Small animal photon counting cone-beam CT on a preclinical radiation research platform to improve radiation dose calculation accuracy

Abstract

Objective.Cone beam CT (CBCT) in preclinical small animal irradiation platforms provides essential information for image guidance and radiation dose calculation for experiment planning. This project developed a photon-counting detector (PCD)-based multi(3)-energy (ME-)CBCT on a small animal irradiator to improve the accuracy of material differentiation and hence dose calculation, and compared to conventional flat panel detector (FPD)-based CBCT.Approach.We constructed a mechanical structure to mount a PCD to an existing preclinical irradiator platform and built a data acquisition pipeline to acquire x-ray projection data with a 100 kVp x-ray beam using three different energy thresholds in a single gantry rotation. We implemented an energy threshold optimization scheme to determine optimal thresholds to balance signal-to-noise ratios (SNRs) among energy channels. Pixel-based detector response calibration was performed to remove ring artifacts in reconstructed CBCT images. Feldkamp-Davis-Kress method was employed to reconstruct CBCT images and a total-variance regularization-based optimization model was used to decompose CBCT images into bone and water material images. We compared dose calculation results using PCD-based ME-CBCT with that of FPD-based CBCT.Main results.The optimal nominal energy thresholds were determined as 26, 56, and 90 keV, under which SNRs in a selected region-of-interest in the water region were 6.11, 5.91 and 5.93 in the three energy channels, respectively. Compared with dose calculation results using FPD-based CBCT, using PCD-based ME-CBCT reduced the mean relative error from 49.5% to 16.4% in bone regions and from 7.5% to 6.9% in soft tissue regions.Significance.PCD-based ME-CBCT is beneficial in improving radiation dose calculation accuracy in experiment planning of preclinical small animal irradiation researches.

Keywords: cone beam CT; photon counting; small animal imaging.

Figure 1:

Installation of the PCD on the SmART preclinical radiation platform. Photos showing gantry at 0 degree (left) and 45 degree (right).

Figure 2:

SNR ratios γ as a function of (T₂, T₃) combinations; red point indicates the location of the minimum SNR ratio.

Figure 3:

Left: Detector calibration fitting for one selected pixel in the three energy channels. Right: histograms of R² for the three energy channels.

Figure 4:

One projection image at θ = 0 degree (left) and an axial cross section of reconstructed CBCT images of energy channel CH₁ (right) with and without detector calibration. Arrows indicate artifacts. Display window of CBCT: [0, 1.0] cm⁻¹.

Figure 5:

Comparison of mean and variance of photon counts in a randomly selected pixel (left) and two ROIs (middle). Right: cumulative histogram and theoretical cumulative distribution function of Poisson distribution of a selected point indicated by the yellow arrow in the middle figure.

Figure 6:

Top row: CBCT images of the CT calibration insert phantom in the three energy channels. Blue and red circles indicate the regions of water and bone basis materials used in material decomposition. Middle and Bottom rows: CBCT images of the plastinated mouse phantom on transverse and sagittal planes respectively. Display window [0,0.5] cm⁻¹ for all images.

Figure 7:

Comparisons of linear attenuation coefficients abstracted from ME-CBCT images and FPD based CBCT image with computed values. The gray dot-line along diagonal indicates μ_exp = μ_comp.

Figure 8:

Solid water (top) and bone (bottom) maps of the CT calibration phantom (second column) and the plastinated mouse phantom (third and fourth columns).

Figure 9:

Z_eff as a function of the penalty parameter λ. Horizontal dash lines are the ground truth. Vertical line indicates the selected λ value of 250.

Figure 10:

(a) MC dose calculation results. (b) Dose profiles along the vertical dash lines in (a).