Information for Patient requiring special examinations


>>Magnetic Resonance Imaging (MRI)

How does an MRI scanner work

A MRI (Magnetic Resonance Imaging) scanner consists a large magnet housing, a table, coil(s), & associated computers. The magnet housing consists of the magnet proper & the gradient core. The table traverses through the middle (core) of the magnet to place the part of the body to be examined, at the center of the magnet, where the magnet field is most homogeneous.

The Physical Basics of MRI

There are billions of atoms in a human being. The nucleus of an atom spins, or precesses, on an axis. One can think of the nucleus of an atom as a top spinning off its axis.

In common MRI study, we are only interested in hydrogen atoms in the human body. Each of these hydrogen atom precesses along a certain random diection. However, placing these hydrogen atoms in a strong magnetic field changes the directions of these hydrogen atoms. Each hydrogen atom tends to line up, either along or against, the direction of the external magnetic field. If we add up all the hydrogen atoms that precess along the direction of the external magnetic view, the total will almost equates the total of the hydrogen atoms that precess against the direction of the external magnetic field. However, there is still a few excess hydrogen atoms that precess in one direction (the number depends on the environment of these hydrogen atoms & the strength of the magnetic field)

Inside the magnet housing, in addition to the main magnet, there are three sets of gradient magnets. They are turned on and off very rapidly so as to alter the main magnetic field within a small volume of the body. This is basis for imaging the body in different planes.
Apart from the main magnet, the MR scanner also come with coils. These coils work to transfer energy to the precessing hydrogen atoms. They do it by applying radiofrequency (RF) pulses that are specific only to hydrogen atoms. The RF pulses change the net precession direction of the hydrogen atoms. These RF pulses also change the frequency of the net spin. That specific frequency is called the Larmour frequency. When the RF pulses are turned off, the hydrogen atoms return to their natural (original) alignment within the magnetic field and release their excess applied energy. These energies are pick up by the same coils. These are sent to the computer system, where the signals are configured into images.


The Basis of MRI Medical Imaging
MRI is very effective in providing information on the anatomy (structure) & pathology (abnormalities) of the body. Hydrogen atoms are present in most type of molecules within the human bodies. Hydrogen atoms in different types of molecules precess differently. These different types of hydrogen atoms will be presented as difference in signal in a MRI image. Together with the positions & the shapes of the organs, these different signal appearances are the basic for anatomic information of MRI. Similary, by using different applied magnetic strength with the coil, the MRI system can cause tissues in the body to take on different appearances. This is the basic for recognition of pathology.

Similarly, by manipulating the volume, characteristics, & strength of the applied local magnetic field, MRI systems can also image flowing blood. This is the basic for MR angiograms.


Precautions while inside the MR suite
Strict precautions should be observed at all times within the MRI suite.
Within a strong magnetic field, metal objects can become dangerous projectiles. The magnetic force exerted on an object increases exponentially as it nears the magnet. Prior to allowing a patient or support staff member into the scan room, he or she is thoroughly screened for metal objects

Metallic fragments in the eye can move within a strong metallic field to cause eye damage or blindness.
  • Strong metallic fields can disable or adversely affect the workings of pacemakers.
  • Aneurysm clips in the brain may be displaced. This may tear the artery
    Some dental implants are magnetic
  • Most orthopedic implants, even though they may be ferromagnetic, are fine because they are firmly embedded in bone
  • surgical staples in most parts of the body are fine -- once they have been in a patient for a few weeks (usually six weeks), enough scar tissue has formed to hold them in place.
  • cochlear implants and ocular prostheses
  • denture

 

Credit cards, bank cards, Octopus cards, and anything else with magnetic encoding will be erased by MRI system. .. Even metal
There are no known biological hazards to humans from being exposed to magnetic fields of the strength used in medical imaging today.
Most facilities prefer not to image pregnant women. This is due to the fact that there has not been much research done in the area of biological effects on a developing fetus. The first trimester in a pregnancy is the most critical because that is the time of the most rapid cellular reproduction and division. The decision of whether or not to scan a pregnant patient is made on a case-by-case basis with consultation between the MRI radiologist and the patient's obstetrician. The benefit of performing the scan must outweigh the risk, however small, to the fetus and mother. Pregnant MRI technologists can still work in the department. In most cases, they are simply kept out of the actual scan room during their pregnancy.

