#13945 - Radiography - Veterinary Practical Techniques

1. Safety and legislation

   1. X-rays and gamma rays

X-rays have high energy and short wavelength, giving them the ability to penetrate materials and cause changes at the atomic level i.e. ionisation. Ionising radiation results in potentially damaging effects on living tissue. They have no mass, charge, cannot be seen or felt and travel in a straight line at the speed of light.

Genetic effects of ionising radiation include increased risk of DNA mutation. Somatic effects include skin erythema, bone marrow hypoplasia, testicular sterility, ovarian sterility, abortion, cataracts and life-shortening.

Rapidly dividing cells are most susceptible.

2. Gamma scintigraphy

Gamma scintigraphy is the injection of unstable radioactive nucleotide into a patient. This decays, releasing gamma rays. The unstable nucleotide is usually technetium 99. Gamma rays are detected using a gamma camera, which converts the information into light and produces a computer image.

The unstable nucleotide is attached to a carrier substance which has increased uptake in areas of disease.

It is often used for musculoskeletal imaging in horses. Technetium is often attached to MDP (methylene diphosphonate) which is taken up into areas of actively remodelling bone creating a ‘hot-spot’. This is useful in identifying bone disease such as stress fractures.

It can also be used to investigate thyroid tissue in cats.

Technetium has a short half-life (6 hours) and decays rapidly (12-48 hours). It should be contained in a lead-lined container and injected in a lead-lined syringe barrel. The animal should be handled as little as possible during the procedure and isolated afterwards. Technetium is excreted in the urine, so the urine must be caught and stored for 48 hours.

3. Radioiodine treatment

Radioiodine treatment is used for treating feline hyperthyroidism. Iodine 131 is injected and concentrated in the thyroid gland. It emits beta-particles and gamma rays (radiation hazard), with a half-life of 8 days. Cats therefore need to be isolated and hospitalised for at least 14 days.

4. Legislation

Radiography is governed by the Ionising Radiations Regulations 1999.

A health and safety executive must be notified if a vet practice owns or uses an X-ray machine. A controlled area must be defined, and local rules drawn up and followed. The practice must appoint: a radiation protection advisor, who is external to the practice, holds an RPA certificate of competence and has advanced knowledge of radiation; and a radiation protection supervisor, who is a member of staff within the practice and is responsible for day-today supervision and ensuring local rules are followed.

Local rules are a code of conduct for performing radiography that detail equipment, procedures, restrictions and define controlled areas.

A controlled area is one where there is significant risk and is determined by the radiation protection advisor. It should contain the primary beam by either 4 inches of brick or 1mm of lead.

Monitoring badges may be worn by staff involved in radiation on a regular basis.

Protective clothing, such as lead gowns and gloves, only protects against scatter radiation, not the primary beam.

When using radiography, the need must be justified, the exposure to personnel minimal and no dose limit should be exceeded.

Scattered radiation can travel in any direction and is hazardous to personnel. Scatter is increased with increases in kV, larger irradiated area (collimation) and the thickness of the area being radiographed.

Grids may be used when radiographing body areas greater than 10-15cm thick. They consist of alternating strips of plastic and lead which filter out x-ray photons that are not passing in a forwards motion, thus reducing scattered radiation reaching the X-ray cassette and improving radiographic contract. As the grid absorbs some of the primary beam, a high exposure (increased mA) is required.

2. General principles of radiography

   1. Small animal radiography

Restraint can be provided by sedation, general a and positioning aids (N.B never tie an animal that is only sedated – use sandbags).

Sedation for small animals could include:

• Acepromazine and opioid

• Alpha-2 agonist and opioid

• Ketamine and benzodiazepines in cats

General anaesthesia is required for much of musculoskeletal imaging, good quality thoracic radiographs and most contrast studies excepting oral barium.

Movement blur can be minimised by using short exposures. This involves increasing the mA and decreasing the time, using a high kV for the thorax and using fast film plate combinations. Grids may also be used.

1. Large animal radiography

Large animal radiography often involves horizontal beam studies, which present a major safety hazard. Almost every case is imaged standing.

Adequate sedation is required. Alpha-2 agonists and opioids are a good combination.

2. The radiographic process

This can remembered by:

• Pink – positioning. The area of interest should be positioned as close to the cassette as possible and in parallel to the cassette. Standard radiographic positions should be used.

• Camels – centring. The primary beam should be centred over the area of interest, and collimated to the minimum size necessary.

• Collect – collimation. Collimation is described by the number of unexposed borders that are seen within the film/plate, which is important for safety. The primary beam should always be contained within the cassette.

• Extra – exposure.

• Large – labelling. This can be light markers or digital markers, and includes animal ID, date of examination and the area radiographed.

• Apples – artefacts. Sandbags, troughs etc.

  1. Radiographic contract media

Contrast media are substances administered to the patient which are more radiopaque or more radiolucent than the surrounding tissue.

Negative contrast media have a low physical density and low radiographic opacity, appearing black on radiographs.

Positive contrast media have a high atomic number and high radiographic opacity, appearing white on radiographs.

Plain survey radiographs should always be taken first.

Barium sulphate is used to image the GI tract.

Water soluble iodine preparations are used for imaging of the cardiovascular system, urinary tract, joints, tear ducts and gastrointestinal tract. They may be ionic and suitable for IV administration, or non-ionic and suitable for myelography.

3. Small animal abdominal radiography

Minimum of two orthogonal views required – usually ventro-dorsal and right lateral.

The following are not usually seen on abdominal radiographs:

• Adrenal glands

• Gall bladder

• Ovaries

• Uterus

• Mesentery

• Mesenteric lymph nodes

• Omentum

• Pancreas

• Abdominal aorta

• Abdominal vena cava

The most important landmarks are the liver, stomach, spleen, kidneys, small intestines, colon and urinary bladder.

The size of the kidneys should be:

• 2.5-3.5 x the length of L2 in dogs

• 1.9-2.6 x length of L2 in neutered cats

• 2.1-2.3 x length of L2 in entire cats

4. Small animal thoracic imaging

A minimum of two orthogonal views are required – usually right lateral and dorso-ventral.

Right lateral recumbancy is preferred as the heart position is more consistent. Caudo-dorsal rib heads should be superimposed over each other as an indication of good position and minimal axial rotation.

Dorso-ventral views are preferred as the heart lies in the anatomically correct position. Ventro-dorsal views may produce better pulmonary detail.

To avoid movement blur, the timing of the exposure should be at peak inspiration, or in the expiratory pause in a conscious panting animal. High kV and low mA values should be used.

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