What Is A CT Scan?

  • Kitty ReevesBachelor of Science - BSc, Physics, University of Birmingham
  • Saba Amber BSc, Manchester Metropolitan University, UK

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Introduction

Computed Tomography (CT) scans or Computed Axial Tomography (CAT) scans, represent a remarkable advancement in medical imaging technology. This tool results in detailed cross-sectional images of the body’s internal structures, aiding in diagnosis and offering healthcare professionals a comprehensive view going beyond the traditional x-ray methods. By utilising a combination of X-rays and complex computer processing, CT scans excel in visualising bones, organs and soft tissue.1 The versatility of the areas able to be imaged makes them a valuable asset in diagnosing a wide range of medical conditions, guiding treatments and facilitating swift assessments in emergency situations. However, along with these benefits, CT scans come with exposure to ionising radiation and excessive energy consumption, reducing tier wider availability and cost-effectiveness.2 As the technologies continue to develop, CT scans remain at the forefront of the radiological imaging industry. 

Historical background

The technique of CT scanning was first invented by Sir Geoffrey Hounsfield, who subsequently won the Nobel Prize for medicine.3 He began with the idea of using X-rays to create a cross-sectional image of the body and, along with physicist Allan Cormack, developed the mathematics to go with it.

Initially, there was only one X-ray source, an X-ray tube, and one detector placed on either side of the body. One set of results would be taken, where X-rays would be fired out the tube, through the body and out the other side to the detector, then the equipment would be rotated around the body ready for another set of results to be taken for the same cross-section.1 This process repeated until there were enough results to piece together a full image, often taking hours and requiring the patient to stay very still for a clean image. Thus, the final images were often blurry and difficult to interpret, containing lots of artefacts (issues obstructing the desired view of the images caused by, for example, motion or scattering of x-rays).4

As the technology developed, through to the current 5th generation scanners, the most prevalent changes were from the X-ray sources and detectors.5 The beam of X-rays is now more of a cone, about 40°-60° wide, which automatically rotates around the patient, either for a specific cross-section or moving down the patient, providing a spiral, or helical, array of images. Artefacts are also now able to be automatically removed via calibration of the equipment. However, as the technology becomes more advanced, naturally, the increasing amount of resources needed makes it more expensive, limiting the availability of CT scanners to specialised hospitals and clinics. 

Principles of CT scanning

CT scanners send X-rays through the body from specific angles, which are received with different energies and in different directions (scattering). These detections are compared to the original X-rays emitted and, by being mathematically processed by an attached computer, produce a final image of the ‘slice’, approximately 1-10 mm thick, of the body they travelled through.1 This is possible due to a material constant called ‘attenuation’, which means the percentage of energy that will be absorbed when X-rays travel through it. It depends on the material and the distance the X-rays travel through that material, so allows the computer to calculate what may have been in the X-rays paths as they travelled through your body from the attenuation values of materials we know such as blood, muscle and bone. If enough angles of one cross-section of the body are collected, an algorithm can determine the material present at each point of the slice.

To reconstruct the image, there are two possible techniques: analytical and iterative reconstruction.6 Analytical reconstruction uses a filtered back projection, which maps how the different materials could affect the X-rays. By doing this for all X-rays, the computer can determine exactly when a material starts and stops in the body. This is a quicker method and is used more commonly in emergency situations. Iterative reconstruction, however, attempts to predict the image, uses it to calculate what should have been detected and finds the difference.7 The difference is added to the image and the process repeats until there is no longer any difference. This leads to a lower amount of noise in the image (a clearer image), thus requiring less X-rays to produce an adequate image. The different methods are used for different scenarios as they produce two slightly different images. 

Components of a CT scanner

A CT scanner features a large ring of equipment which rotates around you and will be lying on your back on the patient table.1 The X-rays come from an X-ray tube, which converts electrical energy into X-rays by firing electrons at phosphorus, which then releases the rays directed at you.8 Unfortunately, it only releases about 1% of the energy from the electrons as X-rays, and mostly produces heat, meaning that large amounts of energy are needed to emit enough X-rays for one scan. 

