OPTIMAL MAGNETIC RESONANCE IMAGING OF THE SPINE

RUTH DENNIS

Magnetic resonance (MR) imaging is generally considered to be the best imaging modality for the spine because

of its excellent tissue contrast and multiplanar imaging capability; however, good technique is vital in order to

avoid nondiagnostic or even misleading images. The possibility of imaging in multiple planes and using many

different pulse sequences means that judgment is required in order to gain maximum diagnostic information

within a reasonable scanning time. Spinal MR imaging technique for small animals is reviewed with emphasis on

technical aspects including patient positioning, selection of pulse sequences, and image planes. r 2011Veterinary Radiology & Ultrasound, Vol. 52, No. 1, Supp. 1, 2011, pp S72–S80.

Key words: dog, intervertebral disc disease, magnetic resonance imaging, spinal cord.

Introduction

MAGNETIC RESONANCE (MR) imaging is generally con-

sidered to be the best imaging modality for spinal

conditions affecting veterinary patients. High MR tissue

contrast enables depiction of the spinal cord parenchyma,

cerebrospinal fluid (CSF) and epidural fat surrounding the

spinal cord, vertebral venous structures, and ligaments.

Both the outline and internal architecture of the vertebrae

are visible, although fine bone detail is less well demon-

strated than with CT. The intervertebral discs are visible and

the effects of disc pathology can be observed directly rather

than being inferred from the width of the intervertebral

space or myelographic abnormalities. Conditions that can-

not be diagnosed using myelography, such as nerve root

pathology, foraminal disc extrusion, and paraspinal soft-tis-

sue inflammation, may also be detected with MR. Although

certain conditions, such as degenerative myelopathy, may

not affect the MR image, the overall high sensitivity of MR

imaging allows other differential diagnoses to be ruled out.

Clinical indications for spinal MR imaging include de-

formity, spinal or paraspinal pain, ataxia, paresis or para-

lysis, spinal masses, swellings or draining sinus tracts, and

need for vertebral measurements.1 MR imaging is also used

as a screening test for conditions in dogs considered at risk

because of their breed, for example Chiari-like malforma-

tion and syringomyelia in the Cavalier King Charles spaniel.

Contraindications for spinal MR imaging include car-

diac pacemaker and presence of metal close to the volume

to be scanned, which risks movement, local heating, and

distortion of the image as a result of susceptibility artifact.

Previous surgery within the field of view (FOV) may pre-

clude MR imaging due to the presence of metallic implants

or fragments from drill bits,2–4 although depending on the

type of metal, field strength, andMR sequences used, image

distortion may be fairly localized5,6; for example, a titanium

total hip prosthesis does not usually interfere with lumbo-

sacral scanning. Microchips can cause significant distortion

of spinal images and may need to be removed before di-

agnostic scans can be obtained (although it should be noted

that microchips are not erased by the magnetic field).

It is vital that a careful clinical examination and neu-

rolocalization is performed before MR imaging. MR imag-

ing should not be used indiscriminately to image the entire

spine because of the potential for false positive results, such

as subclinical disc lesions in patients with metabolic, or-

thopedic, or nonspinal neurologic conditions that mimic

myelopathy. False negatives may also occur with condi-

tions that do not affect the MR image (e.g., degenerative

myelopathy) or when MR technique or interpretation is

inadequate. MR images should always be interpreted with

knowledge of the clinical history.

Radiofrequency (RF) Coil Selection

Phased-array spine coils are ideal for medium and large

dogs in dorsal recumbency (Fig. 1). These coils consist of

multiple adjacent transmit/receive coils each of which pro-

cesses signal from its own small FOV before the informa-

tion is combined to form a single, longer FOV with reduced

overall noise. The best images are acquired when the lon-

gitudinal FOV is matched to the length encompassed by

active RF coil segments. Signal intensity falls off with in-

creasing distance from the coil; hence the spine should be as

close as possible to the coil surface. Signal drop-off can be

compensated for using imaging options such as surface coil

intensity correction (used on GE systems), which evens out

the signal intensity with distance from the coil. Human

phased-array torso coils are an option for dogs that must be

Address correspondence and reprint requests to Ruth Dennis, at theabove address. E-mail: ruth.dennis@aht.org.ukReceived June 25, 2010; accepted for publication November 15, 2010.doi: 10.1111/j.1740-8261.2010.01787.x

From the Animal Health Trust, Lanwades Park, Kentford, Newmarket,Suffolk CB8 7UU, UK.

