Radiography is an imaging technique that produces high-quality anatomic images by using x-rays. General radiography is currently a major part of hospital imaging departments and includes abdomen, chest and extremity examinations.
The use of digital radiography has rapidly increased in recent years. Computed radiography provided a cost-effective transition mode from the traditional film (from the year 1895) to the direct digital radiography (DDR), by using conventional x-ray equipment. Direct digital radiography is a cassette-less imaging system and is ideal for applications where high throughput is of primary importance. The direct digital radiography system should allow usage of all general radiography diagnostic applications.
The major components of a digital radiography system are as follows:
1. X-ray generator
2. X-ray assembly
3. Table trolley or other device to support the patient
4. Support for the x-ray tube assembly
5. Detector which converts the x-rays to an image
6. Acquisition workstation to process and display the image
There is a variety of technologies on which the direct digital radiography is based:
1) Indirect conversion detector: x-rays are converted to light scintillations and light is converted to electric signals.
2) Direct Conversion Detector: x-rays are directly converted to electric signals.
3) Linear Scanning Detectors: A fan beam of x-ray scans the examined area synchronously with a slot of detectors.
Due to the structure of the detectors, indirect and direct x-ray conversion detectors are frequently referred to as flat panel detectors (FPD’s). There are also portable digital cassettes available, either sold as part of a system or can be retrofitted to an existing CR or film/screen room. Portable detectors can be used in conjunction with an x-ray mobile unit. Such detectors can be connected to a review work station by wire or via a radio link.
Most of the digital detectors will need some level of environmental control. This may be in terms of operating temperature range, rate of exchange of temperature and/ or relative humidity.
As the original image from the detector is likely to be unsuitable for operating; image processing needs to be applied. A flat field correction is usually applied to the raw image to account for variations in the detector sensitivity across its full area. Also, a number of individual pixels may be defective.
The majority of direct digital radiography units are provided with automatic exposure control (AEC) to provide the selected dose to the detector. This may use a conventional AEC detector or the actual image detector to determine the correct dose level. It is essential that the AEC operates in a reliable and consistent manner and that it is correctly set up for the detector of the exposure.
Optimization is the process of identifying the necessary radiation dose level to provide adequate clinical information for a particular examination. Optimization depends on a range of both clinical and technical factors.
In modern digital radiography systems there is an inbuilt detector dose indicator. The detector dose indicator (DDI) gives an indication of the level of radiation exposure received by the detector. This is useful for monitoring that the exposure is in the correct range for optimal image quality and for undertaking QA.