MR IMAGING IN ORTHOPAEDICS

MR IMAGING OF THE SPINE AND CLINICAL APPLICATIONS :

MRI is the most useful imaging modality for evaluation of the spine. It can be routinely used in degenerative spinal diseases, spinal infections, marrow diseases, congenital anomalies, craniovertebral junction . This article gives an overview of the utilities of MRI in imaging of spinal pathologies as far as an orthopaedic surgeon is concerned.

DEGENERATIVE DISEASES OF THE SPINE

Degenerative diseases of the spine is the commonest condition causing backache, neck pain, radiculopathy and radiculo -myelopathy. Depending upon the location the symptoms vary. Degenerative spinal disease itself is an ill defined concept, better understanding has been possible due to advent of various newer imaging modalities specially MRI.

Indications for imaging:

# Failure of appropriate conservative therapy.
# When there is evidence of deficit.
# In patients having excruciating pain.
# When the diagnosis is in doubt.
# For follow up evaluation.

Choice of imaging modalities:

Plain X-ray still has a role in degenerative spinal diseases if it is properly done. Along
with AP and lateral views, the, oblique views are also very helpful sometimes in demonstrating the neural foraminae. It guides the clinician to choose the subsequent modalities. Conditions like infection, neoplasia, spondylolisthesis etc. can be excluded. Disc narrowing , vacuum phenomena or associated osteophytosis however does not rule out any cord or nerve root impingement and only reflects the consequences of disc degeneration.

Myelography with or without CT cuts is another helpful modality in the evaluation of the degenerative diseases.It is possible to evaluate any intrinsic or extrinsic pressure effect on the neural elements,though the morphology of the lesion can not be made with certainity.Still better information can be gathered with addition of CT cuts with the myelography.The greatest disadvantage of myelography is its invasive nature and associated side effects.

MRI is the best imaging modality as it removes all the disadvantages faced by the clinician in other imaging modalities.It is superior to other modalities because-
– It is totally noninvasive.
– Its multiplanar capability-axial,sagittal,coronal and oblique projection can be obtained.
– soft tissues and bony elements can be equally imaged
– Early detection of disease enabling the clinician to reduce the morbidity and mortality.

MRI of disc degeneration:

Normal disc is composed of two basic component-Nucleus pulposus and annulus fibrosus Central nucleus pulposus is composed of gelatinous material consisting of water and proteoglycans. Annulus fibrosus surrounds the nucleus and has two layers.Inner portion is composed of fibrocartilage whereas the outer fibre are composed of lamellated collagen fibres.The annulus is anchored to the adjacent vertebral bodies by sharpeys fibres

In a normal disc the nucleus pulposus is seen as hyperintense signal on T2 weighted images due to increased water content.There is a T2 hypointense cleft(intranuclear cleft) in the centre of the nucleus which represents residual notochordal remnant. The annulus is hypointense on T2 weighted images due to ligaments. Normal disc is well hydrated and with advancing age there is loss of hydration, the process is termed as desiccation. Abnormal desiccation may start at an early age also. Though it is not clearly known why disc desiccation starts with aging vartious theories have been put foroward.These include –decreased diffusion,cell viability and activity and proteoglycan synthesis. On MRI the desiccated discs are hypointense on T2 weighted images and there is loss of biconvex contour of the disc. The intranuclear cleft becomes prominent.
Disc degeneration can be divided into two types-

Intervertebral osteochondrosis(chondrosis)-When the degenerative process involves the nucleus pulposus the process is called intervertebral chondrosis and osteochondrosis when it involves the adjacent bone also. This is reflected in MRI by disc desiccation,end plate changes,intervertebral disc herniation(schmorls node),vacuuming and reduced disc height.There are three types of end plate changes depending upon the MR morphology.Type I change reflect water,type II change reflects fatty change and type III change signifies sclerotic change.

Spondylosis deformans :- When the degenerative process involves the annulus fibrosus the process is called spondylosis deformans. Here there is abnormal traction in the osseous attachment of the annular fibres leading to osteophytic lipping. Disc space and end plates are preserved here.

