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DIAGNOSTIC IMAGING IN RABBITMEDICINE – RADIOGRAPHY

Author : ELISABETTA MANCINELLI

Categories : Vets

Date : May 19, 2014

Radiology is a common diagnostic procedure in any field of clinical veterinarypractice – and exotic animal medicine is no exception. Alongside physical examination,haematology and biochemistry analysis, radiography and ultrasonography are usefuldiagnostic tools – often complementary and certainly the most commonly performeddiagnostic imaging.

The range of potential abnormalities that might be faced in clinical practice and detected bydifferent imaging modalities is very broad. An array of other imaging techniques – such asendoscopy, CT and MRI – are also available in veterinary medicine that may help in achieving adefinitive diagnosis. Radiography will be discussed in the first part of this article, whereasultrasonography and more advanced imaging modalities will be considered in part two.

Radiography

X-rays are a type of ionising electromagnetic radiation that travel through space at the speed oflight. 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 torecord the radiographic image on a conventional screen-film system. Three parameters influencethe quality and quantity of x-rays produced: milliamperage (mA), exposure time and kilovoltagepeak (kVp).

Milliamperage

mA indicates the number of electrons produced by the cathode. Increasing the mA will increase thenumber of electrons that collide with the target and, therefore, the quantity of x-ray produced.

Exposure time

Exposure time – usually a fraction of a second – indicates the period of x-ray production. Increasingthe exposure time will also increase the number of x-rays produced. Usually, because of the rapidrespiratory rate of exotic animals, short exposure times (less than 1/60sec) are needed.

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Kilovoltage peak

kVp is the speed at which the electrons hit the target. Its increase means the electrons between theanode 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 theskull).

Usually, an x-ray machine capable of producing 40kV to 70kV, 300mA and exposure times of 0.008seconds to 0.16 seconds is recommended in exotic animal practice (Redrobe, 2001). A tubeallowing 90° rotation for horizontal beam radiography is also very useful. The focalfilm distancegenerally 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 aradiographic film placed behind it. There is different absorption depending on tissue compositionand thickness.

Bones absorb most of the x-ray photons by photoelectric processes. This is because bones have ahigher electron density than soft tissues. The x-rays passing through the soft tissue leave a latentimage in the photographic film.

When the film is processed, the parts of the image corresponding to higher x-ray exposure aredark, leaving a white shadow of bones on the film. The x-rays not absorbed by the patient passthrough and are recorded on a film-screen within a lightproof cassette. The cassette contains oneor 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 theyprovide sharper images and better details for our small sized patients requiring longer exposuretime (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 andproduces white areas on radiographs. It is, therefore, said to be radiopaque or radiodense. Airabsorbs 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 – fromleast 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 somedisease processes in soft tissue. Some examples are the very common chest x-ray – which can beused to identify lung diseases such as pneumonia, neoplasia or pulmonary oedema – and theabdominal x-ray, which can detect gastrointestinal impaction or obstruction, free air (from visceralperforations) and free fluid (ascites), or the presence of soft tissue masses.

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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 softtissues such as the brain or muscle. X-rays are also commonly used in dentistry, as x-ray imagingis useful in the diagnosis of common oral problems in rabbits.

Digital radiography

Digital radiography (DR) is a form of x-ray imaging where digital x-ray sensors are used instead oftraditional photographic film. Advantages include time efficiency through bypassing chemicalprocessing and the ability to digitally transfer and enhance images. DR is essentially filmless x-rayimage capture. In place of x-ray film, a digital image capture device is used to record the x-rayimage and make it available as a digital file that can be presented for interpretation and saved aspart of the patient’s medical record.

The advantages of DR over film include immediate image preview and availability, a wider dynamicrange – which makes it more forgiving for over and underexposure – as well as the ability to applyspecial image processing techniques that enhance overall display of the image. There are twobasic digital imaging systems: computed radiography and digital radiography.

