Download (CT) and magnetic resonance imaging (MRI) in small

Clinical applications of computed tomography (CT) and magnetic resonance imaging (MRI) in small animals
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COMMISSIONED PAPER (B)
Clinical applications of computed tomography
(CT) and magnetic resonance imaging (MRI) in
small animals
I. Gielen1, A. Van Caelenberg1, H. van Bree1
INTRODUCTION
Computed Tomography (CT or CAT scan) and Magnetic Resonance Imaging (MR or MRI) are modern
imaging procedures that have been established in human medicine for many years. Today, both
procedures are often available in veterinary medicine and have proven their value in the detection
of neurologic, orthopaedic, oncologic, and other diseases.
This paper was commissioned by FECAVA for
The Diagnostic Imaging Special issue of EJCAP.
Definitions of CT and MR
In contrast to conventional radiology where overlying
structures are superimposed, CT uses x-rays and
computers to make views of the patient in transverse
slices without superimposition. After the initial
examination, transverse slices can be reconstructed in
other planes using specific computer software (Fig. 1).
With MR, the animal is placed in a magnetic field and an
image is created, by using radio waves, which is based
on the water molecules within the patient. With MR,
the radiologist is looking for water and as pathologic
processes generally have a high water content, they
can be easily identified. In addition to transverse slice
images, MR can obtain images in all directional planes
of the patient. This procedure does not use any radiation
and is considered one of the safest techniques in medical
imaging (Fig. 2). The major disadvantage is the long
examination time which makes general anaesthesia
necessary. Also, MR equipment is more expensive to
purchase and maintain than CT equipment.
Figure 1: During CT examination, the patient is positioned
on the table. The table moves through the gantry of the CT
device in which the X-ray tube rotates and cross sectional
images are made.
Figure 2: Low-field, open MR system with a permanent
magnet.
Department of Medical Imaging of Domestic Animals and Small Animal Orthopaedics,
CT-MR Unit, Ghent University, Belgium
Corresponding author E-mail [email protected]
1
Clinical applications of computed tomography (CT) and magnetic resonance imaging (MRI) in small animals
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Figure 3: The main advantages
of CT and MR are avoiding
superimposition of surrounding
tissue by obtaining crosssectional images. This scheme
shows a cross-section (B)
respectively at the level of the
skull and the brain (A).
The advantages of CT and MR
The main advantages of CT and MR include the ability
to avoid the superimposition of different structures and
the ability to obtain cross-sections or “cross-sectional”
imaging (Fig. 3). Both procedures are complementary, but
generally, CT is indicated for bone pathology and MR for
soft-tissue pathology. MR can more clearly distinguish
between normal and abnormal tissue, such as a tumour
or inflammation. With the new “multi-slice” CT devices,
which scan a volume, the examination takes only a few
minutes, so studies can be done in a sedated animal.
An MR study especially one using low-field equipment
can take 45 minutes to an hour to perform and usually
requires general anaesthesia. It may therefore not be an
appropriate choice for patients in a poor condition.
With both procedures, contrast can be applied to improve
the detection of possible lesions. Contrast studies usually
require the use of general anaesthesia to avoid motion
artefacts.
Figure 4: 3D CT reconstruction of the skull of a cat. A
unilateral luxation of the left temporomandibular joint is
present (arrowhead).
Figure 5: A: Transverse CT image (A) and sagittal reconstructed image (B) of the head of a dog after intravenous contrast
administration (post-contrast). A: Ventral to the base of the skull a soft tissue opacity (arrowheads) is seen. The border of
the mass shows moderate contrast enhancement which contains contrast fluid. B: The reconstructed sagittal image shows
a prominent nasopharyngeal soft tissue mass (arrowhead). Histology diagnosed a fibrosarcoma.
Clinical applications of computed tomography (CT) and magnetic resonance imaging (MRI) in small animals
Figure 6: Transverse CT image at the level of the caudal
parts of the nasal cavities of a dog showing a completely
obliterated left nasal cavity. Several areas show bony
destruction (arrowheads) and the ethmoid turbinates are
destroyed. Extension of the mass is present at the level
of the nasopharyngeal meatus (asterisk). These signs are
indicative for neoplasia of the left nasal cavity.
Indications
CT and MR, although complementary, each have their
own advantages; the expected pathology dictates
which procedure should be used. CT should be used to
detect skeletal disorders (Fig. 4), common small animal
joint diseases, and oncologic diseases. In the latter
application, CT can not only determine the size and
the extent of tumours (Fig. 5) but also detect small
metastases in the thorax. CT is also very accurate in
diagnosing nasal, sinus and dental diseases (Fig. 6) in
both small animals and horses.
