Frequently Asked Questions

What is MRI?
 
The phenomenon of nuclear magnetic resonance (NMR) was discovered in 1945 by Felix Bloch of Stanford University and Edward Purcell of Harvard University. Working independently, Bloch and Purcell discovered that certain substances, when placed in a magnetic field, are able to absorb and emit radiofrequency electromagnetic radiation. The frequency of absorption and emission was related to the physicochemical structure of the substance being studied. This provided a mechanism for scientists to probe the structure of unknown compounds and NMR became a standard analysis tool in chemistry and physics laboratories. Bloch and Purcell were awarded the Nobel prize for Physics1952 for their discovery of NMR.

 In the 1980s, NMR imaging became known as magnetic resonance imaging (MRI) because of concerns about the negative connotation of the word nuclear. MRI has a tissue contrast sensitivity which is many times greater than other imaging techniques. The images produced by early MRI systems provided unprecedented anatomic detail and MRI quickly gained widespread acceptance for medical imaging.

What does the MRI equipment look like?

The traditional MRI unit is a large tube surrounded by a circular magnet, in which the patient lies without moving for several seconds at a time. The patient is placed on a wheeled bed that is moved into the magnet. In recent years, patient-friendly units have been designed, and examination in such units is becoming increasingly available. These machines are both shorter and wider than a conventional MRI unit and do not fully enclose the patient. Some of the newer C-shaped units are even open on all sides and are thus very attractive to patients who tend to be claustrophobic. A drawback is that image quality is not as consistently good.

Examples of the MRI equipment that may be used are shown at the top of this page.

How does the procedure work?

Exposing the patient to radio waves in a strong magnetic field generates data that are used by a computer to create images of tissue slices that may be viewed in any plane or from any direction. The magnetic field lines up atomic particles in the tissues called protons, which are then spun by a beam of radiofrequency waves and produce signals that are picked up by a receiver in the imager. It is these signals that are processed by the computer to produce images. The resulting images are very sharp and detailed and are thus able to demonstrate tiny changes from the normal pattern that are caused by disease or injury. Special settings are used to image various structures, such as arteries in the case of MRA.

How is the procedure performed?

The patient is placed on a special table and positioned inside the opening of the MRI unit. A typical exam consists of two to six imaging sequences, each taking two to 15 minutes. Each sequence provides a specific image orientation and a specified degree of image clarity or contrast. Depending on the type of exam being done, the total time needed can range from 10 to 60 minutes, not counting the time needed to change clothing, have an IV put in, if required per your referring physician for your type of study.  When contrast material is needed, a substance called gadolinium is given by IV injection during one of the imaging sequences. It highlights blood vessels, making them stand out from surrounding tissues.

The radiologist and technologist leave the examining room during the actual imaging process, but the patient can communicate with them at any time using an intercom. Some centers permit a friend to stay nearby, or a parent if a child is being examined. When the exam is completed you will be asked to wait to make sure that more images are not needed.