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scanned in lateral recumbency. Human extremity (knee)

coils are suitable for the spine of cats and small dogs.

Handling, Restraint, and Positioning

Most dogs and cats are anesthetized for MR imaging,

although heavy sedation may suffice for brief procedures

such as syringomyelia screening. Spinal patients should be

handled with care.

Most are scanned in dorsal recumbency, which places

the spine close to the surface coil and minimizes spinal

motion due to breathing, but large or narrow dogs may be

scanned in lateral recumbency if they are difficult to po-

sition in dorsal recumbency. It is essential that the patient

does not move during the scan, hence use of positioning

aids such as sandbags, foam wedges, and tape (Fig. 1). The

spine must be as straight as possible in the sagittal plane

and should be repositioned if initial localizer images show

curvature; however, suboptimal positioning for initial MR

imaging may have to be accepted in the presence of de-

formity or muscle spasm (e.g., scoliosis associated with disc

extrusion or syringomyelia).

Obtaining MR images with the patient in different pos-

tures may demonstrate dynamic lesions; for example, sagittal

MR images of the ventroflexed lumbosacral junction may be

compared with images obtained in dorsiflexion as a means of

detecting instability (Fig. 2). Traction using weights may be

applied to the neck of dogs with disc protrusion associated

with cervical spondylopathy in order to assess the degree to

which the lesion is dynamic.7,8 Because MR imaging takes

several minutes, it is essential to avoid postures that could

put pressure on the spinal cord, such as cervical ventroflex-

ion in a dog with suspected atlantoaxial subluxation.

Pulse Sequences

Many combinations of pulse sequence, scan plane, and

slice thickness/orientation are possible for spinal MR

imaging, hence judgment is required in order to gain max-

imum diagnostic information within a reasonable scanning

time. Aesthetically pleasing images are not necessarily the

most useful for diagnosis. The author’s approach to the

spine is as follows:

� dorsal T2-weighted (T2W) images, both for their di-agnostic value and as a basis for accurate placementof sagittal slices;

� sagittal T2W images;� transverse T2W images through any suspected

abnormalities identified in dorsal or sagittal imagesor according to the neurolocalization;

� pre- and postcontrast T1-weighted (T1W) images (thelatter usually with fat suppression) and/or gradient–echo (GE) images depending on the suspected nature ofany lesion identified, in planes as required for diagnosis;and

� in patients with signs of back pain and no visible spinallesions, short tau inversion recovery (STIR) images areobtained in the dorsal plane to look for paraspinalsoft-tissue pathology.

A comparison of lesion signal intensity in identical slices

obtained using different pulse sequences aids characteriza-

tion of lesions.

Three-Plane Localizer (Scout Images)

Localizer images should be searched for lesions while the

first sequence is running. Localizers are GE sequences in

Fig. 1. A dog positioned in dorsal recumbency on a spine-array coil formagnetic resonance imaging of the cervical spine.

Fig. 2. Sagittal T2-weighted magnetic resonance images of degenerative lumbosacral stenosis with instability between L7 and S1 in a 9-year-old Germanshepherd dog; (A) extended position and (B) ventroflexed (TR 3000ms, TE 8ms 3, ST 3mm).

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which normal cortical bone has a very low signal; hence,

osteolytic neoplasia may be readily apparent (Fig. 3).

Occasionally, masses may be seen affecting paraspinal soft

tissues, the thorax or abdomen.

