Image processing is very important in digital radiograph as it influences the way the image appears and involves noise reduction with enhanced signal-to-noise ratio.

Dr TBS Buxi, Head, Department of CT Scan and MRI, Sir Ganga Ram Hospital

Digital radiography is the new age radiography with filmless technology using digital detectors and various readout techniques to make it possible to fully digitise radiographic data and use PACS for image acquisition, interpretation, distribution and storage. From the days of Kruger in 1977, when the first digital subtraction angiography was performed, to the introduction of cassette based storage phosphor image plates, it took a long time for the first DR systems to appear with CCD slot scan system. The progress was then fast with the introduction of selenium drum, then the flat panel detector systems with amorphous silicon, amorphous selenium and finally the gadolinium oxide sulphide scintillators which made it possible for digital dynamic flat panel detectors for digital fluoroscopy and angiograpy.

CR systems use storage-phosphor image plates with a separate image readout process while DR systems use systems to convert x-rays into electrical charges by a direct readout process. In a CR system, the image plate is exposed to x-rays which is stored in the detective layer. This detective plate is scanned with a laser beam; stored energy is set free and light emitted is collected by photomultipliers and converted into electrical charges. Image processing is very important in digital radiograph as it influences the way the image appears and involves noise reduction with enhanced signal- to-noise ratio. The spatial resolution determines the quality of the images and this is proportional to the pixel size, image matrix and the detector size. The smaller the pixel size, the larger the matrix and better the image. Detective quantum efficiency (DQE) refers to the efficiency of a detector in converting incident x-ray energy into an image signal. The ideal detector would have a DQE of 1. As of now, the DQE of digital detectors is limited to about 0.45 at 0.5 cycles/mm.

New storage phosphors and scanning systems are being applied to revolutionise computed radiography. New phosphors have their crystals grown in a needle shape and are coated on a glass or aluminum substrate without any binding material between the crystals for tighter phosphor packing and reduced pixel size, resulting in DQE matching those for indirect conversion flat-panel detector systems . In addition, images are scanned line by line, resulting in shorter scanning times or the line scanners could scan for longer times and produce higher signal. The new technologies can reduce the radiation exposure by up to 50 percent while preserving the image quality. The new portable devices with flat panel detector systems especially in fluoroscopy will be more flexible and might even replace CR systems. Improvement in the DQE and signal-to-noise ratio of detectors may lead to further reduction of exposure or improvement in image quality. The architecture of the readout arrays could be optimised by reducing the size of the circuit and pixels.

The future of radiography is digital. With revolutionary advances digital radiography offers higher patient throughput and dose efficiency with reduction in radiation exposure, consistently reproducible images with higher diagnostic quality and interface with PACS and the world of teleradiology.

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