radiography Radiography is an imaging technology, imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical ("diagnostic" radiog ...

, focal plane tomography is

tomography Tomography is imaging by sections or sectioning that uses any kind of penetrating wave. The method is used in radiology, archaeology, biology, atmospheric science, geophysics, oceanography, plasma physics, materials science, cosmochemistry, ast ...

(imaging a single plane, or slice, of an object) by simultaneously moving the

X-ray generator An X-ray machine is a device that uses X-rays for a variety of applications including medicine, X-ray fluorescence, electronic assembly inspection, and measurement of material thickness in manufacturing operations. In medical applications, X-ra ...

and

X-ray detector X-ray detectors are devices used to measure the flux, spatial distribution, spectrum, and/or other properties of X-rays. Detectors can be divided into two major categories: imaging detectors (such as photographic plates and X-ray film (photograp ...

so as to keep a consistent exposure of only the plane of interest during image acquisition. This was the main method of obtaining tomographs in

medical imaging Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to revea ...

until the late-1970s. It has since been largely replaced by more advanced imaging techniques such as CT and

MRI Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and rad ...

. It remains in use today in a few specialized applications, such as for acquiring orthopantomographs of the jaw in

dental radiography Dental radiographs, commonly known as X-rays, are radiographs used to diagnose hidden dental structures, malignant or benign masses, bone loss, and cavities. A radiographic image is formed by a controlled burst of X-ray radiation which penetra ...

. Focal plane tomography’s development began in the 1930s as a means of reducing the problem of

superimposition Superimposition is the placement of one thing over another, typically so that both are still evident. Superimpositions are often related to the mathematical procedure of superposition. Audio Superimposition (SI) during sound recording and repro ...

of structures which is inherent to

projectional radiography Projectional radiography, also known as conventional radiography, is a form of radiography and medical imaging that produces two-dimensional images by X-ray radiation. The image acquisition is generally performed by radiographers, and the images a ...

. It was invented in parallel by, among others, by the French physician

Bocage Bocage (, ) is a terrain of mixed woodland and pasture characteristic of parts of northern France, southern England, Ireland, the Netherlands, northern Spain and northern Germany, in regions where pastoral farming is the dominant land use. ' ...

, the Italian radiologist Alessandro Vallebona and the Dutch radiologist Bernard George Ziedses des Plantes.

Technique

Focal plane tomography generally uses mechanical movement of an

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

source and film in unison to generate a tomogram using the principles of

projective geometry In mathematics, projective geometry is the study of geometric properties that are invariant with respect to projective transformations. This means that, compared to elementary Euclidean geometry, projective geometry has a different setting (''p ...

. Synchronizing the movement of the radiation source and detector which are situated in the opposite direction from each other causes structures which are not in the

focal plane In Gaussian optics, the cardinal points consist of three pairs of points located on the optical axis of a rotationally symmetric, focal, optical system. These are the '' focal points'', the principal points, and the nodal points; there are two ...

being studied to blur out.

Limitations

The blurring provided by focal plane tomography is only marginally effective, since it only occurs in the ''X'' plane. Moreover, since focal plane tomography uses plain X-rays, it is not particularly effective at resolving soft tissues. The increased availability and power of computers in the 1960s and 70s gave rise to new imaging techniques such as CT and MRI which use computational (in addition to or in lieu of mechanical) methods to acquire and process tomographic image data, and which do not suffer from the limitations of focal plane tomography.

Variants

Initially focal plane tomography used simple linear movements. The technique advanced through the mid-twentieth century however, steadily producing sharper images, and with a greater ability to vary the thickness of the cross-section being examined. This was achieved through the introduction of more complex, pluridirectional devices that can move in more than one plane and perform more effective blurring.

Linear tomography

This is the most basic form of conventional tomography. The

X-ray tube An X-ray tube is a vacuum tube that converts electrical input power into X-rays. The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. In contras ...

moved from point "A" to point "B" above the patient, while the detector (such as cassette holder or "bucky") moves simultaneously under the patient from point "B" to point "A". The

fulcrum A fulcrum (: fulcra or fulcrums) is the support about which a lever pivots. Fulcrum may also refer to: Companies and organizations * Fulcrum (Anglican think tank), a Church of England think tank * Fulcrum Press, a British publisher of poetry * Fu ...

, or pivot point, is set to the area of interest. In this manner, the points above and below the focal plane are blurred out, just as the background is blurred when panning a camera during exposure. Rarely used, and has largely been replaced by

computed tomography A computed tomography scan (CT scan), formerly called computed axial tomography scan (CAT scan), is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers or ...

(CT).

Poly tomography

This was achieved using a more advanced X-ray apparatus that allows for more sophisticated and continuous movements of the X-ray tube and film. With this technique, a number of complex synchronous geometrical movements could be programmed, such as hypocycloidic, circular, figure 8, and elliptical.

Philips Koninklijke Philips N.V. (), simply branded Philips, is a Dutch multinational health technology company that was founded in Eindhoven in 1891. Since 1997, its world headquarters have been situated in Amsterdam, though the Benelux headquarter ...

Medical Systems for example produced one such device called the 'Polytome'. This pluridirectional unit was still in use into the 1990s, as its resulting images for small or difficult physiology, such as the inner ear, were still difficult to image with CTs at that time. As the resolution of CT scanners got better, this procedure was taken over by CT.

Zonography

This is a variant of linear tomography, where a limited arc of movement is used, resulting in less blurring than linear tomography. It is still used in some centres for visualising the

kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...

during an intravenous urogram (IVU), though it too is being supplanted by CT.

Panoramic radiograph

Panoramic radiography is the only common tomographic examination still in use. This makes use of a complex movement to allow the radiographic examination of the

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

, as if it were a flat bone. It is commonly performed in dental practices and is often referred to as a "Panorex", though this is a trademark of a specific company and not a generic term.

References

External links

*{{Commonscatinline Diagnostic radiology Tomography