T2 Weighting

With high-field MR systems, T2W is the ‘‘workhorse’’

sequence, and may be all that is required for diagnosis of

disc disease and spinal infarcts. T2W images of the spine

have high contrast, with hydrated disc nuclei, CSF,

epidural fat, and most pathological processes appearing

hyperintense due to high hydration (Fig. 4A). Lesions im-

pinging on the subarachnoid space are readily detected; for

example, T2W scans provide more accurate information

about the severity and extent of extruded disc material than

T1W or STIR images.9 Differentiating extradural and ex-

tramedullary–intradural lesions may be difficult, although

acquisition of T2W images in both dorsal and sagittal

planes may show widening of the subarachnoid space with

the latter. Heavily T2W sequences using longer TR and TE

(‘‘T2-myelogram’’) are favored by some users both for de-

tection of intradural location of lesions10 and as a general

overview of the spine, although resolution is lower than

with normal T2W images (Fig. 4B).

Within the spinal cord parenchyma, T2W hyperintensity

denotes pathologic increased water content as a result of

various lesions, including edema, myelitis, gliosis, myelom-

alacia, demyelination, necrosis, hydromyelia, syringomy-

elia, neoplasia, and hemorrhage at certain stages of its

evolution11; corresponding T1W images of the affected

area are often unremarkable. The presence and extent of

cord hyperintensity is a prognostic factor in dogs with

compressive disc extrusion,12 acute, noncompressive disc

extrusion,13 and ischemic myelopathy.14

T1 Weighting

T1W images generally have high signal-to-noise ratio

(SNR), which enables excellent depiction of anatomy, al-

though with lower contrast than T2W images. CSF in T1W

images is slightly hypointense compared with the cord, but

epidural fat remains hyperintense; hence comparison of T1W

and T2W images allows CSF and fat to be differentiated.

Bone detail is good in T1W images, and disc width and

vertebral body sclerosis are more accurately assessed than

with T2W images (Fig. 5). Only the most marked spinal cord

lesions, such as syringomyelia, are visible in T1W images.

Contrast Enhancement and Subtraction

Contrast enhancement of the spinal cord reflects in-

creased permeability of the blood–spinal cord barrier as a

result of pathology, particularly neovascularization associ-

ated with neoplasia. Postcontrast T1W images delineate

the extent of neoplasia better than other sequences10 (Fig.

6). Myelitis usually causes less contrast enhancement than

neoplasia. Subtle contrast enhancement may be evident

only on subtraction images.

Fig. 3. Sagittal magnetic resonance image from a three-plane localizer ofa dog with lymphoma in which severe osteolysis of the vertebral body of C7is evident despite low resolution.

Fig. 4. (A) Sagittal T2-weighted magnetic resonance image of the lumbosacral junction of a 2-year-old Boxer dog with discospondylitis. The lumbosacraldisc space is narrowed and the vertebral endplates are irregular, with hyperintensity of subchondral bone. A streak of high signal intensity material in the discspace represents hydrated, necrotic disc material. There is mild dorsal disc protrusion and marked ventral spondylosis. Two normal intervertebral discs are alsoseen (TR 3000ms, TE 83ms, ST 3mm) (reproduced with permission, Veterinary Comparative Orthopaedics and Traumatology). (B) Heavily T2-weightedmagnetic resonance image of a 6-year-old Beagle cross with a disc extrusion at C2-3, showing marked contrast between the hyperintense cerebrospinal fluid andthe other tissues analogous to a myelogram (TR 6000ms, TE 255ms, ST 2.5mm).

S74 DENNIS 2011

Outside the spinal cord, contrast enhancement reflects

increased vascularity of lesions such as meningitis, extra-

medullary–intradural and extradural masses, disco-

spondylitis, paraspinal inflammation, abscesses, and sinus

tracts.15,16 Areas of nonenhancement indicate avascular

areas such as spinal empyema, paraspinal fluid accumula-

tions, and foreign bodies.17 Again, subtraction may be

helpful, especially for areas of contrast enhancement within

or close to fat such as radiculitis or panniculitis; however, it

must be remembered that contrast enhancement is a non-

specific finding. Contrast enhancement around disc extru-

sions due to provoked meningitis or granulation tissue may

mimic other pathology.18,19 In such cases, careful scrutiny

of the images for other signs compatible with disc disease

should avoid misdiagnosis.

