CT Scan (Computed Tomography Scan)

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Introduction

CT scan (or Computed tomography scan) is a computerized x-ray imaging technique in which a narrow beam of x-rays is focused on a person and rotated quickly around the body that produces signals that are processed by the computer to create cross-sectional tomographic images (slices). CT scans are also called Computed Axial Tomography (CAT) scans. The series of X-rays and a computer is used to generate 3 dimensional (3D) images of soft tissues and bones. The scanner uses a thin beam of radiation to scan from different angles of the body. It then provides the scanned images to a computer that creates separate cross-sections or slices, of the internal body.

Indications of CT Scan

CT scan is a crucial diagnostic procedure specially performed for diagnosing different illnesses and analyzing injuries. Some of the indications for CT scans are as follows:

To diagnose muscle disorders, bone fractures/deformities, and infections
To spot the location of tumors, lesions including cancer
To examine the internal structures of the body and blood vessels
To determine the extent of internal injuries and bleeding/hematoma
To guide various procedures like biopsies and surgeries
To evaluate the effectiveness of treatments for certain medical/surgical conditions, including heart disease and cancer

Advantages of CT Scan

CT scanning procedure is painless, noninvasive, and has high accuracy.
It can simultaneously produce images of blood vessels, soft tissue, and bone.
It provides very detailed images than a conventional x-ray or Ultra sonogram.
The imaging procedure is fast and simple. So it is very effective in emergency cases, to reveal internal bleeding and injuries thus helpful in saving lives.
A CT scan is affordable for a variety of clinical issues.
It is less sensitive to the movement of the patient than MRI.
It can be done in patients with any kind of implanted medical device.
It provides real-time imaging that guides needle biopsies and aspirations involving the lungs, liver, abdomen, pelvis, bones, etc.
It helps in diagnosis and thus decreases the need for diagnostic/exploratory surgery and biopsies.
Radiation is not retained in a patient’s body after a CT scan.

Types of CT Scan

Spiral CT

Spiral CT or helical CT is the scanning method in which the entire X-ray tube revolves around the middle axis of the body area being scanned.
The drawback of this type of CT is the weight and passivity of the equipment which limits the spinning speed of the equipment.
Some designs of machines use two X-ray sources and a detector range neutralized by an angle, as a method to improve temporal resolution.

Electron beam tomography (EBT)

EBT is a particular form of CT scan in which a large X-ray tube is made so that only the pathway of the electrons, traveling between the electrodes (cathode and anode) of the X-ray tube, is rotated using deflection coils.
Its major advantage is that sweep speeds can be faster, allowing for clear imaging of mobile structures, such as the heart and blood.

Dual-source CT

Two X-ray tube detector system is used in dual-source CT, whereas conventional CT uses only one tube. It is a modern and sophisticated type of scan.
This scanner permits quick scanning with a higher temporal resolution by obtaining a complete CT slice in just half rotation.
Fast imaging also reduces blurring due to motion, and high heart rates and is beneficial for ill patients who have difficulty holding their breath or who can’t take heart-rate-decreasing medication.

CT perfusion imaging

CT perfusion imaging is a specific form of CT to assess the flow of blood, blood circulates time, and blood volume of an organ, by injecting a contrast agent.
This is useful to detect poor brain perfusion and stroke in the brain better than a conventional spiral CT scan.

Contraindications of CT Scan

Pregnancy: can be done if the benefits of the test overweigh the potential risks
Known history of iodine allergy
Hypothyroidism or an enlarged thyroid gland
Planned radioiodine treatment of thyroid cancer
Gastrointestinal perforation (for oral or rectal dye/solutions)

Procedure of CT Scan

Before the Procedure

Patients need to wear a hospital gown and remove all metal objects including jewelry from the body.
Contrast agent: Contrast agents help soft tissue to appear clearly by blocking the X-rays. Thus, they appear white on the scan, highlighting organs, blood vessels, liver, gallbladder, or other structures.
- Contrast materials are usually made of iodine or barium sulfate and can be given via oral, intravenous route, or as Enema.
- Oral contrast material enhances scans of the digestive tract and enemas are given to visualize intestines.
The patient may need to withhold oral feedings a few hours before the scan requiring a contrast agent.
Some patients are advised to take plenty of fluids before and after the procedure to remove the contrast material from the body via the kidney.
If a patient has diabetes and is taking metformin, it must be held 24 hours before and 48 hours after the procedure or as prescribed by the doctor.
Children who cannot stay calm may require sedatives during the procedure.

