Computing effective dose in cardiac CT

Abstract

We present a method of estimating effective doses in cardiac CT that accounts for selected techniques (kV mAs(-1)), anatomical location of the scan and patient size. A CT dosimetry spreadsheet (ImPACT CT Patient Dosimetry Calculator) was used to estimate effective doses (E) using ICRP 103 weighting factors for a 70 kg patient undergoing cardiac CT examinations. Using dose length product (DLP) for the same scans, we obtained values of E/DLP for three CT scanners used in cardiac imaging from two vendors. E/DLP ratios were obtained as a function of the anatomical location in the chest and for x-ray tube voltages ranging from 80 to 140 kV. We also computed the ratio of the average absorbed dose in a water cylinder modeling a patient weighing W kg to the corresponding average absorbed dose in a water cylinder equivalent to a 70 kg patient. The average E/DLP for a 16 cm cardiac heart CT scan was 26 microSv (mGy cm)(-1), which is about 70% higher than the current E/DLP values used for chest CT scans (i.e. 14-17 microSv (mGy cm)(-1)). Our cardiac E/DLP ratios are higher because the cardiac region is approximately 30% more radiosensitive than the chest, and use of the ICRP 103 tissue weighting factors increases cardiac CT effective doses by approximately 30%. Increasing the x-ray tube voltage from 80 to 140 kV increases the E/DLP conversion factor for cardiac CT by 17%. For the same incident radiation at 120 kV, doses in 45 kg adults were approximately 22% higher than those in 70 kg adults, whereas doses in 120 kg adults were approximately 28% lower. Accurate estimates of the patient effective dose in cardiac CT should use ICRP 103 tissue weighting factors, and account for a choice of scan techniques (kV mAs(-1)), exposed scan region, as well as patient size.

Image quality in coronary CT angiography: challenges and technical solutions

“…It is preferably estimated by using a detailed computational organ dose model that accounts for selected techniques, anatomical location of the scan and patient size. 15,16 For practical reasons, a rough estimate can also be provided by using a general dose-length product to E conversion coefficient, the so-called k-factors that are determined by the anatomical location. The reported use of conversion factors that are established for conventional chest CT k 5 0.014-0.017 mSv mGy 21 cm 21 tend to severely underestimate the E in coronary CTA, where a value of k 5 0.026 mSv mGy 21 cm 21 is more appropriate.…”

A comparison of coronary CTA and stress testing using high-efficiency SPECT MPI for the evaluation of chest pain in the emergency department

“…21,22 The lower Huda et al 22 conversion factor of 0.026 was chosen to be used for all CTA calculations. The effective dose for MPI was determined from administered activity using separate conversion factors for stress and rest imaging provided in the International Commission on Radiological Protection (ICRP) Publication 80.…”

“…Using two scanner-specific cardiac conversion factors found in the literature for calculating effective dose from the dose-length product, the mean effective dose for the CTA was 12.6 ± 8.6 and 13.5 ± 9.2 mSv. 21,22 The lower Huda et al 22 conversion factor of 0.026 was subsequently used for all CTA calculations. The lowest effective dose was 1.8 mSv and the highest was 43.0 mSv with a median of 9.4 mSv (IQR 7.1-15.0 mSv).…”

“…There is ongoing controversy on how to calculate effective dose from CTA so we used two of the recently published conversion factors derived specifically for cardiac studies to be as current as possible. 21,22 The older, more commonly used conversion factor of 0.014 was originally from the 2004 European Guidelines for Multislice Computed Tomography Appendix C based on work by Shrimpton et al using single-slice scanners. 34 Effective dose from nuclear cardiology procedures was estimated using the most recent ICRP effective dose conversion factors for Tc99m sestamibi, which are based on tissue-weighting factors from ICRP Publication 80, while CTA conversion factors were based on tissue-weighting factors from ICRP Publication 103.…”

Radiation dose at CT coronary angiography: how low can we go?

“…Dose length product (DLP) was 28 milligrays per centimetre (mGy.cm) and this equates to an effective dose of 0.8 millisievert (mSv) (specialist software 2 was used to determine effective dose for the patient, based on tissue factors as per the recommendations of the International Commission on Radiological Protection [ICRP 103]) 3 with corrections for scanned region and patient size. 4 In over four years and more than 900 scans this was our lowest DLP for CTCA. Post-processing on GE Advantage Windows v4.4 workstation using cardiac autoanalysis was uncomplicated.…”