What is MRI ?

MRI stands for Magnetic Resonance Imaging. It's a non-invasive, revolutionary process that enables doctors to "see" right through bone to the soft tissue inside your body, without surgery and without the radiation that is used in x-rays and CT scans. In simple terms, this is how it works: the MRI scanner creates a strong pull that aligns your body's protons in the same direction. Next, a radio signal is beamed into the magnetic field, causing the protons to move out of alignment. When the beam stops, the protons line up once again, releasing energy as they move. Different types of tissue release slightly different signals, which are measured by a receiver coil. A computer translates the measurements into a highly accurate image of your internal anatomy.

Conditions that may only be apparent from physical signs and symptoms can be clearly seen using MRI. Diagnostic-quality MR images provide highly accurate information to your physician, who is then able to prescribe the appropriate treatment.

The level of detail an MRI scan provides means there can be much greater accuracy in the early detection and treatment of disease. Early treatment is often less expensive and much more successful - which means MRI can save both time and money and, in some cases, early detection can also save a life.

In addition, MRI exams use no radiation and pose no known health concerns. Best of all, the procedure is virtually painless and may eliminate the need for diagnostic surgery.

In conjunction with radio wave pulses of energy, the MRI scanner can pick out a very small point inside the patient's body and ask it, essentially, "What type of tissue are you?" The point might be a cube that is half a millimeter on each side. The MRI system goes through the patient's body point by point, building up a 2-D or 3-D map of tissue types. It then integrates all of this information together to create 2-D images or 3-D models.

MRI provides an unparalleled view inside the human body. The level of detail we can see is extraordinary compared with any other imaging modality. MRI is the method of choice for the diagnosis of many types of injuries and conditions because of the incredible ability to tailor the exam to the particular medical question being asked. By changing exam parameters, the MRI system can cause tissues in the body to take on different appearances. This is very helpful to the radiologist (who reads the MRI) in determining if something seen is normal or not. We know that when we do "A," normal tissue will look like "B" -- if it doesn't, there might be an abnormality. MRI systems can also image flowing blood in virtually any part of the body. This allows us to perform studies that show the arterial system in the body, but not the tissue around it. In many cases, the MRI system can do this without a contrast injection, which is required in vascular radiology.

To understand how MRI works, let's start by focusing on the "magnetic" in MRI. The biggest and most important component in an MRI system is the magnet. The magnet in an MRI system is rated using a unit of measure known as a Tesla. Another unit of measure commonly used with magnets is the gauss (1 tesla = 10,000 gauss). The magnets in use today in MRI are in the 0.5-tesla to 2.0-tesla range, or 5,000 to 20,000 gauss.

MRI is useful in:

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.
These are but a few of the many of reasons to perform an MRI scan.

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. CT is limited to one plane, the axial plane (in the loaf-of-bread analogy, the axial plane would be how a loaf of bread is normally sliced). An MRI system can create axial images as well as images in the sagitall plane (slicing the bread side-to-side lengthwise) and coronally (think of the layers of a layer cake) or any degree in between, without the patient ever moving. If you have ever had an X-ray, you know that every time they take a different picture, you have to move. The three gradient magnets discussed earlier allow the MRI system to choose exactly where in the body to acquire an image and how the slices are oriented.

MRI vs CT

A computed tomography(CT) scanner uses X-rays, a type of ionizing radiation, to acquire its images, making it a good tool for examining tissue composed of elements of a relatively 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) which have been artificially enhanced with contrast agents containing elements of a higher atomic number than the surrounding flesh (iodine, barium). MRI, on the other hand, uses non-ionizing radio frequency signals to acquire its images and is best suited for non-calcified tissue.

Both CT and MRI scanners can generate multiple two-dimensional cross-sections (slices) of tissue and three-dimensional reconstructions. Unlike CT, which uses only X-ray attenuation to generate image contrast, MRI has a long list of properties that may be used to generate image contrast. By variation of scanning parameters, tissue contrast can be altered and enhanced in various ways to detect different features. (See Application below.)

MRI can generate cross-sectional images in any plane (including oblique planes). CT is limited to acquiring images in the axial (or near axial) plane. However, the development of multi-detector CT scanners with near-isotropic resolution produces data that can be retrospectively reconstructed in any plane with minimal loss of image quality.