Scanning K-edge subtraction (SKES) imaging with laser-compton x-ray sources

Abstract

Background: K-edge subtraction (KES) imaging is a dual-energy imaging technique that enhances contrast by subtracting images taken with x-rays that are above and below the K-edge energy of a specified contrast agent. The resulting reconstruction spatially identifies where the contrast agent accumulates, even when obscured by complex and heterogeneous distributions of human tissue. This method is most successful when x-ray sources are quasimonoenergetic and tunable, conditions that have traditionally only been met at synchrotrons. Laser-Compton x-ray sources (LCSs) are a compact alternative to synchrotron radiation with a quasimonoenergetic x-ray spectrum. One limitation in the clinical application of KES imaging with LCSs has been the extensive time required to tune the x-ray spectrum to two different energies.

Purpose: We introduce an imaging technique called scanning K-edge subtraction (SKES) that leverages the angle-correlated laser-Compton x-ray spectrum in the setting of mammography. The feasibility and utility of this technique will be evaluated through a series of simulation studies. The goal of SKES imaging is to enable rapid K-edge subtraction imaging using a laser-Compton x-ray source. The technique does not rely on the time-consuming process of tuning laser-Compton interaction parameters.

Methods: Laser-Compton interaction physics are modeled using conditions based on an X-band linear electron accelerator architecture currently under development using a combination of 3D particle tracking software and Mathematica. The resulting angle-correlated laser-Compton x-ray beam is propagated through digitally compressed breast phantoms containing iodine contrast-enhanced inserts and then to a digital flat-panel detector using a Matlab Monte Carlo propagation software. This scanning acquisition technique is compared to the direct energy tuning method (DET), as well as to a clinically available dual-energy contrast-enhanced mammography (CEM) system.

Results: KES imaging in a scanning configuration using an LCS was able to generate a KES image of comparable quality to the direct energy tuning method. SKES was able to detect tumors with iodine contrast concentrations lower than what is clinically available today including lesions that are typically obscured by dense fibroglandular tissue. After normalizing to mean glandular dose, SKES is able to generate a KES image with equal contrast to CEM using only 3% of the dose.

Conclusions: By leveraging the unique quasimonochromatic and angle-correlated x-ray spectrum offered by LCSs, a contrast-enhanced subtraction image can be obtained with significantly more contrast and less dose compared to conventional systems, and improve tumor detection in patients with dense breast tissue. The scanning configuration of this technique could accelerate the clinical translation of this technology.

Keywords: K‐edge subtraction; contrast‐enhanced mammography; dual‐energy; laser‐Compton.

FIGURE 1

(a) Attenuation coefficients of iodine, fibroglandular tissue, and adipose tissue in the energy range used in this study. (b) Diagram of laser‐Compton interaction. A well‐timed overlap of a laser pulse and an accelerated electron bunch will under Compton scattering to produce x‐rays in the direction of electron beam propagation. (c) Laser‐Compton intensity heat map with mean energy contours overlay. (d) Integrated laser‐Compton energy spectrum over different half‐angle collimations. More aggressive collimation blocks low energy photons at larger half‐angles resulting in decreasing energy bandwidth, but also causes lower overall flux.

FIGURE 2

(a) Illustration of the SKES configuration. The dashed area represents the blocked regions allowing for separation of energies above and below the K‐edge. A 1‐dimensional scan will expose the object to both energies allowing for a KES reconstruction. (b) Rendering of a physical beam block machined out of tungsten designed for a circular x‐ray beam aperture. (c) Visualization of patient/beam interfacing. (d) Demonstration of scanning via 1‐dimensional patient movement through the Compton x‐ray beam.

FIGURE 3

Graphical summary of breast phantoms used in this study including their morphology, distribution of fibroglandular tissue, and tumor/lesion placement.

FIGURE 4

Comparison of spectra between the three methods investigated in this study and their proximity to the iodine K‐edge. The spectra are normalized to peak intensity for each respective x‐ray source.

FIGURE 5

Illustration of spectral variation in the dimension normal to the scanning dimension. The low‐energy spectrum has little variation while the high‐energy spectrum changes more in flux than it does in bandwidth.

FIGURE 6

Single energy images (HE/LE) along with combined KES images for the dense phantom simulations with 1.02 mg/cc iodine tumors under simple subtraction reconstruction (KES) and weighted log subtraction reconstruction (wKES). The only post‐processing applied to the single exposure HE/LE images was flat‐field correction. HE, high energy; KES, K‐edge subtraction; LE, low energy.

FIGURE 7

Summary of KES images for each phantom and x‐ray source investigated for two different lesion concentrations of iodine contrast using simple subtraction reconstruction. Each image has been normalized to have unity peak value. KES, K‐edge subtraction.

FIGURE 8

Summary of wKES images for each phantom and x‐ray source investigated for two different lesion concentrations of iodine contrast using weighted log subtraction reconstruction. Each image has been normalized to have unity peak value. KES, K‐edge subtraction.