Radiography in veterinary practice- a review and update by Kimberly Palgrave

ABSTRACT: Radiography is a commonly used diagnostic tool in veterinary practice. A fundamental understanding of how radiographs are created enables the user to select the most appropriate exposure factor settings during radiograph production, in order to achieve optimal image quality. There are currently three types of radiography systems available for use in veterinary practice including conventional film-screen radiography, computed radiography [CR] and direct digital radiography (DR). Digital radiography systems, including both CR and DR, offer a number of distinct advantages over conventional film-screen radiography systems.

Radiography is an essential diagnostic tool for the veterinary practitioner. Although the technology associated with generation of X-rays has remained relatively unchanged since its discovery, the process of radiograph production has evolved significantly. In particular, the advent of digital radiography systems has revolutionised the efficiency of radiograph production and dissemination.

This article will provide a brief review of the fundamental aspects of radiograph production and an update on the various types of radiography systems currently available for use in veterinary practice.

What are X-rays?

X-rays were discovered by Wilhelm Conrad Roentgen in 1895. He discovered that the application of a high voltage to a cathode-ray tube resulted in the fluorescence of phosphorescent material in the room and determined that this resulted from exposure to a previously unknown form of electromagnetic radiation which he termed ‘X’-rays.

Generation of X-rays

X-ray photons are produced as a result of electrons hitting metal while travelling at high speed. This is achieved through the application of an electric current to a cathode (negative electrode) via a high voltage power source (electricity) which enables electrons to be released from the cathode into the X-ray tube. These electrons, being negatively charged, are attracted to the anode (positive electrode or target). When the electrons strike the metallic target (anode) in the tube, X-rays and heat are produced (Figures 1a & 1b).

Figure la: Schematic representation of an X-ray tube, demonstrating electrons travelling from the negatively charged cathode to the positively charged anode to produce X-rays

Figure 1b: Photograph of an X-ray generator which houses the X-ray tube

The X-ray tube is a primary component of a generator. A generator is sometimes referred to simply as the ‘X-ray machine’ and may be permanently mounted above the X-ray table or be portable for use on farms, stables or other locations outside of the practice.

How X-rays work

The usefulness of radiography as a veterinary diagnostic tool is dependent on the ability of a radiographic image to adequately display the differences between tissue types within the patient. This ability is dependent on the mechanisms by which X-rays interact with body matter. There are three possible destinations of X-rays once they have been generated (Figure 2) in that they may:

1. pass through tissue

2. be absorbed by tissue

3. undergo scatter.

Figure 2: Schematic diagram of the interaction of X-rays with tissue: 1. pass through tissues; 2. be absorbed by tissues; and 3. undergo scatter

Preferential absorption of X-rays by various body tissues is useful for the production of a diagnostic radiograph. Conversely, scattering of X-rays is detrimental to radiographic image quality, as it results in a generalised ‘fog’ or greyness of the film. Additionally, scattered X-rays contribute to the overall exposure of staff members to harmful radiation and are, therefore, undesirable.

Radiographic exposure factors

The radiographic appearance of various body tissues is influenced by a number of factors which determine the character of X-rays produced by the generator. These exposure factors are integral to image quality and should, therefore, be manipulated accordingly in order to achieve a diagnostic radiograph of optimal quality

Milliamperage (mA)

Milliamperage is the current that is applied to the cathode of the X-ray tube to produce X-rays. The higher the mA, the greater the number of X-rays produced.

Time (seconds)

The exposure time is the length of time that X-rays are being produced during each exposure. The longer the exposure time, the greater the number of X-rays produced.

mAs

The milliamperage (mA) and time (s) are often combined on the generator settings as the mAs. Therefore, to achieve a given number of X-rays per exposure, as mA is increased, exposure time is shortened, and vice versa.

Kilovoltage (kV)

The voltage applied across the X-ray generator at the time of X-ray production is known as the kV. Increasing the kV results in increased energy of the X-rays produced and, therefore, the ability of the X-ray beam to penetrate the patients tissues also increases.

