¶ … Conventional Tomography outlining the various aspects, issues and methods used. It has 10 sources. The field of medical imaging has been in existence for over one hundred years but new research and scientific breakthroughs have changed both its image and its role. Radiology is not only diagnostic but is expanding to encompass curative techniques as well. The most common radiological investigation remains the conventional X-ray but a wide range of new and more efficient modalities have also been available for the past several decades. Amongst these methods is tomography, specifically in reference to this paper, Conventional Tomography.

Conventional Tomography:

Principle

Radiographs deal with the internal anatomy of bodies, commonly used to view bones, calcified material, and soft tissue masses. This area has branched out since the use of fluorescent dyes but still is limited by the fact that a conventional radiography is unable to display within the framework of two-dimensional x-ray picture all the information found in the three-dimensional figure under view. Objects superimpose losing detail and effectiveness which is what paved the way for the advent of tomography as far back as the 1930's. (Author not available, 2003).

Tomography is a method whereby selected planes in the patient's body can be examined with greater clarity and perspective. Logical other names for Conventional Tomography include body section radiography, planigraphy, laminography or stratigraphy. It differs from Conventional radiography in that a single plane is selected and kept in focus, as structures above and below are blurred and out of focus. This is done by moving the x-ray tube in one direction over the patient. The tube and film are linked and synchronized to move in opposite directions. This process uses the motion of the X-ray focal spot and image receptor to produce radiographs in which only one specific plane remains in sharp focus. Irrelevant details from other segments become blurred and thus insignificant. (Author not available, 2003).

In ordinary radiography however a two-dimensional picture of a three-dimensional object is produced causing the image of all structures throughout the thickness of the patient in the direction of the x-ray beam being superimposed on the same film. (Author not available, 2003).

It is the ratio of the tube-object distance to the object-film distance that determined the level at which the tomographic 'cut' was taken. The thickness of the section in turn is determined inversely by the length of the tube-film travel. Thus extremely thin sections and thicker ones called zonograms could all be recorded. (Heitzman 2000)

Types

Over the years a variety of tomography techniques have been developed as radiologists modified X-ray sources and movement styles to get optimum results. One of these types is known as linear tomography which uses the fact that by changing the relative motion of the film and tube, the focal plane can be adjusted upward or downward. (Author not available, 2003, Novelline 1997). Rectilinear tomography was most common with the images produced popularly referred to as planigrams. A disadvantage to this method was its inability to also blur the linear structures that lay in the same direction as the tube-film excursion. (Author not available, 2003, Novelline 1997).

Other movement types such as circular, elliptical, figure-8, hypocycloidal, trispiral have all been used. Each different type of motion affects the blurring of a specific plane and thus has different advantages and indications. Hypocycloidal motion became the standard of reference for analog tomography. The sections produced were thin and at times difficult to orient to the portion of the body being examined. (Author not available, 2003, Novelline 1997).

Eventually the forerunner to transverse tomography was evolved and techniques that 'cut' the body into cross-sections rather than longitudinally discovered. Geographic distortions were prominent with this technique and transverse tomography never really became popular. (Author not available 2003, Novelline 1997).

Indications

There are various specific and less particular indications for the use of conventional tomography but mostly it has been replaced by more efficient methods. This technique may be useful in determining fracture healing, evaluation of pulmonary nodules, evaluating the kidneys in excretory urography, and evaluating the integrity of spinal fusion. (Author not available 2003)

It is still widely used in the assessment of musculoskeletal disease. Certain structures such as the vertebral column, sternum, ribs, and sella turcica are difficult to isolate and examine in ordinary X-rays. Their size, orientation in the body, or presence of surrounding tissues all serve to complicate results. (Author not available 2003)

This is similarly true of particular joints for e.g. The sternoclavicular, temporomandibular, sacroiliac, costovertebral, apophyseal, atlanto-occipital, atlantoaxial,...

