Results of an edge detection algorithm and pseudo-color rendering.

Radiography has been around for a long time in marine geology, but up until recently it has involved film. The spatial and depth (gray scale) resolution of film is great, but it has many draw backs, not least of which involves its somewhat inconvenient viewing modality. That is, there must be a better way than holding the radiograph up to a fluorescent light... To improve on film based radiography, and especially to allow easy post-processing of x-radiographs (shown above), we tried for many years to develop a digital x-radiography system. Finally, through support from a DURIP award, we were able to put together a state-of-the-art system.

There are two main elements to our digital x-radiography system: an x-ray source and a detector (see below). The x-ray source is a Lorad LPX-160 industrial x-ray generator that comprises a tube head, a liquid cooling unit and a control unit. The tube head is an end grounded, exposed anode configuration with a thin beryllium window, and is capable of producing x-ray energies up to 160 kV (1-kV increments) with a tube current up to 5 mA (0.1-mA increments). An important characteristic of the x-ray source is that the voltage and line current during an exposure are stable to within ± 1%, thus the cumulative x-ray flux of sequential exposures is steady. This stability is demonstrated by the extremely small variability in brightness of a calibration module (~ 1% C.V.).

The most important component of the system is the x-ray detector: a dpiX Flashscan 30 imager. This unit is a high-resolution amorphous silicon (a-Si) detector that utilizes a Kodak Lanex Fine® phosphor screen coupled to an array of a-Si photodiodes that detect visible light and thin-film transistors that connect the photodiodes to the readout electronics. Salient characteristics of the imager are a 29.3 by 40.6 cm (WxH) active area, a pixel spot size of 127 mm resulting in an image size of 2,304 by 3,200 pixels, and a 12-bit (i.e., 4096 gray levels) depth resolution. The resultant digital x-radiographs are indistinguishable from those collected on x-ray film, yet they are acquired in near real time and are immediately available for post-acquisition image processing.

Ancillary equipment associated with the system includes a computer running Windows NT, a 14-mm thick lead sheet that limits the beam to the imager’s active area and a lead-lined x-ray cabinet with safety interlocks that houses the tube head and the detector. Both the x-ray source and the imager are controlled remotely, thereby facilitating shipboard use of the system. A typical imaging sequence consists of exposing the detector at 70 kV and 5 mA for a total of 28 s. During the first ~5 s of the exposure the tube head is ramping up to the full voltage and tube current output, at +6 s the imager is actuated, which leads to a 6.4-s long period during which charges are accumulated. Three successive scans are accumulated and the average brightness of each pixel sent through a shielded RS-485 to the computer. A calibration module that consists of varying thicknesses of glass is present within each image.