Computed Tomography

Summary

Computed tomography (CT) is an imaging modality that relies on X-rays and computer algorithms to provide high-quality image data. CT scanners are an integral part of medical health care in the developed world. Physicians rely on CT scanners to acquire important anatomical information on patients. CT images provide a three-dimensional view of the body that is both qualitative and quantitative. CT scans are often used in the diagnosis of cancer, stroke, bone disorders, lung disease, atherosclerosis, heart problems, inflammation, and a range of other diseases and physical ailments, such as a herniated disc and digestive problems.

Definition and Basic Principles

Computed tomography (CT, also called computer-aided tomography, or CAT) is an imaging modality that uses X-rays and computational and mathematical processes to generate detailed image data of a scanned subject. X-rays are generated through an evacuated tube containing a cathode, an anode, and a target material. The high voltage traveling through the tube accelerates electrons from the cathode toward the anode. This is very similar to a light bulb, with the addition of a target that generates the X-rays and directs them perpendicularly to the tube. As electrons interact with the target material, small packets of energy, called photons, photons, are produced. The photons have energies ranging from 50 to 120 kilovolts, characteristic of X-ray photons. The interaction of X-ray photons with a person's body produces a planar image with varying contrast depending on the density of the tissue being imaged. Bone has a relatively high density and is more readily absorbed by X-rays, resulting in a bright image on the X-ray film. Less dense tissues, such as lungs, do not absorb X-rays as readily, and as a result, the image produced is only slightly exposed and therefore dark.

In computed tomography, X-rays are directed toward the subject in a rotational manner, generating orthogonal, two-dimensional images. The X-ray tube and the X-ray detector are placed on a rotating gantry, which allows the X-rays to be detected at every possible gantry angle. The resulting two-dimensional images are processed through computer algorithms, and a three-dimensional image of the subject is constructed. Because computed tomography relies on ionizing radiation, it has an associated risk of inducing cancer in the patient. The radiation dose obtained from CT procedures varies considerably depending on the patient’s size and the imaging type.

Background and History

X-rays were discovered by Wilhelm Conrad Röntgen in November 1895, during his experiments with cathode-ray tubes. He noticed that when an electrical discharge passed through these tubes, a certain kind of light was emitted that could pass through solid objects. Throughout his experimentations with this light, Röntgen began to refer to it as an X-ray, as x is the mathematical term for an unknown. By the early 1900s, medical use of X-rays was widespread. They were also used to entertain people by providing them with photographs of their bodies or other items. In 1901, Röntgen was awarded the first Nobel Prize in Physics for his discovery of X-rays. During World War II, X-rays were frequently used on injured soldiers to locate bullets or bone fractures. The use of X-rays in medicine increased drastically by the mid-twentieth century.

The development of computer technology in the 1970s made it possible to invent computed tomography. In 1972, British engineer Godfrey Hounsfield and South African physicist Allan Cormack, who was working at Tufts University in Massachusetts, independently developed computed tomography. Both scientists were awarded the 1979 Nobel Prize in Physiology or Medicine for their discovery.

How It Works

The first generation of CT scanners was built by Electric and Musical Industries (EMI) in 1971. The CT scanner consisted of a narrow X-ray beam (pencil beam) and a single detector. The X-ray tube moved linearly across the patient and subsequently rotated to acquire data at the next gantry angle. The process of data acquisition was lengthy, taking several minutes for a single CT slice. By the third generation of CT scanners, numerous detectors were placed on an arc across from the X-ray source. The X-ray beam in these scanners is wide (fan beam) and is covered by the entire area of detectors. At any single X-ray emission, the entire subject is in the field of view of the detectors, and therefore linear movement of the X-ray source is eliminated. The X-ray tube and detectors remain stationary, while the entire apparatus rotates about the patient, resulting in a drastic reduction in scan times. Most medical CT scanners in the world are of the third-generation type.

Mode of Operation. The process of CT image acquisition begins with the X-ray beam. X-ray photons are generated when high-energy electrons bombard a target material, such as tungsten, placed in the X-ray tube. At the atomic level, electrons interact with the atoms of the target material through two processes to generate X-rays. One mode of interaction is when the incoming electron knocks another electron from its orbital in the target atom. Another electron from within the atom fills the vacancy, and as a result, an X-ray photon is emitted. Another mode of interaction occurs when an incoming electron interacts with the nucleus of the target atom. The electron is scattered by the strong electric field in the nucleus of the target atom, and as a result, an X-ray photon is emitted. Both modes of interaction are very inefficient, resulting in considerable energy being dissipated as heat. Cooling methods need to be considered in the design of X-ray machines to prevent overheating in the X-ray tube. The resulting X-ray beam has a continuous energy spectrum, ranging from low-energy photons to the highest-energy photons, which corresponds with the X-ray tube potential. However, since low-energy photons increase the dose to the body and do not contribute to image quality, they are filtered from the X-ray beam.

