|NDT.net - December 2002, Vol. 7 No.12|
The General Electric (GE) RevolutionTM family of a-Si flat panel digital x-ray detectors demonstrate outstanding sensitivity and resolution. The DXR-500 version, with 100- micron pixel pitch, has sufficient resolution to provide MTF over 20% at 5 lp/mm in a variety of industrial digital radiography applications. This detector exhibits linear performance over a wide range of exposure, from much less than 1 mR to over 60 mR incident dose at 80 kV with 0.32 cm of Al beam filtration. Noise performance is dominated by photon counting statistics, allowing effective frame averaging in challenging applications. The available image contrast from the new detectors depends on the x-ray energy selected, the dose captured, and the number of frames averaged. In practice, the desired image quality is balanced against the throughput requirements of the application. However, the total exposure time required with the detector typically is much less than that employed for industrial film applications, and can be as little as 10–20 % that of the film exposure while maintaining comparable or improved image quality. Thus, in film replacement applications, the dose efficiency of the detectors allows for extremely short exposure times and correspondingly high throughput in high-volume applications. This efficiency makes microfocus radiography practical when higher spatial resolution is required. These features enable inspection systems using these devices to provide excellent return on investment rates and short payback periods in a wide range of industrial applications.
Over the past decade, GE has invested over $150MM to develop the RevolutionTM family of flat panel digital x-ray detectors for medical applications such as mammography, angiography, and digital chest x-rays. GE Medical Systems has introduced a variety of products based on this technology. Key detector performance attributes required for success in these medical applications include outstanding dose efficiency and fine detail contrast. These attributes provide an excellent basis for corresponding performance in a variety of industrial radiography applications.
This development effort has also resulted in the availability of a new industrial detector, currently being introduced by GE Inspection Technologies. The DXR-500 is suited to a broad spectrum of industrial applications. Characteristics are shown in Table 1.
The GE DXR-500 detector is mounted in a rigid cast housing, and uses an external heat exchanger to provide thermal stability, allowing response calibrations to be performed only very infrequently and enabling effective utilization of fast, low-noise readout electronics.
|DXR-500||100||2304 X 1920||23 X 19||300||14/16384|
|Table 1: Physical characteristics of GE industrial flat panel detector.|
|Fig 1: The GE DXR-500 detector was adapted for industrial use from a high- performance medical device currently installed in hospital digital mammography systems worldwide. The detector (shown at upper left) provides useful resolution to at least 5 lp/mm without geometric magnification (upper right). The image at bottom of a standard test object demonstrates the detector’s large field of view and sensitivity, and shows the ability to detect flaws smaller than 0.005” diameter in realistic inspection conditions.|
A variety of detailed performance characterization measurements have been performed under a set of typical industrial radiography conditions for the DXR-500 detector. These include linearity, dose sensitivity, spatial resolution, and contrast sensitivity. Earlier results (1) showed that this detector provided enhanced probability of detection for small flaws in casting inspection applications.
Linearity of detector response is a key factor in producing high-quality digital radiographic images. Unlike conventional x-ray film, all flat panel detectors exhibit some degree of spatial inhomogeneity in their x-ray response. In particular, there is typically some slight variation in pixel-to-pixel sensitivity. To produce “smooth” images, these variations must be measured and a correction procedure applied. This correction is commonly referred to as “flat-fielding” or normalization, and is typically accomplished by applying a linear transformation on a pixel-by-pixel basis to the raw image data, using offset and gain calibration data.
In order for the normalization procedure to work over a wide range of exposure conditions, the detector’s basic response needs to be extremely linear over the detector’s useful dynamic range. Linearity was characterized by illuminating the DXR-500 detector with an industrial x-ray source. A series of offset corrected images was acquired at each dose level, and the mean signal was calculated over a small region in the center of the detector. The images were collected at a source-detector distance of 58 cm, using a Kevex KM16010-EA microfocus tube operating at 80 kV, with an additional beam filter consisting of 0.32 cm aluminum placed adjacent to the beryllium tube exit window. A Radcal Corp. model 9015 dose meter with model 10X5-180 probe was used to measure incident dose on the detector, by placing it approximately midway between source and detector, then using a inverse-square correction factor to calculate the incident dose at various mA settings. As shown in Fig. 2, the GE DXR-500 detector exhibits extremely linear behavior over its full operating range, with R2 > 0.999 from a linear regression fit.
