Computed tomography (CT scan or CAT scan) is a noninvasive diagnostic imaging procedure that uses a combination of X-rays and computer technology to produce horizontal, or axial, images (often called slices) of the body. A CT scan shows detailed images of any part of the body, including the bones, muscles, fat, and organs. CT scans are more detailed than standard X-rays.

In standard X-rays, a beam of energy is aimed at the body part being studied. A plate behind the body part captures the variations of the energy beam after it passes through skin, bone, muscle, and other tissue. While much information can be obtained from a standard X-ray, a lot of detail about internal organs and other structures is not available.

In computed tomography, the X-ray beam moves in a circle around the body. This allows many different views of the same organ or structure. The X-ray information is sent to a computer that interprets the X-ray data and displays it in a two-dimensional (2D) form on a monitor.

CT scans may be done with or without "contrast." Contrast refers to a substance taken by mouth or injected into an intravenous (IV) line that causes the particular organ or tissue under study to be seen more clearly. Contrast examinations may require you to fast for a certain period of time before the procedure. Your physician will notify you of this prior to the procedure.

CT scans of the spine can provide more detailed information about the vertebrae (bones of the spine) and other spinal structures and tissues than standard X-rays of the spine, thus providing more information related to injuries and/or diseases of the spine.

Other related procedures that may be used to diagnose problems of the spine include X-rays , magnetic resonance imaging (MRI) of the spine , myelogram and positron emission tomography (PET) scan of the spine.

Anatomy of the spinal column
Anatomy of the spine
The spinal column is made up of 33 vertebrae that are separated by spongy disks and classified into distinct areas.

The cervical area consists of seven vertebrae in the neck.

The thoracic area consists of 12 vertebrae in the chest area.

The lumbar area consists of five vertebrae in the lower back area.

The sacrum has five, small fused vertebrae.

The four coccygeal vertebrae fuse to form one bone, called the coccyx or tailbone.

The spinal cord, a major part of the central nervous system, is located in the vertebral canal and reaches from the base of the skull to the upper part of the lower back. The spinal cord is surrounded by the bones of the spine and a sac containing cerebrospinal fluid. The spinal cord carries sense and movement signals to and from the brain and controls many reflexes.

What are the reasons for a CT scan of the spine?
A CT scan of the spine may be performed to assess the spine for a herniated disk, tumors and other lesions, the extent of injuries, structural anomalies such as spina bifida (a type of congenital defect of the spine), blood vessel malformations, or other conditions, particularly when another type of examination, such as X-rays or physical examination, is not conclusive. CT of the spine may also be used to evaluate the effects of treatment of the spine, such as surgery or other therapy.

There may be other reasons for your physician to recommend a CT scan of the spine.

What are the risks of a CT scan?
You may want to ask your physician about the amount of radiation used during the CT procedure and the risks related to your particular situation. It is a good idea to keep a record of your past history of radiation exposure, such as previous CT scans and other types of X-rays, so that you can inform your physician. Risks associated with radiation exposure may be related to the cumulative number of X-ray examinations and/or treatments over a long period of time.

If you are pregnant or suspect that you may be pregnant, you should notify your physician. Radiation exposure during pregnancy may lead to birth defects. If it is necessary for you to have a CT of the spine, special precautions will be made to minimize the radiation exposure to the fetus.

Nursing mothers should wait 24 hours after contrast material is injected before resuming breastfeeding.

If contrast media is used, there is a risk for allergic reaction to the media. Patients who are allergic to or sensitive to medications should notify their physician. Studies show that 85 percent of the population will not experience an adverse reaction from iodinated contrast; however, you will need to let your physician know if you have ever had a reaction to any contrast media, and/or any kidney problems. A reported seafood allergy is not considered to be a contraindication for iodinated contrast.

Patients with kidney failure or other kidney problems should notify their physician. In some cases, the contrast media can cause kidney failure. The effects of kidney disease and contrast agents have attracted increased attention over the last decade, as patients with kidney disease are more prone to kidney damage after contrast exposure. Also, patients taking the diabetes medication metformin (Glucophage) should alert their doctor before having IV contrast, as it may cause a rare condition called metabolic acidosis. If you take metformin, you will be asked to stop taking it at the time of the procedure and then wait for 48 hours after your injection. A blood test to check kidney function may be required before you can start taking metformin again.

