Erratum: Characterization of a GaAs photon-counting detector for mammography.

[This corrects the article DOI: 10.1117/1.JMI.8.3.033504.].

Characterization of a GaAs photon-counting detector for mammography.

Purpose: The purpose of this study was to evaluate the potential of a prototype gallium arsenide (GaAs) photon-counting detector (PCD) for imaging of the breast. Approach: First, the contrast-to-noise ratio (CNR) using different aluminum/poly(methyl methacrylate) (PMMA) phantoms of different thicknesses were measured. Second, microcalcification detection accuracy using a receiver operating characteristic study with three observers reading an ensemble of images was measured. Finally, the feasibility of using a GaAs system with two energy bins for contrast-enhanced mammography was investigated. Results: For the first two studies, the GaAs detector was compared with a commercial mammography system. The CNR was estimated by imaging 18-, 36-, and 110 - µ m -thick aluminum targets placed on top of 6 cm of PMMA plates and was found to be similar or better over a range of exposures. We observed a similar performance of detecting microcalcifications with the GaAs detector over a range of clinically applicable dose levels with a small increase at lower dose levels. The results also showed that contrast-enhanced spectral mammography using a GaAs PCD is feasible and beneficial. Conclusions: Results from this study suggest that performance with this new detector seems either slightly improved or equivalent to a commercial mammography system that used an energy-integrated detector. No attempt at optimizing exposure techniques for the GaAs detector was performed. Further research is needed to determine optimal acquisition parameters for the GaAs detector and to develop more sophisticated material decomposition algorithms that promise to provide improved quantitative estimates of iodine uptake.

Differentiation of Crystals Associated With Arthropathies by Spectral Photon-Counting Radiography: A Proof-of-Concept Study.

OBJECTIVES: The aims of this study were to test whether spectral photon-counting radiography (SPCR) is able to identify and distinguish different crystals associated with arthropathies in vitro and to validate findings in a gouty human third toe ex vivo. MATERIALS AND METHODS: Industry-standard calibration rods of calcium pyrophosphate, calcium hydroxyapatite (HA), and monosodium urate (MSU) were scanned with SPCR in an experimental setup. Each material was available at 3 different concentrations, and a dedicated photon-counting detector was used for SPCR, whereas validation scans were obtained on a clinical dual-energy computed tomography (DECT) scanner. Regions of interest were placed on SPCR images and consecutive DECT images to measure x-ray attenuation characteristics, including effective atomic numbers (Zeff). Statistical tests were performed for differentiation of Zeff between concentrations, materials, and imaging modalities. In addition, a third toe from a patient with chronic gouty arthritis was scanned with SPCR and DECT for differentiation of MSU from HA. RESULTS: In both SPCR and DECT, significant differences in attenuation and Zeff values were found for different concentrations among (P < 0.001) and between different materials (P < 0.001). Overall, quantitative measurements of Zeff did not differ significantly between SPCR- and DECT-derived measurements (P = 0.054-0.412). In the human cadaver toe, gouty bone erosions were visible on standard grayscale radiographic images; however, spectral image decomposition revealed the nature and extent of MSU deposits and was able to separate it from bone HA by Zeff. CONCLUSIONS: Identification and differentiation of different crystals related to arthropathies are possible with SPCR at comparable diagnostic accuracy to DECT. Further research is needed to assess diagnostic accuracy and clinical usability in vivo.

Detection and Characterization of Monosodium Urate and Calcium Hydroxyapatite Crystals Using Spectral Photon-Counting Radiography: A Proof-of-Concept Study.

OBJECTIVE: To assess the potential of spectral photon-counting (PC) radiography (SPCR) for the detection and characterization of monosodium urate (MSU) and calcium hydroxyapatite (HA) crystals, based on effective atomic number (Zeff) values derived from specific X-ray attenuation characteristics at different energy levels. METHODS: Suspensions of either pure agar, synthetic MSU (200 mg/ml) or HA (100 and 150 mg/ml) crystals in agar were sealed in industry-standard polystyrene vials and supported on a 2.5-mm-thick plastic table. Samples were scanned using a vendor microfocus X-ray tube and a spectral PC detector prototype with four energy thresholds per acquisition (15, 25, 30, and 35 keV). Material decomposition calibration was performed using polymethyl methacrylate (PMMA) and polyvinylchloride (PVC) slabs. Using a custom post-processing software based on polynomial material decomposition, Zeff of the respective samples were computed. All samples were additionally scanned using dual-energy CT (DECT, 80 kV and tin-filtered 150 kV) and analyzed with a proprietary post-processing algorithm for gout. RESULTS: MSU crystal suspension attenuated significantly less than both HA samples. MSU and HA suspensions differed significantly in Zeff (mean ± SD: 7.74 ± 0.28 vs. 9.43 ± 0.41, p < .001). Zeff values from SPCR were comparable to DECT-based reference values (p = 0.16) and were independent of the radiation dose level (0.18 - 18 mAs, p = 1). DISCUSSION: This in vitro feasibility study demonstrates the potential of SPCR for discriminating MSU from HA crystal suspensions based on Zeff differences. Further studies have to corroborate these initial findings ex vivo and in vivo, and to compare the diagnostic performance of SPCR with DECT in imaging of crystal-associated arthropathies.

A generalized quantitative interpretation of dark-field contrast for highly concentrated microsphere suspensions.

In X-ray grating interferometry, dark-field contrast arises due to partial extinction of the detected interference fringes. This is also called visibility reduction and is attributed to small-angle scattering from unresolved structures in the imaged object. In recent years, analytical quantitative frameworks of dark-field contrast have been developed for highly diluted monodisperse microsphere suspensions with maximum 6% volume fraction. These frameworks assume that scattering particles are separated by large enough distances, which make any interparticle scattering interference negligible. In this paper, we start from the small-angle scattering intensity equation and, by linking Fourier and real-space, we introduce the structure factor and thus extend the analytical and experimental quantitative interpretation of dark-field contrast, for a range of suspensions with volume fractions reaching 40%. The structure factor accounts for interparticle scattering interference. Without introducing any additional fitting parameters, we successfully predict the experimental values measured at the TOMCAT beamline, Swiss Light Source. Finally, we apply this theoretical framework to an experiment probing a range of system correlation lengths by acquiring dark-field images at different energies. This proposed method has the potential to be applied in single-shot-mode using a polychromatic X-ray tube setup and a single-photon-counting energy-resolving detector.