Using a commercial clinical CT scanner (GE lightspeed), nine CT scans were performed on a 20 cm diameter plastic
pipe filled with water. The mAs was varied from 10 to 400mAs and the beam energy was varied from 80 to140kVp. For
each scan three volume datasets were reconstructed using different filters. Noise power spectrum (NPS) curves were
measured to examine the effect of varying kVp, mAs and reconstruction filter on the noise content. Sixteen slices from
each of the reconstructed volumes were used to compute the NPS; the central 192x192 pixels of each slice were split into
four overlapping regions of interest (ROI) of 128x128 pixels. A total of 64 ROI were used per scan. The magnitude
squared of the 2D Fourier transform of each ROI was computed. The mean of the 64 2D results was averaged over radial
frequency, yielding a 1D NPS. The overall shape of the NPS was dependent on the reconstruction filter used. The
magnitude of the curves decreased with the increase of mAs or kVp. kVp, mAs, and the reconstruction filter can be
adjusted to modulate the amount of noise present in resulting CT volumes, but the effect these values have on the patient
must be considered. The relationship between NPS and the Noise Equivalent Quanta (NEQ) makes trends in NPS
important and is the motivation for this evaluation and future research.
Spatial resolution is one of the most crucial parameters for an imaging system. The modulation transfer function (MTF) was physically measured using wire images on a prototype breast CT scanner previously1. In this study, the effects on MTF from different components in the imaging chain, including the x-ray focal spot distribution, detector lag, and x-ray detector MTF were physically measured. The contributions of these three factors and gantry motion affecting CT system resolution were studied using computer simulation. The CT system MTF was computed and the role of each factor was studied independently. The simulated MTF results demonstrated that the x-ray focal spot size and detector MTF have an effect on the system resolution, while the scanner motion degrades only the azimuthal MTF, with greater degradation occurring further from isocenter where greater rotational velocities occur. The azimuthal MTF of this system has a cutoff frequency of 2.0 cycles/mm at the isocenter but degrades to 1.0 cycles/mm at the periphery. The radial MTF has a cutoff frequency of 2.0 cycles/mm, at both the isocenter and periphery. The comparison between the computer simulated and physically measured MTF values demonstrates reasonable accuracy in the simulation process. The results from computer simulation also suggest ways in which the spatial resolution can be improved by system modification.
While mammography is the gold standard for breast cancer screening worldwide, it is widely recognized that mammography has limitations, especially in women with dense breasts. In response to the need for a more sensitive approach to breast cancer screening, a CT scanner specifically for breast imaging in the pendant geometry was designed, fabricated, and is currently in clinical evaluation. The spatial resolution and noise properties are discussed, and breast images from a normal volunteer and a patient with breast cancer demonstrate very promising breast CT image quality from a qualitative perspective.
The implementation of contrast-enhanced dual-energy digital subtraction mammography may lead to better identification of breast tumors, and thus provide a lower cost and more widely available alternative to breast MRI. This technique involves the acquisition of low- and high-energy images after the IV administration of iodinated contrast agent. In this study, the effect of the beam energy (kVp) was examined using the CNR2/dose metric, where CNR is the contrast-to-noise ratio and dose implies the mean glandular dose. The mean glandular dose was calculated using parameterized normalized glandular dose coefficients (DgN), which allowed the computation of the mean glandular dose for the modeled spectra considered in this study, coupled with incident kerma measurements. Optimization studies were performed using a dedicated cone-beam breast CT scanner designed and fabricated in our laboratory, with the system operating in stationary imaging mode. A flat tissue-equivalent phantom (7.5 cm in thickness) was placed at the isocenter of the scanner, and an air gap of 34.5 cm was used in lieu of a grid. Dilute iodine-based contrast agent was introduced into the phantoms using plastic vials. Data were acquired from 40 to 90 kVp at 10 kVp intervals. Due to the low mA available on the breast CT system, a large number of images (1000) were acquired in fluoroscopic mode, which allowed us to match the dose and noise properties for each kVp combinations by changing the number of images used for averaging. Preliminary results demonstrate that the best CNR2/dose is achieved with a 50 kVp low-energy image and a 90 kVp high-energy image. Consequently, radiation doses for contrast-enhanced mammography should be far lower than regular mammography. Since the spatial resolution requirements should also be lower than regular mammography, dual-energy contrast-enhanced mammography, when performed using the optimal technique factor, may indeed provide very similar diagnostic information as breast MRI but at significantly reduced costs.
A digital radiography system comprised of a large field of view (43x43cm) high luminance CsI scintillator, optically coupled to a 4096x4096 element CCD sensor with 12:1 demagnification was evaluated by measuring the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). Detector evaluation was performed using IEC standard #62220-1 methodologies for beam quality. In this study, RQA-5 (21 mm Al added filtration, 74 kVp, 7.1 mm half value layer (HVL)) and RQA-9 (40 mm Al added filtration, 119 kVp, 11.5 mm HVL) qualities were used at several incident exposures from <0.1 mR to >50 mR. Two detector modes of operation included high resolution (HR) and high efficiency (HE), with 108 and 216 μm pixel dimensions, respectively. The detector system responded to 60 mR incident exposure prior to saturation for the HR mode and up to 30 mR in the HE mode. The pre-sampled MTF(f) had 50% modulation at 0.95 mm-1 (HR) and 0.85 mm-1 (HE); and 10%MTF(f) was reached at 2.4 mm-1 (HR) and 2.0 mm-1 (HE). At a frequency of 0.5 mm-1, the DQE was 40% to 50%, and at 1 mm-1 was 12% to 20% for HR and HE modes, respectively. The DQE at low exposures was substantially better for the HE mode. Little dependence of the DQE on beam energy was found, but the RQA 9 beam had lower values. Above a frequency of 2 mm-1 the DQE dropped to zero, attributed to low MTF. Results suggest that patient radiation exposures equivalent or better than a conventional 400 speed screen-film detector can be achieved for many imaging procedures with sufficient SNR and spatial resolution required for a wide range of diagnostic radiography applications.
A pendant-geometry, cone-beam breast CT scanner has been constructed and is undergoing thorough testing in our facility. The system is capable of acquiring 30 frames/sec in 2×2 binning mode (1024×768 pixels) using a flat panel detector coupled to a thallium-doped cesium iodide scintillator. The DQE of the detector system for RQA5 and RQA9 x-ray beam qualities were computed, and the low frequency DQE values were 65% and 57% respectively at approximately 16 μR/frame. The results also shown that minor improvements in DQE are achieved for exposures greater than 16 μR/frame. It is expected that the scanner will be available for the imaging of human volunteers in the first half of 2004.