MRI creates cross-section pictures of your insides. But MRI uses strong magnets instead of radiation to make the images. An MRI scan can take cross-sectional slices (views) from many angles, as if someone were looking at a slice of your body from the front, from the side, or from above your head. MRI creates pictures of soft tissue parts of the body that are sometimes hard to see using other imaging tests.MRI is very good at finding and pinpointing some cancers. An MRI with contrast dye is the best way to see brain tumors. Using MRI, doctors can sometimes tell if a tumor is not cancer (benign) or cancerous (malignant).MRI can also be used to look for signs that cancer may have spread (metastasized) from where it started to another part of the body.MRI images can also help doctors plan treatment such as surgery or radiation therapy.Special MRI machines, now available in some hospitals, are designed just for looking inside the breast. This is called an MRI with dedicated breast coils. Breast MRI is recommended along with mammograms to look for breast cancer in women at high risk for breast cancer. At this time MRI is not used by itself to detect breast cancer early. Breast MRI can also be used in women who have already been diagnosed with breast cancer to better determine the actual size of the cancer and to look for any other cancers in the breast.
How does it work?
An MRI scanner is a cylinder or tube that holds a very strong magnet weighing several tons. As you lie on a table within the tube, the device surrounds you with a powerful magnetic field. The magnetic force causes the nuclei (centers) of hydrogen atoms in your body to line up in one direction. Once the atoms are lined up, the MRI machine gives off a burst of radiofrequency waves. These waves cause the hydrogen nuclei to change direction. When they return to their original position, they give off certain signals that the scanner detects. Hydrogen nuclei in the body tissues change direction in different ways. A computer takes the signals from these changes and converts them into a black and white picture.
Contrast materials can be put in through a vein to improve the quality of the image. Once absorbed by the body, these agents speed up the rate at which tissue responds to the magnetic and radio waves. As a result, the signals produce stronger, clearer pictures.
MRI scans are usually done on an outpatient basis in a hospital or clinic. You will lie down on a narrow, flat table. The technologist may use straps or pillows to make you comfortable and help keep you from moving. The table then slides into a narrow cylinder. The part of your body that is being looked at will be in the center of the cylinder.
The test is painless, but you have to lie still inside the cylinder with its surface a few inches from your face. You may be asked to hold your breath or keep very still during certain parts of the test. The machine may make loud, thumping and whirring noises, much like the sound of a washing machine, as the magnet switches on and off. Some facilities let you wear earplugs or headphones to block this noise out during testing.
Special machines that are less restrictive may be easier for some people. These open MRI machines replace the narrow cylinder with a larger ring. This design lessens the banging sound and the feeling of lying in an enclosed space. But the device does not create as strong a magnetic field. Although open MRI technology is improving, the pictures may not be as clear or detailed as they are with standard MRI. Sometimes, this may require retesting on a standard MRI machine.
You may be given contrast material in a vein or have to swallow it. The contrast material used for an MRI exam is called gadolinium. Let the technologist know if you have any kind of allergies.
Breast MRI uses a special machine that only does this test, and contrast material is often used. A woman will lie face down on a platform specially designed for the procedure. The platform has openings for each breast that allow them to be scanned without being compressed. The platform contains the sensors needed to capture the MRI image. It is important to remain very still throughout the exam.