MR is used mainly to diagnose neurologic disorders and
soft-tissue musculoskeletal disorders, because the softtissue contrast is very high.
Both procedures offer added value in determining the
extent and severity of lesions and the stage of the
patient’s cancer, which is necessary when planning the
patient’s treatment and to determine its prognosis.
For both procedures, a thorough knowledge of anatomy
in the transverse, dorsal and sagittal planes of each body
region is necessary.
Brain and skull
MR and CT are both widely used to diagnose brain
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Figure 7: Transverse CT image of the temporal lobe of
the brain of a dog. There is a loss of normal anatomical
symmetry, a prominent midline shift is visible and the
ventricles are distorted. This phenomenon is called a mass
effect, an indication for a space occupying process of the
brain.
diseases in animals. The first reports of CT scans of small
animals were published in the 1980s and dealt with the
normal CT brain anatomy and various types of tumours
detected in dogs and cats. In the past few years, there
has been a huge increase in the literature about MR brain
anatomy and disorders. Because MR offers better softtissue contrast, it is more suitable than CT to diagnose
brain tumours and to visualize secondary tumour features,
such as oedema, cyst formation, and necrosis. Other
subtle changes, such as displacement of the ventricular
system, depth of the gyri, and a hernia of the temporal
lobe and cerebellum are more easily diagnosed with
MR. However, most space-occupying processes can be
detected with CT, because the mass displaces the normal
anatomical structures (mass effect) and disturbs the
normal brain symmetry (Fig. 7). CT cannot clearly show
lesions within the medulla oblongata, cerebellum and
the piriform lobe; this is because this area is surrounded
by highly opaque structures. These bony structures, such
as the hard petrous temporal bones of the skull base,
create an artefact, called Beam Hardening, which makes
demonstration of lesions in the caudal fossa of the skull
difficult, or even impossible (Fig. 8). In these cases, MR
is indicated.
Unfortunately, different kinds of masses can create
almost identical images, so it is not always possible
to distinguish between neoplastic and non-neoplastic
diseases. Certain tumours, such as meningiomas, can
be easily recognized, but in most cases, a biopsy is
necessary to determine the exact nature of the lesions.
Clinical applications of computed tomography (CT) and magnetic resonance imaging (MRI) in small animals
Figure 8: Beam hardening at the level of the caudal fossa
of a dog. The white arrow points to a black horizontal
area where diagnosis becomes impossible. This artefact
is present when low energy protons are absorbed by
radiopaque structures (e.g. skull base and temporal bone).
Some specific features:
Meningiomas are extra-axial lesions with a higher
density than the rest of the brain parenchyma.
They usually show a homogeneous contrast
enhancement and are well delineated. Sometimes
regions of calcification are present; this type of
tumour hyperostosis can be seen in 50 percent of
cat meningiomas. A typical feature of this tumour,
which is often seen on MR, is the presence of a socalled “Dural tail,” which connects the tumour to the
neighbouring meninges (Fig. 9).
Astrocytoma and oligodendroglioma are intra-axial
lesions that usually have a very variable pattern.
After intravenous contrast administration, the image
produced can show peripheral contrast uptake with a
central hypo-dense region, as well as heterogeneous,
non-uniform contrast uptake. Peripheral contrast
uptake is a relatively non-specific sign that could
indicate a brain abscess or inflammation which can
appear similar; MR can distinguish between these
two conditions.
Choroid plexus tumours are typically intraventricular, well-defined, hyper-dense masses
that are enhanced after intravenous contrast
administration (Fig. 10).
Large pituitary tumours, which can be identified
by their typical localization at the level of the sella
turcica, show a uniform contrast uptake (Fig. 11).
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Figure 9: Sagittal MR (T1- weighted) post contrast image
of the brain of a cat. A meningioma is seen as a large
hyper-intense structure (asterisk). The presence of a ‘dural
tail’ (arrowhead) is consistent with this type of tumour.
Figure 10: Dorsal MR image of the brain of a dog, with
an intraventricular mass (T1- weighted, post contrast).
Histology revealed a coronoid plexus tumor.
Non-neoplastic processes, such as infection and
inflammation, can be visualized by CT and MR. However,
even with MR, differentiation of neoplastic versus
non-neoplastic space-occupying lesions is not always
clear. The presence of multifocal, granulomatous lesions
in several parts of the brain is specific for primary
inflammatory disorders such as granulomatous meningoencephalitis (Fig. 12). Multifocal regions of reduced
opacity are typical for necrotizing encephalitis which is
mainly seen in Yorkshire terriers.
