COMMENTS ON CENTRAL NERVOUS SYSTEM AND RADIOLOGICAL ANATOMY

Radiological Anatomy

Radiological anatomy is a medical specialty that employs the use of imaging to both diagnose and treat disease visualised within the human body. It explores various tissue densities to be able to determine structures. It could either be invasive or non-invasive. Radiologists use an array of imaging techniques which include:

• Conventional radiography (X-ray)

• Ultrasound

• Computerized Tomography

• Magnetic Resonance Imaging

• Interventional Radiology

• Radiation Therapy

The acquisition of medical imaging is usually carried out by the radiographer or radiologic technologist. The radiologist then interprets or "reads" the images and produces a report of their findings and impression or diagnosis.

Central Nervous System

The nervous system is organized into two parts: the central nervous system, which consists of the brain and the spinal cord, and the peripheral nervous system, which connects the central nervous system to the rest of the body. In the central nervous system, the brain and spinal cord are the main centres where communication of nervous information occur. Both the brain and spinal cord are covered with a system of membranes, called meninges and are suspended in the cerebrospinal fluid; they are further protected by the bones of the skull and the vertebral column.

The central nervous system is composed of large numbers of excitable nerve cells and their processes, called neurons, which are supported by specialized tissue called neuroglia. The long processes of a nerve cell are called axons or nerve fibres. The interior of the central nervous system is organized into gray and white matter. Gray matter consists of nerve cells embedded in neuroglia; it has a gray color. White matter consists of nerve fibres embedded in neuroglia; it has a white colour due to the presence of lipid material in the myelin sheaths of many of the nerve fibres. The billions of neurons in the brain are connected to neurons throughout the body by trillions of synapses

Brain: The brain contains more than 90% of the body's neurons. The brain has been divided into 3 different areas: the hindbrain, the midbrain, and the forebrain.

The hindbrain is found in even the most primitive vertebrates. It is made up of the cerebellum, the pons, and the medulla. The medulla is a narrow structure nearest the spinal cord; it is the point at which many of the nerves from the left part of the body cross to the right side of the brain and vice versa. The medulla controls such functions as breathing, heart rate, and blood pressure. The pons, located just above the medulla, connects the top of the brain to the cerebellum. Chemicals produced in the pons help maintain our sleep-wake cycle. The cerebellum is divided into 2 hemispheres and handles certain reflexes, especially those that have to do with balance. It also coordinates the body's actions.

The midbrain lies between the hindbrain and forebrain and is crucial for hearing and sight.

The forebrain is supported by the brain stem and buds out above it, drooping somewhat to fit inside the skull. It consists of the thalamus, the hypothalamus, and the cerebral cortex. The thalamus relays and translates incoming messages from the sense receptors—except those for smell. The hypothalamus governs motivation and emotion and appears to play a role in coordinating the responses of the nervous system in times of stress.

The cerebral hemispheres, located above the thalamus and hypothalamus, take up most of the room inside the skull. The outer covering of the cerebral hemispheres is known as the cerebral cortex. The cerebral hemispheres are what most people think of when they think of the brain. They are the most recently evolved portion of the brain, and they regulate the most complex behaviour. Each cerebral hemisphere is divided into 4 lobes, delineated by deep fissures on the surface of the brain.

The occipital lobe of the cortex, located at the back of the head, receives and processes visual information. The temporal lobe, located roughly behind the temples, is important to the sense of smell; it also helps us perform complex visual tasks, such as recognizing faces.

The parietal lobe, which sits on top of the temporal and occipital lobes, receives sensory information, in the sensory projection areas, from all over the body and figures in spatial abilities. The ability to comprehend language is concentrated in 2 areas in the parietal and temporal lobes.

The frontal lobe is the part of the cerebral cortex responsible for voluntary movement and attention as well as goal-directed behaviour. The brain starts response messages in the motor projection areas, from which they proceed to the muscles and glands. The frontal lobe may also be linked to emotional temperament

These 4 lobes are both physically and functionally distinct. Each lobe contains areas for specific motor sensory function as well as association areas. The association areas are areas that are free to process all kinds of information and make up most of the cerebral cortex and enable the brain to produce behaviours requiring the coordination of many brain areas. The brain lies in the cranial cavity and is continuous with the spinal cord through the foramen magnum. It is surrounded by 3 meninges: the dura mater, the arachnoid mater, and the pia mater; these are continuous with the corresponding meninges of the spinal cord. The cerebrospinal fluid surrounds the brain in the subarachnoid space.

