Diagnostic Imaging - Veterinary Referral Centre

Magnetic resonance imaging or MRI is a very new type of imaging technique discovered America in the 1970s and which only became available as a clinical tool in the 80s.

MRI uses a magnetic field rather than x-rays or ultrasound waves to produce an image. MRI has some similarities to computed tomography or CT in that the patient lies flat on a table which is then moved into a circular tunnel and the images produced are slices or cross sections through the patient.

The magnetic field used can be of different strengths broadly categorised as high or low field. Irrespective of the strength of the magnetic field, the radiation used is not of the ionising type as seen in x-rays. To the best of our knowledge, the magnetic fields produced do not cause any significant damage to living cells and are considered extremely safe.

In addition to its safety, MRI’s major advantage is its ability to show marked differences in soft tissue and fluid contrast for example differences between the cerebrospinal fluid surrounding the brain and the brain itself, between grey and white matter within the brain or between tumours, oedema or swelling within the brain and surrounding normal tissue. Similarly, the intervertebral discs in the spine, the spinal cord itself and the nerves exiting the spine may be clearly seen.

MRI is now being increasingly used in veterinary medicine in particular to image the brain and spine. It is frequently used in the investigation of seizures, brain tumours, encephalitis and meningoencephalitis (inflammation of the brain and its surrounding membranes), ‘stroke’ or haemorrhage. The internal architecture of the brain could be clearly seen including abnormalities such as hydrocephalus, the location and extent of tumours and secondary changes such as swelling or haemorrhage. Abnormalities in the shape of the skull such as caudal occipital malformation and secondary changes such as syringohydromyelia can be assessed noninvasively.

Bulging or rupture of the intervertebral discs is a common problem in dogs and is best appreciated on MRI. Another major advantage of MRI is it’s non-invasive nature. Prior to the advent of MRI, a dye would have to be injected into the spinal canal surrounding the spinal cord in order for many these changes can be seen on an x-ray (myelogram) or CT study. Secondary involvement of the spinal cord such as swelling or haemorrhage is also clearly seen on MRI. MRI is also used in the investigation of infection of the intervertebral discs or discospondylitis.

MRI may also be the best method in the investigation of diseases of the nasal cavity, sinuses, orbit and the middle and inner ear. Nasal tumours, inflammation and infection are common in dogs. MRI is tremendously useful in identifying the presence and extent of disease, the potential involvement of the surrounding nasal bones and skull bones and in differentiating between the different types of disease.

Many people are familiar with the use of MRI in the investigation of shoulder and knee pain in people. Similarly MRI is very useful in dogs and cats also. The joint fluid, ligaments, tendons and the cartilage and underlying bone be clearly visualised in a non-invasive fashion. MRI is particularly useful in the investigation of some chronic unexplained lameness conditions in dogs. Stress fractures, bone bruises, tendon or ligament tears, joint effusions, peripheral nerve sheath tumours such as brachial plexus or lumbosacral plexus tumours for example may be best evaluated with MRI. Myositis or inflammation of muscle tissue is also best seen on an MRI scan.

Because of its inherent ability in differentiating soft tissue contrast, MRI is frequently employed in the investigation of soft tissue lumps or masses or in investigating chronic discharging wounds or sinuses. Examples include investigation of swellings of the neck or in the search for foreign bodies such as pieces of wood, grass awns or small pieces of glass.

MRI is also used less frequently in the evaluation of abdominal, chest and heart disease.

The field of diagnostic imaging is the most rapidly growing field in both human and veterinary medicine over the last 40 years. Although x-rays have been around for over 100 years, ultrasound, computed tomography (CT) and magnetic resonance imaging (MRI) have only been developed over last 40 years or so.

X-rays are present throughout the universe passing through space but are only present in very low quantities surrounding us as we go about our daily duties on the surface of planet Earth. However, we knew nothing about them until Wilhelm Roentgen discovered their existence in a laboratory in Germany in 1895. When he discovered them however, he very quickly recognised their huge potential in the field of medicine. X-rays would for the first time in the history of the human race, allow mankind to look inside the human body and indeed animal bodies without having to cut them open! It was this discovery that resulted in Roentgen being awarded the first Nobel Prize for Physics in 1901.

Computed tomography or CT uses the same x-rays that Roentgen discovered in the 1890s but in a different way to produce a different type of image. The ability to use x-rays to produce a CT image was only made possible by the advent of another tremendous invention – the computer. Many people nowadays are familiar with CT having seen it on television or indeed having had a CT scan themselves. The patient lies flat on a table in the centre of a tunnel. So what happens now?

Imagine yourself for a moment at the centre of a large Ferris wheel like the London Eye for example. Except this Ferris wheel has 360 gondolas… You are lying still on your back looking up at the sky and all around you – above, below and to either side – are all these gondolas, each packed with sharphooters armed with magic x-ray guns. Once the operator presses the button to start the wheel, all the sharpshooters start firing at you at once and continue doing so as the wheel rotates around you. Luckily, you don’t feel anything because the x-rays pass through you magically. Phew! OK, what happens next?

Because x-rays are being fired from 360° around the patient instead of a single static point, things are far more complicated than with a regular x-ray examination. The computer ‘sees’ the x-rays passing through the patient from all these different directions and is then able to construct a picture of a slice of the patient just as you would slice through an orange. This is very different to a regular x-ray which is more like a photograph of the orange for example. This provides a major advantage over regular x-rays as the image avoids the superimposition of tissues which is inevitable on a regular x-ray. Because the CT scan produces an image of a slice or cross-section through the patient, it is often referred to as cross-sectional imaging. Magnetic resonance imaging or MRI is also a form of cross-sectional imaging because it to produces images of slices through the body.

