CR IMAGE (drt) in hindi

CR Image

 We have previously described the process of photostimulable luminescence which is exploited in Computed Radiography, where the absorption of radiation causes electrons to become trapped at intermediate energy levels. Phosphors having this property are therefore referred to as Storage Phosphors.

A latent X-ray image can be recorded using a plate coated with crystals of barium fluorohalide compounds which contain trace amounts of europium. The general form of these compounds is BaFlX:Eu, where X can be Cl, Br or I or a mixture thereof. Radiographic information is recorded by elevating electrons to traps in the energy gap and the number of filled traps is proportional to the amount of radiant energy absorbed.

 When stimulated by visible radiation of wavelength around 600 to 680 nm (i.e. red light), the crystals luminesce as the electrons return to their ground state in the range 300-500 nm (blue).

Laser Readout

Fig. 4.2: CR scanning plate.

The plate can be scanned in a raster pattern with a finely focused laser beam, e.g. he/ne and a scanning mirror - see Figure 4.2.

 The laser light stimulates the release of electrons from the traps giving rise to the emission of light which is collected by a light guide and fed to a photodetector, e.g. a photomultiplier tube.

 The signals generated by the photodetector as the plate is being scanned are amplified and digitized by an analogue-to-digital converter (ADC).

The image is in essence built up point by point and line by line to give a digital image resolution of up to 4k x 4k pixels. Pixel size is typically 0.1 μm. 

The imaging cycle is completed by flooding the plate with a high intensity sodium discharge lamp that erases any remnants of the latent image and essentially prepares the plate for reuse - see Figure 4.3.

Fig. 4.3: The CR imaging process.

The spatial resolution of computed radiography is influenced by factors such as the phosphor plate thickness, the readout time and the diameter of the laser beam, which is typically about 100 μm. Note that divergence through scattering of the laser light in the body of the phosphor layer will broaden the stimulated area to beyond this diameter.

Note that the nature of the CR imaging process limits its application to single-shot radiographic imaging. In clinical practice, the process is generally part of a workflow where patient radiographs are recorded as traditionally in film/screen radiography, but with images now generated with a latent image read-out device and automatically sent to a quality control workstation for image evaluation, annotation and transfer to a PACS for reporting.

Imaging Plate

Fig. 4.4: Implications of the finite width of laser beam and phosphor layer on the spatial resolution in CR.

The CR imaging plate is very similar to intensifying screens in its structure. It consists of tiny phosphor grains of about 5 μm embedded in an organic binder which is coated onto a substrate material.

 The plate is turbid and as a result scatters light (both excitation laser and stimulated light) strongly and isotropically - see Figure 4.4.

 Thus, the light diffusion limits the useful thickness of the phosphor layer.

 An improvement in X-ray absorption efficiency can indeed be obtained by increasing the thickness of the phosphor layer. However,

lateral diffusion of light in the phosphor layer will increase in proportion to the layer thickness, impairing spatial resolution; and

sensitivity will not increase all that much when the layer exceeds a certain thickness, because most of the light stimulated deep in the layer will not reach the surface for detection.

This sensitivity limitation can be overcome by making the substrate of the imaging plate from a transparent material and to detect the photo-stimulated luminescence from both the front and the back sides of the plate.

 This requires two light-collection systems, but only one laser beam.

 The improved sensitivity results since only ~30% of the photostimulated light is collected using the conventional single laser approach. In addition, sensitivity improvements have been achieved using a structured phosphor, e.g. CsBr:Eu - see the discussion below - as the imaging plate.

A fundamental limitation of CR is the time required to read the latent image.

 Since the decay time of the phosphor luminescence is ~0.7 μs typically, the readout of a 3,000x3,000 pixel image can take over half a minute to complete. 

An improvement can be obtained by line scanning, where a full line of pixels is stimulated and read out simultaneously instead of single pixels, as described above.

 This line-scanning approach requires a linear array of laser light sources, e.g. laser diodes, as well as a linear array of photodetectors as wide as the imaging plate, and gives rise to readout times of less than 10 seconds. Furthermore, the linear scanning mechanism can be built into the image receptor cassette.

Note that clinical radiography with CR plates has been found to generate a range of unique artefacts, which are the subject of a pictorial review in Cesar et al.

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