Disc herniation:

Disc herniation means displacement of the intervertebral disc from its normal position.Disc may herniate in any direction-anterior,lateral ,posterior and in craniocaudal direction. Various names have been applied and classifications have been made for different entities by different authorities in the comminuted method. Disc herniation has been subdivided into three main groups. Protrusion, extrusion(including sequestration) and intravertebral herniation. In conventional teaching this classification is based on the integrity of the annular/posterior longitudinal ligament complex.A herniated disc is called protruded when the ligaments are intact.When there is break in the ligament complex in a herniated disc it is termed as disc extrusion.However the ligament complex may not be possible to visualize,hence various authorities try to use various names for these entities.Sometimes these terms become confusing .To obviate these confusions a consensus has been made in the nemenclature (after P.Milette). This has been endorsed by –American Society of Spine Radiology,American Society of Neuroradiology,North American Society Spine Society,American Association of Neurological Surgeons,Congress of Neurological Surgeons and American Academy of Orthopaedic Surgeon. A simplified version of the terminology has been discussed here :-
According to the morphology (based on the shape of the displaced disc material) a herniated disc is called
1. Protrusion: If the greatest distance in any plane ,between the edges of the disc material is less than the distance between the edges of the base in the same plane than this called protrusion.
2. Extrusion: If the greatest distance in any plane,between the edges of the herniated disc is more than the distance between the edges at the base is called extrusion.Sequestration is a type of extrusion where the extruded fragment looses its contact with the parent disc.
3. Intravertebral: It means craniocaudal herniation of the disc material through the break in the end plates(Schmorls node).
According to the location a herniated disc may be Central,right/left central,lateral recess(subarticular zone),foraminal(pedicule zone) and extraforaminal (far lateral zone).

Spinal infections:

Infection of the spine may be broadly classified into two main groups-tubercular and non-tubercular. Non-tubercular infections may be due to pyogenic,fungal,parasitic and viral infecting agents.

Tubercular infection:

Lower thoracic and upper lumbar region is the commonest site for tubercular infection follwed by cervicodorsal region.Tubercular infection can involve the vertebral body,disc,epidural space,cord parenchyma,posterior elements and paraspinal soft tissues in combined or isolated form.

MR features of tubercular spinal infections:

MRI can detect the ongoing tubercular infection in the spine at a stage when it is not appreciable in plain X-ray or CT scan. The vertebral involvement may not show any abnormal signal intensity in T2 weighted images. On T1 weighted images the vertebral lesion becomes hypointense. The disc height is relatively maintained so that sometimes it mimics neoplasia. There may be skip lesion, epidural soft tissue lesion, paraspinal muscle involvement ect. Reduction of disc height, reactive sclerosis, vertebral body destruction, gibbus formation are found in advanced cases. In the spinal cord it may produce cord tuberculoma or in early cases myelitis. Involvment of the posterior elements favours the diagnosis of tubercular infection.

Pyogenic spondylodiscitis:

Pyogenic infection of the spine or disc can occur by haematogenous route or by direct inoculation of the disc or the vertebral body. In pyogenic spinal infection ,infection centres around the disc. Disc is vascular in children and in haematogenous infections the disc is first to be involved and the endplates and vertebra are secondarily involved. As in adult the disc gets supply from the endplates thus the disc is involved after the end plates.
On MRI there is irregulartities in the end plates with altered signal in the end plates.The disc height is invariably reduced with strong hyperintense signal in the disc.The vertebral body shows increased signal on T2 and reduced signal on T1 weighted images.There is inhomogenous soft tissue extension.

Brucella spondylitis:

In brucella there is destruction of the anterior aspect of superior end plate of the vertebral body.Healing starts simultaneously in the form of sclerosis and in typical cases there is spillage of healing bones forming large anterior osteophytes.Then the sclerosis extends to the body.Another typical finding of brucella is the peripheral vacuum phenomena.Here there is collection of aerocele in between the disc and superior end plate anteriorly.

