Feasibility of megavoltage portal CT using an electronic portal imaging device (EPID) and a multi-level scheme algebraic reconstruction technique (MLS-ART)

Abstract

Although electronic portal imaging devices (EPIDs) are efficient tools for radiation therapy verification, they only provide images of overlapped anatomic structures. We investigated using a fluorescent screen/CCD-based EPID, coupled with a novel multi-level scheme algebraic reconstruction technique (MLS-ART), for a feasibility study of portal computed tomography (CT) reconstructions. The CT images might be useful for radiation treatment planning and verification. We used an EPID, set it to work at the linear dynamic range and collimated 6 MV photons from a linear accelerator to a slit beam of 1 cm wide and 25 cm long. We performed scans under a total of approximately 200 monitor units (MUs) for several phantoms in which we varied the number of projections and MUs per projection. The reconstructed images demonstrated that using the new MLS-ART technique megavoltage portal CT with a total of 200 MUs can achieve a contrast detectibility of approximately 2.5% (object size 5 mm x 5 mm) and a spatial resolution of 2.5 mm.

A bench‐top megavoltage fan‐beam CT using ‐photodiode detectors. II. Image performance evaluation

“…However, since computed tomography ͑CT͒ provides better low contrast resolution ͑LCR͒ than two-dimensional ͑2D͒ electronic portal imaging devices, it is the more desirable method for imaging patients in the treatment position. 1, 2 The combination of a linear accelerator and a diagnostic CT scanner, 3 a kilovoltage ͑kV͒ x-ray tube mounted on the C arm of a linear accelerator in combination with an indirect detection active matrix flat panel detector, 4 use of a megavoltage ͑MV͒ beam either in fan beam 1,2,5-7 or cone beam [8][9][10][11][12][13][14][15] configuration are present examples of implementing three-dimensional ͑3D͒ CT imaging in the treatment position.…”

Monte Carlo study of a highly efficient gas ionization detector for megavoltage imaging and image-guided radiotherapy

“…However, MVCT imaging is being investigated as a three-dimensional ͑3D͒ alternative. Many systems have been proposed for multislice cone beam CT using purpose-built detectors [2][3][4] or using commercial EPIDs such as fluoroscopic systems, [5][6][7] liquid-filled ionization chamber 8 or an amorphous silicon flat-panel detector. 9 The first tomotherapy prototype utilises an arc-shaped one-dimensional detector array for fan-beam megavoltage CT. [10][11][12] Two approaches combining the treatment unit with an onboard kilovoltage CT system are under development.…”

A preliminary investigation of local tomography for megavoltage CT imaging

“…To improve upon EPID-based imaging, extensive effort has been directed toward developing megavoltage computed tomography ͑MV CT͒. [1][2][3][4][5][6] MV CT is performed by simultaneously rotating the treatment source and detector around the patient in analogy to the operation of a third-generation diagnostic CT scanner. Consequently, MV CT does not require dedicated hardware and significant modifications to existing radiation therapy treatment machines.…”

“…MV CT may not be clinically viable using current commercially available radiation detectors because it would impart an unacceptably large radiation dose to the healthy tissues of the patient. 2,3,10 Given the relative inefficiency of commercially available radiation detectors, one way to circumvent this problem is to limit the number of beam angles ͑i.e., projection measurements͒ that are used to reconstruct the image. For example, in Ref.…”