Once a useful filtered X-ray beam is generated, the beam is directed toward the subject, while an image receptor (film or detector) is placed in the beam direction past the subject to collect the X-ray signal and provide an image of the subject. As X-rays interact with the body's tissues, the X-ray beam is attenuated by different degrees, depending on the density of the material. High-density materials, such as bone, attenuate the beam drastically and result in a brighter X-ray image. Low-density materials, such as lungs, cause minimal attenuation of the X-ray beam and appear dark on the X-ray image because most of the X-rays strike the detector.

Image Acquisition. In CT, X-ray images of the subject are taken from many angles and reconstructed into a three-dimensional image that provides an excellent view of the scanned subject. At each angle, X-ray detectors measure the X-ray beam intensities, which are characteristic of the attenuation coefficients of the material through which the X-ray beam passes. Generating an image from the acquired detector measurements involves determining the attenuation coefficients of each pixel within the image matrix and using mathematical algorithms to reconstruct the raw image data into cross-section CT image data.

Applications and Products

The power of computed tomography to provide detailed visual and quantitative information on the object being scanned has made it useful in many fields and suitable for numerous applications. Aside from disease diagnosis, CT is also commonly used as a real-time guide for surgeons to accurately locate their target within the human body. It is also used in the manufacturing industry for nondestructive evaluation of manufactured products and in security.

Disease Diagnosis. The most common use for CT is in radiological clinics, where it is used as an initial procedure to evaluate specific patients' complaints or symptoms, thereby providing information for a diagnosis, and to assess surgical or treatment options. Radiologists, medical professionals specialized in reading and analyzing patient CT data, look for foreign bodies such as stones, cancers, and fluid-filled cavities revealed by the images. Radiologists can also analyze CT images for the size and volume of body organs and detect abnormalities that suggest diseases and conditions involving changes in tissue density or size, such as pancreatitis, bowel disease, aneurysms, blood clots, infections, tuberculosis, narrowing of blood vessels, damaged organs, and osteoporosis. During the COVID-19 pandemic, CT systems played a role in diagnosing and managing coronavirus in some patients, especially those with complications or comorbidities. Later-generation CT systems provide results within seconds, unlike earlier CT systems.

In addition to disease diagnosis, CT has been used by private radiological clinics to provide full-body scans to symptom-free people who desire to obtain a CT image of their bodies to ascertain their health and to detect any conditions or abnormalities that might indicate a developing problem. However, the use of CT imaging for screenings in the absence of symptoms is controversial because the X-ray radiation used in CT has an associated risk of cancer. The dose of radiation deposited by a CT scan is between fifty and two hundred times the dose deposited by a conventional X-ray image. Although the association between CT imaging and cancer induction is not well established, its casual use remains an area of considerable debate.

In 2021, Philips Healthcare developed the Spectral CT 7500 CT system, which uses artificial intelligence (AI) to provide high-quality images that improve disease description and reduce rescans.

Guided Biopsy. A biopsy is a time-consuming, sometimes inaccurate, and invasive procedure. Traditionally, doctors obtained a biopsy (sample) of the tissue of interest by inserting a needle into a patient at the approximate location of the target tissue. Real-time CT imaging allows doctors to observe the location of the biopsy needle within the patient. Therefore, doctors can obtain a more accurate tissue biopsy in a relatively short time and without using invasive procedures.

CT Microscopy. The resolution of clinical CT scanners is limited by practical scan times for the patients and the size of the X-ray detectors used. In the case of small animals, higher resolutions can be obtained using smaller detectors and longer scan times. Micro-computed tomography (micro-CT), also known as X-ray Microscopy (XRM) or CT microscopy, has rapidly developed in the early twenty-first century to study disease pathology in animal models of human disease. Numerous disorders can be modeled in small animals, such as rats and mice, to better understand the disease biology or assess the efficacy of emerging treatments or drugs. Traditionally, animal studies involve killing the animal at a specific time and processing the tissue to view it under the microscope. However, the development of CT microscopy has allowed scientists and researchers to investigate disease pathology and treatment efficacy at very high resolutions while the animal is alive, reaching one-fifth of the resolution of a light microscope.