There may be other risks depending upon your specific medical condition. Be sure to discuss any concerns with your physician prior to the procedure.

How do I prepare for a CT scan?
If you are having a computed tomography angiography (CTA) with Johns Hopkins radiology, you will be given specific instructions when you make your appointment.

PRECAUTIONS : If you are pregnant or think you may be pregnant, please check with your doctor before scheduling the exam. Other options will be discussed with you and your doctor.

CLOTHING : You may be asked to change into a patient gown. If so, a gown will be provided for you. A locker will be provided to secure personal belongings. Please remove all piercings and leave all jewelry and valuables at home.

CONTRAST MEDIA : CT scans are most frequently done with and without a contrast media. The contrast media improves the radiologist's ability to view the images of the inside of the body.

Some patients should not have an iodine-based contrast media. If you have problems with your kidney function, please inform the access center representative when you schedule the appointment. You may be able to have the scan performed without contrast media or have an alternative imaging exam.

You will be asked to sign a consent form that will detail the risks and side-effects associated with contrast media injected through a small tube places in a vein called an intravenous (IV) line.

The most common type of CT scan with contrast is the double contrast study that will require you to drink a contrast media before your exam begins in addition to the IV contrast. The more contrast you are able to drink, the better the images are for the radiologist to visualize your digestive tract.

ALLERGY : Please inform the access center representative when you schedule your CT scan if you have had an allergic reaction to any contrast media. IV contrast will not be administered if you have had a severe or anaphylactic reaction to any contrast media in the past. If you had mild to moderate reactions in the past, you will likely need to take medication prior to the CT scan. These plans will be discussed with you in detail when you schedule your exam. Any known reactions to a contrast media should be discussed with your personal physician.

EAT/DRINK : If your doctor ordered a CT scan without contrast, you can eat, drink and take your prescribed medications prior to your exam. If your doctor ordered a CT scan with contrast, do not eat anything three hours prior to your CT scan. You are encouraged to drink clear liquids. You may also take your prescribed medications prior to your exam.

DIABETICS : Diabetics should eat a light breakfast or lunch three hours prior to the scan time. Depending on your oral medication for diabetes, you may be asked to discontinue use of the medication for 48 hours after the CT scan. If you have a CT scan with Johns Hopkins radiology, detailed instructions will be given following your examination.

MEDICATION : All patients can take their prescribed medications as usual.

Based on your medical condition, your doctor may request other specific preparation.

Patient in CT scnanner
What happens during a CT scan?
CT scans may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your physician's practices.

Generally, a CT scan follows this process:

You may be asked to change into a patient gown. If so, a gown will be provided for you. A locked will be provided to secure all personal belongings. Please remove all piercings and leave all jewelry and valuables at home.

If you are to have a procedure done with contrast, an intravenous (IV) line will be started in the hand or arm for injection of the contrast media. For oral contrast, you will be given a liquid contrast preparation to swallow. In some situations, the contrast may be given rectally.

You will lie on a scan table that slides into a large, circular opening of the scanning machine. Pillows and straps may be used to prevent movement during the procedure.

The technologist will be in another room where the scanner controls are located. However, you will be in constant sight of the technologist through a window. Speakers inside the scanner will enable the technologist to communicate with and hear you. You may have a call button so that you can let the technologist know if you have any problems during the procedure. The technologist will be watching you at all times and will be in constant communication.

As the scanner begins to rotate around you, X-rays will pass through the body for short amounts of time. You will hear clicking sounds, which are normal.

The X-rays absorbed by the body's tissues will be detected by the scanner and transmitted to the computer. The computer will transform the information into an image to be interpreted by the radiologist.

It will be important that you remain very still during the procedure. You may be asked to hold your breath at various times during the procedure.

If contrast media is used for your procedure, you may feel some effects when the media is injected into the IV line. These effects include a flushing sensation, a salty or metallic taste in the mouth, a brief headache, or nausea and/or vomiting. These effects usually last for a few moments.

