Megavoltage cone beam computed tomography (MVCBCT) is routinely used for visualizing anatomical structures and implanted fiducials for patient positioning in radiotherapy. MVCBCT using a 6 MV treatment beam with high atomic number (Z) target and flattening filter in the beamline, as done conventionally, has lower image quality than can be achieved with a MV beam due to heavy filtration of the low-energy bremsstrahlung. The unflattened beam of a low Z target has an abundance of diagnostic energy photons, detected with modern flat panel detectors with much higher efficiency given the same dose to the patient. This principle guided the development of a new megavoltage imaging beamline (IBL) for a commercial radiotherapy linear accelerator. A carbon target was placed in one of the electron primary scattering foil slots on the target-foil slide. A PROM on a function controller board was programed to put the carbon target in place for MVCBCT. A low accelerating potential of 4.2 MV was used for the IBL to restrict leakage of primary electrons through the target such that dose from x rays dominated the signal in the monitor chamber and the patient surface dose. Results from phantom and cadaver images demonstrated that the IBL had much improved image quality over the treatment beam. For similar imaging dose, the IBL improved the contrast-to-noise ratio by as much as a factor of 3 in soft tissue over that of the treatment beam. The IBL increased the spatial resolution by about a factor of 2, allowing the visualization of finer anatomical details. Images of the cadaver contained useful information with doses as low as 1 cGy. The IBL may be installed on certain models of linear accelerators without mechanical modification and results in significant improvement in the image quality with the same dose, or images of the same quality with less than one-third of the dose.

Quantitative reconstruction of cone beam X-ray computed tomography (CT) datasets requires accurate modeling of scatter, beam-hardening, beam profile, and detector response. Typically, commercial imaging systems use fast empirical corrections that are designed to reduce visible artifacts due to incomplete modeling of the image formation process. In contrast, Monte Carlo (MC) methods are much more accurate but are relatively slow. Scatter kernel superposition (SKS) methods offer a balance between accuracy and computational practicality. We show how a single SKS algorithm can be employed to correct both kilovoltage (kV) energy (diagnostic) and megavoltage (MV) energy (treatment) X-ray images. Using MC models of kV and MV imaging systems, we map intensities recorded on an amorphous silicon flat panel detector to water-equivalent thicknesses (WETs). Scattergrams are derived from acquired projection images using scatter kernels indexed by the local WET values and are then iteratively refined using a scatter magnitude bounding scheme that allows the algorithm to accommodate the very high scatter-to-primary ratios encountered in kV imaging. The algorithm recovers radiological thicknesses to within 9% of the true value at both kV and megavolt energies. Nonuniformity in CT reconstructions of homogeneous phantoms is reduced by an average of 76% over a wide range of beam energies and phantom geometries.

We describe a focused beam-stop array (BSA) for the measurement of object scatter in imaging systems that utilize x-ray beams in the megavoltage (MV) energy range. The BSA consists of 64 doubly truncated tungsten cone elements of 0.5 cm maximum diameter that are arranged in a regular array on an acrylic slab. The BSA is placed in the accessory tray of a medical linear accelerator at a distance of approximately 50 cm from the focal spot. We derive an expression that allows us to estimate the scatter in an image taken without the array present, given image values in a second image with the array in place. The presence of the array reduces fluence incident on the imaged object. This leads to an object-dependent underestimation bias in the scatter measurements. We apply corrections in order to address this issue. We compare estimates of the flat panel detector response to scatter obtained using the BSA to those derived from Monte Carlo simulations. We find that the two estimates agree to within 10% in terms of RMS error for 30 cm x 30 cm water slabs in the thickness range of 10-30 cm. Larger errors in the scatter estimates are encountered for thinner objects, probably owing to extrafocal radiation sources. However, RMS errors in the estimates of primary images are no more than 5% for water slab thicknesses in the range of 1-30 cm. The BSA scatter estimates are also used to correct cone beam tomographic projections. Maximum deviations of central profiles of uniform water phantoms are reduced from 193 to 19 HU after application of corrections for scatter, beam hardening, and lateral truncation that are based on the BSA-derived scatter estimate. The same corrections remove the typical cupping artifact from both phantom and patient images. The BSA proves to be a useful tool for quantifying and removing image scatter, as well as for validating models of MV imaging systems.

To measure and compare the contrast to noise ratio (CNR) as a function of dose for the CBCTs produced by the mega‐voltage (MV) imaging beam line (IBL) and the treatment beam line (TBL), and to compare the dose to target and various critical structures of pediatric patients for the IBL CBCT versus standard TBL orthogonal port films.Two Siemens Oncor linear accelerators were modified at our institution such that the MV‐CBCT would operate under an investigational IBL rather than the standard 6MV TBL. Prior to the modification, several CBCTs of an electron density phantom were acquired with the TBL at various dose values. After the modification, another set of CBCTs of the electron density phantom were acquired for various doses using the IBL. The contrast to noise ratio (CNR) for each tissue equivalent insert was calculated. In addition, a dosimetric study of pediatric patients was conducted comparing the 1 cGy IBL CBCT and conventional TBL orthogonal pair port films.The CNR for eight tissue equivalent inserts at five different dose settings for each type of CBCT was measured. The CNR of the muscle insert was 0.8 for a 5 cGy TBL CBCT, 1.1 for a 1.5 cGy IBL CBCT, and 2.8 for a conventional CT. The CNR of the trabecular bone insert was 2.9 for a 5 cGy TBL CBCT, 5.5 for a 1.5 cGy IBL CBCT, and 14.8 for a conventional CT. The IBL CBCT delivered approximately one‐fourth the dose to the target and critical structures of the patients as compared to the TBL orthogonal pair port films.The IBL CBCT improves image quality while simultaneously reducing the dose to the patient as compared to the TBL CBCT. A 1 cGy IBL CBCT, which is used for bony anatomy localization, delivers one‐fourth the dose as compared to conventional ortho‐pair films.PACS number: 87.57.Q, 87.57.cj, 87.53.Jw

The IBL has the advantage of improved image quality at the same dose, or reduced dose for the same image quality, over the TBL.