Scale-based scatter correction for computer-aided polyp detection in CT colonography - PubMed (original) (raw)
Scale-based scatter correction for computer-aided polyp detection in CT colonography
Jiamin Liu et al. Med Phys. 2008 Dec.
Abstract
CT colonography (CTC) is a feasible and minimally invasive method for the detection of colorectal polyps and cancer screening. Computer-aided detection (CAD) of polyps can improve consistency and sensitivity of virtual colonoscopy interpretation and reduce interpretation burden. However, high-density orally administered contrast agents have scatter effects on neighboring tissues. The scattering manifests itself as an artificial elevation in the observed CT attenuation values of the neighboring tissues. This pseudoenhancement phenomenon presents a problem for the application of computer-aided polyp detection, especially when polyps are submerged in the contrast agents. The authors have developed a scale-based correction method that minimizes scatter effects in CTC data by subtraction of the estimated scatter components from observed CT attenuations. By bringing a locally adaptive structure, object scale, into the correction framework, the region of neighboring tissues affected by contrast agents is automatically specified and adaptively changed in different parts of the image. The method was developed as one preprocessing step in the authors' CAD system and was tested by using leave-one-patient-out evaluation on 56 clinical CTC scans (supine or prone) from 28 patients. There were 50 colonoscopy-confirmed polyps measuring 6-9 mm. Visual evaluation indicated that the method reduced CT attenuation of pseudoenhanced polyps to the usual polyp Hounsfield unit range without affecting luminal air regions. For polyps submerged in contrast agents, the sensitivity of CAD with correction is increased 24% at a rate of ten false-positive detections per scan. For all polyps within 6-9 mm, the sensitivity of the authors' CAD with scatter correction is increased 8% at a rate of ten false-positive detections per scan. The authors' results indicated that CAD with this correction method as a preprocessing step can yield a high sensitivity and a relatively low FP rate in CTC.
Figures
Figure 1
The mean of CT attenuation of a polyp: (a) Polyp without pseudoenhancement (patient in prone scan and the polyp is surrounded by luminal air) and (b) polyp is pseudoenhanced (the same patient in supine scan while the same polyp is submerged in contrast agents).
Figure 2
Illustration of local structure size or scale. (a) Volume of interest (VOI) of a slice CTC image. Note that the scale is usually small in the vicinity of boundaries. (b) The corresponding VOI of scale image. In this image, intensity at a location is proportional to the scale value at that location.
Figure 3
Implementation of scatter correction in the authors’ CAD system.
Figure 4
Patient and polyp flowchart.
Figure 5
The mean of CT attenuation of polyps without pseudoenhancement (the first column, patient in supine∕prone scan and the polyp is surrounded by luminal air), pseudoenhanced polyps (the second column, the same patient in prone∕supine scan while the same polyp is submerged in contrast agents), and corrected polyps (the third column).
Figure 6
Examples of TP, FN, and FP detections. The first row shows the 2D and 3D displays of one TP detection (an 8 mm polyp) in CAD with scatter correction which is FN in CAD without scatter correction. The second row shows one FN detection (a 9 mm polyp) in CAD with and without scatter correction. This polyp is at the air–fluid boundary and scatter effects are low. The third row shows one FP detection on a haustral fold in CAD with and without scatter correction.
Figure 7
Flowchart of quantitative evaluation.
Figure 8
FROC curves of our CAD system without and with the application of correction: (a) 6–9 mm polyps submerged in contrast agents; (b) 6–9 mm polyps surrounded by luminal air; (c) all polyps measuring 6–9 mm. The curves show the benefits of the scatter correction for submerged polyps. Polyps that are surrounded by luminal air are unaffected by the correction.
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