Focus-film distance (FFD)

The focus-film distance is the distance between the X-ray source (generator) and the cassette or as it is commonly known, the ‘plate’. As this distance increases, the intensity of the X-ray beam decreases.

Effects of alterations to exposure factors

It is important to realise that it is the combination of exposure factors which ultimately determines the overall exposure or ‘darkness’ of the radiographic image. In other words, if a radiograph is deemed to be underexposed or too ‘light’, increasing either the mAs or the kV will result in greater radiographic exposure. However, it is vital to not only assess a radiograph for adequate exposure, but also the quality of the image produced. This will enable individual exposure factors to be altered appropriately to optimise image contrast, brightness and clarity.

For example, if mAs is too low, the resultant image will appear grainy owing to inadequate numbers of X-rays reaching the cassette/plate (Figure 3). However, if the overall image exposure is acceptable, increasing the mAs to resolve the grainy appearance must be accompanied by a concurrent decrease in the kV in order to maintain the same level of image ‘darkness’.

Figure 3: Radiographs of the equine foot demonstrating A) appropriate mAs setting resulting in adequate radiographic exposure, and B) grainy radiographic appearance owing to the use of a mAs setting which is too low

The use of higher kV settings results in increased penetration of the tissues, producing an image with lower contrast between tissue types and a more uniformly grey appearance. Conversely, lower kV techniques result in images with higher contrast, reducing the ‘shades of grey’ and producing a more black and white image.

For example, when taking radiographs of the thorax – where there is high natural subject contrast (between bone, soft tissue and gas) – using a high kV technique will decrease contrast between tissue types and enhance the detail of the soft tissues of the lung fields (Figure 4).

Figure 4: Lateral canine thoracic radiograph demonstrating the use of high kV exposure technique to enable improved visualisation of soft tissue thoracic structures (achieved through decreasing natural subject contrast)

The higher kV technique also enables a lower mAs to be used, thereby reducing movement blur from breathing (caused by a shorter exposure time).

Compared to the thorax, the abdomen has poor natural contrast because abdominal organs are generally uniformly comprised of soft tissue. Therefore, the kV should be reduced in order to maximise the difference in contrast between abdominal organs (Figure 5).

Figure 5: Lateral feline abdominal radiograph demonstrating the use of low kV exposure technique to enable evaluation of soft tissue abdominal organs (achieved through enhancing natural subject contrast)

Finally, it is important to realise that even a relatively small increase in FFD requires a significant increase in mAs to avoid underexposure. Common examples of inadvertent alterations to FFD include using surfaces of varying heights, such as the ‘X-ray table’ in the small animal clinic setting or standing at varying distances from a large animal patient while using a portable generator.

Types of radiography systems

There are currently two categories of radiography system available: conventional film-screen radiography and digital radiography. Digital radiography includes both computed radiography (CR) and direct digital radiography (DR).

Conventional film-screen radiography

Film-screen radiography is the traditional method of producing radiographic images in which the exposed film undergoes wet- processing. The primary components of film-screen radiography include the X-ray cassette, intensifying screens, radiographic film and film processing.

• X-ray cassette – the X-ray cassette is a flat, light-tight box with clips (Figure 6)

Figure 6: A conventional film-screen X-ray cassette

• intensifying screens – radiographic film is more sensitive to visible light than X-rays. Therefore intensifying screens are commonly used to convert X-rays into visible light, thereby reducing the number of X-rays necessary to produce a diagnostic quality radiograph (Figure 7).

Figure 7: Intensifying screens inside a conventional film-screen X-ray cassette

• radiographic film – radiographic film consists of a polyester base, coated with an emulsion of gelatine containing fine silver halide crystals. The crystals are sensitive to X-rays, ultraviolet and visible light, as well as physical pressure, chemicals and gasses.

