Is the CT dangerous?

The revolution brought about by the introduction of computerized tomography in medical diagnosis is often compared to the discovery of X-rays by Roentgen. The benefits of the images it produced radically changed radiological practice. Finally, the internal structures of the patient could be seen without the overlap they experience in conventional radiology. Furthermore, it was the first application of digital procedures in the field of medical physics.

The theoretical foundations of CT were established in 1917 by the Austrian mathematician J. Radon, while working on gravitational theory. He demonstrated that a two or three-dimensional object can be reproduced from an infinite set of projections obtained from its different angles. Hypothesis, which remained hidden for a long time despite its usefulness for medicine.

Years later, in 1963, A. M. Cormack raised the problem that would lead to the concept of CT, obtaining information on three-dimensional structures by transmitting X-rays through them. Unaware of Radón's work, he developed his own mathematical equations to solve it, but these were not very well received. However, that did not stop him from sharing the Nobel Prize in 1979 with Sir Godfrey Hounsfield, the British engineer who designed the first CT scanner for clinical use.

This team, specialized in head scans, was installed at the Atkinson Morley Hospital in 1971, and produced the first human image on October 4 of that same year. The results of the patient submitted to the study were presented by the physicist G. N. Hounsfield and the neuroradiologist J. Ambrose at the British Institute of Radiology in 1972, the date of the birth of CT. Subsequently, the American Ledley introduced the first total-body, with which it was possible to test any area of the body.

En los años 70, la gran demanda de equipos potenció la inversión de recursos destinados a su evolución tecnológica con el principal objetivo de reducir los tiempos de exposición. Los cambios que se efectuaron tanto en la geometría del sistema tubo de rayos X-detector como en las partes mecánicas que proporcionaban los movimientos, definieron las diferentes generaciones de TC.

Currently, as a result of all these advances, we have equipment capable of acquiring information from large anatomical volumes at high speed and spatial resolution, with minimal reconstruction times. Some devices whose diagnostic advantages have skyrocketed their use in recent years, creating new clinical applications outside of radiodiagnosis services: hemodynamic units, operating rooms, endoscopy units, etc… This great proliferation of equipment and fields of application makes that the population subjected to this type of examination is high and is increasing. Therefore, despite the fact that the individual risk associated with CT is small, the large number of individuals to which it is applied makes it an appreciable risk that must be analyzed and taken into special consideration.

CT equipment emits X-rays, which is high-energy electromagnetic radiation, the interaction of which with living organisms can lead to potentially harmful biological effects. The study of its danger will be carried out based on the effective dose, which is the magnitude that quantifies the overall risk and is defined as the absorbed dose (absorbed energy per unit of mass) multiplied by factors characteristic of the type of radiation and the different radiosensitivity of the organs and tissues of the body. The SI unit of measurement for absorbed dose is the Gray (Gy) and the effective dose is the Sievert (Sv).

With regard to the biological effects on living matter, taking into account the nature of the damage they produce, they are classified as deterministic and probabilistic. Deterministic effects only appear from a threshold dose. Above this value, a very significant number of cells die or stop dividing, which causes morphological and functional damage to the organ or tissue. Given that the threshold doses are very high (1-2 Gy), we will not take them into consideration in CT scans (they only appear in Fluoro-CT practices that involve a scan and much longer times). Regarding the probabilistic effects, they lack a dose threshold and are the result of the transformations of unrepaired cells. The probability of their occurrence is proportional to the effective dose received and, therefore, the way to minimize their appearance in radiographic techniques will consist in reducing the dose given to the patient.

At this point, however, it should be noted that the proportionality between the increase in stochastic effects and the radiation dose (above the natural background dose) responds to a model called "linear without threshold" that is built from the extrapolation of the results obtained for people subjected to higher doses and dose rates. Despite the fact that there is no evidence of effects below 100 mSv, the International Commission on Radiological Protection estimates that this model is the most practical for managing the risk of radiation exposure, in accordance with the "prevention principle" ( UNESCO, 2005). [comments 2 and 3]

The doses received by the patient during a CT scan are among the highest of all radiodiagnostic techniques. If we add to this the continuous increase in the frequency and complexity of these tests in recent years, we have an increase in the doses administered to the population and a greater risk of suffering probabilistic biological effects (assuming the aforementioned "linear without threshold" model). . It is important to keep in mind that, contrary to what some radiologists think, several studies have shown that the doses associated with multislice CT equipment (MCCT), tomographs with more than one detector crown, are higher than those of CT (single cut). This is due, among other factors, to the direct transfer of CT equipment protocols, without proper optimization, to the ease of performing longer scans in less time, which sometimes leads to an unnecessary increase in the radiated field, to the increase in phases of the explorations with contrast, etc…

In order to reduce the appearance of biological effects, the principles of radiological protection (RP) must be followed, which, in the case of the patient, correspond to the justification for the execution of the test, the optimization of the doses used and the establishment of reference dose levels.


An x-ray is only justified if it provides a net benefit against the individual detriment it may cause. This assessment must take into account the diagnostic advantages it entails as well as the benefits it implies for the health of individuals and society. Both the prescribing physician requesting the test and the clinical radiology specialist who will perform or supervise it should be involved in this justification process. Both will assess the need for the examination, contemplating the possibility of using alternative tests that involve a lower dose or that do not use ionizing radiation. The final decision, however, remains in the hands of the specialist in clinical radiology (RD 815/2001).

The goal of this RP principle is to avoid all unnecessary radiation exposures. This is extremely important in the pediatric patient whose relative risk is much higher than that of adults. If we represent a graph, published by the International Commission on Radiological Protection (ICRP), with the percentage of fatal cancer risk as a function of age and sex, we observe the considerable increase in the percentage as the age at the time of diagnosis decreases. The exhibition. However, we must not forget that the probability of death from radiation-induced cancer is much lower than that due to cancer caused by other factors.

