Veterinary · Clinical · also Veterinary Radiology
X-ray physics and radiation safety, positioning and interpretation, contrast studies, ultrasonography (including reproductive), endoscopy and the basics of CT and MRI.
In the exams: ECFVG / NEB CPE radiographic positioning section; NAVLE items with images (about 15-20%); ICAR AIEEA-PG 13 (Surgery & Radiology)
Diagnostic imaging is the part of veterinary practice that looks inside a living animal without cutting it open. Radiography uses X-rays to record differences in how tissues absorb radiation; ultrasonography uses reflected sound to show soft tissue in real time; endoscopy puts a camera inside a hollow organ or body cavity; computed tomography (CT) and magnetic resonance imaging (MRI) produce cross-sectional images. Each modality answers a different question, and much of the skill lies in choosing the right one, making a diagnostic image with the least risk to patient and staff, and reading it systematically.
The subject matters in every kind of practice. A mixed practitioner radiographs a dog with a cough, a lame horse's fetlock and a cow suspected of hardware disease in the same week; a dairy veterinarian scans hundreds of cows a month for pregnancy; an emergency clinician uses a focused ultrasound scan to find free abdominal fluid within minutes of a road accident. Poor technique wastes radiation, repeats exposures and produces films that hide lesions, and poor radiation practice injures people, so the physics and the law are as practical as the interpretation.
Imaging is examined worldwide. It sits inside the Surgery and Radiology papers of India's ICAR PG and PhD entrance exams and state veterinary officer papers, the diagnostics domains of the BCSE and PAVE, image-based items on the NAVLE, a timed radiographic positioning station in the ECFVG clinical skills exam, and the imaging topics of the Australasian, British, Nepali, Philippine, South African and Chinese exams. Specialists sit the ACVR or ECVDI diplomas. This manual covers the principles and practice-level interpretation that all of these expect. Radiation law differs by country, so it is described in general terms with examples; always follow your national regulator and your practice's local rules.
X-rays are electromagnetic radiation of very short wavelength and high energy, able to pass through tissue and to ionise atoms, which is why they are useful and why they are harmful. In an X-ray tube, a heated filament in the cathode releases electrons by thermionic emission. The filament is usually tungsten, set in a negatively charged focusing cup that directs the electron stream. A high potential difference, measured in kilovolts, accelerates the electrons across a vacuum inside a glass or metal envelope to strike the target on the anode. The target is tungsten because it has a high atomic number (74), which makes X-ray production more efficient, and a very high melting point, which lets it survive the heat.
When the electrons strike the target, two kinds of X-ray are produced. Bremsstrahlung ("braking") radiation arises when an electron is slowed and deflected by a nucleus; it gives a continuous spectrum of energies and makes up most of the beam. Characteristic radiation arises when an incoming electron knocks out an inner-shell electron and an outer-shell electron drops into the gap, releasing a photon of an energy fixed by the target element. Less than 1% of the electrons' energy becomes X-rays; about 99% becomes heat. Heat is therefore the limiting factor in tube design and in how many exposures can be made in quick succession. Large machines use a rotating anode, which spreads the heat over a ring of target, and an oil bath around the tube to conduct it away.
The focal spot is the area of the target struck by electrons. A small focal spot gives a sharper image, because the X-rays behave more like a point source and the blurred edge around each structure (the penumbra, or geometric unsharpness) is smaller. A large focal spot tolerates more heat. The line focus principle resolves this conflict: the target face is angled, so the actual area struck is long and narrow, but seen from the patient's side the effective focal spot is much smaller. Many machines offer a fine focus for small parts and extremities and a broad focus for large exposures.
Because the target is angled, X-rays leaving towards the anode side pass through more target material and are partly absorbed. The beam is therefore less intense on the anode side than on the cathode side. This is the heel effect. It matters when a body part varies in thickness: the thicker end should be placed towards the cathode.
Two exposure factors are set on the control panel. The kilovoltage peak (kVp) determines the energy, and therefore the penetrating power or quality, of the beam; it has the greatest effect on image contrast. The milliampere-seconds (mAs), the tube current multiplied by exposure time, determines how many X-ray photons are produced, the quantity of the beam, and therefore the overall blackening or density of a film. Doubling mAs doubles the number of photons. A short exposure time with high mA is chosen when motion, such as panting or breathing, would otherwise blur the image.
Every practice should use a written exposure chart made for its own machine and image receptor, with settings for each region by measured thickness. The part is measured with calipers at the thickest point in the path of the beam, in centimetres. Two rules of thumb help when building or adjusting a chart. Santes' rule estimates kVp as twice the thickness in centimetres, plus the source-to-image distance in inches, plus a grid factor when a grid is used; it is an approximation and is refined by trial. The 15% rule states that increasing kVp by about 15% has roughly the same effect on film density as doubling the mAs, and reducing kVp by about 15% is roughly offset by halving it. Raising kVp in this way lowers contrast, because more photons penetrate every tissue.
The intensity of radiation falls with the square of the distance from the source (the inverse square law). If the source-to-image distance is doubled, the intensity at the image falls to one quarter, so the mAs must be multiplied by four to keep the same density. The same law is the basis of distance as a radiation-protection measure, discussed in the next chapter.
An image forms because tissues attenuate the beam to different degrees. In the diagnostic range two interactions matter. In the photoelectric effect a photon is completely absorbed by an inner-shell electron; the probability rises steeply with the atomic number of the tissue (roughly with its cube) and falls as photon energy rises. Photoelectric absorption produces contrast between bone, which contains calcium, and soft tissue, and it dominates at low kVp. In Compton scatter a photon strikes an outer-shell electron, loses part of its energy and changes direction. Scatter adds no useful information, fogs the image and is the main source of radiation dose to people in the room. It increases with higher kVp and with thicker, larger volumes of tissue.