You should notify the technologist if you feel any breathing difficulties, sweating, numbness or heart palpitations.

When the procedure has been completed, you will be removed from the scanner.

If an IV line was inserted for contrast administration, the line will be removed.

While the CT procedure itself causes no pain, having to lie still for the length of the procedure might cause some discomfort or pain, particularly in the case of a recent injury or invasive procedure such as surgery. The technologist will use all possible comfort measures and complete the procedure as quickly as possible to minimize any discomfort or pain.

What happens after a CT scan?
If contrast media was used during your procedure, you may be monitored for a period of time for any side effects or reactions to the contrast media, such as itching, swelling, rash, or difficulty breathing. Notify the radiologist or your physician if you experience any of these symptoms.

Sperm penetration tests check to see if a man's sperm can move through cervical mucus and the fallopian tubes to join with (fertilize) an egg. This test is usually done when a couple is having trouble getting pregnant (infertility).

There are different sperm penetration tests.

Sperm mucus penetration. This test checks to see if a man's sperm can move through the cervical mucus. This test is not done very often because there are other fertility tests that give more information about the fertility problem.
Sperm penetration assay. This test checks to see if a man's sperm can join with an egg. (This is also called the hamster zona-free ovum test or hamster test.) Sperm are mixed with hamster eggs in a laboratory. The number of sperm that penetrate the egg is measured. This test is done most often at special fertility centers that do in vitro fertilization .

Sperm penetration tests may be done:

When other tests have not found a cause for infertility.
To see how the sperm function when a semen analysis test has shown some problems. Problems could include the sperm moving slowly or looking abnormal.
To test sperm before doing in vitro fertilization.

Talk to your doctor about any concerns you have about the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form [PDF] (What is a PDF document?) .

Sperm mucus penetration test
For women, this test must be done during ovulation . Follow your doctor's instructions for checking the level of luteinizing hormone (LH) in your urine. When you check your LH level, do the urine test in the mid- to late morning. Do not drink any fluids that morning until you have done the test. If your test shows that you are ovulating, call for a doctor's visit for the next day.

For men, the semen sample is collected after the cervical mucus sample is taken. You should not release your sperm (ejaculate) for 2 days before the test. It is important to not go longer than 5 days before the test without ejaculating.

Sperm penetration assay (SPA)
The SPA only requires a semen sample to be taken.

For men, it is important that you do not release your sperm (ejaculate) for 2 days before the test. But do not go longer than 5 days before the test without ejaculating.

Sperm mucus penetration test
For the sperm mucus penetration test, samples of the woman's cervical mucus and the man's semen will be collected. In the lab, the semen is added to the mucus in a tube. After 90 minutes, the distance the sperm have moved is measured.

For the woman: a sample of cervical mucus is collected from the woman during a pelvic exam. To learn more, see the topic Pelvic Examination .

For the man: a semen sample is collected by masturbation. First, you should urinate and then wash and rinse your hands and penis before you collect the semen in a sterile cup. Do not use lubricants or condoms when you collect the sample. If you collect the semen sample at home, be sure to get it to the lab or clinic within 1 hour. Keep the sample at body temperature and out of direct sunlight. Do not collect the sample by having sex and then withdrawing when you ejaculate, because vaginal fluid may be mixed with the sperm.

Sperm penetration assay (SPA)
For the SPA, a semen sample is collected. The sperm are mixed with hamster eggs in a laboratory. The number of sperm that can penetrate an egg is measured.

For the man: a semen sample is collected by masturbation. First, you should urinate and then wash and rinse your hands and penis before you collect the semen in a sterile cup. Do not use lubricants or condoms when you collect the sample. If you collect the semen sample at home, be sure to get it to the lab or clinic within 1 hour. Keep the sample at body temperature and out of direct sunlight. Do not collect the sample by having sex and then withdrawing when you ejaculate, because vaginal fluid may be mixed with the sperm.

