Industrial and security imaging can be challenging because it covers a wide range of applications with many different requirements. Performance requirements include X-ray energy ranges from 20 kV to 15 MV; resolution requirements from microns to centimeters; 1D, 2D, and 3D applications with broadly different requirements; as well as frame rates from 600 fps, to 0.01 fps.
To overcome these challenges, Varex is developing a modular strip detector based on amorphous silicon (a-Si) technology for use in a wide variety of imaging applications at various energies and doses. Expandable to different sizes and compatible with most scintillators, the a-Si modular strip detector offers cost reduction compared to custom scintillator arrays and expanded capabilities at many energies.
Design Configurations and Performance
The a-Si modular strip detector being developed is based on a 15 cm x 15 cm digital radiography Pax Scan Imager and is modularly expandable to very large fields of view (FOV). It offers a single set of readout configurable for multiple applications at a lower cost due to its volume and a-Si technology. The detector will also include an integrator that can stock the same imager as well as change the scintillator as required, and it includes an integrator friendly, single software API for many hardware configurations. In addition, a-Si modular strip detectors will decrease downtime because of the availability of replacement parts.
So far, a few prototypes have been built and tested with promising results. Improvements from the Varex CST package, including scatter reduction kernels, beam hardening correction, and resolution enhance, have provide improved image performance. The a-Si modular strip detector will also easily integrate and sync with pulsed sources such as linear accelerators.
To date, three configurations have been tested. Configuration A (side entry) is designed to cover a large area and includes high energy CT (HECT), security line scanning and high energy or high dose line scanning applications. With this configuration, the resolution is limited by pixilation of the 1D array or 127 microns.
Configuration B (front entry) offers high energy line scanning and time delay integration (TDI) imaging. TDI can increase the scanning speed, improve the contrast to noise ratio (CNR), and increase the signal to noise ratio (SNR). This design can accept a variety of 1D and 2D thick scintillator arrays, and the resolution is limited by pixilation of the scintillator or 127 microns.
The third configuration, C, offers arrays and a scintillator less than 3 mm thick; 20 kV to 450 kV; and TDI imaging. With Configuration C, the resolution is limited to 127 microns or array pitch.
The prototype a-Si modular strip detector is a modified 1515DXT-I imager with a standard 127 micrometer pitch but should be able to run in excess of 200 frames per second (fps) and in coarsest resolution mode at over 600 fps in 2x2 binning mode, allowing for imaging to occur on a pulse-to-pulse basis for most accelerators. The module will be designed to be tiled for scaling to large field of view applications.
In our tests the prototype a-Si modular strip detector, which includes a fast 2-D zoom mode (100x1152), was read out at 100Hz. Different scintillators were switched in and out for our measurements, including a standard scintillator, DRZ+TM, a scintillating glass array of LKH5, and a cadmium tungstate array. The X-ray source for the 320 and 450 kV measurements was 450 kV HPX-450-11. Currently, we are testing a variety of scintillators for comparison that include DRZ+, CsI, CdWO4, and scintillating glass arrays.
Preliminary results have been obtained for a polished CdWO4 array with a 1.6 mm pixel pitch, 3 mm thickness, and with 26.5 micrometer septa separating the pixels. A polished scintillating glass array with 1.176 mm pitch, 3 mm thickness, and 26.5 micrometer septa has also been tested.
Measurements for CNR were performed per the ASTM E2737 for the different scintillators at 320 kV and 450 kV. In the future, we plan to test at 950 kV as well. Additionally, contrast improvement was demonstrated for real-time motion radiography using a shift and stitch time integration technique. For most measurements, the source to object distance (SOD) was 80 cm and the source to imager distance (SID) was 128.6 cm, with a geometric magnification of 1.6 and exposure time for all images around 6 minutes.
Varex’s modular strip detector has the potential to meet the vastly differing requirements for industrial and security imaging, at a lower cost than custom, low volume, high performance imagers.
The LKH5 glass offers adequate performance for many high energy imaging applications at about 1/40th the cost CdWO4, and the a-Si modular strip detector provides substantial improvements in CNR for thicker scintillators when the resolution requirements are relaxed and not over specified. In addition, the modular strip detector can utilize both standard GOS scintillators as well as various 1D and 2D pixelated arrays.
In the future, Varex plans to modify the a-Si configuration so that the imager will be end-to-end buttable and image at higher energies 1 MV–15 MV. Plans are also in place to test full scale arrays as the application need arises.