INVESTIGATIONS OF THE CENTRAL AND PERIPHERAL NERVOUS SYSTEMS

SECTION II INVESTIGATIONS OF THE CENTRAL AND PERIPHERAL NERVOUS SYSTEMS




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.



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



NORMAL SCAN










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.











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.



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.




ULTRASOUND





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Jul 16, 2016 | Posted by in NEUROSURGERY | Comments Off on INVESTIGATIONS OF THE CENTRAL AND PERIPHERAL NERVOUS SYSTEMS

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