What is semen analysis?
Semen analysis, also known as a sperm count test, analyzes the health and viability of a man's sperm. Semen is the fluid containing sperm (plus other sugar and protein substances) that's released during ejaculation. A semen analysis measures three major factors of sperm health:

the number of sperm
the shape of the sperm
the movement of the sperm, also known as sperm motility
Doctors will often conduct two or three separate sperm analyses to get a good idea of sperm's health. According to the American Association for Clinical Chemistry (AACC), the tests should be conducted at least seven days apart and over the course of two to three months. Sperm counts can vary on a daily basis. Taking an average of the sperm samples can give the most conclusive result.

Why undergo semen analysis?
Test for male infertility
A semen analysis is often recommended when couples are having problems getting pregnant. The test will help a doctor determine if a man is infertile. The analysis will also help determine if low sperm count or sperm dysfunction is the reason behind infertility.

Test for vasectomy success
Men who have had a vasectomy undergo semen analysis to make sure no sperm are in their semen. In a vasectomy, the tubes that send sperm from the testicles to the penis are cut and sealed as a permanent form of birth control. After a vasectomy, doctors often recommend that men take a sperm analysis once a month for three months to ensure that sperm is no longer present in their semen.

How to prepare for semen analysis
Your doctor will let you know what you should do in preparation for the semen analysis. It's very important to follow these instructions for accurate results.

To get the best sample:

Avoid ejaculation for 24 to 72 hours before the test.
Avoid alcohol, caffeine, and drugs such as cocaine and marijuana two to five days before the test.
Stop taking any herbal medications, such as St. John's wort and echinacea, as instructed by your healthcare provider.
Avoid any hormone medications as instructed by your healthcare provider.
Discuss any medications you're taking with your doctor.

You'll need to provide your doctor with a semen sample for a semen analysis. There are four main ways to collect a semen sample:

masturbation
sex with a condom
sex with withdrawal before ejaculation
ejaculation stimulated by electricity
Masturbation is considered the preferred way to get a clean sample.

Getting a good sample
Two main factors are crucial to having a good testing sample. First, the semen must be kept at body temperature. If it gets too warm or too cold, the results will be inaccurate. Second, the semen must be delivered to the testing facility within 30 to 60 minutes of leaving the body.

Test interference
Some factors can negatively affect the test, including:

semen coming into contact with spermicide
taking the test when you're ill or stressed
lab technician error
contamination of the sample
There are no known risks associated with a sperm analysis.

If semen analysis results aren't within normal limits and handling of the specimen isn't a factor, your doctor may also consider whether you're taking the following substances, which can affect your sperm count:

alcohol
caffeine
herbs, such as St. John's wort
prescription drug use of medicines known to reduce sperm count, such as cimetidine
recreational drug use
tobacco
Testing your semen at home
Home semen tests are available. However, they only test for sperm count. They don't analyze sperm motility or shape. Find at-home sperm analysis tests here.

Results for at-home tests are usually available within 10 minutes. A normal sperm count (above 20 million sperm per milliliter of semen) from a home test doesn't necessarily mean that a man is fertile, since it doesn't consider all the possible causes of male infertility.

If you are concerned about your fertility, it's best to get a lab test done by a medical professional. This will give you a more comprehensive evaluation of your fertility.

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What are normal results?
After your semen sample is collected, your test results should be ready within 24 hours to one week, depending on the laboratory you go to. When a doctor reviews sperm analysis test results, there are many factors to consider. An analysis after vasectomy looks for the presence of sperm, but the analysis to look for fertility issues is much more in depth. Your doctor will take each of the following results into account:

Sperm shape
A normal result for sperm shape is that more than 50 percent of sperm are normally shaped. If a man has greater than 50 percent of sperm that are abnormally shaped, this reduces his fertility. A laboratory may identify abnormalities in the sperm's head, midsection, or tail. It's also possible the sperm could be immature and therefore not able to effectively fertilize an egg.

Movement
For a normal result, more than 50 percent of sperm must move normally an hour after ejaculation. Sperm movement, or motility, is important to fertility because sperm must travel to fertilize an egg. An automated system analyzes the sperm for movement and rates them on a scale of 0 to 4. A score of 0 means the sperm are not moving, and a score of 3 or 4 represents good movement.

pH
A pH level should be between 7.2 and 7.8 to achieve a normal result. A pH level higher than 8.0 could indicate the donor has an infection. A result less than 7.0 could indicate the specimen is contaminated or that the man's ejaculatory ducts are blocked.

