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Basics of musculoskeletal magnetic resonance imaging

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Abstract

MRI is a cross-sectional imaging modality, which uses the spins of hydrogen protons to generate anatomical images and, with some techniques, functional information. One of the advantages of MRI over CT is that in MR studies no ionizing radiation is used, but its main advantage is its superior soft tissue contrast when compared with all other modalities. This chapter explains terminology; fundamental principles of magnetic resonance imaging; indications; safety; technique; contrast media; protocols; principles of interpretation; abnormalities, artefacts and pitfalls.

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Figures

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4.1 High-field MRI scanner. The cylindrical part of the scanner contains a superconducting magnet, which is cooled by liquid nitrogen. During scanning of a patient the table moves to the centre of the magnetic field within the bore of the magnet. The MRI scanner needs to be situated within a dedicated room surrounded by a copper Faraday cage.
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4.2 RF coils used for MRI. The choice of coil used depends upon the area of anatomy being imaged. Coil construction varies with coil type, with some coils transmitting as well as receiving RF signals. Coil choice is important to ensure diagnostic images are acquired.
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4.3 MRI of the brain of a normal 12-year-old dog. (a) T2W and (b) T1W transverse sequences. (a) On the T2W images, fluid (e.g. CSF) has a high (hyperintense) signal (arrowed). T2W images have high contrast, and many types of pathology are more easily detected on this sequence. (b) The CSF is hypointense (arrowed) on T1W images. T1W images are complementary to T2W images and are good for showing anatomy.
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4.4 Transverse plane T2W (a), T1W (b) and FLAIR (c) MR images of the brain of a cat with an intracranial abscess secondary to otitis media. The free fluid within the abscess gives higher signal on the T1W and FLAIR images compared with normal CSF. This is due to the effects of macromolecules within the abscess, which result in altered relaxivity of the proton spins within water. The water in the abscess behaves as if it is bound, compared with the unbound state of water in the CSF.
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4.6 Sagittal MR images of the stifle of a 5-year-old Boxer with cranial cruciate ligament (CCL) disease and osteoarthrosis. (a) PDW and (b) PDW with FATSAT sequences. On the FATSAT image there are focal areas of high signal intensity (arrowed) within the subchondral bone adjacent to the insertion of the CCL. These changes represent oedema, possibly with haemorrhage and microfractures. Note how the changes are much easier to see on the FATSAT image.
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4.7 (a) MRI-compatible anaesthetic machine and monitoring equipment, with (b) slave units outside the MRI scan room to monitor the patient remotely when it is within the MRI scanner.
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4.8 Sagittal plane T1W MR image of the stifle of a dog with cranial cruciate disease which had been treated using the modified Maquet procedure (MMP). The hypointense MMP wedge is clearly visible but the associated implants do not interfere with the ability to assess the intra-articular structures. (Courtesy of R Janiak, Chantry Vets)
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4.9 (a) Sagittal plane T2W MR images of the stifle of a dog with cranial cruciate disease and (b) a 5-year-old Cavalier King Charles Spaniel with syringomyelia. The focal image distortion (arrowed) is due to susceptibility artefacts caused by (a) tibial tuberosity advancement (TTA) implants and (b) a microchip. In these two cases the artefacts did not prevent the studies from being diagnostic.
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4.10 Veterinary ear defenders are available in a range of sizes and are useful for minimizing the risk of hearing damage to the patient during high-field MRI scanning.
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4.11 A 9-year-old Boxer with brain masses (not visible). (a) High-field and (b) low-field MR transverse plane T2W images. Although both studies are diagnostic, higher-field MRI allows better image resolution.
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4.13 Sagittal T2W MR image of an 11-year-old Lurcher with neck pain. The patient has been poorly positioned for the study, hindering evaluation. Because of the failure to position the neck straight it is difficult to assess the size and signal intensity of the caudal cervical spinal cord. Poor patient positioning may result in missed diagnoses.
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4.14 (a) Dorsal, (b) sagittal and (c) transverse plane T2W MR images of an 11-year-old Fox Terrier with a right sciatic nerve mass (arrowed) and aortic aneurysm (dashed arrow). In this case the severe atrophy of the right gluteal muscles (arrowheads) secondary to the sciatic nerve mass was not visible on the sagittal plane images and the dilated aorta was not readily identified on the dorsal plane images. The enlarged sciatic nerve was best shown on the transverse plane image.
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4.15 (a) T1W, (b) T2W and (c) post-contrast T1W transverse plane MR images of the brain of a 4-year-old Boxer with a suprasellar extra-axial mass, presumed to be a meningioma. On the post-contrast image there is enhancement of the mass (arrowed), which appears hyperintense when compared with the pre-contrast images. There is also normal enhancement of the choroid plexi, which lie outside the blood–brain barrier. Although the lesion is clearly visible on the T2W image, defining the boundaries of the mass is difficult.
