1) What is a CT?
- CT (Computed Tomography) scanning—or CAT (Computer Axial Tomography) scanning-is a noninvasive medical imaging method that uses tomography to diagnose and to treat medical conditions. It combines special x-ray equipment with sophisticated computers to produce multiple images or pictures of the inside of the body. The CT scanner uses digital geometry processing to generate a 3-dimensional image of the inside of an object after taking many 2-dimensional x-ray images around a single axis of rotation. Simply put, CT scans are 3-D images complied from many pictures of the same area taken from different angles. These cross-sectional images, then, can be examined on a computer monitor, printed or transferred to a CD. When compared to regular x-ray exams, CT scans provide greater clarity and detail of internal organs, bones, soft tissues, and blood vessels. Additionally, though CT imaging does involve x-rays, the diagnostic benefits generally outweigh the risks of x-ray radiation exposure. With CT scans, radiologists can more easily diagnose diseases such as cancers, cardiovascular disease, infectious disease, appendicitis, trauma, and musculoskeletal disorders.
- CT imaging is one of the best and fastest tools for studying the chest, abdomen and pelvis because of its detailed, cross-sectional views of all types of tissue. Thus, it is often the preferred method for diagnosing many different cancers (lung, liver, kidney, and pancreatic cancer) as the images allow physicians to confirm the presence of tumors and measure their sizes, precise locations, and extents of their involvement with other nearby tissues. As a result, CT examinations play significant roles in the detection, diagnosis and treatment of vascular diseases that may lead to stroke, kidney failure or even death. It is also invaluable in diagnosing and treating spinal problems and injuries to the hands, feet, and other skeletal structures because of CT image’s ability to show even the very small bones as well as surrounding tissues such as muscle and blood vessels.
- Physicians often use the CT examination to:
i. quickly identify injuries to the lungs, heart and vessels, liver, spleen, kidneys, bowel or other internal organs in cases of trauma.
ii. guide biopsies and other procedures such as abscess drainages and minimally invasive tumor treatments.
iii. plan for and assess the results of surgery, such as organ transplants or gastric bypass.
iv. stage, plan and properly administer radiation treatments for tumors as well as monitor response to chemotherapy.
v. measure bone mineral density for the detection of osteoporosis.
2) What happens during a CT?
- Most imaging centers or hospitals provide patients with gowns. Usually patients undress down to their underwear and wear the gowns. If the place does not provide gowns, patients should wear loose-fitting clothes.
- Beforehand, any women who suspects to be pregnant should tell her doctor. Additionally, as metal interferes with the workings of the CT scanner, patients will need to remove all jewelry and metal fastenings. Some doctors may ask patients to not eat or to even refrain from consuming liquids for a specific time before the scan.
- After getting dressed, patients will be asked to lie down on a motorized examination table that goes into a giant doughnut-like machine. Straps and pillows may be used to maintain the correct position and to hold patients still during the exam. Depending on the part of the body being scanned, patients may be asked to lay face-down or sideways or to keep their hands over their heads.
- Mostly, scanners are fast enough to scan children without sedation. However, in special cases, sedations may be needed for children who cannot hold still. For adults, although the scan is painless, some may feel uncomfortable lying still for what may seem like a long time to them. Thus, patients can always ask their doctors for a mild sedative if they think the whole experience will be stressful.
- Additionally, some patients are given a contrast dye or substance which is either swallowed or injected. This substance merely improves the picture of some blood vessels or tissues. Thus, it is crucial that patients tell their doctors beforehand if they are allergic to contrast materials. Then, doctors may provide some medications that will reduce allergic reactions to contrast materials.
- Next, after patients have lied down on the table, it will move quickly through the scanner to determine the correct starting position for the scans. Then, the table will move slowly through the machine to perform the actual CT scanning. (Note: depending on the type of CT scan, the machine may make several passes).
- When entering the CT scanner, patients may see special light lines on their bodies, which ensure that you are properly positioned. Additionally, you may also hear slight buzzing, clicking and whirring sounds as the scanner revolves around you during the imaging process—this is completely normal.
- During the exam, you will be alone as the technician will leave the exam room. The technologists will always be able to see, hear and talk to you at all times using an intercom. However, with pediatric patients, a parent may be allowed in the room but is required to wear a lead apron to minimize radiation exposure.