Although an asymmetric increase in the size of one of
the lateral ventricles is often associated with pathology,
it is also a common finding in healthy dogs and must
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and cats have been developed. Biopsy of brain masses
larger than 6-9 mm in diameter is now possible with the
correct equipment.
Figure 11: Transverse post contrast CT image of a pituitary
macroadenoma in a dog. A large, smoothly delineated
mass with homogeneous contrast enhancement is visible
extending dorsally from the pituitary fossa.
CT is the best procedure to detect an acute stroke which
appears as a homogeneous, hyper-dense area associated
with a mass effect during the first 72 hours; the lesion
then becomes isodens and, after a week, hypodens. On
MR, T1 weighted images show a hypo-intense pattern
in the acute stage; a hyper-intense aspect becomes
visible in the chronic stage. Cerebral infarction is usually
detectable on certain MR sequences (Fig. 13).
Intracranial lesions that can be visualised include fluidfilled spaces in the brain. Hydrocephalus can be seen
with both imaging techniques and the aetiology can
sometimes be determined.
Figure 12: Transverse MR image (T2- weighted) of
the brain of a toy breed dog with granulomatous
meningoencephalomyelitis (GMEM). Multiple hyper-intense
areas (arrowheads) are visible within the brain parenchyma.
always be related to the clinical picture. There is limited
available information about ventricle size and normal
variants in the different breeds.
To determine the type of brain tumour, a biopsy or
surgical excision of the mass remains the best and only
appropriate option. Analysis of cerebrospinal fluid is used
to differentiate infectious lesions from neoplasia, but it
has its limitations.
The use of CT-guided brain biopsy with the free hand
is described in the dog. The most accurate method
for biopsy of cerebral masses in dogs is CT-guided
stereotactic biopsy. The animal’s head is placed in a
frame whilst the orientation and location of the biopsy
site is determined by coordinates derived from the CT
images. Several CT-guided stereotactic devices for dogs
Figure 13: Transverse MR image (T2- weighted). In this dog
a unilateral wedge shaped hyper-intensity is seen at the
level of the cerebellum. This finding is consistent with a
cerebellar infarction (white arrow).
In trauma cases, CT can easily demonstrate skull fractures
(Fig. 14), whereas MR is more sensitive for both intraand extra-cerebral parenchymal lesions.
Tumours of the skull are visible on CT where bone lysis,
secondary to a soft-tissue process that surrounds and
invades the skull, is often present.
In a Chiari-like malformation, the size of the skull’s caudal
fossa is not in proportion to the volume of the cerebellum
and brainstem. In these cases, the cerebellum may protrude
Clinical applications of computed tomography (CT) and magnetic resonance imaging (MRI) in small animals
Figure 14: Transverse CT image of the caudal nasal cavity
of a dog. Multiple fractures (arrowheads) are seen at
the level of the frontal sinus. Within the frontal sinuses
soft tissue opacities are present (most likely due to
haemorrhages).
caudally through the foramen magnum, blocking the
cerebrospinal fluid flow, which may cause fluid-filled cavities
called syringomyelia to develop inside the spinal cord. MR
must be performed to determine this condition (Fig. 15).
CT usually adds no value to detect inflammation and
tumours of the cranial nerves, whereas MR is the best
procedure to allow visualisation of most disorders of the
cranial nerves (Fig. 16).
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Figure 16: Transverse MR image (T2- weighted) of a dog with
a tumour the of the trigeminal nerve (arrow heads). Note
the atrophy of the ipsilateral temporalis muscle (asterisk).
In both CT and MR, intravenous contrast agents can be
administered to:
a) Determine the perfusion of the tissues; and
b) Distinguish between normal and abnormal tissue.
Contrast studies are extremely useful for specific
examination of the meninges, the choroid plexus,
and the pituitary gland, all of which contain
fenestrated capillaries that allow the contrast agent
to enter their interstitium. In normal brain tissue,
the contrast agents do not cross the blood-brain
barrier, so they cannot get into the parenchyma.
However, with lesions, including inflammation
and tumours, the agents will cross the blood-brain
barrier and the pathological tissue will be enhanced.
Contrast agents used for MR contain a paramagnetic
substance, gadolinium, such as Magnevist.® Contrast
agents used for CT contain iodine such as Ultravist.®
Nose, sinus, retrobulbar region,
tympanic bullae, teeth and jaw
Figure 15: Transverse MR image (T2 weighted) of a dog
with Chiari malformation. The cerebellar tissue is visible
through the foramen magnum (arrowhead) and associated
syringomyelia is present (asterisk).
The diagnostic information obtained with CT and MR in
terms of location, extent and characterisation of lesions
in the nasal cavity, paranasal sinuses, retrobulbar region,
jaw, temporomandibular joints, and tympanic bullae is
more accurate than those obtained with conventional
radiographs.