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4.16 Transverse plane (a) T2W, (b) FLAIR and (c) T1W post-contrast MR images of the brain of a dog with presumed meningitis. The abnormal meninges can only be seen on the post-contrast image (arrowed).
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4.17 (a) Sagittal T1W and (b) T1W post-arthrography MR images of the shoulder of a 9-year-old Border Collie with a tear of the biceps tendon. The absence of the biceps tendon (arrowed) within the tendon sheath is clearly seen on the arthrogram. MRI arthrography is most commonly performed in the shoulder, when it may be required to allow visualization of tears to the glenohumeral ligaments, subscapularis tendon or biceps tendon.
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4.18 When interpreting MR images it is helpful to have (a) multiple or large monitors which (b) allow multiple sequences to be compared at the same time. Viewing software should allow images from different planes and sequences to be cross-referenced.
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4.19 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 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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4.20 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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4.21 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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4.23 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 NJ, Pettitt RA and McConnell JF ( ) Pneumorrhachis in a Rottweiler. , 608–611, with permission)
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4.24 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 difficult on MRI and these are easier to visualize on the radiograph. (Courtesy of Torrington Orthopaedics)
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4.25 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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4.26 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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4.27 (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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4.28 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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4.29 (a) Dorsal and (b) transverse plane T2W MR images of the brain of a 5-month-old West Highland 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. Histopathological 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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4.30 Transverse plane FLAIR MR images of a dog with an intra-axial brain mass within the left piriform lobe presumed to be a glioma. Surrounding the mass is extensive vasogenic oedema (arrowed) which is preferentially within the white matter and spares the adjacent cortical grey matter.
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4.31 Transverse plane T2W MR image of the brain of a dog with postictal changes (arrowed) within the frontal lobes. The changes are due to cytotoxic oedema and are confined to the cortical grey matter with no involvement of the adjacent white matter.
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4.32 Transverse plane (a) T2W, (b) T1W pre- and (c) post-contrast, and (d) dorsal plane STIR MR images of the head of a dog with confirmed cellulitis and panniculitis. The abnormal fat (arrowed) is easiest to recognize on the T1W and STIR images. The presence of extensive enhancement of the tissue is suggestive of inflammation rather than oedema.
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4.34 Transverse plane (a) T2W and (b) T2*GE MR images of the brain of a dog with a cerebral microbleed (arrowed). The T2*GE images are much more sensitive for demonstrating haemorrhage than fast-spin echo sequences. The lesion appears larger on the T2*GE image because of ‘blooming’ caused by susceptibility artefacts due to the haemorrhage.
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4.35 Dorsal plane T2W MR images of the lumbar spine of a dog. The images were acquired with the frequency-encoding direction (a) right–left and (b) craniocaudal. The black line (arrowed) and white line (arrowheads) at the boundary of the epidural fat and CSF in the subarachnoid space represent chemical-shift artefact and occur in the frequency-encoding direction. Swapping the frequency-encoding direction from (a) right–left to (b) a craniocaudal direction moves the artefact 90 degrees and allows better assessment of the subarachnoid space, meninges and epidural fat, which appear symmetrical.
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4.36 (a) Lateral radiograph and (b) sagittal plane T2W MR image of a dog with an acute extrusion of the L5–6 disc. The extruded disc (arrowed) can be seen to be calcified on the radiograph, and on the MRI the calcified tissue (arrowhead) appears signal void. Densely calcified tissue is usually signal void/hypointense on all sequences.
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4.37 (a) Dorsal plane STIR, and transverse plane (b) T2W, (c) T1W pre- and (d) post-contrast MR images of the head of a dog with masticatory myositis. The areas of high T2 signal (arrowed) within the muscle with abnormal enhancement (arrowheads) represent oedema/inflammation. It is the distribution of the changes, which are confined to the masticatory muscles, that is indicative of masticatory myositis.
