Brian Rodricks

Improved imaging performance of a 14"x17" direct radiography system using a Se/TFT detector

Progress is discussed on the improvement of a Direct RadiographyTM solid state, flat panel, digital detector designed for use in general radiographic applications. This detector, now known as DirectRayTM, operates on the principle of direct detection of X-ray photons with a selenium photoconductor and consists of 500 micrometer thick amorphous selenium coupled to an amorphous silicon thin-film-transistor (TFT) readout array. This device is fabricated with a 14 X 17-inch (35 X 43-cm) active imaging area, corresponding to 2560 X 3072 pixels having dimensions of 139 micrometer X 139 micrometer and a geometrical fill factor of 86%. Improvements include a TFT array design upgrade with reduced noise characteristic, lower-noise readout electronics, and improved interfaces. Clinical radiographic images are currently being generated with the DirectRay detector using an X-ray exposure level equivalent to that of a 400 speed screen- film combination while maintaining the superior spatial resolution that is inherent in the direct conversion method. An effective sensor restoration technique has been implemented that eliminates the potential for selenium memory artifacts after a high dose. New results on NPS, MTF, DQE and signal linearity are presented. Detectability of low contrast objects using FAXiL test objects as well as the results of clinical studies are discussed.

Digital Camera Imaging System Simulation

A digital camera is a complex system including a lens, a sensor (physics and circuits), and a digital image processor, where each component is a sophisticated system on its own. Since prototyping a digital camera is very expensive, it is highly desirable to have the capability to explore the system design tradeoffs and preview the system output ahead of time. An empirical digital imaging system simulation that aims to achieve such a goal is presented. It traces the photons reflected by the objects in a scene through the optics and color filter array, converts photons into electrons with consideration of noise introduced by the system, quantizes the accumulated voltage to digital counts by an analog-to-digital converter, and generates a Bayer raw image just as a real camera does. The simulated images are validated against real system outputs and show a close resemblance to the images captured under similar condition at all illumination levels.

Radiographic imaging characteristics of a direct conversion detector using selenium and thin film transistor array

Progress on the development of a semiconductor-based, direct-detection, flat-panel digital radiographic imaging device will be discussed. The device consists of a 500 micrometers thick amorphous selenium sensor coupled to an amorphous silicon thin-film-transistor (TFT) readout matrix. This detector has an active imaging area of 14 inches X 17 inches, 3072 X 2560 pixels with dimensions 139 micrometers X 139 micrometers and a geometrical fill factor of 86 percent. Charges generated primarily as a consequence of photoelectric interaction between the incoming x-rays and Se are integrated on storage capacitors that are located at each pixel. The high electric field applied across the Se minimizes the lateral spreading of the signal resulting in a significantly higher spatial resolution when compared to conventional film/screen systems used for general radiography. The sensor array is read out one pixel line at a time by manipulating the source and gate lines of the TFT matrix. Data are digitized to 14 bits. This paper will discuss the statistical photon counting analysis performed on an early prototype device. Measurements will include modulation transfer function, detector quantum efficiency, linearity, and noise analysis. Image analysis will include small contrast object visibility studies using a Faxil x-ray test object T016. Advantages of this flat-panel electronic sensor over conventional systems are discussed.

Development of a novel high-resolution direct conversion x-ray detector

The development of a new high spatial resolution x-ray detector system is described. The prototype detector is based on a patented detector technology that utilizes selenium for the x-ray conversion material, charge storage capacitors, and a thin film transistor (TFT) array for reading out the charge image. This experimental detector consists of a 512 X 512 matrix with a pixel pitch of 70 microns. The selenium layer deposited on the TFT array is 250 microns thick. With a low absorption entrance window the system is optimized for an energy range of 10 - 30 keV, and is designed for applications that require high spatial resolution and low noise. This presentation describes the imaging performance of the detector using the DQE and MTF metrics. Example images of phantoms are shown. Previously, we demonstrated a practical flat-panel self-scanned digital radiography system based on amorphous selenium and TFT technology. This system is being used clinically for chest radiography and general musculoskeletal imaging, and in industrial applications. The current work demonstrates the feasibility of adapting this technology for applications requiring higher spatial resolution.

Filtered gain calibration and its effect on DQE and image quality in digital imaging systems

Digital imaging systems require offset and gain calibration to normalize the behavior of individual pixels. This normalization corrects for imperfections in the system and also external variables that have effects on uniformity. Imaging metrics like Detective Quantum Efficiency (DQE) and Modulation Transfer Function (MTF) define how sensitivity and resolution are transferred through the system. Gain calibration can result in a loss of DQE due to the noise associated with its application. The typical technique to minimize this noise is to average several gain calibration procedures so that the introduced noise is minimized. This paper discuses the effects of gain calibration on DQE. It measures DQE as a function of the number of gain calibration procedures averaged and contrasts it with a novel technique that uses a single filtered gain calibration. It demonstrates that noise filter techniques, applied to a single gain calibration, regains the loss in DQE without any degradation in resolution. This paper also compares imaging performance of a system using a filtered gain map against a system that has many gain calibrations averaged. The technique is demonstrated using a Thin-Film-Transistor (TFT)-based large area medical imaging system.

