A feasibility study for megavoltage cone beam CT using a commercial EPID.

This study used a standard commercial electronic portal imaging device (EPID) area detector attached to an isocentric linear accelerator and the Feldkamp algorithm to produce cone beam tomographic reconstructions. The EPID has a active area of 32.5 x 32.5 cm2, and can record 12-bit images using two monitor units (MU), with a resolution of 2.1 x 2.0 mm2 FWHM. Since the EPID was not large enough to record the full patient projection at about 1.5 geometric magnification, it was necessary to offset the detector to collect half-cone projections. Corrections are required to convert pixel values into units of exit dose and to realign the projections to overcome the +/- 4 mm support arm sag. With a geometric magnification of 1.5 the sensitive volume is a cylinder of radius 21 cm and length 17 cm. Unfortunately, the patient couch contains metal bed support rails that lie just outside this cylinder, and produce streak artefacts in the reconstruction. Using 90 views the system delivers a central dose of 90 cGy, and has a density resolution of 4%.

[1]  H H Barrett,et al.  Statistical limitations in transaxial tomography. , 1976, Computers in biology and medicine.

[2]  J. H. Hubbell,et al.  Photon mass attenuation and energy-absorption coefficients , 1982 .

[3]  P M Evans,et al.  Scattered radiation in portal images: a Monte Carlo simulation and a simple physical model. , 1996, Medical physics.

[4]  F. Natterer,et al.  Sampling in Fan Beam Tomography , 1993, SIAM J. Appl. Math..

[5]  B. F. Logan,et al.  The Fourier reconstruction of a head section , 1974 .

[6]  J. H. Hubbell,et al.  Pair, Triplet, and Total Atomic Cross Sections (and Mass Attenuation Coefficients) for 1 MeV-100 GeV Photons in Elements Z=1 to 100, , 1980 .

[7]  S Webb,et al.  Three-dimensional x-ray microtomography for medical and biological applications. , 1990, Physics in medicine and biology.

[8]  W Swindell,et al.  A 4-MV CT scanner for radiation therapy: the prototype system. , 1982, Medical physics.

[9]  T W Griffin,et al.  Cone-beam CT for radiotherapy applications. , 1995, Physics in medicine and biology.

[10]  L. Burkinshaw,et al.  Tomographic Reconstruction from Experimentally Obtained Cone-Beam Projections , 1987, IEEE Transactions on Medical Imaging.

[11]  Philippe Rizo,et al.  Comparison of two three-dimensional x-ray cone-beam-reconstruction algorithms with circular source trajectories , 1991 .

[12]  R. Boellaard,et al.  The dose response relationship of a liquid-filled electronic portal imaging device. , 1996, Medical physics.

[13]  Philip M. Evans,et al.  Monte Carlo simulations of CsI(Tl) scintillation crystals for use in a three-dimensional megavoltage CT scanner , 1994 .

[14]  P. Munro,et al.  A review of electronic portal imaging devices (EPIDs). , 1992, Medical physics.

[15]  S Webb.  A modified convolution reconstruction technique for divergent beams. , 1982, Physics in medicine and biology.

[16]  A Brahme,et al.  Radiotherapeutic computed tomography with scanned photon beams. , 1987, International journal of radiation oncology, biology, physics.

[17]  M C Schell,et al.  Input/output characteristics of a matrix ion-chamber electronic portal imaging device. , 1994, Medical physics.

[18]  S Webb,et al.  The effect of reversing projection data in fan-beam reconstruction tomography. , 1985, Physics in medicine and biology.

[19]  D. G. Lewis,et al.  A megavoltage CT scanner for radiotherapy verification. , 1992, Physics in medicine and biology.

[20]  J. Boone,et al.  An analytical edge spread function model for computer fitting and subsequent calculation of the LSF and MTF. , 1994, Medical physics.

[21]  R H Huesman,et al.  The effects of a finite number of projection angles and finite lateral sampling of projections on the propagation of statistical errors in transverse section reconstruction. , 1977, Physics in medicine and biology.

[22]  G. N. Ramachandran,et al.  Three-dimensional reconstruction from radiographs and electron micrographs: application of convolutions instead of Fourier transforms. , 1971, Proceedings of the National Academy of Sciences of the United States of America.

[23]  W Swindell,et al.  Computed tomography with a linear accelerator with radiotherapy applications. , 1983, Medical physics.

[24]  M van Herk,et al.  A liquid ionisation detector for digital radiography of therapeutic megavoltage photon beams. , 1985, Physics in medicine and biology.

[25]  J M Boone,et al.  Comparison of x-ray cross sections for diagnostic and therapeutic medical physics. , 1996, Medical physics.

[26]  L. Feldkamp,et al.  Practical cone-beam algorithm , 1984 .