Five radiographic opacities are recognised, from least to most opaque: gas, fat, soft tissue or fluid (which look the same), mineral or bone, and metal. A structure is only visible where its border lies against a tissue of different opacity. When two structures of the same opacity touch, the border between them disappears; this is border effacement, also called the silhouette sign. It explains why a heart silhouette vanishes in pleural effusion and why serosal detail is lost when the abdomen contains free fluid or when a young or very thin animal lacks abdominal fat.
Grids reduce scatter reaching the receptor. A grid is a plate of thin lead strips separated by radiolucent spacers; the grid ratio is the height of the strips divided by the gap between them. Higher ratios remove more scatter but need more exposure. A grid is generally recommended when the part is more than about 10 cm thick. A moving grid (a Potter-Bucky diaphragm) blurs out the grid lines. A focused grid placed upside down, or used at the wrong distance, absorbs useful beam at the edges (grid cut-off).
In film-screen radiography, the film carries an emulsion of silver halide crystals (mainly silver bromide). The cassette holds intensifying screens whose phosphor converts X-rays to light, so far less radiation is needed than with film alone. Older screens used calcium tungstate; rare-earth phosphors such as gadolinium oxysulphide are faster. Exposure creates an invisible latent image. The developer reduces exposed silver halide to black metallic silver; the fixer (a thiosulphate solution) dissolves the unexposed silver halide and hardens the emulsion; washing removes chemicals so the film does not discolour. Film has a narrow exposure latitude, so both under- and over-exposure spoil the image.
Most practices now use digital radiography. Computed radiography (CR) uses a cassette containing a photostimulable phosphor plate that stores the energy; a laser in a reader releases it as light, which is digitised, and the plate is erased for reuse. Direct digital radiography (DR) uses a flat-panel detector. Indirect DR has a scintillator (such as caesium iodide) that converts X-rays to light, read by an array of thin-film transistors; direct DR uses a photoconductor such as amorphous selenium that converts X-rays straight to charge. Images are stored and exchanged in the DICOM standard and archived in a picture archiving and communication system (PACS). Digital systems have wide latitude and the brightness can be adjusted after exposure, which saves repeats but encourages "dose creep", the habit of over-exposing because the image still looks good. Each system reports an exposure indicator that should be checked.
A diagnostic image needs adequate density (or signal), appropriate contrast, sharp detail and correct geometry. Contrast is the difference between adjacent opacities. A low-kVp technique gives short-scale, high contrast, suited to bone; a high-kVp technique gives long-scale, low contrast with more shades of grey, suited to the thorax, where many soft-tissue and gas densities must be distinguished.
Sharpness is lost through movement, a large focal spot, a long object-to-image distance, and screen or detector blur. Magnification equals the source-to-image distance divided by the source-to-object distance, so it is reduced by keeping the part as close to the receptor as possible and the source as far away as practical. Distortion, an unequal magnification of different parts, occurs when the part or the beam is not aligned: the part should be parallel to the receptor and the central ray perpendicular to both.
Ionising radiation damages cells mainly through DNA damage, directly or through free radicals formed from water. Rapidly dividing cells are most sensitive: gonads, bone marrow, lens of the eye, skin, and the developing embryo and foetus. Effects are of two kinds. Deterministic (tissue) effects, such as skin erythema, cataract or radiation burns, appear only above a threshold dose and become more severe as the dose rises. Stochastic effects, cancer and heritable genetic change, have no known safe threshold; the probability of the effect, not its severity, rises with dose. Radiation protection therefore aims to keep deterministic effects from happening at all and to keep stochastic risk as low as reasonably achievable.
Absorbed dose is the energy deposited per kilogram of tissue, measured in grays (1 Gy = 1 J/kg; the older unit is the rad, and 1 Gy = 100 rad). Equivalent dose weights absorbed dose by the type of radiation, and effective dose further weights it by the sensitivity of each tissue; both are measured in sieverts (Sv; older unit the rem, 1 Sv = 100 rem). For X-rays the radiation weighting factor is 1, so 1 Gy gives an equivalent dose of 1 Sv. Staff doses are usually reported in millisieverts (mSv) or microsieverts.
The International Commission on Radiological Protection (ICRP) sets the framework most countries follow: justification (every exposure must do more good than harm), optimisation (doses kept as low as reasonably achievable, ALARA) and dose limitation. ICRP recommends an occupational effective-dose limit of 20 mSv a year averaged over five years, with no more than 50 mSv in any single year; 1 mSv a year for members of the public; and, since 2011, an occupational limit of 20 mSv a year to the lens of the eye averaged over five years. Once a worker declares a pregnancy, the dose to the embryo or foetus should be kept to about 1 mSv for the remainder of the pregnancy. National law adopts these values with local variations, and a well-run veterinary practice keeps staff doses far below them.
The three physical tools are time, distance and shielding. Minimise the time the beam is on and the number of exposures by getting technique and positioning right the first time. Maximise distance, because of the inverse square law; stand at least 2 m from the primary beam where the room allows, and use an exposure switch on a cable long enough to do so. Use shielding: lead-lined walls or mobile screens, and personal protective equipment such as lead-rubber aprons, thyroid shields, gloves and glasses. Aprons are commonly 0.25 to 0.5 mm lead equivalent. Protective clothing attenuates scatter; it does not make it safe to put hands in the primary beam, and no part of any person, even gloved, should be in the primary beam. Collimate the beam to the area of interest, with the collimation edges visible on every image, because a smaller field produces less scatter and a better image.
Restraint is the central veterinary problem. Wherever possible the animal is positioned with sedation or general anaesthesia and with sandbags, foam wedges, troughs and ties, and nobody stays in the room. Many regulators and national guidance documents treat manual restraint as exceptional, to be justified case by case, and forbid it by persons under 18 years and pregnant staff. Large-animal radiography often needs a person to hold the cassette; long cassette holders and careful beam direction keep them out of the primary beam.
