Computed Tomography and Radiation Side-effects

Modern medical use of computerised tomography (CT) is increasing. On this writing we explore its side-effects associated with CT scanning. It will surely help the Indonesian doctors as well as doctors around the world.

By its nature, CT involves larger radiation doses than the more common, conventional x-ray imaging procedures. Although, when a CT scan is justified by its medical need, the associated risk is small relative to the diagnostic information obtained, however the risk is still enough to think how to get rid of that.

Introduction

Computed Tomography (CT) is a high-tech imaging methodology that gives doctors the ability to look at the internal anatomy of the body in a great detail. In its simple definition CT is considered 3 dimensional X-ray. CT scans work by producing three-dimensional images of the inside of the human body by using X-rays taken from different viewpoints surrounding the human body.

3d reconstruction of the brain

Figure 1. 3D reconstruction of the brain and eyeballs from CT scanned DICOM images (courtesy [12])

Despite its interesting capability to differentiate normal structures from abnormal structures such as tumours, aneurysms, infection etc., CT is well criticized for being a radiation dose intensive modality. Excluding the possibility of radiation overdose that may occur when a CT scan machine is not calibrated correctly [13], the ionizing radiation in the form of x-rays used in CT scans are energetic enough to directly or indirectly damage DNA [2]. Such damage to the DNA occasionally leads to cancer. Even though no large-scale epidemiologic studies of the cancer risks associated with CT scans have been reported so far, on the basis of risk estimates and data on CT used from 1991 through 1996, it has been estimated that about 0.4% of all cancers in the United States may be due to CTs performance in the past [3]. By adjusting this estimate for current CT use, this estimate might now be in the range of 1.5 to 2.0% [2], however, this estimate is disputed in [4]. Having mentioned that no large-scale study on cancer risks associated with CT scans have been done so far, one such study is just beginning [5], in order to know the exact percentage we have to wait for some years. This writing is to make you aware of CT scans, its benefits and shortcomings along with the future of CT.

basics of CT

Figure 2. The basics of CT (courtesy: [9])

The History of CT

The mathematical principles of Tomographic reconstruction were investigated by Johann Radon in 1917 [6]. The Austrian mathematician Radon showed mathematically that a function could be reconstructed from an infinite set of its projections. In 1937, a Polish mathematician, named Stefan Kaczmarz, developed a method to find an approximate solution to a large system of linear algebraic equations [7]. This led the foundation to another powerful-reconstruction method called Algebraic Reconstruction Technique (ART) which was later adapted by Sir Godfrey Hounsfield as the image reconstruction mechanism in his famous invention, the first commercial CT scanner in 1972. It is worth mentioning that the first commercially viable CT scanner was invented by Sir Godfrey Hounsfield in Hayes, United Kingdom, at EMI Central Research Laboratories using X-rays. The first EMI-Scanner was installed in Atkinson Morley Hospital in Wimbledon, England, and the first patient brain-scan was done on 1 October 1971 [8]. It was publicly announced in 1972.

Since the first CT scanner, CT technology has vastly improved. There are improvements in speed, slice count, image quality and so on. Scanners now produce images much faster and with higher resolution enabling doctors to diagnose patients more accurately and perform medical procedures with greater precision.

Why do we need CT?

CT scans are used to diagnose and monitor a variety of different health conditions including brain tumours, certain bone conditions and injuries to internal organs such as the kidneys, liver or spleen.

CT scans are also often used to look inside the body before another procedure takes place, such as radiotherapy treatment or a biopsy (where a small tissue sample is taken so that it can be examined under a microscope).

Because of the inherent high-contrast resolution of CT, differences between tissues that differ in physical density by less than 1% can be distinguished in CT and thus enabling doctors to diagnose patients more accurately.

Quantitative Measure of Radiation Doses associated with CT

Various measures are used to describe the radiation dose delivered by CT scanning, the most relevant being absorbed dose, effective dose, and CT dose index (or CTDI) [9].

The absorbed dose is the energy absorbed per unit of mass and is measured in grays (Gy). One gray equals 1 joule of radiation energy absorbed per kilogram. The organ dose (or the distribution of dose in the organ) will largely determine the level of risk to that organ from the radiation. The effective dose, expressed in sieverts (Sv), is used for dose distributions that are not homogeneous (which is always the case with CT); it is designed to be proportional to a generic estimate of the overall harm to the patient caused by the radiation exposure. The effective dose allows for a rough comparison between different CT scenarios but provides only an approximate estimate of the true risk.

A typical plain film x-ray involves radiation dose of 0.01 to 0.15 mGy, while a typical CT can involve 10–20 mGy for specific organs, and can go up to 80 mGy for certain specialized CT scans [11]. The table 1 reports average radiation exposures, however, there is a wide variation in radiation doses between similar scan types, where the highest dose could be as much as 22 times higher than the lowest dose [10].

Table 1. Radiation doses associated with CT/ X-ray (courtesy [12])

radiation doses
Adverse side-effects of CT

The ionizing radiation in the form of x-rays used in CT scans is energetic enough to directly or indirectly damage DNA. These types of DNA damages are occasionally not corrected properly by cellular repair mechanisms and occasionally lead to cancer [12]. It is estimated that 0.4% of current cancers in the United States are due to CTs performed in the past and this may increase to as high as 1.5–2% with 2007 rates of CT usage [2]. Even though the additional risk is still low compared to the background risk of dying from cancer of ~20%[10], it should not be overlooked. The most common cancers caused by radiation exposure are thought to be lung cancer, breast cancer, thyroid cancer, stomach cancer and leukemia [12].

