American Medical Association Family Medical Guide - American Medical Association [90]
Prostate cancer
This is a view of a male pelvis showing an enlarged, cancerous prostate gland (green). The bones of the pelvis are the red areas on either side of the prostate. The rectum is the circular blue area below the prostate. The bladder is the yellow oval (slightly indented by the enlarged prostate) directly above the prostate.
THE PROCEDURE
Before you have a CT scan, a technician may inject a contrast medium (dye), such as iodine, into a vein in your arm to help get a better image of the blood vessels and any tumors. If your abdomen and pelvis are being scanned, you will be asked to drink a weak solution of barium sulfate (another contrast medium) to expand your intestines and improve the image. The technician will also position you (or the scanner) to get the best image possible. You will be asked to lie very still on a table while you are inside the scanner. CT scanning is painless. However, the procedure may take an hour or more, depending on the number of angles and exposures required.
Liver cancer
The large brownish mass at left is a liver to which cancerous tumors (red spots) have spread from the colon. The spine (at the top) and the ribs (surrounding the internal organs) are yellow.
Pancreatic cancer
CT scans are useful for revealing small pancreatic tumors or swelling of the pancreas caused by inflammation. Here, a cancerous tumor of the pancreas (green) is evident. Also seen are the ribs and spine (white); the aorta (red), which is the major artery that carries blood away from the heart; and a vena cava (blue), one of the two major veins that drain blood from the body into the right side of the heart.
MRI
Magnetic resonance imaging (MRI) makes cross-sectional images by measuring changes in the body’s natural magnetic field as parts of the body are exposed to strong magnets and various radio frequencies. Using these techniques, a doctor can examine the structure and appearance of internal organs. Like computed tomography (CT) scanning (see previous page), MRI uses a computer to construct images from information recorded by the scanner. In MRI, however, the information is not provided by X-rays. Instead, the person being examined is placed inside a powerful magnet, which arranges the nuclei of some of the hydrogen atoms in the body in a precise pattern (like iron shavings around a magnet). A pulse of radio waves is then passed through the person’s body, moving the nuclei of the aligned hydrogen atoms briefly out of alignment. The nuclei then return to their original pattern, emitting radio signals as they do so. Different tissues such as tumors emit a more or less intense signal. These signals are detected by the machine and analyzed by the computer. The information is then used by the computer to construct an image. MRI can sometimes produce more detailed three-dimensional images than CT.
MRI technology includes magnetic resonance angiography (MRA) and magnetic resonance spectroscopy (MRS). MRA, like other types of angiography (see page 110), is used to evaluate blood flow but does not use dyes or radioactive tracers. MRS is different from conventional MRI in that MRS uses a continuous band of radio waves to excite hydrogen atoms in a variety of chemical compounds other than water. The compounds absorb and emit radio energy at certain frequencies (or spectra) that can be used to identify them. A color image is created by assigning a color to each distinct spectral emission. MRS is used to produce color images of brain function and to identify the chemical composition of diseased tissue.
Functional magnetic resonance imaging (fMRI)—also called brain mapping—uses the same MRI scanner hardware to provide noninvasive images of the brain’s activity, and to detect changes resulting