Staff who work with radiation wear personal dosimeters, such as film badges, thermoluminescent dosimeters (TLD, usually lithium fluoride) or optically stimulated luminescence (OSL) badges, in the position their local rules specify, usually on the trunk under the apron, and send them for reading at fixed intervals. An area around the X-ray machine where special procedures are needed is designated a controlled area, with warning signs and lights, and entry limited while the machine is energised.
The law differs by country but has a common pattern: the practice must register or license its X-ray equipment with a national regulator, appoint a responsible person to supervise day-to-day radiation safety, obtain advice from a qualified expert, write local rules, keep dose records and train staff. Examples include the Ionising Radiations Regulations in the United Kingdom, where a practice appoints a Radiation Protection Supervisor and consults a Radiation Protection Adviser; the Atomic Energy Regulatory Board (AERB) in India, which licenses medical and veterinary X-ray equipment under the Atomic Energy Act; and state radiation-control programmes in the United States, guided nationally by the National Council on Radiation Protection and Measurements, which publishes a report on radiation protection in veterinary medicine. Check your own national regulator for the current rules.
A radiographic projection is named by the point where the central ray enters the body followed by the point where it exits, using standard directional terms agreed by veterinary radiology bodies. Ventrodorsal (VD) means the beam enters the ventral surface and leaves dorsally, so the animal lies on its back; dorsoventral (DV) means the animal lies on its sternum. A lateral projection is named by the side nearer the receptor: a right lateral view is taken with the animal in right lateral recumbency. On the limbs, cranial and caudal are used proximal to the carpus and tarsus, and dorsal, palmar (forelimb) and plantar (hindlimb) are used from the carpus and tarsus distally. On the head, rostral replaces cranial. Oblique views combine terms, for example dorsolateral-palmaromedial oblique (DLPMO), with the angle stated in degrees.
Radiographs are hung or displayed in a standard orientation so that everyone reads them the same way. Lateral views are displayed with the animal's head to the viewer's left. VD and DV views of the trunk are displayed with the head at the top and the animal's right side on the viewer's left. Limbs are displayed with the proximal end at the top. Every image must carry the patient identity, date and a left or right marker placed in the field at the time of exposure; adding a side marker after the event is unreliable.
Two views at right angles (orthogonal views) are the minimum for almost every region, because a radiograph compresses three dimensions into two. A lesion seen on one view only may be superimposed or misplaced. Both sides are often compared in young animals, whose growth plates are easily mistaken for fractures.
For the thorax, the exposure is made at peak inspiration, when the lungs are most aerated and contrast is best. The field extends from the thoracic inlet to the first lumbar vertebra, centred roughly at the caudal border of the scapula, with the forelimbs pulled cranially so they do not overlie the cranial thorax. In lateral recumbency, the dependent lung loses air and becomes more opaque, so a lesion in the dependent lung can be hidden. For this reason a metastasis check uses three views: left lateral, right lateral and a VD or DV. A lesion is best seen in the uppermost (non-dependent) lung. The DV view keeps the heart in a more consistent position and shows the caudal lobar vessels well, and it is safer for a dyspnoeic animal, which should not be forced onto its back. The VD view shows the accessory lung lobe and the right middle lobe region better.
For the abdomen, the exposure is made at end-expiration, when the diaphragm is cranial and the abdominal organs are least crowded. The field runs from the diaphragm to the hip joints, centred just caudal to the last rib. Food and faeces obscure detail, so elective studies follow a fast and, when needed, an enema. The right lateral view is standard for suspected gastric dilatation-volvulus because it shows gas in the displaced pylorus.
The spine is radiographed with the animal under general anaesthesia so that it can be positioned truly straight, with foam pads under the neck and sternum so the vertebral column is parallel to the table. Several short, tightly collimated views centred over the region of interest are better than one long view, because the divergent beam distorts disc spaces away from the centre.
Limb radiographs are centred on the joint or bone of interest, include the joint above and below for long-bone fractures, and are collimated tightly. Mediolateral and craniocaudal (or dorsopalmar/dorsoplantar) views are standard, with flexed and stressed views where needed. The elbow is examined in extended and flexed mediolateral views and a craniocaudal view; a flexed mediolateral view shows the anconeal process. The skull needs general anaesthesia and a range of views, including open-mouth, oblique views for dental arcades and tympanic bullae, and a rostrocaudal view for the frontal sinuses.
The hip-extended VD view is used for screening hip dysplasia in official schemes run by kennel clubs and veterinary associations in many countries. The dog lies on its back, the pelvis is symmetrical, the femurs are extended parallel to each other and to the table, and the patellae are centred over the femoral trochlea. Most schemes require sedation or anaesthesia for correct positioning. Schemes differ in grading systems and minimum age, and some use additional stress views, such as distraction radiography, to measure joint laxity. Exact requirements must be taken from the scheme being used.
Equine radiography is done mostly in the standing, often sedated, horse with portable or mobile equipment. Limb views are taken with the foot on a positioning block so that the beam can be centred on the region of interest, and the cassette is held in a holder with a long handle. The foot is prepared by removing the shoe when needed, cleaning the sole and packing the frog sulci with a radiolucent material of similar opacity to hoof (such as a soft putty) to avoid gas-line artefacts crossing the navicular bone.
Standard views of the foot include the lateromedial, the dorsoproximal-palmarodistal oblique (upright pedal, with the beam angled at about 65 degrees to the ground and the toe on a tunnel block) to show the navicular bone and distal phalanx, and the palmaroproximal-palmarodistal oblique ("skyline") view of the navicular bone. The fetlock, carpus and tarsus each need at least four views: lateromedial, dorsopalmar or dorsoplantar, and two obliques, because chip fractures and osteochondral fragments hide on the curved margins.
