CT scan

A computed tomography scan, commonly known as a CT scan, is a sophisticated medical imaging technique. It allows doctors to obtain highly detailed internal images of the human body. Formerly called a computed axial tomography scan or CAT scan, this technology provides a way to see inside without surgery. It is particularly important for patients with metallic implants or pacemakers. For these individuals, magnetic resonance imaging, or MRI, is contraindicated, meaning it cannot be safely used. CT scans are versatile tools used for medical diagnosis and can even image non-living objects.

The mechanism of a CT scan relies on the interaction between X-rays and body tissues. Inside a large ring called a gantry, an X-ray tube rotates around the patient. Opposite the tube, a row of detectors measures X-ray attenuation. Attenuation is the process where different tissues, like bone or muscle, absorb different amounts of X-ray energy. The scanner takes many measurements from various angles as the tube spins. A computer then processes these measurements using tomographic reconstruction algorithms. This mathematical process produces tomographic images, which are cross-sectional "slices" of the body.

Different scanning methods exist based on how the image data is acquired. Sequential CT, also called step-and-shoot CT, involves a table that moves in discrete steps. The table moves to a location, stops, and then the X-ray tube rotates to take a single slice. This process repeats, but the stopping and starting increases the total scanning time. In contrast, spiral CT, or helical CT, is much more common in modern medicine. In this method, the entire X-ray tube spins continuously around the central axis. While spiral CT is dominant due to lower costs, the bulk and inertia of the equipment can limit its rotation speed.

Advanced versions of CT technology offer even more specific details. Electron beam tomography, or EBT, uses deflection coils to spin only the path of electrons. This allows for much faster sweep speeds, which reduces blur in moving structures like the heart. Dual energy CT, or spectral CT, uses two different energy levels to create two sets of data. One advanced version is the dual source CT, which uses two X-ray tube detector systems mounted at a 90-degree angle. This allows the machine to acquire a full slice in only half a rotation, which helps reduce motion blurring in patients with high heart rates.

The history of this technology is tied to significant scientific breakthroughs. CT scanning was developed during the 1970s. This era saw the rise of computer-assisted tomography as a vital medical tool. In 1979, the Nobel Prize in Physiology or Medicine was awarded for these developments. The prize was shared by British electrical engineer Godfrey Hounsfield and South African-American physicist Allan MacLeod Cormack. Their work transformed how we visualize the internal structures of living organisms.

Doctors use CT scans to investigate many different parts of the body. In the head, scans can detect infarction, which is a stroke, or identify tumors and hemorrhages. In the lungs, high resolution CT produces detailed samples of the lung parenchyma, or tissue. This is helpful for evaluating chronic conditions like emphysema or fibrosis. For the heart, coronary CT angiography uses an iodine-based contrast agent to visualize arteries. This helps doctors assess coronary artery disease or look for calcium deposits.

Some specialized scans combine different types of imaging for better results. CT perfusion imaging assesses blood flow through vessels by injecting a contrast agent. This can calculate blood transit time and organ blood volume. Another example is the PET-CT, a hybrid modality. It combines a positron emission tomography scanner with a CT scanner in one gantry. This allows the functional data from the PET scan to be precisely aligned with the anatomical data from the CT scan. This combination is particularly helpful in detecting various types of cancers.

Medical use of CT has grown dramatically over the last twenty years. In the United States, an estimated 72 million scans were performed in 2007. By 2015, that number had increased to more than 80 million scans. While CT is used for screening, such as CT colonography for colon cancer risk, many professional organizations advise caution. They express concern regarding the radiation dose applied during frequent scans. However, the ability to see through the body with such precision remains a cornerstone of modern medicine.