beam quality 4 , 6 , 33 , 36 , 37 . These were tested to judge if the new solution is applicable in low‐dose‐level dosimetry.…”

“…Investigations are carried out by modifying the system dedicated to relative high doses by incorporating chemical components, like benzoic acid 3 , 9 , 31 , 32 , 33 in its preparation. Comparisons between the developed FXG system with conventional dosimetry systems are accomplished with the aim of validating it as a reliable dosimetry system.…”

Ultrasound-modulated optical computed tomography of biological tissues

“…13,14 However, our hybrid optical imaging technique is quite different in physics from those pure optical imaging techniques. In those applications, the scanning beam is a laser beam and the projection data are the intensity of the transmitted ballistic light, which cannot penetrate relatively thick biological tissues.…”

Dosimetry tools and techniques for IMRT

“…221,222 The practical implication is that Fricke gel dosimeters must be scanned within 2 h postirradiation, presenting logistical difficulties in a clinical setting. Most Fricke gel dosimetry has utilized MR as the readout method; however, Kelly et al 204 introduced xylenol-orange dye to create a Fricke gel compatible with optical-CT imaging. Recently, a promising new radiochromic material, PRESAGE, 223 has been introduced and detailed studies of the basic dosimetric properties have confirmed its promise for 3D dosimetry.…”

“…Recently, new 3D dosimetry materials have been proposed with striking performance characteristics. 204,205 The three principal categories of 3D dosimeters are polyacrylamide ͑PAG͒ gels, 206 Fricke gels, 204 and radiochromic plastics, 205 and the two main methods of dose-readout are magnetic resonance ͑MR͒ imaging 207,208 and optical-computedtomography ͑optical-CT͒. 203,204,209 Other readout techniques are under evaluation, including x-ray-CT and ultrasound, but are not discussed further here as they are more preliminary.…”