Common questions about CT
Surely, your radiologist or treating physician has already explained a lot about the exam to you. However, sometimes there are unanswered questions that only occur to you later, after the initial discomposure has passed. We have collected the most common questions below.
I have claustrophobia—will I be lying in a “tube”?
You will not be lying in a “tube,” that is the MRI exam. This exam takes place in a “ring” through which the patient can see during the exam. The CT exam takes only a few minutes, unlike an MRI exam. The scanning unit of the computed tomography system, the gantry, is like a 70 cm thick disc with an opening in the middle through which the patient is moved and can see. Even people with claustrophobia don’t really have a problem with this. If claustrophobia should occur anyway, a sedative can be given, with typically excellent results. This does limit the ability to drive, however, so in this case you should not drive yourself to the appointment.
Why do I need a contrast agent?
Two types of contrast agent are used. Contrast agent injected into the bloodstream accumulates wherever there is strong blood flow. This includes most tumors, but also inflammatory processes. In internal organs with strong blood flow, in contrast, disease processes are often evident from their lower blood flow in comparison with healthy tissue. Benign and malignant areas can typically be differentiated with contrast agents. In addition, contrast agents are typically given to drink before a gastrointestinal exam. This dyes the walls of the intestine so that they can be distinguished from other structures in the very crowded abdominal cavity.
Where does the contrast agent go?
Contrast agent injected into the bloodstream accumulates wherever there is strong blood flow. This includes most tumors, but also inflammatory processes. In internal organs with strong blood flow, in contrast, disease processes are often evident from their lower blood flow in comparison with healthy tissue. Benign and malignant areas can typically be differentiated with contrast agents. In addition, contrast agents are typically given to drink before a gastrointestinal exam. This dyes the walls of the intestine. The contrast agent in the blood stream is eliminated by the kidneys within 5 minutes and is completed eliminated from the body within about 6 hours. It can be problematic if kidney function is degraded. The simplest sign of this is a change in the lab value for creatinine. This is typically reviewed before the CT exam. The second contrast agent, the one that is swallowed, is absorbed to a limited degree, but mostly it remains in the intestine and is eliminated in this manner. Contrast agents in other areas of the body are absorbed into the blood or lymph and then eliminated through the kidneys. Bile duct contrast agents do enter the blood, but are eliminated by the liver through the bile duct and then enter the intestine.
Do I always need a contrast agent?
No, most exams of the spinal column, skeletal system, joints, teeth, and paranasal sinuses, and CT bone density scans (QCT) do not need contrast agents. For exams of the brain, the soft tissues in the throat, and the chest and abdominal cavities, however, contrast agent is typically administered in order to get optimal results.
When should a CT not be performed? Are there contraindications?
During pregnancy, CT must not be performed due to the radiation load. Alternative examination methods, such as ultrasound, can often be helpful here or in special cases. Other contraindications apply only to the use of contrast agent. The exam is then performed without the contrast agent, if needed. For most types of hyperthyroid function, a history of severe intolerance of the contrast agent, severe cardiac circulatory diseases, severely limited kidney function, and “plasmocytoma” bone marrow disease, no contrast agent should be administered.
What do I need to do for a cardio CT?
Please bring a current letter from your physician and current blood test results. Avoid caffeinated beverages or performance-enhancing agents on the day of the exam. Erectile dysfunction medications (such as Viagra) should be avoided for at least 24 hours prior to the exam.
Do I need to purge for a virtual colonoscopy?
Yes, purgative measures are needed for virtual colonoscopy, just like for conventional colonoscopy, in order to be able to adequately evaluate the colon.
What is low-dose CT?
Low-dose CT uses a CT technology that requires a much lower dose of radiation. The quality of the images is not as good, however, and they appear less sharp. This loss of quality is acceptable in many cases, however, because the exam is not used on patients who have concrete suspicion of new malignant growth (cancer), but rather as a screening tool for patients with a certain probability of such disease (e.g., to discover lung cancer in long-term smokers.) Low-dose computed tomography can also be used to monitor progression with a known diagnosis. In comparison with standard CT of the lungs, the dose can be reduced to 1/10 of the standard level.