Exposure margin in skeletal radiography and its effect on tube tilt compensation

To examine the rationale for and necessity of adjusting the film focal distance when X-ray angulation is employed during a radiographic study, and to determine the sensitivity of human interpreters to overall differences in film density and the correlation, if any, with digital analysis of radiographic films.

The study was performed in the radiology department at Logan College of Chiropractic in St. Louis, Missouri. A standard X-ray phantom of the hand and wrist and several aluminum step wedges were exposed from 10% to 250% of baseline exposure intensity using equipment commonly found in a field doctor's practice.

The films were independently rated for overall image quality on a visual analog scale by five board-certified chiropractice radiologists. The films were also digitized, and selected regions analyzed on a computer.

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, as would be predicted from theoretical film response characteristics. The range of "acceptable" image quality was determined to lie between -40% and +60% of the baseline technique.

This study validated the empirically derived tube tilt correction factor of 1 in of vertical adjustment per 5 degrees of tube angulation for tube tilts greater than or equal to 20 degrees. Underexposure impaired the interpretation of radiographs more than overexposure. Experienced human observers were able to reliably discriminate exposure differences on the order of 10% relative change when presented with complex grey-scale images, such as plain film radiographs.