Performance evaluation of a prototype high resolution digital radiographic/near real-time fluoroscopic computerized...

A new prototype digital radiographic/near real-time fluoroscopic and computerized tomographic (CT) imaging system has been developed and its performance is evaluated for future implementation in a radiotherapy simulator.

The new imaging modality uses a slow scan cooled charge-coupled device (CCD) camera with 2 k x 2 k x 12 bit image resolution. X-ray images formed by a Gd2O2S:Tb flat screen are directly viewed by the CCD camera via a 45 degrees angled mirror and a high speed objective lens. For CT image reconstruction, digital data from a slit image on the CCD array obtained from time-controlled x-ray scans around the object are transferred to a second tomographic computer for processing and display.

In the digital radiographic mode, and for a 43.2 x 43.2 cm field size at the phosphor plane, the spatial resolution of the system is 2.3 +/- 0.1 lp/mm (1 sigma) as measured at the 4% level of the modulation transfer function (MTF), whereas the contrast resolution has a value of 0.5%. In the fluoroscopic mode, the system may be operated at a maximum rate of eight frames/s at a relatively lower spatial and contrast resolution. For CT scans, although the potential intrinsic spatial resolution at the isocenter is 0.35 mm, display-limited spatial resolutions of 1.2 and 1.6 mm were obtained for 30 and 40 cm reconstruction circle diameters, respectively. A contrast resolution of 1% at 0.015 Gy entrance dose was achieved, and CT reconstruction circles of up to 51 cm were attainable.

The new CCD-based imaging system is capable of delivering high-quality digital radiographic and CT images for radiotherapy simulation, whereas the near real-time fluoroscopic mode yields acceptable flicker-free lower quality images at object speeds typical of simulation motions.