Computed radiography

Computed radiography (CR) uses very similar equipment to conventional radiography, but theimaging plate is run through a special laser scanner, or CR reader, which reads and digitises theimage. 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 digitalimage that can be enhanced for soft copy diagnosis or further review. CR and DR can both presentan image very quickly after exposure, but CR generally involves the use of a cassette that housesthe imaging plate, similar to traditional film-screen systems, to record the image. DR insteadcaptures the image directly on to a flat panel detector, without the use of a cassette. The digitalinformation 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 patientbeing x-rayed is viewed in real time on a monitor or display. In its simplest form, a fluoroscopeconsists 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 devicevideo camera, allowing the images to be recorded and played on a monitor (Capello and Lennox,2008).

Obtaining radiographs

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The majority of exotic patients, including rabbits, can be placed directly on the radiographiccassette (no grid is needed). A grid may be used for larger breeds where the body thickness islikely 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, theserecommendations are not very useful when using mammography films. A chart simply based onmeasurement/area of interest is inadequate when working on exotic species (tissues ofherbivorous species differ from those of carnivorous ones), therefore it is highly recommendedevery 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 NominaAnatomica 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 itshead and the cassette underneath the patient, is a ventrodorsal projection. Oblique views are moredifficult, but are named in the same way – beam entry to beam exit and adding, to the directionalterms, the degree of obliquity.

Collimators are lead plates on the x-ray machine that permit the area of interest to be outlined andirradiated by the x-ray beam, so scattering can be reduced, providing better quality imaging andlimiting personnel exposure (Capello and Lennox, 2008).

Patient positioning

Correct positioning of the patient is critical to obtain quality diagnostic radiographic imaging and toreduce the risk of diagnostic errors. Sedation or general anaesthesia are safer and less stressfulfor the patient than restraint of the conscious animal. They can facilitate positioning, reducing theneed for repeated exposures due to movements of the patient (Capello and Lennox, 2011). Whenpharmacologic restraint is contraindicated, radiographs can be taken by covering the rabbit’s eyeand using sandbags to evoke an immobile state. This procedure should be immediatelydiscontinued if the patient shows any signs of stress. Trancing or “tonic immobility” is, in fact, afear response and, furthermore, the period of time for which this immobile state is maintained isvariable. In some cases, this may be preferable to inducing anaesthesia for a short, non-painfulprocedure. Endotracheal intubation may also be considered if a long sequence of x-rays has to betaken, but preferably not performed when examining the head to avoid artefacts and complicateimage 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 thepatient to the radiographic film, without interfering with the images. Appropriate radiographicinterpretation usually requires more than one view to be taken for each anatomic area studied.

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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 improveassessment of unilateral lesions.

Appropriate radiation safety measures have to be taken in any situation to avoid, or reduce the riskof, personnel exposure.

Total body

The whole body projection generally includes all structures (perineal area too) except for the tailand 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 beideal 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 thepatient is placed in dorsal (or ventral for the dorsoventral) recumbency; head and limbs in neutralposition.

Head

Lateral view

The patient is placed in right or left lateral recumbency, with the head flat and horizontal. Thephiltrum should be parallel to the cassette. Pieces of radiolucent foam can be used to appropriatelyraise the head if necessary. The x-ray beam is centred just rostral and ventral to the eye.

Oblique projections

The patient is placed in right or left lateral recumbency, with the head slightly tilted anti-clockwise of10° to 20° from a true lateral position.

Dorsoventral projection

The patient is placed in ventral recumbency, with the neck hyperextended and secured with tapeso the head is parallel to the cassette. The x-ray beam is directed between the eyes in the midline.

Skyline view (craniocaudal)

The patient is positioned in dorsal recumbency, with the head hyperflexed so the palate isperpendicular to the cassette and the head appears symmetrical and not deviated laterally.Respiration may be impaired if this position is maintained for too long.

Thorax

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Lateral projection

The patient is placed in left or right lateral recumbency, with the thoracic limbs extended craniallyand held with a sandbag or tape. The neck is extended gently. The beam is directed on the caudalborder of the scapulae, with the field of the x-ray beam including cranial abdomen and caudal neck.

