INVESTIGATIONS OF THE CENTRAL AND PERIPHERAL NERVOUS SYSTEMS

Published on 12/04/2015 by admin

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Last modified 12/04/2015

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SECTION II INVESTIGATIONS OF THE CENTRAL AND PERIPHERAL NERVOUS SYSTEMS

SKULL X-RAY

With the development of more advanced imaging techniques, skull X-ray is now less often used, but may still provide useful information.

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More specific views are available, but in practice have been replaced by other imaging techniques, e.g.

Base of skull (submentovertical) – cranial nerve palsies
Optic foramina – progressive blindness
Sella turcica – visual field defects
Petrous/internal auditory meatus – sensorineural deafness.

COMPUTERISED TOMOGRAPHY (CT) SCANNING

The development of this non-invasive technique in the 1970s revolutionised the investigative approach to intracranial pathology. A pencil beam of X-ray traverses the patient’s head and a diametrically opposed detector measures the extent of its absorption. Computer processing, multiple rotating beams and detectors arranged in a complete circle around the patient’s head enable determination of absorption values for multiple small blocks of tissue (voxels). Reconstruction of these areas on a two-dimensional display (pixels) provides the characteristic CT scan appearance. For routine scanning, slices are 3–5 mm wide. The latest ‘spiral’ or ‘helical’ CT scanners use a large bank of detectors (multislice) and the patient moves through the field during scanning so that the X-ray beams describe a helical path. This considerably reduces scanning time and is of particular value when slices of 1–2 mm thickness provide greater detail. These ‘high definition’ views permit coronal and sagittal reconstructions and allow detailed examination of certain areas e.g. the orbit, pituitary fossa and cerebello-pontine angle.

Selecting different window levels displays tissues of different X-ray density more clearly. Most centres routinely provide two images for each scanned level of the lumbar spine, one to demonstrate bone structures, the other to show soft tissue within and outwith the spinal canal.

An intravenous iodinated water-soluble contrast medium is administered when the plain scan reveals an abnormality or if specific clinical indications exist, e.g. suspected arteriovenous malformation, acoustic schwannoma or intracerebral abscess. Intravenous contrast shows areas with increased vascularity or with impairment of the blood– brain barrier.

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Note: diagram illustrates individual slices. In the latest generation scanners, the beam describes a helical pathway around the head.

NORMAL SCAN

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Spinal CT scanning

If MRI is unavailable, CT of the spine can demonstrate the bony canal, intervertebral foramen and disc protrusion. After instilling some intrathecal contrast, CT scanning clearly demonstrates lesions compressing the spinal cord or the cervico-medullary junction.

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Coronal and sagittal reconstruction

CT imaging in the coronal plane is difficult and in the sagittal plane, virtually impossible. Two dimensional reconstruction of a selected plane may provide more information, but requires CT slices of narrow width e.g. 1–2 mm.

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Coronal CT scanning

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In the absence of the latest scanners and good reconstruction, full neck extension combined with maximal angulation of the CT gantry permits direct coronal scanning.

CT angiography

Helical scanning during infusion of intravenous contrast provides a non-invasive method of demonstrating intracranial vessels in 2 and 3-D format. The ability to rotate the image through 360° more clearly demonstrates vessels and any abnormalities. Many reports claim that 3-D CT angiography is as accurate as conventional angiography in detecting small aneurysms.

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CT perfusion imaging

Following the infusion of contrast it is possible to construct brain perfusion maps. Ischaemic regions receive less contrast and appear as low density areas. This technique can be of value in predicting outcome from acute stroke.

Xenon-enhanced computed tomography (XE-CT)

Inhaled stable xenon mixed with O2 crosses the intact blood–brain barrier. CT scanning detects changes in tissue density as xenon accumulates producing quantitative maps of regional blood flow. This technique determines the degree and extent of cerebral ischaemia.

Interpretation of the cranial CT scan

Before contrast enhancement note:

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After contrast enhancement:

Vessels in the circle of Willis appear in the basal slices. Look at the extent and pattern of contrast uptake in any abnormal region. Some lesions may only appear after contrast enhancement.

MAGNETIC RESONANCE IMAGING (MRI)

For many years, magnetic resonance techniques aided chemical analysis in the food and petrochemical industries. The development of large-bore homogeneous magnets and computer assisted imaging (as in CT scanning) extended its use to the mapping of hydrogen nuclei (i.e. water) densities and their effect on surrounding molecules in vivo. Since these vary from tissue to tissue, MRI can provide a detailed image of both head and body structures. The latest echo-planer MR imaging permits rapid image acquisition.

Physical basis

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A variety of different radiofrequency pulse sequences (saturation recovery (SR), inversion recovery (IR) and spin echo (SE)) combined with computerised imaging produce an image of either proton density or of T1 or T2 weighting depending on the sequence employed.

Normal MRI images (T1/T2 weighting in relation to normal grey/white matter)

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Interpretation of abnormal MRI image

Look for structural abnormalities and abnormal intensities indicating a change in tissue T1 or T2 weighting in relation to normal grey and white matter. (A prolonged T1 relaxation time gives hypointensity, i.e. more black; a prolonged T2 relaxation time gives hyperintensity, i.e. more white).

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Paramagnetic enhancement

Some substances e.g. gadolinium, induce strong local magnetic fields – particularly shortening the T1 component. After intravenous administration, leakage of gadolinium through regions of damaged blood–brain barrier produces marked enhancement of the MRI signal, e.g. in ischaemia, infection, tumours and demyelination. Gadolinium may also help differentiate tumour tissue from surrounding oedema.

