Computed Tomography: History, Uses, and Patient Risks

Introduction

Computed tomography — more commonly known as a CT scan — is an X-ray technique used to produce very detailed images of internal organs located in various parts of the body, such as the head, chest, and abdomen. Doctors use the images produced through this procedure to help diagnose and treat diseases. Other terms for the technique include computerized tomography and computerized axial tomography (CAT). While conventional X-ray exams produce two-dimensional images, CT scanning uses an X-ray-sensing unit that rotates around the body and a large computer to create cross-sectional images of the body's interior. This paper addresses vital educational information including a brief history of computed tomography, its medical uses, and the effects it may have on the patient.

Brief History of Computed Tomography

British engineer Godfrey Hounsfield of EMI Laboratories, England, and Allan Cormack of Tufts University in Massachusetts invented computed tomography. Their work led to the installation of the first clinical CT scanners between 1974 and 1976. The original systems were limited to imaging only the head, but soon "whole body" systems with larger patient openings became available. CT scanning came into wide use beginning in 1980. There are now approximately 6,000 CT scanners installed in the United States and about 30,000 installed worldwide.

The first CT scanner developed by Hounsfield in his laboratory took several hours to acquire the raw data for a single scan, or "slice," and took days to reconstruct a single image from that raw data. This is a far cry from the efficiency of the latest multi-slice CT systems, which can collect up to four slices of data in about 350 milliseconds and then reconstruct a matrix image from millions of data points in less than a second. As an example, an entire chest scan can be completed in five to ten seconds using the most advanced multi-slice CT systems.

Throughout its history, advancements in computerized tomography have produced great improvements in speed, patient comfort, and image resolution. As scan times have become faster, more anatomy can be imaged more quickly and efficiently. The extreme speed of scanning eliminates artifacts caused by patient motion such as normal breathing or peristalsis. CT exams are now quicker and more patient-friendly than ever before. Tremendous research and development has been directed toward providing exceptional image quality while ensuring the lowest possible X-ray dose for diagnostic purposes.

Benefits and Uses of Computed Tomography

Computed tomography has been a powerful diagnostic tool for more than thirty years. It is a noninvasive way to examine internal organs and tissues. "Advances with helical and subsequent multidetector technology have offered expanding and diverse opportunities" (Frush, 2003), and doctors use CT scans to diagnose many conditions, such as tumors, infections, blood clots, and broken bones (MayoClinic.com, 2003). A CT scan also helps in diagnosing some diseases that might otherwise require surgery. For example, doctors can use a CT scan to guide catheters to an abscess in the body and then drain pus from the infected area.

Computed tomography is used for a variety of clinical purposes, including:

• Diagnosing muscle and bone disorders, such as osteoporosis. Bone disorders are easier to detect with CT than with traditional X-ray technology. Many hairline fractures that were previously overlooked — and therefore left untreated — can now be identified, preventing further loss of bone integrity.
• Pinpointing the location of a tumor, infection, or blood clot.
• Guiding procedures such as surgery, biopsy, and radiation therapy.
• Detecting and monitoring diseases such as cancer or heart disease, and tracking the progression of a condition.
• Detecting internal injuries and internal bleeding.
• Revolutionizing surgical approaches to the posterior ethmoid sinuses since the introduction of computed tomography.

There are many revolutionary advances in the field of sinus surgical procedures. Following the introduction of computer-aided image guidance in 1993, the endoscopic view and the CT view have been combined to provide a three-dimensional triplanar perspective of surgical anatomy, making computer-aided imaging an invaluable tool in critical anatomic regions such as the sphenoid sinus (Wagner & Conti, 1991).

Wagner and Conti (1991) note that "the sphenoid sinus, located in a recessed position in the skull base and surrounded by a host of vital neurologic structures, may be amenable to image-guided surgery in both primary and revision surgical cases. Laterally, the sphenoid sinus is flanked by the cavernous sinus, which houses the oculomotor nerve, the trochlear nerve, the ophthalmic division of the cranial nerve, and the internal carotid artery."

Traditionally, the sphenoid sinus was accessed by two principal methods: bicoronal craniotomy or transnasal entry. Both approaches carry significant risks, and expected post-operative complications include brain retraction and possibly permanent anosmia. However, with the use of computed tomography, "the anatomy of the paranasal sinuses constitutes a complex three-dimensional structure that deserves the accurate spatial representation afforded by computer-aided image-guided endoscopic sinus surgery to avoid surgical pitfalls" (Wagner & Conti, 1991).

CT exams can be performed even when the patient already has an implanted pacemaker or cardioverter defibrillator. If the patient is pregnant or suspects they may be, their doctor should be informed, as it may be advisable to postpone the procedure or choose an alternative exam that does not involve radiation. CT scanning is also used on healthy individuals. Many people undergo whole-body CT scans to detect cancer and other medical conditions in their earliest stages, before any symptoms develop. Early detection is a key factor in positive health outcomes.

Cancer screening has also benefited from the use of CT scans. Wagner and Conti (1991) note that lesions may not appear on traditional imaging such as MRI, and lesions of disparate pathology may appear similar on those scans. CT data is more accurate, and complementary imaging technologies such as positron emission tomography (PET) and single photon emission tomography can provide information about the biochemical function of the brain by utilizing emissions from radioactive tracers to construct an image of where those tracers are concentrated in living tissue. Through appropriate tracer selection, detailed information about the location of key biochemical processes may be obtained. These imaging technologies, used together, yield information about cancer in its living, dynamic state and complement the pathological analysis of processed tissue specimens (Wagner & Conti, 1991).

Conclusion

Continuous advancements are constantly being made with the use of computed tomography. Researchers are experimenting with various levels of radiation to reduce risk, and exploration of imaging applications and increased scanning speed remains an active area of development. CT scanning has greatly improved the quality of life for many patients, and the inclusion of noninvasive procedures has allowed patients to feel more comfortable and less apprehensive about undergoing imaging. While the technology is already remarkable, the field of medical imaging continues to evolve, and further advancements are on the horizon.

References

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