In the nasal cavity, all structures are clearly visible with
CT and MR, but CT is more accurate in detecting the
location and spread of chronic nasal disease, as well as
showing any degradation of the cribriform plate (Fig. 17).
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Indications for CT and MR of the Brain
CT
MR
Mainly bony lesions
Mainly soft tissue lesions
Congenital anomalies
Congenital anomalies, including hydrocephalus,
arachnoid cysts
Infection/inflammation such as osteomyelitis
Infection/inflammation such as neospora infection, granulomatous encephalitis
Haemorrhage, acute and chronic
Haemorrhage, sub-acute and chronic
Neoplasia, usually visible after contrast administration or
bony changes, including bone hyperostosis in meningioma
Neoplasia, including astrocytoma, brainstem
tumours
Skull trauma with bone destruction
Skull trauma, usually parenchymatous lesions
Common nasal disorders have typical features. In nasal
aspergillosis, destruction of the nasal turbinates creates
large cavities (Fig. 17) which can be identified. In
neoplasia of the nasal cavity, mass-like lesions associated
with bony destruction will be seen. Additionally, the
degree of destruction of the frontal bone, which will
expand into the orbit, can be accurately determined (Fig.
18). In non-specific rhinitis, non-destructive processes
will often effect both nasal cavities, but not the
paranasal sinuses.
Disease in the retro-bulbar area is difficult to diagnose
with radiographs; CT and MR allow orbital lesions and
retro-bulbar masses to be more easily identified (Fig.
19). Exophthalmos and soft-palate swelling associated
with the orbit or skull can be evaluated. CT can diagnose
multi-lobular tumours and whether there is involvement
of the zygomatic arch and frontal bone, whereas MR
allows the soft-tissue components to be evaluated in
more detail.
CT and MR allow visualisation of the base of the skull
and the tympanic bullae without superimposition of
other structures. They can also accurately diagnose otitis
media and interna in dogs and cats at an earlier stage
than conventional radiography. Within the tympanic
bullae, MR can establish any subtle increase in softtissue opacity and fluid. Patients with otitis interna
can develop secondary meningitis, which is visible
with MR. CT, especially contrast CT, can provide useful
Figure 17: Adult dog with nasal aspergillosis. A: Transverse CT image of the caudal parts of the nasal cavities in a dog.
In the right nasal cavity soft tissue and fluid density are present (asterisk). In the dorsal part of the right nasal cavity
there is destruction of the nasal turbinates and the maxilla (arrowhead). B: Transverse CT image at the level of the
frontal sinuses showing mucosal thickening in the right frontal sinus. There is suspected destruction of the cribriform plate
dorsally (arrowhead). C: Dorsal CT reconstructed image confirming lysis of the cribriform plate (arrowhead). Evaluation of
the cribriform plate in this type of lesion is very important to determine prognosis and therapeutic planning.
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Figure 20: Post-contrast transverse CT image of a
cholesteatoma of the right tympanic bulla in a dog. The
right tympanic bulla is completely filled with soft tissue
opacity. Lysis of the wall and os petrosum is present.
Intracranial invasion is seen with contrast enhancement at
the border of the mass (black arrow).
information about the external ear canal, the inner ear,
the nasopharynx and the extent of the involvement of
the bony bullae (Fig. 20).
Figure 18: Post-contrast dorsal reconstructed CT image
of the head and nasal cavities in a dog. The right frontal
sinus is filled with soft tissue density. Due to excessive
growth, the mass extended into the orbital region (orange
arrow) with exophthalmia of the right eyeball and into the
skull with lysis of the cribriform plate (blue arrow). Notice
also the midline shift of the large hemispheres probably
due to oedema.
Disorders of the mandible and maxilla and any eventual
involvement of the teeth can be evaluated with both CT
and MR. CT provides clear images of the bony components
(Fig. 21), while MR provides more detail about the
expansion of the soft-tissue structures.
Figure 19 A: Dorsal MR image (T2- weighted) of the retrobulbar region in a dog. At the left retrobulbar region a hyper-intense
structure is seen (arrowheads). Part of the frontal sinus is missing and there is invasive growth into the sinus (*). B: Dorsal
MR image (T1- weighted, post-contrast). The contrast enhancement at the level of the mass is suggestive for a tumour.
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Figure 21: Preoperative CT to delineate the extension of an odontoma in a dog. (A: transverse, B and C: dorsal and sagittal
reconstructed images). At the level of the rostral mandible new bone formation and osteolysis are visible (arrowheads).
