PART 3.2 - RADIOGRAPHIC TECHNIQUE
What is radiographic technique?
When training to become a diagnostic radiographer you would learn how to position the patient and equipment in the correct manner and select the correct X-ray exposure factors (kV/mAs) to achieve a diagnostic radiograph. Within paleoradiography this is equally important in order to create images which minimise image distortion and allow comparisons with pre-existing imaging. There are a number of variables which the investigator must consider, some of which we have already covered:
Exposure factors:
Tube current (mAs) controls the quantity of X-rays being emitted from the X-ray tube.
Tube voltage (kV) controls the strength of the X-ray beam, allowing it to pass through objects of greater density.
Specimen / object positioning:
This includes the way the object is placed in relation to the direction of the X-ray beam. With (living) patients I would position them in such a way to demonstrate their anatomy. This may include showing the shape or alignment of bones to check for fractures or pathologies. The demonstration of joints (i.e. knee, hip, shoulder) requires accurate positioning for consistent results.
Within paleoradiography the 'patient' is much more cooperative but typically disarticulated, with bones separated and lacking connective tissue. Advantages include the lack of overlying soft tissues or bones which may make diagnosis more difficult, but the disadvantage includes the lack of these to support the bone into standard radiographic positions. The images below show the positioning of different human anatomy using soft sponge supports.
Late Roman skull from Canterbury, UK. The skull has suffered fragmentation and therefore has become rather difficult to perform standard radiographic views.
Examples of specimen positioning for radiography. Fragmented remains makes this difficult!
X-ray tube positioning:
Clinical X-ray rooms are incredibly versatile, unlike Faxitron-like systems where the X-ray tube is fixed (see right). The investigator is able to increase or decrease the distance between the specimen and image detector. With clinical X-ray systems it is also possible to adjust the angle of the X-ray tube, turning it horizontal if necessary. The reasons for adjusting the angle of an X-ray tube is dependent upon the object being imaged. I have encountered various items which, for whatever reason, could not be turned upon their side or moved from their supports. In such instances I would move the image detector and turn the X-ray tube accordingly. A standard distance for radiography is 1 metre between the X-ray tube and detector.
For an example of a Faxitron being used in archaeological practice visit this website by Wessex Archaeology in the United Kingdom. The image below demonstrates the radiography of a soil block containing a boar's skull. This was particularly difficult to image and required experimentation with the X-ray tube positioning.
Image detector positioning:
In tandem with X-ray tube positioning goes image detector positioning. Ordinarily the detector would be parallel to the X-ray tube to minimise image distortion. However, in some instances the investigator may be physically restricted or otherwise unable to maintain a parallel configuration.
A variety of imaging positions where the detector is not parallel to the X-ray tube. Whereas such imaging may produce a distorted image it may also demonstrate areas of the object that could not be seen before.
X-ray tube collimation:
The X-ray tube has dials to control the size of area being exposed to X-rays (aka 'collimation'), as shown in the video 'An X-ray room' within Part 1. The X-ray tube assembly has a light beam diaphragm which emits light to the same extent as the collimation, so the operator can tell where the X-rays will go. Within the hospital setting we would reduce the collimated area to a minimum to ensure the patient does not undergo excessive radiation exposure. Within paleoradiography we would not be so concerned with radiation but there are other benefits. The main benefit is having smaller file sizes with less 'dead' areas on the image. Proper use of collimation provides cleaner images that are typically easier to interpret.
Use of radiographic markers and other accessories:
You will notice that most radiographs have a 'R' or 'L' upon them. These are called radiographic markers and denote the side of the item being imaged. For example, if I were to X-ray a right femur I would place a 'R' alongside the specimen. Because images can be flipped or inverted, it is important to have a radiographic marker to confirm the laterality (i.e. which side). However, this is normally only important for biological specimens and I only put a marker on non-biological items (metalwork, ceramics) as a point of reference.
Other accessories include placing legends or arrows directly upon the X-ray detector / cassette / film. These are typically made out of lead and were traditionally used to highlight aspects of the specimen or to show a name/number on the image. Such practices are uncommon now due to the emergence of digital imaging, which allows editing of the image after acquisition.
The article by Mitchell and Dittmar (2021) demonstrates the novel use of a grid during imaging to locate bone lesions for biopsy. By imaging the grid and bone together the authors were able to count each wire on the initial radiograph and pin-point the area for biopsy.
This archaeological specimen has several areas of lucency (low density areas of bone) associated with spread of cancer (lytic metastases). The researchers placed a sieve on top of the bone and then used the position of the wires to estimate where to biopsy the bone.
My research
I have conducted research to investigate the radiographic technique needed for investigating human dry bones. The article is open access and can be viewed here.
In the study I collaborated with Adelina Teoaca, an osteoarchaeologist from Canterbury Archaeological Trust. We took X-rays of 92 individuals from an excavation in St Albans (United Kingdom). Whilst Adelina wanted to visualise Harris lines to assess for biological stress, I used the opportunity to refine the radiographic technique needed.
We found that an exposure factor of 55kV and 5mAs was suitable for most bones. We also advocate the use of sponge supports to position the bones accurately. We provide a list of recommendations for future radiographic surveys of skeletal remains by archaeologists.
Why is this important to paleoradiography?
An investigator must be aware of all the different variables involved within radiography when conducting imaging. Each factor may impact upon the quality of the image and therefore its research value.
Diagnostic radiographers must learn the radiographic technique for the entire body along with methods to overcome problems. This involves having different radiographic projections or 'views'. It is my belief that many of these views can be applied to human bones in archaeology, so that we create comparable radiographs.
The importance of radiographic technique is covered next.
Reading Task:
Read the journal article 'Testing “Saintly” Authenticity: Investigations on Two Catacomb Saints'.
By Amelie Alterauge, Thomas Becker, Brigitta Berndt, Christian Jackowski, and Sandra Lösch (2016).
RadioGraphics
(2016) Volume 36, Pages 573-579.
DOI: 10.1148/rg.2016150008
The article can be found using this link to a copy on ResearchGate.
A very interesting article about the radiographic imaging of skeletal remains for two saints in Switzerland. Both were imaged in situ, without the ability to move them to accommodate the image detector. Take note of the Radiographic Imaging section on page 574, which describes the radiographic technique.
Estimated reading time: 20 minutes
