When your doctor says, "Let's do some tests," don't panic. Learn the difference between CTs and PETs. Here is a guide to diagnostic tests and some common conditions they detect - from sinus infections to breast cancer. These tests can be used to diagnose many other conditions not mentioned here.

Breast cancer If you're over 40, your physician has probably referred you for a mammogram, a low-dose X-ray that can detect breast tumors too small to feel. Your breast is rested on a flat surface and compressed to spread the breast tissue for a clear picture. A radiologist may use digital mammography or computer-aided detection (CAD), new technologies that provide enhanced breast images.

An ultrasound will help distinguish between a harmless cyst and a solid mass. High frequency sound waves sent to the breast echo back to a computer that reads the waves as an image. If you are at high risk of breast cancer, you may undergo magnetic resonance imaging (MRI). The MRI uses magnetic fields, not radiation, to create a 3-D image, providing a sharp contrast between normal and abnormal tissues.

Doctors often recommend positron emission tomography (PET) to see if cancers have recurred or spread. For this test, you receive a very small amount of radioactive material. Cancer cells are more active than normal cells and are more likely to absorb the radioactive material, indicating where cancer may be growing.

Chest pain A range of tests can help detect heart disease. An exercise stress test shows how well your heart and blood vessels respond to exercise. While walking on a treadmill, you will be connected to an electrocardiogram (EKG), which monitors your heart rate during exercise and recovery. For a more thorough evaluation, your stress test may be combined with nuclear scanning, where a small amount of radioactive thallium is injected before and during the stress exercise. The thallium is absorbed by the heart and identifies areas that have an inadequate blood supply or that are scarred by a heart attack.

Another test is the echocardiogram, which is a heart ultrasound. Also called a stress echo, it allows your physician to see your heart's pumping chambers, valves and the large veins and arteries.

Osteoporosis Thinning bones after menopause can cause osteoporosis, which leads to bone fractures. Your doctor may suggest a dual energy X-ray absorptiometry (DEXA) scan. The DEXA bone scan is a painless, enhanced X-ray that measures bone density in the hip and spine. You lie on a table while the arm of the machine passes over your body.

Sinus Infection Sinus infections, or sinusitis, are common, but if the infection isn't responding to treatment, your doctor may suggest a computed tomography (CT) scan. The CT scan can pick up mucosal thickening, polyps or other abnormalities.

You lie on a platform inside a donut-shaped machine while the scanner rotates around you, taking hundreds of X-rays that are combined into a computerized image of a slice of your body. The CT scan takes only a few minutes to examine many parts of the body, from the smallest bones to muscle tissue and blood vessels. You may be injected with a "contrast dye," or iodine, to make your organs more visible. The test is painless. It is invaluable in diagnosing and treating spinal problems and bone fractures and is used to help diagnose cancers and guide radiation treatment. CT scans are also used to diagnose and treat vascular diseases such as stroke, and in emergency trauma to identify internal injuries.