As with all imaging modalities, the images should be eval-uated for technical quality to see whether the study is diagnostic and to identify artefacts. Questions to be answered before interpretation include:
• Was the correct area imaged – especially if the scan is normal?
• Were the correct sequences, planes and slice thicknesses used?
• Are there any artefacts?
• Did contrast enhancement occur as expected?
MRI is unique in that a large amount of information relating to the tissue characteristics can be obtained from the images. By comparing the signal intensity of lesions and tissues on different pulse sequences, it is possible to infer characteristics of the tissue (e.g. oedema, free fluid, fat content, presence of haemorrhage) (Figures 4.19 to 4.21). The signal intensity is displayed on a greyscale; the more intense the signal the lighter the shade displayed.
The signal intensity of normal tissues differs for different pulse sequences (Figure 4.22) and depends on the amount of fat and free water, the tissue structure and the number of hydrogen protons in the tissue.
Most pathology is best seen on T2W (‘fluid-sensitive’
sequences), and for muscle and bone disease fat suppres-sion techniques (STIR or spectral FATSAT) are valuable.
Many diseases (infectious and inflammatory diseases, neoplasia, etc.) result in an increase in water content.
Areas of high water content appear hyperintense (bright) on T2W images and with reduced signal (dark) on T1W images.
Cortical bone and normal ligaments are normally signal free or, as usually stated, ‘signal void’ owing to the very short relaxation times of the tissues. One limitation of MRI compared with CT or radiography is in differentiating mineralization from gas and ligaments because both can appear as low signal regions on MRI (Figures 4.23 and 4.24). In cancellous bone and the medullary cavities of long bones the signal intensity is dependent upon the proportions of red and yellow bone marrow and varies with age.
Transverse plane (a) T2W, (b) T1W and (c) T2 GE MR images of the brain of a 1-year-old Golden Retriever with an epidural haematoma (white arrow) secondary to Angiostrongylus vasorum infection. By comparing the signal intensity of the lesion on different pulse sequences it is possible to determine that the lesion contains proteinaceous fluid (homogeneous appearance, high signal on T2W and mid-signal on T1W), with a fluid–fluid level (arrowhead) with peripheral haemorrhage (low signal on T2 GE and high signal peripherally on the T1W images (black arrows)).
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(a)
(b)
(c)
Transverse plane (a) T2W, (b) FLAIR, (c) T1W and (d) T1W post-contrast MR images of a 10-year-old Shih Tzu with a presumed brainstem arachnoid cyst (arrowed). The lesion is isointense to the normal CSF (arrowheads) on all pulse sequences, which shows that the tissue is similar in characteristics to CSF (i.e. low protein, free fluid) and demonstrates the
importance of comparing the appearance of lesions on multiple pulse sequences.
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(a) (b)
(c) (d)
Dorsal plane (a) STIR and transverse plane (b) T2W, (c) T1W and (d) T1W post-contrast with FATSAT MR images of the axilla of a 5-year-old Labrador Retriever with infiltrative lipoma (arrowed). It is possible to be certain that the mass is a lipoma because the tissue is isointense to normal fat on all sequences.
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(a) (b)
(c) (d)
Tissue T2W T1W a d T2*GE
Fat Very hyperintense to grey
matter. Similar signal to free fluid
Very hyperintense to normal
brain Very hypointense yper- or hypointense
depending upon scan parameters
Free water with low
protein (e.g. CSF) Very hyperintense to grey
matter Very hypointense to grey
matter Very hyperintense Very hyperintense
igh-protein fluid Variable, usually hypointense
to CSF yperintense to CSF Variable, usually hypointense
to CSF Variable, usually hypointense
to CSF
Grey matter Mid–high signal Mid signal Mid–high signal Mid–high signal
White matter Medium intensity, lower
signal than grey matter Mid–high signal, slightly
hypointense to grey matter ypointense to grey matter ypointense
Muscle Low signal Mid signal Low signal Mid–high signal
Cortical bone Signal void Signal void Signal void Signal void
Ligaments Signal void Signal void (except with
magic angle artefact) Signal void Signal void
Gas Signal void and often
susceptibility artefact Signal void and often
susceptibility artefact Signal void and often
susceptibility artefact Signal void with susceptibility artefact
Signal intensities of tissue on different MR pulse sequences. CSF = cerebrospinal fluid; FATSAT = fat saturation; GE = gradient echo;
STIR = short tau inversion recovery; T1W = T1-weighted; T2W = T2-weighted.
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(a) (b) (c)
Transverse (a) T2W and (b) T1W MR images and (c) corresponding bone window CT image of an 8-year-old Rottweiler with spinal cord compression due to pneumorrhachis (air within the spinal canal). On the MR images it is not possible to determine whether the signal void (arrowed) is due to gas or mineralization. Differentiating gas from calcification is easily done on CT, which is one advantage of this modality over MRI.