Fungal spondilytis:

Fungal spondylitis may be caused by various organisms like blastomycosis, cryptococcosis and coccidiodomycosis. Destructive lesions associated with sclerotic margins, paraspinal masses, multiple sites of involvement with preservation of the disc space are the hallmark of fungal spondylitis.

MRI of failed back surgery syndrome:

Intractable pain and chronic disability after surgery is referred to as failed back surgery syndrome. It is a major cause for increased morbidity and mortality in patients with lumbar disc surgery.The main causes can be divided into two groups-immediate and delayed.

Immediate causes:

Haemorrhage: Onset of symptoms in haematoma occurs within few days.The haematoma occurs in the epidural space and compresses the thecal sac.The severity of symptoms will depend on the size of the haematoma. On MRI the haematoma is bright on T1 weighted images and dark on T2 weighted images.

Disc space infection: Disc space infection(discitis) is the most difficult situation for the surgeons. as there is severe pain and it takes long time to control. The symptoms starts as early as first week and as long as 2 years. On MRI the disc become hyperintense, there is T1 hypointensity in the end plates. These hypointensities my extend to entire vertebral body. On T2 weighted images these are moderately hyperintense. There is T2 hyperintense lesions extending to the epidural space compressing the thecal sac .Soft tissue lesions may be seen in the paraspinal soft tissues.

Pseudomeningocele: may result from dural tear during surgery.It protrudes posteriorly .The sac is hyperintense on T2 and hypointense on T1 weighted images.

Delayed causes:

Disc related causes: The simple cause of persistant pain after surgery may be due to operation in a wrong level. The main disc causing symptoms may be left untreated.There may be recurrent disc at the same level or in a different disc level causing symptoms. Lastly a portion of the disc(residual) may be left during surgery which may cause recurrence of symptoms.

Canal stenosis: There may be congenital canal stenosis associated with herniated disc so that even after removal of the disc patient may not impoove .Again there may be stenotic changes in the central,foramina or recess due to bony overgrowth after facetectomy,late spur formation and post-operative spondylolisthesis.

Epidural fibrosis :Epidural scarring/fibrosis is of uncertain clinical significance, when it is present it may or may not produce any symptoms. On MRI most of the it can be demonstrated easily. In difficult cases intravenous contrast study may be necessary.

Arachnoiditis: Spinal arachnoiditis is a nonspecific response to a variety of stimulus .It is chracterisized by root adhesion, clumping and sometimes cord swelling. There may be deformity of the thecal sac in late cases.

MRI of marrow disease:

MRI is the best imaging modality in the study of structural and morphological anatomy of bone marrow.Earliest changes in the marrow can be detected which are not picked up in X-ray or CT scan so that early intervention can be made.

MRI of normal marrow: Bone marrow has three components. Trabecular bone, Red marrow and Yellow marrow. Trabecular bone acts as a support to the red and yellow marrow. Red marrow is responsible for haemopoiesis and yellow marrow gives nutritional support to red marrow. On T1weighted images yellow marrow appears bright as subcutaneous fat and T2 weighted images it is hyperintense but less than T1.Red marrow is of intermediate signal intensity in both T1 and T2 weighted images.

Marrow pathology:. Abnormalities in the bone marrow can be divided into –

Marrow proliferative disease.
Marrow replacement disorders
Marrow depletion
Vascular
Miscellaneous diseases.

Marrow proliferative diseases:

In this group of diseases there is abnormal proliferation of the cells that exists normally in the bone marrow. Marrow proliferative diseases may be benign or malignant.Common conditions causing marrow proliferation are leukaemia, multiple myeloma, primary amyloidosis, myelodysplasia, mastocytosis and sometimes reconversion of yellow marrow.

