Projection x-ray images

Radiography, fluoroscopy and fluorography

The evolution of radiology during the past two decades has been tremendous, not least due to the introduction of computed tomography (CT) and ultrasonography (US) in the seventies, and magnetic resonance imaging (MRI) in the eighties. These new modalities all provide sectional images, i.e. two-dimensional displays of tissue slices. However, the majority of examinations performed at most radiology departments still produce traditional projection images.

The techniques used in projection X-ray imaging may be divided into three main groups: 1) direct analogue techniques, 2) indirect analogue techniques, and 3) digital techniques.

Direct, analogue techniques

With these techniques, the final X-ray image is created directly on a detector medium, i.e., without any complicating intermediate steps. The medium may be a radiographic film or a fluorescent screen. The film and the screen are both analogue detectors of X-rays, which means that their response to a steady and continuous increase in radiation dose, is also steady and continuous, as opposed to stepwise. The radiographic film responds with blackening, the fluorescent screen by emitting visible light (fluorescence).

The two main direct, analogue techniques are: a) direct radiography, and b) direct fluoroscopy.

Direct radiography

This is the original, traditional means of radiography where the X-rays, after having passed through the patient, create an image directly on a photographic film (Fig. 1).

The film is covered, usually on both sides, by a photographic emulsion. The emulsion consists of a layer of gelatine containing tiny silver bromide crystals. (Average linear dimension is approx. 1 m.) The emulsion is sensitive to photons having a wide range of energies; X-rays, ultraviolet radiation, and visible light may all blacken the film. The silver bromide crystals are ionised by the photon energy. The number of silver ions (Ag+) thus created, varies with the number of photons transmitted to the film; the higher the radiation dose, the higher the number of silver ions. The varying density of the silver ions creates a latent image within the emulsion, the images only become visible after treatment with a liquid developer. When the film is developed, black metallic silver is precipitated from those crystals containing silver ions. The non-ionised silver bromide crystals remain unchanged and invisible. After being developed, the film is washed, fixed, and dried. The fixative removes the silver bromide crystals, leaving the metallic silver behind, thus making the film insensitive to light.

In this way, the visible image on the radiographic film is related to varying degrees of blackening, caused by the varying density of the microscopic black silver granules. The darkest areas in the images have been subjected to the highest radiation intensity or dose, and the image is thus a so-called negative.

One might believe that the blackening of the radiographic film is caused solely by irradiation with X-rays, and indeed this was the case in the earliest days of radiography. Today however, the film is nearly always placed within a cassette between two fluorescent screens, so-called intensifying screens. The screens are effective absorbers of X-ray photons (photo-electric absorption). In the process, light photons are emitted, and these photons are the main cause of blackening of the film. The response of the screens to X-rays is linear; a certain increase in radiation dose is followed by a corresponding increase in emitted light intensity.

The radiographic cassette is 10-1,000 times as effective as film alone, and the use of cassettes therefore allows a considerable reduction in radiation dose. The cassette protects the film from external light and makes it possible to undertake the radiographic exposure in daylight and then bring the film to the darkroom for development. The cassette may also contain a grid to reduce secondary or scattered radiation to the film, and the back of the cassette may have a layer of lead to stop the X -ray photons.

There are different kinds of intensifying screens and radiographic films. Some screens emit blue light (calcium tungstate and some lanthanides), and others emit green light (various lanthanides). Correspondingly, there are films that are especially sensitive to blue light or green light, respectively. The newer lanthanide screens may be 3-5 times as sensitive to radiation as the older calcium tungstate screens, which means less dose to the patient in otherwise comparable circumstances.

A film-screen combination has a characteristic curve, showing the variation of the blackening (density) of the photographic emulsion with exposure (Fig. 2). In radiography, the structures of interest should lie within the middle, linear part of the curve. Here, the contrast amplifying effect of the film has its maximum. The slope of the linear part of the curve is called the gamma, and film-screen combinations having a large gamma will yield high-contrast images. Parameters such as sensitivity, spatial resolution, and noise are to a large extent determined by the  

intensifying screens. A thick screen containing large fluorescent crystals is fast (i.e., has a short exposure time) due to its high sensitivity. The advantage to the patient is low radiation dose; the disadvantages are low spatial resolution and high noise. Slow screens are thinner with smaller fluorescent crystals that give higher spatial resolution and less noise, the radiation dose needed is higher, however. When low patient dose is important, as in imaging pregnant women, fast intensifying screens should be used.

Direct radiography using radiographic cassettes is still quantitatively the most important radiological modality. The technique is also called full-size radiography because the anatomy is shown in its original size, with some added geometric enlargement. The technique provides static images with the highest spatial resolution of all radiological modalities. Full-size radiography is too slow for movie-like (cine) displays; to give the impression of smooth motion, at least 25 images per second are needed. With the use of so-called serial changers it is possible, however, to perform full-size radiography with an exposure frequency of e.g. 2 images per second. Serial changers may be compared to the automatic film advance of modern cameras. The film is drawn into the primary radiation beam for exposure, and then transported into a receiving cassette for later development. Full-size serial radiography is used in angiography (contrast-enhanced X -ray imaging of blood vessels) to follow the flow of contrast medium in the vascular tree.