Along with the radiation side-effects the radio-contrast agents (that are injected some time to highlight different tissue types bringing out vessels, tumors, inflammation, cysts, etc) may lead reactions such as nausea, vomiting and an itching rash and so on [12].

It has been observed that person’s age plays a significant role in the subsequent risk of cancer [12]. Estimated lifetime cancer mortality risks from an abdominal CT of a 1-year-old are 0.1% or 1:1000 scans [10]. The risk for someone who is 40 years old is half that of someone who is 20 years old with substantially less risk in the elderly.

Figure 3 represents the observed phenomenon in [9] that shows cancer risks decrease with increasing age.

estimated dependance lifetime of radiation

Figure 3. Estimated Dependence of Lifetime Radiation-Induced Risk of Cancer on Age at Exposure for Two of the Most Common Radiogenic Cancers (courtesy [9])

Avoiding/minimizing radiation side-effects?

Being a non-radiation intensive modality MRI is gaining popularity over CT nowadays in many cases. However still in many medical complications that CT is the one and only diagnostic modality. Research is going on to make CT less and less dose intensive. Some interesting works have already been proposed that are able to reconstruct CT from significantly less number of X-ray projections, results in significant radiation dose diminution. Detail elaboration on this regard is beyond the scope of this writing; interested readers are requested to have a look on [1, 14].

Conclusion

The advent of computed tomography (CT) has revolutionized diagnostic radiology. Since the inception of CT in the 1970s, its use has increased rapidly. It is estimated that more than 62 million CT scans per year are currently obtained in the United States, including at least 4 million for children [2].

estimated number of ct scans

Figure 4. Estimated Number of CT Scans Performed Annually in the United States (courtesy[2])

The widespread use of CT probably represents the most advance in diagnostic radiology. However, as compared with plain-film radiography, CT involves much higher doses of radiation, resulting in a marked increase in radiation exposure in the population. From an individual standpoint, when a CT scan is justified by medical need, the associated risk is small relative to the diagnostic information obtained, however this risk is still enough to think off an alternative of CT or CT with least possible radiation doses.

References
[1]. Xueli  Li  and  Shuqian  Luo,  “A  compressed  sensing-based  iterative  algorithm  for  CT  reconstruction  and  its possible application to phase contrast imaging,” BioMedical Engineering OnLine , 10-73 (2011).
[2]. Brenner DJ., Hall EJ., “Computed tomography–an increasing source of radiation exposure,” N. Engl. J. Med. 357 (22), 2277–84 (2007). doi:10.1056/NEJMra072149. PMID 18046031.
[3]. Berrington de Gonzalez A, Darby S., “Risk of cancer from diagnostic X-rays: estimates for the UK and 14 other countries,” Lancet 363, 345-351 (2004).
[4]. Tubiana M.,  “Comment on Computed Tomography and Radiation Exposure,” N. Engl. J. Med. 358 (8), 852–3 (2008). doi:10.1056/NEJMc073513. PMID 18287609.
[5]. Giles J., “Study warns of ‘avoidable’ risks of CT scans,” Nature 431, 391-391 (2004).
[6]. Radon J., “Uber die Bestimmung von Funktionen durch ihre Integralwerte Langs Gewisser Mannigfaltigkeiten (English translation: On the determination of functions from their integrals along certain manifolds),” Ber. Saechsische Akad. Wiss. 29 (262), (1917).
[7]. Kaczmarz S., “Angenäherte Auflösung von Systemen linearer Gleichungen,” Bulletin International de l’Académie Polonaise des Sciences et des Lettres. Classe des Sciences Mathématiques et Naturelles. Série A, Sciences Mathématiques 35, (1937).
[8]. BECKMANN, E. C., “CT scanning the early days,” TheBritish Journal of Radiology 79 (937), 5–8 (2006). doi:10.1259/bjr/29444122. PMID 16421398.
[9]. D. J. Brenner, E. J. Hall, “Computed Tomography — An Increasing Source of Radiation Exposure,” New England Journal of Medicine 357, 2277-2284 (2007).
[10]. Furlow B., “Radiation dose in computed tomography,” Radiologic technology 81 (5), 437–50 (2006). PMID 20445138.
[11]. Hall EJ., Brenner DJ., “Cancer risks from diagnostic radiology,” The British journal of radiology 81 (965), 362–78 (2008). doi:10.1259/bjr/01948454. PMID 18440940.
[12]. X-ray computed tomography. Available: http://en.wikipedia.org/wiki/X-ray_computed_tomography#cite_ref-crfdr_26-3
[13]. CT CAT Scan Radiation Overdose. Available: http://www.yourlawyer.com/topics/overview/CT_CAT_Scan_Radiation_Overdose
[14]. Sajib  Saha,  Murat  Tahtali,  Andrew  Lambert,  Mark  Pickering,  “Multi-axial CT reconstruction from few view projections”, Proc. SPIE 8500, Image Reconstruction from Incomplete Data VII, 85000A (2012). doi:10.1117/12.928899.

Sajib Saha

Sajib Saha is a PhD student in Electrical Engineering at University of New South Wales (UNSW), Canberra, Australia. He finished his BSc. in Computer Science and Engineering from Khulna University, Bangladesh in 2007 and MSc. in Color in Informatics and MEdia Technology (CIMET) with Erasmus mundus scholarship from France and Norway in July, 2011. He carried out his master thesis in Technicolor R&D, France.

In August 2011 he has joined the medical image processing group in UNSW, Canberra as a PhD research student.

Mr. Saha has attended and presented papers in numerous international conferences worldwide. Image Processing, Object recognition and Tracking, Medical Image Analysis are his area of interests.

Leave a Reply

Your email address will not be published. Required fields are marked *