An MRI Image Of Vertebral Body Biology Essay

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INTRODUCTION

Magnetic resonance imaging (MRI) of spine is a procedure to assess the spinal condition. The examination can able the radiologist to evaluate the tissue condition of the spines. The spinal MRI is superior in visualising the soft tissue that related to the spine such as the intervertebral disc, the spinal nerves, the surrounding muscle and fats, and also can show the extension of the spinal nerves, (Hanafi, 2008) . The superiority of MRI in soft tissue contrast resolution has given advantage especially in differentiating the healthy tissue and pathological tissues that corresponds to the vertebral column.

Figure 1 : An MRI image of vertebral body

The examination of spinal column is done individually. Since the spinal column is divided into several major parts such as cervical vertebra, thoracic vertebral, lumbar vertebral, sacrum and coccyx. The assessment of these parts is done separately and however, the method used in the examination is nearly the same. The parameter of the scanning is different related to which area the radiologist request to scan. The parameters such as the field of view (FOV), slice thickness, and matrix are usually changing with correspond to the specific vertebral, (Hanafi, 2008)

ANATOMY OF SPINES

Image 2 : Anatomy of the Human Spine

The vertebral column is constructed by individual bones called vertebrae. The identification of each vertebral given by their location along the spinal column. There are 7 cervical vertebrae, 12 thoracic, 5 lumbar, 5 sacral fused into 1 sacrum, and 4 to 5 small coccygeal vertebrae fused into 1 coccyx. And it mark up about 24 body of vertebra, and 1 fused sacrum and 1 fused coccyx. The first seven vertebral are situated within the neck. And they are called as cervical vertebra. The first cervical vertebra called as atlas. The atlas articulated with the occipital bone of the skull. The function of this articulation is to support the head and forms the pivot point. This pivot point also includes the involvement of the second cervical vertebra which called as axis. This point will able our head to rotate from side to side and perform flexion and extension. The cervical vertebra only has name for the first two cervical vertebras only, meanwhile the remaining 5 of the cervical vertebra do not have names.

Image 3 : Diagram of cervical spine

The second part of the vertebra is the thoracic vertebra. Each of the thoracic vertebra form joints with each other by superior and inferior articular process. This processes articulate the upper and lower vertebral body. The thoracic vertebra also articulates with the ribs to form the thoracic cage. This ribs is articulate with facets in the vertebra body.

Image 4 : Diagram of Thoracic spine

The third part of vertebra column is the lumbar vertebra. The lumbar vertebra is the largest and strongest bones in spinal column. Lumbar vertebra consist 5 vertebra bodies. It also act as the last terminal of the spinal nerve which is the nerve end at the level of L1 and L2.

The spinal column has four natural curvatures. And the naming of these curvatures is corresponds to the vertebra related to it. The cervical spine naturally appears curve forward, and the thoracic spine curving backward. Meanwhile the lumbar curvature is forward and the sacrum curvature is to the backwards.

Image 5 : Diagram of Lumbar spine

INDICATION

These are the common indication for cervical spine MRI examination

1) Cervical Myelopathy

2) Cervical radiculopathy

3) Cervical cord compression or trauma

4) Assessment of extent of spinal infection or tumour

5) Diagnosis of Chiari malformation and cervical syrinx

6) MS plaques within the cord

(Westbrook, C,2008)

These are common indication for thoracic spine MRI procedure

1) Thoracic spine disc disease

2) Thoracic cord compression

3) Visualisation or assessment of MS plaque in the thoracic cord

4) Thoracic cord tumour

5) To visualise the inferior extent of cervical syrinx (Westbrook, C,2008)

These are common indication for lumbar spine MRI procedure

1) Disc prolapsed with cord or nerve root compression

2)Spinal dysraphism ( to assess cord termination, syrinx, diastematomyelia)

3) Discitis

4) Evaluation of the conus in patient with appropriate symptoms

5) Failed back syndrome

6) Arachnoiditis (Westbrook, C,2008

CONTRAINDICATION

The contraindication for MRI is different comparing to other modalities. This is because in MRI, the radiographer or MRI personnel are responsible to the patient safety. This is because, the facility is using very high magnetic force and every single ferromagnetic material will be attracted toward the MRI gantry on very high velocity. Thus, the patient must be screen first before the patient entering the MRI suite.

The main aspect of safety to the patient in MRI suite is the magnetic safety, (Westbrook, C, 2008). It is very important that all the personnel include the patient are screened and prevented upon entering the magnetic field area. All hospital that install the MRI modality must have proper screening policy.

The radiographer or MRI operator must screen the patient and check for

a) Pacemaker

The patient with pacemakers is absolute contraindicated to any MRI procedure. This is because; the magnetic field strength will switch the reed function of the pacemaker. And this will lead to the cardiac arrhythmias and also can cause cardiac arrest.

