Importance of Computed tomography and dose related risks

CT is used in pediatric patients as well as adults, and its use has increased rapidly since the technology’s inception in the 1970s.

Computed tomography (CT) is used extensively in diagnostic radiology, primarily for examination of human soft tissues.CT scans produce serial cross-sectional images of the body and generate three-dimensional views which facilitate detailed examination of specific anatomical and pathological areas of concern. Children are being increasingly referred for CT examinations. Increased demand for CT in children is partially due to the advent of fast scanning techniques. Fast helical/multi-slice scanning can negate the need for sedation and allows the evaluation of younger or less co-operative children. The tremendous rise in the use of CT imaging is also related to the development of advanced and reliable diagnostic radiology techniques. CT is now a standard diagnostic tool for pediatric cancer detection, trauma, renal calculi, appendicitis, and heart conditions.CT has reduced the failure rate of laparotomy from 18% in 1997 to less than 5% currently, and also decreased the cost related to number of inpatient days per patient. In certain instances, it has also obviated the need for exploratory surgery.

CT: ADVANTAGES OVER OTHER IMAGING MODALITIES

Ultrasound is very useful in pediatrics, since image quality and resolution improve with a smaller patient size. Ultrasound can also be used to image almost any area of the body, with the exception of those composed mainly of bone or air. MRI uses magnetic fields and radio waves to create a set of 2D slices of the body and thus does not expose the patient to ionizing radiation. It’s use in children, however, is constrained by the fact that patients need to remain absolutely still as even small amounts of motion can affect the image quality. Younger children often require sedation, necessitating specialized equipment and staff which may not be accessible in all imaging centers.

Benefits of CT

CT differentiates overlying structures much better than X-ray techniques and allows greater contrast differentiation than other imaging modalities. Many medical conditions are more accurately imaged and diagnosed using CT, for example, vascular diseases with the potential to cause renal failure, stroke, or death. Thus CT is the best imaging option in many cases, and if the protocol is well optimized, the value of the information obtained will offset the risks associated with the relatively large radiation dose. CT examinations are fast and simple; in emergency cases, they can reveal internal injuries and bleeding quickly enough to help save lives.

CT has been shown to be a cost-effective imaging tool for a wide range of clinical problems.

CT is less sensitive to patient movement than MRI.

CT imaging provides real-time imaging, making it a good tool for guiding minimally invasive procedures such as needle biopsies and needle aspirations of many areas of the body, particularly the lungs, abdomen, pelvis and bones. Sometimes ultrasound is substituted for CT as a method of imaging in these procedures in children.

A diagnosis determined by CT scanning may eliminate the need for exploratory surgery and surgical biopsy.

No radiation remains in a patient's body after a CT examination.

X-rays used in CT scans should have no immediate side effects

Using a multi detector CT unit to examine children is faster than the older CT scanners, reducing the need for sedation and general anesthesia.

New technologies that will make even faster scanning possible are becoming increasingly available. For children this means shorter imaging times and less time required to hold still in order to produce clear images. Also, shorter scan times will make it easier for children to hold their breath during critical parts of the exam.

Risks associated with CT

The risk of serious allergic reaction to contrast materials that contain iodine is rare in children, and almost always mild.

Radiation is necessary to obtain CT images. It is known that high levels of radiation may cause cancer. However, CT scans result in a low-level exposure. Whether such levels cause cancer is debatable but because it is possible, every effort is made to limit the amount of radiation children may receive from a CT scan.

ACHIEVING AN OPTIMAL RADIATION DOSE

Radiation doses from CT scanning are considerably larger than those from corresponding conventional radiography procedures. For example, a planar anterior-posterior abdominal X-ray examination results in a dose to the stomach of approximately 0.25 mGy, approximately 2% of the corresponding dose from an abdominal CT scan, and a CT scan of the chest delivers 100 times the radiation of a conventional chest X-ray.

    Bio-effects of radiation      

The bio-effects associated with radiation exposure can be divided into two main groups: deterministic risk and stochastic effects.

The deterministic risk is a function of radiation dose delivered to an organ or body region. Deterministic effects of radiation are seen above a threshold dose, with higher doses promoting more severe effects; these are rarely seen in diagnostic radiology.

Stochastic effects are dependent upon a complex series of events, including cell transformation. Stochastic effects may appear as a cancer in the patient or as genetic abnormalities in their children.The probability of seeing stochastic effects increases with the amount of radiation but the severity of the effect is independent of the dose of radiation received. Oncogenesis is a major stochastic effect of CT radiation exposure. Children’s organs and tissues are highly sensitive to the oncogenic effects of radiation because they contain a large proportion of cells that are dividing and reproducing. The radiation-induced risk is also higher in pediatric patients due to wider and increased cellular distribution of red bone marrow and their greater post-exposure life expectancy. The effective radiation doses received by children are about 50% higher than those received by adults due to their smaller body size and related attenuation. At ages up to 10 years, children are more sensitive than adults by a factor of three, as their longer expected life span is combined with the higher radiation sensitivity of the developing organs. 

Researchers from the National Cancer Institute and the Society of Pediatric Radiology in the US estimated the risk of dying from cancer to be 1 in 550 following abdominal CT and 1 in 1500 for a brain CT performed in infancy, approximately 0.35% more cancer deaths than expected in the general population

Efforts towards dose reduction in CT have been recommended by major international organizations such as the International Commission on Radiological Protection the International Atomic Energy Agency and the European Commission. These agencies recommended the implementation of CT dose guidance levels for the most frequent examinations to promote strategies for the optimization of CT doses.

Alliance for Radiation Safety in  Pediatric Imaging

It recommends the following steps to prevent excessive dose exposure to pediatric patients:

(1) Acquisition of new CT equipment should be supported by validation of the protocol to help ensure that patient doses are “As Low As Reasonably Achievable”

(2) Any increase in dose must be justified by a corresponding improvement in diagnostic information, and where possible, use iodinated contrast medium to perform CT examinations at lower kV values with no loss of diagnostic information.

CT RADIATION DOSE AWARENESS AMONG PATIENTS AND HEALTHCARE PROFESSIONALS

A majority of the hospital protocols involve explanation of CT radiation risk to patient or his/her family.

ADDRESSING THE PROBLEM

In order to protect pediatric patients from undue exposure to radiation, The FDA recommends that Medical imaging professionals should use techniques that are adjusted to administer the lowest radiation dose that yields an image quality adequate for diagnosis or intervention (i.e., radiation doses should be "As Low as Reasonably Achievable") In order to facilitate dose adjustment for pediatric patients, some equipment manufacturers have incorporated automatic exposure control (AEC) in their CT scanners. An AEC adjusts dose according to patient size and optimizes radiation dose within a single patient using dynamic tube current. Enhancing understanding of the factors that affect patient doses in CT should be considered the first step in optimization strategies. The need to train radiology personnel, establish protocols, and continuously monitor the performance of CT equipment to control patient CT doses is of utmost importance.

CONCLUSION

Over the past two decades, CT scanning rates have increased greatly, and this has increased the average radiation dose delivered to pediatric patients. Dose reduction is being implemented by CT manufacturers, but medical imaging professionals must not rely on this alone. Improvements to CT protocols, referral practices and imaging professionals’ education are needed to minimize the amount of unnecessary CT dose that is delivered. By undertaking these changes and with continual vigilance, the benefits of CT can be obtained at low radiation dose and the minimum of harmful effects to pediatric patients.