We report on the development of silicon strip detectors for energy-resolved clinical mammography. Typically, X-ray integrating detectors based on scintillating cesium iodide CsI(Tl) or amorphous selenium (a-Se) are used in most commercial systems. Recently, mammography instrumentation has been introduced based on photon counting Si strip detectors. The required performance for mammography in terms of the output count rate, spatial resolution, and dynamic range must be obtained with sufficient field of view for the application, thus requiring the tiling of pixel arrays and particular scanning techniques. Room temperature Si strip detector, operating as direct conversion x-ray sensors, can provide the required speed when connected to application specific integrated circuits (ASICs) operating at fast peaking times with multiple fixed thresholds per pixel, provided that the sensors are designed for rapid signal formation across the X-ray energy ranges of the application. We present our methods and results from the optimization of Si-strip detectors for contrast enhanced spectral mammography. We describe the method being developed for quantifying iodine contrast using the energy-resolved detector with fixed thresholds. We demonstrate the feasibility of the method by scanning an iodine phantom with clinically relevant contrast levels.
We report on our efforts toward the development of silicon (Si) strip detectors for energy-resolved clinical breast
imaging. Typically, x-ray integrating detectors based on scintillating cesium iodide CsI(Tl) or amorphous selenium (a-
Se) are used in most commercial systems. Recently, mammography instrumentation has been introduced based on
photon counting silicon Si strip detectors. Mammography requires high flux from the x-ray generator, therefore, in order
to achieve energy resolved single photon counting, a high output count rate (OCR) for the detector must be achieved at
the required spatial resolution and across the required dynamic range for the application. The required performance in
terms of the OCR, spatial resolution, and dynamic range must be obtained with sufficient field of view (FOV) for the
application thus requiring the tiling of pixel arrays and scanning techniques. Room temperature semiconductors,
operating as direct conversion x-ray sensors, can provide the required speed when connected to application specific
integrated circuits (ASICs) operating at fast peaking times with multiple fixed thresholds per pixel, provided that the
sensors are designed for rapid signal formation across the x-ray energy ranges of the application at the required energy
and spatial resolutions. We present our methods and results from the optimization of prototype detectors based on Si
strip structures. We describe the detector optimization and the development of ASIC readout electronics that provide the
required spatial resolution, low noise, high count rate capabilities and minimal power consumption.
Developments in room temperature solid state imaging arrays for energy-resolved single photon counting in medical x-ray imaging are discussed. A number of x-ray imaging applications can benefit from these developments including mammography which requires very good spatial resolution. Energy resolved photon counting can provide reduced dose through optimal energy weighting and compositional analysis through multiple basis function material decomposition. Extremely high flux can occur in x-ray imaging and energy integrating detectors with a large dynamic range and good detection efficiency have been conventionally used. To achieve the benefits of energy resolved photon counting, imaging arrays with a large count rate range and good detection efficiency are required. Si based semiconductor radiation detectors with strip anode arrays electrically connected to application specific integrated circuits (ASICs) can provide fast, efficient, low-noise performance with good energy and spatial resolution for use in mammography however this can only be achieved with a careful optimization of the Si sensors and ASICs together. We have designed and constructed a Si imaging array, with a 1 x 1024 grid of electrical 100 micron wide strip contacts inter connected to multi channel ASICs, with a counting range up to 1 x 10<sup>6</sup> counts per second per pixel.
Developments in room temperature cadmium telluride (CdTe) based solid state imaging arrays for energy-resolved
photon-counting in medical x-ray imaging are discussed. A number of x-ray imaging applications can benefit from these
developments including mammography, radiography, and computed tomography (CT). Energy-resolved photon-counting
can provide reduced dose through optimal energy weighting, compositional analysis through multiple basis
function material decomposition, and contrast enhancement through spectroscopic x-ray imaging of metal nanoparticles.
Extremely high flux can occur in x-ray imaging and energy integrating detectors have been conventionally used. To
achieve the benefits of energy resolved photon counting, imaging arrays with a large count rate range and good detection
efficiency are required. Compound semiconductor radiation detectors with pixellated anode arrays electrically connected
to application specific integrated circuits (ASICs) can provide fast, efficient, low-noise performance with adequate
energy resolution however this can only be achieved with a careful optimization of the CdTe sensors and ASICs
together. We have designed and constructed CdTe imaging arrays, 3 mm thick with a grid of electrical contacts inter-connected
to a multi-channel channel ASICs. Arrays with a pixel pitch of 0.5 mm have achieved a counting range up to
20 million counts per second per square mm. Additionally, ASICs with a two dimensional array of pads has been
fabricated and tested by connecting the inputs to 1 mm pitch CdTe sensors demonstrating 7 keV full width at half
maximum energy resolution across a dynamic range of 30 keV to 140 keV for clinical CT.
