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.

Efficient scatter correction using asymmetric kernels

“…Several methods have been proposed for estimating scatter in raw projection data. These include scatter deconvolution 5,6 , scatter modeling 7 , modulators 8,9 , beam stop arrays 10 , two scans 11 and a detector shadowing technique 12 . The scatter deconvolution method, also known as scatter kernel superposition (SKS) has the advantages of not requiring extra hardware, preserving the entire field-of-view, and being computationally efficient, but has not proven to be as accurate as some of the other techniques.…”

Scatter correction for full‐fan volumetric CT using a stationary beam blocker in a single full scan

“…The measurement-based methods typically insert sparse x-ray beam blockers (e.g., lead) between the x-ray source and the object, and obtain scatter samples inside the resultant shadows on the detector. 8,[20][21][22][23] The scatter distribution of the whole-field is then accurately estimated using interpolation/extrapolation on the sampled scatter data since scatter distributions have dominant lowfrequency components. 8,24 These methods are simple and efficient, and achieve effective scatter correction without prior knowledge of the imaged object.…”

Low-Dose and Scatter-Free Cone-Beam CT Imaging Using a Stationary Beam Blocker in a Single Scan: Phantom Studies

“…On the other hand, methods of direct scatter measurement conveniently obtain accurate scatter estimates with negligible computational cost [17]. In the measurement-based method, a beam blocker is typically inserted between the X-ray source and the object, and scatter signals are estimated inside the detector shadows of the beam blocker [15, 29–31], where primary signals are fully attenuated. The scatter distribution of the whole field is then obtained via interpolation/extrapolation on the scatter samples inside the shadows, since scatter distributions have dominant low-frequency components [15, 32, 33].…”