|Fig 2: The GE DXR-500 detector exhibits extremely linear behavior (R2 > 0.999) up to an incident dose of approximately 60 mR (for a common industrial beam spectrum). Its efficiency at low doses allows for high quality imaging with exposure times much faster than required for typical industrial film radiography applications.|
Every x-ray detection system, whether film or digital, has a useable dynamic range. On the lower end, detector noise or sensitivity sets a minimum dose threshold below which no useful information can be discerned. At the upper end, saturation limits the maximum effective dose. A wider dynamic range thus allows for a greater number of x-ray photons to be detected with less added noise, resulting in higher signal-to-noise levels in each exposure. Utilizing doses outside of the useable range typically produces unacceptable images, even with film.
Dynamic range was characterized using a method that relates the detector performance to the fundamental statistical limit for photon noise (2). Because the Poisson distribution governs the temporal distribution of photon statistics, the variance of the measured signal for a particular pixel is anticipated to be proportional to mean signal (3). A series of uncorrected images was acquired at various dose levels, and the temporal variance was calculated (as the square of the standard deviation) on a pixel-by-pixel basis As shown in Fig. 3, the GE DXR-500 detector exhibits photon-limited behavior across its useful dynamic range (over 10000:1) from offset to saturation with minimal additive noise.
|Fig 3: The GE DXR-500 detector exhibits response noise behavior characteristic of fundamental photon counting statistics over a dynamic range greater than 10000:1.|
Resolution was characterized using a thin, square tungsten shim placed at a slight angle (~5°) with respect to the principal axes of the detector, and series of corrected images was acquired at 90 kV. Following the standard recipe, edge-response functions along each edge were Fourier transformed and averaged to derive the Modulation Transfer Function (MTF) of the detector. For the 100-micron pixel pitch of the detector, the Nyquist frequency is 5.0 lp/mm. The resulting MTF (Fig. 4) indicates excellent resolution for imaging of small features, with significant modulation (over 20%) at 5.0 lp/mm.
|Fig 4: The measured MTF of the GE DXR-500 detector remains over 20% even at the Nyquist frequency of 5.0 lp/mm, providing the ability to effectively image small features in industrial radiography applications.|
The DXR-500 is capable of detecting a variety of defects in common industrial nondestructive testing (NDT) applications. Examples of standard (Fig. 5) and digitally processed (Fig. 6) images are shown below.
|Fig 5: Tube Weld Inspection Performance. This digital radiograph of a thin-wall weld test specimen clearly shows weld flaw indications, including some less than 0.005” in diameter. The detection perfomance can be improved or detuned to optimize throughput and minimum detectable indication size of the GE DXR-500, depending on application requirements.|
|Fig 6: This edge-enhanced digital radiograph of a thin-walled weld test specimen from a DXR-500 detector clearly shows a variety of weld flaw indications. Less than 8.5% of the available image area is shown here.|
GE recently installed an automated system (Fig. 7) for x-ray inspection of turbine blade weld repairs, developed to effectively utilize the performance of the DXR-500 detector. This system is uses a robotic manipulator to position the parts in the x-ray beam above the detector. This inspection system, shown in Fig. 7, rapidly acquires digital radiographs at multiple orientations. The resulting images are automatically transferred to a review station in a modified DICOM-based format using standard DICOM network protocols to ensure reliability. This system reduces x-ray inspection cycle times by 50% over the previous film-based inspection process, and eliminates over $30,000 in film, chemical, and disposal costs per year.
Fig 7: GE DXR-500 system for turbine engine airfoil inspection. Parts are loaded
by the operator through a automatic sliding door |
Because of their sensitivity, linearity, low noise, and dynamic range, the GE a-Si family of detectors provides the basis for fast, high-performance digital radiography in a variety of industrial non-destructive testing applications.
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