Various projections are optimal for the different joints. (Author not available 2003)
Conventional Tomography is also indicated in various musculoskeletal problems such as detection and delineation of fractures of the vertebral column, tibial plateau, carpus, and femur; identification of osteochondral defects; and evaluation of fracture healing. It is of assistance in assessing a patient with chronic osteomyelitis by identifying areas of active disease and in cases of neoplasm, by identifying a nidus of an osteoid osteoma, matrix calcification in cartilaginous tumors, and soft tissue extension of malignant disease. (Author not available 2003)

In the investigation of primary and secondary pulmonary neoplasms it was revealed that the evaluation of the presence of neoplasia is better accomplished by conventional examination, while extent of disease is best assessed by Computed Tomography (CT).(Mintzer 1979)

Recent progress in dental tomographic imaging has spread to dentistry. Research in the field of dentistry has shown that that CVT is superior to computed tomography in the diagnosis of single structural bone changes but comparable for comprehensive diagnosis of temporomandibular joint disease. (Tanimoto 1990) Further research has demonstrated that the use of tomograms increases the efficacy images, with respect to the prediction of appropriate implant size, by a factor of 2.5 in Dentistry. Conventional cross-sectional tomography has been recommended for treatment planning before installation of single tooth implants. (Schropp 2001) New breakthroughs and research are at their peak in this area.

Changes over Time

With the development of methods such as Computed Tomography (CT, CAT scans), Magnetic resonance Imaging (MRI), Fluoroscopy, Conventional tomograms arc less likely to be requested today. As we have noted above indications for CVT still remain and indeed are being enhanced as radiologists improve and modify the existing instruments and techniques. One of these innovations has been the digitalization of the images. The use of storage phosphor image plates with laser scanners or CCD sensor systems etc. has been increasingly common in medicine and has recently entered the field of Dentistry as well.

That X-rays have an effect on living cells is an acknowledged fact. The controversy arises at the time of deciding the type, and quantity that is hazardous, the duration of exposure, and the exact consequences. (Flake 2003) It is known that x-rays are absorbed, and change the structure of tissues. If exposed for long enough x-rays have been known to even cause burns or destruction. (Flake 2003)They have been found to cause mutations in DNA especially in germ cells of the reproductive organs. The radiation dose of genetic significance from diagnostic radiology is estimated to be about twenty millirem/year. (Flake 2003) The main danger of X-ray exposure arises from its silent and slow progression. Years may go by without detection of side-effects and sometimes radiation may not be held accountable for lack of conclusive evidence. (Flake 2003) major disadvantage of the CVT technique is the fact that the whole region of interest is exposed at the same time with a continuously varying beam direction and the necessity for a large imaging surface. (Maue-Dickson 1981) In conventional tomography research shows that exposure increases almost arithmetically with the number of contiguous sections scanned. (Maue-Dickson 1981)

With the arrival of digital techniques the exposure to harmful chemicals and radiation are reduced because less retakes are needed. The efficacy of the overall health care system is improved as this has created links to efficient file archiving. (Preston 1998)

Intensified tomography was a new method that served to enhance the conventional tomographic images of soft tissues such as the brain. The three basic modifications in this system included a specially designed filter to attenuate selectively the x-ray beam at the source, an improved blurring movement to reduce system artifacts, and enhanced image contrast by the superimposition of several identical images on the same film or television screen. (Perilhou 1982) The results of research were positive even in comparison with computed tomography. It was proposed that this method could be useful as a screening tool in areas where the CT is unavailable. (Perilhou 1982)

Conclusion

Conventional Tomography evolved as far back as the 1930's and was especially used in areas such as chest radiology by Ziedses des Plantes. (Heitzman 2000) Its advantages were in being able to produce sharp images in selected plains with no obstructive artifacts. It was made more efficient and effective over the decades but newer and better techniques have replaced it in many areas.

The radiology of 50 years ago was a primitive science compared with the radiology of today. (Heitzman 2000)

Medical Imaging allows us to see inside the human body and to make judgments based on what we see. Whilst we…