19 May 2014
Elisabetta Mancinelli
Job Title
Radiology is a common diagnostic procedure in any field of clinical veterinary practice – and exotic animal medicine is no exception. Alongside physical examination, haematology and biochemistry analysis, radiography and ultrasonography are useful diagnostic tools – often complementary and certainly the most commonly performed diagnostic imaging.
The range of potential abnormalities that might be faced in clinical practice and detected by different imaging modalities is very broad. An array of other imaging techniques – such as endoscopy, CT and MRI – are also available in veterinary medicine that may help in achieving a definitive diagnosis. Radiography will be discussed in the first part of this article, whereas ultrasonography and more advanced imaging modalities will be considered in part two.
X-rays are a type of ionising electromagnetic radiation that travel through space at the speed of light. The high velocity electrons collide with a metal target – the anode – producing the x-rays. When x-rays strike certain inorganic materials they cause a brief flash of light, which is used to record the radiographic image on a conventional screen-film system. Three parameters influence the quality and quantity of x-rays produced: milliamperage (mA), exposure time and kilovoltage peak (kVp).
mA indicates the number of electrons produced by the cathode. Increasing the mA will increase the number of electrons that collide with the target and, therefore, the quantity of x-ray produced.
Exposure time – usually a fraction of a second – indicates the period of x-ray production. Increasing the exposure time will also increase the number of x-rays produced. Usually, because of the rapid respiratory rate of exotic animals, short exposure times (less than 1/60sec) are needed.
kVp is the speed at which the electrons hit the target. Its increase means the electrons between the anode and the cathode are accelerated. More penetrating x-rays are therefore produced. Usually, the higher end of kVp is used for thicker or superimposed areas (for example, radiography of the skull).
Usually, an x-ray machine capable of producing 40kV to 70kV, 300mA and exposure times of 0.008 seconds to 0.16 seconds is recommended in exotic animal practice (Redrobe, 2001). A tube allowing 90° rotation for horizontal beam radiography is also very useful. The focalfilm distance generally used is 90cm, reduced to 80cm for enlarged radiographs.
When an x-ray image is produced, short x-ray pulses illuminate the body, or part of it, with a radiographic film placed behind it. There is different absorption depending on tissue composition and thickness.
Bones absorb most of the x-ray photons by photoelectric processes. This is because bones have a higher electron density than soft tissues. The x-rays passing through the soft tissue leave a latent image in the photographic film.
When the film is processed, the parts of the image corresponding to higher x-ray exposure are dark, leaving a white shadow of bones on the film. The x-rays not absorbed by the patient pass through and are recorded on a film-screen within a lightproof cassette. The cassette contains one or two intensifying screens that convert the x-ray into visible light, which exposes the x-ray film. High-resolution mammography x-ray films are especially advantageous for exotics because they provide sharper images and better details for our small sized patients requiring longer exposure time (Capello and Lennox, 2008; Capello and Lennox, 2011).
The terms “radiopacity” or “radiodensity” are often used to define the radiographic density (greyscale) seen on a radiograph. Bone, for example, absorbs large amounts of radiation and produces white areas on radiographs. It is, therefore, said to be radiopaque or radiodense. Air absorbs few x-rays, produces dark areas on a radiograph and is, therefore, said to be radiolucent. Fat, water and soft tissue have intermediate densities. The five basic radiographic densities – from least to most opaque – are air, fat, water (soft tissue), bone and metal. All soft tissues, though, cannot be distinguished from each other on a conventional radiograph.
X-rays are useful in the detection of pathology of the skeletal system, as well as for detecting some disease processes in soft tissue. Some examples are the very common chest x-ray – which can be used to identify lung diseases such as pneumonia, neoplasia or pulmonary oedema – and the abdominal x-ray, which can detect gastrointestinal impaction or obstruction, free air (from visceral perforations) and free fluid (ascites), or the presence of soft tissue masses.
X-rays may also be used to detect pathology such as kidney or bladder stones, which are often (but not always) visible (radiopaque). Traditional plain x-rays are less useful in the imaging of soft tissues such as the brain or muscle. X-rays are also commonly used in dentistry, as x-ray imaging is useful in the diagnosis of common oral problems in rabbits.
Digital radiography (DR) is a form of x-ray imaging where digital x-ray sensors are used instead of traditional photographic film. Advantages include time efficiency through bypassing chemical processing and the ability to digitally transfer and enhance images. DR is essentially filmless x-ray image capture. In place of x-ray film, a digital image capture device is used to record the x-ray image and make it available as a digital file that can be presented for interpretation and saved as part of the patient’s medical record.