Fat Suppression�

Use of spectral fat suppression enables normal fat in the

bone marrow, epidural space, and fascial planes to be dis-

tinguished from other hyperintense tissues, which may be

pathological.20,21 Fat suppression is routinely used for

postcontrast T1W spinal imaging, in which contrast me-

dium in the vertebral venous plexus, together with nulling

of the epidural fat signal, creates a myelogram-like effect

that aids detection of small disc lesions (Fig. 7).

STIR

The STIR sequence is a fat-suppressed T2W scan that is

particularly useful for low-field MR systems, in which

spectral fat suppression is not possible. STIR sequences

depict clearly fluids and tissues with prolonged T2 relax-

ation, but suppress any substance with short T1 relaxation,

including fat, proteinaceous fluid, subacute hemorrhage,

and gadolinium. Because of nulling of signal from gado-

linium, STIR images are usually acquired before contrast

medium administration.22 STIR images have high contrast

Fig. 5. Sagittal T1-weighted magnetic resonance image of the caudal cer-vical spine of a 6-year old Border Collie with a narrowed intervertebralspace at C6-7 and disc protrusion outlined by epidural fat. The adjacentportions of the vertebral bodies of C6 and C7 show reduced signal intensity,probably reflecting sclerosis and/or bone marrow edema (TR 450ms, TE17ms, ST 3mm).

Fig. 6. Dorsal postcontrast T1-weighted magnetic resonance image of thecervical spine of a 5-year-old Domestic Short-haired cat with a meningioma.Contrast enhancement delineates the mass clearly and shows its extra-medullary location (TR 600ms, TE 11ms, ST 3mm).

Fig. 7. Sagittal postcontrast T1-weighted magnetic resonance image withfat suppression in an 8-year-old Whippet with signs of severe neck pain.There is a nonmineralized disc extrusion at C6-7 and cord compression.Epidural fat signal has been nulled and the hyperintensity outlining theextruded material is contrast medium in the internal vertebral venous plexus(TR 460ms, TE 10ms, ST 3mm).

�See accompanying article ‘‘Value of fat suppression in gadolinium-enhanced magnetic resonance neuroimaging,’’ pp. S85–S90.

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but lower resolution than spin-echo sequences, and they

are prone to movement artifacts from respiration and ar-

terial pulsation. STIR images are useful as a means of ex-

amining large regions of the spine, in order to detect

paraspinal soft-tissue pathology (Fig. 8). Images in the

dorsal plane allow comparison of right and left over a large

area, which is advantageous.

T2� GE

The T2� (‘‘T2 star’’) GE sequence produces images in

which bone and other calcified material appear black and

soft tissues are of fairly uniform, medium signal intensity.

It is therefore an excellent sequence for depicting lesions

infiltrating or destroying bone, which appear hyperintense,

and for lesions affecting vertebral bodies, such as narrowed

intervertebral space width, spondylosis, and fractures

(Fig. 9). Calcified disc material and ligaments such as the

dorsal longitudinal ligament also have a very low signal

compared with adjacent soft tissues, hence the T2� GE se-

quence clearly demonstrates most types of disc pathology.

T2� GE sequences are particularly prone to susceptibil-

ity artifacts, which appear as distortion or void resulting

from local variations in the magnetic field created by ferro-

or paramagnetic materials. Hemoglobin degradation prod-

ucts have paramagnetic effects,23 hence T2� GE images are

sensitive for the detection of hemorrhage19 (Fig. 10). T2�

GE sequences should be used before paramagnetic contrast

medium administration.24 The T2� GE sequence aids de-

tection of hemorrhagic spinal lesions including disc extru-

sions accompanied by rupture of the venous sinuses,25

hematomyelia secondary to lumbar puncture,26 iatrogenic

brainstem injury during cisternal puncture,27 hemorrhagic

myelomalacia,28 and intramedullary disc extrusion.29 How-

ever, calcified disc material cannot be accurately distin-

guished from blood because both produce signal void.

Other lesions which are readily detected as areas of signal

void and which have relevance for spinal MRI include

foreign bodies, gas in sinus tracts, and vacuum phenom-

enon in degenerate discs under traction.