Most imaging modalities use injectable contrast. MRI contrast works by altering the local magnetic field in the tissue being examined

• Diagnosing multiple sclerosis (MS)
• Diagnosing tumors of the pituitary gland and brain
• Diagnosing infections in the brain, spine or joints
• Visualizing torn ligaments in the wrist, knee and ankle
• Visualizing shoulder injuries
• Diagnosing tendonitis
• Evaluating masses in the soft tissues of the body
• Evaluating bone tumors, cysts and bulging or herniated discs in the spine
• Diagnosing strokes in their earliest stages


The fact that MRI systems do not use ionizing radiation is a comfort to many patients, as is the fact that MRI contrast materials have a very low incidence of side effects. Another major advantage of MRI is its ability to image in any plane

  • There are many people who cannot safely be scanned with MRI (for example, because they have pacemakers), and also people who are too big to be scanned.
  • There are many claustrophobic people in the world, and being in an MRI machine can be a very disconcerting experience for them.
  • The machine makes a tremendous amount of noise during a scan. The noise sounds like a continual, rapid hammering.
  • MRI scans require patients to hold very still for extended periods of time. MRI exams can range in length from 20 minutes to 90 minutes or more. Even very slight movement of the part being scanned can cause very distorted images that will have to be repeated.
  • Orthopedic hardware (screws, plates, artificial joints) in the area of a scan can cause severe artifacts (distortions) on the images.

 

 

Common Diseases of the BRAIN

The brain is affected by a variety of disease processes. The common diseases of the brain are


• Stroke
• Bleed
• Tumor

Due to the thick bone of the encasing skull, MR is the most effective method to study the brain. However, in a few circumstances, CT may be needed.


Stroke is the sudden loss of a brain function. It is caused by a lack of blood flow to part of the brain. The flow of blood through a blood vessel can be blocked in two ways. One is by obstructing the vessel. The other is by interrupting the vessel. Thus, there are two main types of stroke, ischemic stroke and hemorrhagic stroke. Ischemic strokes, which account for about 80 percent of all strokes, are caused by an obstruction in an artery. The obstruction can arise from the wall of the artery or a clot within the lumen of the artery. The obstructive wall abnormality usually begins as an atherosclerotic plaque. In atherosclerosis, fatty deposits build up on the inner wall of an artery. As the fatty deposit grows, it narrows the lumen through which blood can flow. A clot can arise anywhere between the heart & the brain. In fact, as atherosclerotic plaques enlarge, raw areas (ulcers) can form on their surfaces. The fragments from these ulcers may break loose & travel to a small vessel, where they obstruct the flow of blood in that vessel


A special type of ischemic stroke is transient ischemic attack (TIA). In a TIA, also known as a mini-stroke, stroke-like symptoms develop but disappear within five minutes to 24 hours. A TIA occurs when a clot develops at the site of an atherosclerotic deposit but dissolves right away, or an embolism lodges in a narrowed vessel but is soon dislodged on its own. Regardless of the cause, the oxygen deprivation is not severe enough to kill brain cells, and the cells are able to bounce back from their injury. About 10 percent of ischemic strokes are preceded by TIAs.


MR is the method of choice to detect ischemic stroke. With the use of modern medication, early ischemic stroke may be cured, without leaving permanent problems.
Hemorrhagic strokes account for the remaining 20 percent of all strokes. They occur when weakened blood vessels within the brain rupture and bleed into the surrounding tissue. Blood vessels can be weakened in several ways. An aneurysm is a focal bulge in the wall of the blood vessel. With time, this bulge expands, resulting in a balloon-like abnormality. As it enlarges, its wall is progressively thinned & weakened. It will eventually rupture. Another vessel abnormality is an arteriovenous malformation (AVM). It is a cluster of enlarged, structurally weak blood vessels that forms at or before birth.