During the Procedure

The patient must lie flat on the scanner table. Straps and foam pillows may be used to position and help keep the body still.
The table is slid into the cylindrical hole of the machine.
The patient will be asked to remain still and hold his breath frequently during the procedure.
The patient can communicate with the technologist in case of difficulties like breathing problems, palpitation, sweating, etc.
Many images are taken depending on the condition or indication and part of the body.
The equipment makes clicks and buzzes while taking the images which is normal.
It may take a few minutes to half an hour or more according to the type of scan.

After the Procedure

The person may need to stay in the diagnostic center for a brief period if s/he has received the iodinated contrast material or sedation during the procedure.
A follow-up appointment is done with a consultant physician or surgeon regarding the report of the scan and further management of the illness.

Precautions of CT Scan

Allergic reactions to contrast dyes: Usually the reaction is mild with itchiness or rashes but in a few cases, the dye may trigger a life-threatening reaction. A history of allergies to medications, seafood, or iodine should be explored before the procedure.
Rarely contrast materials can lead to kidney problems. So the history of any kidney problems should be explored before the CT scan.

Interpretation

The presentation of tissues on a CT scan is expressed in terms of ‘density.’

Darker structures like air, fat, and cerebrospinal fluid are black (hypodense or low density).
Brighter structures like bone, calcification, and contrast are white (hyperdense or high density).
Brain tissues are grey (isodense).
Blood can be confusing because its appearance differs according to its duration.
- Acute blood is hyperdense
- Subacute blood is isodense
- Chronic blood (after many weeks old) is hypodense

Summary

A computerized tomography (CT) scan merges a series of X-ray images taken from various angles around the body and computer processing is done to create slices (cross-sectional images) of the bones, soft tissues as well as blood vessels of the body.
A CT scan is very useful in urgent cases for those who may have internal injuries from accidents or other types of trauma.
A CT scan can visualize almost whole part of the body and is utilized to diagnose diseases to plan various medical, surgical, and/or radiation treatments.
There are a few risks associated with CT scans like radiation exposure and allergies to contrast agents.

References

Patel, P. R., & Jesus, D.O. (2022). CT Scan. StatPearls Publishing. https://pubmed.ncbi.nlm.nih.gov/33620865/
Goldman, L.W. (2007, Sep). Principles of CT and CT Technology. Journal of Nuclear Medicine Technology, 35 (3) 115-128; DOI: https://doi.org/10.2967/jnmt.107.042978
Zainab T. et al (2020). A critical review of medical imaging techniques (CT and PET scans) in the medical field. IOP Conf. Ser.: Mater. Sci. Eng. 870 012043 DOI 10.1088/1757-899X/870/1/012043
National Institute of Biomedical Imaging and Bioengineering (2022). Computed-Tomography(CT) Scan. Retrieved on 2023, March 11from https://www.nibib.nih.gov/science-education/science-topics/computed-tomography-ct
Cleveland Clinic. CT Scan. Retrieved on 2023, March 11 from https://my.clevelandclinic.org/health/diagnostics/4808-ct-computed-tomography-scan
NHS.UK. (2021). CT Scan. Retrieved on 2023, March 08 from https://www.nhs.uk/conditions/ct-scan/
Weatherspoon, D. (2022). CT Scan. Healthline. Retrieved on 2023, March 08 from https://www.healthline.com/health/ct-scan