Fluid-Attenuated Inversion Recovery (FLAIR)w

The FLAIR sequence is used frequently when imaging

the brain because it nulls the signal from CSF, which in-

creases conspicuity of small lesions close to CSF and aids

diagnosis of lesions that have high signal components in

T2W images that must be distinguished from CSF. In the

spine, FLAIR images are also used to investigate suspected

fluid collections (e.g., subarachnoid cyst or syrinx); how-

ever, FLAIR images generally have lower resolution than

corresponding images obtained using other sequences, and

flow artifacts in CSF may result in unpredictable hetero-

geneous signals, which complicate image interpretation.30

Fig. 8. Dorsal short tau inversion recovery magnetic resonance image ofa 6-year-old Labrador with signs of generalized spinal pain and pyrexia.There is an extensive, mottled, hyperintense signal in subcutaneous fat due tosevere panniculitis (TR 3420ms, TE 27ms, TI 120ms, ST 6mm).

Fig. 9. Sagittal T2� gradient-echo magnetic resonance image of thelumbosacral spine of a 3-year-old German Shepherd dog with cauda equinasyndrome. There is lumbosacral disc protrusion and a well-defined, miner-alized fragment adjacent to the dorsal margin of the cranial endplate of S1.The diagnosis was sacral osteochondrosis (TR 400ms, TE 15ms, flip angle201, ST 3mm).

wSee accompanying article ‘‘Optimal MR imaging of the brain,’’pp. S15–S22.

S76 DENNIS 2011

MR Angiography (MRA)

MRA may be used to image both arteries and veins,

including the internal vertebral venous plexus, and is oc-

casionally useful for spinal MR imaging (Fig. 11). MRA

was used to examine an arterial anomaly in the cervical

spine of a dog.31

Intrathecal Gadolinium Administration

A small amount of gadolinium-based MR contrast me-

dium mixed with CSF and administered by cisternal punc-

ture was used to detect two small dural tears associated

with brachial plexus avulsion in a dog.32 This technique

could also be used to demonstrate communication or oth-

erwise between cyst-like lesions and the subarachnoid

space; however, it must be noted that gadolinium is cur-

rently not licensed for intrathecal use in man or animals.

Image Planes

For most lesions, it is wise to obtain images in all three

planes: sagittal, transverse, and dorsal. Reliance on only one

or two planes may result in incomplete depiction of lesions.

Sagittal Plane

Sagittal images should always be included in a spine

scan, as this is usually the most helpful plane, especially as

an overview of the area of interest. Sagittal images are the

easiest to relate to radiographs when both imaging tech-

niques are used. Comparisons of adjacent disc spaces and

vertebrae are relatively easy and landmarks such as verte-

brae C1, C2, T1, and the lumbosacral junction are readily

identified in sagittal images. Positional scans (traction,

flexion, etc.) are usually obtained in the sagittal plane.

Transverse Plane

Transverse images are usually acquired after sagittal, in

order to obtain more information about a lesion in the

orthogonal plane. Transverse images may also show lesions

that are not evident on the sagittal scan (e.g., lateral cer-

vical stenosis), so absence of findings in sagittal images

should not preclude imaging in the transverse plane. When

no lesion is detected in sagittal images, it may be efficient to

obtain transverse images only at the discs rather than

throughout the whole area of interest to save time; how-

ever, if lesions such as dispersed epidural disc material,

hemorrhage, or cord parenchymal changes are present,

contiguous slices to include normal spine both cranial and

caudal to the lesion are indicated to define their full extent.

Fig. 10. Sagittal T2� gradient-echo magnetic resonance image of the lum-bar spine of a 5-year-old German Shepherd dog with epidural hemorrhage dueto rupture of the ventral venous sinuses following acute disc extrusion at L3-4.The hemorrhage is seen as an irregular signal void extending for more thantwo vertebral lengths. The L3-4 disc space is narrowed and contains residualmineralized disc material (TR 400ms, TE 15ms, flip angle 201, ST 3mm).

