The study was designed as a retrospective, descriptive study.

European guidelines suggest the usage of the Computed Tomography Dose Index_volume (CTDI_vol) and dose length product (DLP) as CT dose descriptors.¹⁰ CTDI₁₀₀ is a linear measure of the dose distribution in a 10 cm ionization chamber inserted into a 16 cm phantom for paediatric CT. The weighted CTDI (CTDI_w) is calculated by establishing the CTDI₁₀₀ for the centre and the periphery of a cylinder and combining these. In helical CT scanning, the dose is inversely related to the pitch (number of rotations of the gantry per distance moved by the examination bed). Computed Tomography Dose Index_w divided by the pitch equals CTDI_vol, which is expressed in the international system of units (SI units) as milligray (mGy). Dose length product is the product of the length of the scanned area with the CTDI_vol¹³ and is expressed in milligray-centimetre (mGy*cm).

The data collected were the CTDI_vol and DLP values for each CT examination in paediatric patients (age less than 15 years) during the 6-month period from 01 November 2016 to 30 April 2017 at the following hospitals: Charlotte Maxeke Johannesburg Academic Hospital (CMJAH) and Rahima Moosa Mother and Child Hospital (RMMCH). These hospitals are situated within the City of Johannesburg Municipality in Gauteng province, South Africa.

Data were retrieved from the Picture Archiving and Communication System (PACS) from CMJAH and the local area network at RMMCH. Data were categorised and tabled for each CT scanner and further categorised according to the type of study and the age group. The CT brain data were also categorised into three different time categories as follows: weekdays (Monday to Friday from 08:00 to 16:00), after-hours (00:00–08:00 and 16:00–24:00 from Monday to Friday) and weekends and public holidays (00:00–24:00 on Saturday and Sunday, as well as on public holidays).

The categories were chosen as per recommendations made by the European Diagnostic Reference Levels for Paediatric Imaging Workshop in 2013.¹⁰ All CT investigations were included and categorised according to the anatomical region of interest. The data could not be categorised according to indication, as the indication was not available on the database. The age groups were divided into 0 to < 1 year; 1 year to < 5 years, 5 years to < 10 years and 10 years to < 15 years. The European Commission suggests categorising the CT body examinations according to weight, but patients’ weight was not available from the database. The time of day and day of the week was recorded for each study.

The distribution of the CT examinations in this study sample was calculated using frequencies.

For statistical analysis of the CTDI_vol and DLP values, only the single-phase CT examinations were included in the study sample. Radiation output data for CT orbits, CT paranasal sinuses, CT musculoskeletal, CT whole spine and CT peripheral angiography were excluded from further analysis as the sample sizes were too small for statistical significance.

Data sets were categorised for CT brain, CT temporal bones, CT neck, CT cervical spine, CT chest, CT trunk and CT abdomen. The dose distribution for CT brain in each age category was determined for each hospital. Data for each of the other CT examination types were combined for the two hospitals.

The mean, average, median and 75th percentile of the data distribution, with confidence intervals (CIs), were then calculated for each category for each examination type. Local DRLs are defined as the 75th percentile of the data distribution.¹⁰ The local DRLs in this study were the 75th percentile value for each category, rounded up to the nearest single digit for CTDI and the nearest 5 for DLP.

The data were then compared to similar studies with local and national DRLs.

In addition, the CT brain results were compared between the two hospitals according to the difference in CTDI_vol and DLP 75th percentile values between the two hospitals for each age group and time category, using the Fisher’s exact test. Furthermore, the quantile regression method was used to compare the 75th percentiles in different groups by calculating the 95% CI for the difference between percentiles. If the 95% CI for the difference did not contain 0, the percentiles were significantly different.

Ethical approval to conduct the study was obtained from the Human Research Ethics Committee (Medical) of the University of the Witswatersrand (approval number: M170634).

The audit period for the 6 months from 01 November 2016 to 30 April 2017 included 1031 paediatric CT examinations from RMMCH and CMJAH.

