Coronary CTA Using Radiation Exposure Similar to CXR
Coronary CTA Using Radiation Exposure Similar to CXR
This study is the first to demonstrate the feasibility of ultra-low-dose CCTA using MBIR for noise reduction in a broad spectrum of patients as seen in daily clinical routine over a wide range of BMI. Despite a substantial radiation dose reduction, median image quality was not significantly different to standard CCTA and 97.8% of coronary segments were interpretable yielding comparable or even higher image quality scores than reported in other CCTA studies.
The comparable image quality of ultra-low-dose vs. standard CCTA was enabled by an effective noise reduction by MBIR resulting in an even lower image noise in ultra-low-dose compared with standard CCTA. In combination with the shift towards higher beam attenuation by iodine in low tube voltage scanning, this resulted in an SNR substantially higher than in standard CCTA. Despite increasing noise due to less photon emission at such low tube current and peak voltage, this containment of radiation dose was enabled by one of the latest iterative reconstructions algorithms such as MBIR, which incorporates modelling of the photon and noise statistics, but also involves modelling of system optics. Recently, CCTA with reasonable image quality and a radiation dose as low as 0.06 mSv has been achieved in a study population with <100 kg using a similar modern iterative reconstruction algorithm from another vendor (SAFIRE, Siemens Healthcare, Forcheim, Germany). However, their reported SNR for the coronary arteries averaged 5, whereas in our study this value averaged 27–29.
It is relevant to point out that the median effective radiation dose of 0.21 mSv achieved with the present protocol is even substantially lower than, for example, the effective radiation dose for standard coronary calcium score scanning. Coronary computed tomography angiography at such low exposure opens new doors for non-invasive assessment. This is particularly remarkable as our protocol resulted in a median exposure of 0.21 mSv, close to the dose of a postero-anterior and lateral chest X-ray, which is reported to range from 0.05 to 0.24 mSv in the literature.
It is generally accepted that the strength of CCTA lies in its high negative predictive value for CAD. Consequently, the consensus is to consider the use of CCTA mainly in low-to-intermediate probability populations due to its excellent ability to rule out CAD. As such populations, however, are inherently characterised by a low risk for cardiac events it is unlikely that any diagnostic or therapeutic procedure will further improve the outcome. This put the bars very high for any technique to keep a positive balance of harms and benefits for any diagnostic tool, evoking a vivid discussion on the potential carcinogenic risk of CCTA and its justification for a purely diagnostic test. As a consequence, this has fuelled an intense search for strategies to minimise radiation exposure while maintaining image quality. In the past years radiation dose reduction has been successfully achieved by introducing scanning protocols with prospective ECG triggering, limiting the beam to a narrow diastolic phase. The new protocol represents yet another milestone for further substantial dose reduction from CCTA as new reconstruction algorithms for noise reduction allow decreasing tube current and voltage. This further shifts the tip of the benefit-to-harm balance favourably towards clinical benefits, where potentially even screening and monitoring of CAD therapy effects may no longer appear prohibitive for radiation safety concerns. It appears foreseeable that the presented development of latest iterative reconstruction algorithms will soon enter the clinical arena for CCTA and its implementation on different scanners from multiple vendors will allow a widespread use offering substantial decrease in radiation from CCTA to a large patient population in the near future.
It may be perceived as a potential limitation of this study that diagnostic accuracy and stenosis measurement was not compared with invasive coronary angiography. However, the high accuracy of standard CCTA has been previously established and, therefore, it seems reasonable to use it as a ground of truth reference standard. Another limitation is that the MBIR algorithm is currently not yet approved for clinical use in CCTA, whereas it is commercially available for non-cardiac use since several years. Furthermore, prevalence of coronary calcifications in the present study population was moderate. It cannot be excluded that in patients with higher prevalence of massive coronary calcifications image quality may be impaired, particularly in ultra-low-dose CCTA. However, we are in line with the general recommendations to use CCTA preferentially in patients with low CAD risk profile and, thus, lower prevalence of coronary calcifications. Finally, larger studies may be helpful to establish how the promising results of the present study can be implemented into daily routine.
In conclusion, CCTA acquisition with diagnostic image quality is feasible at an ultra-low radiation dose of 0.21 mSv, e.g. in the range reported for a postero-anterior and lateral chest X-ray.