Fig. 11. Dorsal plane magnetic resonance angiography image using a 3Dtime-of-flight fast spoiled gradient-echo sequence in an 8-year-old Labradorwith signs of neck pain and cranial vena cava syndrome due to a large cranialmediastinal mass. There is marked dilation of the internal vertebral venousplexus (TR 5.5ms, TE 1.9ms, flip angle 401).

S77OPTIMAL MR IMAGING OF THE SPINEVol. 52, No. 1, Supplement 1

Dorsal Plane

The dorsal plane appears to be underused in veterinary

MR imaging, although it is analogous to a ventrodorsal

radiograph, which should always be part of a spinal ra-

diographic study. In the author’s clinic, it is usually the first

plane to be acquired and has many advantages:

� It is a more detailed image than a localizer on whichto accurately prescribe sagittal images.

� It enables comparison of the two sides of the spine.� It allows identification of transitional or vestigial ribs

at the thoracolumbar junction before surgery in thisarea.

� It gives the clearest image of transitional vertebrae orlateral subluxation (Fig. 12).

� It demonstrates the craniocaudal length of lateralizedlesions most accurately.

� It aids examination of lateralized lesions, such as pa-raspinal or paravertebral muscle atrophy.10

� In the cauda equina region, it gives the best image ofspinal nerves and intervertebral foramina.

� It is ideal for an overview of the soft tissues, especiallyusing STIR sequences (Fig. 8).

The dorsal plane is most valuable where the spine is rel-

atively straight in the midcervical, thoracic, and lumbar

areas. Where the spine curves, dorsal plane images are less

helpful, although thick slices can still be used as an accu-

rate localizer.

FOV and Slice Thickness

FOV selection depends on factors including size of the

patient, type of coil used, and the aim of the study. A large

FOV gives a higher SNR but lower resolution than a small

FOV. When using a very large FOV for sagittal spinal

images, it may be difficult to obtain sufficiently straight

positioning throughout. Generally, sagittal images cover-

ing a craniocaudal spinal length of 20–30 cm in medium

and large dogs are recommended. In the transverse plane,

only the spine and immediately surrounding soft tissues

should be included on the FOV unless a larger area, such as

the brachial plexus, is under investigation.33 Where para-

spinal soft tissues may be abnormal a large FOV dorsal

plane STIR image is helpful.

Selection of slice thickness also represents a compromise

between SNR and resolution. For most spinal imaging,

2–4mm slice thickness is used, depending on the size of the

patient, the area to be covered, and the nature of the lesion.

In selected cases, a second scan with smaller FOV and slice

thickness may be used to give more precise information

once a lesion has been identified.

Artifacts

Discussion of MR artifacts is outside the scope of this

article; useful reviews have been published elsewhere.34,35 It

may not be possible to abolish an artifact but its effect can

sometimes be reduced by certain manoeuvers, such as al-

tering the phase- and frequency-encoding directions. Arti-

facts of particular relevance to spinal MR imaging are

motion (Fig. 13), susceptibility2–4 (Fig. 14), flow artifacts,

chemical shift, and truncation.

Landmarks and Interpretation

The tomographic nature of spinal MR images means

that it can be difficult to locate a lesion precisely with re-

spect to vertebral segment, although this is necessary for

spinal surgery. On the sagittal scan, certain vertebrae are

characteristic in shape, and the ribs and transverse pro-

cesses can be identified in dorsal images of the thoraco-

lumbar spine, although care must be taken not to overlook

vestigial or unilateral ribs. Near the thoracolumbar junc-

Fig. 12. Dorsal plane T2-weighted magnetic resonance image of a 4-year-old Irish red setter with lateral subluxation between L1 and L2 after being hitby a car (TR 3800ms, TE 88ms, ST 3mm).

S78 DENNIS 2011

tion, the celiac and cranial mesenteric arteries are clear

landmarks; although their position varies slightly between

individuals, their location in one scan sequence can be used

to identify the closest vertebra (usually L1) on subsequent

sequences performed on that patient.36 Similarly, lesions

such as spondylosis or disc degeneration may be used as

cross-referencing features.

ACKNOWLEDGMENT

Disclosure: The author declares no conflict of interest.

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