Improving small animal cone beam CT resolution by mitigating x-ray focal spot induced blurring via deconvolution

Abstract

Objective.Modern preclinical small animal radiation platforms utilize cone beam computerized tomography (CBCT) for image guidance and experiment planning purposes. The resolution of CBCT images is of particular importance for visualizing fine animal anatomical structures. One major cause of spatial resolution reduction is the finite size of the x-ray focal spot. In this work, we proposed a simple method to measure x-ray focal spot intensity map and a CBCT image domain deblurring model to mitigate the effect of focal spot-induced image blurring.Approach.We measured a projection image of a tungsten ball bearing using the flat panel detector of the CBCT platform. We built a forward blurring model of the projection image and derived the spot intensity map by deconvolving the measured projection image. Based on the measured spot intensity map, we derived a CBCT image domain blurring model for images reconstructed by the filtered backprojection algorithm. Based on this model, we computed image domain blurring kernel and improved the CBCT image resolution by deconvolving the CBCT image.Main results.We successfully measured the x-ray focal spot intensity map. The spot size characterized by full width at half maximum was ∼0.75 × 0.55 mm²at 40 kVp. We computed image domain convolution kernels caused by the x-ray focal spot. A simulation study on noiseless projections was performed to evaluate the spatial resolution improvement exclusively by the focal spot kernel, and the modulation transfer function (MTF) at 50% was increased from 1.40 to 1.65 mm^-1for in-plane images and 1.05-1.32 mm^-1for cross-plane images. Experimental studies on a CT insert phantom and a plastinated mouse phantom demonstrated improved spatial resolution after image domain deconvolution, as indicated by visually improved resolution of fine structures. MTF at 50% was improved from 1.00 to 1.12 mm^-1for in-plane direction and from 0.72 to 0.84 mm^-1for cross-plane direction.Significance.The proposed method to mitigate blurring caused by finite x-ray spot size and improve CBCT image resolution is simple and effective.

Keywords: cone beam CT; deconvolution; image resolution; x-ray spot size.

Figure 1:

(a) SmART preclinical radiation platform. (b) Illustration of the configuration for x-ray focal spot intensity map measurement.

Figure 2:

Geometry definition.

Figure 3:

(a) Measured focal spot intensity map and (b) its projections along ζ- and η- directions.

Figure 4:

Top: f_δ,ζ(x, y, x₀, y₀) image obtained with a fixed (x₀, y₀) = (0, 0) and various ζ. Middle: k(x, y, z = 0, x₀, y₀, z₀ = 0) computed with various $r_0=\sqrt{x_0^2+y_0^2}$. Bottom: K(x−x₀, y−y₀, 0) along y = 0 (left-most), x = 0 (second to left) and diagonal (middle) directions. Second to right: Blurring kernel K(x−x₀, y−y₀, 0). Right-most: Blurring kernel K(0, 0, z − z₀).

Figure 5:

(a) CBCT of the simulated ideal, raw and deblurred images, and the difference between deblurred and raw images. Top and bottom rows are in-plane and cross-plane images. Display window [0, 0.2] cm⁻¹ for phantom images and [−0.05, 0.05] cm⁻¹ for difference image. Squares indicated locations of zoomed-in views. (b) MTFs of in-plane direction (top) and cross-plane direction (bottom) of different images.

Figure 6:

(a) CBCT images of the calibration insert phantom. Top and bottom rows are in-plane and cross-plane images. Display window [−1000,300] HU for phantom images and [−100,100] HU for difference images. (b) MTFs of in-plane direction (top) and cross-plane direction (bottom) of different images.

Figure 7:

Top row: In-plane CBCT images of plastinated mouse phantom. From left to right, original image, deblurred image, and difference. Second to last rows: raw images, deblurred images, and differences of sagittal and coronal CBCT images. Display window [−1000,300] HU for CBCT images and [−100,100] HU for difference images. Arrows indicate regions of improvement. Square regions are zoom-in views in Figure 8.

Figure 8:

Top and middle rows: zoom-in regions of raw and deblurred images of plastinated mouse phantom. Display windows are [−1000 300] HU for the top row and [−950,−600] HU for the middle row. Bottom row: line profiles of plastinated mouse indicated by solid green lines in the top and middle row images.

Figure 9:

(a) Blurring kernel K(x − x₀, y − y₀, 0) and (b) K(0, 0, z − z₀) computed at voxel size of 0.05 mm