CT scan

A computed tomography scan (CT scan), formerly known as a computed axial tomography scan (CAT scan), is a medical imaging technique used to get detailed pictures of what’s inside the body. People who do this work are called radiographers or radiology technologists. CT scanners have a special X-ray tube that spins around the body and a row of detectors in a gantry that measure how X-rays pass through different parts. These measurements are taken from many angles and then a computer uses special math, called tomographic reconstruction, to create clear pictures, like slices, of the inside of the body. CT scans can help doctors look at people even if they have metal in their bodies or devices like pacemakers, for whom magnetic resonance imaging (MRI) might not be safe (contraindicated).

Since it was created in the 1970s, CT scanning has become a very useful way to see inside things. It is mostly used to help doctors find out what is wrong when someone is sick, but it can also be used on objects that are not alive. In 1979, the Nobel Prize in Physiology or Medicine was given to British electrical engineer Godfrey Hounsfield and South African-American physicist Allan MacLeod Cormack for creating computer-assisted tomography.

Types

There are different kinds of CT scanners based on how they take pictures and work.

Sequential CT

Sequential CT, also called step-and-shoot CT, is a scanning method where the table moves step by step. The table stops at a spot, the X-ray tube turns, and one picture is taken. Then the table moves again for the next picture. Because the table stops between pictures, it takes longer to finish the scan.

Spiral CT

Spiral CT, also known as helical CT, spins the whole X-ray tube around the body part being scanned. This type is very common because it has been made for a long time and costs less. However, the equipment is heavy and can’t spin very fast. Some designs use two X-ray sources to get better pictures of moving parts like the heart.

Electron beam tomography

Main article: Electron beam computed tomography

Electron beam tomography (EBT) is a special kind of CT where only the path of electrons between the cathode and anode of the X-ray tube is spun using special deflection coils. This makes the pictures clearer for moving parts like the heart. But these scanners are not made very often because they cost more and are bigger.

Dual energy CT

Dual energy CT uses two different energy levels to create two sets of data. There are two main ways to do this:

• Dual source CT uses two X-ray tubes in one system. This lets it scan faster and get clearer pictures, especially for the heart. It works well for patients who find it hard to hold their breath.
• Single source with energy switching uses one tube that changes energy levels quickly.

CT perfusion imaging

Main article: CT Perfusion

CT perfusion imaging checks blood flow through blood vessels after a special contrast agent is injected. It can measure blood flow, time it takes for blood to move, and how much blood is in an organ. It is especially useful for checking the brain for problems like stroke earlier than regular CT scans.

PET CT

Main article: PET-CT

PET-CT combines a positron emission tomography (PET) scanner and an X-ray CT scanner in one machine. This lets doctors see both the shape of organs and how they are working. It helps in finding different types of cancers by showing where there is more activity in the body.

Medical use

Since the 1970s, CT scans have become a key tool in medical imaging. They help doctors see inside the body more clearly than regular X-rays or ultrasound. CT scans are now used not just to find problems but also for checking on people who might be at risk for certain diseases, like those with a higher chance of colon cancer or heart disease.

CT scans are used for many parts of the body. For the head, they help find strokes, tumors, bleeding, and injuries to bones. For the neck, they help look at lumps and check the thyroid gland. In the lungs, CT scans can spot changes that regular X-rays might miss. They can also show blood vessels throughout the body, helping to find blockages or clots. For the heart, CT scans help see the arteries and check for heart disease. In the belly and pelvis, they help diagnose conditions like Crohn’s disease, bleeding, and cancer. For bones and joints, CT scans can show detailed images of fractures and other injuries.

Other uses

Industrial use

Industrial CT scanning uses X-ray equipment to create 3D images of objects, both inside and outside. This helps industries check for problems in parts, measure sizes, and study how things are put together. It is also used to protect valuable museum items by looking at them without causing damage.

Aviation security

CT scanning is used in places like airports to help find dangerous materials such as explosives. This technology started at Shannon Airport in March 2022 and has allowed more liquids to be carried. Other airports, like Heathrow, plan to use it too.

Geological use

X-ray CT helps scientists study rocks and soil by showing what is inside drill samples. Different materials appear differently in the images, making it easier to understand the geology.

Paleontological use

CT scanning lets scientists study fossils without harming them. This way, they can see tiny details and examine fragile parts that would otherwise be too hard to study.

Cultural heritage use

CT scans help protect important historical objects. They let experts see inside items like old scrolls or mechanical devices without touching them. This has been useful for studying things like the Antikythera mechanism and hidden texts in ancient scrolls.

Microorganism research

Scientists use CT scanning to study tiny organisms, like fungi, that can damage wood. This helps them understand how these organisms move and grow.

Timber sawmill

Sawmills use CT scanners to find defects in wood, like knots, which helps them make better use of the timber. While this technology is very useful, it costs a lot to start using.

Interpretation of results

The result of a CT scan is a three-dimensional image made of tiny blocks called voxels. These images can be shown in different ways, such as thin slices, projections, or full 3D views.

When we look at these images, we see shades of gray that tell us about the different tissues inside the body. For example, water shows up as a certain shade, while air and bone show up as very dark or very light shades. To make these images easier to read, doctors use a technique called "windowing" to focus on the important details.

