New cone-beam computed tomographic (CBCT) mammography system designs are presented where the detectors provide high spatial resolution, high sensitivity, low noise, wide dynamic range, negligible lag and high frame rates similar to features required for high performance fluoroscopy detectors. The x-ray detectors consist of a phosphor coupled by a fiber-optic taper to either a high gain image light amplifier (LA) then CCD camera or to an electron multiplying CCD. When a square-array of such detectors is used, a field-of-view (FOV) to 20 x 20 cm can be obtained where the images have pixel-resolution of 100 μm or better. To achieve practical CBCT mammography scan-times, 30 fps may be acquired with quantum limited (noise free) performance below 0.2 μR detector exposure per frame. Because of the flexible voltage controlled gain of the LA's and EMCCDs, large detector dynamic range is also achievable. Features of such detector systems with arrays of either generation 2 (Gen 2) or 3 (Gen 3) LAs optically coupled to CCD cameras or arrays of EMCCDs coupled directly are compared. Quantum accounting analysis is done for a variety of such designs where either the lowest number of information carriers off the LA photo-cathode or electrons released in the EMCCDs per x-ray absorbed in the phosphor are large enough to imply no quantum sink for the design. These new LA- or EMCCD-based systems could lead to vastly improved CBCT mammography, ROI-CT, or fluoroscopy performance compared to systems using flat panels.
In order to satisfy the high resolution (3 to 10 cycles/mm) imaging requirements in neurovascular image-guided interventional (IGI) procedures, a micro-angiographic fluoroscope (MAF) is being developed to enable both rapid sequence angiography (15 fps) at high exposure levels (hundreds of μR/frame) as well as fluoroscopy at high frame rates (30 fps) and low exposure levels (5 to 20 μR/frame). The prototype MAF consists of a 350-μm-thick CsI(Tl) scintillator coupled by a 2:1 fiber-optical taper to an 18 mm diameter variable-gain light image intensifier with two-stage microchannel plate (MCP) viewed by a 12-bit, 1024x1024, 30 fps CCD camera with digital interface board. The optical set-up enables variation of effective pixel-size from 31 to 50 micron. The first frame lag of the MAF in fluoroscopic 30 fps mode (2:1 binning) was less than 0.8% at exposures of 5-23 μR/frame. MTF, NPS, and DQE in angiographic mode were measured for IEC standard spectrum RQA 5. At spatial frequencies of 4 and 10 cycles/mm the MTF was 14% and 1.5%, and the DQE was 12% and 1.2%, respectively, while the DQE(0) was 60%. Acquisition software was developed to acquire 15 fps angiography and 30 fps fluoroscopy for real-time dark field and flat field correction or real-time roadmapping. Images obtained with the MAF in small animal IGI procedures are demonstrated. The linearity versus x-ray intensity and MCP working range effects has been studied. We plan to expand the current 3.6 cm diameter field of view to 6 cm in the next model of the MAF.
New neuro-interventional devices such as stents require high spatial-resolution image guidance to enable accurate localization both along the vessel axis as well as in a preferred rotational orientation around the axis. A new high-resolution angiographic detector has been designed with capability for micro-angiography at rates exceeding the 5 fps of our current detector and, additionally, with noise low enough and gain high enough for fluoroscopy. Although the performance requirements are demanding and the detector must fit within practical clinical space constraints, image guidance is only needed within a approximately 5 cm region of interest at the site of the intervention. To achieve the design goals, the new detector is being assembled from available components which include a CsI(Tl) phosphor module coupled to a fiber-optic taper assembly with a two stage light image intensifier and a mirror between the output of the fiber taper and the input to a conventional high performance optical CCD camera. Resulting acquisition modes include 50-micron effective pixels at up to 30 fps with the capability to adjust sensitivity for both fluoroscopy and angiography. Estimates of signal at the various stages of detection are made with quantum accounting diagrams (QAD).
A new high spatial resolution micro-angiographic camera will enable routine viewing within a region of interest of detailed vascular structure unable to be seen with current full field of view (FOV) angiographic detectors. Such details include perforator vessels, vessel contractility or compliance, and condition and location of 50 micron or smaller stent wires. Although the basic CsI(Tl) phosphor-optical taper-CCD design of the new ROI micro-angiographic camera is essentially the same as that of the pre-clinical prototype, many of the physical parameters are much improved. The FOV is 5 cm X 5 cm vs. the previous 1 cm X 1 cm; the phosphor thickness is 350 - 400 micron vs. the previous 100 micron; the taper ratio is now 1.8 rather than 3.0 (2.8X improvement in light collection). The pixel size is either 25 or 50 micron. Additionally, detector noise may now be carefully considered in the camera design as may mechanical supporting mechanisms, methods to synchronize image acquisition with exposure and the effects of other physical factors such as exposure parameters, tube loading, focal spot size and geometric unsharpness. It is expected that this new capability should allow improved treatments and further development of smaller interventional devices and catheter delivery systems.