Radiography in food animals is limited by patient size, equipment output and economics, so it is used for questions it can answer quickly. The commonest indications are limb lameness (fractures, septic arthritis of the distal interphalangeal joint, osteomyelitis of the distal phalanx), traumatic reticuloperitonitis and neonatal problems. The reticulum is radiographed in the standing cow with a lateral view centred low over the cranial abdomen just behind the elbow; high-output equipment is needed. The film may show a wire perforating the reticular wall, gas or fluid lines in an abscess, or a magnet with metal attached, which indicates that the magnet is doing its job. Calves, sheep, goats and small camelids can be radiographed in lateral recumbency, much like large dogs, and their thorax and abdomen can be examined in full.
Birds are radiographed under brief general anaesthesia, which is safer than struggling restraint for a species with air sacs and no diaphragm. A standard study is a whole-body VD view, with the wings extended and the keel superimposed on the spine, and a lateral view with the wings pulled dorsally and the legs caudally. High-detail, low-exposure settings and fine focus are used. Common findings include heavy-metal particles in the ventriculus (gizzard), egg binding, fractures and enlarged organs.
In chelonians (tortoises and turtles), the shell hides the organs on a VD view, so three views are standard: dorsoventral, horizontal-beam lateral and horizontal-beam craniocaudal. The horizontal-beam views keep the lungs uncompressed and show each lung field separately, which is essential for diagnosing pneumonia. Rabbits and rodents need detailed skull views for dental disease, which is one of their commonest problems; oblique views separate the cheek teeth arcades.
Interpretation begins before looking for disease: check the patient's identity, species, age, the side marker and the views available, then assess whether exposure and positioning are adequate. Then read the whole image in a fixed order, not just the region of the clinical complaint, because incidental findings, such as a lung nodule on an abdominal film, change management. Each abnormality is described using the Roentgen signs: number, size, shape, margination, opacity, location and function. Only then are the signs matched to a list of differential diagnoses, ranked by the history and clinical signs. Writing a report forces this discipline: findings, then conclusions, then recommendations such as further views, ultrasonography or sampling.
On a lateral thoracic view of a dog, the heart normally spans about two and a half to three and a half intercostal spaces, depending on breed and chest conformation. A more objective measure is the vertebral heart score (VHS): the long axis (from the carina to the apex) and the short axis (the greatest width perpendicular to it, in the central third) are each measured against the thoracic vertebrae beginning at T4, and the two values are added. A commonly quoted canine mean is about 9.7 vertebrae, but normal values vary markedly between breeds, and many breeds have their own published reference ranges; in cats values are lower. VHS is a screening measurement, not a diagnosis.
Signs of chamber enlargement are learned as patterns. Left atrial enlargement lifts the caudal trachea and left mainstem bronchus dorsally on the lateral view and may separate the mainstem bronchi on the DV view. Left ventricular enlargement gives a taller heart and a straight caudal border. Right heart enlargement increases sternal contact on the lateral view and gives a rounded right border, like a reversed letter D, on the DV. Pulmonary vessels are assessed by comparing the artery and vein that run together to each lung lobe: enlarged veins suggest left-sided congestive heart failure, enlarged arteries suggest pulmonary hypertension or heartworm disease. A generalised globoid cardiac silhouette with very sharp margins suggests pericardial effusion, which ultrasonography confirms.
Lung changes are classified into patterns. An alveolar pattern is fluid or cells filling the air spaces; its hallmarks are air bronchograms (air-filled bronchi outlined by opaque lung) and the lobar sign (a lobe outlined sharply against aerated lung). Causes include pneumonia, oedema, haemorrhage and atelectasis; location helps, since bacterial aspiration pneumonia in dogs is often cranioventral and cardiogenic oedema in dogs is typically perihilar and caudodorsal, though in cats oedema is often patchy and irregular. An interstitial pattern is increased opacity of the lung tissue itself; it may be unstructured (hazy) or structured nodular, as in metastasis or fungal disease. A bronchial pattern shows thickened bronchial walls as rings end-on and parallel lines side-on, typical of feline asthma and chronic bronchitis. A vascular pattern is a change in vessel size.
Pleural effusion produces fissure lines between lobes, lung lobes retracted from the chest wall, rounded lung margins at the costophrenic angles and a heart border that is effaced by fluid. Pneumothorax produces lungs retracted from the chest wall with no vascular markings peripherally and, on a lateral view, the heart apparently lifted away from the sternum because the collapsed lungs no longer support it in position. A diaphragmatic hernia shows an incomplete diaphragmatic line, abdominal organs or gas-filled bowel in the thorax and abdominal organs missing from their normal place. Megaoesophagus appears as a gas- or fluid-filled tube dorsal to the heart, often with ventral displacement of the trachea, and predisposes to aspiration pneumonia.
Serosal detail, the sharp outline of organs against intra-abdominal fat, is the first thing assessed. Loss of serosal detail means free fluid, peritonitis, carcinomatosis, or a lack of fat in young or emaciated animals. A mottled or streaky loss with gas bubbles outside the bowel indicates pneumoperitoneum; free gas after recent surgery can be normal for some days, but free gas without such a history suggests a ruptured hollow organ and is a surgical emergency.
In gastric dilatation-volvulus the stomach is distended with gas, and on the right lateral view the pylorus is displaced dorsally and to the left, divided from the fundus by a soft-tissue band so that the stomach looks compartmentalised (the "double bubble" or "reversed C" appearance). Simple dilatation without volvulus lacks the compartment. Mechanical small-intestinal obstruction produces two populations of bowel, some grossly dilated and some normal. In dogs a small-intestinal diameter greater than about 1.6 times the height of the fifth lumbar vertebral body at its narrowest point suggests obstruction; cats use different, smaller thresholds. Linear foreign bodies cause plication, with the bowel bunched and containing small, tear-shaped gas bubbles.
