A CCD-based optical CT scanner for high-resolution 3D imaging of radiation dose distributions: equipment specifications, optical simulations and preliminary results

Abstract

Methods based on magnetic resonance imaging for the measurement of three-dimensional distributions of radiation dose are highly developed. However, relatively little work has been done on optical computed tomography (OCT). This paper describes a new OCT scanner based on a broad beam light source and a two-dimensional charge-coupled device (CCD) detector. A number of key design features are discussed including the light source; the scanning tank, turntable and stepper motor control; the diffuser screen onto which images are projected and the detector. It is shown that the non-uniform pixel sensitivity of the low-cost CCD detector used and the granularity of the diffuser screen lead to a serious ring artefact in the reconstructed images. Methods are described for eliminating this. The problems arising from reflection and refraction at the walls of the gel container are explained. Optical ray-tracing simulations are presented for cylindrical containers with a variety of radii and verified experimentally. Small changes in the model parameters lead to large variations in the signal intensity observed in the projection data. The effect of imperfect containers on data quality is discussed and a method based on a 'correction scan' is shown to be successful in correcting many of the related image artefacts. The results of two tomography experiments are presented. In the first experiment, a radiochromic Fricke gel sample was exposed four times in different positions to a 100 kVp x-ray beam perpendicular to the plane of imaging. Images of absorbed dose with slice thickness of 140 microm were acquired. with 'true' in-plane resolution of 560 x 560 microm2 at the edge of the 72 mm field of view and correspondingly higher resolution at the centre. The nominal doses measured correlated well with the known exposure times. The second experiment demonstrated the well known phenomenon of diffusion in the dosemeter gels and yielded a value of (0.12 +/- 0.02) mm2 s(-1) for the diffusion coefficient of the xylenol orange/iron complex. Finally, the overall implications of the above findings for dosimetry using OCT are discussed.

3D dosimetry by optical-CT scanning

“…For many radiation deliveries this is not the case as beams impinge on the flask in an axial manner. Several correction techniques to minimize refractive wall artifacts have since been proposed [13,17,24]. One method involves forming subtraction images of post and pre-irradiation optical-CT images of the same gel phantom, to subtract out common artifacts (e.g.…”

Dosimetry tools and techniques for IMRT

“…228 For MR measurements requiring high accuracy and highspatial resolution, long imaging times ͑several hours͒ may be required to achieve the low uncertainty and high spatial resolution associated with standard dosimeters. An alternative readout technique, optical-CT, 166,203,204,209,227,[231][232][233][234][235] has been proposed for imaging PAG gels; an approach analogous to first-generation x-ray CT. In optical-CT the x-ray source is replaced by a visible laser and the x-ray detector replaced with a light-sensitive photodiode.…”

“…Much faster broad-beam and cone-beam configurations have been developed for radiochronic gels and plastics, where the scatter component is much less. 232,237 For these approaches corrections may be necessary for spectral warping 238 and residual scatter. 232,239 …”

An improved method for the removal of ring artifacts in high resolution CT imaging

“…In hardware based approach, the detector array is moved during data acquisition to reduce the non-uniform sensitivity of different detector elements. Then an average response of all the detector pixels is calculated to suppress the ring artifacts [8,9]. But in this approach special hardware arrangement is needed.…”