- For the best results, patients need to lie very still. Thus, during the scanning, they may be asked to hold their breaths. Motions like breathing or body movements can degrade the quality of the examination by leading to artifacts on the images. This is similar to blurring seen on a photograph of an object in motion. This will make it difficult for radiologists to interpret the images.
- When the examination is completed, you will be asked to wait until the technologist verifies that the images are of high enough quality for accurate interpretation. The CT examination is usually completed within 30 minutes. The portion requiring intravenous contrast injection usually lasts only 10 to 30 seconds.
3) What should I do to prepare for a CT?
- Generally, no special preparation is required for a CT scan. For hospitals without gowns for patients to wear during the scan, wear comfortable, loose clothing. It is also crucial to remove any metal (jewelry, dentures, eyeglasses, belt buckles, and metal zippers and buttons) that can interfere with the images prior to the exam. At times, patients may be asked to limit eating and drinking liquids (water, coffee, tea, broth, etc.) for several hours before the appointment for a CT scan. If a contrast material (a liquid enhancing the imaging of certain organs or blood vessels) needs to be administered, patients may be required to fast (eating no food) for several hours or to use an enema to cleanse the colon prior to his/her appointment. Also remind doctors if you have allergic reactions to contrast materials or if you think that you may be pregnant.
4) What are the risks?
- CT scanning is painless, noninvasive and accurate. CT scans are fast and simple; for which, in emergencies, they can reveal internal injuries and bleedings quickly enough to help save lives. Additionally, CT scans can be performed even if you have an implanted medical device of any kind. No radiation will remain in a patient’s body after a CT exam. As for the x-rays used in CT scans, they usually have no immediate side effects.
- In other words, there is always a slight chance that patients be exposed to excessive radiation. However, the benefit of an accurate diagnosis far outweighs this risk.
- For pregnant women, CT scanning is generally not recommended unless medically necessary due to potential risks to the baby. Nursing mothers should wait 24 hours after contrast material injection before resuming breast-feeding.
- For children, they are more sensitive to radiation. As a result, CT exam for children should only be necessary if it is essential to make a diagnosis. If a child does undergo a CT scan, repeated CT exams are not recommended and should always be done with low-dose techniques.
5) Are there any after effects?
- No. CT scanning itself causes no pain. Though discomfort may exist from having to remain still for several minutes. Nevertheless, if claustrophobic or have chronic pain, technologists or nurses (under the guidance of a physician) may offer mediation to help patients tolerate the CT scanning procedure.
- If swallowed a contrast material, patients may find the taste mildly unpleasant. They can also expect to experience a warm, flushed sensation or a sense of abdominal fullness and increasing need to expel the liquid (if given by enema). However, most patients can easily tolerate this as the mild discomfort does not last long.
- Thus, after a CT exam, patients can return to their normal activities unless received contrast material (for which patients will be given special instructions).
6) What happens with the scan results?
- When the scan is complete, the couch glides out of the scanner and patients will be permitted to get dressed (if in a gown). However, radiologists usually ask patients to wait while images are reviewed, in case more images are necessary. After the CT images are reviewed, patients are free to go. Generally, patients can assume normal activities at once unless a contrast agent has been used. If used, doctors may provide specific after-care instructions.
- A physician, usually a radiologist with supervising and interpretation expertise, will analyze the images and send a signed report to patients’ primary care physicians or the physicians who referred you for the exam.
- A follow-up examination may be necessary. In this case, your doctor will explain the exact reason for requesting another exam. Follow-up examinations are sometimes the best way to detect if treatment is working or if an abnormality is stable over time.
7) What is the difference between an MRI and CT scan?
- There are two basic differences between an MRI and CT.
- First, CT uses x-rays to detect and record the radiation absorbed by difference tissues. Then, it sends the data to a computer to transform into images. On the other hand, MRI does not use x-rays, but employs a powerful magnetic field. This field will monitor the nuclei of hydrogen atoms in water (the most abundant element in the body). Then, when subjected to MRI’s magnetic field, the hydrogen protons are knocked out of alignment and emit a radiofrequency signal that is detected by the MRI machine and processed into images.
- Second, CT scans highlight solid structures like calcium deposits or kidney stones, while showing little differentiation in soft tissues. MRI scans emphasize detailed tissue structures with their differences in water content.
- To see a comparison chart, click here.