In hemorrhagic stroke, the escaped blood compresses nearby blood vessels, cutting off blood flow and depriving the surrounding tissue of oxygen. Though hemorrhagic strokes occur less frequently than ischemic strokes, they tend to affect larger areas of the brain. Symptoms of a hemorrhagic stroke may be more sudden and more severe. The patient usually complains of severe headache. These strokes also carry a greater risk of death than ischemic strokes.
Early hemorrhagic stroke is best to detect with CT. However, with aneurysm, MR can often detect these without use of invasive procedures.


Bleeding can occur inside, as well as outside the brain. The bleed that occurs outside the bleed is usually due to subdural bleed. The bleeding is in the space between the brain & the brain’s covering. Blood can slowly accumulate, with progressive symptoms.


Subdural bleeds (hematomas) are most frequently the result of a head injury. They can occur spontaneously in the elderly, but this is less common. Subdural hematomas are seen in approximately 15% of all head traumas. Those who are very young or very old, those on anticoagulation therapy, & those with chronic alcohol use are more at rik. Tiny "bridging veins" that run between the surface of the brain and its outer covering (dura) stretch and tear, allowing blood to collect. Depending on the time of development of the subdural hematoma, it is classified as acute, subacute, and chronic. These patients usually complain of headache & may have change of mental state.


Tumor may occur in the brain or its covering. These tumors may arise from the brain or elsewhere. Again, MR is the best method to detect these tumors. However, contrast may be needed.

 

Common Diseases of the Spine

There are many pathologic processes affecting the spinal column. The common ones are
• Slipped disc
• Abnormal curvature

Slipped disc is a common cause for low back pain. Each disc consists of a central gelatinous material surrounded by layers of fibrous tissue. As one ages, these fibrous layers develop cracks which extend from the central gelatinous material to the periphery. The gelatinous material extends along these cracks & drags the surrounding fibrous tissue into the spinal canal or the intervertebral canal. The spinal cord &/or the spinal nerves may be pinched. If a spinal nerve is pinched, one may experience pain along the area supplied by this nerve. Each spinal nerve supplies a certain portion of the body. In sciatica, the L5 spinal nerve is pinched.

Radiographs cannot show discs. MR is the method of choice to see & characterize abnormality of the disc.


Abnormal curvatures of the spinal column are common. These conditions may be caused by weak ligaments, by poor posture habits, by disease or congenital abnormalities of the spinal column, by injury, or by spasm of the back muscles.


Hunchback is an abnormal hump of the thoracic spine. This hump is the result of several abnormal curvatures. These curvatures may be either a kyphosis, which is an accentuation of the normal posterior curvature, or a combination of kyphosis or extreme lateral curvature (scoliosis), which is known as kyphoscoliosis. Hunchback may be due to one or many collapsed vertebrae. Collapse may occur in elderly people, particularly women, whose bones may become soft and brittle (osteoporosis), causing a dorsal kyphosis. As a result of the spinal deformity in hunchback, the ribs become contorted, compressing or displacing the lungs and other structures within the chest cavity and thrusting the collarbone and shoulder blades into distorted positions. To compensate for such abnormal curvatures, the body develops deformities in the hips and other parts of the body in its effort to maintain balance.
Fortunately, one can detect osteoporosis before it results in hunchback. There are many types of equipment that are advertised to be able to detect osteoporosis. Many of these claims are, at best, mis-representations. It is well established among the medial specialists dealing with osteoporosis that DEXA bone densitometers are the most reliable type of equipment to assess bone density. Also, WHO has established that whole body DEXA bone densitometer is the equipment of choice.

 

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