Magnetic resonance imaging: spine
- MRI Magnetic resonance imaging: basic principles is the best imaging modality for investigation of many spinal diseases, due to its multiplanar imaging capability (cross-sectional images can be obtained in any plane) and the high resolution of the resulting images .
- CT Computed tomography (CT) is also helpful, but soft tissue contrast is poorer, and images in the sagittal and dorsal planes may require reformatting from the transverse plane.
Advantages of MRI over myelography and CT
- It is non-invasive, as no subarachnoid injection of contrast medium is required.
- It can differentiate types of spinal cord swelling, eg solid and cystic.
- It will show lesions which do not produce a mass effect and which therefore are not seen on myelography Radiography: myelography.
- It detects early disk degeneration and other soft changes in the disk space, such as diskospondylitis Diskospondylitis.
- Acquisition of images in the transverse plane provides a dimension that cannot be assessed radiographically.
- Spinal nerves can be seen.
- The investigation is often quicker than myelography, dependant on scanner type, and is physically easier as no repositioning of the patient is needed.
- If the clinical signs are multifocal, the brain can easily be scanned at the same time.
- Paraspinal soft tissues can be assessed for involvement in pathology.
- MRI provides superior contrast resolution than CT and is better suited for imaging soft tissues, such as the spinal cord, nerve roots, and intervertebral disks.
Disadvantages of MRI compared with myelography and CT
- It is more expensive, and is often not immediately available for acute cases requiring urgent surgery.
- Localization of lesions with respect to the meninges may be harder, eg differentiation of extradural and intradural lesions.
- Stressed and traction views are more difficult; in particular, stressed views which may compress the cord further cannot be justified because of the length of time for which the position must be maintained.
- There is reduced fine bone detail compared with radiography, and periosteal new bone is hard to see (although medullary and subchondral bone changes are better demonstrated).
- Whole spine screens can take a long time.
- CT provides superior spatial resolution and is better suited for imaging bone.
- Thinner slice acquisition (submillimeter) is possible with CT.
- CT is cheaper to buy, requires fewer maintenance requirement and associated expenses as well as rapidity of imaging.
- CT can be used to successfully guide needle aspirations or biopsies once a lesion is localized, but this is not generally done with MRI guidance because of prolonged imaging times and inability to use metal implements for tissue sampling.
- Interpretation requires considerable experience, and false positive diagnoses are easy to make.
- Resolution may be inadequate for some small cats.
Restraint and positioning
- As with MRI of other areas, the patient must remain still during scanning, and so general anesthesia General anesthesia: overview is required.
MRI compatible anesthetic monitoring equipment is required.
- Metallic objects should not be present in the scanning field of view, and so previous spinal surgery using implants prevents MRI.
- Microchips will produce a small area of signal void and image distortion, but this does not usually impair diagnosis. The chips code is not affected by the magnetic field and the chip will not be moved in the tissues.
- It is important that the spine is as straight as possible in the sagittal plane ; lateral curvature of the spine results in poor or non-diagnostic sagittal scans as only short sections of the spine are visible on each image.
- Positioning aids such as troughs and soft wedges are used as in radiography.
- Most animals are best scanned in dorsal recumbency, using an appropriate radiofrequency (RF) coil according to the size of the patient and the area under investigation - extremity coil for cats. The spinal segment(s) to be imaged must be in close contact with the radiofrequency coil for satisfactory signal to noise ratio.
- Animals with spinal pain should be given pain-killers as part of the anesthetic protocol, to abolish muscle spasm, to reduce the likelihood of movement during scanning and to prevent exacerbation of the clinical signs after prolonged recumbency.
As with myelographs care should be taken with handling anesthetized animals with suspected spinal injury to avoid further injury.
- For lumbar and caudal thoracic spine the animal should be placed tail first into the scanner. For central spine and cranial thoracic spine they should be placed head first into the scanner.
Care should be taken that annotation of the images (left and right) is correct.
- Initial pilot or scout images are obtained; the number of slices and the planes used varies with the scanner.
- If the spine is seen to be laterally curved or rotated, an attempt should be made to reposition the animal.
- A definitive dorsal plane scan should be obtained first with slices parallel to the vertebral canal and spinal cord.
- Sagittal images should be placed on a dorsal plane image that shows the spine clearly, in order to align the slices accurately with the spine .
- Images should be obtained in all three planes (sagittal , transverse , and dorsal ) although the former two are usually more helpful. Transverse images are placed through any areas of interest.
- Sagittal views are often used as the survey series to localize a specific site of pathology as a long segment of the spine can be imaged.
- Transverse views are typically limited to a specific site of pathology as imaging of extended lengths of spine in this plane is time intensive.
- Suitable slice thickness varies from 2-4 mm.
