Factors that can be Attributable to Radiation Dose Reduction among Pediatric Age Group Undergoing Brain Computed Tomography (Practices at KHMC, Jordan)
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Correspondence:
P.O Box 182721 Amman 11118.
E-mail: narimannsour@yahoo.com

INTRODUCTION

The use of brain computed tomography has increased rapidly in the past two decades¹, however it is generally felt that up to one third of CTs performed on children are not pertinent to either the diagnosis or management, nor is it necessarily the best test³. Children are not only more sensitive to radiation than adults, but they will have more years in which cancerous changes might occur³. Dr Levatter et al mentioned that nowadays the rate of increase of CT examination is probably higher in children, who are more sensitive to radiation induced cancer⁴. For patient protection we should use the right technical parameters to avoid excessive, harmful and unnecessary radiation dose for these investigated children by CT scan and the clinicians should always be concerned and be strongly attentive to minimize CT scan radiation dose for children. To reduce the radiation dose, appropriate strategies have been developed to optimize scanning practices based on clinical indications, the age or body size of the patients, the area being investigated, and low radiation settings⁵. Technical developments with automated exposure control⁶ can also help in optimizing the relationship between image noise and radiation dose (balance between image quality and radiation dose).

Various quantitative measures are used to describe the radiation dose delivered by CT scanning, the most relevant being absorbed dose, and effective dose.
The absorbed dose is the energy absorbed per unit of mass and is measured in grays (Gy). The organ dose (or the distribution of dose in the organ) will largely determine the level of risk to that organ from the radiation. For risk estimation, the organ dose is the preferred quantity.

The effective dose, expressed in sieverts (Sv), is designed to be proportional to the estimate of the overall harm to the patient caused by the radiation⁷.

In June 2004, 150 brain CT scans obtained in 89 females and 61 males, referred to the radiology department for different causes, and a brief clinical history was obtained. Adjustments made in the exposure parameters to determine the amount of radiation children who might receive from CT scan.
We performed brain CT scan using a modified pediatric CT scan protocol, by changing exposure parameters to assess their effect on radiation dose. Brain computed tomography was done using GE Light Speed Plus machine (GE Healthcare, CT, USA). Images were obtained using a multi-slice spiral computed tomography (CT) system of 5 mms slice thickness without automatic selection of effective -mAs (E-Mas). All HCTs were reviewed by a radiology specialist. Radiation dose and exposure factors (scanning parameters) were analyzed. Scanning parameters that affect radiation dose include peak kilovoltage, (tube current-milliampere -second), pitch, number of slices and slice thickness.
Patients were categorized into four groups according to the applied modified scanning protocol. We modified in our study just one exposure parameter: reduced mA, reduced kVp, and increased pitch and slice thickness, which are inversely proportional to radiation dose. However adjustment of two or three exposure parameters is also possible.

From 150 patients referred to our radiology department 32.2% underwent brain CT scan for head injury, 21.1% for abnormal movements including convulsions, 14.4% with chronic headache and 31.3% for developmental delay, psychiatric disorders and miscellaneous reasons. 83.4% of brain CT scan results were normal. We reviewed the literature regarding radiation dose reduction during brain CT scan, depending on many scanning parameters and exposure factors. These scanning parameters are tube current (milliampere per second), kilovoltage peak (kvp), pitch, slice thickness and number of slices. In our study we classified children into four groups to assess the effect of these factors on dose reduction by reducing or elevating one of these parameters while maintaining the other parameters unchanged. However proper modification to maintain proper image quality is mandatory. We then analyzed the radiation dose (recorded by ct machine) that the patient received, with the new modified scanning exposure.
Patients groups were:
The first group included children scanned with modified and reduced tube current with mAs ranging from 90-280 (n=90, 60% of the investigated children), with a median tube current of 159 mAs. The second group of children with high pitch ranged from 1-2 (n=38 (25.3%), and the third with low KVp 120-140 ( n=12 (8%) and the last group was children with applied modified scanning protocols with increasing number of slices from 18-22 slices and slice thickness from 5-8 mm slice thickness (n=10(6.6%). We found that low mA was the most common technique used by (60%) up to 80 mAs and the most important scanning parameter that can significantly decrease received radiation dose during ct scan, followed by high pitch up to 1.5 (25.3%), low kilo voltage peak 140 (6%) and decreased number of slices to 18 slices and slice thickness up to 5 mm (2%). There is also a trend to increase slice thickness as the age of the children increased but we usually use a slice thickness of 5mm.

Age-based adjustments were made, however, 11-26% of CT examinations of children younger than 9 years are performed using less than 150 mA. We found little variation in the kilovoltage used. For 34% of patients less than 140 kVp was used for brain scans, and 66% routinely used 140 kVp for brain scanning among the pediatric population. Other modifications included shielding of radio sensitive organs, avoiding multiphase examinations, using automatic modulation of tube current, using thicker collimation and these can be very helpful in radiation dose reduction. The radiation dose CTDI measured in mlligrays, displayed on the CT monitor) was calculated by the CT scan machine automatically, after we did adjustments and modification of exposure parameters DLP ranged from 200 mGy -2100.

