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Sunday, July 10, 2011

BRAIN: CT, MRI - Noninvasive imaging of the central nervous system - Reshared Knol

Source Knol: BRAIN: CT, MRI

by Christopher Hess, Assistant Professor, Neuroradiology, University of California San Francisco, San Francisco, CA

Derk D Purcell, Radiologist at CPMC Radiology


Introduction
The complexity of the organ that determines how a person thinks, moves, feels and remembers is overshadowed only by its unique vulnerability. The brain is hidden from direct view by the protective bony covering of the skull, which not only shields it from injury but also hinders the study of its function in both health and disease. The cells in the arteries that supply the brain are so tightly bound that even most normal cells in the bloodstream are prevented from crossing the so-called “blood-brain barrier,” thereby rendering the normal chemistry of the brain invisible to the routine laboratory blood tests that are often used to evaluate the heart, liver or kidneys.

Computed tomography (CT) and magnetic resonance imaging (MRI) have revolutionized the study of the brain by allowing doctors and researchers to look at the brain noninvasively. Like other organs in the human body, the structure of the brain is highly organized according to its function. Different parts of the brain play specific roles that govern different activities, such as movement, cognition, and vision. These imaging techniques have allowed for the first time the noninvasive evaluation of brain structure, allowing doctors to infer causes of abnormal function due to different diseases.


How are brain images made with CT?
Computed tomography is based on the measurement of the amount of energy that the head absorbs as a beam of radiation passes through it from a source to a detector. Within a CT scanner, the radiation source and detector are mounted opposite one another along a circular track, or gantry, allowing them to rotate rapidly and synchronously around the table on which the patient lies. As the x-ray source and detector move around the patient’s head, measurements consisting of many projections through the head are obtained at prescribed angles and stored on a computer. The table moves horizontally in and out of the scanner in order to cover the entire head [1, 2].

The “tome” in tomography is the Greek word for “slice.” At the core of the scanner is a computer that not only controls the radiation source, the rotation of the x-ray tube and detector, and the movement of the table, but also generates anatomical slices, or tomograms, from the measured projections. The mathematical technique that allows an image of the head to be recovered from its projections is referred to as the backprojection algorithm [3]. Because the patient is positioned horizontally on the table, the backprojection algorithm yields slices that are transaxial, which means the slices are oriented at right angles to the long axis of the body.

The primary physical quantity that is captured with CT is density, or mass per unit volume. Prior to display and storage of CT images, pixel intensities are mapped to a standard numerical scale to allow reliable discrimination between different densities of tissue such as air, water, fat, bone, and various brain constituents. When the images are reviewed on a computer, the intensities are further modified by a process referred to as windowing in order to optimally depict the density of different tissues for visual display. Extremely dense material, such as metal or bone, appears bright on CT images, whereas tissue that is less dense, like fat or water, appears dark.

Magnetic resonance imaging relies upon signals derived from water molecules, which comprise between 70% and 80% of the average human brain. This ubiquitous biological molecule has two protons, which by virtue of their positive charge act as small magnets on a subatomic scale. Positioned within the large magnetic field of an MR scanner, typically 30 to 60 thousand times stronger than the magnetic field of the earth, these microscopic magnets collectively produce a tiny net magnetization that can be measured outside of the body and used to generate very high-resolution images that reveal information about water molecules in the brain and their local environment.

Protons placed in a magnetic field have the interesting property that they will absorb energy at specific frequencies, and then re-emit the energy at the same frequency. To measure the net magnetization, a coil placed around the head is used to both to generate electromagnetic waves and measure the electromagnetic waves that are emitted from the head in response. Unlike CT, which uses x-rays with very high frequency energy, MRI uses electromagnetic waves in the same portion of the electromagnetic spectrum as broadcast FM radio.

MRI is also a tomographic imaging modality, in that it produces two-dimensional images that consist of individual slices of the brain. Images in MRI need not be acquired transaxially, and the table or scanner does not move to cover different slices in the brain. Rather, images can be obtained in any plane through the head by electronically “steering” the plane of the scan. Precise spatial localization is achieved through a process termed gradient encoding [4]. The switching on and off of these magnetic field gradients are the source of the loud clicking and whirring noises that are heard during an MRI scan. While this process requires more time than CT scanning, imaging can be performed relatively rapidly using modern gradient systems [5].

Image intensity in MRI depends upon several parameters. These are proton density, which is determined by the relative concentration of water molecules, and T1, T2, and T2* relaxation, which reflect different features of the local environment of individual protons. The degree to which these parameters contribute to overall image intensity is controlled by the application and timing of radiofrequency energy through different pulse sequences. The most commonly used pulse sequences in brain imaging preferentially emphasize T1 relaxation, T2 relaxation, T2* relaxation or proton density. Specialized pulse sequences can sensitize images to flowing blood, minute changes in local brain oxygen content, or even to the microscopic movement of water molecules within the brain. Each pulse sequence confers a different contrast weighting to the image, such that when combined, the aggregate intensities from the different pulse sequences allow inference about the properties and local environment of the brain tissue being studied. For example, using MRI, one can infer the phase (solid or liquid), content (fat, water, air, blood) or movement (static or pulsatile) of a given structure in the brain

What is a contrast agent?
Contrast media, or “dyes” are used both in brain CT and MRI to provide another mechanism for modulating image intensity beyond what is possible using intrinsic tissue contrast. These are externally administered pharmaceutical agents given during an imaging examination to highlight normal or diseased brain structures. The additional information provided by a contrast agent may or may not be necessary to make an accurate diagnosis.