Image 6 : Diagram of a pacemaker in human body

b) Aneurysm clip

The aneurysm clip commonly made from the ferromagnetic material. And the ferromagnetic material must not be scanned in MRI. This is because it can alter the position of the clip and cause ruptures the blood vessel. Recently, the non ferromagnetic aneurysm clip has been produce, but unfortunately, even though this clip can be scanned in MRI, it still can cause deviation especially in position of the clip. And as the result, the non ferromagnetic clip is still contraindicated in MRI scanning.

Image 7 : Diagram of an aneurysm clip

c) Intra ocular foreign bodies

d) Metal devices or metal prosthesis

e) Cochlear implants

f) Spinal implants

g) Possibility of early pregnancy

e) any jewellery, cards, and other metal accessories

INSTRUMENTS PREPARATION

For Cervical Spine Procedure

1) Posterior cervical neck coil / volume neck coil / multi-coil array spinal coil

Image 8 : Diagram of head and neck coil

2) Immobilisation pads and straps

Image 9 : Diagram of Immobilisation pads and straps

3) Pe gating lead if required

4) Ear plugs

For Thoracic Spine Procedure

1) Posterior spinal coil / multi-coil array spinal coil

Image 10 : Diagram of a surface coil that placed under patient body

2) Pe gating lead if required

3) Ear plugs

For Lumbar Spine Procedure

1) Posterior spinal coil / multi-coil array spinal coil

Image 11 : Dedicated surface coil for thoracic and lumbar spine MR

2) Foam pads to elevate the knees

3) Ear plugs

COILS TYPE

Generally, there are two types of coils used in spinal imaging.

1) Linearly polarised

2) Circularly polarised.

The circularly polarised coil (CP Coil) is mere preferable because the construction of this type of coil. This coil constructed to produce more signal and this will provide better signal to noise ratio (SNR).

Usually, there is one type of coils that used in MR spine imaging. It construction is in form of a flat box with lumbar or thoracic support and can directly attached with patient table. These coils can generate a maximum field of view of 24 to 40 cm (Stichnoth, et al. 2006).

The coils for cervical spine usually design to have support to the patient's neck. The field of view of this coils also extended to cover the cranoicervical junction area. (Stichnoth,et al.2006).

For cervical coil also, some manufacturers also produce the circular solenoid coil that can be wrap around the neck. This coil are designed to produce smaller field of view thus will increase resolution of the image. Flexible coils also can be used for patient who cannot position in normal cervical coil.

PATIENT CONSIDERATION

For cervical region

Some patient will faced with the difficulty to place their neck over the posterior neck coil, especially for the case of fixed deformity. It is important to place the neck as closed as possible to acquire as much signals and to increase signal to noise ratio (SNR). The pads is placed under the patient's shoulder to flat the spine and therefore it is possible to put the back of the neck closer to the coils. For those with cervical cord trauma, cord compression or tumours are often severe disabled. Great care must be applied when removing or transferring the patients on to the examination couch and on to the bed. They should move as little as possible at a time and gentle. For wise, let the patient move the body by themselves if they able to cooperate. Due to excessive loud produce by the gradient noise, ear plug should be given to the patient. (Westbrook, C,2008)

For thoracic region

Patient indicated with spinal cord trauma might be in severely disabled and in great pain. Thus, the examination should be undergoing in less time and with proper imaging sequence. (Westbrook, C,2008)

For lumbar region

Many patients that undergo the lumbar spine examination usually in severe pain. Especially for those with prolapsed lumbar disc. The technologist or radiographer must treat the patient calmly and make the patient comfortable as possible. The back of the patient must be supported with pads supporting and their knees is best in flexed position. Place a small pad below the lumbar curvature will help to elevate the sciatica and other type of back pain. Give earplug to the patient as during the procedure, some sequence will produce very loud gradient noise. (Westbrook, C,2008)

PATIENT POSITIONING

For Cervical Spine Procedure

The patient will lie supine (head first) on the table or couch and the neck coil is placed under the cervical region. The coils are usually designed to fit the back of the head and neck so that the patient will automatically centered to the coils. If the flat coil is used, a support pads must be placed under the patient shoulder so that the curvature of the cervical spine will be reduced. It is also purpose to bring the cervical closer to the coil. The position of the coil should cover from the base of skull and extent to the sternoclavicular joint in order to cover all seven cervical spines. The position of the patient also must be aligned with the longitudinal alignment (midline of the couch). Meanwhile the horizontal alignment is ensured by horizontal light laser pass through the hyoid bone (above the Adam's apple). The patient's head is mobilised by using the foam pads and retention straps. If required, the gating lead is attached to the patient.

