Magnetic resonance imaging (MRI)
Magnetic resonance imaging (MRI) is a test that uses a magnetic field and pulses of radio wave energy to make pictures of organs and structures inside the body. In many cases MRI gives different information about structures in the body than can be seen with an X-ray, ultrasound, or computed tomography (CT) scan. MRI also may show problems that cannot be seen with other imaging methods.
For an MRI test, the area of the body being studied is placed inside a special machine that contains a strong magnet. Pictures from an MRI scan are digital images that can be saved and stored on a computer for more study. The images also can be reviewed remotely, such as in a clinic or an operating room.
How MRI works
MRI machines make use of the fact that body tissue contains lots of water, and hence protons (H+ ions), which get aligned in a large magnetic field.. Each water molecule has two hydrogennuclei or protons. .When a person is inside the powerful magnetic field of the scanner, The protons aligned with the direction of the field. A radio frequency current is briefly turned on, producing a varying electromagnetic field. This electromagnetic field has just the right frequency, known as the resonancefrequency, to be absorbed and flip the spin of the protons in the magnetic field. After the electromagnetic field is turned off the bulk magnetization becomes re-aligned with the static magnetic field. During this relaxation, a radio frequency signal is generated, which can be measured with receiver coils.
Information in 3D space can be obtained by applying additional magnetic fields during the scan. This is the idea of K-space ,( 3d image compiled from multiple 2d images).This 3d image can also produce images in any plane of view. The image can be rotated and manipulated by the doctor to be better able to detect tiny changes of structures within the body.
Protons in different tissues return to their equilibrium state at different relaxation rates. Different tissue variables, including spin density, relaxation times, and flow and spectral shifts can be used to construct images. By changing the settings on the scanner, this effect is used to create contrast between different types of body tissue or between other properties,
MRI scans require a magnetic field with two properties, uniform field density and strength. The magnetic field cannot vary more than 1% and field strength ranges (depending on the scanner) from 0.2 to 3 Teslas in strength in currently clinically used scanners,
Why It Is Done
Magnetic resonance imaging (MRI) is done for many reasons. It is used to find problems such as tumors, bleeding, injury, blood vessel diseases, or infection. MRI also may be done to provide more information about a problem seen on an X-ray, ultrasound scan, or CT scan. Contrast material may be used during MRI to show abnormal tissue more clearly. An MRI scan can be done for the:
Head. MRI can look at the brain for tumors, an aneurysm, bleeding in the brain, nerve injury, and other problems, such as damage caused by a stroke. MRI can also find problems of the eyes and optic nerves, and the ears and auditory nerves.
Chest. MRI of the chest can look at the heart, the valves, and coronary blood vessels. It can show if the heart or lungs are damaged. MRI of the chest may also be used to look for breast or lung cancer.
Blood vessels. Using MRI to look at blood vessels and the flow of blood through them is called magnetic resonance angiography (MRA). It can find problems of the arteries and veins, blocked blood vessel, or the torn lining of a blood vessel . Sometimes contrast material is used to see the blood vessels more clearly.
Abdomen and pelvis. MRI can find problems in the organs and structures in the belly, such as the liver, gallbladder, pancreas, kidneys, and bladder. It is used to find tumors, bleeding, infection, and blockage. In women, it can look at the uterus and ovaries. In men, it looks at the prostate.
Bones and joints. MRI can check for problems of the bones and joints, such as arthritis, problems with the temporomandibularHYPERLINK " joint, bone marrow problems, bone tumors, cartilage problems, torn ligaments or tendons, or infection. MRI may also be used to tell if a bone is broken when X-ray results are not clear. MRI is done more commonly than other tests to check for some bone and joint problems.
Spine. MRI can check the discs and nerves of the spine , disc bulges, and spinal tumors.
How does a patient obtain the results of the MRI scan
After the MRI scanning is completed, the computer generates visual images of the area of the body that was scanned. These images can be transferred to film (hard copy). A radiologist is a physician who is specially trained to interpret images of the body. The interpretation is transmitted in the form of a report to the practitioner who requested the MRI scan. The practitioner can then discuss the results with the patient and/or family
(In research and industry, MRI is known as NMR – nuclear magnetic resonance. It's more or less the same process, but the medical establishment prefers the term MRI because some patients are scared off by the word nuclear.)
Specialized MRI scans
Real-time MRI refers to the continuous monitoring ("filming") of moving objects in real time.
Diffusion MRI is done by analysing the diffusion of water molecules in biological tissues Clinically, diffusion MRI is useful for the diagnoses of conditions (e.g., stroke) or neurological disorders (e.g., Multiple Sclerosis), and helps better understand the connectivity of white matter axons in the central nervous system .
MRI versus CT
The use of X-rays, a type of ionizing radiation, by computed tomography (CT) scanner, to acquire images, make computed tomography a good tool for examining tissue composed of elements of a higher atomic number than the tissue surrounding them, such as bone and calcifications (calcium based) within the body (carbon based flesh), or of structures (vessels, bowel). MRI, on the other hand, uses non-ionizing radio frequency (RF) signals to acquire its images and is best suited for soft tissue (although MRI can also be used to acquire images of bones, teeth and even fossil