Estimation of radiation dose and establishment of local diagnostic reference levels for computed tomography of head in pediatric population

Abstract

BACKGROUND:

Pediatric population is more sensitive to the effects of radiation than adults. Establishing diagnostic reference level (DRL) is an efficient dose optimization technique implemented by many countries for reducing radiation dose during Computed Tomography (CT) examinations.

OBJECTIVES:

To estimate radiation dose and establish a new local diagnostic reference level for CT head examination in the pediatric population.

MATERIALS AND METHODS:

We prospectively recruited 143 pediatric patients referred for CT head examination with age ranging from 0–5 years old. All patients had undergone CT head examination using the standard pediatric head protocol. Volumetric CT dose index (CTDIvol) and dose length product (DLP) were recorded. The effective dose was first calculated. Then, 75th percentile of dose indices was calculated to establish DRLs.

RESULTS:

DRLs in terms of CTDIvol and DLP are 23.84 mGy, 555.99 mGy.cm for patients <1 years old and 28.65 mGy, 794.99 mGy.cm for patients from 1–5 years old, respectively. Mean effective doses for <1 years old patients and 1–5 years old patients are 2.91 mSv and 2.78 mSv respectively.

CONCLUSION:

The study concludes that DRL in terms of CTDIvol is lower but DRL in terms of DLP and the effective dose is higher compared to a few other studies which necessitate the need for dose optimization.

Keywords

Diagnostic reference levels pediatric computed tomography volumetric computed tomography dose index dose length product effective dose estimation

1 Introduction

Computed tomography (CT), is a valuable imaging tool used in radiology for diagnosing various diseases and injuries. In developed countries, the usage of CT has raised from 2% to 9% in the fourteen years. Pediatric CT examination has also been increased in recent years with the advancement in CT scan technology. Pediatric CT head examination is the most commonly performed CT examination to look for the signs of skull fractures and bleeding which could be life-threatening if they are not detected and treated [1 –3]. However, children exposed to radiation at the youngest age are more vulnerable to the risk of radiation [4 , 30].

Due to the increased use of CT scanner and adverse effects of radiation, pediatric CT is a public health concern [6]. Children with CT examination involving bone marrow dose of 30 mGy and higher are at increased risk of developing leukemia. Similarly, children receiving a brain dose of 50 mGy or higher are at greater risk of developing brain cancer [7]. Therefore, optimization of radiation dose with acceptable diagnostic image quality is considered important in pediatric CT examinations.

In 1996, the International Commission on Radiological Protection (ICRP) introduced “Diagnostic Reference Levels (DRL)” for optimizing the radiation dose during medical diagnostic examinations in ICRP publication 73 [8]. The ICRP defines DRL as “a form of investigation level used as a tool to aid in optimization of protection in the medical exposure of patients for diagnostic and interventional procedures”. DRL is used to check whether the amount of radiation delivered for specific diagnostic examination under routine conditions is high or low. Establishing DRL is an efficient dose optimization tool used by many countries for ensuring the radiation dose is as low as reasonably achievable without compromising diagnostic image quality during imaging procedures. There are local, national and regional DRLs depending on the selected group of dose survey [9, 10]. Limited studies conducted dose surveys for pediatric CT head examinations in India specifically with less than five years age group. Therefore, this study was undertaken to estimate radiation dose and establish local DRL in Karnataka for CT head examination in the pediatric population.

2 Materials and methods

2.1 Subjects

This is a prospective study. The approval for the study was taken from the institutional research committee and institutional ethics committee (IEC) of Kasturba hospital, Manipal (IEC 610/2018). The study included pediatric patients referred for CT head examination from January 2019–2021 with age ranging from 0–5 years old. The patients were categorized into two groups depending on their age as less than one year and 1–5 years old. A total of 143 patients with 73 patients in less than one-year age group and 70 patients in the 1–5 years old age group were included. Patients with implanted metallic devices and CT images showing motion artifacts were excluded from the study. The informed consent was obtained from the parents /legal guardians. The demographic details of the patients such as age, gender, weight, and head circumference were recorded from each patient before undergoing CT head examination.

2.2 CT image acquisition

All CT scans were performed on a 128 Slice CT scanner (Incisive, Philips Health care). For less than one-year age group 100 kVp and 200 mAs and 1–5 years old age group 120 kVp and 250 mAs was used with all other scanning parameters constant for both age groups for CT head examination (Table 1). The images from both the groups were reconstructed with iDose⁴ level 3 Iterative reconstruction (IR) algorithm.

