THE IMAGING PLATE

The imaging plate is coated with a photostimulable phosphor (PSP), which captures X-rays once they have passed through the patient or object being imaged (Fig. 13.2). The PSP material has been ‘doped’ with small amounts of impurities which alter the physical properties of its crystalline structure.

Figure 13.2 Construction of the imaging plate showing the photostimulable phosphor (PSP) layer.

A PSP material has a high absorbance at the energies used for diagnostic radiology and is able to store the irradiated energy and then release it as visible light in response to the stimulation of a beam of laser light within the reader. The family of europium doped barium fluorohalides fits these criteria – BaFX, where X can be chlorine (Cl), bromine (Br) or iodine (I).

As the X-rays pass through the PSP material, they interact with the electrons in the crystalline structure, giving them energy, which enables them to enter the conduction band. Some electrons return immediately to the valence band, but others remain ‘trapped’ in the forbidden zone between the two bands (Fig. 13.3). This trapped signal is proportional to the amount of radiation incident on the plate; these electrons form the latent image in the PSP.

Figure 13.3 Schematic sequence of images of conduction and valence bands showing ‘trapped’ electrons.

RETRIEVING THE LATENT IMAGE

To retrieve the latent image from the PSP it is placed in a CR reader, where a laser scans it (Fig. 13.4). The laser gives the electrons enough energy to return and leave the traps and to decay down to the ground or valence state. As these electrons move down in energy a blue light is emitted. This light is collected by a light guide and directed towards the photomultiplier tube. The light moves through the photomultiplier, is then amplified and then the signal is digitised using an analogue-to-digital converter, allowing the temporary storage of the image in digital format. This can then be sent to a monitor for viewing or to a printer.

Figure 13.4 Schematic of a computed radiography (CR) plate reader.

ERASURE OF THE PLATE

Unfortunately the read out with the laser does not clear all the trapped electrons, as there are traps at many different energy levels that do not respond to the energy of the laser light beam. The remaining electrons are removed using a bright white light, which has enough energy to pass through the PSP and all the traps will respond. This removes any residual signal from any previous images. It should also be exposed to this bright light, a ‘primary’ erase, to prevent build-up of background signals over time.

CARE OF THE IMAGING PLATE AND CASSETTE

All manufacturers have a suggested cleaning and maintenance procedure and it is important to follow their guidance and use the recommended products in order not to invalidate any warranty or guarantee.

CR plates should be checked regularly for damage, as part of a departmental quality assurance programme, and removed from use or repaired if damage is found. The imaging plates also need regular checks because they are removed from the cassette each time they are read – edges tend to get damaged, cracks can appear and they can be vulnerable to scratches. Once again any damaged plates must be removed from service. There have been reported problems with cassette catches opening and the plates dropping out, so these must be checked and cleaned before being returned to service. Imaging plates have an estimated life of 10 000 exposures (manufacturer dependent), so it is important to make sure cassettes are rotated and used evenly throughout a department.

With CR the detector is separate from the imaging device and incorporates extra stages to the production of an image, digital radiography removes the need for a cassette and a reader from this process.

IMAGING THE PATIENT USING CR

The patient is registered to each imaging plate used by means of an individual barcode on each plate, using the patients’ hospital number. X-rays are then taken, the operator inputs into the CR reader the views that have been taken (e.g. chest X-ray (CXR), abdominal X-ray (AXR)) and the plates are then placed in the CR reader, which picks up the barcode and allocates images to the correct patient, along with the correct protocol for image processing (Fig. 13.5).

Figure 13.5 Computed radiography (CR) reader for single cassette use.

The CR reader then removes the plate from the cassette and this is read out as previously described. The image is subsequently viewed on an acquisition workstation where it is cropped and markers applied (i.e. L or R, PA, AP, etc.). The image can be checked for quality and dose indicators, such as sensitivity or exposure index (manufacturer dependent). Once the radiographer is satisfied that the image is correctly identified, manipulated and annotated, it is then sent on to the network for reporting and storage for retrieval at a later date from anywhere within the network (Fig. 13.6).

Figure 13.6 The sequence of events during a diagnostic examination using computed radiography.

INDIRECT DIGITAL RADIOGRAPHY

Indirect digital radiography (IDR) uses a phosphor (caesium iodine, CsI), which is coated over an active matrix array (AMA) of amorphous silicon (a-Si) (Fig. 13.7).

Figure 13.7 Amorphous silicon detector.

DIRECT DIGITAL RADIOGRAPHY

Direct digital radiography (DDR) does not use a phosphor material; the X-ray photons are directly converted into an electrical charge. The detector material used is amorphous selenium (a-Se) which is coated on a thin-film-transistor (TFT) array (Fig. 13.8).

Figure 13.8 Amorphous selenium detector.

A bias voltage is applied across the detector structure and as the X-rays pass through they directly generate positive and negative charges which are in proportion to the level of X-ray exposure. The positive charge is drawn to the capacitor where it is stored and readout by the customised electronics within the array. The imaging plate is made up from millions of these detectors causing variations between them. Software is available to smooth out the differences between the detectors on the plate. This smoothing process is known as ‘stitching’. As with CR, the patient images are checked, annotated and sent onto the computer network for reporting and storage.

IMAGE DISPLAY

As with the image production, the viewing display must be seen as the final part of this process for the reporting workstations. These displays must be able to manage large data files, be able to show the full resolution and greyscale levels of the image and have a fast response to manipulation of the images. For subsequent image reviewing the high quality is not so important so standard PC monitors are acceptable.

Viewing conditions must be optimised where possible for reporting workstations: they must be in an area of low ambient light, with little or no reflection off the screen from other light sources. They should be part of a quality assurance programme to ensure viewing conditions remain satisfactory throughout the monitor’s lifespan.

The reporting monitors must be checked daily using a recognised test pattern such as the SMPTE test pattern (Fig. 13.10). There should be checks for distortion and centring, by verifying that all 11 luminance patches are visible. It should be checked that the 5% and 95% patches can be seen and the visibility of the high and low contrast line pair patterns should be assessed.

Figure 13.10 SMPTE test pattern.