focal plane tomography

Focal plane tomography generally uses mechanical movement of an X-ray source and film in unison to generate a tomogram using the principles of projective geometry.{{cite book|last=Littleton|first=J.T.|title=A History of the Radiological Sciences|publisher=American Roentgen Ray Society|url=http://www.arrs.org/publications/HRS/diagnosis/RCI_D_c15.pdf|accessdate=11 January 2014|chapter=Conventional Tomography|archive-date=11 March 2017|archive-url=https://web.archive.org/web/20170311175532/http://www.arrs.org/publications/HRS/diagnosis/RCI_D_c15.pdf|url-status=dead}} 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 being studied to blur out.

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.

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.

File:Focal plane tomography.png

This is the most basic form of conventional tomography. The X-ray tube 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".{{cite book|last1=Allisy-Roberts|first1=Penelope|last2=Williams|first2=Jerry R.|title=Farr's Physics for Medical Imaging|publisher=Elsevier Health Sciences|isbn=978-0702028441|page=76|url=https://books.google.com/books?id=EHODwuD73XMC&pg=PA76|language=en|year=2007}} The fulcrum, 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 (CT).

{{external media

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| video1 = [https://www.youtube.com/watch?v=c4SyiuGfWjU Polytome Myelogram Study, 1969] (YouTube) – LA Foundation of Otology film depicting a Polytome being used in a myelography exam to diagnose an acoustic neuroma tumor. Nowadays, this type of tumor would be evaluated using a contrast-enhanced MRI scan.

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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 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.{{cite journal|last1=Lane|first1=John I.|last2=Lindell|first2=E. Paul|last3=Witte|first3=Robert J.|last4=DeLone|first4=David R.|last5=Driscoll|first5=Colin L. W.|title=Middle and Inner Ear: Improved Depiction with Multiplanar Reconstruction of Volumetric CT Data|journal=RadioGraphics|date=January 2006|volume=26|issue=1|pages=115–124|doi=10.1148/rg.261055703|pmid=16418247}}

This is a variant of linear tomography, where a limited arc of movement is used, resulting in less blurring than linear tomography.{{cite journal|last1=Ettinger|first1=Alice|last2=Fainsinger|first2=Maurice H.|title=Zonography in Daily Radiological Practice|journal=Radiology|date=July 1966|volume=87|issue=1|pages=82–86|doi=10.1148/87.1.82|pmid=5940479}} It is still used in some centres for visualising the kidney during an intravenous urogram (IVU),{{cite journal|last1=Daniels|first1=S.J.|last2=Brennan|first2=P.C.|title=A comparison of tomography and zonography during intravenous urography|journal=Radiography|date=May 1996|volume=2|issue=2|pages=99–109|doi=10.1016/S1078-8174(96)90002-4}} though it too is being supplanted by CT.{{cite journal|last1=Whitfield|first1=Ahn|last2=Whitfield|first2=HN|title=Is There a Role for the Intravenous Urogram in the 21st Century?|journal=Annals of the Royal College of Surgeons of England|date=January 2006|volume=88|issue=1|pages=62–65|doi=10.1308/003588406X83168|pmid=16460641|pmc=1963625}}{{cite book|last1=Whitley|first1=A. Stewart|last2=Jefferson|first2=Gail|last3=Holmes|first3=Ken|last4=Sloane|first4=Charles|last5=Anderson|first5=Craig|last6=Hoadley|first6=Graham|title=Clark's Positioning in Radiography 13E|publisher=CRC Press|isbn=9781444165050|page=526|url=https://books.google.com/books?id=51xECgAAQBAJ&pg=PA526|language=en|date=2015-07-28}}

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, as if it were a flat bone.{{cite book|last1=Ghom|first1=Anil|title=Textbook of Oral Radiology|date=2008|publisher=Elsevier India|isbn=9788131211489|page=460|edition=1st|url=https://books.google.com/books?id=mO6Z07lHQO4C&pg=PA460|language=en}} 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.

• Tomography

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Category:Diagnostic radiology

Category:Tomography