Table 1
Scanning parameters used for pediatric CT head examination

Scanning Parameters Standard dose protocol

<1 years old 1–5 years

Tube voltage (kVp) 100 120

Tube current (mAs) 200 250

Slice thickness (mm) 3 3

Increment(mm) 3 3

Pitch 0.60 0.60

Rotation time (s) 0.50 0.50

Matrix 512×512 512×512

Collimation (mm) 64×0.625 64×0.625

FOV (mm) 250 250

Reconstruction mode iDose⁴ level 3 iDose⁴ level 3

Scanning Parameters	Standard dose protocol
Tube voltage (kVp)	100	120
Tube current (mAs)	200	250
Slice thickness (mm)	3	3
Increment(mm)	3	3
Pitch	0.60	0.60
Rotation time (s)	0.50	0.50
Matrix	512×512	512×512
Collimation (mm)	64×0.625	64×0.625
FOV (mm)	250	250
Reconstruction mode	iDose⁴ level 3	iDose⁴ level 3

2.3 Radiation dose measurements

Radiation dose indices such as “Volumetric Computed Tomography Dose Index (CTDIvol)” and “Dose Length Product (DLP)” were noted from the CT console after each scan. The effective dose (E) was calculated by multiplying the DLP with the age-specific conversion factor (k) (k = 0.011 for 0 year, 0.0067 for 1 years old, and 0.0040 for 2 years, 3 years, 4 years and 5 –years respectively) [11, 12].

2.4 Statistical analysis

Statistical analysis was performed using RX64 4.1.1 software. Demographic characteristics such as gender, age, head circumference and body weight were analysed using descriptive statistics such as frequency, mean and standard deviation. According to the ICRP recommendation, the DRL values are set at the 75th percentile of the median of distributions of the CTDIvol and DLP [10]. Therefore, the 75th percentile (third quartile) of CTDIvol (m.Gy) and DLP (mGy. cm) was calculated to establish local DRL. To find the correlation between demographic data (body weight and head circumference) and radiation dose (CTDIvol, DLP, and effective dose) spearman’s correlation coefficient was calculated.

3 Results

3.1 Participant characteristics

A total of 143 pediatric patients were included in the study with 73 patients in less than one-year age group and 70 patients in the 1–5 years old age group. Demographic details of patients are shown in Table 2.

Table 2
Demographic details of patient

Demographic data <1 year 1–5 years old

Male (n) 42 35

Female (n) 31 35

Age (Mean±SD) 7.73±4.82 (in months) 3.5±1.06 (in years)

Head circumference (Mean±SD) 42.74±4.71 (cm) 49.82±1.53 (cm)

Body weight (Mean±SD) 7.51±2.53 (kg) 15.00±2.61 (kg)

Demographic data	<1 year	1–5 years old
Male (n)	42	35
Female (n)	31	35
Age (Mean±SD)	7.73±4.82 (in months)	3.5±1.06 (in years)
Head circumference (Mean±SD)	42.74±4.71 (cm)	49.82±1.53 (cm)
Body weight (Mean±SD)	7.51±2.53 (kg)	15.00±2.61 (kg)

3.2 Radiation dose assessment

The minimum, 25th percentile, median, mean, 75th percentile, maximum for pediatric patients undergoing CT head examination were calculated and are summarized in Table 3. The mean effective dose for less than 1year and 1–5 years old age group was 2.91±0.41 mSv and 2.78±0.85 mSv respectively. DRL was established by calculating the 75th percentile of CTDIvol and DLP. DRL for pediatric CT head in terms of CTDIvol and DLP was 23.84 mGy, 555.99 mGy.cm for less than one year, and 28.65 mGy, 794.99 mGy.cm for 1–5 years olds age group respectively. CTDI vol and DLP were higher for the age group of 1–5 years olds (Fig. 1).