Computed tomography brain examinations (755/1031; 73.23%) were the most common CT examination, followed by CT of the abdomen, which amounted to 82/1031 (7.95%) (see Table 1).

From all the study types conducted during the study period, there were only seven study types with enough cases in different age groups to allow data distribution calculation. These included CT brain, CT temporal bones, CT neck, CT cervical spine, CT trunk, CT chest and CT abdomen (total number for analysis = 905). The 75th percentile of both the CTDI_vol and DLP of each of these examination types in the various age groups is demonstrated in Table 2, with a 95% CI.

The CT brain data sets were used to compare the two different hospitals, as well as to evaluate for potential variation in different time categories. At CMJAH, there was an increase in the 75th percentile of the data distribution in the weekend and after-hours group compared to regular weekdays. The greatest increase in dose was in the 0–1-year after-hours group with a 150.56% (691.3 mGy*cm vs. 275.9 mGy*cm) increase in DLP compared to the 0–1-year group during routine weekdays. The second most significant increase in dosage was in the 5–10-year weekend group. Here, there was a 78.07% (760.7 mGy*cm vs. 427.2 mGy*cm) increase in DLP compared to 5–10 years routine weekday group (see Tables 3 and 4).

Similarly, the data from RMMCH demonstrated an increase in CTDI_vol and DRL for after-hours and weekends compared to regular weekdays, although the increase was not as significant as it was at CMJAH. The most pronounced increase in dose was in the 1–5-year after-hours group, with an increase in DLP of 40.46% (570.7 mGy*cm vs. 406.3 mGy*cm). The second highest increase in dose compared to routine weekdays was in the 0–1-year after-hours category, with an increase of 25.92% (418.3 mGy*cm vs. 332.2 mGy*cm) in DLP (see Tables 5 and 6).

The comparison of the dosages during CT brain investigation between the two hospitals revealed in general a lower DLP at a lower DLP at CMJAH for the 0–1y, 1–5y and 5–10y groups, compared to RMMH (statistically significant in the 0–1y and 5–10y groups only, as 0 is not included in the 95% CI). The 10–15-year stratified groups demonstrated lower DLP values at RMMCH compared to CMJAH (see Figure 1).

The combined DLP and CTDI_vol 75th percentile values were compared to DRLs from the European guidelines, UK, Germany, Japan, Kenya and Brazil^{10,14,15,16,17,18} (see Tables 7 and 8).

The CTDI_vol and DLP values for CT brain were found to be less than the comparative DRL values in most cases, except for the DLP values compared to the European DRL in the age category for 10–15 years.

The CTDI_vol and DLP values for CT chest were higher than the European DRLs, but better than those from Japan, Kenya and Brazil. The exception was the increased value compared to Brazil in the 0–1-year age category.

The CTDI_vol values for CT abdomen were lower than the international DRLs, except for the 0–1-year category, which was higher than the European Diagnostic Reference Levels (EDRL) and Brazilian values. The 0–1-year category for CT abdomen DLP values was also higher than those from Brazil. Although only the 10–15-year category DLP values were within the EDRL range, the rest of the values were lower than that of the other international DRLs.

The higher number of investigations at the CMJAH compared to RMMCH was expected, as it is considered a central hospital in South Africa, a level 1 trauma centre and major referral centre in the country.¹⁹ The CT brain percentage of total investigations was marginally higher in comparison to international studies in the developed world, whereas the CT abdomen percentage compared to the CT utilisation trends in other countries was similar.^5,6,20 The reason for the higher percentage of CT brains performed at the studied facilities is likely because the initial neuroimaging investigation in the public health sector of South Africa for a child presenting with the first episode of convulsion is a CT brain instead of a magnetic resonance imaging (MRI) brain, as recommended by the American Academy of Neurology.²¹ Computed tomography brain for neurological disease in South Africa is a reasonable initial radiological investigation, as the incidence of neurological infections is higher than that of developed countries.²² Magnetic resonance imaging availability and anaesthetic support are limited in the South African public health sector. Furthermore, CMJAH is a level 1 trauma centre and will have an increased percentage of CT brains for trauma indications.