The liver is judged by the position and angle of the gastric axis; a small liver shifts the stomach cranially, as in portosystemic shunts. Kidney size is compared with the length of the second lumbar vertebra (L2), with normal ranges that differ between dogs and cats. Urinary calculi of struvite and calcium oxalate are usually radiopaque, whereas urate and cystine calculi are often radiolucent or only faintly opaque and may need contrast studies or ultrasonography. Mineral in the urethra of a male dog is often trapped just caudal to the os penis, so the whole urethra must be included in the field.
Bone reacts in only two ways, by losing mineral (lysis) or by producing new bone, and the pattern of these reactions indicates how aggressive a lesion is. Aggressive features include cortical destruction, a long, poorly defined zone of transition between normal and abnormal bone, moth-eaten or permeative lysis, and interrupted or spiculated periosteal reaction (sunburst patterns, or a Codman triangle where new bone lifts off at the edge). Non-aggressive lesions have a short, sharp zone of transition and smooth, solid periosteal reaction. Aggressive lesions are chiefly neoplasia or osteomyelitis, which can look alike; sampling decides.
In dogs, primary bone tumours, most often osteosarcoma, typically arise in the metaphyses of long bones in large and giant breeds, classically the distal radius, proximal humerus, distal femur and proximal tibia ("away from the elbow, towards the knee"). Osteosarcoma rarely crosses the joint, whereas synovial tumours and infection may affect both sides of a joint. Thoracic radiographs are taken to look for metastasis before treatment.
Developmental diseases have characteristic signs. Panosteitis gives patchy increased medullary opacity in the diaphyses of long bones in young, large-breed dogs. Hypertrophic osteodystrophy produces a radiolucent line in the metaphysis parallel to the physis. Hypertrophic osteopathy produces palisading periosteal new bone along the distal limbs, usually secondary to a thoracic mass. Hip dysplasia shows joint incongruity, subluxation, a shallow acetabulum and later osteoarthritis. Elbow dysplasia includes a fragmented medial coronoid process, ununited anconeal process, osteochondrosis of the humeral condyle and joint incongruity; the fragmented coronoid itself is often hard to see, and secondary osteophytes and sclerosis are the usual clues. Physeal fractures in young animals are classified by the Salter-Harris system, types I to V, according to whether the fracture involves the physis alone or also the metaphysis, epiphysis or both. Degenerative joint disease shows osteophytes, subchondral sclerosis and joint space narrowing.
In horses, radiographs commonly show osteoarthritis, osteochondrosis fragments, chip fractures of the carpus and fetlock, and changes of the navicular bone and distal phalanx. In chronic laminitis, the distal phalanx may rotate away from the dorsal hoof wall or sink distally, and lateromedial views are used to measure its position.
Contrast media change the opacity of a structure so that it can be distinguished from surrounding tissue. Positive contrast agents are more opaque than soft tissue because they contain elements of high atomic number: barium sulphate for the gastrointestinal tract, and iodinated compounds for the vascular, urinary and other systems. Negative contrast agents, air or carbon dioxide, are less opaque. A double-contrast study uses a small amount of positive medium to coat the mucosa and gas to distend the lumen, showing mucosal detail.
Barium sulphate is inert, inexpensive and coats the gut well, but it must not be used when perforation of the gut is suspected, because barium in the peritoneal cavity causes severe granulomatous peritonitis and adhesions, and it is dangerous if aspirated in large volumes. Iodinated agents are either ionic or non-ionic, and high- or low-osmolar. Non-ionic low-osmolar agents cause fewer adverse reactions and are required for myelography, where ionic agents are neurotoxic. Hyperosmolar iodinated agents given orally draw water into the gut, which dilutes them, can dehydrate small or sick patients and cause pulmonary oedema if aspirated. Intravenous iodinated contrast carries a risk of adverse reactions, from vomiting to hypotension, and of contrast-induced kidney injury, particularly in dehydrated patients; patients should be hydrated before the study. Specific products and doses are chosen from the product information and a current formulary.
An oesophagram (barium swallow) evaluates megaoesophagus, strictures, foreign bodies and motility; with suspected aspiration risk, small volumes are used and fluoroscopy shows swallowing in real time. An upper gastrointestinal series follows a positive contrast medium through the stomach and small intestine with radiographs taken at intervals, looking for filling defects, obstruction, abnormal wall thickness and transit time; gastric emptying times vary with species, meal and sedation, so films are compared with the patient's own study rather than with a single fixed value.
The urinary tract offers several studies. Excretory urography (intravenous urography) outlines the kidneys and ureters: an early nephrogram phase shows the renal parenchyma opacified, and a later pyelogram phase shows contrast in the renal pelvis and ureters, useful for ectopic ureters, ureteral obstruction and renal size and shape. Cystography, positive, negative or double contrast, shows bladder rupture, wall thickening, masses and calculi. Retrograde urethrography in males, and vaginourethrography in females, show urethral strictures, tears and calculi. In a patient with suspected bladder or urethral rupture after trauma, positive contrast leaking into the peritoneal cavity or periurethral tissue confirms the diagnosis.
Myelography injects non-ionic iodinated contrast into the subarachnoid space, at the cisterna magna or in the lumbar region, to outline the spinal cord. It classifies lesions as extradural (such as disc extrusion, with the contrast column deviated and thinned), intradural-extramedullary (a "golf tee" widening at the edge of the mass) or intramedullary (the cord swollen and the contrast columns thinned on both views). Seizures after myelography are a recognised complication, especially after cisternal injection and in large dogs. In practices with access to CT and MRI, myelography has largely been replaced by these techniques.
Diagnostic ultrasound uses sound waves well above the range of human hearing, usually in the range of about 2 to 18 megahertz (MHz). A transducer contains piezoelectric crystals, which vibrate and emit sound when a voltage is applied and generate a voltage when returning echoes strike them. The transducer sends a short pulse and then listens. The machine assumes that sound travels at an average of 1540 metres per second in soft tissue and calculates the depth of each reflector from the time taken for the echo to return: depth equals speed multiplied by time, divided by two, because the sound travels to the reflector and back.
