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Chiropractic Research Review

Adjusted Exposure Margin = More Precise Skeletal X-Rays

Appropriate radiographic technique is essential in screening for postural alterations, identifying pathology, and evaluating the effects of care. Common radiographic examinations may require angulation of the x-ray tube to create the best image of the body structures under examination.

This study examined the rationale for and necessity of adjusting the film focal distance when x-ray angulation is employed during a radiographic study.

A standard x-ray phantom of the hand and wrist and several aluminum step wedges were exposed from 10-250% of baseline exposure intensity, using equipment commonly found in clinical practice. The films were independently rated for overall image quality on a visual analog scale by five board-certified chiropractic radiologists.

The panel of radiologists was able to consistently grade exposure intensity differences of approximately 10% relative change between films. The perceived density was directly proportional to the logarithm of the exposure intensity. The range of "acceptable" image quality was determined to lie between -40% and +60% of the baseline technique.

These results suggest the need for a tube tilt correction factor of one inch of vertical adjustment per five degrees of tube angulation for tube tilts greater than or equal to 20 degrees. Experienced observers can reliably discriminate exposure differences when presented with complex grey-scale images such as plain film radiographs.

Andersen KK, Carstensen HM. Exposure margin in skeletal radiography and its effect on tube tilt compensation. Journal of Manipulative and Physiological Therapeutics, May 1998;21(4), pp246-51. Reprints: Tel: (800) 638-6423; Fax: (410) 528-8596

Chiropractic Research Review

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