Volume
The volume of semen for a normal result should be greater than 2 milliliters. A low semen volume could indicate a low amount of sperm to fertilize an egg. An excess fluid volume could also mean the amount of sperm present is diluted.

Liquefaction
It should take 15 to 30 minutes before semen liquefies. While semen is initially thick, its ability to liquefy, or turn to a watery consistency, helps sperm to move. If semen does not liquefy in 15 to 30 minutes, fertility could be affected.

Sperm count
The sperm count in a normal semen analysis should be between 20 million to over 200 million. This result is also known as sperm density. If this number is low, conceiving can be more difficult.

Appearance
The appearance should be whitish to gray and opalescent. Semen that has a red-brown tint could indicate the presence of blood, while a yellow tint could indicate jaundice or be a medication side effect.


What do abnormal results mean?
Abnormal sperm will have trouble reaching and penetrating eggs, making conception difficult. Abnormal results could indicate the following:

infertility
infection
hormonal imbalance
disease, such as diabetes
gene defects
exposure to radiation
If your results come back at abnormal levels, your doctor will probably suggest that you take additional tests. These tests include:

genetic tests
hormone testing
urinalysis after ejaculation
taking a tissue sample from your testicles
anti-sperm immune cells testing
Outlook after a semen analysis
A semen analysis that's the most conclusive requires careful collection and analysis of multiple specimens. The test can provide a variety of information that could help determine factors that affect your fertility. If your test results are abnormal, your doctor may recommend that you see a fertility specialist.

What do abnormal results mean?
Abnormal sperm will have trouble reaching and penetrating eggs, making conception difficult. Abnormal results could indicate the following:

infertility
infection
hormonal imbalance
disease, such as diabetes
gene defects
exposure to radiation
If your results come back at abnormal levels, your doctor will probably suggest that you take additional tests. These tests include:

genetic tests
hormone testing
urinalysis after ejaculation
taking a tissue sample from your testicles
anti-sperm immune cells testing

Outlook after a semen analysis
A semen analysis that's the most conclusive requires careful collection and analysis of multiple specimens. The test can provide a variety of information that could help determine factors that affect your fertility. If your test results are abnormal, your doctor may recommend that you see a fertility specialist.

Overview
A fetal ultrasound (sonogram) is an imaging technique that uses sound waves to produce images of a fetus in the uterus. Fetal ultrasound images can help your health care provider evaluate your baby's growth and development and monitor your pregnancy. In some cases, fetal ultrasound is used to evaluate possible problems or help confirm a diagnosis.

The first fetal ultrasound is usually done during the first trimester to confirm the pregnancy and estimate how long you've been pregnant. If your pregnancy remains uncomplicated, the next ultrasound is typically offered during the second trimester, when anatomic details are visible. If a problem is suspected, a follow-up ultrasound or additional imaging tests, such as an MRI, might be recommended.

There are two main types of fetal ultrasound exams:

Transvaginal ultrasound. With this type of fetal ultrasound, a wandlike device called a transducer is placed in your vagina to send out sound waves and gather the reflections. Transvaginal ultrasounds are used most often during early pregnancy. This type of ultrasound also might be done if a transabdominal ultrasound didn't provide enough information.
Transabdominal ultrasound. A transabdominal fetal ultrasound is done by moving a transducer over your abdomen.
Various other types of transabdominal ultrasounds are available, including:

Specialized sonographic evaluation. This type of exam might be needed in specific circumstances, such as when a fetal abnormality is known or suspected. In this situation, a more detailed evaluation can provide additional information about the abnormality.
3D ultrasound. This exam provides a two-dimensional display of three-dimensional data. This type of ultrasound is sometimes used to help health care providers detect facial abnormalities or neural tube defects.
Doppler ultrasound. A Doppler ultrasound measures slight changes in the ultrasound waves as they bounce off moving objects, such as blood cells. It can provide details about a baby's blood flow.
Fetal echocardiography. This exam provides a detailed picture of a baby's heart. It might be used to confirm or rule out a congenital heart defect.
Why it's done
First trimester ultrasound examination is done to evaluate the presence, size and location of the pregnancy, determine the number of fetuses, and estimate how long you've been pregnant (gestational age). Ultrasound can also be used for first trimester genetic screening, as well as screening for abnormalities of your uterus or cervix.