A CT image differs from a regular x-ray image in another fundamental way. A regular x-ray shows fluid and all types of soft tissue (blood, urine, muscle, liver, spleen) as the same colour or opacity on the x-ray film. A CT image however is able to differentiate, not only between fluid and soft tissue, but different types of soft tissue. Thus, blood in the heart can be differentiated from the heart wall, tumour is from the surrounding liver, bleeding or blood clots in the brain and so on.

In veterinary medicine, CT scans are becoming more available to patients and vets and are being used more commonly as the primary method to diagnose orthopoedic diseases such as poorly-mineralised or fragmented medial coronoid process, distal humeral OCD, elbow incongruency and incomplete ossification of the humeral condyles (IOHC) or in the search for spread of cancer to the lungs. Complex anatomic areas such as the skull or spine are frequently evaluated in more detail with CT as it avoids the problem of superimposition seen with regular x-rays. The chest and abdomen are also evaluated more frequently when more detail is required subsequent to an abnormality being seen initially on a regular x-ray or ultrasound examination.

We are all now familiar with the use of x-rays in human and veterinary medicine. Hospitals and veterinary surgeries use x-rays daily to help diagnose disease. They can be used to diagnose fractures of bone, cancer or pneumonia in the lung, to investigate heart disease and causes of abdominal pain for example. In addition nowadays some human and veterinary hospitals use other forms of imaging including ultrasound, computed tomography (CT) and magnetic resonance imaging (MRI) as part of their radiology or diagnostic imaging departments.

Care has to be taken when x-rays are used. Many people are familiar with the use of lead gowns when an x-ray is being taken for example. This is to protect parts of the body that are not being imaged from being exposed to the x-rays themselves. The risk is miniscule because the dose of x-rays from an x-ray procedure in a hospital or verterinary surgery is tiny. But there is no point is taking risks!

The reason that x-rays are potentially harmful is that they have the ability to ionise cells or atoms that they pass through. This means that they have enough energy to knock an electron out of the atom or cell (a bit like a billard ball being hit by the white ball) thereby damaging or killing it. This is only of significant risk if the dose of x-rays is high. Indeed this harmful property is put to very good use in both human and veterinary medicine in the form of radiation therapy. This is where x-rays and sometimes gamma rays, are focused carfully on a particular area and used to kill tumour cell so helping to treat and potentially cure cancer.

In veterinary medicine, x-rays are used commonly as the first step in diagnosing disease of bone such as arthritis, trauma, cancer, congenital and developmental abnormalities and metabolic bone disease. As far as nasal, orbital, spine, skull and brain disease is concerned, CT and MRI are more and more commonly being used as the primary investigative tool.

Examples of such bone disease include arthritis or degenerative joint disease, osteomyelitis (inflammation or infection of the bone and bone marrow), fractures, or dislocations, osteosarcoma, chondrosarcoma, osteochondritis dissecans (OCD), elbow dysplasia (ununited anconeal process, fragmented medial coronoid process, distal humeral OCD, elbow incongruency), incomplete ossification of the humeral condyles (IOHC), ununited medial humeral epicondyle, retained ulnar cartilage core, hip dysplasia, panosteitis, nutritional or renal secondary hyperparathyroidism, metaphyseal osteopathy, hypertrophic osteopathy and craniomandibular osteopathy.

In the chest, x-rays are frequently used in evaluating the lung to check for diseases such as cancer, pneumonia, pulmonary oedema (“fluid on the lung”) or haemorrhage, lungworm and collapse or torsion of the lung lobes. Pneumothorax (free air) and pleural effusion (free fluid) can also be assessed. The outline of the diaphragm can be checked for a diaphragmatic hernia. The size and shape of the heart can be evaluated and secondary lung congestion assessed. Ultrasound is also commonly used in the investigation of the heart (echocardiography) and sometimes the chest.

In the abdomen, the presence of free air or free fluid can also be assessed and the size, shape, number, margination, location and radiographic opacity of abdominal organs checked. Increasingly, ultrasound is being used in the primary investigation of abdominal disease.

ADRENALS

• Neoplasia (primary or metastatic)
• Cushing’s disease

CARDIOVASCULAR

Congenital:

• pulmonic & aortic stenosis
• PDA
• VSD & ASD
• mitral and tricuspid dysplasia

Acquired :

• endocardiosis
• endocarditis
• cardiomyopathy(dilated & hypertrophic)
• pericardial disease (e.g. effusion, neoplasia)
• thrombosis

EYE

• foreign bodies
• intraocular or retrobulbar masses
• retinal detachment

EFFUSION

• pericardial
• peritoneal
• pleural

GENITAL

• orchitis/epididymis
• pregnancy
• prostatomegaly
• pyometra
• testicular torsion

GIT

• chronic diarrhoea
• foreign bodies
• intussusception
• neoplasia (primary or metastatic)
• tenesmus/dyschesia
• vomiting

LIVER

• biliary obstruction
• cirrhosis
• lipidosis
• neoplasia (primary or metastatic)
• portosystemic shunts

MUSCULOSKELETAL

• abscesses
• cellulitis
• foreign bodies
• ligament/tendon strain
• masses

PANCREAS

• neoplasia (primary or metastatic)
• pancreatitis

SPLEEN

• Haematoma
• Infarcts
• neoplasia(primary or metastatic)
• splenic rupture

THYROID & PARATHYROID

• neoplasia

URINARY

• chronic or acute renal failure
• cystitis
• ectopic ureters
• hydronephrosis/hydroureter
• neoplasia(kidney or bladder, primary or metastatic)
• polycystic kidneys
• pyelonephritis
• tenesmus/dysuria
• urolithiasis (bladder, ureteral or renal)

ULTRASOUND-GUIDED ASPIRATION AND BIOPSY

• thoracic and abdominal masses or fluid pockets