Focal plane tomography

In radiography, focal plane tomography is tomography (imaging a single plane, or slice, of an object) by simultaneously moving the X-ray generator and X-ray detector 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 until the late-1970s. It has since been largely replaced by more advanced imaging techniques such as CT and MRI. It remains in use today in a few specialized applications, such as for acquiring orthopantomographs of the jaw in dental radiography. Focal plane tomography’s development began in the 1930s as a means of reducing the problem of superimposition of structures which is inherent to projectional radiography. It was invented in parallel by, among others, by the French physician Bocage, the Italian radiologist Alessandro Vallebona and the Dutch radiologist Bernard George Ziedses des Plantes. 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. 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.