Nondestructive Evaluation. Computed tomography has gained wide use in numerous manufacturing industries for nondestructive evaluation of composite materials. Nondestructive evaluation is used to inspect specimens and ensure the integrity of manufactured products, either through sampling or through continuous evaluation of each product. CT requirements for nondestructive testing differ from those for medical imaging. For medical imaging, scan times must be short, radiation exposure must be minimal, and patient comfort throughout the procedure must be considered. For nondestructive evaluation, patient comfort and radiation exposure are not important issues. However, keeping scan times short is advantageous, especially for large-scale industries. Furthermore, the X-ray energy for scanning industrial samples can vary significantly from the energy used for patient imaging, since patient composition is primarily water and industrial samples can have a wide range of compositions and associated densities. Engineers have custom-designed CT scanners for specific materials, including plastics, metals, wood, fibers, glass, soil, concrete, rocks, and various composites. The capability of CT to provide excellent qualitative image data and accurate quantitative data on the density of the specimen has made it a powerful tool for nondestructive evaluation. CT is used in the aerospace industry to ensure the integrity of various airplane components and in the automotive industry to evaluate the structure of wheels and tires. In addition to industrial applications, CT is commonly used in research centers to further their imaging and analytical power.

Photon-counting detector (PCD) CT is a technology with advanced features such as reducing electronic noise, beam-hardening, and metal artifacts. It has high spatial resolution and the capability to image with and differentiate among various CT contrast materials.

Security. Beginning in the early 2020s, airports across the globe began using CT scanners at security. In 2019, the Transportation Security Administration (TSA) purchased 300 checkpoint scanners to test at several airports. The scanners were well-received, and in the following years, the TSA continued purchasing and implementing more CT scanners across the United States. Using CT scanners allows security operators to view bag contents from a three-dimensional, high-resolution perspective, helping them better detect prohibited or dangerous items. Operators can rotate digital images of suspicious items on their screens and inspect them from every angle without physically opening a passenger's bag.

Careers and Course Work

CT is an imaging modality that exists primarily in the healthcare sector and is available in every major hospital and most small hospitals in the developed world. CT is also widely distributed in hospitals in the developing world. Experts in operating and maintaining CT scanners who live in developed nations are often recruited to work in less-developed nations.

Those who wish to pursue careers in CT can take many paths. A degree in physics or engineering with a specialization in biomedical or electrical engineering can provide a solid grounding in the electronics and hardware of CT. Engineers can find work in hospitals or in industries that design and manufacture CT scanners. Degrees in mathematics and computer science provide the necessary background for working with image reconstruction algorithms and advancing software-related operation of CT scanners. Computer programmers can find work in the research and development sector of CT industries. Various institutions such as Jem College in California, Beckfield College in Kentucky, and Yale University in Connecticut offer courses in ultrasound technology, diagnostic sonography, and anatomy.

A graduate degree in medical physics provides theoretical and practical experience in medical imaging, from data acquisition to image reconstruction, interpretation, and troubleshooting. Medical physicists often work in imaging facilities and hospitals, where they usually supervise the personnel operating CT scanners. A degree in medicine specializing in radiology provides theoretical and practical experience in understanding human anatomy, pathology, and physiology; interpreting CT images; and diagnosing specific disease conditions. Technical colleges can provide education in the operation of CT scanners. Graduates with a technical degree in CT imaging often work as technologists in hospitals, responsible for patient scheduling and CT operations. CT is invaluable in the medical sector, and career prospects have been good. Aspirants can work as CT technologists and radiologic technologists.

Social Context and Future Prospects

The number of CT scanners and scans performed has risen considerably since the 1980s. By the early twenty-first century, thousands of scanners in the United States performed 70 to 90 million scans annually. The rapid and wide acceptance of CT scanners in healthcare institutions sparked controversy in the media and among healthcare practitioners regarding the radiation doses being delivered through the scans. The risk of cancer induction rises with increased exposure to radiation, and this risk must be carefully weighed against the benefits of a CT scan. Also, studies have revealed that some people are allergic to contrast agents. Most reactions are mild with an itch or a rash. In rare cases, it can lead to a life-threatening reaction. In 2020, the amount of contrast and radiation doses were reduced with advanced software in CT systems that can provide isotropic images. Research continues to develop more automated and easily accessible scanners. The issue of cancer induction is more alarming when CT procedures are performed on young children or infants. Some studies have found that the incidence of cancer in people exposed to CT scans in childhood or adolescence is up to 24 percent greater than in the unexposed population. Studies have recommended that CT scanning of children should not be performed using the same protocols as used for adults because children are generally more sensitive to radiation than adults.

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