Dorsoventral projection

The rabbit is in ventral recumbency on the cassette, with the thoracic limbs extended cranially andtaped securely to either side of the head, to minimise superimposition of the scapulae andassociated musculature on the cranial thorax. Remember to position the ears to avoidsuperimposition with the thoracic cavity. This position is more comfortable and less stressful thanthe ventrodorsal – especially in rabbits with dyspnoea. Centre the x-ray beam on the middle of thethoracic spine, along the midline.

The x-ray should be taken, in laterolateral (for maximum information both right and left) anddorsoventral standard views, when the patient is at maximum inspiration to enhance evaluation ofthe lung fields.

Abdomen

Lateral projection

The rabbit is in right or left lateral recumbency, with the pelvic limbs extended caudally to avoidsuperimposition 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.

Ventrodorsal projection

With the patient in dorsal recumbency on the cassette, the thoracic limbs can be extended craniallyor left in a neutral position; the pelvic limbs can be slightly abducted or extended caudally andsecured with tape. Centre the x-ray beam in the midline of the middle portion of the lumbar spine toinclude the caudal thorax and the whole pelvis.

Thoracic limbs

The rabbit is in right or left lateral recumbency, with the limb of interest on the cassette and thecontralateral extended cranially or caudally to avoid superimposition. Caudocranial/palmardorsaland craniocaudal/dorsopalmar views should be taken.

Pelvis and pelvic limbs

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The rabbit is positioned in lateral recumbency, with the legs kept parallel and hyperextendedcaudally. A slight oblique view of the pelvis can be obtained by holding the leg on the cassetteparallel and hyperextending the controlateral caudally or caudally with slight abduction, to preventsuperimposition of the femurs.

A ventrodorsal projection of the pelvis and craniocaudal of the femurs can be obtained with therabbit placed in dorsal recumbency. The limbs can be left in a neutral position, slightly abducted toobtain a “frog position” of the hip joint or extended caudally for a more standard view of the hipsand the head of the femurs. Ideally, the distal femurs should be rotated internally, keeping thediaphyses parallel to perform an appropriate radiographic study of the pelvis.

Contrast media

Liquid barium sulphate can be used (3ml/kg to 10ml/kg) by mouth or via an orogastric tube forupper gastrointestinal (GI) tract contrast study, which can be useful to identify gastric hairballs inthe stomach or other foreign bodies along the upper GI tract. Iodinebased solutions can be usedinstead if there is suspicion of GI perforation because barium can be responsible for severeinflammatory reactions in the abdominal cavity.

Iothalamate sodium or meglumine (2ml/kg) can be given intravenously to perform an excretoryurogram. Meglumine can also be injected directly into the bladder via a urinary catheter to performa cystography and identify filling defects or other wall abnormalities. Iodinebased contrast mediacan also be used for myelography and urinary contrast study, as well as fordacryocystorhinography (contrast study of the nasolacrimal duct; Capello and Lennox, 2008;Redrobe, 2001).

Radiographic interpretation

Knowledge of the normal radiographic anatomy is necessary to be able to appreciate radiographicchanges due to disease processes. The same principles used for radiographic interpretation inother 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.

References

Redrobe S (2001). Imaging techniques in small mammals, Sem Av Exot Pet Med 10(4):187-197.Capello V and Lennox A M (2008). The basics of radiology. In Clinical Radiology of ExoticCompanion Mammals, Wiley-Blackwell, Ames, Iowa: 1-51.Capello V and Lennox A M (2011). Diagnostic imaging of the respiratory system in exotic

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companion mammals, Vet Clin North Am Exot Anim Pract 14(2): 369-389.Ludewig E, Pees M and Morgan J P (2012). Clinical technique: digital radiography in exoticpets – important practical differences compared to traditional radiography, J Exot Pet Med 21(1): 71–79.

Figure 1. Contrast media and air can be injected into the bladder for double contrast studies.

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Figure 2 (above) and Figure 3 (below). Laterolateral and ventrodorsal view of the abdomen of thesame rabbit with gastrointestinal syndrome.

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Figure 4. Patient positioning is of utmost importance to enable radiographic image interpretation.

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Figure 5. Laterolateral projection of the skull of a rabbit with dental disease.

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