The Principle

In digital luminescence radiography, the film–screen system is replaced as a data-storage medium by a storage screen, which, for example, consists of a layer of phosphorus compound. Special cassettes with formats identical to those of conventional systems are required. The cassettes equipped with phosphorus storage screens can be exposed in conventional radiographic examination units, making the procedure economic. The radiographic absorption profile is saved as a temporary image by the activation of electrons to a higher energy level on the phosphorus screen. The read-out process involves scanning with a beam of a helium laser ( Fig. 2.1 ). The energy absorbed within the phosphorus crystals during x-ray exposure is released locally as luminescence according to the information in the virtual image. The distribution of the emitted light is stored as an electrical signal via photomultipliers. The digitally stored DLR images can be postprocessed on a computer work-station.

Technical Procedure

Storage screens have high sensitivity for x-rays and, in principle, enable radiation exposure to be reduced. Since the signal-to-noise ratio decreases with lower doses, reduction of the dose in diagnostic imaging of the hand is only possible to a limited degree. Dose reductions of about 15% can be achieved in follow-up radiography in traumatology and in pediatric radiography. Radiation quality and doses applied in digital luminescence radiography are generally comparable to those of the conventional film–screen technique.

Images that have been over- or underexposed in the acquisition process can usually be corrected during postprocessing, since the characteristic gradation curve is linear over a large range in storage-screen systems. The linearity over a large range results of faulty exposure can thus be compensated.

Difficulties in interpretation in skeletal imaging can be caused by overshoot artifacts in the immediate vicinity of materials used for osteosynthesis. Extinction phenomena, which in the worst case can mimic loosening of material or infection, may arise with metallic implants with low image resolution.

Indications

Given its many advantages, digital luminescence radiography with storage screens is currently the favored imaging procedure in projection radiography of the musculoskeletal system. K2 systems with an image matrix of 2048×2048 are used for diagnosis of the hand.

The Principle

In addition to the phosphorus storage-screen technique, projection radiography with electronic flat detectors (semiconductors and flat-panel detectors) provides a further digital procedure for projection radiography. These systems employ either amorphous silicon or selenium as the detector. In silicon or selenium detectors, information about the absorbed x-ray profile is recorded directly, electronically enhanced, and transferred to an analog-to-digital (A/D) converter ( Fig. 2.2 ). Since an intermediate step of optical, electronic, or mechanical scanning is unnecessary, this technology is referred to as direct radiography. Direct radiography has superior quantum efficiency to other imaging systems, resulting in lower image noise at the same radiation dose. This improved quantum efficacy can potentially be used to reduce the radiation dose.

In a further application, real-time images that enable digital fluoroscopy to be performed can be generated with the selenium and silicon detectors because of their active matrix.

Technical Procedure

Positioning methods and x-ray exposures in direct radiography with flat detectors are the same as for conventional projection radiography. The exposure values are equivalent to the common kV and mAs values over wide ranges. After exposure on the flat-panel detector system, the digital image information is immediately available for postprocessing and channeling into a PACS environment. In routine use, the processing is rapid because of the absence of cassettes is an advantage. Static integration of the system into the Bucky table or the use of a wall-mounted cassette holder introduces limitations, however, since both prevent mobile use with the use of individually placed cassettes.

Indications

The advantages of improved image quality, exposure without cassettes, and digital image acquisition and archiving make flat-panel direct radiography the imaging technique of the future. The flat-panel technique is not widely employed at present because the complex production techniques for the crystals result in high investment cost.

The Principle

In digital image-intensifying radiography, image recording is performed with a CCD (charge-coupled device) camera, which is integrated into the image-intensifying unit of a fluoroscopy unit as a recording system. The prototype of digital image-intensifying radiography is digital subtraction angiography (DSA). The resultant image is immediately available on the monitor screen. The digital image acquisition makes postprocessing possible (e.g., mask subtraction, pixel shift, edge enhancement, adjustment of contrast and brightness).

Technical Procedure

Fluoroscopy of the hand is achieved with spot films or angiography series. For high-contrast musculoskeletal imaging (plain osseous and arthrographic series), a matrix size of 1024×1024 with a spatial resolution of 1.8–2.0 Lp/mm is generally sufficient. For minute bony lesions, an image matrix of 2048×2048 is desirable. Digital image-intensifying radiography makes it possible to reduce radiation dose, but this desirable effect in radiation hygiene is achieved at the cost of a reduced signal-to-noise ratio and decreased detail recognition.

Indications

Besides digital subtraction angiography, digital image intensifying radiography is used for fluoroscopic analysis and documentation. Despite reduced spatial resolution, arteriography of the hand (Chapters 5 and 48), movement analysis of the wrist, and three-compartment arthrography Fig. 3.2 can be comfortably performed and documented.

The Principle

Survey views of the entire hand are performed with a film–screen combination providing a sensitivity value of 200 in traumatology. To achieve greater detail resolution, a less-enhancing mammography film–screen combination with a sensitivity value of 25 can be used. At a tube focus size of 0.6 mm, the spatial resolution is 6–8 Lp/mm.

Technical Procedure

The mammography films, contained in special cassettes, are exposed on the Bucky table with a suspended above-table unit (FFD 105cm) without the use of a scattered-radiation grid. Typical exposure values are 55 kV and 6.4mAs.

Indications

Because of their higher radiation dose requirements than those for standard films (up to eightfold greater), mammography film–screen combinations should be employed only when strictly indicated for the diagnostic purposes listed in Table 2.1 .

The Principle

The X-ray focus of mammography equipment has a size of 0.1–0.4mm. The object should be enlarged by at least a factor of 1.5 in the image plane. If the enlargements are made at a focus–film distance of 0.65m and an object–focus distance of 0.34m, the result is an enlargement factor of 2. Skeletal radiographs for investigation of the fine structure of the trabecular bone are performed with an aluminum filter. The conventional molybdenum filter is used to investigate abnormalities of the soft tissues.