(Reproduced from Macdonald N , Pettitt RA and McConnell F (2011) Pneumorrhachis in a Rottweiler. Journal of Small Animal Practice 52(11), 608–611, with permission)
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Sagittal plane (a) PDW and (b) T2 GE MR images and (c) caudocranial radiograph of a 4-year-old Labrador Retriever with lameness due to infraspinatus tendinopathy. There is dystrophic mineralization present within the tendon (arrowed). Identifying small areas of soft tissue mineralization may be di cult on MRI and these are easier to visualize on the radiograph.
(Courtesy of Torrington Orthopaedics)
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(a) (b) (c)
Dorsal plane T2W MR image of the brain of a 3-year-old Shih Tzu. The image has not been acquired in a true dorsal plane, resulting in artefactual asymmetry of the piriform lobes of the brain and the cochlea (arrowed).
When acquiring MR images care needs to be taken to ensure that imaging slices are correctly positioned to allow accurate
interpretation.
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Sagittal plane T2W MR image of the thoracolumbar spine of a dog with a metastatic vertebral tumour (arrowed). The primary tumour (arrowhead) at the heart base would be easily overlooked if a systematic approach to image interpretation is not performed.
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When viewing MR images a systematic approach is recommended to ensure pathology is not missed (Figure 4.25). The images should initially be assessed for accuracy of slice positioning, e.g. comparing the cochlea and temporomandibular joints in the head and assessing patient positioning on the dorsal plane images of the spine (Figure 4.26).
As most diseases result in asymmetrical pathology, detection of lesions is easiest in images which show left and right sides of the body (i.e. transverse or dorsal plane images). Images must be acquired with no rotation to allow accurate comparison of left and right sides because obliquity can mimic pathology (Figure 4.27).
Once the images have been evaluated for positioning and diagnostic quality they can be assessed critically.
Image interpretation is similar to that for radiography and is based on classical Roentgen signs (size, shape, align-ment, margination, etc.) plus signal intensity (absolute and relative to normal tissue). Every structure on the images should be assessed for alterations in shape, size, signal intensity, margination and position. Any localizer images should also be looked at because incidental abdominal or
(a) Transverse and (b) dorsal plane T2W MR images of the brain of a 9-year-old Norfolk Terrier. The brain is normal, but on the transverse plane image there appears to be an abnormal mass of tissue (black arrow) adjacent to the left side of the mesencephalon. This is a pseudolesion due to oblique positioning of the transverse plane images. On the dorsal plane image the ‘abnormal mass’ can be seen to be part of the normal cerebellum (white arrow). By cross-referencing the images the slice location of the transverse plane image (green lines) on the dorsal image, it can be seen that the pseudolesion is due to partial volume averaging and slice obliquity.
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(b)
thoracic disease is occasionally visible and may not be included in the field of view of the other images. When interpreting MRI it is helpful to have a library of normal images for comparison or to refer to an MRI atlas because there are many incidental findings and anatomical variants which are of no clinical significance.
Following administration of contrast media (gadolinium chelates) the images should be assessed to ensure that normal contrast enhancement has occurred. The choroid plexuses, large veins, pituitary gland, nasal mucosa, sali-vary glands and trigeminal nerve ganglion (Figure 4.28) should all enhance in a normal animal. Failure of enhance-ment of a lesion may be due to a lack of blood supply or an intact blood–brain barrier, but may be due to failure of administration of contrast media (e.g. contrast still in cath-eter, leakage from catheter).
Transverse plane post-contrast T1W MR images of the normal brain of a dog. (a) Normal contrast enhancement is visible within the pituitary gland (arrowed), (b) choroid plexuses (arrowed), (c) larger blood vessels and trigeminal nerves ganglia (arrowed).
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(a) (b) (c)
Artefacts are common in MRI; genuine lesions are usually visible on different imaging planes and often on different sequences (see later). When a suspect lesion is detected it should be looked for on other images to ensure it is genuine. Although MRI is more sensitive for detecting pathology than other imaging modalities, it still gives primarily anatomical information, which is often non-specific. Many types of pathology have characteristic imaging findings on MRI, but for many diseases definitive diagnosis requires additional testing or histopathological examination (Figure 4.29).
(a)
(b)
(a) Dorsal and (b) transverse plane T2W MR images of the brain of a 5-month-old West ighland White Terrier presented for investigation of seizures.
The MRI study showed only a small focal area of increased signal intensity within the right frontal lobe (arrowed). The imaging changes are non-specific and could represent infectious inflammatory disease, postictal change or trauma. istopathological examination showed the lesion was due to a necrotizing
meningoencephalitis, which was more extensive than was visible on the MRI study.
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