The advantages of DR over film include immediate image preview and availability, a wider dynamic range – which makes it more forgiving for over and underexposure – as well as the ability to apply special image processing techniques that enhance overall display of the image. There are two basic digital imaging systems: computed radiography and digital radiography.
Computed radiography (CR) uses very similar equipment to conventional radiography, but the imaging plate is run through a special laser scanner, or CR reader, which reads and digitises the image. The digital images can then be viewed and enhanced using software.
CR and DR are very similar. Both use a medium to capture x-ray energy and produce a digital image that can be enhanced for soft copy diagnosis or further review. CR and DR can both present an image very quickly after exposure, but CR generally involves the use of a cassette that houses the imaging plate, similar to traditional film-screen systems, to record the image. DR instead captures the image directly on to a flat panel detector, without the use of a cassette. The digital information is sent directly to the computer, without the reading step (Ludewig et al, 2012).
CR and DR should not be confused with fluoroscopy – an imaging technique commonly used, where there is a continuous beam of radiation. This is the system where the image of the patient being x-rayed is viewed in real time on a monitor or display. In its simplest form, a fluoroscope consists of an x-ray source and fluorescent screen between which a patient is placed. However, modern fluoroscopes couple the screen to an x-ray image intensifier and charge-coupled device video camera, allowing the images to be recorded and played on a monitor (Capello and Lennox, 2008).
The majority of exotic patients, including rabbits, can be placed directly on the radiographic cassette (no grid is needed). A grid may be used for larger breeds where the body thickness is likely to exceed 10cm. The thickness of the area to be radiographed is measured (in centimetres) to determine the kVp settings.
mA settings depend on the body area being imaged (thorax, abdomen, extremity and so on). Often, a chart with specific recommendations is provided by the manufacturer. More often, these recommendations are not very useful when using mammography films. A chart simply based on measurement/area of interest is inadequate when working on exotic species (tissues of herbivorous species differ from those of carnivorous ones), therefore it is highly recommended every practitioner develops his or her own species-specific chart.
X-ray projections or views are named according to the beam entry and exit point. The Nomina Anatomica Veterinaria provides the correct terminology used to describe radiographic projections. For example, a thoracic radiograph of a rabbit in dorsal recumbency, with the x-ray tube over its head and the cassette underneath the patient, is a ventrodorsal projection. Oblique views are more difficult, but are named in the same way – beam entry to beam exit and adding, to the directional terms, the degree of obliquity.
Collimators are lead plates on the x-ray machine that permit the area of interest to be outlined and irradiated by the x-ray beam, so scattering can be reduced, providing better quality imaging and limiting personnel exposure (Capello and Lennox, 2008).
Correct positioning of the patient is critical to obtain quality diagnostic radiographic imaging and to reduce the risk of diagnostic errors. Sedation or general anaesthesia are safer and less stressful for the patient than restraint of the conscious animal. They can facilitate positioning, reducing the need for repeated exposures due to movements of the patient (Capello and Lennox, 2011). When pharmacologic restraint is contraindicated, radiographs can be taken by covering the rabbit’s eye and using sandbags to evoke an immobile state. This procedure should be immediately discontinued if the patient shows any signs of stress. Trancing or “tonic immobility” is, in fact, a fear response and, furthermore, the period of time for which this immobile state is maintained is variable. In some cases, this may be preferable to inducing anaesthesia for a short, non-painful procedure. Endotracheal intubation may also be considered if a long sequence of x-rays has to be taken, but preferably not performed when examining the head to avoid artefacts and complicate image interpretation (Capello and Lennox, 2008).
Symmetry, with the exception of oblique or other particular projections, is of utmost importance. Pieces of foam, sandbags or tape can be used to facilitate correct positioning and to secure the patient to the radiographic film, without interfering with the images. Appropriate radiographic interpretation usually requires more than one view to be taken for each anatomic area studied. Standard practice is to take two views 90° apart (a lateral and a dorsoventral or ventrodorsal view) of the area of the body of interest. Having then both right and left lateral views can improve assessment of unilateral lesions.
Appropriate radiation safety measures have to be taken in any situation to avoid, or reduce the risk of, personnel exposure.
The whole body projection generally includes all structures (perineal area too) except for the tail and distal part of the limbs. This overview should not be used when a specific study of the thorax, abdomen or limbs is indicated. It has to be remembered that settings – especially kVp – will not be ideal for all sections. The patient is positioned in right or left lateral recumbency (lateral projection) and the beam will not be properly centred on an area of interest. For the ventrodorsal projection the patient is placed in dorsal (or ventral for the dorsoventral) recumbency; head and limbs in neutral position.
