Even though conventional X-rays are commonly used and helpful in many applications, they can only produce a two-dimensional image of a three-dimensional structure (in this case, the human body). So, one downside of conventional X-rays is that we can’t use them to obtain highly detailed images and in-depth information. Also, another problem is that the contrast between soft tissues is limited.
However, with today’s technology, X-ray computed tomography, also known as a CT, solves both of these issues.
What is computed tomography (CT)?
Computed tomography (CT) (also known as computerised tomography) was the first in its field as a non-invasive radiological method that allows medical professionals to view the inside of the whole human body without needing an adjoining structure. CT scans have a more significant advantage over conventional X-rays: When scanning the human body from just one side with a conventional X-ray, soft tissues and organs may be hidden behind the denser bones. However, with CT scanners, medical professionals can take multiple X-rays from different directions to gather much more detailed and useful information.
To achieve different goals, manufacturers modify the structure of CT scanners. However, in general, all CT scanners are similar to each other and consist of a scanning gantry, an X-ray tube, a computer system, and a viewing console for the radiologist.
An X-ray tube is an enclosed electron tube that creates X-rays by accelerating electrons and directing them to impact an anode plate.
A CT Scanner
CT scan procedure
In CT scanning, multiple arrays of X-rays pass through the patient’s body in several directions to produce a scan that gives information about the human body’s interior and depth. These rays pass through a specific region called a slice, and an array of detectors on the other side of the patient’s body registers these X-rays.
There are two different mechanisms via which CT scanners achieve the wanted results.
- The cone beam CT scanner is relatively new compared to the other ones. Cone beam CT scanners use a flat-surfaced detector ray and usually rotate around the patient in less than 240°.
- The fan beam CT scanner is an older mechanism and is found more commonly in today’s medical physics. For this reason, we focus on fan beam CT scanners in this explanation.
Diagram showing the mechanism of a fan-beam CT scanner
The above graphic shows how a CT scanner works to achieve scans from multiple angles. Let’s look at how a CT scanner works step by step.
- The patient lies down on a bed placed in the middle of the CT scanner.
- The X-ray tube and the detectors start rotating together around the patient’s body.
- The X-ray tube produces X-ray beams that pass through the slice and reach the detectors on the other side of the patient.
- The array of detectors registers the X-rays. The attenuation (absorption) of the X-ray beam as it penetrates the human body is what creates contrast in CT imaging. An energy range of 20 to 150 kiloelectronvolts is used for the diagnostic imaging technique.
- As the mechanism rotates around the patient, several images are taken back to back.
- The computer in use processes the image according to the absorption of the X-rays to form a detailed image.
The average linear attenuation coefficient
The contrast of the image depends on the intensity of the X-ray beam that reaches the detector, which then depends on how many rays are absorbed on their way to the detector. This so-called absorption is related to the average linear attenuation coefficient. This can be mathematically expressed as follows:
\[I = I_0 \cdot e^{-\mu x}\]
I0 is the initial intensity of the X-ray beam, µ is the linear attenuation coefficient, and I is the intensity of the X-ray beam after passing through a material with a depth of x centimetres.
Usually, when X-rays are used in diagnostics, muscle has a linear attenuation coefficient of around 0.180m-1. If we re-arrange the equation and put in the linear attenuation coefficient for muscle for each centimetre, we get:
\[\frac{I}{I_0} = e^{-\mu x} = e^{-0.180 \cdot 1} = 0.835\]
If you do the same calculation for blood and bone, you will get 0.837 for blood and 0.619 for bone. As you can see, the results for blood and muscle don’t differ much. So, to have better contrast between blood and muscle, a contrast medium with iodine can be introduced into the patient’s body to point out the blood vessels in the final result.
Check out our explanation on the Absorption of X-Rays.
There is a different type of CT scanner that is similar to the fan beam mechanism called the ring. In this mechanism, the detectors do not rotate in sync with the X-ray tubes but are stationary. The X-ray tube rotates 360° around the patient while the ring-shaped stationary detectors behind the X-ray tube scan the whole process.
Diagram showing the mechanism of a ring CT scanner
Advantages and disadvantages of CT scanners
CT scanners have many advantages in medical physics. One advantage is the CT scanner’s ability to show very small details with density differences of less than one percent.
But despite the advantages, there are some disadvantages to CT scanners that need to be kept in mind. Let’s look at the table below to compare the pros and cons of CT scanners.
Advantages | Disadvantages |
Produces highly-detailed 3D images. | |
Movement of the patient isn’t too much of a concern. | More time consuming than conventional X-rays. |
Removes the superimposition of imagery of unwanted structures. | Required equipment is relatively expensive. |
More affordable and quicker than MRI scanning. | Has the risk of ionising radiation and needs contrast agents. |
Applications of CT scanners
CT scans are used for a variety of purposes in medical physics. Some of these purposes include:
- Locating infections in the human body.
- Diagnosing vascular diseases, problems in the spine, and injuries that cause harm to bones.
- Determining internal injuries caused by trauma or accidents and locating different types of cancer.
- Assisting in biopsies and other processes in medical physics.
CT Scanners - Key takeaways
- Computed tomography (CT) was the first in its field as a non-invasive radiological method that allows medical professionals to view the inside of the human body without needing an adjoining structure.
- An X-ray tube is an enclosed electron tube that creates X-rays by accelerating electrons and directing them to impact an anode plate.
The contrast of a CT scan result depends on the average linear attenuation coefficient.
Generally, there are two types of CT scanners: cone beam CT scanners and fan beam CT scanners.
CT scanners have many advantages in medical physics. They can produce highly detailed, three-dimensional images.
CT scanners are used to diagnose problems in the spine, determine and locate different types of cancer, determine internal injuries caused by trauma or accidents, etc.
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