In the second or third trimester a standard ultrasound is done to evaluate several features of the pregnancy, including fetal anatomy. This exam is typically done between weeks 18 and 20 of pregnancy. However, the timing of this ultrasound might be altered for reasons such as obesity, which could limit visualization of the fetus.

During the second and third trimesters, limited ultrasound evaluation might be needed when a specific question requires investigation. Examples include the evaluation of fetal growth and the estimation of amniotic fluid volume. A specialized or detailed exam is done when an anomaly is suspected based on your history or other prenatal exam results.

Your health care provider might use fetal ultrasound to:

Confirm the pregnancy and its location. Some fetuses develop outside of the uterus, in the fallopian tube. A fetal ultrasound can help your health care provider detect a pregnancy outside of the uterus (ectopic pregnancy).
Determine your baby's gestational age. Knowing the baby's age can help your health care provider determine your due date and track various milestones throughout your pregnancy.
Confirm the number of babies. If your health care provider suspects a multiple pregnancy, an ultrasound might be done to confirm the number of babies.
Evaluate your baby's growth. Your health care provider can use ultrasound to determine whether your baby is growing at a normal rate. Ultrasound can be used to monitor your baby's movement, breathing and heart rate.
Study the placenta and amniotic fluid levels. The placenta provides your baby with vital nutrients and oxygen-rich blood. Too much or too little amniotic fluid the fluid that surrounds the baby in the uterus during pregnancy or complications with the placenta need special attention. An ultrasound can help evaluate the placenta and amniotic fluid around the baby.
Identify birth defects. An ultrasound can help your health care provider screen for some birth defects.
Investigate complications. If you're bleeding or having other complications, an ultrasound might help your health care provider determine the cause.
Perform other prenatal tests. Your health care provider might use ultrasound to guide needle placement during certain prenatal tests, such as amniocentesis or chorionic villus sampling.
Determine fetal position before delivery. Most babies are positioned headfirst by the end of the third trimester. That doesn't always happen, though. Ultrasound imaging can confirm the baby's presentation so that your health care provider can discuss options for delivery.
Fetal ultrasound should be done only for valid medical reasons. Fetal ultrasound isn't recommended only to determine a baby's sex. Similarly, fetal ultrasound isn't recommended solely for the purpose of producing keepsake videos or pictures.

If your health care provider doesn't suggest a fetal ultrasound but you'd like the reassurance an ultrasound can provide, share your wishes with your care provider so that you can work together to determine what's best for you and your baby.

Risks
Diagnostic ultrasound has been used during pregnancy for many years and is generally considered safe when used appropriately. The lowest amount of ultrasound energy that provides an accurate assessment should be used.

Fetal ultrasound also has limitations. Fetal ultrasound might not detect all birth defects — or might incorrectly suggest a birth defect is present when it's not.

How you prepare
You might be asked to drink a certain amount of fluid or avoid urinating before a fetal ultrasound, depending on the type of ultrasound. When scheduling your ultrasound, ask your health care provider for instructions.

Also be aware that fetal ultrasound can be done through the vagina (transvaginal) or over the abdomen (transabdominal), depending on why it's being done or the stage of your pregnancy. If you're having a transabdominal ultrasound, consider wearing loosefitting clothing so that you can easily expose your abdomen.

What you can expect
During the procedure
During a transabdominal fetal ultrasound, you'll recline on an exam table and expose your abdomen. Your health care provider or technician will apply a special gel to your abdomen. This will improve the conduction of sound waves and eliminate air between your skin and the transducer.

Your health care provider or technician will move or scan the transducer back and forth over your abdomen. The sound waves reflected off your bones and other tissues will be converted into images on a monitor.