- A 0.5 mm gap between slices or interleaved (no gap) acquisition is typical to avoid image cross-talk and resultant decreased signal to noise.
- As MRI is extremely sensitive to motion artifact secondary to respiration and blood flow, saturation bands may be placed over the abdomen and thorax to suppress the effects of motion from these areas during scanning.
- T2-weighted scans are usually most informative, although pre- and post-contrast administration T1W scans should be obtained in cases of suspected neoplasia or inflammatory disease. T2 gradient echo and fat-suppressed scans may also be helpful.
- Images should be reviewed carefully on the computer screen for optimum interpretation.
- If hard copies are sent elsewhere for interpretation, the individual images should be of a reasonable size and not too small.
- It is vital that an anatomical landmark such as a distinctive vertebrae, eg T1, the sacrum on sagittal images or T13 on dorsal images, can be identified for lesion localiztion prior to surgery.
- On T2W images, the nucleus pulposus of the intervertebral disks, epidural fat and CSF is hyperintense (bright) and the spinal cord and nerve roots are mid-signal . T2W images often give more useful information of the spinal cord than do T1W scans due to greater contrast. A thin, discrete line of very low signal between CSF and epidural fat represents a combination of the dura and chemical shift artifact .
- On T1W intervertebral disks are hypointense, and usually show a central, vertical band of lower signal . Although epidural fat remains hyperintense, CSF is of low signal and is not easy to differentiate from cord parenchyma.
- With STIR (short tau inversion recovery) or fat saturation, the typically hyperintense fat (on both T1W and T2W sequences) will be of low signal intensity. If a sequence is T2W, CSF can be more readily differentiated from epidural fat. If a sequence is T1W, a contrast enhancing lesion can be more readily differentiated from fat.
- On gradient echo scans bony and ligamentous structures produce a signal void (appear black) allowing easy assessment of disk space width, endplate erosion and annular bulging. It is also frequently used to detect areas of hemorrhage.
- In the mid lumbar area the lumbar intumescence is visible , and caudal to this the cauda equina is seen as a collection of individual nerves .
- Venous sinuses are visible ventral to the spinal cord on transverse images.
- Degeneration of the lumbosacral disk (signal reduction on T2W images) and mild dorsal bulging is a common finding and is often of no significance.
Congenital spinal disease
- Atlanto-axial malformation Spine: congenital vertebral malformations is rare in cats. Although instability is better demonstrated radiographically, MRI will show not only bone changes but also abnormalities in the dens ligaments and associated spinal cord compression and/or contusion. C2 is often displaced craniodorsally.
- Subarachnoid cyst-like Spine: arachnoid cyst lesions are well-demonstrated on T2W scans, usually in cervical or thoracic spine, in which the CSF accumulation is hyperintense. The dilated subarachnoid space is typically teardrop-shaped. The use of the term "cyst" to describe these lesions is a misnomer, as these outpouchings are not lined by epithelium. Very rare in cats.
Degenerative spinal disease
- Spondylosis is less readily seen than on radiographs, best seen on the T2 GRE sequence. Small spondyles around intervertebral foramina can be seen with MRI. Not usually of significance in cats.
- Osteoarthrosis of dorsal spinal articulations is also less clear on MRI than on radiographs, but resulting compression of the spinal cord is easily detected. Such dorsolateral compression may be missed or under-estimated on myelography if oblique projections are not obtained. It can be hard on MRI to differentiate bone and soft tissue proliferation (eg ligamentum flavum), as both are of very low signal intensity. However, the distinction is usually only of academic interest. Cysts (hyperintense on T2W, hypointense on T1W) may be seen associated with the facet joints and may cause compression on occasion.
Intervertebral disk disease
- Degeneration (dehydration) of disks is seen as reduction in signal intensity on T2W scans and calcified material produces a signal void on all sequences. Reduction in disk space width can also be seen but is less obvious than with radiography. Dorsal displacement of disk material, hypertrophy of the dorsal longitudinal ligament and spinal cord compression can be seen although it may not be possible to distinguish disk extrusion from disk protrusion Intervertebral disk disease.
- Laceration of the vertebral venous sinuses may result in epidural hemorrhage which can extend for a considerable distance from the disk space. This usually apears as amorphous, ill-defined tissue in the epidural space of mid-low signal.
- Whilst disk protrusion is readily recognized the significance may be difficult to determine unless cord compression is obvious as many animals have multiple bulging disks.
- Many disk lesions are lateralized, and this can easily be recognized, especially in transverse scans. Likewise, involvement of the intervertebral foramina and nerve roots can be seen.
- In some cases, changes in the overlying spinal cord can be seen hyperintensity on T2W images denotes contusion, myelomalacia or chronic damage.