Evaluation of 10MeV proton irradiation on 5.5 Mpixel scientific CMOS image sensor

We evaluate the effects of 10 MeV proton irradiation on the performance of a 5.5 Mpixel scientific grade CMOS image sensor based on a 5T pixel architecture with pinned photodiode and transfer gate. The sensor has on-chip dual column level amplifiers and 11-bit single slope analog to digital converters (ADC) for high speed readout and wide dynamic range. The operation of the sensor is programmable and controlled by on-chip digital control modules. Since the image sensor features two identical halves capable of operating independently, we used a mask to expose only one half of the sensor to the proton beam, leaving the other half intact to serve as a reference. In addition, the pixel array and the digital logic control section were irradiated separately, at dose rates varying from 4 rad/s to 367 rad/s, for a total accumulated dose of 146 krad(Si) to assess the radiation effects on these key components of the image sensor. We report the resulting damage effects on the performance of the sensor including increase in dark current, temporal noise, dark spikes, transient effects and latch-up. The dark signal increased by about 55 e-/pixel after exposure to 14 krad (Si) and the dark noise increased from about 2.75e- to 6.5e-. While the number of hot pixels increased by 6 percent and the dark signal non uniformity degraded, no catastrophic failure mechanisms were observed during the tests, and the sensor did not suffer from functional failures.

A CMOS-based Large-Area-High-Resolution Imaging System for High-Energy X-Ray Applications

CCDs have been the primary sensor in imaging systems for x-ray diffraction and imaging applications in recent years. CCDs have met the fundamental requirements of low noise, high-sensitivity, high dynamic range and spatial resolution necessary for these scientific applications. State-of-the-art CMOS image sensor (CIS) technology has experienced dramatic improvements recently and their performance is rivaling or surpassing that of most CCDs. The advancement of CIS technology is at an ever-accelerating pace and is driven by the multi-billion dollar consumer market. There are several advantages of CIS over traditional CCDs and other solid-state imaging devices; they include low power, high-speed operation, system-on-chip integration and lower manufacturing costs. The combination of superior imaging performance and system advantages makes CIS a good candidate for high-sensitivity imaging system development. This paper will describe a 1344 x 1212 CIS imaging system with a 19.5μm pitch optimized for x-ray scattering studies at high-energies. Fundamental metrics of linearity, dynamic range, spatial resolution, conversion gain, sensitivity are estimated. The Detective Quantum Efficiency (DQE) is also estimated. Representative x-ray diffraction images are presented. Diffraction images are compared against a CCD-based imaging system.

A simple edge device method for determining the presampling modulation transfer function (MTF) of flat field digital detectors

The presampling modulation transfer function (MTF) of a digital imaging system is commonly determined by measuring the system’s line spread function (LSF) using a narrow slit or differentiating the detector’s edge spread function (ESF) with an edge device. The slit method requires precise fabrication and alignment of a slit as well as a high radiation exposure. The edge method [3] is a complicated image processing procedure, requiring determination of the edge angle, reprojection, sub-binning, smoothing and differentiating the ESF, and spectral estimation. In this paper, a simple method is employed to evaluate the MTF using an edge device. The image processing procedures required by this method involve simply the determination of the over-sampling rate and the Fourier transform of the modified ESF. Differentiation and signal to noise ratio (SNR) improvement are jointly applied in the Fourier domain. The MTFs obtained by this simple method are compared to the theoretical MTF and the previously proposed more complicated edge method. The experimental results show that the proposed method provides a simple, accurate and convenient measurement of the presampling MTF for digital imaging systems.

Filtered-gain calibration and its effect on frequency-dependent DQE and image quality in se-based general radiography and full-field mammographic digital imaging

This paper will describe details of and results for a frequency-dependent filtered gain calibration technique that optimizes DQE, yet does not reduce MTF performance which is important to both systems.

CMOS digital intra-oral sensor for x-ray radiography

In this paper, we present a CMOS digital intra-oral sensor for x-ray radiography. The sensor system consists of a custom CMOS imager, custom scintillator/fiber optics plate, camera timing and digital control electronics, and direct USB communication. The CMOS imager contains 1700 x 1346 pixels. The pixel size is 19.5um x 19.5um. The imager was fabricated with a 0.18um CMOS imaging process. The sensor and CMOS imager design features chamfered corners for patient comfort. All camera functions were integrated within the sensor housing and a standard USB cable was used to directly connect the intra-oral sensor to the host computer. The sensor demonstrated wide dynamic range from 5uGy to 1300uGy and high image quality with a SNR of greater than 160 at 400uGy dose. The sensor has a spatial resolution more than 20 lp/mm.