Table 3
Radiation dose indices for pediatric CT head

CTDIvol (mGy) DLP (mGy.cm) Effective dose (mSv)

<1 year 1–5 years old <1 year 1–5 years old <1 year 1–5 years old

Minimum 6.90 25.29 215.34 500.10 1.44 2.00

25th percentile 7.30 26.36 299.70 613.5 2.00 2.45

Median 19.33 26.63 467.80 698.93 3.13 2.79

Mean 16.85 27.35 435.74 697.03 2.91 2.78

75th percentile 23.84 28.65 555.99 794.99 3.72 3.18

Maximum 23.97 29.74 595.91 895.80 3.99 3.58

	CTDIvol (mGy)	DLP (mGy.cm)	Effective dose (mSv)
Minimum	6.90	25.29	215.34	500.10	1.44	2.00
25th percentile	7.30	26.36	299.70	613.5	2.00	2.45
Median	19.33	26.63	467.80	698.93	3.13	2.79
Mean	16.85	27.35	435.74	697.03	2.91	2.78
75th percentile	23.84	28.65	555.99	794.99	3.72	3.18
Maximum	23.97	29.74	595.91	895.80	3.99	3.58

Fig. 1

Box plot showing the distribution of radiation dose for less than one year and 1–5 years old age group (A) CTDIvol by age (B) DLP by age (C) Effective dose by age.

The CTDIvol showed strong correlation with head circumference and body weight for less than 1 years old (r = 0.9111, r = 0.9535; p < 0.001) and 1–5 years old (r = 0.9316, r = 0.9058; p < 0.001) age group (Fig. 2). The DLP showed strong correlation with head circumference and body weight for less than 1 years old (r = 0.9126, r = 0.9215; p < 0.001) and 1–5 years old (r = 0.9078, r = 0.8927; p < 0.001) age group (Fig. 3). The effective dose showed strong correlation with head circumference and body weight for less than 1 years old (r = 0.9126, r = 0.9215; p < 0.001) and 1–5 years old (r = 0.9078, r = 0.8927; p < 0.001) age group (Fig. 4).

Fig. 2

Scatter plot matrix showing relationship between CTDIvol and Head circumference, weight for less than 1 year (A) and 1–5 year age group (B).

Fig. 3

Scatter plot matrix showing relationship between DLP and Head circumference, weight for less than 1 year (A) and 1–5 year age group (B).

Fig. 4

Scatter plot matrix showing relationship between effective dose and Head circumference, weight for less than 1 year (A) and 1–5 year age group (B).

4 Discussion

Assessment of radiation dose and establishment of DRL is a useful tool for optimizing the radiation dose in CT, especially for the pediatric population. In the current study, for less than 1year age group, CTDIvol, DLP and effective dose was ranging from 6.90 mGy–23.97 mGy, 215.34 mGy.cm–595.91 mGy.cm and 1.44 mSv–3.99 mSv respectively. For 1–5 years old age group CTDIvol, DLP and effective dose was ranging from 25.29 mGy–29.74 mGy, 500.10 mGy.cm–895.80 mGy.cm and 2.00 mSv –3.58 mSv respectively. DRLs established in terms of CTDIvol and DLP in this study were compared with the recently published studies that have established DRL for pediatric CT head examination (Table 4).

Table 4
Comparison of DRL (CTDIvol and DLP) and mean effective dose among various studies

Age group This study EC [12] Saravanakumar et al. [13] Gao et al. [14] Rawashdeh et al. [15] Muhammad et al. [16] Kanda et al. [17] Benmessaoud et al. [18] Jayasooriya et.al. [19] Zamani et al. [20]

CTDIvol (mGy)

<1 years old 23.84 25 20 26 49 24 30 26.98 30 25

1–5 years old 28.65 38 38 35 55 34 40 28.88 37.9 28

DLP (mGy.cm)

<1 years old 555.99 370 352 340 744 427 480 461.64 535.50 352

1–5 years old 794.99 505 505 558 982 622 660 540.06 984 355

Effective dose (mSv)

<1 years old 2.9 – – 1.9 – 3.0 – 3.6 – –

1–5 years old 2.7 – – 2.3 – 2.2 – 2.9 – –

Age group	This study	EC [12]	Saravanakumar et al. [13]	Gao et al. [14]	Rawashdeh et al. [15]	Muhammad et al. [16]	Kanda et al. [17]	Benmessaoud et al. [18]	Jayasooriya et.al. [19]	Zamani et al. [20]
CTDIvol (mGy)
<1 years old	23.84	25	20	26	49	24	30	26.98	30	25
1–5 years old	28.65	38	38	35	55	34	40	28.88	37.9	28
DLP (mGy.cm)
<1 years old	555.99	370	352	340	744	427	480	461.64	535.50	352
1–5 years old	794.99	505	505	558	982	622	660	540.06	984	355
Effective dose (mSv)
<1 years old	2.9	–	–	1.9	–	3.0	–	3.6	–	–
1–5 years old	2.7	–	–	2.3	–	2.2	–	2.9	–	–

