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Computerized Axial Tomography: A Revolutionary Medical Imaging Tool

by Harmony Phuong Huynh

Published on April 2, 2011

A Life-Saving Tool

In the aftermath of the shooting of Congresswoman Gabrielle Giffords in January 2011, many were intrigued by the miraculous recovery of her headshot wound. After all, one would expect the worst after a gunshot in the head. The CT or CAT scan, used interchangeably for the term Computerized Axial Tomography, proved crucial to her remarkable recovery. A CT scan is a non-invasive procedure that uses x-rays to generate cross-sectional views or three-dimensional images of specific organs or regions of the body, allowing structural abnormalities to be detected. Through use of the CT scan, Giffords' neurosurgeons were able to determine the exact location of the brain injury and whether or not there was any progressive swelling during her treatment.

According to Dr. Nakaji, a neurological surgeon at St. Joseph's Hospital and Medical Center in Arizona, advances in treating brain trauma have vastly improved since JFK's assassination in 1963. In an interview with Phoenix NewTimes, Najiki claimed that in the 1960s, physicians "were still capable of opening up [the skull] and letting off pressure, but there weren't even CAT scans at the time to see where the bullet was" (D'Andrea).

U.S. National Library of Medicine

Figure 1: The patient lies on an examination table that slides through the aperture in the CT scanner.

The CT scanner is a large x-ray machine with a tunnel in the middle, giving it a donut-shaped appearance. As shown in Figure 1, the patient lies on an examination table that moves into the tunnel during the scan. The scanner captures cross-sectional images of the body that are analogous to horizontal "slices" of a loaf of bread (Stoppler).

CT scans of the brain and other areas of the body are used widely now, making it possible to detect any abnormalities and trauma-induced damage. In Giffords' case, CT scans proved key to enabling neurosurgeons to detect regions of swelling in the brain and to decrease pressure inside the skull — one of the most crucial aspects of treating brain trauma.

History of the CT Scan

In 1972, the invention of the CT scan is credited to British electrical engineer Godfrey Hounsfield from Electric and Musical Industries (EMI) and South African physicist Allan Cormack from Tufts University (Imaginis Corporation, "Brief History of CT"). Hounsfield and Cormack both received the 1979 Nobel Prize in Physiology or Medicine for their revolutionary invention though the two scientists had never met and worked independently (Allison). In 1955, Cormack was recruited by a hospital in Cape Town, South Africa and was asked to create a map demonstrating the amount of x-ray absorption by different parts of the body. He was unimpressed by x-ray images, realizing that simple x-rays do not provide clear distinction between soft tissue, bones, and organs. In 1957, Cormack joined the faculty of Tufts University, where he developed a mathematical model to compile the information from multiple x-rays taken at different angles into one highly accurate image (Thomas).

In 1951, before Cormack's work on the mathematical model, Hounsfield began to work in EMI, and in 1962, the company signed a contract with the Beatles before the band become a worldwide phenomenon. The group's fame gained the EMI millions of dollars, allowing Hounsfield to conduct independent research. Hounsfield's interest in both computers and x-rays gave him the idea of scanning the human head from multiple angles and producing three dimensional images of the brain on a computer. His research led to the development of the first CT scan (Whittle).

The first CT scanners were installed in 1974 and could only provide imaging of the head (Imaginis Corporation, "Brief History of CT"). In 1975, Georgetown Professor Robert Ledley invented the whole body CT scanner (Allison). Performing CT scans on patients became a prevalent medical practice in 1980. Since then, the scanners have been improved in speed, clarity, and patient comfort, allowing this imaging tool to detect internal abnormalities with incredible accuracy. Currently, there are 6,000 CT scanners installed in the United States and 30,000 scanners installed throughout the world, allowing about 62 million CT scans to be performed each year (Imaginis Corporation, "Brief History of CT").