Back and spinal disorders
Today CT and MR are replacing conventional radiography
and myelography for the diagnosis of spinal cord
diseases. Both techniques give different, but
complementary, information about the spinal cord and its
surrounding structures.
CT is very useful for detecting bony changes such as
osteolysis and calcification. It also gives an accurate
representation of the anatomy of the vertebrae and
facet joints (Fig. 22). The presence of gas between
the vertebrae in the intervertebral area, known as the
vacuum phenomenon, is a sign of degeneration of the
intervertebral disc and a frequent finding on CT.
Abnormalities of the disc with the occurrence of
mineralized and non-mineralized material, spinal tumours
and cervical spondylosis can be evaluated with CT, which
is as accurate as myelography for detecting extradural
lesions, but less accurate in differentiating intramedullary
from extradural lesions.
Mineralized material of an intervertebral disc in the spinal
canal can be detected with CT without myelography (Fig.
23); however, material with soft-tissue density may not
be visible on native CT. In these cases, CT, myelography,
or MR is indicated.
CT is more accurate than conventional myelography for
the diagnosis and localization of an intervertebral disc
herniation; a lateralized disc herniation, in particular, will
be more visible with CT and/or MR than with myelography
(Fig. 24). Also, post-processing CT images can be
obtained in the sagittal and dorsal plane providing more
information.
Figure 22: Sagittal reconstructed CT image of a young
dog with osteochondritis dissecans of the sacrum. At the
dorsocranial aspect of the first sacral vertebra a separate
bone fragment is seen. A bone defect and sclerosis of the
dorsocranial aspect of the sacrum can also be noticed.
MR is the procedure of choice to evaluate the soft
tissues within the spinal canal such as the ligaments,
intervertebral discs, and the spinal cord itself. MR
provides additional information about the spinal
parenchyma and the peri-vertebral soft tissues and
distinguishes between the extradural and intradural parts
of nerve sheath tumours, the intervertebral foramen,
and extradural spaces. Intramedullary lesions can also be
assessed (Fig. 25).
In cases of spinal arachnoid cysts, both CT and
MR provide more information in more detail than
Clinical applications of computed tomography (CT) and magnetic resonance imaging (MRI) in small animals
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Figure 23: Dog with a herniated disc at the level of the first and second lumbar vertebrae. (A: transverse, B and C: a
sagittal and dorsal CT reconstructed image). There is severe extradural compression by an extruded mineralized disc. The
reconstructed images allow accurate lesion localization.
CT and MR are also used to evaluate the extension of
nerve sheath tumours such as brachial plexus tumours in
the axillary region. The lumbosacral plexus can be seen as
a soft- tissue mass cranial to the sacrum. The lumbosacral
nerves and the sciatic nerve can also be evaluated.
Intravenous administration of contrast helps to identify
the vascular structures (Fig. 27), but the differentiation
between a nerve sheath tumour and a neuritis is still a
challenge and not always possible.
With CT, the presence and extent of haemorrhage in the
spinal canal can be properly evaluated in the acute stage.
With MR, sometimes an infarct in the spinal cord can
be seen. These lesions are intramedullary with a hyperintense appearance (Fig. 28).
Figure 24: Transverse MR image of a herniated disc in
the region of the cervical spine (white arrow). Prominent
compression of the nerves at the level of the intervertebral
foramen is present in this dog.
myelography. This is achieved by visualisation of the
caudal edge of the cyst, the topographic position, and
the degree of spinal cord compression.
CT and MR are valuable in the diagnosis and evaluation of
lumbosacral lesions. Remodelling of bone, intervertebral
disc bulging, and evidence of compression of the cauda
equina, the facet joints, and the sacro-iliac intervertebral
foramina can all be evaluated without superimposition
(Fig 26).
The individual nerve roots of L5-S3 can be visualized and
tracked to the exit point at their foramen. The epidural
fat provides an inherent contrast medium.
CT can clearly demonstrate bony lesions caused by trauma
to the spinal canal. By contrast, lesions in the spinal cord
are not always clearly visible and are sometimes missed.
Intravenous contrast enhances images of inflammatory
and neoplastic lesions and helps distinguish these lesions
from disc material and haemorrhage.
The use of different MR sequences, combined with or
without an intravenous injection of a paramagnetic
contrast agent (e.g., Magnevist®), allows visualization
of a large number of spinal cord lesions and their
surrounding structures.
Thorax
CT is the best imaging procedure to detect and evaluate
masses, malformations and effusion in the thoracic cavity.
The exact size and shape of a mass can be determined, as
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Figure 25: A: Sagittal MR image
(T2 weighted) of a dog. A hyperintense area is visible at the spinal
cord at the level of the first 2
cervical vertebrae. B: Transverse
image (T1 post-contrast) showing
prominent contrast enhancement
of a cervical meningioma.
