Rock Creek Imaging MRI

Computerized Tomography (CT Scan)

What is CT scan?

Computed Tomography (CT) imaging, also known as "CAT scanning" (Computed Axial Tomography), combines the use of a digital computer together with a rotating x-ray device to create detailed cross sectional images or "slices" of the different organs and body parts such as the lungs, liver, kidneys, pancreas, pelvis, extremities, brain, spine, and blood vessels. For many patients, CT can be performed on an outpatient basis without requiring admittance to the hospital.

Among the various imaging techniques such as MR and x-ray, CT has the unique ability to image a combination of soft tissue, bone, and blood vessels. For example, x-ray imaging of the head can only show the dense bone structures of the skull. X-ray angiography of the head only depicts the blood vessels of the head and neck and not the soft brain-tissue. Magnetic resonance (MR) imaging does an excellent job of showing soft tissue and blood vessels, but MR does not give as much detail of bony structures such as the skull. CT images of the head allow physicians to see soft-tissue anatomic structures like the brain's ventricles or gray and white matter. Physician then can selectively "window" the digital CT images on the computer monitor to look at the soft tissue, then the bone and then the blood vessels, as needed.

How long is a CT scan?

Most CT examinations are similar and include the following steps:

Patients are encouraged to bring something to read or do in case there are any delays prior to their CT exam. Patients should wear comfortable, loose fitting clothing for their CT exam. Patient preparation for a CT examination involves removing any articles of clothing or jewelry that might degrade the CT images, such as belts, earrings, bras, glasses, dentures, hairpins, etc. Zippers and snaps common in many clothes can also cause image degradation. In some cases, the patient may be asked to wear a patient gown (such as CT imaging of the body). Many CT examinations require the oral or intravenous administration of a contrast agent, a liquid material that enhances the images of the organs and/or blood vessels.
The patient is positioned by the technologist on the specialized CT table. The area of interest, for example the chest, is positioned inside the CT gantry opening, usually with the help of cross-hair type positioning lights. Some types of CT examination, for example a "coronal CT of the sinuses" may require the patient to be positioned in a special head holder that extends the head and neck. For such precise CT examinations, soft straps may be used to help position and immobilize the patient's head.
Once the patient is comfortably and correctly positioned, the technologist leaves the CT patient room. For many types of CT, for example CT of the chest or abdomen, the patient is asked to hold their breath and remain perfectly still for a few moments up to about 30 seconds while the technologist acquires the CT data set. During the CT examination, the technologist and patient can communicate at any time using an intercom. The CT technologist controls the CT examination from a workstation console in the control room next to the room with the CT scanner. The technologist watches the patient during the study and monitors the progress of the image acquisition during the study.
Patients should lie still and relax during a CT study. Patients may hear the CT scanner rotating during the study. Depending upon the type of study being performed and the model of CT scanner being used, the whirring noise may be very quite or more noticeable. The table will move very slowly during the CT data acquisition. Depending upon the type of study being performed, the table will either move in several small increment of a few millimeters (for example for a CT of the brain), or the table will move in one large continous step, for example 20 or 30 cm for a CT of the lungs. The gantry of the CT scanner may tilt in either direction during CT exams such as head, sinus, inner ear and spine imaging. This allows the CT images to be angled to better image anatomy such as spinal disks.
Some CT studies will require the technologist to come into the patient room in the middle of the exam to administer the contrast media via injection
Once the CT examination is complete, the technologist will ask the patient to get dressed and wait while the CT images are reviewed, either on film or monitor.

Information about Intravenous and Oral Contrast Used in CT

For many computed tomography examinations patients may be asked to take a special contrast agent (orally, rectally or via injection). Intravenous, oral and rectal CT contrast are pharmaceutical agents (liquids) and are sometimes referred to as "dye". CT contrast is used to make specific organs, blood vessels and/or tissue types "stand out" with more image contrast to better show the presence of disease or injury. Thus CT contrast highlights specific areas of the resultant CT image or "dyes" it.

Note: It is important that patients consult the staff member performing their CT exam for specific instructions to follow when contrast will be used. The information contained herein is only a general guideline.

There are four types of contrast agent used in CT:

The type that is given via intravenous (through a vein) injection
The type that is given orally
The type that is given rectally
A much less common type of contrast used in CT is inhaled as a gas and used for special lung and brain imaging. This technique (called Xenon CT) is only available at a small number of locations throughout the world and is only performed for rare cases.

CT Contrast Given Via Intravenous Injection

Intravenous contrast is used in CT to help highlight blood vessels and to enhance the tissue structure of various organs such as the brain, spine, liver and kidneys. "Intravenous" means that the contrast is injected into a vein using a small needle. Some imaging exams of the abdomen and gastrointestinal system use both the intravenous iodine and orally administered barium contrast for maximum sensitivity.

The intravenous CT contrast is clear like water and has a similar consistency. It is typically packaged in glass bottle or vial. A sterile syringe is used to draw it from the bottle or a power injector is used to administer the contrast. Typically between 75 cc to 150 cc (about 2.5 oz. to 5 oz) of contrast is used depending upon the patient's age, weight, area being imaged and cardiovascular health.

What Preparation is Needed Before Receiving Intravenous Contrast?

Sometimes it is necessary to not drink anything for an hour to several hours before the exam. The preparation time varies depending on the actual exam as well as the imaging center's requirements. Always ask the staff member for exact guidelines.

Oral CT Contrast

Oral contrast is often used to enhance CT images of the abdomen and pelvis. There are two different types of substances used for oral CT contrast. The first, barium sulfate, is the most common oral contrast agent used in CT. The second type of contrast agent is sometimes used as a substitute for barium and is called Gastrografin.

Barium contrast looks like and has a similar consistency as a milk shake. It is mixed with water and depending on the brand used, may have different flavors (for example, strawberry or lemon). Gastrografin contrast is a water-based drink mixed with iodine and has a tinted yellow color. When given orally, gastrografin may taste bitter.

Patients usually need to drink at least 1000 to 1500 cc (about three to four 12 oz. drinks) to sufficiently fill the stomach and intestines with oral contrast.

What Preparation is Needed Before Taking Oral CT Contrast?

It is important to eliminate as much food as possible from the stomach and intestines in order to help the sensitivity of the CT exam using oral contrast. Food and food remains can mimic disease when the oral contrast is present. Thus a regimen of not eating and/or drinking for several hours before the CT exam is required. The preparation time varies depending on the actual exam as well as the imaging center's requirements. Patient preparation varies from center to center, depending on a number of issues. Some types of oral CT contrast are taken at home well before the CT examination.