Focused beam-stop array for the measurement of scatter in megavoltage portal and cone beam CT imaging.
ABSTRACT 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.
Conference Proceeding: Assessment of x-ray scatter for the micro-CT subsystem of the FLEX Triumph™ preclinical PET-CT scanner[show abstract] [hide abstract]
ABSTRACT: This work aims at assessing x-ray scatter and more specifically the scatter to primary ratio (SPR) for the micro-CT subsystem of the FLEX Triumph™ preclinical PET-CT scanner. Two approaches were used: the experimental single blocker method and Monte Carlo (MC) simulations using the MCNP4C code. For the experimental setup, five cylindrical blockers with diameters ranging between 3.0 and 11.65 mm were used to assess the SPR using a polyethylene phantom (d=50mm). The beams energy influence was studied by scanning the phantom at 30, 50 and 80 kV (mag=1.3). Likewise, additional acquisitions at 50 kV were performed with a magnification of 2.0 to evaluate the impact of geometrical magnification and using a Plexiglas phantom (d=25mm) filled with water at 50 kV (mag=1.3) to assess the influence of the size and composition of the phantom. For each condition, the five blocker SPR results were linearly interpolated to obtain the SPR without the blocker. Central and peripheral SPR values were obtained by rotating the source and detector and were fitted with a Gaussian function. MC simulations were carried out using the MCNP4C code where the estimates were also fitted with a Gaussian function. The comparison showed that MC simulations can reproduce well the experimental results, at least in the region inside the phantom. The highest difference was obtained with the small phantom in the peripheral regions. The maximum SPR (0.562) was obtained at 30kV and magnification of 1.3 using the large phantom. The full SPR profile was calculated using MC simulations and used to express its dependency on beam energy and phantom diameter (quadratic), and air gap (asymptotic). The obtained results are in good agreement with theoretical predictions. MC is a very good alternative to experimental measurements of x-ray scatter in micro-CT imaging and can be used to validate novel scatter correction techniques. This will also improve the accuracy of CTAC on preclinical PET-CT systems.Nuclear Science Symposium Conference Record (NSS/MIC), 2010 IEEE; 12/2010