The strength of an echo depends on the difference in acoustic impedance between two tissues; acoustic impedance is density multiplied by the speed of sound in that tissue. Soft tissues differ only slightly, so most sound passes on and weak echoes return from many depths. Soft tissue and gas, or soft tissue and bone, differ so much that almost all the sound is reflected, which is why ultrasound cannot see through gas-filled lung or bowel or into bone, and why the hair is clipped, the skin cleaned with alcohol or water and coupling gel applied to remove air between transducer and skin.
Frequency is a trade-off. Higher frequency means shorter wavelength (wavelength equals speed divided by frequency) and better axial resolution, but greater attenuation and less penetration. High frequencies are used for superficial structures, tendons, eyes, cats and small dogs; low frequencies for deep structures in large dogs, horses and cattle.
Linear-array transducers produce a rectangular image, suited to superficial structures, tendons and transrectal reproductive work in large animals. Curvilinear (convex) transducers give a wide, fan-shaped field used for the abdomen. Phased-array (sector) transducers have a small footprint that fits between ribs and are used for echocardiography.
B-mode (brightness mode) is the standard two-dimensional grey-scale image. M-mode (motion mode) plots movement of structures along one line against time and is used to measure cardiac chambers and wall motion; fractional shortening is calculated as the difference between the left ventricular internal diameter in diastole and in systole, divided by the diastolic diameter, multiplied by 100. Doppler ultrasonography detects the frequency shift of echoes from moving blood. Colour Doppler maps direction and mean velocity over an area; pulsed-wave Doppler measures velocity at a chosen point but cannot measure very high velocities without aliasing; continuous-wave Doppler measures high velocities along a line but cannot localise them. The simplified Bernoulli equation converts a peak velocity into a pressure gradient: the pressure difference in mmHg is four times the velocity squared in metres per second.
Operators adjust overall gain, time-gain compensation (which amplifies echoes from deeper tissue to offset attenuation), depth, focal zone and frequency. Echogenicity is described relative to other tissue: anechoic (black, as with clear fluid), hypoechoic, isoechoic and hyperechoic (bright, as with fat, fibrous tissue, bone surface or gas). In the normal dog, the renal medulla is least echogenic, the renal cortex is usually iso- or hypoechoic to the liver, the liver is hypoechoic to the spleen, and the renal sinus fat is brightest. In cats, the renal cortex can be normally hyperechoic because of fat within tubular cells.
Many ultrasound artefacts are diagnostic. Acoustic shadowing is an anechoic band deep to a strongly reflecting or absorbing structure: a clean, black shadow behind a calculus or bone, or a dirty, grey shadow with reverberation behind gas. Acoustic enhancement is increased brightness deep to a fluid-filled structure, such as the gall bladder, a cyst or the urinary bladder, because fluid attenuates less than tissue; it helps distinguish a cyst from a solid hypoechoic nodule. Reverberation produces repeated, equally spaced lines deep to a strong reflector such as gas; comet-tail and ring-down artefacts are forms of it. Mirror-image artefact duplicates a structure on the far side of a strong curved reflector; the classic example is liver and gall bladder appearing to lie beyond the diaphragm-lung interface, mimicking a thoracic lesion. Edge shadowing occurs at the curved margins of rounded structures. Slice-thickness and side-lobe artefacts can place false echoes inside the bladder or gall bladder, mimicking sediment. Knowing these artefacts is necessary before lesions are called.
Abdominal ultrasonography evaluates the size, shape, echogenicity and architecture of the liver, spleen, kidneys, adrenal glands, bladder, prostate, gastrointestinal wall layers, pancreas and lymph nodes, and guides fine-needle aspiration and biopsy. The normal intestinal wall has five alternating layers, and loss of layering suggests infiltrative disease such as neoplasia. Focused assessment with sonography for trauma (FAST) is a short, standardised scan for free fluid at defined abdominal windows (between the diaphragm and liver, near the spleen and left kidney, over the bladder, and in the hepatorenal region) and, in the thoracic version, for pleural and pericardial fluid and pneumothorax. Loss of the normal "glide sign" of lung sliding along the chest wall suggests pneumothorax, and vertical B-lines increase with interstitial fluid.
Echocardiography measures chamber size and function, valve motion and blood flow. In dogs, the ratio of left atrial to aortic diameter on a right parasternal short-axis view is used to assess left atrial size, with values below about 1.5 generally regarded as normal. Pericardial effusion is seen as an anechoic space around the heart, and right atrial collapse indicates tamponade.
Transrectal ultrasonography with a linear transducer, commonly 5 to 7.5 MHz, is the routine tool of dairy and beef herd fertility programmes. It shows the ovaries and uterus more accurately than rectal palpation alone and gives earlier and more information about pregnancy. Follicles appear as round anechoic structures; the preovulatory follicle in a cow is usually well over a centimetre across. A corpus luteum is a grey, moderately echogenic structure, which may contain a normal fluid-filled central cavity that must not be mistaken for a cyst. Follicular cysts are anovulatory fluid-filled structures larger than a normal preovulatory follicle, with thin walls, that persist; luteinised cysts have thicker, echogenic walls. Definitions and size thresholds vary between textbooks, and ultrasound findings should be combined with the herd's reproductive records and, where available, progesterone measurement.
Pregnancy can be diagnosed reliably from about 28 to 30 days after insemination, when an embryo with a heartbeat can be seen within fluid in the uterine horn; fluid alone, without an embryo, is not proof of pregnancy. Early diagnosis lets non-pregnant cows be re-bred sooner. Embryonic and foetal loss is common in the first two months, so many programmes recheck pregnant cows later. Ultrasonography also identifies twins, estimates foetal age from measurements such as crown-rump length, confirms foetal viability by heartbeat, and allows foetal sex determination by the position of the genital tubercle, usually in a window of roughly the second month of gestation (about days 55 to 70). Uterine fluid with echogenic particles in a cow after calving suggests endometritis or pyometra.
