PART 2.4 - IMAGE BRIGHTNESS / CONTRAST
Photodensitometry
Radiographs demonstrate a greyscale range extending from black to white, with a multitude of greys in between dependent upon the number of X-rays interacting with the image receptor. The density of an object affects its 'whiteness' upon a radiograph, with very low density items showing as black (similar to air) and very high density items presenting as white (similar to metal). When looking at X-rays we instantaneously apply this logic and correlate black areas with low density, grey areas are mid-density and white areas with high density. This can be termed qualitative assessment of density, where we use a non-metric method of estimating density rather than a quantitative approach.
In modern clinical practice patients undergo DEXA scans (Dual-energy X-ray absorptiometry) which quantify bone density by comparing absorption of X-rays between soft and hard tissues. Computed tomography can also be used to estimate bone density in a similar manner and is termed quantitative computed tomography (QCT). For an example of QCT being used in archaeology to generate bone mineral density view this article. For an example of both DEXA and QCT being used to estimate bone density in archaeology check out this article for a prehistoric hunter-gatherer from Southern Patagonia.
Prior to DEXA and QCT the world used photodensitometry as a method of providing a patient's bone density. Photodensitometry is a method of quantifying the amount of 'whiteness' upon a radiograph to estimate relative density using a reference material of known density. An aluminium step-wedge served this purpose, and shall be demonstrated within the video below.
The aluminium step-wedge:
Within photodensitometry the investigator requires something of known density upon the image which can be used as a reference material. The aluminium step-wedge serves this purpose, with a range of thicknesses along its length which appears as differing levels of 'whiteness' upon a radiograph (see below). It is typically used within radiography as a quality control test tool, but in photodensitometry it can be used to compare relative densities. Any changes to X-ray exposure including strength (kV) or quantity (mAs) would affect the appearances of both the specimen and step-wedge in unison. Likewise, any digital adjustments to contrast and brightness would be replicated for both the specimen and step-wedge.
An aluminium step-wedge, typically used for radiography quality control, but also useful for photodensitometry.
In the past photodensitometry was completed by estimating the amount of light coming through the X-ray film at specific regions of anatomy (for example distal radius). Estimations were made by the eye and were subjective, hence the alternative name optical densitometry. With modern digital X-ray formats the risk of bias can be reduced by measuring the pixels in a region of interest (ROI).
How is density calculated using photodensitometry?
With digital radiography the 'whiteness' of each step of the step-wedge is measured, providing pixel values for a region of interest (ROI) as demonstrated in the image above. This is plotted against the thickness of the aluminium. By measuring the same size of ROI on a bone we can generate pixel values that can be compared to the aluminium step-wedge. Using regression analysis we can work out the relative density of the bone because we know the density of aluminium (2.7 grams per cubic centimetre). Not all bone is uniform, some areas have greater density than others, hence investigators normally select the same position across all skeletons to produce comparable results. Because radiographs demonstrate anatomy in two dimensions the results of photodensitometry are given in grams per centimetre squared (g/cm2).
This article by Bernadette Manifold provides a description of photodensitometry methodology using medieval sub-adult bones in England. In her study she used chemical processed film to generate her radiographs which were photographed for conversion to digital files. Using an image processing program she measured the whiteness for the proximal, mid-shaft and distal portion of bone (radius & femur). By measuring the whiteness for her step-wedge using the same variables (kV/mAs) she was able to provide an indication of density upon the radiographs.
What limitations are there?
Within archaeology there are three major limitations.
1. Soil infiltration. Any contaminants that infiltrate the bone cavities will change the relative density of the bone. Soil, sand and other matter will have greater density than the air that normally occupies these areas. One option is to select anatomical regions that lack infiltration or air-filled spaces.
2. Survivorship of anatomy for comparable data. A group of skeletons from a burial site may not have enough of the same piece of anatomy (femur, humerus, radius, etc) to generate meaningful data. Relying upon one set of data from an individual skeleton risks higher bias and lower generalisability across a population.
3. Taphonomic changes to bone. Bones undergo changes whilst interred within the ground due to weathering, precipitation, leaching or impregnation of minerals. Bones selected for analysis make the assumption that they have not undergone significant change in density since time of death. For an extensive account on how bone changes whilst in the earth please see this article by Kendell et al.
To date, I have not yet seen the application of photodensitometry with non-organic remains such as metal objects, leather or wood. In theory an investigator may estimate the density of light metal objects, although identification of the material through X-ray fluorescence would provide the same information (described in Part 4).
Summary of photodensitometry:
Provides a quantitative measurement of density using differences in greyscale
Based upon relative estimations with an aluminium step-wedge
Primarily used on bone as an indicator of individual health