Your health care provider or technician will measure your baby's anatomy. He or she might print or store certain images to document important structures. You'll likely be given copies of some of the images.

Depending on your baby's position and stage of development, you might be able to make out a face, hands and fingers, or arms and legs. Don't worry if you can't "see" your baby. Ultrasound images can be hard for an untrained observer to decipher. Ask your health care provider or technician to explain what's on the screen.

The procedure for other types of fetal ultrasound exams is similar. If you're having a transvaginal ultrasound, however, you'll be asked to change into a hospital gown or undress from the waist down. You'll recline on an exam table and place your feet in stirrups. The transducer will be covered in a plastic sheath, like a condom, and be lubricated with gel. Your health care provider or technician will place the transducer in your vagina.

What Is a Slit Lamp Examination?
When you visit your eye doctor, she's not just checking to see if you can read the third line on the eye chart clearly. She's also making sure your eyes are healthy.

To do this, many doctors use a slit lamp. It's a special microscope and light that lets your doctor see your eyes in 3-D, both inside and out. She'll use it along with an ophthalmoscope to look at the back of your eye.

A slit-lamp exam is usually done during a regular checkup with your eye doctor.

What Should I Expect?
The slit lamp is several pieces of equipment combined into one device. It includes a binocular microscope on a base that moves in an arc, an adjustable light source, and a frame to rest your head on and hold it steady during the exam.

Your doctor has a lot of flexibility with the light. She can narrow and widen it, increase its brightness, and filter it with colors. By doing this, your doctor can focus on particular parts of your eyes and face.

If you drive to your appointment, you may want to bring someone along to take you home. Some eye exams include dilating your pupils. Until they return to normal size, the outside world may seem overly and even uncomfortably bright for a few hours. Your vision may be a bit blurry too.

What Is the Doctor Looking At?
Before the exam starts, you'll be asked to remove your glasses or contact lenses. You'll put your chin and forehead against rests to keep your head steady. Your doctor may also put a few drops of dye in your eyes to highlight things she wants to look at. She'll then turn out the room lights and turn on the slit lamp.

During the exam, your doctor will look through the microscope, adjusting the light from the slit lamp to view certain parts of your eyes. Things she'll look at:

The skin around the eye. Your doctor can check the area for skin diseases and abrasions.

Your eyelids and eyelashes. Styes (oil gland infections), folliculitis (hair follicle infections), and tumors are some of the conditions your doctor will look for.

The surface of the eye. This includes the tissue under your eyelids and over the whites of your eyes. These areas can be swollen or infected. This can be caused by sexually transmitted diseases, allergies, or viruses

The sclera. This is the protective outer layer of the eyeball. Next to the sclera is the episclera, which helps keep it healthy. These areas can get diseases related to allergies, autoimmune disorders (where the body attacks itself), and gout (a type of arthritis).

The cornea. This is the layer of the eye that helps focus your vision. A slit-lamp exam may show your cornea isn't as clear as it used to be. A number of things can cause your vision to blur.

The iris. This is the colored disc that surrounds the pupil and changes to allow more or less light into your eye. It can be affected by a variety of diseases and conditions, including (in rare cases) tuberculosis, leukemia, and even forms of arthritis.

The lens. Cataracts (a clouding of the lens) are diagnosed by examining this part of the eye. It's located behind the pupil.

In addition, when the slit lamp is coupled with a special magnifying lens, your doctor will be able to view the retina and the optic nerve located in the back of your eye. Before doing this exam, he'll dilate your pupils with eye drops.

Looking at the retina and optic nerve can help your doctor figure out if you have glaucoma or if diabetes is affecting your eyes. The examination may also show anemia (low levels of red blood cells), tumors, blood clots, and hardening of the arteries caused by high blood pressure.

Though eye exams make some people squeamish, the procedures should be pain-free.

The Results
Your doctor should tell you what she learned during your eye exam immediately.

If your exam reveals you may have a disease affecting other parts of your body, your eye doctor may recommend you see your regular doctor. If you have a specific eye condition, she may give you a prescription or suggest further testing.