In mares, transrectal ultrasonography follows follicle growth to time breeding. During oestrus, oedema of the endometrial folds gives a characteristic "cartwheel" or "wheel spoke" pattern in cross-section, which fades as ovulation approaches. The preovulatory follicle is large compared with that of the cow, often several centimetres across, with size varying by breed and individual. The embryonic vesicle can be detected from about 11 to 14 days after ovulation as a small spherical anechoic structure that moves within the uterus until it becomes fixed, usually at about 16 to 17 days, at the base of one uterine horn. The embryonic heartbeat is usually visible from about the fourth week.
Twin pregnancy is a major cause of abortion in mares, because the equine placenta rarely supports two foals to term. Mares are therefore scanned early, and one vesicle of a twin pair is crushed manually, ideally before fixation, when the vesicles are mobile and can be separated.
In bitches and queens, transabdominal ultrasonography confirms pregnancy, estimates gestational age and assesses foetal viability. Gestational sacs can be detected in the bitch from about three weeks after the luteinising hormone surge and heartbeats shortly afterwards; a scan at about 25 to 30 days is commonly advised for a reliable answer. Ultrasonography is poor at counting large litters. Radiography counts foetuses better after skeletal mineralisation, which becomes visible from about 42 to 45 days after the LH surge, and a late-pregnancy radiograph helps plan whelping by revealing litter size and very large foetuses. Foetal heart rate is normally well above the mother's; persistently slow foetal heart rates indicate foetal distress, and published guidance on intervention thresholds should be consulted. Ultrasonography also diagnoses pyometra, showing a fluid-distended uterus, and cystic endometrial hyperplasia, ovarian cysts, prostatic enlargement, cysts and abscesses, and testicular tumours.
In sheep and goats, transabdominal scanning of the right inguinal region, or transrectal scanning in early pregnancy, is used for pregnancy diagnosis and for counting foetuses so that ewes carrying twins or triplets can be fed accordingly, which reduces pregnancy toxaemia. Commercial flock scanning is usually done within a window around the middle third of gestation, when foetuses are large enough to count but not so large as to overlap.
Endoscopy is the direct visual examination of a hollow organ or body cavity through an instrument inserted via a natural opening or a small incision. Rigid endoscopes, built from a series of rod lenses, give excellent optical quality and are used for arthroscopy, laparoscopy, thoracoscopy, rhinoscopy, otoscopy and cystoscopy in females. Flexible endoscopes, either fibreoptic or video endoscopes with a camera chip at the tip, can be steered around bends and are used for the gastrointestinal and respiratory tracts. A flexible endoscope has channels for air or water insufflation, suction and passing instruments such as biopsy forceps, retrieval baskets and snares. Diameter, working length and channel size are matched to the species: an equine gastroscopy needs an endoscope about three metres long, whereas a cat's bronchi need a very narrow one.
Endoscopes are expensive and delicate. After every use they are cleaned immediately, leak-tested before immersion, manually cleaned through every channel, and then high-level disinfected or sterilised according to the manufacturer's instructions and the intended use; instruments that enter sterile tissue, such as arthroscopes and laparoscopes, must be sterile.
Gastrointestinal endoscopy is used in small animals to find and remove oesophageal and gastric foreign bodies, avoiding surgery, and to take mucosal biopsies in chronic vomiting and diarrhoea. It requires fasting, general anaesthesia and careful insufflation, because overdistending the stomach of a small patient can compress the caudal vena cava and impair breathing. In horses, gastroscopy after fasting is the standard way to diagnose and grade equine gastric ulcer syndrome, recording lesions in the squamous and glandular mucosa separately.
Respiratory endoscopy in horses is done standing, usually without sedation when upper airway function is being assessed, because sedatives change laryngeal function. It examines the nasal passages, ethmoid region, pharynx, larynx, guttural pouches and trachea. Recurrent laryngeal neuropathy, which almost always affects the left side, reduces abduction of the arytenoid cartilage and is graded on standard scales. Dynamic problems that appear only at exercise, such as dorsal displacement of the soft palate or dynamic collapse of airway structures, are examined by endoscopy during treadmill or overground exercise. Guttural pouch mycosis and empyema are diagnosed by passing the endoscope into the pouch.
In small animals, bronchoscopy allows airway inspection and bronchoalveolar lavage, rhinoscopy investigates nasal discharge and removes foreign bodies, cystoscopy examines the urethra and bladder and can retrieve small calculi, and video otoscopy examines and flushes the ear canal. Laparoscopy and thoracoscopy allow biopsy and minimally invasive surgery, such as laparoscopic ovariectomy and gastropexy, and in horses standing laparoscopy is used for cryptorchid castration and ovariectomy.
A CT scanner rotates an X-ray tube and an opposing detector array around the patient, recording attenuation from many angles, and a computer reconstructs cross-sectional slices. Modern multidetector helical scanners acquire a volume in seconds and allow reconstruction in any plane and in three dimensions, which removes the superimposition that limits radiography. Each pixel is given a CT number in Hounsfield units (HU), scaled so that water is 0 HU and air is about −1000 HU; fat is negative (roughly −100 HU), soft tissue slightly positive, and bone several hundred to over a thousand. Because the eye can distinguish only a limited number of grey shades, the image is displayed through a window: the window width sets the range of HU shown in grey, and the window level sets its centre. A narrow soft-tissue window, a wide lung window and a wide, high bone window each show different tissues well.
Intravenous iodinated contrast highlights vessels and tissues with increased blood supply or broken blood-brain barrier. CT is the preferred technique for many problems: nasal and sinus disease, middle ear disease, complex fractures and joint disease such as elbow dysplasia, detection of pulmonary metastases (CT detects smaller nodules than radiographs), CT angiography of portosystemic shunts, surgical planning for tumours, and the equine head. CT uses ionising radiation, and patient doses are higher than for a radiograph, so the same justification and protection principles apply. Most veterinary patients need general anaesthesia or deep sedation to stay still, although some systems scan standing horses' heads and limbs.