We report results from the development of a second-generation CdTe direct-conversion compound-semiconductor x-ray
detector for photon-counting clinical CT. The first-generation detector has 512 pixels with a 1 mm pitch and is vertically
integrated with the readout. A 32-row multi-slice CT system using first-generation detectors has been used for clinical
low-dose CT applications. To provide adequate performance for whole-body diagnostic CT we have designed and
fabricated new 0.25 mm<sup>2</sup> pixels to increase the maximum output to greater than 20 Mcps per mm<sup>2</sup> while preserving
sufficient energy resolution for photon-counting CT. In addition to the need for dynamic range, CT places stringent
uniformity and temporal response requirements on the detector. We have measured detector parameters including the
dynamic range, energy resolution, noise floor, stability, and temporal response. Temporal response is determined by
rapid cycling of the input flux with shutter driven attenuators. Cycling between high and low flux generates reproducible
counts, within counting statistics, with a response time less than 1 ms. Stability is determined by measuring uniformity corrected flood images repeatedly over a time interval exceeding whole-body diagnostic CT scan times. Long exposure to uniform flux generates a number of counts which drift in some pixels slightly in excess of counting statistics. These results demonstrate the potential for these detectors to achieve whole-body CT.
We demonstrate the feasibility of using a dual-modality fluorescence and x-ray computed tomography (CT)
system for quantitative molecular imaging with phantom studies. A CCD based non-contact FT system,
which can take measurements from multiple views was built.
High-resolution X-Ray CT was used to obtain
structural information from the phantom. A 3.6 mm diameter fluorescence inclusion was deeply embedded
in the heterogeneous optical background. The results demonstrated that the fluorophore concentration can
only be obtained accurately when guided by the a priori information provided by the x-ray CT.
We report on a characterization study of a multi-row
direct-conversion x-ray detector used to generate the first photon
counting clinical x-ray computed tomography (CT) patent images. In order to provide the photon counting detector with
adequate performance for low-dose CT applications, we have designed and fabricated a fast application specific
integrated circuit (ASIC) for data readout from the pixellated CdTe detectors that comprise the photon counting detector.
The cadmium telluride (CdTe) detector has 512 pixels with a 1 mm pitch and is vertically integrated with the ASIC
readout so it can be tiled in two dimensions similar to those that are tiled in an arc found in 32-row multi-slice CT
systems. We have measured several important detector parameters including the maximum output count rate, energy
resolution, and noise performance. Additionally the relationship between the output and input rate has been found to fit a
non-paralyzable detector model with a dead time of 160 nsec. A maximum output rate of 6 × 10<sup>6</sup> counts per second per
pixel has been obtained with a low output x-ray tube for CT operated between 0.01 mA and 6 mA at 140 keV and
different source-to-detector distances. All detector noise counts are less that 20 keV which is sufficiently low for clinical
CT. The energy resolution measured with the 60 keV photons from a <sup>241</sup>Am source is ~12%. In conclusion, our results
demonstrate the potential for the application of the CdTe based photon counting detector to clinical CT systems. Our
future plans include further performance improvement by incorporating drift structures to each detector pixel.
Bulk single crystals of CdZnTe compound semiconductor is used for room temperature
radiation detection in commercial radiation sensors. A large volume of detector material
with low defect density is required for increasing the detection efficiency. Manufacture of
such a bulky detector-quality material with low defect density is expensive. In this
communication, synthesis of nanowires arrays of CdZnTe that can be used for detecting
low energy radiation is reported for the first time. CdZnTe ternary compound
semiconductor, referred as CZT, was electrodeposited in the form of nanowires onto a
TiO<sub>2</sub> nanotubular template in non-aqueous electrolytes using a pulse-reverse process at
130 °C. Very high electrical resistivity of the CZT nanowires (in the order of 10<sup>10</sup> Ω-cm)
was obtained. Such a high resistivity was attributed to the presence of deep defect states
such as cadmium vacancies created by the anodic cycle of the pulse-reverse
electrodeposition process. Stacks of series connected CZT nanowire arrays were
impressed with different bias potentials. The leakage current was in the order of tens of
PicoAmperes. When exposed to a radiation source (Am -241, 60 keV), the current flow
in the circuit increased. The preliminary results indicate that the CZT nanowire arrays can
be used as radiation detector materials at room temperature with a much low bias
potential (0.7 - 2.3 V) as against 300 - 500 V applied to the bulk detector materials.