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What are computed tomography (CT) scans? How can they help detect cancer?

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Cancers diagnosed: Most

Why performed: CT is a valuable tool for screening for cancer, locating tumors, performing guided biopsy, identifying cancer that has spread, staging cancer, planning and monitoring cancer treatment, and checking for cancer recurrence. CT can identify blood vessels that support a tumor, as well as the tumor’s shape, size, location, and volume. CT scans are used with procedures to diagnose cancer, such as needle-guided biopsies. They are used in cancer treatments, such as radiofrequency ablation and interstitial therapy.

CT images are archived in computers or printed form. Images are printed on film or with laser imaging. They can be sent and viewed online within a facility and can be compared for changes over time.

Conventional CT was developed in the early 1970s. CT uses controlled amounts of thin, high-energy radiation beams. Images are taken from a variety of angles. A computer calculates information about the images and produces a slice image of a specific area of the body. CT can be taken of the entire body or a specific section. Dye is used to enhance images to show more detail and contrast. The contrast dye can be swallowed, received intravenously, or administered in an enema.

CT has evolved to become faster and to use lower doses of radiation. Spiral or helical CT allows images to be collected quickly. The rapid process helps compensate for movements or breathing that otherwise can blur images. Additionally, spiral CT produces thinner slices of images that show even greater detail. A computer stacks the images to create three-dimensional, rotational pictures of a tumor. Spiral CT is especially useful for imaging liver, lung, and pancreatic cancer.

Recent technology allows doctors to use CT with endoscopy to create virtual endoscopy, virtual bronchoscopy, and virtual colonoscopy or CT colonography. The process uses a computer to create and manipulate three-dimensional images to create a “fly-through” view of an organ, as seen on endoscopy.

CT is used to guide surgeons precisely during procedures. CT is used with needle aspiration biopsy for tumors, such as for breast cancer biopsies. The CT guidance eliminates the need for invasive exploratory surgery.

CT is used to guide certain radiation treatments. For example, interstitial therapy uses CT guidance to insert radioactive material into a tumor. CT is also used to create a three-dimensional image of a tumor to allow external beam radiation to precisely target the tissue and spare as much healthy tissue as possible.

Patient preparation: CT scans may not be appropriate for some people. Patients should let their doctor know if they have diabetes or kidney disease or if they are pregnant. Some facilities conduct a pregnancy test on all women prior to scanning because the radiation can be dangerous to a developing fetus. Children, patients with movement disorders, or patients that are claustrophobic may receive a mild sedative prior to the test. In extreme situations, people may be sedated if they are not able to remain still while the images are taken. Some CT machines may not be used for people that weigh more than three hundred pounds because of size parameters of the machinery. A small percentage of patients are allergic to contrast dye. Patients should let their doctor know if they are allergic or sensitive to contrast dye, iodine, or shellfish.

CT scans are outpatient procedures performed at a hospital radiology department or freestanding radiology center. Patients will receive instructions prior to their procedure, which vary depending on which procedure is conducted. Patients may be instructed not to eat or drink for several hours before the CT. Some patients need to use an enema or laxatives. Patients may need to drink a liquid contrast agent before arriving at the clinic or upon arriving at the clinic.

Patients wear an examination gown or robe for the procedure. They must remove metal objects that can interfere with the imaging process, such as jewelry, hearing aids, hair clips, glasses, or dentures.

Steps of the procedure: Patients are led into the imaging room. A radiology technologist conducts the CT. Patients lie on a narrow table. The technologist may use positioning devices to help patients maintain specific postures for the test. Head-stabilizing devices may be used for brain scans. Patients may be instructed to change positions throughout the procedure.

During the CT scan, the radiologist technician steps into a room that is separated by a glass partition. The radiologist is in constant contact with the patient via a microphone and intercom system. The radiologist technician operates the CT machine.

To take a CT, the table slides into the doughnut-shaped hole of the scanner. Patients must remain motionless while the images are taken. Patients are instructed to take a deep breath and hold it until they are told to exhale. The CT scanner has an X-ray component that emits beams at specific angles. The beams pass through the body and are detected by equipment on the opposite side. The equipment makes buzzing or clicking noises while the scanner is employed.

Patients receiving a contrast material may have a CT scan first, receive the contrast material, and then receive a final CT scan. An intravenous (IV) line is inserted into the hand or forearm to deliver injected contrast material. Contrast material may be swallowed. An enema is also used to insert contrast material for gastrointestinal-related CT scans. The contrast material may make patients feel temporarily warm or flushed.

The actual CT scan is short, but the preparation and positioning time makes the procedure last from about ten to thirty minutes. Spiral CTs are shorter in length. Patients are asked to wait until it is confirmed that the images are clear. Patients are observed for allergic reactions for a short time following the procedure.

After the procedure: There are usually no aftercare procedures following CT. If the patient has received sedating medication, a friend or relative must drive the patient home.

Risks: Patients may experience an allergic reaction to the contrast dye. The symptoms include hives, shortness of breath, nausea, wheezing, itching, or a bitter taste in the mouth. Severe allergic reaction or anaphylactic shock is a rare risk. Radiation from CT is higher than that of standard X rays. CT is not recommended for pregnant women.

Results: CT images are read by a radiologist and conveyed to the ordering doctor. The doctor may view the images as well. CT images depict anatomical structures in black, white, and shades of gray. Air, soft tissue, hard tissue, fluids, and the contrast agent show details that otherwise cannot be seen. Abnormal results show different characteristics from what is expected, such as tumors, cysts, cancer metastasis, tumor density, tumor composition, enlarged lymph nodes, or atypical fluid accumulations. In some cases, CT can help differentiate between types of tumors.

Bibliography

Applegate, Kimberly. “Pregnancy Screening of Adolescents and Women Before Radiologic Testing: Does Radiology Need a National Guideline?” Journal of American College of Radiology 4.8 (2007): 533–36. Print.

"Computed Tomography (CT) Scans and Cancer." National Cancer Institute. Natl. Cancer Inst., 16 July 2013. Web. 10 Sept. 2014.

Ishikawa, Susumu., et al. “Mass Screening of Multiple Abdominal Solid Organs Using Mobile Helical Computed Tomography Scanner: A Preliminary Report.” Asian Journal of Surgery 30.2 (2007): 118–21. Print.

Pearce, Mark S., et al. "Radiation Exposure from CT Scans in Childhood and Subsequent Risk of Leukaemia and Brain Tumours: A Retrospective Cohort Study." Lancet 4 Aug. 2012: 499–505. Print.

Sone, Shusuke., et al. “Long-Term Follow-up Study of a Population-Based 1996–1998 Mass Screening Programme for Lung Cancer Using Mobile Low-Dose Spiral Computed Tomography.” Lung Cancer 58.3 (2007): 329–41. Print.

Storrs, Carina. "How Much Do CT Scans Increase the Risk of Cancer?" Scientific American. Scientific American. 18 June 2013. Web. 10 Sept. 2014.

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