Since their invention, many engineers and scientists have improved the speed and scanner design, which has led to the creation of four generations of scanners. The first generation scanner, which is based on Hounsfield's original vision, was a head-only scanner and placed the patient's head through a rubber membrane into a water-filled box, compromising patient comfort. The second generation scanner, developed in 1974, was the first waterless, full-body CT scanner which involved multiple x-ray beams and multiple detectors and operated with faster speed. The third generation scanner, which is currently the most common one used in medical practice, involves a wide x-ray beam and an array of detectors that rotate together, allowing the machine to capture data at a multitude of angles. This CT scanner has a scanning speed of 0.5 second per rotation. In the fourth generation scanner, the beam and detectors are attached to a rotating ring, which prevents unnecessary artifacts of the machine from appearing in CT images (Goldman; Platten).

How does the CT scan work?

Bjecas/Wikimedia Commons

Figure 2: Components of the CT Scanner. T: X-ray tube; D: X-ray detectors; X: X-ray beam; R: Direction of gantry rotation.

The internal structures of a CT scanner are shown in Figure 2. Inside the scanner is a rotating frame called a gantry that contains x-ray tubes on one side and an array of detectors on the opposite side. While the table slides into the machine, the gantry rotates, and the allowing the x-ray beam to pass through the patient's body at numerous angles while the detector determines the absorption of x-ray photons by the body, which is dependent on the body's internal structural characteristics. The absorption information gathered by the detector during one 360 degree rotation is compiled into one cross-sectional image, or an x-ray "slice," called a tomogram. As the table slides through the scanner, the gantry continues to rotate and capture absorption data. This data is processed through mathematical algorithms, creating multiple x-ray images on a film that can be viewed on the computer, as shown in Figure 3. The captured images are also compiled into three-dimensional volumes displayed on the computer (Imaginis Corporation, "How Does CT Work").

Uses of the CT Scan

The CT scan has become a revolutionary tool in the field of medicine because of its versatile uses and ability to detect abnormalities in multiple regions of the body. The scan is used to analyze structures in the head, neck, spine, chest, abdomen, and pelvis. The results allow physicians to evaluate the appearance of the brain (shown as a film in Figure 3), lungs, liver, heart, and other soft tissue organs in detailed cross-sectional images. The scan also allows examination of the bones and blood vessels (Davis).

National Heart Lung and Blood Institute

Figure 3: Atherosclerosis of a coronary artery leading to coronary artery disease.

The head CT scan is done to determine if there is a stroke, hemorrhage, blood clot, infection, or any other abnormality in the brain or a fracture in the skull. CT scans of the spinal cord can define the bony structure of the vertebrate, including the anatomy of intervertebral discs. Moreover, the scan can even be used to assess a patient's bone density to determine if he or she has osteoporosis. On occasion, contrast material, such as an x-ray dye is injected into the spinal fluid to enhance the image and the various structural relationships of the spinal cord, the spine, and nerves (Stoppler). When a chest x-ray detects an abnormality, CT scans of the chest provide more detailed images of the lungs and ribs to identify any possible cysts, tumors, or infections. In the abdomen, CT scans aid in defining organ anatomy, allowing physicians to visualize the gallbladder, liver, spleen, pancreas, kidneys, aorta, adrenal gland, ovaries, and uterus (Davis).

CT scans are often done to determine the absence or presence of infection, tumors, damage due to trauma, or benign anatomical anomalies. Oncologists frequently use the CT scans to determine if patients with newly diagnosed cancer have metastasis of tumor cells from the primary tumor site to other regions in the body. When patients with metastatic cancer receive hormonal therapy, chemotherapy, radiation, or other treatments, CT scans are done to determine if the treatment is effectively killing the tumor cells or preventing further metastasis (Oncology Hematology Care). When a person suffers a traumatic injury, such as in Giffords' case, a CT scan can be done to determine the extent of the damage. If a person is suffering from unusual pain or dizziness, a physician might recommend a CT scan to search for any abnormalities possibly causing those symptoms. When a simple x-ray detects an abnormality, a CT scan can be done to examine the abnormality with greater clarity, allowing physicians to determine if the patient has a tumor, clot, or other problem (Davis).

