VACIS High Resolution System

Introduction
While the VACIS systems are well-suited to the original goal of narcotics detection, there is opportunity to improve the performance with respect to new priorities such as locating bombs, etc. To this end, one parameter of interest is improved spatial resolution. Fundamental simultaneous design constraints include maintaining or improving system throughput, minimizing the scope of the changes, and, of course, minimizing cost.
After analyzing many different possible techniques and detectors to achieve these design goals in the VACIS product line, we have decided to use block detectors, of the type used in nuclear medicine PET studies. This provides a high-resolution option (patent pending) for the VACIS product line. These detectors, with associated image reconstruction software, will enable an improvement in spatial resolution of approximately a factor of three while maintaining throughput. VACIS systems are photon limited by design, so thehighest resolution will only be achieved with low-attenuation objects. As attenuation increases, it is planned that the software will re-bin the data into larger virtual pixels to achieve a step-wise tradeoff between pixel count density and spatial resolution.
Configuration
A series of analytic models, ray-tracing code runs, and Monte Carlo code runs indicate that for the VACIS product line, a pixel size of approximately 6 mm is ideal for achieving maximum resolution. Factors that preclude smaller pixels include out-scattering from the pixels (effectively reducing contrast), source brightness and scan timeA significant amount of work was performed to design the block detector unit as a nearly self-contained, field replaceable unit (see Figure 10). Integral to the detector housing is the HV power supply, analog preamplifier, ADCs, an FPGA and an embedded micro-controller. The block detector inputs are low-voltage DC power and two master control lines on a single DB-15 connector. The output is an Ethernet data stream on a standard RJ-45 connector.