The body takes nutrients from food and converts them to energy. After the body has taken the food that it needs, waste products are left behind in the bowel and in the blood.

The kidneys and urinary system keep chemicals, such as potassium and sodium, and water in balance, and remove a type of waste, called urea, from the blood. Urea is produced when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea is carried in the bloodstream to the kidneys.

Two kidneys, a pair of purplish-brown organs, are located below the ribs toward the middle of the back. Their function is to:

• Remove liquid waste from the blood in the form of urine

• Keep a stable balance of salts and other substances in the blood

• Produce erythropoietin, a hormone that aids the formation of red blood cells.

• Regulate blood pressure

The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule.

Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney.

What are the reasons for a CT scan of the kidney?

A CT scan of the kidney may be performed to assess the kidneys for tumors and other lesions, obstructions such as kidney stones , abscesses, polycystic kidney disease , and congenital anomalies, particularly when another type of examination, such as X-rays or physical examination, is not conclusive. CT scans of the kidney may be used to evaluate the retroperitoneum (the back portion of the abdomen behind the peritoneal membrane). CT scans of the kidney may be used to assist in needle placement in kidney biopsies .

After the removal of a kidney, CT scans may be used to locate abnormal masses in the empty space where the kidney once was. CT scans of the kidneys may be performed after kidney transplants to evaluate the size and location of the new kidney in relation to the bladder.

There may be other reasons for your doctor to recommend a CT scan of the kidney.

What are the risks of a CT scan?

You may want to ask your doctor 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 doctor. 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 doctor. Radiation exposure during pregnancy may lead to birth defects.

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 doctor. You will need to let your doctor 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 doctor. In some cases, the contrast media can cause kidney failure, especially in patients with underlying kidney problems or dehydration. Patients taking the diabetes medication metformin (Glucophage), or its derivatives, who receive contrast are at increased risk of developing a condition called metabolic acidosis, or an unsafe change in blood pH, and the drug may be halted for 48 hours after the procedure.

There is a small chance of contrast material leakage from the IV line (known as contrast extravasation), which may cause swelling, stinging pain, or skin damage at the IV site.

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

Certain factors or conditions may interfere with the accuracy of a CT scan of the kidney. These factors include, but are not limited to, the following:

• Metallic objects within the abdomen, such as surgical clips

• Barium in the intestines from a recent barium study

• Recent tests involving media or other foreign substances

How do I prepare for a CT scan?

If you are having a computed tomography angiography (CTA) or virtual colonoscopy 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.

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:

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

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

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

11. 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.

If you notice any pain, redness, and/or swelling at the IV site after you return home following your procedure, you should notify your doctor as this could indicate an infection or other type of reaction.

Otherwise, there is no special type of care required after a CT scan of the kidney. You may resume your usual diet and activities unless your doctor advises you differently.

Your doctor may give you additional or alternate instructions after the procedure, depending on your particular situation.