The patient is placed in right or left lateral recumbency, with the head flat and horizontal. The philtrum should be parallel to the cassette. Pieces of radiolucent foam can be used to appropriately raise the head if necessary. The x-ray beam is centred just rostral and ventral to the eye.
The patient is placed in right or left lateral recumbency, with the head slightly tilted anti-clockwise of 10° to 20° from a true lateral position.
The patient is placed in ventral recumbency, with the neck hyperextended and secured with tape so the head is parallel to the cassette. The x-ray beam is directed between the eyes in the midline.
The patient is positioned in dorsal recumbency, with the head hyperflexed so the palate is perpendicular to the cassette and the head appears symmetrical and not deviated laterally. Respiration may be impaired if this position is maintained for too long.
The patient is placed in left or right lateral recumbency, with the thoracic limbs extended cranially and held with a sandbag or tape. The neck is extended gently. The beam is directed on the caudal border of the scapulae, with the field of the x-ray beam including cranial abdomen and caudal neck.
The rabbit is in ventral recumbency on the cassette, with the thoracic limbs extended cranially and taped securely to either side of the head, to minimise superimposition of the scapulae and associated musculature on the cranial thorax. Remember to position the ears to avoid superimposition with the thoracic cavity. This position is more comfortable and less stressful than the ventrodorsal – especially in rabbits with dyspnoea. Centre the x-ray beam on the middle of the thoracic spine, along the midline.
The x-ray should be taken, in laterolateral (for maximum information both right and left) and dorsoventral standard views, when the patient is at maximum inspiration to enhance evaluation of the lung fields.
The rabbit is in right or left lateral recumbency, with the pelvic limbs extended caudally to avoid superimposition of the femurs over the caudal abdominal area. Centre the x-ray beam in the mid-abdomen, with the field of the beam extending from the caudal thorax to include the whole pelvis.
With the patient in dorsal recumbency on the cassette, the thoracic limbs can be extended cranially or left in a neutral position; the pelvic limbs can be slightly abducted or extended caudally and secured with tape. Centre the x-ray beam in the midline of the middle portion of the lumbar spine to include the caudal thorax and the whole pelvis.
The rabbit is in right or left lateral recumbency, with the limb of interest on the cassette and the contralateral extended cranially or caudally to avoid superimposition. Caudocranial/palmardorsal and craniocaudal/dorsopalmar views should be taken.
The rabbit is positioned in lateral recumbency, with the legs kept parallel and hyperextended caudally. A slight oblique view of the pelvis can be obtained by holding the leg on the cassette parallel and hyperextending the controlateral caudally or caudally with slight abduction, to prevent superimposition of the femurs.
A ventrodorsal projection of the pelvis and craniocaudal of the femurs can be obtained with the rabbit placed in dorsal recumbency. The limbs can be left in a neutral position, slightly abducted to obtain a “frog position” of the hip joint or extended caudally for a more standard view of the hips and the head of the femurs. Ideally, the distal femurs should be rotated internally, keeping the diaphyses parallel to perform an appropriate radiographic study of the pelvis.
Liquid barium sulphate can be used (3ml/kg to 10ml/kg) by mouth or via an orogastric tube for upper gastrointestinal (GI) tract contrast study, which can be useful to identify gastric hairballs in the stomach or other foreign bodies along the upper GI tract. Iodinebased solutions can be used instead if there is suspicion of GI perforation because barium can be responsible for severe inflammatory reactions in the abdominal cavity.
Iothalamate sodium or meglumine (2ml/kg) can be given intravenously to perform an excretory urogram. Meglumine can also be injected directly into the bladder via a urinary catheter to perform a cystography and identify filling defects or other wall abnormalities. Iodinebased contrast media can also be used for myelography and urinary contrast study, as well as for dacryocystorhinography (contrast study of the nasolacrimal duct; Capello and Lennox, 2008; Redrobe, 2001).
Knowledge of the normal radiographic anatomy is necessary to be able to appreciate radiographic changes due to disease processes. The same principles used for radiographic interpretation in other species can be applied for rabbits. Therefore, any change in symmetry, size, shape, number, location, margins and opacity of body parts on each view need to be carefully assessed. Radiographic images need to be evaluated also, considering the clinical signs of the patient.
Figure 2 (above) and Figure 3 (below). Laterolateral and ventrodorsal view of the abdomen of the same rabbit with gastrointestinal syndrome.
Figure 4. Patient positioning is of utmost importance to enable radiographic image interpretation.
Figure 5. Laterolateral projection of the skull of a rabbit with dental disease.
Elisabetta Mancinelli
Job Title