MR Angiography (MRA)

Rapidly flowing protons can create different intensities from stationary protons and the resultant signals obtained by special sequences can demonstrate vessels, aneurysms and arteriovenous malformations. Vessels displayed simultaneously, may make interpretation difficult, but selection of a specific MR section can demonstrate a single vessel or bifurcation. By selecting a specific flow velocity, MRA will show either arteries or veins. The resolution has improved with 3 Tesla MRA, but may still miss aneurysms seen on intra-arterial DSA (see page 45).

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Diffusion-weighted MRI (DWI)

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Images are based on an assessment of thermally driven translational movement of water and other small molecules within the brain. In acute ischaemia, cytotoxic oedema restrains diffusion. The degree of restricted diffusion is quantified with a parameter termed the apparent diffusion coefficient (ADC). ADC values fall initially, then normalise and prolong as ischaemic tissues become necrotic and are replaced by extracellular fluid. DWI shows size, site and age of ischaemic change. Whilst the volume normally increases within the first few days, the initial lesion size correlates best with the final outcome. This image shows restricted diffusion in a left middle cerebral artery infarct 3 hours from the onset.

Perfusion-weighted MRI (PWI)

Images are obtained by ‘bolus tracking’ after rapid contrast injection. A delay in contrast arrival and reduced concentration signifies hypoperfusion of that brain region. Soon after onset, ischaemic changes on PWI appear larger than on DWI. The difference between PWI and DWI may reflect dysfunctional salvagable tissue (ischaemic penumbra see page 245). Early resolution of the PWI abnormality indicates recanalisation of an occluded vessel, whereas in those who do not recanalise the DWI volume expands to fill a large part of the original PWI lesion.

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Functional MRI (fMRI)

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The oxygenated state of haemoglobin influences the T2 relaxation time of perfused brain. A mismatch between the supply of oxygenated blood and oxygen utilisation in activated areas, produces an increase in venous oxygen content within post capillary venules causing signal change due to blood oxygenation level dependent (BOLD) contrast. Improved spatial and temporal resolution has increased the scope of functional imaging, leading to greater understanding of normal and abnormal brain function. A demonstration of the exact proximity of eloquent regions to areas of proposed resection, helps minimise damage.

Magnetic resonance spectroscopy (MRS)

Spectroscopic techniques generate information on in vivo biochemical changes in response to disease. Concentrations of chemicals of biological interest are minute but measurement can be undertaken in single or multiple regions of interest of around 1.5cm3. N-acetylaspartate (a neuronal marker) and lactate are studied by 1H-MRS, whilst adenosine triphosphate phosphocreatine and inorganic phosphate are measured by 31P-MRS. MRS is gradually emerging from being a research tool to play a role in tumour characterisation, the confirmation of metabolic brain lesions and the study of degenerative disease.

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1H-MRS from both regions of normal brain and from a grade II astrocytoma. The tumour trace shows a high choline peak, due to high membrane turnover, a grossly reduced peak of N-acetylaspartate and the presence of lactate, confirming anaerobic metabolism.

Diffusion tensor imaging (DTI) – tractography

As with diffusion-weighted MRI, diffusion tensor imaging utilises the movement of water. Water diffuses more rapidly in the direction aligned with the internal structure. Each MR voxel has a rate of diffusion and a preferred direction. The structure of the white matter tracts facilitates movement of water through the brain in the direction of the tract (anisotropic diffusion). Tractography is the technique which makes use of this directional information. For each voxel a colour-coded tensor (or vector) can be created which reflects 3-dimensional orientation of diffusion, the colour reflecting the direction. By this means neural tracts can be demonstrated along their whole length.

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This imaging technique has demonstrated interruption of white matter tracts in patients who have suffered a traumatic diffuse axonal injury. Of even more clinical value is the technique’s ability to show whether intrinsic tumours infiltrate or deflect crucial structures such as the corticospinal tracts, potentially of value in pre-operative planning and performing tumour resection. The accuracy of DTI tractography and its use as an operative guide still requires validation.

ULTRASOUND

Extracranial

When the probe (i.e. a transducer) – frequency 5–10 MHz, is applied to the skin surface, a proportion of the ultrasonic waves emitted are reflected back from structures of varying acoustic impedance and are detected by the same probe. These reflected waves are reconverted into electrical energy and displayed as a two-dimensional image (β-mode).

When the probe is directed at moving structures, such as red blood cells within a blood vessel lumen, frequency shift of the reflected waves occurs (the Doppler effect) proportional to the velocity of flowing blood. Doppler ultrasound uses continuous wave (CW) or pulsed wave (PW). The former measures frequency shift anywhere along the path of the probe. Pulsed ultrasound records frequency shift at a specific depth.

Duplex scanning combines β-mode with doppler, simultaneously providing images from the vessels from which the velocity is recorded.

Colour Coded Duplex (CCD) uses colour coding to superimpose flow velocities on a two dimensional ultrasound image.

Applications: assessment of extracranial carotid and vertebral arteries.

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Intracranial – transcranial Doppler ultrasound

By selecting lower frequencies (2 MHz), ultrasound is able to penetrate the thinner parts of the skull bone.

Combining this with a pulsed system gives reliable measurements of flow velocity in the anterior, middle and posterior cerebral arteries and in the basilar artery.

Applications:

Assessment of intracranial haemodynamics in extracranial occlusive/stenotic vascular disease. Detection of vasospasm in subarachnoid haemorrhage.

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ANGIOGRAPHY

Many neurological and neurosurgical conditions require accurate delineation of both intra- and extracranial vessels. Intra-arterial injection of contrast, imaged by digital subtraction (DSA), remains the gold standard for imaging intracranial vessels.

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Digital subtraction angiography (DSA)

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