The increase in CT of the cervical spine after the age of 1 year is expected in a level 1 trauma centre. The number of temporal bone CT investigations in the 5–10- and 10–15-year age groups is consistent with previous studies, which demonstrated the majority of patients with temporal bone pathology to be between the ages of 11 and 20 years.²³

Both hospitals demonstrated an increase in the DLP values of CT brain during after-hours and some of the weekend categories. During after-hours, there is less staff present on the floor and often fewer senior staff to guide procedures, which could lead to an incorrect choice of parameters or selection of scan area, with a resultant increase in radiation dose to the patient. The increase in values is more significant in CMJAH than in RMMCH. The Radiology Department at RMMCH is almost an exclusive paediatric radiology department, with staff trained in paediatric radiology. On the contrary, the Radiology Department at CMJAH is a large combined adult and paediatric academic radiology department. At CMJAH, there are dedicated time slots for paediatric CT examinations during the week, but after-hours urgent paediatric CTs are performed in between adult patient CTs, which could lead to an incorrect parameter and CT protocol selection when examining children. Previous research has shown that probability exists for a significant DLP variation between radiographers even in the setting of a dedicated paediatric hospital.²⁴

Although the finding of increased DLP values in the 0–1-year age group was considered not to be statistically significant, follow-up investigation in this age group is suggested, as the findings might suggest clinical significance.

The increased CTDI_vol and DLP values for RMMCH compared to CMJAH could be ascribed to hardware and software variables between the two departments. Charlotte Maxeke Johannesburg Academic Hospital Philips machines have more detector rows (64 and 128) as well as utilisation of the i-Dose software by Philips.^25,26

The CTDI_vol and DLP values for CT brain compare well against previously established DRLs and suggest consistent application of well-developed protocols at the different facilities.

Most of the higher CTDI_vol and DLP values in the 0–1-year age group for CT abdomen were associated with higher kV settings. The South African Society of Paediatric Imaging suggests the reduction of kV settings in paediatric examinations.²⁷ The CT abdomen studies with CTDI_vol and DLP values comparable to international DRL ranges were performed with a reduction of kV from 120 to 100. It is suggested that the CT abdomen protocol for both RMMCH and CMJAH should be reviewed, adjusted and applied to all cases.

Although the CTDI_vol and DLP values for CT chest compare well against some of the international DRLs, it is also suggested that the protocols for CT chest should be reviewed and adjusted.

A discrepancy in the comparison of CTDI_vol and DLP values for a specific examination in a specific age category is likely because of a larger-than-expected pre-selected scan area for the particular study.^24,25 This and the fact that the CT brain values compared better than the CT chest and abdomen values could be a result of using age, rather than weight, as an input parameter for CT chest and abdomen examination in children.¹⁰

From the data analysis, DRLs are proposed for the most frequent examinations. Local DRLs are not suggested for 0–1-year age group, except for CT brain, as the 75th percentile values are higher than the older age groups and compare unfavourably to international DRLs (Table 9).

In recent similar international studies and according to the European guidelines, it is proposed that DRLs could be presented in a graph format instead of tabular format. See the DRL graph and curve for CT abdomen in Figure 2 and for CT brain in Figure 3. This type of graph is created by plotting all the different values for each age on an x:y scatter plot, establishing the 75th percentile for each age and creating a polynomial, exponential graph.²⁸

Presentation of a DRL in a graph format can aid in the comparison of results and also could be an easy visual reference in the department.

One of the limitations of this study is that the European guidelines suggest that body CTs should be categorised according to weight, but the weights were not documented on PACS for RMMCH and CMJAH during the study period. Further limitations included the significant percentage of multi-phasic CT scans, specifically in the 0–1-year age group, which limited the statistical significance of the findings.