PART 3.4 - RADIOGRAPHIC TECHNIQUE
Radiogrammetry
Radiogrammetry is a method of measuring bone anatomy directly upon a radiograph to estimate bone loss due to age or pathology. Within Part 2 we looked at photodensitometry where an investigator would measure the amount of 'whiteness' upon an X-ray and compare it to an aluminium step-wedge. The 'whiteness' of a bone on a radiograph could then be compared to the different thicknesses of the aluminium. Using regression analysis we could then estimate the bone mineral density of the individual.
Radiogrammetry typically uses measurements of the cortical bone thickness within the metacarpal bones (hands). Patients with bone loss will exhibit decreased cortical thickness. This is important as it demonstrates the health of the individual, along with signs of biological stress (diet, disease or genetic factors).
Radiogrammetry is still used within modern medicine to identify patients with osteoporosis and the risk of bone fractures in the future. Check out this article concerning radiogrammetry of children.
The bones of the hand are used because there is minimal geometric unsharpness (image distortion) due to the proximity of the bones to the image detector during radiography. Hand X-rays are easy to perform and require low radiation when compared to larger anatomy (like the pelvis). Measuring points of interest upon a digital X-ray provides a measurement which can be deemed accurate, with some studies suggesting only 1% error margin due to distortion.
Automated software programs exist, using artificial intelligence to locate and measure cortical thickness for clinical use (in the hospital). In Part 4 I have produced a short video to demonstrate how to generate measurements using digital radiography files (DICOM).
The image shown above is taken from Wilczek et al (2013) available here.
Why is radiogrammetry useful for paleoradiography?
Interest in radiogrammetry within archaeology has increased in recent years with application on excavated skeletal remains. The method is reproducible, relatively cheap and may be applied on whole and fragmented bones (although the metacarpals are still preferentially used). The article by Gilmour et al (2021) explores the use of fragmentary archaeological metacarpals (unfortunately, the article is not free to access).
The lack of soft tissue with skeletonised remains allows for accurate radiography of the bone, with minimal image distortion. Alternative methods include sectioning of bones and direct measurement of the cortical thickness, which avoids radiography altogether. Depending on your viewpoint, the destructive nature of this method may not be ethically acceptable for the human remains. The process of radiogrammetry can be performed manually using printed film, although digital radiographs are considerably easier due to the added ability of image manipulation and digital callipers.
Other considerations include the accuracy of measuring cortical thickness and the presence of artefacts (soil) within the bones which may lead to inaccurate measurements. Normally, investigators would remove obviously deformed or taphonomically damaged bones from the same selection. Radiogrammetry is primarily used upon metacarpals, which may limit the scope for sample size depending on bone survival.
For more information about radiogrammetry you can check out one of the original articles in 1969 which describes the methodology. For an overview, the article by Adams (2010) gives a good introduction as well. These articles are not open access, but your institutional library or local library may be able to obtain a copy.
Archaeological examples of human metacarpals being used for radiogrammetry.