Impact of a novel exponential weighted 4DCT reconstructionalgorithm.

Eric D Morris,Joshua P Kim,Carri K Glide‐Hurst,Paul Klahr

doi:10.1002/acm2.12423

Abstract

PurposeThis work characterizes a novel exponential 4DCT reconstruction algorithm (EXPO), in phantom and patient, to determine its impact on image quality as compared to the standard cosine‐squared weighted 4DCT reconstruction.MethodsA motion platform translated objects in the superior–inferior (S‐I) direction at varied breathing rates (8–20 bpm) and couch pitches (0.06–0.1) to evaluate interplay between parameters. Ten‐phase 4DCTs were acquired and data were reconstructed with cosine squared and EXPO weighting. To quantify the magnitude of image blur, objects were translated in the anterior–posterior (A‐P) and S‐I directions for full‐width half maximum (FWHM) analysis between both 4DCT algorithms and a static case. 4DCT sinogram data for 10 patients were retrospectively reconstructed using both weighting factors. Image subtractions elucidated intensity and boundary differences. Subjective image quality grading (presence of image artifacts, noise, spatial resolution (i.e., lung/liver boundary sharpness), and overall image quality) was conducted yielding 200 evaluations.ResultsAfter taking static object size into account, the FWHM of EXPO reconstructions in the A‐P direction was 3.3 ± 1.7 mm (range: 0–4.9) as compared to cosine squared 9.8 ± 4.0 mm (range: 2.6–14.4). The FWHM of objects translated in the S‐I direction reconstructed with EXPO agreed better with the static FWHM than the cosine‐squared reconstructions. Slower breathing periods, faster couch pitches, and intermediate 4DCT phases had the largest reductions of blurring with EXPO. 18 of 60 comparisons of artifacts were improved with EXPO reconstruction, whereas no appreciable changes were observed in image quality scores. In 18 of 20 cases, EXPO provided sharper images although the reduced projections also increased baseline noise.ConclusionExponential weighted 4DCT offers potential for reducing image blur (i.e., improving image sharpness) in 4DCT with a tendency to reduce artifacts. Future work will involve evaluating the impact on treatment planning including delineation ability and dose calculation.

Highlights

The advent of four‐dimensional computed tomography (4DCT) has enabled a more accurate characterization of the internal target volume (ITV)[1,2,3] of moving tumors while reducing motion artifacts.4,5 4DCT offers the ability to characterize tumor motion and reconstruct its trajectory over a patient's breathing phase.[6]
We evaluate the impact of applying, in phantom and patient, an exponential weighting factor to the current standard of care in order to generate an exponential 4DCT reconstruction algorithm (“EXPO”)
Our work builds upon these preliminary results by presenting a detailed theoretical basis for the algorithm, evaluates its performance in controlled phantom experiments with a large variety of acquisition parameters, and performs a quantitative and qualitative comparison between conventional and EXPO 4DCT reconstructions for a patient cohort, with the overarching goal of evaluating the potential of using EXPO reconstruction to sharpen the boundaries of moving targets for radiation therapy

Summary

| INTRODUCTION

The advent of four‐dimensional computed tomography (4DCT) has enabled a more accurate characterization of the internal target volume (ITV)[1,2,3] of moving tumors while reducing motion artifacts.4,5 4DCT offers the ability to characterize tumor motion and reconstruct its trajectory over a patient's breathing phase.[6]. Work presented by Shen et al suggested that using EXPO yielded improved volume estimation and reduced motion artifact in 4DCT.[10] Our work builds upon these preliminary results by presenting a detailed theoretical basis for the algorithm, evaluates its performance in controlled phantom experiments with a large variety of acquisition parameters, and performs a quantitative and qualitative comparison between conventional and EXPO 4DCT reconstructions for a patient cohort, with the overarching goal of evaluating the potential of using EXPO reconstruction to sharpen the boundaries of moving targets for radiation therapy

| MATERIALS AND METHODS

| RESULTS

| DISCUSSION