There is a collimator and a filter between you and the source, to protect you from excess radiation, and the detectors are on the opposite side of the ring. There are between 8 and 64 rows of detectors, each containing up to 2000 individual detectors, creating precise and accurate images.8  As there are so many, it is important the detectors are small in size and cheap to make. The incoming X-rays are recorded and sent to the computer as an electrical signal for it to interpret. 

Procedure and patient preparation

Before coming in for a scan, your doctor may ask you to stop eating and only drink clear liquids for a few hours before, ensuring an unobstructed image. Some scans require a contrast agent to form a clear picture, which may be ingested before the scan as a drink, injected into the blood or inserted into your bottom (enema).1 If you are claustrophobic, your doctor may give you a sedative to help you keep calm during the scan, and almost always your clothing will be removed and replaced with a hospital gown. 

The scans usually only take 10-20 minutes. The doctor operating the machine (radiographer) will be in the next room with a computer reviewing the detections but will be able to communicate with you over an intercom. If you use a contrast agent, you may be required to wait for up to an hour afterwards to ensure you are not having an adverse reaction to it, otherwise, you are safe to leave and carry on as normal.9 

Types of scans

There are two main types of CT scans, each used for different scenarios.10 Specialised scans, such as a virtual colonoscopy or cardiac CT, look at a particular structure of something stationary in the body and can be used for any area. Functional CT scans measure a change in metabolism, for example, the movement of urine through the kidneys, and usually only scan the torso or head.11 The use of contrast, as well as the type of scan, is particular to what each scan is referencing. 

The most common reason for a CT scan is to assess chronic back pain or a spinal injury but is also used as a diagnosis tool for conditions such as cancer, strokes and organ damage.1 It can also be used as a screening tool to spot tumours or lesions for early detection or preventative measures or to assess, plan or manage any treatments. Dual energy CT scans refer to using two types of X-ray waves, to thoroughly assess the cross-section. 

Advantages and limitations

CT scans have a range of advantages making them a desirable tool for any hospital. They consistently produce clear and detailed images in a relatively short amount of time compared to other imaging techniques such as positron emission tomography (PET) scans, enabling them to be used in critical situations. Further, they can be used to produce qualitative and quantitative results,12 so can be used to compare and monitor variables or characteristics over time. 

However, due to the nature of the scan, there is some mild radiation exposure.13 One scan has a 1/2000 chance that you will develop cancer later in life, the equivalent of between a few months and a few years of background radiation exposure, which rises after multiple scans. Due to this, pregnant women should not have a CT scan unless it is an emergency to protect the foetus. The procedure is expensive and requires very specific equipment not always widely available. Artefacts, although mostly removable, can cause unclear or misleading images, leading to unusable results. This particularly affects those who have difficulty keeping still, such as young children or claustrophobics. 

Technological advances

Artificial Intelligence (AI) is currently being developed to aid in the iterative reconstruction of CT and MRI images, including calculating the correct contrast in images.14 Further development includes implementing AI-automated methods into radiology imaging to analyse changes in scans over time. Other developments include multiple x-ray tubes being used within one CT scanner, leading to a quicker scan time and less mechanical motion and reducing artefacts and mechanical motion.15 

Summary

This article introduces CT scans as a leading form of medical imaging. It used the difference in emitted and received X-rays travelling through all angles of the body to reconstruct a cross-sectional image. The preparation and procedure vary from scan to scan but commonly involves drinking clear liquids, removal of clothing and possibly the insertion of contrast to aid the image clarity. CT scans are valuable for functions including diagnoses, treatment monitoring and screening, with different forms of scan catering to different scenarios. The scans are quick and non-invasive; however, the cost and mild radiation exposure often limit their wider accessibility. Current advancements include the incorporation of AI for image construction and the addition of multiple x-ray tubes.

