Contributors: Fraser J McConnell
Species: Feline | Classification: Miscellaneous
- In human medical imaging digital radiography has superseded convention radiography in many hospital departments. Over the last 10 years digital systems have been installed in many veterinary hospitals, particularly in equine units.
- Film based radiography has been around for approximately 100 years, so why are things changing now? Conventional radiographs have many good points. Because they have been around for so long we are familiar with how they look, can interpret what the different opacities mean, and readily recognize film faults and errors with technique. Conventional X-ray film has an intrinsically high resolution exceeding that of many digital systems. Equipment purchase is not expensive and radiographs are relatively cheap to produce.
- However, there are also disadvantages associated with conventional film-based radiography. It may be difficult or impossible to correctly expose all areas on a radiograph. A darkroom and chemicals are required to process the radiographs and exposure problems and film faults are common. Film is also bulky to store and radiographs are easily lost and difficult to distribute. It was to overcome some of the problems associated with conventional film- based systems, particularly in large human hospitals, that digital imaging systems were developed and introduced in the early 1980s.
Advantages and disdvantages of digital systems
- Digital images have many advantages over conventional systems:
- No need for a darkroom.
- Errors in exposure can still result in diagnostic images.
- Images can be manipulated to enhance lesions.
- Storage and distribution of images is easier.
- Ongoing cost savings are made (no need to purchase film or chemicals and reduced number of non-diagnostic films).
- There is a significant cost to the initial purchase of digital set-ups (//www.vetlexicon.com£25K+) and although some digital systems allow dose reductions (a big plus feature in human radiography), overexposure is easy to overlook since a diagnostic image will still be produced in an overexposed film.
- Recent surveys of veterinary practices have shown that conventional veterinary radiographs have a high prevalence of faults. Up to 50% are under or overexposed, (either too dark or too light). Overexposure in particular is extremely common. The other problem with conventional film is that it is difficult, or impossible, to have a single radiograph that is optimally exposed for tissues that vary greatly in depth, eg the abdomen of a deep-chested dog.
- It is hard to achieve a correctly exposed image with conventional radiographs due to the characteristic curve of the radiographic film. The characteristic curve is simply the optical density (ie how light or dark the radiograph is) plotted against the log of the exposure. With conventional film the curve is S shaped and the useful part of the curve is the straight line (see ). Therefore there is a narrow range of exposure factors that can give a diagnostic radiograph. If the radiograph is incorrectly exposed then parts of the film will be too dark or too light and the information carried by the pattern of the photons will be lost and cannot be recovered.
- One of the advantages of digital radiology is that there is a much wider range of exposures over which a useful image can be obtained (dynamic range), compared to conventional film. In digital systems the relationship between exposure and signal is linear. Digital systems have between 4 and 10 times more latitude compared to conventional film. Unless the image is grossly over or underexposed a useful image can be obtained with digital systems. With a digital system it is possible to alter the brightness and contrast of the final image to evaluate both the spine and soft tissues without having to take separate radiographs.
- However digital systems are not always better. With some, mainly cheaper, digital systems image quality can be poor and inferior to conventional films. One reason for this is cheaper systems may produce lower resolution images. The resolution of an image is determined by the size of the pixels - the smaller the pixel the higher the image resolution. However, if pixels are smaller the image contains more pixels and this increases the size of the digital file. More computing power is required to handle the files, and data storage and distribution becomes more expensive. The average pixel size on a digital radiograph is //www.vetlexicon.com150 micrometers and a large 35/43 cm plate has //www.vetlexicon.com15 megapixels.
- The other main factor that determines the file size and image quality is how many shades of gray can be measured, or displayed, on the radiograph. A conventional radiograph has a continuous scale from white to black. The number of shades of gray shown on a radiograph is determined by the grayscale range which is in turn determined by the number of bits. A 1 bit grayscale has 2 shades (black and white) a 2 bit grayscale has 4 shades of gray. Digital radiographs are typically acquired with 14-16 bit grayscale but are displayed on monitor at 12 bit which allows up to //www.vetlexicon.com4000 shades of gray to be detected and displayed. The size of the grayscale also affects file size - with increased numbers of bits increasing file size.
- If we look at the digital X ray image immediately after it has been made but without any processing by the computer the whole 4000+ shades of gray are displayed. This makes the image appear dark and lacking in contrast. All digital systems therefore perform image processing before displaying the image. The quality of image processing can have a marked effect on image quality. Poor processing of good original data can produce a poor image.
- The computer receives the raw data and converts the linear relationship to an S shaped curve similar to that of conventional film. The final image displayed therefore appears like a conventional radiograph. However the original data is retained and the shape and slope of the curve can be altered to produce images with different contrast or to highlight different parts of anatomy. The same image can be adjusted to show bone detail or to accentuate soft tissues. This is called applying post-processing curves.
- The image may be manipulated in other ways before it is displayed. Filters may be applied to the data to make the image appear smoother or to enhance edges. Although a useful feature this is also one of the problems with digital systems - data manipulation can have a marked effect on the appearance of the image. In fact images taken on different systems may have very different appearances.
- One of the advantages of digital systems is that we dont need to print film - saving in cost and storage space. Viewing and reporting of digital images is normally done using a soft copy ie the image is viewed on a computer monitor. The monitor should be in a room with low ambient light to reduce glare and reflections on the screen. To display the image at full size with its original resolution we need to have high resolution medical grade monitors. Black and white monitors are used because they allow the full gray scale of //www.vetlexicon.com4000 shades to be displayed. The monitors are usually LCD and have to have higher levels of brightness and contrast compared to most monitors. Unfortunately medical grade 2 megapixel monitors are expensive (£3-4K) but are essential to get the most from a digital system. A standard PC monitor does not have adequate size or resolution to view a 35x43 cm image with its original resolution and it is possible to miss lesions if the images are viewed on these monitors.
- Digital systems do not eliminate artifacts, we just get different ones and it should not be thought that digital systems make the importance of good technique any less, it can be even more important.