MRI findings:

If the changes are very less or the density of the abnormal cells are less MR may be normal.
Signal intensity varies with the disease pattern.In leukaemia the marrow becomes slightly hyperintense on T2 and hypointense of T1 weighted images.In multiple myeloma T1 images will be hypointense and in 50% of patients T2 images may be hypointense.In mastocytosis and myelofibrosis ,it stimulates fibrosis and marrow show reduced signal on both T1 and T2 weighted images.In all these conditions as the haemopoiesis is hampered due to reduction in the affective red marrow ,there is associated reconversion of the existing yellow marrow.
In haemolytic anaemia there is reconversion of yellow marrow.There may deposition of haemosiderin in the bone marrow causing reduced signal in spin echo images.

Marrow replacement diseases:

In this group of diseases there is replacement of the existing normal bone marrow by cells not present normally in the bone marrow.The conditions include metastases,lymphoma,,infectionand primary bone tumours. MRI can be used as screening modality and frequently used to monitor the response to therapy.

MRI features : On T1 weighted images the marrow will be hyperintense,On T2 weighted images it gives variable signal depending upon the infiltrates.Lyphoma and other sclerotic metastases gives hypointense signal on T2 weighted images.

Marrow depletion:

Marrow depletion can occur in aplastic anaemia, post radiation and chemotherapy. The red marrow will replaced by yellow marrow. These changes are observed when MRI is performed for different purpose.Focal areas of red marrow can be seen if there is patchy marrow regeneration.

Marrow oedema:

Various conditions produce marrow oedema. Examples are osteoporosis, trauma (bone contusion)transient osteoporosis, ischaemia, infection, tumour and degenerative joint abnormalities. On T2 weighted images it is very hyperintense and on T1 weighted images these of are intermediate signal intensity.

OsteoporoticVs Pathological frature :

In osteoporotic collapse marrow signal aleteration involves only the vertebral body. The posterior elements are relatively spared. In pathological collapse the posterior elements may also show altered signal intensity.

In osteoporotic collapes no soft tissue involvement is seen. But in pathological collapse the soft tissue involvement is usually seen.

In pathological compression the posterior margin of the involved vertebra is convex.

Fracture line(s) may be seen in osteoporotic collapse.

Spinal dysraphism:

Spinal dysraphism refers to a group of diseases where there is incomplete closure of the spine involving skin, bones, cartilage and neural elements. It is of two types-spina bifida aperta when the spinal contents come out through the osseous defect and produces a visible swelling. Examples-meningocele and meningomyelocele. Second type is spina bifida occulta(occult spinal dysraphism).In this type the malformationis not visible from outside as it dose not produce any swelling. Examples of this condition are-meningocele, dorsal dermal sinus, spinal lipomas, tethered cord ,diastematomyelia, neuroenteric cyst etc. With the advent of MRI there has been better understanding and early detection of these conditions followed by early intervention and hence better prognosis.

Miscellaneous conditions: MRI is the best imaging modality for evaluation of Craniovertebral junction anomalies. Due to its multiplannar capability and better soft tissue contrast it is the modality of choice in evaluating such cases. The spinal intramedullary and extramedullary tumours are best assessed by MRI. Various cord lesions of the cord like syringomyelia, myelitis, pyogenic and tubercular infections are also easily assessed by MRI. . MRI is invaluable in spinal trauma especially when the bony elements are apparently normal. Haemorrhagic and non haemorrhagic contusions are only detected on MRI.

References:

1.Appeal.B: Nomenclature and classification of lumbar disc pathology.Rivista di Neurologica 14:147-152,2001.
2.Milette PC,Fontine S et al:Differenting lumbar disc protrusions,disc bulges and discs with normal contour but abnormal signal intensity.Spine 24:44-53,1999.
3.Kaplan,Helms,Dussault,Anderson and Major.Musculoskeletal MRI. W.B.Saunders company.2001.
4.Wilmink JT.Clinical and Radiological aspects of degenerative disc disease and spinal stenosis,Rivista di neurological 14:163-166,2001.
5.Osborn Anne G. Nonneoplastic disorders of the spine and spinal cord,Diagnostic Neuroradiology Mosby year book,Inc.1994.
6.Tali ET,Imaging of spinal infectious diseases,Rivista di neurological.14:177-187,2001.

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