References

  1. Patel PR, De Jesus O. Ct scan. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Nov 24]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK567796/
  2. Power SP, Moloney F, Twomey M, James K, O’Connor OJ, Maher MM. Computed tomography and patient risk: Facts, perceptions and uncertainties. World J Radiol [Internet]. 2016 Dec 28 [cited 2023 Nov 24];8(12):902–15. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5183924/
  3. Sir godfrey hounsfield - the university of nottingham [Internet]. [cited 2023 Nov 24]. Available from: https://www.nottingham.ac.uk/microct/about-us/sir-godfrey-hounsfield.aspx
  4. Barrett JF, Keat N. Artifacts in ct: recognition and avoidance. RadioGraphics [Internet]. 2004 Nov [cited 2023 Nov 24];24(6):1679–91. Available from: http://pubs.rsna.org/doi/10.1148/rg.246045065
  5. Schulz RA, Stein JA, Pelc NJ. How CT happened: the early development of medical computed tomography. J Med Imaging (Bellingham) [Internet]. 2021 Sep [cited 2023 Nov 24];8(5):052110. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8555965/
  6. Image reconstruction techniques [Internet]. [cited 2023 Nov 24]. Available from: https://www.imagewisely.org/Imaging-Modalities/Computed-Tomography/Image-Reconstruction-Techniques
  7. Stiller W. Basics of iterative reconstruction methods in computed tomography: A vendor-independent overview. Eur J Radiol. 2018 Dec;109:147–54. Available from: https://www.sciencedirect.com/science/article/abs/pii/S0720048X18303747
  8. Flohr T. CT Systems. Curr Radiol Rep [Internet]. 2013 Mar 1 [cited 2023 Nov 24];1(1):52–63. Available from: https://doi.org/10.1007/s40134-012-0005-5
  9. Baerlocher MO, Asch M, Myers A. Allergic-type reactions to radiographic contrast media. CMAJ : Canadian Medical Association Journal [Internet]. 2010 Sep 9 [cited 2023 Nov 24];182(12):1328. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2934800/
  10. Garvey CJ, Hanlon R. Computed tomography in clinical practice. BMJ [Internet]. 2002 May 4 [cited 2023 Nov 24];324(7345):1077–80. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1123029/
  11. Beek EJ van, Hoffman EA. Functional imaging: ct and mri. Clinics in chest medicine [Internet]. 2008 Mar [cited 2023 Nov 24];29(1):195. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2435287/
  12. Cheng Z, Qin L, Cao Q, Dai J, Pan A, Yang W, et al. Quantitative computed tomography of the coronavirus disease 2019 (COVID-19) pneumonia. Radiology of Infectious Diseases (Beijing, China) [Internet]. 2020 Jun [cited 2023 Nov 24];7(2):55. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7186132/
  13. Huang R, Liu X, He L, Zhou PK. Radiation exposure associated with computed tomography in childhood and the subsequent risk of cancer: a meta-analysis of cohort studies. Dose Response [Internet]. 2020 May 5 [cited 2023 Nov 24];18(2):1559325820923828. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7218306/
  14. Paudyal R, Shah AD, Akin O, Do RKG, Konar AS, Hatzoglou V, et al. Artificial intelligence in ct and mr imaging for oncological applications. Cancers (Basel) [Internet]. 2023 Apr 30 [cited 2023 Nov 24];15(9):2573. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10177423/
  15. Hsieh J, Flohr T. Computed tomography recent history and future perspectives. Journal of Medical Imaging [Internet]. 2021 Sep [cited 2023 Nov 24];8(5). Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8356941/

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This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Kitty Reeves

Bachelor of Science - BSc, Physics, University of Birmingham

Kitty Reeves is a recent physics graduate from the university of Birmingham, where she focussed on medical and biological physics. She has previous experience working on machine learning to aid in MRI brain scans and has taught physics and maths for several years.

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