CT is an important imaging modality for examining children, and its use is increasing rapidly. Given the recent attention to radiation risks and CT in children, the need for adjustment in scanning protocol in this population would be helpful⁸. Physicians, CT technologists, CT manufactures and other medical organizations share the responsibility to reduce radiation doses to children and efforts should be made to decrease the number of CT studies that are prescribed.

In the evaluation of scanning protocols used for pediatric patients we found that the CT dose should be reduced to be as low as reasonably achievable to meet clinical needs, therefore CT dose reduction will require a combination of approaches³.
Current guidelines do not recommend obtaining brain CT scan for children, unless the history and physical examination indicate that, otherwise every child requires an accurate, efficient, and optimal, diagnostic work-up, avoiding excessive testing and radiological investigations which is potentially harmful. CT scan should not be ordered for children below ten years indiscriminately⁹. Richard Smart et al mentioned that it is both economically and ethically desirable to restrict the use of diagnostic radiation to only those who will benefit from it¹⁰. If CT scanning parameters used for pediatric patients are not adjusted on the basis of examination type, age and/or size of the child, then some patients will be exposed to an unnecessarily high radiation dose during CT examinations¹¹.

Special considerations are also required to protect children who are generally more sensitive to the short and longterm detrimental effects of radiation exposure⁹. Prudent clinicians should order only those studies that result in clinically important information and efforts should be made to minimize radiation exposure¹². CT radiation doses need to take into account patient age and the selected X-ray technique, cross sectional areas and mean Housenfield unit (HU). The radiation dose reduction to particular organs from any given CT study depends on many factors including replacement of CT use with other imaging modalities such as ultrasonography and magnetic resonance imaging (MRI) which have less radiation dose, and decrease in the number of CT studies that are prescribed. We found also that the automatic exposure-control option on the latest generation of scanners also helps in radiation dose reduction. Multiple factors can affect radiation dose and the most important are the number of scans, the tube current and scanning time in milliamp-seconds (mAs), size of the patient, the axial scan range, the scan pitch or advancement of the scanning plane through patients, the degree of overlap between adjacent CT slices, the tube voltage in the kilovolt peaks (kVp), and the specific design of the scanner being used. Pitch and number of slices, and slice thickness were inversely proportional to radiation dose, while the Ma (current tube) is directly proportional to the radiation dose We found little variation in the kilovoltage used.
Finally we used a reconstruction as recommended by the manufactures for brain ct scan¹³.
Many of these factors are under the control of the radiologist or radiology technician. The mA-s are the most important factor affecting dose reduction, because increased dose per milliampere-second, increased radiation risk and increased exposure risk with p` 0.001 For helical CT at a fixed X-ray energy, and scanning time, the radiation dose to the patient is directly related to the X-ray tube current¹⁴. The dose is directly proportional to the selected tube current-time product; therefore a reduction in mAs by 50% results in a reduction of a dose by half¹³ and inversely proportional to number of slices, slice thickness and pitch. In our department during brain CT scanning the tube current ranged from 90 to 280 mAs, with a median tube current of 159 mAs. Kilo voltage of 120 may not be the optimal level for examining infants⁸ so we use a typical 140 kvp X-ray beam
Several studies have suggested that a technique with significant reduction in exposure parameters (milliampere -seconds) could be adopted for pediatric ct protocol without significant loss of information¹. Adjustment of pediatric protocol, means that children should not be scanned using adult exposure parameters, so we should use lower Ma-s, followed by high pitch which is inversely proportional to the radiation dose (a decrease in pitch by half increases the dose by two), low peak kilovoltage, fewer slices and lesser slice thickness and lower radiation dose settings. So if we are using a CT scanner without automated dose adaptation, we should look up tables with reference to suitable brain ct scan parameters, especially for children. Finally we found that by applying these modifications to the scanning protocol we can achieve low radiation dose and minimize it to lower levels, and this confirms the importance of careful selection of technical parameters for each type of examination¹¹. However inappropriate reduction of radiation exposure causes artifact noise and loss of signal intensity, sometimes resulting in poor image quality¹⁰.
Therefore the radiologists must be attentive to their responsibility to maintain an appropriate balance between diagnostic image quality and radiation dose.

Major national and international organizations responsible for evaluating radiation risk, established immediate and long term strategies to minimize radiation exposure in children.
These include:
perform only necessary CT examination and
adjust exposure parameters for pediatric CT based on: child size/ weight;
Region scanned: the region of the body scanned should be limited to the smallest necessary area,
organ systems scanned: lower mA settings should be considered for skeletal and lung imaging and long term strategies including, encourage development and adoption of pediatric ct protocols, educate working staff through journal publications and conferences within and outside radiology specialties, conduct further research to determine the relationship between CT quality and dose, to customize CT scanning for individual children, to optimize exposure settings and to assess the need for CT in an individual patient. An estimate made by Brenner et al estimated a lifetime increased risk of cancer for children younger than 15 years that results from CT scans, that 600,000 abdominal and head ct examinations annually on children under the age of 15 years could result in 550 cases of cancer attributable to ct radiation¹⁴.

In the light of rapidly increasing frequency of pediatric CT examinations, dose reduction while preserving the value of CT examination and image quality, is a challenging task.
Therefore, if a ct scan has to be done on a child, radiologists need to ensure that the dosage is reduced to the minimal appropriate levels without loss of diagnostic information by adjusting and modifying the applied pediatric CT scanning protocols, using low radiation dose settings.

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