Figure 26: Sagittal MR image (T2- weighted) at the
lumbosacral region of a dog with lumbosacral stenosis.
Dorsal and ventral (arrowhead) compression of the cauda
equina is seen at the level of L7-S1. At this level the disc is
dehydrated and the disc is bulging into the vertebral canal.
Figure 28: Sagittal MR image (T2- weighted) of the cervical
spine of a dog. A delineated hyper-intensity is seen in
the spinal cord at the level of the 6th cervical vertebra
(arrowhead). This is suggestive for a fibrocartilaginous
infarct.
Figure 27: A: Transverse MR image (T1- weighted) of the lumbosacral region of a dog with neoplasia of the lumbosacral
plexus. A hyper-intensity is seen at the level of the nerves extending from the lumbosacral plexus (arrowhead). B:
Transverse MR image (T1 weighted) showing prominent muscle atrophy at the level of the gluteus and iliopsoas muscles
(*). C: Dorsal MR image (STIR). At the level of the extending nerves L4-L5 and L6, and also at the level of the muscles, a
hyper-intense signal is visible.
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Indications for CT and MR in spinal disorders
CT
MR
Mainly bony lesions
Mainly soft-tissue lesions
Disc pathology, including mineralized disc material
Disc pathology
Neoplastic lesions, usually with bone involvement
Spinal cord tumour, including intramedullary
neoplasia
Ischemic myelopathy
Discospondylitis, usually osteolysis of endplates
Infection/inflammation among others, including
myelitis, meningitis, discospondylitis , epidural
empyema
Cervical spondylopathy
Congenital spinal anomalies such as atlanto-axial
malformation, hemivertebrae
Congenital spinal anomalies, including
caudal occipital malformation syndrome with
syringohydromyelia, Chiari malformation in Cavalier
King Charles spaniels
Spinal trauma, including fractures, dislocations
Spinal trauma, usually affecting the spinal cord
Lumbosacral stenosis
Lumbosacral stenosis
well as the presence of early mineralization within a mass
(a sign of neoplastic change), and the extent of spread
into the surrounding tissues (Fig. 29). This information is
important when planning surgery.
Due to the large air-tissue contrast, the lung structures
can be seen in great detail. Changes to the pleura and
the medial aspect of the ribs can also be evaluated.
Various windows can be used to evaluate soft tissues in
the mediastinum, the body wall and the bone.
Masses in the thorax can be differentiated from
mediastinal or pleural fluid. CT images of similar
views with and without an intravenous contrast agent
can differentiate between the blood vessels of the
mediastinum and other structures. CT is considered the
most sensitive method for the detection of pulmonary
metastases. Nodules can be detected from 2mm (Fig. 30).
MR is used less frequently for thoracic diseases because
structures that contain air give no signal and appear
black, and respiratory and cardiac motion blur the images.
Abdomen
Abdominal CT produces very good anatomical images of
abdominal organs and blood vessels, although it is used
less frequently for abdominal pathology, except in very
obese animals. This is largely due to the extensive use of
ultrasound to make clinical diagnoses in the abdomen. CT
is often used to evaluate the involvement of surrounding
vital structures in cases with abdominal masses; to
evaluate lesions in the spinal and pelvic canals; for
early detection of renal carcinoma; and to differentiate
between cysts and solid tumours.
A bolus injection of contrast medium can differentiate
between solid, vascular or avascular renal masses,
and cysts. CT is also indicated to image the adrenal
glands and to evaluate the size, shape and topography
of adrenal masses. In cases of hyperadrenocorticism,
CT distinguishes between unilateral and bilateral
enlargement of the adrenal glands (Fig. 31) and any
infiltration of the vena cava. Additionally, with CT
guidance, fine-needle aspiration of an adrenal mass can
also be performed.
CT images obtained at the time of maximal expiration
provide a detailed overview of the normal parenchyma
of the pancreas, but the normal biliary ducts cannot be
visualised. The main technical problem in CT studies of the
pancreas is the delineation of adjacent organs, particularly
the liver, spleen and stomach. CT with intravenous contrast
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Figure 31: Post-contrast transverse CT image of the
abdomen of a dog with a left adrenal gland tumour. Medial
to the left kidney a contrast enhancing soft tissue mass is
seen (white arrowhead).
Figure 29: Transverse CT image of the thorax of a dog with
chondrosarcoma. A soft tissue mass present is centred
on the rib cage with osteolysis of the rib. Intrathoracal
extension of the process can be evaluated.
