Evaluation of the microangiographic fluoroscope (MAF) using generalized system performance metrics

Abstract

Purpose:The performance of a newly developed, high resolution, microangiographic fluoroscope (MAF) (35 μm pixel pitch and 300 μm thick CsI phosphor) was evaluated using a generalized linear system analysis and compared with that of a standard amorphous Si thin film transistor flat panel detector (FPD) (194 μm pixel pitch and 600 μm thick CsI phosphor). The linear system metrics such as modulation transfer function (MTF), noise power spectrum, and detection quantum efficiency (DQE) are commonly used to gauge the intrinsic detector performance in the detector plane. However, these linear system metrics do not provide information about the image receptor performance in a real system since they do not include the effects of other parameters such as focal spot distribution, scatter radiation, and geometric unsharpness, which may compromise detector performance characteristics. Use of generalized linear system metrics [generalized modulation transfer function (GMTF), generalized normalized noise power spectrum (GNNPS), and generalized detection quantum efficiency (GDQE)] that include these effects gives a more meaningful, complete, and appropriate evaluation of detector performance as part of the imaging system. Methods: A uniform head equivalent phantom was used to simulate realistic clinical parameters and x-ray spectra. The detector MTFs were measured using the slanted edge method and the focal spot MTFs were measured using a pinhole assembly. The scatter MTF was simulated and the scatter fraction was measured for a head-equivalent phantom. The generalized system metrics were calculated for different combinations of three choices of focal spots and three different magnifications with two different air-gaps. The performance of the MAF was also illustrated using stent images obtained with different focal spots under similar conditions. Results: Results for the generalized metrics provide a quantitative description of the performance of the imaging system for both detectors. This generalized analysis demonstrated that both detectors have similar imaging capabilities at lower spatial frequencies, but that the MAF has superior performance over the FPD at higher frequencies even when considering focal spot blurring and scatter. Conclusions: This generalized performance analysis demonstrates the significance of focal spot size, magnification, and scatter on the system performance metrics (GMTF, GNNPS, and GDQE). Although the ideal detector performance characteristics of the MAF are not fully realized due to these other system factors, it still retains an advantage in DQE at high spatial frequencies over the FPD. Similar studies based on the generalized linear system metrics can serve as an efficient tool to evaluate total system capabilities under different realistic conditions to enable optimal design for specific imaging tasks.

Photon-counting hexagonal pixel array CdTe detector: Spatial resolution characteristics for image-guided interventional applications

“…6,9 Characterization of a MAF employing a 300 µm thick CsI:Tl scintillator coupled to a CCD with 35 µm pitch via a light image intensifier and fiberoptic taper showed a limiting resolution of 4 cycles/mm. 11 The observed 10% MTF of ∼12 cycles/mm with the investigated detector in the SPC mode compares favorably with the added advantage of effectively eliminating electronic noise through appropriate selection of the energy threshold. Further improvement in MTF is possible with appropriate selection of energy threshold, when there is prior knowledge of the x-ray spectrum.…”

“…Hence, high-resolution small field-of-view devices, referred to as microangiographic fluoroscope (MAF), have been developed. 11 Photon-counting detectors allow for discrimination and removal of system (electronic) noise with appropriate selection of energy threshold. Cadmium telluride (CdTe) has been studied for x-ray and gamma ray detection due to its high atomic number (Z Cd = 48 and Z Te = 52), high density (5.85 g/cm 3 ), and large band-gap (1.5 eV).…”

Region-of-interest cone beam computed tomography (ROI CBCT) with a high resolution CMOS detector

“…The full potential of a high resolution detector in x-ray imaging is not utilized fully due to the limitation and degrading effect of the finite size focal spot and geometric magnification. [14-15]. Use of a smaller focal spot will provide improvement in the overall imaging capability of the detector.…”

Initial investigations of a special high-definition (Hi-Def) zoom capability in a new detector system for neuro-interventional procedures

“…During the deployment of such devices it is important to provide good live visualization of the treatment area to the interventionalist. Towards this goal two high resolution fluoroscopes based on Charge Couple Devices (HRF-CCD) and Complementary Metal Oxide Semiconductors (HRF-CMOS) were developed and previously reported [1,2]. Both of these devices were mounted on the existing flat panel detector supports using a mechanical changer unit.…”