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PART 2.3 - IMAGE BRIGHTNESS / CONTRAST

Object density and grayscale



Having explored how X-rays are generated and the effect of changing tube current or voltage I shall now progress onto how radiographs represent the interaction of X-rays with matter. In short, dense matter will stop more X-rays from passing through the specimen and the corresponding area on the radiograph will be white. For your knowledge, the main X-ray receptors currently in existence are:


Chemical film (sometimes called analogue film) - The oldest method with clear links to the discovery of X-rays in 1885. An unexposed film with a layer of light or X-ray photon sensitive substance is placed within a protective envelope to exclude light, with an optional X-ray cassette depending upon technique (imagine a flat, metal box). Once the X-rays have been exposed the film is processed using developer and fixer solutions. The film is dried and then ready for viewing.


Computed Radiography (CR) - Similar to above except that the film has been replaced with a receptor and is reusable. It is still reliant upon detection of X-rays with light or X-ray photon sensitive substances, but the receptor can be reset or wiped to be used again. Once the CR cassette has been exposed to X-rays the receptor is removed, read and converted from a latent image to a computed (digital) image. The receptor is replaced within the cassette, ready for immediate re-use. Images may be digitally stored and analysed.


Digital Radiography (DR) - Interactions between X-rays and the digital detector are instantaneously converted into a digital output via electrical output. The use of film and receptors have been eliminated.


I recognise that some organisations or service providers within archaeology or museums still use chemical film for their imaging. There are several reasons for this but the primary reason is that it is far cheaper than CR and DR, with digital detectors costing £50,000 or more. For low-throughput imaging the use of chemical film makes financial sense. There's also the argument for image quality, with chemical film still providing excellent image resolution.


For the purposes of this short course I will relay the effect of tube current. voltage and the benefits of paleoradiography through the use of digital detectors. The video below provides several demonstrations to illustrate how the density of an object affects radiographic appearances. It also touches upon how digital images can be adjusted, allowing changes to brightness or contrast.





Why is this important to paleoradiography?


The relationship of object density and image brightness is directly related to the previous key concept - radiographs are 2D representations of 3D objects.


We have the ability to change the characteristics of X-ray beams to image objects of different densities, but the cumulative density of individual structures within a specimen also affects their appearances on the X-ray. We also have the ability to digitally change the appearances of resultant radiographs by adjusting contrast and brightness, however this may change our perception of relative density within the image.


The images below help to illustrate the various factors affecting image grayscale within radiography.




The chest X-ray. Useful for identifying infections of the lungs, broken bones and possibility of heart failure. When looking at this X-ray I instantaneously know the lungs are not filled with fluid because they are both black. Likewise, I know the overlying areas of white structures are the ribs. The heart is not as dense as bone but shows up white because of its size. Collectively, the heart demonstrates the same or greater density than a single rib.
The chest X-ray. Useful for identifying infections of the lungs, broken bones and possibility of heart failure. When looking at this X-ray I instantaneously know the lungs are not filled with fluid because they are both black. Likewise, I know the overlying areas of white structures are the ribs. The heart is not as dense as bone but shows up white because of its size. Collectively, the heart demonstrates the same or greater density than a single rib.


It does not matter which way you take an X-ray, the collective densities will demonstrate as the same greyscale upon the radiograph. This is an oversimplistic statement, as there will be different levels of image distortion involved, but the overall message is the same. The diagram shown above has three objects of different densities, but the resultant image is the same no matter which direction the X-rays are passed.
It does not matter which way you take an X-ray, the collective densities will demonstrate as the same greyscale upon the radiograph. This is an oversimplistic statement, as there will be different levels of image distortion involved, but the overall message is the same. The diagram shown above has three objects of different densities, but the resultant image is the same no matter which direction the X-rays are passed.


The item being radiographed may have multiple densities in different quantities. A large low-density object may collectively have the same radiographic appearances as a small high-density object.
The item being radiographed may have multiple densities in different quantities. A large low-density object may collectively have the same radiographic appearances as a small high-density object.


Reading Task:


Read the journal article 'Arctic paleoradiology: Portable radiographic examination of two frozen sailors from the Franklin Expedition (1845-1848)'.


By Derek Notman, Lawrence Anderson, Owen Beattie and Roger Amy (1987).

​

American Journal of Roentgenology

(1987) Volume 149, Pages 347-350.

https://doi.org/10.2214/ajr.149.2.347


The article can be found using this link. The article is shown on the website, alternatively it can be downloaded as a PDF for offline reading.


A wonderful article demonstrating the use of radiography in the field. The authors conducted radiographic imaging for two members of the Franklin Expedition at the site of their frozen graves. Take note of the difference in 'whiteness' between the chest X-rays before and after they have been temporarily thawed.


Estimated reading time: 14 minutes






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