Establishment of diagnostic reference levels in cardiac computed tomography

Abstract The aim of this study was to determine diagnostic reference levels (DRLs) for cardiac computed tomography (CCT) in Jordan. Volume computed tomography dose index (CTDIvol) and dose–length product (DLP) were collected from 228 CCTs performed at seven Jordanian hospitals specialized in cardiac CT. DRLs for cardiac CT were defined at the 75th percentile of CTDIvol and DLP. CTDIvol and DLP were collected from 30 successive cardiac CT in each center except for one center (18 scans). The 75th percentile of the CTDIvol and the DLP of the centers calculated from mixed retrospective and prospective gated modes were 47.74 milligray (mGy) and 1035 mGy/cm, respectively. This study demonstrated wide dose variations among the surveyed hospitals for cardiac CT scans; there was a 5.1‐fold difference between the highest and lowest median DLP with a range of 223.2–1146.7 mGy/cm. Differences were associated with variations in the mAs and kVp. This study confirmed large variability in CTDIvol and DLP for cardiac CT scans; variation was associated with acquisition protocols and highlights the need for dose optimization. DRLs are proposed for CCT; there remains substantial potential for optimization of cardiac CT examinations for adults in Jordan.

In order to achieve optimization, guidance on appropriate levels of patient exposure is required. Basic safety standards have been provided by the International Commission on Radiological Protection (ICRP), World Health Organization (WHO), and International Atomic Energy Agency (IAEA) in order to optimize the protection of patients during all radiological procedures, including CT. 6 These include a recommendation for DRLs 7 or guidance levels to be put in place, monitored, and used to improved radiological procedures. 8  DRLs are not dose limits, but rather guidelines. Where they are regularly exceeded, corrective action should be taken. DRLs have been effective in reducing radiation dose, with radiation levels being reduced by 50% in the UK since their adoption. 9,10 As dose variation for the same examination can reach up to 23-fold between centers for non-CT X-ray examinations 2 and 13-fold for CT examinations. 11 Strategies such as DRLs are required to reduce dose variation between centers. The method of establishing DRLs starts with a determination of radiation dose levels delivered for specific examinations in several hospitals in an individual country (or state). These data are then used to compute examination-specific DRLs for that country, state or region, usually in terms of the  12 The dose for cardiac CT in Jordan is currently not known, so we cannot compare our performance internally or internationally. The current work aims to address this gap.

| MATERIALS AND METHODS
In 2018, a retrospective survey was performed across seven Jordanian hospitals that routinely perform CCT. Data from 228 cardiac CT scans were collected over a 3-month period from May to August.
These cardiac CT scans were performed on a wide range of scanners with different number of slices from three manufacturers (GE, Siemens, and Philips). These are representative of CT scanners being used across Jordan. According to the Jordanian regulations, CT scanners undergo a quality control program that includes daily, monthly, quarterly, and annual checking of the assessment of the radiation dose. 13 Dose in CTDI vol and DLP was provided in 30 successive cardiac CT scans in each center except for one, which provided 18 scans. There survey respondents used two scanning modes, prospective electrocardiographic (ECG) gating modes (PGM) where the scanner monitors the patient's ECG and retrospective ECG-gating modes (RGM) where the patient is scanned continuously, while only certain portions of the scan are used to reconstruct the image. Participants were all adults between 17 to 75 yr and all genders were included. In line with a methodology previously published, 9,14 protocol and scan sequence details were grouped in the survey ( Table 1) To standardize weight for the sampled population and in line with international recommendations, the survey included only those patients who weighed between 60 and 80 kg. 9

2.A | Radiation dose data
On modern CT scanners CTDI vol and DLP are provided for every sequence and examination. However, on older equipment it was necessary to calculate the dose using exposure and procedural data.
The patient dose data, CTDI vol and DLP values, were extracted from the picture archiving and communication system (PACS). A summary of the two relevant dose parameters is given below along with methods for calculating these factors. CTDI vol is defined as weighted CTDI (CTDI w ) divided by CT pitch and provide an estimation for average phantom dose for a complete spiral CT scan. 15

2.B | Weighted CT Dose Index -CTDI w
This describes the radiation dose delivered per unit cranio-caudal (z) axis thickness. Significant previous work has provided methodologies so that the baseline dose value CTDI w can be calculated for specific examinations and from this baseline value other important dose metrics can be calculated 4 (2) where pitch factor is the distance the table moves in the z axis divided by the slice number and each slice thickness.
CTDI w is calculated from Formula 1.

2.D | Dose Length Product (DLP)
The dose measurements above do not consider the total length of the patient who has been irradiated during each examination sequence. This is calculated using Formula 3.
where CTDI vol is calculated from Formula 2.

2.E | Data analysis
The minimum, maximum, and the first, second, and third quartiles ACKNOWLEDGMENT Appreciation and thanks are due to Jordan University of Sciences and Technology for their research grant (grant no. 20180322).

CONF LICT OF I NTEREST
There is no conflict of interest declared in this article.