The projection image provided by direct analogue radiography contains all radiopaque structures of the three-dimensional object being imaged. Obtaining several projection views of the object (e.g. frontal, lateral, oblique), shows the spatial relationships of the various structures to better advantage, and improves visualisation of the anatomy. Traditional radiography may, however, also provide "sectional" images. The technique is called (traditional) tomography and involves movement of the X-ray tube and film in such a way that only a thin plane through the patient, parallel to the film, is imaged sharply. Structures located in planes other than that being examined (closer or more distant to the film) are subjected to blurring due to gross movement unsharpness (Fig. 3). Traditional tomography is fundamentally different from the more modern tomographic and cross-sectional imaging modalities, such as US, CT, MRI, SPECT, and PET (see later). The sectional images provided by these new modalities contain information from thin slices of tissue only. The traditional tomographic image, on the other hand, also contains blurred information from all tissues above and below the sharply imaged plane. The importance and use of traditional tomography has greatly diminished with the introduction of the new imaging modalities.

Direct fluoroscopy

Traditional fluoroscopy or screening, common in clinical practice until the mid-1960s is now obsolete. The transmitted X -ray beam fell on a fluorescent screen, resulting in a dynamic projection light image. The image could be observed directly by the radiologist, who was protected from transmitted X-rays by a sheet of lead glass. The technique was especially used to study physiological movements such as swallowing, respiration and cardiac contractions. To keep the exposure rate to the patient at tolerable levels (levels today considered too high), the screen luminance was extremely low, in fact so low that approximately 15 minutes of dark-adaptation was needed by the radiologist prior to fluoroscopy. Traditional direct fluoroscopy has long since been replaced by indirect fluoroscopy employing X-ray image intensifiers and TV-technique.

Indirect, analogue techniques

In modern fluoroscopy, the primary projection image is created on a fluorescent screen, quite similar to the direct techniques. The screen image, however, is not observed directly. The screen is part of an X-ray image intensifier that enhances the brightness (luminance) of the primary image by a factor of about 5,000 (Fig. 4).

The minified and intensified image that emerges from the intensifier, may be recorded via lenses by a TV camera and shown on a monitor. The image may also be reflected by a mirror to a small-film still camera (70 mm, 100 mm or 105 mm film format), or cine camera (16 mm or 35 mm film format) (Fig. 4). Filming with a still camera, so-called spot filming, is also called fluorography, and the spot film itself a fluorogram. The patient dose in fluorography is about 1/10 the dose in full-size phy; the image quality, however, (especially spatial resolution) is markedly inferior. Cine-fluorography provides mo vie-like images with a time resolution of e.g. 50 images per second. Cine-fluorography using 35 mm film is still commonly used in angiographic studies of the heart and coronary arteries (although digital techniques are gradually replacing the analogue ones). The cine-film is usually shown projected on a screen by a movie projector.

Digital techniques

All radiological techniques and modalities are analogue at the starting point of imaging. The light intensity in a fluorescent screen, the electric current induced by X-rays in the CT detector, by the echo in the ultrasound transducer, or by the magnetism in the MR receiver coil, are all analogue, continuous responses. The last three modalities, computed tomography (CT), ultrasonography (US) and magnetic resonance (MR) imaging, are still considered digital techniques because the analogue response (the electric current) is digitised (given certain numerical values). Digital techniques may be applied in projection X-ray imaging as well, and the term digital radiography is commonly used in this restricted sense only.

A "true" digital image is composed of a digital matrix, i.e., rows and columns of numbers. The numbers may represent echo strength in an ultrasound image, X-ray attenuation in a CT image, tissue magnetism in an MR image or light intensity from a fluorescent screen in digital projection X-ray imaging. To visualise the image, the digital matrix is transformed into a matrix of visible picture elements, pixels, where each pixel is given a shade of grey according to the corresponding number in the digital matrix. (See the Digital radiography chapter.)

There are several ways to produce digital projection X-ray images. Following exposure to X-rays, special imaging plates retain a latent image of stored energy. By scanning the imaging plate with a laser beam, the energy is released as light or luminescence, where the light intensity is proportional to the absorbed dose of X-ray photons. The emitted light is recorded by a photo detector as analogue signals; the signals are digitised, and the image may be presented in a grey scale format on a monitor or hard copied by a laser printer.

An alternative digital technique is to digitise the analogue video signal coming from the TV camera in a X-ray image-intensifier-television system (Fig. 4). The digitised image may be displayed on a TV monitor (digital fluoroscopy), or it may be photographed by a small-film camera (digital fluorography). A variant of this technique is used in angiography for subtraction of images. In addition to displaying digital angiograms, the technique may also be used to subtract the data contained in an image having no radiographic contrast medium in the blood vessels from an image of the exact same area containing contrast medium within the blood vessels. The result is a selective and improved visualisation of the vessels; all other structures, e.g. bone, are more or less subtracted. This technique is called digital subtraction angiography (DSA).

The spatial resolution of digital images is inferior to full-size radiographs due to the relatively large pixel size. The digital format still has many advantages. Key words are digital archiving (on magnetic and optical disks), communication (via network, phone and satellite), and post-processing and manipulation (e.g. change of contrast, grey scale, and size; calculation of distance, area, and volume; measurement of pixel intensities).

Hans-Jørgen Smith