In the current study, DRL in terms of CTDIvol was lower than the previously published studies [13 –21] for both the age group except the DRL reported by Saravanakumar et al. [22] for less than one-year age group which was slightly lower (20 mGy) than the present study. However, DRL in terms of DLP for both the age group was higher compared to DRL reported by recently published studies [13 –21] except the DRL from Jordan s16] which was higher than the current study. The 75th percentile of DLP for less than one age group, reported by Jayasooriya et al. [19] was lower but for 1–5 year age group, DRL was higher than the present study. As DLP is directly proportional to the scan length, the higher DLP in the current study might be due to the over-ranging beam during scanning as it is challenging to differentiate fine anatomical details of the craniocervical junction and upper cervical spine on lateral scout image of CT head especially in pediatric patients with 1–5 years old ages group. Similar findings were reported by Muhammed et al. [16] and Vawda et al. [21].

In the current study, the mean effective dose for less than one-year group was lower than the studies reported by Muhammed et al. [16] and Benmessaoud et al. [18] but higher than Gao et al. [14]. Similarly, the mean effective dose of the 1–5 years old age group was higher than Gao et al. [13], Mohammed et al. [16] but lower than Benmessaoud et al. [18]. Few studies have reported that the effective dose to the pediatric patients from CT head examination was lower in the older age group compared to the younger age group [16 , 23–25]. The high effective dose for the younger age group could be due to the smaller and proximity of adjacent organs such as the thyroid and lungs that increase the scattered radiation. Similar findings were observed in the current study, where the mean effective dose for less than one age group (2.91 mSv) was higher than the mean effective dose of the 1–5-year age group (2.78 mSv).

In CT, the patient radiation dose is controlled by technical factors such as kilovoltage (kVp), tube current exposure time product (mA.s), pitch, slice thickness, scan length, bow-tie filter, patient positioning, and variation in vendor models [26]. Modern CT scanners are equipped with automatic tube current modulation for optimization of radiation dose in CT. The use of iterative reconstruction techniques would help in the reduction of radiation dose with acceptable diagnostic image quality [27, 30]. The difference in DRLS (CTDIvol and DLP) and mean effective dose between various studies included in our article is due to the variations in use of single [13], dual [13] and multislice CT scanner such as 4-slice [18 , 22], 16-slice [18 , 22], 64-slice [13 , 22], 128 slice [16, 22], 160 slice [19]; scan range of 20–24 mm [22], 11.91–15.0 mm [16] and 13.0–15.0 mm [20]; selection mode of acquisition axial [13, 22] and helical scanning [13 –22]; tube potential of 80 kVp [22], 100 kVp [13 , 22] and 120 kVp [13 –22]; tube current of 210–230 mAs [16], 120–300 mAs [14], 100–130 mAs [22], 80–120 mAs [20]; pitch of 0.4 –0.64 [16], 1.00 [14] and 0.6–1.1 [22], rotation time of 1 second [14] and 0.75 seconds [20]; reconstruction algorithms such as filtered back projection [13], iDose⁴ iterative reconstruction [16], ASIR–Adaptive Statistical Iterative Reconstruction [14].

Establishing local DRLs will help in identifying the unwanted high radiation dose delivered to the patient and helps in modifying scanning protocols and optimizing radiation dose. DRLs need to be updated periodically (3–5 years old). Few studies recommend establishing DRLs based on patient’s weight, height, Body mass index as an alternative to age-based grouping [28].

In the present study, we found that there was a positive correlation between the CTDIvol, DLP, effective dose and body weight, head circumference for both the age group. A phantom study conducted by Shohji et al. [29], reported that SSDE (size-specific dose estimation) showed the strongest correlation with head circumference compared to age and weight.

The study has a few limitations. First, the radiation dose was estimated from only one center and included pediatric patients with age ranging from 0–5 years old. Future studies can be conducted including multiple centers and increasing the age group (6–18 years). Secondly, the study only estimated the radiation dose but the image quality was not assessed. Studies can be done in the future, for the evaluation of subjective and objective image quality.

5 Conclusion

The study concludes that DRL in terms of CTDIvol was lower compared to other published studies for both the age group. But the DRL in terms of DLP and the effective dose was higher compared to a few of other studies. As the pediatric population is more sensitive to radiation, the established DRL in this study would contribute as a guide to optimize the radiation dose for pediatric CT head examination.

Footnotes

Acknowledgments

None.

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