As regards the tests that are already carried out in a justified way, they should be revised whenever new information is found on their efficacy or their consequences, or alternative tests of lower risk appear.

In order to compare CT technology with respect to conventional radiology and verify the importance of the proper selection of the exam to be performed, the effective doses for both techniques are indicated in the following table. Although the values may seem high, it must be borne in mind that the annual doses from natural radiation are in the range of 1 to 10 mSv.

The goal of this RP principle is to avoid all unnecessary radiation exposures. This is extremely important in the pediatric patient whose relative risk is much higher than that of adults. If we represent a graph, published by the International Commission on Radiological Protection (ICRP), with the percentage of fatal cancer risk as a function of age and sex, we observe the considerable increase in the percentage as the age at the time of diagnosis decreases. The exhibition. However, we must not forget that the probability of death from radiation-induced cancer is much lower than that due to cancer caused by other factors.

The request for an examination must obtain a result that contributes to modifying the diagnostic-therapeutic behavior of the physician or to confirming the diagnosis. Unfortunately not all of the bush is oregano and in some cases patients are exposed to radiation unnecessarily. The main causes of this unjustified overexposure are the following:

  1. Repetition of tests carried out previously: It is essential to know the existing radiographs and find out if the exploration is necessary. The ease of storing and sending results with the introduction of digital radiology can minimize this problem.
  2. Request for excessive additional tests that in some cases may provide irrelevant or highly unlikely results. Here it should also be noted that some patients feel calmer if more tests are performed on them. We have to forget this idea, a greater number of tests does not imply a better diagnosis, you just have to do the necessary ones.
  3. Lack of all the clinical information necessary to analyze in depth what needs to be looked for with diagnostic tests. Before considering any treatment, it is necessary to have the entire history
  4. Prescription of examinations with a greater frequency than the evolution of the disease which, therefore, will not serve to modify the treatment.
  5. Request for inadequate tests due to ignorance of the different diagnostic techniques that can be applied. It is recommended to seek advice from a clinical radiology specialist on the most suitable test.

To avoid these bad practices, it is necessary that all Radiodiagnosis Care Units have the criteria for justifying radiological examinations in their Quality Assurance Program. Guidelines are available for its preparation, such as the European Commission publication "Radiation Protection 118: Guide to indications for the correct application of diagnostic imaging tests".

In the case of Hospitals or Clinics, it is recommended to inform all doctors who refer patients to the Radiodiagnosis Service about the justification criteria, especially newcomers and residents. Likewise, clinical sessions are a good framework to communicate them.


The justification of the exhibition, as we have seen, is fundamental but it is not enough. To minimize the risk, it is also necessary to take measures to optimize the doses given so that they are as low as possible compatible with obtaining the required diagnostic information. This principle is called ALARA (as low as reasonably achievable).

The doses involved in CT examinations represent the majority percentage in the doses associated with radiographic examinations. Their values are given by the technical characteristics of the equipment and by the exploration parameters. The optimization of these parameters is a complex task, since it depends on the type of application, the size of the patient and the model of the tomograph. As an aid, there are a series of European and national guides that recommend starting protocols for different explorations:

Quality criteria in multi-slice CT

  • 2004 – MSCT Quality Criteria – European Commission
  • 2004 – MSCT Pediatric Quality Criteria – European Commission

Quality criteria in conventional CT (single slice)

  • 1999 – European Guidelines On Quality Criteria For Computed Tomography – European Commission – EUR 16262
  • 1998 - Criteris de Qualitat Tècnica I Asistencial de Les Exploracions Amb Convencional Computed Tomography – Agència d’Avaluació de Tecnología Mèdica – Servei Català de la Salut (Catalan)

Other organizations also have protocols for CT scans, both conventional and multi-slice:



The International Commission on Radiological Protection (ICRP), in its 1990 recommendations, states that, for common diagnostic procedures, the use of dose restrictions selected by the appropriate professional or regulatory authority should be considered. It also recommends a certain flexibility in its application, in order to allow higher doses when indicated by well-founded clinical evaluation. Putting this recommendation into practice requires the establishment of reference levels of doses given to the patient against which the values obtained in each particular diagnostic center will be endorsed.

Diagnostic Reference Levels (DRLs) contribute to optimizing the protection of patients by trying to prevent them from being exposed to unnecessarily high doses. Your establishment is part of the regular quality assurance program. In accordance with the recommendations of ICRP 73, NRD is understood as a level established for type examinations of groups of patients of standard size or standard mannequins. In the event that they are systematically exceeded, the procedure and the equipment used will be reviewed to adopt, if necessary, corrective measures.

To determine it, national studies have been carried out in various countries on the doses associated with the different examinations and the value below which 75% of the centers are found has been determined. It has been estimated that if 75% of the centers can carry out examinations with an adequate quality with that dose value, the remaining 25% can correct their equipment and/or examination techniques to reduce them.

It should be noted that these are not dose limits that are prohibited from being exceeded, but rather a research tool to detect unusually high dose levels and take appropriate measures to optimize them. Reference values are never applied to individual patients.

In the event that NRD is not available for some examinations, the initial values will be taken as a reference for subsequent controls until reference values are defined.

Each center should determine its dose levels for each type of examination as mean values observed for representative samples of each group of patients. These should be compared with reference levels set at the national or international level.

Some of the reference dose levels appear in:

  • European Guide EUR 16262 – European guidelines on quality criteria for computed tomography
  • Guidance on Diagnostic Reference Levels (DRLs) for Medical Exposures. EC PR 109
  • ICRP Publication 87: Computed Tomography dose management
  • Spanish Protocol for Quality Control in Radiodiagnosis. 2011 revision
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