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4.38 Transverse plane T2W MR image of the head of a dog with a left-sided trigeminal nerve tumour (arrowed). The severe atrophy of the masticatory muscles on the left is typical of chronic denervation. In acute cases of peripheral nerve injury there may be minimal or no atrophy, but the distribution of the denervation changes can be helpful in indicating which nerves are dysfunctional.
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4.39 Transverse plane T2W MR image of the head of a dog with a soft tissue sarcoma (arrowed) within the right temporalis muscle, with invasion of the mandible. The muscle masses are variable in appearance and biopsy is required to determine tumour type and in some cases to differentiate neoplasia from other myopathies.
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4.40 (a) Dorsal plane STIR, transverse plane (b) T2*GE, T1W (c) pre- and (d) post-contrast MR images of the spine of a Great Dane with confirmed vertebral osteomyelitis. The lesion is relatively aggressive, with bony lysis (arrowed) visible as areas of increased signal in the T2*GE image. There is lysis of the spinous process and articular processes with loss of visualization of the normally thin, low-signal cortical bone on the T1W images. The lack of a defined mass and the diffuse changes within multiple bones and adjacent soft tissues are more typical of inflammatory disease than neoplasia, but biopsy is required for definitive diagnosis.
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4.41 Transverse plane (a) T2W, (b) T1W and (c) T2*GE MR images and (d) lateral radiograph of the head of a dog with presumed congenital generalized osteosclerosis/osteopetrosis. There is reduced signal intensity of the cancellous bone (arrowed) on all sequences and thickening of the cortical bone. Increased opacity and cortical thickening of the bone are also visible on the radiograph.
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4.42 (a) Transverse plane T2W MR image and (b) bone window CT image of a dog with a severely comminuted fracture of the T1 vertebra. Spinal cord compression, soft tissue changes and larger fractures can be seen on the MRI but identifying the smaller fragments and determining the fracture configuration accurately is difficult. Visualizing the fractures and smaller mineralized fragments is easier on the CT.
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4.43 MR images of a cat with multiple pelvic fractures and left sacroiliac subluxation. (a) Sagittal plane T2*GEs, (b) dorsal plane 3D-DESS (double-echo steady state) and (c) reformatted sagittal plane 3D-DESS. T2*GE images are useful when evaluating bone images on MRI because of the high contrast between bone and the adjacent soft tissues. On T2*GE images most bone is of very low signal, and this technique has been called ‘black bone imaging’. It allows easier recognition of small fractures, bony lysis and new bone formation compared with other MRI sequences. For orthopaedic imaging, thin-slice 3D sequences are valuable because they allow reformatting of images into multiple planes (similar to CT). In this cat the 3D images were acquired in the dorsal plane (b) and were (c) reformatted into the sagittal plane.
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4.44 (a) Transverse and (b) sagittal plane T2W MR images of a 5-year-old Akita with a normal brain. On the transverse plane image there is a small T2 hyperintense focus (white arrow) apparently within the pons. This can be seen to be artefactual on the sagittal plane images, and is due to partial volume averaging. By cross-referencing the slice location of the transverse plane image (green lines) on the sagittal plane image it can be seen that the pseudolesion is due to voxels at the rostral end of the pons (black arrow) containing both CSF and brain tissue.
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4.45 (a) Transverse and (b) dorsal plane T1W MR images of the brain of a 10-year-old Boxer. The transverse plane image (a) shows a curvilinear ‘lesion’ (arrowed) within the vermis of the cerebellum and brainstem. On the corresponding (b) dorsal plane image, there is no visible lesion at the same location (arrowed). The apparent lesion is not visible on the (c) transverse plane T2W image at the same level. The pseudolesion in the cerebellum is due to a pulsatility artefact from the adjacent venous sinuses (large arrowhead). Smaller pulsatility artefacts (small arrowheads) are present ventrally within the soft tissues.
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4.46 (a) Sagittal PDW MR image of the shoulder of a normal dog. The focal increased signal intensity (arrowed) within the biceps tendon is caused by the magic angle effect and is artefactual. (b) On the dorsal T2W image at the same level the biceps tendon appears normal with no hyperintensity.
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4.47 Transverse plane T2W MR image of the thoracic spine of a 5-year-old Japanese Spitz. The multiple curvilinear lines (arrowed) overlying the lateral parts of the abdomen are due to phase wrap artefact.
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4.48 Transverse plane (a) T2W, (b) FLAIR and (c) T1W MR images of a dog with congenital hydrocephalus. The areas of abnormal signal (arrowed) within the CSF of the lateral ventricles on the T2W (a) and FLAIR (b) images represent flow artefacts. They are not visible on the corresponding T1W (c) image. Transverse plane (c) T1W MR image of a dog with congenital hydrocephalus. The areas of abnormal signal (arrowed) within the CSF of the lateral ventricles on the T2W (a) and FLAIR (b) images that represent flow artefacts, are not visible on the corresponding T1W (c) image.

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