- With chronic compression, the spinal cord may atrophy, and the surrounding subarachnoid space remains patent despite a reduction in cord cross-sectional area. Such animals may not benefit from decompressive surgery.
- Acute non-compressive hydrated nucleus pulposus extrusion (also known as high velocity, low volume disk extrusions or Type III disks) are occasionally seen on MRI. A small amount of disk material escapes explosively and causes a focal contusion of the adjacent spinal cord, with peracute clinical signs. MRI shows that there is no spinal cord compression, indicating that decompressive surgery is not necessary. Intramedullary protrusions may be seen with disk material penetrating the spinal cord resulting in severe spinal cord traumas. A linear tract may be seen extending from the disk unto the spinal cord.
- Spinal infarcts (fibrocartilaginous emboli) Fibrocartilaginous embolism produce similar peracute clinical signs. The MR appearance may be similar, with a focal cord hyperintensity on T2W. However, whilst the lesion from an acute non-compressive hydrated nucleus pulposus extrusion must be opposite an abnormal disk, that of an infarct may occur anywhere.
Cauda equina syndrome
- MRI is often the only means of assessing the cauda equina fully, since radiographic contrast techniques may fail to demonstrate the area of interest.
- Lumbosacral stenosis (various causes) is easily seen, although as with the cervical spine, stressed views are not easy to obtain.
- Disk disease and cauda equina compression due changes in other structures can be seen.
- However, there is poor correlation between clinical signs (lumbosacral discomfort on palpation is unreliable and often falsely positive) and MRI changes. It is easy to over-diagnose LS disease, because disk degeneration and protrusion is a common, clinically-silent condition in animals.
- MRI is especially valuable in cases of unilateral hindlimb lameness due to foraminal stenosis, as spinal nerve compression can be seen or inferred in some cases. The foramina appear narrowed and there is loss of the normal hyperintense peri radicular fat surrounding the nerve roots. This is best seen on transverse and parasagittal image . Nerve root swelling may be seen in severe cases.
- Swelling and parenchymal change due to cauda equina neoplasia and inflammation may be recognized, although the changes on MRI are non-specific and lumbar CSF analysis should be performed.
- Diskospondylitis Diskospondylitis often occurs at the lumbosacral junction (see below).
- MRI is an ideal technique for assessing bony and soft tissue lesions, providing that the patient can withstand the necessary general anesthetic Spine: fracture / luxation.
- MRI is very much safer than radiography because the patient need not be repeatedly manipulated, and there is no risk of myelographic contrast medium coming into contact with damaged cord parenchyma.
- Three-dimensional information about fractures and subluxations is given, although interpretation in complex fractures can be hard.
- If surgical fixation is performed, repeat MRI is precluded by the presence of metallic implants.
Inflammatory spinal disease
- Diskospondylitis can be diagnosed on MRI before being evident on radiographs.
- Typically there is material of very high signal on T2W scans in the disk space. This is of higher signal intensity than normal disk material and is less structured.
- Vertebral endplate erosion and subchondral sclerosis are best seen on sagittal scans.
- Contrast enhancement of disk material, paravertebral soft tissues and adjacent vertebrae around the affected site confirms the diagnosis.
- Extension of the process in the epidural space (spinal epidural empyema) can be seen forming an elongated irregular soft tissue structure that is iso to hypointense on T1W images, hyperintense/heterogeneous on T2 W images, with a rim-like or diffuse contrast enlargement.
- Local lymphadenopathy, cellulitis and prostatic lesions may be seen.
- Meningomyelitis may occur in the spinal cord as well as in the brain.
- Lesions are ill-defined and hyperintense on T2W, with varying degrees of swelling.
- Contrast enhancement on T1W scans is sometimes seen.
- Diagnosis is confirmed on CSF analysis, preferably from a lumbar tap.
- Tumors Spinal neoplasia may affect the spinal cord, meninges, peripheral spinal nerves, vertebrae and paraspinal soft tissues. Classification is based on similar criteria to myelography.
- Early bone involvement may be more readily seen on MRI than on radiography, despite the fact that the latter is more sensitive for subtle bony change such as a periosteal reaction.
- Gradient echo images are particularly useful for demonstrating bone involvement with increase in signal of the affected bone.
- Neoplasia produces loss of normal architecture, often with swelling and contrast enhancement of the affected tissues.
- However, MRI changes are non-specific and other disease processes may mimic neoplasia.
- MRI is invaluable for the diagnosis of peripheral nerve tumors, especially when myelography is normal as there is no spinal cord invasion. However, severe clinical signs may precede detectable enlargement of the affected nerve on MRI.
- Gross swelling of the nerve often with cord invasion is seen with PNST. It is often not possible to differentiate true cord invasion by the tumor from severe compression. Widening of the invertebral foramine may be seen on transverse plane image.