(Westbrook, C,2008)

For Thoracic Spine Procedure

Patient lies supine on the examination couch with head first, the positioning of the spinal coil must cover from the top of the shoulder and extent to the lower costal margin to ensure the coverage of all thoracic vertebral. The patient's position must be aligned with the longitudinal alignment light and the horizontal alignment light must pass through at the centre of the coil which corresponds to pass through at the level of fourth thoracic vertebra. The gating lead is attached if required by the examination.

(Westbrook, C,2008)

For Lumbar Spine Procedure

Patient lies supine on the examination couch with head first and the knees are flexed and elevated over the foam pads. The foam pads is given to add comfortness to the patient and also to flatten the lumbar vertebra so that the lumbar vertebra will be more closer to the coil. The position of the coil must cover from the xiphisternum and extent to the sacrum for better coverage of lumbar region. The positioning of the longitudinal light is at the midline of the patient and the horizontal alignment light is pass through the lower costal margin which correspond to the third lumbar vertebra.

(Westbrook, C,2008)

PROTOCOLS

For Cervical Spine Procedure

1) Saggital/Coronal SE/FSE or coherent GRE T2*

This act as the localizer if the 3 plane localization is not provided by the manufacturer.

Coronal localizer: Medium slice or gap is prescribed relative to the vertical alignment positioning light. Start from the posterior aspect of the spinous process and to the anterior borders of the vertebral body. It covers the area of the base of skull and extend to the second thoracic vertebra.

Saggital localizer: medium slice thickness or gap is prescribed on either side of longitudinal alignment light. Starts from the left to the right lateral borders of vertebral body. It cover the area from the base of skull and extend to the second thoracic vertebra.

Saggital SE/FSE T1: Thin slice or gap is prescribed on the either side of the longitudinal alignment light, from the right to the left lateral borders of the vertebral bodies. It covers from the base of skull and extend to the second vertebral body.

Image 12 : Saggital SE/FSE T1

Axial/Oblique SE/FSE T1/T2 or coherent GRE T2*: Thin slice or gap is angled so that they are parallel to the disc space or perpendicular to the lesion. For disc disease, 3 to 4 slice per level is usually enough. But for larger lesion, thicker slice is required and small area covers top and bottom might be necessary.

2) Additional sequence

Saggital / oblique SE/FSE T1

The slice prescription is as for Axial or oblique T2*. And this sequence is add with contrast enhancement for tumour assessment.

Saggital SE/FSE T2 or STIR

This sequence is an alternative for coherent GRE T2*

3D Coherent/incoherent (Spoiled) GRE T2*/T1

Thin slice and a few or medium number of slice locations are prescribed through the region of interest. If proton density or T2* weighting is required, then the coherent or steady state sequence is used. If T1 weighting is required, an incoherent or spoiled sequence is used.

Saggital SE/FSE T1 Or Fast Incoherent (Spoiled) GRE T1/PD

Slice prescription is as for the Saggital T1, T2 and T2*. Exception is only in neck flexion and extension to correlate the potential revalence of spondylotic changes to the manifestation of the disease (sign and symptoms)

3D Balanced Gradient Echo (BGRE)

The contrast characteristics of a BGRE provide for high signal intensity from the cerebrospinal fluid (CSF) which has high T1 and T2 ratio. This will provide an image with high contrast between the CSF and the nerve root. (Westbrook, C, 2008)

For Thoracic Spine Procedure

1­) Saggital/Coronal SE/FSE or coherent GRE T2*

This act as the localizer if the 3 plane localization is not provided by the manufacturer.

Coronal localizer: Medium slice or gap are prescribed relative to the vertical alignment light, from the posterior aspect of the spinous process to the anterior border of the vertebral bodies. It covers the area from the 7th cervical vertebra and extend to the conus.

Saggital localizer: Medium slice or gap are prescribed on the either sides of the longitudinal alignment light , starts from the left towards to the right lateral borders of the vertebral bodies. It covers the area from the 7th cervical vertebra and extend to the conus.

Saggital SE/FSE T1 : The thin slice or gap are prescribed on the either sides of the longitudinal alignment light, it starts from the left to the right lateral borders of the vertebral bodies.

Saggital SE/FSE or Coherent GRE T2*

The prescription of this sequence is as for Saggital T1

Axial / Oblique SE/FSE T1 or Coherent Gradient Echo T2*

Thin slice or gap are angled so that they are parallel to the disc space or perpendicular to the lesion that want to assess. For larger lesion such as tumour or syrinx, thicker slice is needed.

2) Additional Sequence

Saggital / Axial / Oblique SE/FSE T1 with or without contrast.