Contrast agents are most often administered by intravenous injection, but may in certain cases require intrathecal (spinal) injection using a lumbar puncture (spinal tap) procedure. Because they are water soluble they are normally removed from the body by the kidneys, though when the kidneys are diseased the liver may also contribute to their elimination. CT contrast agents are typically iodine-containing compounds that transiently increase the density of structures that they pass through. MRI contrast agents contain the heavy metal gadolinium, which changes the inherent T1 and T2 relaxation parameters of tissues.

The normal path of intravenously-injected contrast agents is through the heart, lungs and arteries of the chest and neck before entering the head. Once in the head, contrast passes first into the arterial system of the brain and its coverings, then through the cerebral microcirculation to supply the brain itself, ultimately passing into the intracranial venous system. Once it has passed into the large veins in the head, contrast is transmitted into the veins of the neck. From there, the contrast enters the heart for the second time, beginning the process of recirculation. The relative timing of the scan with respect to the location of the contrast agent within the blood pool allows detailed imaging of the arteries, brain, or veins.

Contrast agents in neuroimaging are usually given to evaluate blood vessels or to assess the integrity of the blood-brain barrier. In the former case, CT or MR angiography can diagnose cerebral aneurysms, vascular malformations, and narrowed or occluded arteries. In the latter, enhancement of the brain itself is used for the diagnosis of disease. The cells that line the capillaries of the normal brain are tightly bound together to form the “blood-brain barrier,” which allows the passage of oxygen and nutrients into the brain but prevents the transit of disease-causing organisms and large molecules, including contrast agents. Brain tumors, infections and inflammatory processes often disrupt the blood-brain barrier, giving rise to abnormal enhancement within the brain.

What do brain CT and MRI images show?
Interpretation of brain images requires a detailed knowledge of anatomy and a comprehensive understanding of how different diseases affect the brain and its supporting structures. Radiologists are medical doctors who specialize in acquiring and interpreting images; neuroradiologists focus specifically on imaging of the nervous system. These specialists work together with neurologists, neurosurgeons and primary care physicians to use CT and MRI to diagnose disorders of the brain and understand their significance for patients. Several different anatomical structures are routinely visualized with neuroimaging:

THE BRAIN. The gray matter of the brain consists of the cortex that lines the external surface of the brain and the gray nuclei deep inside of the brain, including the thalami and basal ganglia. Within the gray matter lie the cell bodies of the roughly 85 billion neurons that constitute the processing engine for the brain. White matter is comprised of the neuronal axons that interconnect different regions of the brain and serve as the interface between the brain and the rest of the body. Different diseases affect the gray and white matter in distinct patterns.

BLOOD VESSELS. The arterial supply to the brain arises from paired carotid and vertebral arteries in the neck. These four vessels continue into the head and divide into separate anterior, middle and posterior cerebral arteries that provide oxygen and nutrients to different regions of the brain. These vessels are interconnected at the base of the brain through a network of arteries called the circle of Willis. The principal veins within the head, the dural venous sinuses, collect blood that has passed through the brain. Blood vessels may cause symptoms by becoming enlarged or narrowed, occluded, or by supplying vascular malformations in or around the brain.

BRAIN COVERINGS. The brain is not rigidly adherent to the skull. It is surrounded by three layers of covering: the innermost pia mater, the middle arachnoid mater, and the outermost dura mater. Cerebrospinal fluid (CSF), a translucent liquid derived from blood and contained within the space between the pia and arachnoid mater, serves as a chemical and physical cushion for the brain. Blood from ruptured aneurysms, trauma or pus from infections such as meningitis may collect within the spaces between meningeal layers or within the CSF.

THE SKULL. The bones that surround the brain, including the calvarium, facial skeleton and skull base, are collectively referred to as the skull. These structures provide protection for the brain and a rigid frame to support the functions of the face. Because cortical bones contain very little water, they are evaluated reliably only with CT. The marrow within these bones, however, can be seen on MRI images. Bones can be the primary source of disease, or they can be secondarily involved by different infections and tumors, for example.

SURROUNDING TISSUES. The skull is surrounded not only by the scalp, but also fat, muscles, blood vessels, and various special glands. Importantly, the front of the head contains an array of muscles, salivary glands, and lymph nodes that may be primarily or secondarily affected by various disorders. The physical location of abnormalities in these regions often gives clues as to the source of disease.

Still there is lot more material in this long article and there are excellent images. There are references. For further reading visit:

Source Knol: BRAIN: CT, MRI

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