• film processing – film processing is necessary to convert the ‘latent’ image on the radiographic film to a permanent image which will not change on further exposure to light. Processing may be performed manually in tanks or, more commonly these days, by using an automatic processor.

Digital radiography

Digital radiography systems use the same generator to produce X-rays as conventional film-screen radiography systems. However, the image is produced by exposure of a capture device (plate) to X-rays, which are then converted to a digital data signal and displayed on a computer monitor.

There are two main types of digital radiography – computed radiography (CR) and direct digital radiography (DR).

• Computed radiography – unlike conventional film-screen radiography, computed radiography (CR) uses a portable imaging plate containing an interior screen which records the ‘raw’ image created by X-rays passing through the patient (Figure 8).

Figure 8: A computed radiography (CR) plate with interior screen

The exposed plate is manually placed into a specialised reader where the interior screen is scanned by a laser and a digital radiographic image is produced. The final digital image can then be viewed and manipulated using computer software (Figure 9).

Figure 9: A computed radiography system (CR) demonstrating a plate placed into the reading device, with final image displayed on the computer monitor

• Direct digital radiography – direct digital radiography (DR) systems also involve film-less X-ray capture. Unlike a computed radiography system, however, there is no requirement for the user to place a cassette/plate into a specialised reader. The digitised image is sent directly from an X-ray detector plate to the computer workstation, resulting in almost instantaneous image production (Figure 10).

Figure 10: A detector plate used in a direct digital radiography system (DR]
– the cable attached to the plate (top right corner) is directly connected to the computer workstation

• In the small animal practice setting, this plate may be used on the surface of the X-ray table or it may be mounted in the ‘bucky assembly’ which holds the X-ray cassette/plate underneath the table-top (Figure 11). Some DR systems are also designed to be highly portable, enabling rapid acquisition of digital radiographs of small and large animal patients in both practice-based and field environments (Figure 12).

Figure 11: A detector plate in a direct digital radiography system (DRI mounted in the bucky' of an X-ray table – the digital cable which directly connects the plate to the computer workstation is present (exiting the left side of the bucky' at the front of the tablet

Figure 12: A direct digital radiography system (DR) in use in the field environment for radiography of the equine limb – the digital cable attached to the handle region of the detector plate is directly attached to the computer work station/laptop (in the bottom left corner of the image)

Digital image storage and display

Digital radiographs are saved in a specific format called DICOM (Digital Imaging and Communications in Medicine). This format ensures the security of radiographic images taken by preventing tampering with the original image. The storage of images in DICOM format also ensures consistency of file type between all radiographic systems – it is a common language’.

Digital radiographic images saved in DICOM format are then stored within a PACS network. PACS is the Picture Archiving and Communication System and allows stored images to be viewed and disseminated to colleagues, referral centres and clients. PACS also enables the user to perform various functions on the image, such as zooming, contrast and brightness adjustments, annotations and measurements.

Comparison between conventional film-screen and digital radiography

In summary, digital radiography has multiple advantages over film-screen radiography (Figure 13):

Figure 13: Illustration comparing the differences between A a conventional film-screen radiography system; B. a computed radiography system |CR|; and C. a direct digital radiography system |DR|

• faster image acquisition

• wider latitude (range of exposures giving correctly exposed image)

• ‘windowing’ i.e. post-exposure image manipulation (brightness, contrast, etc.)

• ability to perform various functions, such as zoom, pan, crop, measurements/calculations and annotations

• chemical processing is unnecessary

• film storage facilities are not required

• ease of image dissemination.

Author

Kimberly Palgrave BS BVM&S MRCVS

Kimberly Palgrave qualified as a veterinary surgeon from the Royal (Dick) School of Veterinary Studies in 2007. She has worked with a wide variety of species in both general and referral practice. Her primary interests include diagnostic imaging and veterinary education. She is currently clinical development manager for BCF Technology.

To cite this article use either

DOI: 10.1111/j.2045-0648.2012.000143.X or Veterinary Nursing Journal Vol 27 pp 51-55

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