Generalized Objective Performance Assessment of a New High-Sensitivity Microangiographic Fluoroscopic (HSMAF) Imaging System.

The objective performance evaluation metrics, termed Generalized Modulation Transfer Function (GMTF), Generalized Noise Power Spectrum (GNPS), Generalized Noise Equivalent Quanta (GNEQ), and Generalized Detective Quantum Efficiency (GDQE), have been developed to assess total imaging-system performance by including the effects of geometric unsharpness due to the finite size of the focal spot and scattered radiation in addition to the detector properties. These metrics were used to evaluate the performance of the HSMAF, a custom-built, high-resolution, real-time-acquisition detector with 35-mum pixels, in simulated neurovascular angiographic conditions using a uniform head-equivalent phantom. The HSMAF consists of a 300-mum-thick CsI(Tl) scintillator coupled to a 4 cm diameter, variable-gain, Gen2 light image intensifier with dual-stage microchannel plate, followed by direct fiber-optic coupling to a 30-fps CCD camera, and is capable of both fluoroscopy and angiography. Effects of focal-spot size, geometric magnification, irradiation field-of-view, and air-gap between the phantom and the detector were evaluated. The resulting plots of GMTF and GDQE showed that geometric blurring is the more dominant image degradation factor at high spatial frequencies, whereas scatter dominates at low spatial frequencies. For the standard image-geometry and scatter conditions used here, the HSMAF maintains substantial system imaging capabilities (GDQE>5%) at frequencies above 4 cycles/mm where conventional detectors cannot operate. The loss in image SNR due to scatter or focal-spot unsharpness could be compensated by increasing the exposure by a factor of 2 to 3. This generalized evaluation method may be used to more realistically evaluate and compare total system performance leading to improved system designs.