The medulla oblongata is conical in shape and connects the pons superiorly to the spinal cord inferiorly. It contains many collections of neurons, called nuclei, and serves as a conduit for ascending and descending nerve fibres. The pons is situated on the anterior surface of the cerebellum, inferior to the midbrain and superior to the medulla oblongata. The pons, or bridge, derives its name from the large number of transverse fibres on its anterior aspect connecting the 2 cerebellar hemispheres. It also contains many nuclei and ascending and descending nerve fibres.

The cerebellum lies within the posterior cranial fossa of the skull, posterior to the pons and the medulla oblongata. It consists of 2 laterally placed hemispheres connected by a median portion, the vermis. The cerebellum is connected to the midbrain by the superior cerebellar peduncles, to the pons by the middle cerebellar peduncles, and to the medulla by the inferior cerebellar peduncles. The peduncles are composed of large bundles of nerve fibers connecting the cerebellum to the remainder of the nervous system. The surface layer of each cerebellar hemisphere is called the cortex and is composed of gray matter. The cerebellar cortex is thrown into folds, or folia, separated by closely set transverse fissures. Certain masses of gray matter are found in the interior of the cerebellum, embedded in the white matter; the largest of these is known as the dentate nucleus.

The medulla oblongata, the pons, and the cerebellum surround a cavity filled with cerebrospinal fluid, called the fourth ventricle. This is connected superiorly to the third ventricle by the cerebral aqueduct; inferiorly, it is continuous with the central canal of the spinal cord. It communicates with the subarachnoid space through three openings in the inferior part of the roof. It is through these openings that the cerebrospinal fluid within the central nervous system can enter the subarachnoid space.

Spinal Cord: The spinal cord is situated within the vertebral canal of the vertebral column and is surrounded by 3 meninges: the dura mater, the arachnoid mater, and the pia mater. Further protection is provided by the cerebrospinal fluid, which surrounds the spinal cord in the subarachnoid space. The spinal cord is roughly cylindrical and begins superiorly at the foramen magnum in the skull, where it is continuous with the medulla oblongata of the brain. It terminates inferiorly in the lumbar region. Below, the spinal cord tapers off into the conus medullaris, from the apex of which a prolongation of the pia mater, the filum terminale (internum), descends to attach to the back of the coccyx.

Thirty one pairs of spinal nerves are attached by the anterior or motor roots and the posterior or sensory roots along the entire length of the spinal cord. Each root is attached to the cord by a series of rootlets, which extend the whole length of the corresponding segment of the cord. Each posterior nerve root possesses a posterior root or spinal ganglion, the cells of which give rise to peripheral and central nerve fibres.

Radiological Techniques Applicable To Central Nervous System

• Conventional radiography (X-ray)

• Ultrasound

• Computerized Tomography

• Magnetic Resonance Imaging

• Interventional Radiology

• Radiation Therapy

INDICATIONS, CONTRAINDICATIONS AND RISK FACTORS OF THE RADIOLOGICAL ANATOMY OF CENTRAL NERVOUS SYSTEM

Conventional Radiography(X-ray)

Indications:

• Tumours involving the bony spine, meninges, nerve roots or spinal cord • infection involving the bony spine, intervertebral discs, meninges and surrounding soft tissues • inflammation of the arachnoid membrane that covers the spinal cord • spinal lesions caused by disease or trauma Contraindications: • Pregnancy

Risk Factors: • Burns • Malignancy

Computerized Tomography This kind of imaging technique can be used to: • Evaluate the extent of bone and soft tissue damage in patients with facial trauma, and planning surgical reconstruction. • Diagnose diseases of the temporal bone on the side of the skull, which may be causing hearing problems. • Determine whether inflammation or other changes are present in the paranasal sinuses. • Plan radiation therapy for cancer of the brain or other tissues. • Guide the passage of a needle used to obtain a tissue sample (biopsy) from the brain. • Assess aneurysms or arteriovenous malformations through a technique called CT angiography
It can also be used to detect: • Haemorrhage, brain injury and skull fractures in patients with head injuries. • Bleeding caused by a ruptured or leaking aneurysm in a patient with a sudden severe headache. • Blood clot or bleeding within the brain shortly after a patient exhibits symptoms of a stroke. • Stroke, especially with a new technique called Perfusion CT. • Brain tumors. • Enlarged brain cavities (ventricles) in patients with hydrocephalus. • Diseases or malformations of the skull. Contraindications: • Pregnancy • Very young children Risk Factors • Anaphylactic reaction to contrast medium • Malignancy • Varying radiation dose