Figure 32: CT angiogram, dual phase, sagittal reconstructed
image of a cat with an extrahepatic portosystemic shunt.
An anomalous vessel diverges from the portal vein and
travels to the left between the stomach and liver. The
shunt vessel terminates in the caudal vena cava at the
level of the hepatic veins near the diaphragm
for the diagnosis of porto-systemic shunts. The current
devices use a short examination period and provide an
arterial, hepatic, and venous phase image. Post-processing
3D images give a fair representation of the type of portosystemic shunt (Fig. 32).
Figure 30: Transverse CT image of the caudal lung lobes
of a dog with metastatic lung neoplasia. Several poorly
marginated nodules are seen (arrowheads).
offers a combination of topographical and functional
evaluation of the spleen and can accurately diagnose
splenic torsion in the dog. CT with contrast is also useful
It is necessary to administer contrast medium to
differentiate mesenteric masses from normal bowel and
vascular structures. Detailed images of both the small
and large intestine suggest that CT could be used to
evaluate gastrointestinal pathologies, but gastrointestinal
imaging in dogs is limited because it is difficult to assess
the dynamic activity of the intestine. Because of the
high-contrast resolution and lack of superposition, CT is
valuable when evaluating the ureters and their endings in
Clinical applications of computed tomography (CT) and magnetic resonance imaging (MRI) in small animals
22(4) P 97
Figure 34: Transverse CT image (A) of the tarsus of a dog,
at the level of the tarsocrural joint. A subchondral defect
and fragments at the top of the medial talar ridge are
present (arrowhead). At the sagittal reconstructed image
(B) flattening of the medial talar ridge is obviously visible.
These findings are compatible with medial osteochondritis
dissecans of the tarsal joint.
approximately 30.0 percent with conventional x-rays. CT
is the most accurate procedure to evaluate the extent of
osteosarcoma in dogs (Fig.35).
Figure 33: Post-contrast dorsal reconstructed CT image
of the abdomen and pelvis of a dog. An ectopic ureter,
intramural, is seen ending in the proximal part of the
urethra (arrow).
the bladder. An ectopic ureter can be detected with the
administration of intravenous contrast (Fig.33).
The abdomen is usually studied using ultrasound and CT.
MR has limited value in the abdomen.
Skeleton and joints
CT is widely used in human medicine to evaluate skeletal
disorders and muscle and tendon injuries. Today, CT in
animals is more frequently used to diagnose orthopaedic
disorders. CT of the skeleton may help in clinical cases
where standard radiographs are negative or doubtful and
pathology is strongly suspected. Joint pathology in small
animals is often difficult to determine on radiographs,
because of the complex radiographic anatomy of some
smaller joints and the superimposition of bony structures.
CT facilitates the study of complex joints such as the
elbow and the tarsus, because it prevents superimposition
of other structures (Fig. 34). Osteolysis, sclerosis, and
new bone formation can be detected very early on
because of CT’s high sensitivity for bony lesions. CT can
detect changes in density of 0.5 percent, compared to
Figure 35: Transverse CT image at the level of the humeral
head in a dog. There is a lytic region visible at the
lesser tubercle representing a primary bone tumour, an
osteosarcoma.
CT has been proven to be superior to radiography for the
diagnosis of fragmented medial coronoid processes in the
elbow joint (Fig.36). As well as the diagnosis, CT may be
used to determine the degree of elbow incongruity (Fig.
37), although this matter is still under discussion.
CT is also superior for the diagnosis of incomplete
ossification of the humeral condyle (Fig. 38).
CT can be used to assess the dorsal aspect of the
acetabular rim when treating hip dysplasia, an important
criterion when a triple pelvic osteotomy is considered.
Stifle CT is indicated to determine the origin of avulsions
and fragments (Fig. 39). CT can also be used to determine
the volume and density of bones and the orientation of
angular deformities (Fig. 40).
Clinical applications of computed tomography (CT) and magnetic resonance imaging (MRI) in small animals
Figure 36: Transverse CT slice, bone window, of a canine
elbow at the level of the radius and ulna. A small displaced
fragment (arrowhead) is present at the medial coronoid
process of the ulna
22(4) P 98
Figure 38: Transverse CT slice, bone window, at the level
of the distal part of the humerus. A radiolucent area
surrounded by a sclerotic rim is visible (white arrow)
representing an intercondylar fissure.
Figure 37: Transverse slice (A) and sagittal (B) and dorsal (C) reconstructed CT images of an incongruent elbow joint. The
black arrow points out the widened and diverging joint space. The white arrows show the step present, between radius
and ulna.
