PART 3.3 - RADIOGRAPHIC TECHNIQUE
Geometric unsharpness
Geometric unsharpness is the loss of definition upon a radiograph due to geometric factors, i.e. the physical alignment and distance of X-ray tube, specimen and image detector. This is best described within the video shown below where I compare the travel of X-ray to that of light, and the radiograph may be compared to a shadow. I have also provided a series of images with further explanation thereafter.
Depending upon the intent of the investigator, using an incorrect radiographic technique may produce image distortion and degrade the research value of the image. As I have alluded to within some of the previous examples, sometimes the object or specimen cannot be moved or positioned in a certain manner. In such circumstances the investigator must adapt their technique and accept an element of distortion.
These images show the X-rays and bone positioning from the video. Raising or lowering portions of the same bone lead to different radiographic appearances. The correct radiographic technique will allow accurate demonstration of shape, form and therefore anatomy.
These radiographs show the same bone (a tibia) in two views. On the left is an anterior-posterior view, meaning that the X-rays passed through the front (anterior) of the bone and passed out through the back (posterior) to reach the X-ray detector.
On the right is shown a lateral view, or more specifically a medio-lateral view. This is where the X-rays have passed through the medial aspect of the tibia and out of the lateral aspect of the tibia to reach the X-ray detector.
Why does this matter?
Anatomy that is further from the X-ray detector will suffer from greater geometric unsharpness. One way of overcoming this problem is to have standard views (positions) for specific bones. What cannot be accurately seen in one view may be visualised in another view. We accept that radiographs do not provide a perfect representation of the anatomy, but by having standard views we are able to increase the reliability of the resultant imaging.
Why is this important to paleoradiography?
Improper positioning of specimens during radiography may produce images which are spatially inaccurate. Some or all of the images may be distorted and therefore unreliable for image interpretation.
We will never be able to have all of the specimen in direct and close contact with the image detector and there will always be areas with some image distortion. However, if we maintain a logical approach to radiographic positioning we can produce images which are accurate and useful as permanent records.
The use of two radiographic projections (or 'views') from 90 degree angles provide the best visualisation of the specimen. Any areas suffering from image distortion on one view may be rectified on the other view (and vice versa).