Ultrasound This can be used in children to: • Evaluate hydrocephalus, or an enlargement of the ventricles, a condition that can have a number of causes. • Eetect bleeding within the brain tissue or the ventricles. The latter condition is called intraventricular haemorrhage (IVH). • Assess whether there is damage to the white matter brain tissue surrounding the edges of the ventricles, a condition known as periventricular leukomalacia (PVL). • Evaluate for congenital abnormalities • Locate the site of an infection or tumour In adults, it can be used to: • Locate and evaluate tumor masses during brain surgery, facilitating their safe removal • Assess the risk of stroke in patients with sickle cell disease • Measure conditions affecting blood flow to and within the brain e.g stenosis and vasospasm

Risk Factors: • There are no known risk factors.
Magnetic Resonance Imaging This imaging technique is important in the diagnosis of: • Brain tumours • Stroke • Infections • Developmental anomalies • Hydrocephalus — dilatation of fluid spaces within the brain (ventricles) • Causes of epilepsy (seizure) • Haemorrhage in selected trauma patients • Certain chronic conditions, such as multiple sclerosis • Disorders of the eye and inner ear • Disorders of pituitary gland • Vascular problems, such as an aneurysm (a bubble-like expansion of the vessel), arterial occlusion (blockage) or venous thrombosis (a blood clot within a vein).
It can also be used to: • Assess the spinal anatomy and alignment. • Detect congenital anomalies of vertebrae or the spinal cord. • Assess intervertebral disk disease (degenerated, bulging or herniated) and intervertebral joint disease, both frequent causes of severe lower back pain and sciatica (back pain radiating into a leg). • Assess compression of spinal cord and nerves. • Help plan spinal surgical procedures, such as decompression of a pinched nerve or spinal fusion. • Monitor changes in the spine after an operation, such as scarring or infection. • Guide the injection of steroids to relieve spinal pain. • Explore other possible causes of back pain e.g. compression fracture • Image spinal infection or tumors that arise in, or have spread to, the spine. • Assess inflammation of the spinal cord or nerves. Contraindicatons: • Patients with hearing aids • Patients with cardiac pacemakers • Patient with metal pins, screws and surgical staples • Patients with nerve stimulators Risk Factors: • Anaphylactic reactions to contrast medium • Excessive sedation if sedatives are used
Radiation Therapy This is done in the form of intracranial vascular treatment. It is a minimally invasive procedure that, depending on the patient's condition, involves image-guided internal catheter navigation of the body's blood vessels to improve or block blood flow in the brain's arteries and veins, or image-guided delivery of focused internal radiation from a source outside of the body. It is used to block off blood flow in the treatment of: • Aneurysm, a bulge or sac that develops in an artery because the wall of the blood vessel is weak. • Arteriovenous malformation (AVM): a tangle of dilated blood vessels that disrupts normal blood flow in the brain. • Arteriovenous fistula (AVF): an abnormal connection or passageway between an artery and a vein. • Tumours with excessive blood flow It is used to improved blood flow in the treatment of: • Acute stroke • Narrow blood vessels to prevent stroke. • Intracranial vasospasm, or tension/contracting of blood vessels.
Risk Factors: • Anaphylactic reactions to contrast agent • Damage to blood vessels

Conclusion The various imaging techniques discussed above are important in the diagnoses and treatments of various ailments which are closely related to the central nervous system.

References ACR and RSNA (2013). Radiology Info radiology information resource for patients Filler, Aaron (2010). "The History, Development and Impact of Computed Imaging in Neurological Diagnosis and Neurosurgery: CT, MRI, and DTI". Internet Journal of Neurosurgery 7 (1). Herman, G. T., 2009 Fundamentals of computerized tomography: 2nd Ed. Image reconstruction from projection, Springer Novelline, R. (1997) Squire's Fundamentals of Radiology. (5th Ed.) Harvard University Press .