Figure 39: Transverse CT image at the level of the femoral
condyles of a stifle in a young dog. The avulsed proximal
attachment of the cranial cruciate ligament is clearly
visible at the medial part of the lateral condyle with
associated subchondral sclerosis (white arrow).
Figure 40: 3D CT reconstruction of a stifle joint with medial
patella luxation. The depth of the trochlear groove and
angular deformities can be evaluated.
Clinical applications of computed tomography (CT) and magnetic resonance imaging (MRI) in small animals
Figure 41: 3D CT reconstruction, ventral view, of a canine
pelvis. Multiple fractures and fragments are visible
(arrowheads). The orientation and delineation of the
fractures and fragments is very valuable for treatment
planning.
22(4) P 99
Figure 43: Transverse MR image (T1-weighted, post
contrast) of a canine elbow with flexor enthesopathy.
Swelling and hypertrophy is visible at the level of the
medial flexor tendons. Contrast administration shows
prominent enhancement of the flexor tendons and the
surrounding tissue (red circle).
difficult. Arthro-CT, using a small amount of contrast
medium in the joint, is potentially useful to delineate
cartilage and is sometimes used to assess the menisci and
cruciate ligaments (Fig. 42).
Figure 42: Sagittal reconstruction of a CT arthrogram
at the level of a canine stifle, after partial medial
meniscectomy. The cranial pole is missing (black arrow)
and the caudal one seems to be crushed (white arrows).
CT’s ability to view images in different planes makes a
complete evaluation of the joint possible. With fractures,
it enables the orientation of the fracture lines to be
determined, which is necessary when planning fracture
treatment and possible surgery (Fig. 41).
Visualisation of cartilage in small animal joints remains
MR has clear advantages over CT in the visualization of
peri-articular and intra-articular soft-tissue structures.
In the area of the shoulder and elbow joint, injuries of
the muscles, tendons, and joint capsule can be clearly
seen (Fig. 43). However, even with MR, the assessment of
cruciate ligaments and menisci is difficult because they
are so small (Fig. 44).
MR is particularly sensitive to the detection of changes
in the bone marrow, but, again, even with MR, canine
cartilage is difficult to visualise. This is because small
animals have very thin cartilage and the resolution of
the MR equipment is too low to directly visualize it.
The distinction between cartilage and synovial fluid
is not clear, at least not in young dogs (Fig 45). The
intra-articular administration of gadolinium-containing
contrast media may give added value, but is still under
investigation. Intravenous injection of contrast agents
may be useful to detect inflammatory processes and
evaluate the extent of neoplastic processes (Fig. 46).
Clinical applications of computed tomography (CT) and magnetic resonance imaging (MRI) in small animals
22(4) P 100
Figure 45: T1- weighted sagittal MR image of the shoulder
of a young dog. The hypointensity at the caudal part of the
humerus represents an OCD lesion. The cartilage cannot be
distinguished.
Figure 44: Proton density, sagittal, MR image of the stifle
of a dog. Bright fluid can be seen within the torn the
meniscus (arrow).
Conclusion
CT and MR are imaging procedures that provide many
applications in various clinical disciplines. They have
become more frequently available for veterinary use and
are especially valuable for the detection and diagnosis of
brain disorders. They are more accurate than conventional
radiography in evaluating the location and extent of
lesions in the nasal cavity, paranasal sinuses, eye socket,
jaw, temporomandibular joint, and tympanic bullae.
Both procedures are also useful when planning treatment
and surgery for back and spinal cord injuries.
CT is one of the best imaging procedures available for
the detection and description of masses, malformations
and effusions in the thorax, and is considered the most
sensitive procedure to detect pulmonary metastases.
Although ultrasound is considered the imaging procedure
of choice to examine the abdomen, CT does give an
excellent anatomical overview of the organs and blood
vessels.
Figure 46: Sagittal MR image of a synovial cell sarcoma in
a canine elbow. Extensive hyperintensities are visible in the
joint capsule (white arrows). Prominent hyperintensities
are present in the ulna representing lysis.(black arrow).
CT of the skeleton is useful in clinical cases where
standard radiographs are negative or doubtful. CT is
superior to radiography for the diagnosis of coronoid
disease and other pathologies of the elbow and is
generally useful to evaluate joint pathology. MR provides
Clinical applications of computed tomography (CT) and magnetic resonance imaging (MRI) in small animals
better images of the peri-articular and intra-articular
soft-tissue structures.
It is expensive to purchase both CT and MR devices;
however, second-hand CT equipment is now available and
more affordable for private practitioners. MR devices,
even low-field, remain expensive, with substantial
maintenance costs. Moreover, only specialists, who are
also expensive, can make optimal use of each device
because expert knowledge is required for the correct
interpretation of both procedures.
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