Modern CT scans can also create images in different planes, helping doctors see how different parts of the body connect. There are ways to reduce unwanted marks or noise in the images, making them clearer for doctors to read.

Advantages

CT scanning offers several benefits compared to traditional two-dimensional medical photography. It removes overlapping images from areas outside the focus, making it easier to see the specific part being examined. CT scans also provide clearer and more detailed pictures, allowing doctors to notice very small differences in tissues.

Because of this clarity, CT scanning can be used for new kinds of tests. For instance, CT angiography can show blood vessels without needing invasive procedures. It can also perform a virtual colonoscopy, which is more accurate and comfortable for patients than traditional methods. This virtual scan is better at finding tumors and uses less radiation than older techniques.

Adverse effects

Cancer

Main article: Radiation-induced cancer

The radiation used in CT scans can damage body cells, including DNA molecules, which can lead to radiation-induced cancer. The amount of radiation from CT scans can be higher than regular X-rays, but sometimes it is similar. Some studies show that CT scan radiation may cause a small increase in cancer risk, especially for people who have many scans over a short time. However, there is ongoing debate about how big this risk really is.

Contrast reactions

Further information: Iodinated contrast § Adverse effects

In the United States, many CT scans use special liquid called contrast that is put into the body. Most people have mild reactions, like feeling sick or a rash. Severe reactions are rare but can be serious. People with certain health problems, like kidney disease, need special care before having a contrast CT scan.

Scan dose

The amount of radiation used in CT scans can vary a lot. A normal X-ray uses less radiation than a CT scan, but CT scans give more detailed pictures. Doctors use lead to protect themselves and patients from extra radiation.

Radiation dose units

The radiation dose from a CT scan is measured in special units. These units help doctors understand how much radiation someone is getting and what the possible risks might be.

Effects of radiation

Radiation can cause two main types of health effects. One type can harm cells directly, and the other type can increase the chance of cancer later in life. The risk of cancer from a single CT scan is very small.

Excess doses

Sometimes, mistakes can happen that cause people to get too much radiation during a CT scan. This is why doctors and technicians follow strict rules to make sure scans are safe. Even though mistakes are rare, they show why quality checks are important.

Procedure

A CT scan works differently depending on what part of the body is being checked. The person lies on a table, and the table is moved into place. Sometimes a special liquid called contrast is used to help see better. The machine then takes pictures, and a computer puts them together to create detailed images of the inside of the body.

Preparation

Before a CT scan, patients might need to do a few things. They usually sign a paper saying they understand the procedure, take off any metal objects or jewelry, and sometimes change into a hospital gown. The doctor may also check certain things in the blood to make sure it’s safe to use contrast.

Mechanism

Main article: Operation of computed tomography

A CT scan, also known as a computed tomography scan, uses special equipment to take detailed pictures of the inside of the body. It works by rotating an X-ray generator around the person while X-ray detectors on the other side measure how much the X-rays are weakened by different tissues. This creates a set of images that show slices of the body.

These images are made up of tiny squares called pixels, which show how dense different parts of the body are. The pictures can also be changed to show different views and even three-dimensional images, helping doctors see and understand what is happening inside the body.

Contrast

Main article: Contrast CT

Sometimes, a special liquid called contrast media is used during a CT scan. This liquid helps certain parts of the body, like blood vessels, show up more clearly in the pictures. Doctors can take pictures with and without this liquid to get more information about how the body is working.

History

Main article: History of computed tomography

The story of CT scans, also known as computed tomography, begins with important math ideas from 1917. In 1963, a scientist named William H. Oldendorf got a special permission for his idea to look inside objects using energy. The first machine that could really be used in hospitals was made by Godfrey Hounsfield between 1967 and 1972.

At first, these machines were called EMI scanners because they were developed by a company named EMI, famous for its music records. The word "tomography" comes from old words meaning "to write slices," because these scans show detailed pictures of thin sections of the body. Today, doctors just call them CT scans.

Society and culture

The Image Gently Campaign was created to help protect children during medical imaging. It encourages doctors and medical workers to use the safest possible methods and the least amount of radiation when doing imaging tests on children. Many medical groups around the world support this campaign.

There has been a big increase in the number of CT scans done around the world, including in the United States, Europe, and Asia. In some places, like Calgary, Canada, about 12% of people who go to the emergency room with urgent problems get a CT scan. This number can vary depending on the doctor. CT scans are now used more often for checking adults for diseases and for helping diagnose children, especially because the scans can be done very quickly. However, some CT scans may not be needed, and this can be a concern because of the radiation used and the costs involved.

Manufacturers

Some big companies that make CT scan machines come from different countries. These include:

• Canon Medical Systems Corporation from Japan
• Fujifilm Healthcare from Japan
• GE HealthCare from the United States
• Neusoft Medical Systems from China
• Philips from the Netherlands
• Siemens Healthineers from Germany
• United Imaging from China

Research

Scientists are working on a new kind of CT scan called photon-counting computed tomography. Most CT scans today measure X-rays in a way that can sometimes get noisy or less accurate. Photon-counting detectors count the actual number of X-ray particles, which could make images clearer and need less radiation. This new method is just starting to be tested and might help doctors see inside the body better while using less radiation.