In addition to serving as a diagnostic tool, CT scans are used by radiologists while they perform different procedures, including removal of internal body fluids for certain tests, the draining of deep abscesses, and biopsies of cancers. Use of the CT scan in conjunction with such minimally invasive procedures has considerably decreased the need to conduct major surgeries (Alltucker and Wang).

How Patients Prepare for the Scan

Before the CT scan, patients are often asked to drink a contrast liquid, which allows clearer distinctions between and higher visibility of different organs and tissues. The liquid may also allow differentiation between abnormal and normal structures. Patients are asked to avoid solid food four to six hours before the exam if contrast material is used. The contrast liquid can be swallowed, given rectally by enema, or injected intravenously (Radiological Society of North America). If the patient has allergies to iodine-containing contrast, the physician and radiology staff must be notified. All metallic materials and certain clothing around the body are removed since they could interfere with the clarity of the scans (Stoppler).

Potential Risks

Being a minimally invasive procedure, computerized axial tomography is a very low-risk technique. Since the CT scan involves relatively high doses of radiation compared to an x-ray or ultrasound, pregnant women in their first trimester should not undergo a CT scan as it may be detrimental to the fetus. However, most men and nonpregnant women undergo the scan without serious reactions (Stoppler). Patients who receive an intravenous iodine-containing contrast liquid may experience rashes, hives, itching, headaches, and nausea and can take antihistamines to alleviate these symptoms. However, in rare instances, patients may experience anaphylactic reaction, an extreme allergic reaction, in response to the intravenous contrast, which involves severe hives and difficulty breathing. Since the anaphylactic reaction can be life-threatening if left untreated, medicines such as antihistamines, epinephrine, and corticosteroids are usually administered to reverse the reaction (Remedy Health Media; Stoppler).

Another rare side effect is kidney failure due to toxicity of the intravenous contrast material. Patients with diabetes or impaired kidney function are at high risk of kidney failure. Since dehydrated individuals may also experience kidney failure, patients may be told to drink plenty of fluids before the scan. Fortunately, newer, less toxic intravenous contrast fluids have been developed, such as Isovue, to prevent this reaction (Remedy Health Media; Stoppler).

CT Scan vs. X-ray

The prevalent use of CT scans in the medical field is a double-edged sword. Studies have shown that CT scans are more sensitive and accurate detection tools than X-rays. For example, a study conducted in the Cumberland Infirmary in the United Kingdom found that CT scans have higher sensitivity and specificity than chest x-rays in detecting lung metastasis and other head and neck cancers (Arunachalam et al). Despite the CT's increased accuracy, this imaging technique involves significantly higher radiation doses than a single x-ray. A multi-institutional study published in the New England Journal of Medicine found that CT scans cause high cumulative radiation doses in the body (Fazel et al). Since ionizing radiation can cause DNA mutations or create harmful hydroxyl radicals from water molecules, some researchers are alarmed by the frequent use of CT scans (Brenner and Hall).

New and Improved CT scans

Researchers are constantly honing the CT scans to produce images of internal body structures with greater speed and clarity, making the CT a more efficient imaging and diagnosis tool. Helical or spiral CT scans are newer methods that offer quick and accurate visualization of internal organs. While the first to fourth generation scanners require the gantry to stop and reposition itself during the entire scan, the helical CT scan can perform imaging continuously, making the process significantly faster. Some trauma centers have installed the helical scanners to quickly diagnose trauma injuries. In addition, high resolution CT (HRCT) scans are able to detect scarring and inflammation of the lungs (Imaginis Corporation, "Spiral CT and Helical CT"; Stoppler).

Works Cited

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