Discussion
This study is the first to demonstrate the feasibility of ultra-low-dose CCTA using MBIR for noise reduction in a broad spectrum of patients as seen in daily clinical routine over a wide range of BMI. Despite a substantial radiation dose reduction, median image quality was not significantly different to standard CCTA and 97.8% of coronary segments were interpretable yielding comparable or even higher image quality scores than reported in other CCTA studies.
The comparable image quality of ultra-low-dose vs. standard CCTA was enabled by an effective noise reduction by MBIR resulting in an even lower image noise in ultra-low-dose compared with standard CCTA. In combination with the shift towards higher beam attenuation by iodine in low tube voltage scanning, this resulted in an SNR substantially higher than in standard CCTA. Despite increasing noise due to less photon emission at such low tube current and peak voltage, this containment of radiation dose was enabled by one of the latest iterative reconstructions algorithms such as MBIR, which incorporates modelling of the photon and noise statistics, but also involves modelling of system optics. Recently, CCTA with reasonable image quality and a radiation dose as low as 0.06 mSv has been achieved in a study population with <100 kg using a similar modern iterative reconstruction algorithm from another vendor (SAFIRE, Siemens Healthcare, Forcheim, Germany). However, their reported SNR for the coronary arteries averaged 5, whereas in our study this value averaged 27–29.
It is relevant to point out that the median effective radiation dose of 0.21 mSv achieved with the present protocol is even substantially lower than, for example, the effective radiation dose for standard coronary calcium score scanning. Coronary computed tomography angiography at such low exposure opens new doors for non-invasive assessment. This is particularly remarkable as our protocol resulted in a median exposure of 0.21 mSv, close to the dose of a postero-anterior and lateral chest X-ray, which is reported to range from 0.05 to 0.24 mSv in the literature.
It is generally accepted that the strength of CCTA lies in its high negative predictive value for CAD. Consequently, the consensus is to consider the use of CCTA mainly in low-to-intermediate probability populations due to its excellent ability to rule out CAD. As such populations, however, are inherently characterised by a low risk for cardiac events it is unlikely that any diagnostic or therapeutic procedure will further improve the outcome. This put the bars very high for any technique to keep a positive balance of harms and benefits for any diagnostic tool, evoking a vivid discussion on the potential carcinogenic risk of CCTA and its justification for a purely diagnostic test. As a consequence, this has fuelled an intense search for strategies to minimise radiation exposure while maintaining image quality. In the past years radiation dose reduction has been successfully achieved by introducing scanning protocols with prospective ECG triggering, limiting the beam to a narrow diastolic phase. The new protocol represents yet another milestone for further substantial dose reduction from CCTA as new reconstruction algorithms for noise reduction allow decreasing tube current and voltage. This further shifts the tip of the benefit-to-harm balance favourably towards clinical benefits, where potentially even screening and monitoring of CAD therapy effects may no longer appear prohibitive for radiation safety concerns. It appears foreseeable that the presented development of latest iterative reconstruction algorithms will soon enter the clinical arena for CCTA and its implementation on different scanners from multiple vendors will allow a widespread use offering substantial decrease in radiation from CCTA to a large patient population in the near future.
It may be perceived as a potential limitation of this study that diagnostic accuracy and stenosis measurement was not compared with invasive coronary angiography. However, the high accuracy of standard CCTA has been previously established and, therefore, it seems reasonable to use it as a ground of truth reference standard. Another limitation is that the MBIR algorithm is currently not yet approved for clinical use in CCTA, whereas it is commercially available for non-cardiac use since several years. Furthermore, prevalence of coronary calcifications in the present study population was moderate. It cannot be excluded that in patients with higher prevalence of massive coronary calcifications image quality may be impaired, particularly in ultra-low-dose CCTA. However, we are in line with the general recommendations to use CCTA preferentially in patients with low CAD risk profile and, thus, lower prevalence of coronary calcifications. Finally, larger studies may be helpful to establish how the promising results of the present study can be implemented into daily routine.
In conclusion, CCTA acquisition with diagnostic image quality is feasible at an ultra-low radiation dose of 0.21 mSv, e.g. in the range reported for a postero-anterior and lateral chest X-ray.
