Still more advanced technologies can be used to study your heart’s structure and function. These procedures include computed tomogra- phy (CT), magnetic resonance imaging (MRI), magnetic resonance angiography (MRA), and positron emission tomography (PET). Such techniques are used to get more detailed information or to avoid more invasive procedures. These scanners are not available at all hospitals or diagnostic centers and are used only when needed to answer speci?c questions your physician may have.

Computed Tomography

CT scanning is an advanced X-ray technique that can take cross-sectional images of your heart. To have a CT scan done, you lie on a movable table that slides into the tubular CT scanner. Many images are taken from all sides of your body. A computer combines these images to construct a detailed cross section of a structure. Your doctor can assess images of your heart, lungs, or major blood vessels. CT scans are often used to see if calci?cation, a natural reaction to injury, has occurred in your blood vessels as a result of atherosclerosis (see page 152), or in your heart muscle as a result of a heart attack. As with other X-ray techniques, CT scanning passes some radiation through your body, but it is a minimal, safe amount that does not remain in your body after the test.

In some cases, a contrast agent (iodine-based dye) is injected into your bloodstream to get a clearer image. If you are not being injected with the dye, you will be told not to eat for about 2 hours before the test. If you are being injected with the dye, you should not eat for about 4 hours beforehand. In some people, this contrast agent causes hot ?ushing and other allergic symptoms, but this reaction is rare .
You will be asked to put on a hospital gown and lie down on the table. If a contrast medium is being used, an intravenous line will be placed into your arm. The table will be moved slowly into the scanner. The technician will start taking pictures, and you will be asked to lie still and hold your breath brie?y as each image is taken. After the test, you may resume your usual activities.

Electron Beam Computed Tomography

Electron beam computed tomography (EBCT or fast CT) is a faster form of CT scanning that takes images in about one-tenth of a second (compared to 1 to 10 seconds for a conventional
CT scan). Because the heart is always in motion, a conventional CT sometimes creates a blurred image. EBCT is fast enough to avoid this prob- lem. EBCT enables your doctor to detect calci- ?cation in your arteries. EBCT is sometimes used for “whole body screening” for healthy people, but there is no evidence it is effective for that purpose.

Spiral Computed Tomography

Spiral computed tomography (or spiral or hel- ical CT) is another form of fast CT scanning. For a conventional CT, you rest on a table while the scanner is moved slightly for each picture; with spiral CT, you lie on a table that moves slowly through the scanner while it takes images nonstop. These scanners are particu- larly helpful in ?nding aneurysms (ballooning in the wall of a weakened artery) and blood clots in the lungs (pulmonary emboli).

Magnetic Resonance Imaging

MRI is another technology that uses magnetic ?elds and radio signals to form an image. Brie?y, the MRI scanner surrounds your body with a magnetic ?eld that reacts with magnetic elements in your body (such as hydrogen). The reaction causes radio signals from which a computer can construct an image. MRI scans produce images that are similar to those from a CT scan, but no radiation is used, and the MRI shows slightly different tissues. The test is painless, does not involve any injections, and does not pose any known risks. People who have pacemakers or other internal metallic devices cannot have an MRI, but people with arti?cial heart valves that are not magnetically active can have one safely. This test can be performed safely in the second half of pregnancy.
You do not need to prepare in any way for an MRI. You will change into a hospital gown and lie on a table that will be placed in the scan- ner, which is a long, narrow tube. Some people with claustrophobia may find the scanner uncomfortable. However, many scanners are now made with open ends that eliminate this problem. If you are con- cerned about being inside the scanner, talk to your doctor before the test is done; a sedative may be administered to help you relax through the test.
When you are inside the scanner, you may be asked to hold your breath brie?y while images are taken. You may hear loud noises inside the scanner. Sometimes you can listen to music through headphones while you are inside the scanner, but the technician’s instructions will also be transmitted via the headset. After the test, you can go about your usual activities.

Magnetic Resonance Angiography

MRA uses an MRI scanner to analyze the blood vessels leading to the brain, kidneys, and legs. This type of scan is done using different set- tings on the scanner, so the procedure is the same as for an MRI from your point of view. Usually, an MRA is done using gadolinium, a mag- netic contrast agent to which virtually no one is allergic. This contrast agent is given as an injection, usually in your arm, before the scanning is done.

Positron Emission Tomography

PET scanning uses information about the energy released by subatomic particles in your body to form an image. A radioactive substance is injected into your body that will travel to damaged or malfunctioning tissues. These tissues have increased or decreased metabolic activity. The PET scanner detects and measures the radioactive substance in these areas of your body, and a computer constructs images. A PET scan is highly accurate because it shows your heart tissue at work. The uses for this technology are still developing, but it has the poten- tial to show how your heart uses energy at a cellular level. Currently, PET scans are used mainly in research rather than in patient care or diagnosis of heart disease.
You do not need to prepare for a PET scan in any way. You will be asked to remove your clothes from the waist up, and a technician will place a ring of detectors around your chest. You will lie down on a table that will be moved into the PET scanner. The scanner is shaped like a large funnel, and your body will be in the tube. The technician or doctor will take a picture of your heart before the radioactive sub- stance is injected. You need to keep your arms above your head during this part of the test, which takes about 15 to 30 minutes. Then the radioactive material will be injected, usually in your arm. You will have to wait about 45 minutes for the substance to move into your heart. Again, you will be asked to keep your arms over your head while the images are being taken. After the test, you may resume your usual activities.

Multidetector CT Scans

A type of CT scanner with more detectors than a conventional CT machine can be used to provide the same kind of infor- mation about the coronary arteries as an angiogram reveals (see page 146). Because having a CT scan is easier and less expensive than an angiogram, the multidetector CT scan might be used more frequently in the future. A recent application is CT angiography, in which dye is injected and images are made of the coronary arteries that may detect both calcified and noncalcified deposits. CT angiography is being used as a screen- ing tool in high-risk people and as a diag- nostic tool in some hospital emergency departments with specialized chest pain centers. Medical experts are working on standards to guide the use of the new multidetector scanners.