Dual‐energy cone‐beam CT with a flat‐panel detector: Effect of reconstruction algorithm on material classification. Issue 2 (30th January 2014)

• Record Type: Journal Article
• Authors: Zbijewski, W.; Gang, G. J.; Xu, J.; Wang, A. S.; Stayman, J. W.; Taguchi, K.; Carrino, J. A.; Siewerdsen, J. H.
• Is Part Of: Medical physics. Volume 41:Issue 2(2014)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2014-01-30
• Subjects: X‐ray imaging -- Computed tomography -- Reconstruction -- Noise -- Scintillation detectors; biomedical equipment -- computerised tomography -- image classification -- image reconstruction -- medical image processing -- phantoms -- solid scintillation detectors -- X‐ray apparatus; cone‐beam CT -- dual‐energy CT -- model‐based reconstruction -- penalized likelihood -- total variation -- dose reduction -- musculoskeletal imaging; Computerised tomographs -- Biological material, e.g. blood, urine; Haemocytometers -- Measurement of nuclear or x‐radiation -- Digital computing or data processing equipment or methods, specially adapted for specific applications -- Image data processing or generation, in general -- Tubes for determining the presence, intensity, density or energy of radiation or particles -- Devices sensitive to very short wavelength, e.g. x‐rays, gamma‐rays or corpuscular radiation -- Transforming x‐rays -- X‐ray technique -- the detector being a crystal -- the detector being made of plastics -- the detector being a liquid -- with semiconductor detectors -- Plates or blocks in which tracks of nuclear particles are made visible by after‐treatment, e.g. using photographic emulsion, using mica; Medical imaging -- Cone beam computed tomography -- Image reconstruction -- Calcium -- Medical image reconstruction -- Medical image noise -- Image sensors -- Computed tomography -- Calibration -- Radiography; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4863598
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: : Cone‐beam CT (CBCT) with a flat‐panel detector (FPD) is finding application in areas such as breast and musculoskeletal imaging, where dual‐energy (DE) capabilities offer potential benefit. The authors investigate the accuracy of material classification in DE CBCT using filtered backprojection (FBP) and penalized likelihood (PL) reconstruction and optimize contrast‐enhanced DE CBCT of the joints as a function of dose, material concentration, and detail size. Methods: : Phantoms consisting of a 15 cm diameter water cylinder with solid calcium inserts (50–200 mg/ml, 3–28.4 mm diameter) and solid iodine inserts (2–10 mg/ml, 3–28.4 mm diameter), as well as a cadaveric knee with intra‐articular injection of iodine were imaged on a CBCT bench with a Varian 4343 FPD. The low energy (LE) beam was 70 kVp (+0.2 mm Cu), and the high energy (HE) beam was 120 kVp (+0.2 mm Cu, +0.5 mm Ag). Total dose (LE+HE) was varied from 3.1 to 15.6 mGy with equal dose allocation. Image‐based DE classification involved a nearest distance classifier in the space of LE versus HE attenuation values. Recognizing the differences in noise between LE and HE beams, the LE and HE data were differentially filtered (in FBP) or regularized (in PL). Both a quadratic (PLQ) and a total‐variation penalty (PLTV) were investigated for PL. The performance of DE CBCT material discrimination was quantified in terms of voxelwise specificity, sensitivity, and accuracy. Results: : Noise in the HE image was primarily responsible for classification errors within the contrast inserts, whereas noise in the LE image mainly influenced classification in the surrounding water. For inserts of diameter 28.4 mm, DE CBCT reconstructions were optimized to maximize the total combined accuracy across the range of calcium and iodine concentrations, yielding values of ∼88% for FBP and PLQ, and ∼95% for PLTV at 3.1 mGy total dose, increasing to ∼95% for FBP and PLQ, and ∼98% for PLTV at 15.6 mGy total dose. For a fixed iodine concentration of 5 mg/ml and reconstructions maximizing overall accuracy across the range of insert diameters, the minimum diameter classified with accuracy >80% was ∼15 mm for FBP and PLQ and ∼10 mm for PLTV, improving to ∼7 mm for FBP and PLQ and ∼3 mm for PLTV at 15.6 mGy. The results indicate similar performance for FBP and PLQ and showed improved classification accuracy with edge‐preserving PLTV. A slight preference for increased smoothing of the HE data was found. DE CBCT discrimination of iodine and bone in the knee was demonstrated with FBP and PLTV at 6.2 mGy total dose. Conclusions: : For iodine concentrations >5 mg/ml and detail size ∼20 mm, material classification accuracy of >90% was achieved in DE CBCT with both FBP and PL at total doses <10 mGy. Optimal performance was attained by selection of reconstruction parameters based on the differences in noise between HE and LE data, typically favoring stronger smoothing of the HE data, and by using penalties matched to the imaging task (e.g., edge‐preserving PLTV in areas of uniform enhancement).