MRI uses a strong magnetic field and radiofrequency pulses rather than ionising radiation. Hydrogen nuclei (protons), abundant in water and fat, align in the field; a radiofrequency pulse tips them, and as they relax they emit signals that coils receive. Tissues differ in relaxation times, T1 and T2, and sequences are weighted to exploit these differences. On T1-weighted images fat is bright and fluid is dark; on T2-weighted images fluid is bright. Fat-suppression sequences such as STIR (short tau inversion recovery) show oedema and inflammation, and FLAIR (fluid-attenuated inversion recovery) suppresses the signal of cerebrospinal fluid so that lesions next to it stand out. Gadolinium-based contrast agents shorten T1 and enhance areas where the blood-brain barrier is disrupted or tissue is vascular. Scanners are described by field strength in tesla: low-field systems are cheaper, open and slower with lower image quality; high-field systems, commonly 1.5 or 3 tesla, give better detail.
MRI gives the best soft-tissue contrast and is the preferred technique for the brain, spinal cord and intervertebral disc disease, and for soft-tissue structures in the equine foot and limb. Its drawbacks are cost, long scan times, and the need for general anaesthesia in most animals. Safety depends on keeping ferromagnetic objects out of the scanner room: scissors, oxygen cylinders, laryngoscopes and ordinary anaesthetic equipment can become projectiles, and anaesthesia machines and monitors must be labelled as safe for the magnet environment. Implants, pacemakers and metallic foreign bodies must be identified before scanning; microchips and many orthopaedic implants cause local image artefacts.
Nuclear scintigraphy uses a radiopharmaceutical, most often technetium-99m bound to a carrier, given intravenously; a gamma camera records where it concentrates. Bone-phase scintigraphy with a technetium-labelled phosphonate identifies areas of increased bone turnover and is widely used in horses to localise lameness that nerve blocks and radiographs cannot, such as stress fractures. Thyroid scintigraphy with pertechnetate evaluates hyperthyroid cats, and radioactive iodine-131 is the curative treatment for feline hyperthyroidism. Patients remain radioactive for a period after injection, so they are housed in designated areas under licence, and their urine and faeces are handled as radioactive waste according to national rules.
Choosing a modality starts from the question. Radiography remains first-line for bone, thorax and a general abdominal survey. Ultrasonography is best for soft-tissue organ architecture, fluid, the heart and pregnancy, and it guides sampling. CT answers questions of complex anatomy, bone detail and metastasis. MRI answers neurological and soft-tissue questions. Endoscopy views and samples mucosal surfaces and removes foreign bodies. Scintigraphy maps function. Cost, anaesthetic risk, radiation, availability and whether the result will change management are weighed for every case.
In the clinics you will measure a dog with calipers, choose settings from the exposure chart, position for a lateral thorax at peak inspiration and a VD abdomen at expiration, place the side marker, collimate so the edges show, and leave the room before the exposure. You will be asked to name projections correctly, to describe a film using Roentgen signs before naming a disease, and to explain why lead gloves do not make it safe to hold a patient in the beam. On the farm you will hold the transducer during herd pregnancy checks and learn to find the ovaries and a 30-day embryo. Exams test the physics and the law as heavily as interpretation, so learn kVp against mAs, the inverse square law, dose units and limits and the contrast-media contraindications until they are automatic.
Five years in, you decide daily whether a case needs radiographs, ultrasound or referral, and you make the images yourself. You recognise GDV, pneumothorax, pleural effusion, obstruction and aggressive bone lesions on your own films and act immediately; you perform FAST scans in emergencies, scan abdomens for free fluid and pyometra, and run the practice's radiation local rules, dosimetry and equipment checks. In large-animal work you diagnose early pregnancy, cysts and twins transrectally, radiograph fetlocks and feet for lameness and prepurchase examinations, and endoscope horses' airways. You know which studies to send to a specialist radiologist for a second opinion.
After 25 years you set up and audit imaging services: which equipment to buy, digital systems and archiving, exposure charts, quality assurance and service contracts. You act as the practice's radiation protection supervisor or equivalent, write local rules, review dosimeter results, train staff and deal with the regulator. You teach younger colleagues systematic reading and report writing, run herd reproductive programmes built on ultrasonography, and decide when advanced imaging is worth its cost and anaesthetic risk. You recognise the limits of your own interpretation and build referral links with specialist radiologists and teleradiology services.
At PhD level with long experience you work on questions such as quantitative imaging biomarkers, dose optimisation for veterinary CT, breed-specific reference values such as heart scores and organ sizes, standardised grading of hip and elbow dysplasia for breeding schemes, ultrasonographic assessment of embryonic loss and ovarian dynamics in cattle, and image-based artificial intelligence tools whose accuracy must be validated against reference standards. You design studies with proper observer-agreement statistics and sensitivity and specificity against a gold standard, and you advise regulators and breed societies on screening programmes and radiation protection in veterinary settings.
This manual is a practice-level foundation. It does not yet include images, which are central to learning interpretation; readers must pair it with an atlas and real cases. It gives only a few reference measurements (vertebral heart score, left atrial to aortic ratio, small-intestinal diameter ratio) and these vary by breed, method and textbook; organ-size ranges, echocardiographic reference values, foetal-age formulas, foetal heart-rate intervention thresholds and cyst size thresholds are deliberately not tabulated. Contrast media volumes and doses are left to product information and national formularies. Radiation law is described in general terms with a few national examples; the current rules of your own regulator always apply. Not covered in depth: fluoroscopy, dental radiography technique, detailed echocardiography, musculoskeletal ultrasonography of the horse, ophthalmic ultrasonography, CT and MRI protocols, interventional radiology, radiation therapy and specialist-level interpretation expected by ACVR and ECVDI diplomas.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
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