This sequence is best for evaluating the conus and the other spinal cord lesion.

(Westbrook, C,2008)

For Lumbar Spine Procedure

1) Saggital/Coronal SE/FSE or coherent GRE T2*

This act as the localizer if the 3 plane localization is not provided by the manufacturer.

Coronal localizer: Medium slice or gap are prescribed relative to the vertical alignment light, starts from the posterior aspect of the spinous process to the anterior border of the vertebral bodies. The area covered is from the conus to the sacrum.

Saggital localizer: Medium slice or gap are prescribed on the either sides of the longitudinal alignment light, starts from the left towards to the right lateral borders of the vertebral bodies. It covers the area from the conus to the sacrum.

Saggital SE/FSE T1

Thin slice or gap are prescribed on the either sides of the longitudinal alignment light, starts from the left towards to the right lateral borders of the vertebral bodies. And it covers the area from the conus to the sacrum.

Saggital SE/FSE or Coherent GRE T2*

The slice prescription of this sequence is same as the sequence for Saggital T1

Axial / Oblique SE/FSE T1 or Coherent Gradient Echo T2*

Thin slice or gap are angled so that they are parallel to each disc space and it extend from the lower lamina and upper lamina above the disc. The lower three lumbar vertebra are usually examined.

2) Additional Sequence

Axial / Oblique or Saggital SE/FSE T1

This sequence always used for examination with contrast for determining the disc prolapsed versus scar tissue in failed back syndrome. It also used for some tumours. For without contrast examination, it used to assess spinal dysraphism. Chemical or spectral presaturation is beneficial to differentiate between fat and enhance the pathology.

Coronal SE/FSE T1 : This sequence is used for assessing cord tethering or alternative view of conus when saggital is not included.

Axial / Oblique FSE T1 : It used for diagnosing arachnoiditis. As for axial or oblique, the slice is parallel to each disc space and vertebral bodies from the scarum to the conus.

STIR : when FSE provide excellent T2 weighted image of the spine, the signal intensity from the normal fat and the marrow is normally high, even with longer TE. For that reason, marrow disease, such as tumour or fractures may be not be adequately visualised on T2 weighted FSE sequence. A STIR sequence is usually used to visualised the bone marrow abnormalities better.

(Westbrook, C,2008)

PULSE PARAMETERS

SPIN ECHO (SE)

T1 : TE =15ms , TR = 300ms

T2 : TE = 70ms , TR = 2000ms

FAST SPIN ECHO (FSE)

T1 : TE = 20ms , TR = 300ms

T2 : TE = 100ms , TR = 3000ms

Ali, D. M. H. (2008)

ARTEFACT PROBLEMS

The cervical area is often obscured by artefact. Not only the aliasing artefact from the surrounding structure, but the periodic, pulsatile motion of CSF within the spinal canal can produce the phase ghosting. The speed of CSF flow usually faster in cervical area. And therefore the use of flow reducing sequence such as spatial presaturation and GMN is less effective in cervical region. Compared to the lumbar region, the flow of CSF in this area is slower related to other region. On T1 weighted image, spatial presaturation pulses placed S and I to the FOV are usually sufficient. However, in T2 weighted sequence floe artefact is commonly troublesome. The use of FSE also sometimes demonstrate more flow artefact than CSE, and for this circumstances, it is not applied widely to the cervical spine MR procedure. This is more trouble when using T2 weighted image because the flow artefact will totally degrade the image. In addition, selecting an S-I phase direction along with oversampling can also reduce CSF flow artefact in saggital imaging.

In thoracic region, the flow of fluid is usually slower compared to the cervical region. Spatial presaturation is necessary to reduce the flow related artefact. The use of GMN also can minimise the flow artefact but as it increase the signal intensity from the CSF and the minimum TE available, it is usually used in T2 and T2* weighted sequences. FSE is commonly utilised in the thoracic region as it is reduce the scan time. However, when comparing to the SE and GRE, the FSE has higher increase of flow artefact. Therefore, when the flow artefact is too troubling, the SE or GRE may be used. Other source of artefact is the phase ghosting and respiratory motion. Spatial presaturation pulses applied into the FOV is the effective way to reduce it. The PE gated imaging also can be used, but when the PE gating is depend too much to the patient's heart rate, it is too time consuming. The used of PE gating is wisely can be applied when there are too much flow artefact that cannot be tolerate by using other parameters.

In lumbar area, CSF pulsation or motion is not usually give trouble as the flow of the fluid is relatively slow. However the phase artefact from the major blood vessel such aorta and vena cava sometimes obscured the lumbar canal. GMN can be used to reduce the flow artefact, but it is usually reserved for T2 and T2* sequences.

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