Characterization of imaging performance in differential phase contrast CT compared with the conventional CT: Spectrum of noise equivalent quanta NEQ(k)

• Published in: Medical physics (Lancaster) 2012-07, Vol.39 (7), p.4467-4482
• Main Authors: Tang, Xiangyang, Yang, Yi, Tang, Shaojie
• Format: Article
• Language: English
• Subjects: Algorithms; ATTENUATION; Biosensors; Computed radiography; Computed tomography; Computerised tomographs; computerised tomography; COMPUTERIZED SIMULATION; COMPUTERIZED TOMOGRAPHY; DATA ACQUISITION; detective quantum efficiency; Diffraction gratings; Digital computing or data processing equipment or methods, specially adapted for specific applications; DQE; Fourier analysis; Fourier transforms; Humans; Image data processing or generation, in general; Image detection systems; IMAGE PROCESSING; imaging performance; KEV RANGE 10-100; Medical image noise; medical image processing; Medical image reconstruction; Medical imaging; Medical X‐ray imaging; modulation transfer function; Modulation transfer functions; MONOCHROMATIC RADIATION; MTF; NEQ; noise; noise equivalent quanta; noise power spectrum; NPS; PHANTOMS; PHOTONS; Poisson distribution; QUANTUM EFFICIENCY; Radiation Imaging Physics; Radiographic Image Enhancement - methods; Radiographic Image Interpretation, Computer-Assisted - methods; RADIOLOGY AND NUCLEAR MEDICINE; Reproducibility of Results; Sensitivity and Specificity; Signal-To-Noise Ratio; SPECTRA; Tomography, X-Ray Computed - methods; TRANSFER FUNCTIONS; X RADIATION; X-RAY SOURCES; X-RAY TUBES; x‐ray CT; X‐ray detectors; x‐ray differential phase contrast CT; X‐ray imaging; x‐ray phase CT; x‐ray tube and grating‐based phase CT
• ISSN: 0094-2405; 2473-4209; 0094-2405
• DOI: 10.1118/1.4730287

Purpose: Differential phase contrast CT (DPC-CT) is emerging as a new technology to improve the contrast sensitivity of conventional attenuation-based CT. The noise equivalent quanta as a function over spatial frequency, i.e., the spectrum of noise equivalent quanta NEQ(k), is a decisive indicator of the signal and noise transfer properties of an imaging system. In this work, we derive the functional form of NEQ(k) in DPC-CT. Via system modeling, analysis, and computer simulation, we evaluate and verify the derived NEQ(k) and compare it with that of the conventional attenuation-based CT. Methods: The DPC-CT is implemented with x-ray tube and gratings. The x-ray propagation and data acquisition are modeled and simulated through Fresnel and Fourier analysis. A monochromatic x-ray source (30 keV) is assumed to exclude any system imperfection and interference caused by scatter and beam hardening, while a 360° full scan is carried out in data acquisition to avoid any weighting scheme that may disrupt noise randomness. Adequate upsampling is implemented to simulate the x-ray beam's propagation through the gratingsG 1 and G 2 with periods 8 and 4 μm, respectively, while the intergrating distance is 193.6 mm (1/16 of the Talbot distance). The dimensions of the detector cell for data acquisition are 32 × 32, 64 × 64, 96 × 96, and 128 × 128 μm2, respectively, corresponding to a 40.96 × 40.96 mm2 field of view in data acquisition. An air phantom is employed to obtain the noise power spectrum NPS(k), spectrum of noise equivalent quanta NEQ(k), and detective quantum efficiency DQE(k). A cylindrical water phantom at 5.1 mm diameter and complex refraction coefficient n = 1 − δ + iβ = 1 −2.5604 × 10−7 + i1.2353 × 10−10 is placed in air to measure the edge transfer function, line spread function and then modulation transfer function MTF(k), of both DPC-CT and the conventional attenuation-based CT. The x-ray flux is set at 5 × 106 photon/cm2 per projection and observes the Poisson distribution, which is consistent with that of a micro-CT for preclinical applications. Approximately 360 regions, each at 128 × 128 matrix, are used to calculate the NPS(k) via 2D Fourier transform, in which adequate zero padding is carried out to avoid aliasing in noise. Results: The preliminary data show that the DPC-CT possesses a signal transfer property [MTF(k)] comparable to that of the conventional attenuation-based CT. Meanwhile, though there exists a radical difference in their noise pow