The radiation dose in retrospective gated tdcoronary computed td tomography (CCT) Saeed AL Ahmari, Ghormallah AL Zahrani, Sumiah AL Helali, Samir AL Dulikan, Abdullah Bafagih, HibaKhashojji Prince Sultan Cardiac Center
Background CCT is becoming an important cardiac imaging modality, and is been widely used. We aimed to measure the amount of radiation dose that patients exposed to during clinically i ll indicated CCT
Methods All patients were scanned using retrospective gating gon a 64 slice CCT, for clinically indicated reasons. The tube voltage used was 120 kvp All patients received B blockers, and sublingual Isordil Calcium score measurements, & Coronary CT angiogram were performed. The radiation dose was calculated from different parameters in millisievert (msv ).
Results Chest pain was the indication for the CT in 80 Chest pain was the indication for the CT in 80 % of patients
Clinical Parameters Number Age Y Gender M/F DM HTN Hyperliped imia Smooker Family Hx of CAD BMI 1287 52 ± 65/35 % 37 % 58 % 23.6 % 13 % 32 % 29.99 13.2 ±8.5
Results The mean radiation dose was: 15±10 msv
Higher radiation dose in Obese patients 16 14 12 15 12 P<0.002 msv 10 8 6 4 2 35< 35> 0 BMI
High radiation for CABG patients 25 22 20 P<0.0001 msv 15 10 14 CABG Non CABG 5 0 Category 1
Published International radiation dose The PROTECTION I (Prospective Multicenter Study On Radiation Dose Estimates of Cardiac CTA In Daily Practice) trial studied the estimated radiation dose associated with coronary CTA at 50 international study sites. The median effective radiation dose was 12 msv. Small relative differences in estimated radiation dose were correlated with patient related factors (patient weight, absence of sinus rhythm). Larger differences were correlated withuse ofspecific strategies to reduce the study radiation dose and with differences in CT equipment.
Published International Radiation Dose Median effective radiation dose (which is a calculated rather than empirically measured quantity) for coronary CTA with current technology was 12 msv in a cross sectional international study of 50 sites (PROTECTION I) Individual sites in this study varied from a median of 5 to 30 msv. In a 15 hospital imaging registry in Michigan in 2007, prospective use of a set of best practice radiation dose reduction recommendations resulted in a reduction in the average scan effective radiation dose from 21 msv to 10 msv with no reduction in image quality.
Calcium Score is high in Males 120 100 80 P< 0.001 60 40 Male Female 20 0 Category 1
High Calcium score in diabetics 140 120 100 P< 0.0001 80 60 40 DM Non DM 20 0 Category 1
HbA1cCalcium Score Correlates 160 140 120 P<0.01 100 80 8< 60 8> 40 20 0 HbA1c
High Calcium score in Hypertensives 140 120 100 80 60 40 20 P < 0.0001 HTN non HTN 0 Category 1
High Calcium score in Hyperlipidimics 200 180 160 140 P< 0.0001 120 100 60 40 Hyperlipedimic 80 Non Hyperlipedimic 20 0 Category 1
Methods Administrative claims were used to identify cardiac imaging procedures performed from 2005 to 2007 in 952,420 nonelderly insured adults (18 65 y)
Results A total of 90,121 (9.5%) individuals underwent at least 1 cardiac imaging procedure using radiation. Among patients who underwent 1 cardiac imaging procedures, the meancumulative effective dose over 3 years was 23.1 msv Myocardial perfusion imaging accounted for 74% of the cumulative effective dose. The annual population based rate of receiving an effective dose of 3 to 20 msv/year was 89.0 per 1,000; and 3.33 per 1,000 for cumulative doses 20 msv/year. Annual effective doses increased with age and were generally higher among men.
In the U.S. US population, the risk of CVD increases with age and is estimated to affect 40% of the middle aged population, with lifetime risk as high as 70% for those with multiple risk factors
- 60 year oldman hada a lifetime attributable risk of 1 in 1,911 (0.05%) for the development of cancer. Thus, the 60 year old individual is 800 times more likely to die of a cardiovascular event than to have attributable cancer from the test. Because CVD is expected to be the cause of death in more than one third of this population, imaging gin 10% appears reasonable.
Managing the risks of CT, fluoroscopy, and nuclear medicine imaging procedures depends on twoprinciples of radiation protection: appropriate justification for ordering and performing each procedure, careful optimization ofthe radiation dose used during each procedure
cardiac CT angiography performed today use either dose modulation or prospective gating, & the mean dose is estimated to be 10 msv,
Radiation from natural sources Estimate to range from approximately 3 msv/year at sea level and increasing to 7 msv/year with elevation. Airline pilots, although hestimated to be occupationally exposed to 150 msv during their hicareer, have never been demonstrated dto have an increased prevalence of cancer,
Occupational hazard of radiation Interventional cardiologists and radiologists have exposure estimates of 16 to 18 msv/year, but without documented increases in cancer rates.
The best aaabeest available estimates of the ecancer ce risk associated with different levels of radiation were compiled by the National Academy of Science's Biological leffects of Ionizing Radiation (BEIR) VII Phase 2 report. But all of those estimates t are based on studies of Japanese atomic bomb survivors, high altitude air crews, or workers in the nuclearindustry. There are very little direct data from cardiac imaging patients.
Thelinear no threshold no model does not reflect reality. It's not a direct relationship for every patient, because the variables include age, genetic background, and otherpredisposing factors for cancer
And although 50 msv is the generally accepted maximum safe annual exposed dose for radiation workers and hospital staff, there have been cases where people exposed to much lower doses have developed types of cancer most strongly associated with radiation effects
The collective dose received from medical uses of radiation has increased >700% between1980 and 2006
Radiation Dose and Risk of Cancer Single 64 slice CCTA was associated with a lifetime cancer risk estimate of 1 in 143 (0.7 percent) in a 20 year old woman 1 in 466 (0.2 percent) in a 60 year old woman 1 in 686 (0.15 percent) in a 20 year old man 1 in 1241 (0.0808 percent) for a 60 year old man.
Balancing risk and benefit of CCTA 1. Missing the right diagnosis in symptomatic patients by avoiding imaging is a risk 2. Improved quality of life due to optimized therapy is difficult to quantify 3. Improved survival due to optimal therapy vs. possible decrease in survival from radiation induced cancer 4. Benefit/risk ratio greater in symptomatic patients, high risk patients, older individuals, and men 5. Current evidence does not support imaging with ionizing radiation in asymptomatic individuals 6. Benefit/risk ratio is improving as technical developments facilitate low dose imaging with high image quality
Balancing risk and benefit of CCTA In the DIAD (Detection of Ischemia in Asymptomatic Diabetics) study, investigators examined the use of adenosine stress SPECT imaging for CAD screening in 1,123 asymptomatic diabetic patients. The cumulative cardiac event rates averaged only 0.6% per year. Although participants with moderate or large perfusion df defects had hdgreater event rates than those with small perfusion defects or normal SPECT studies, the use of SPECT for screening did not improve patient outcomes.
How to reduce radiation dose Lower tube current (expressed in milliampere [ma] or the product of tube current and exposure time, expressed in milliampere seconds [mas]). Lower tube voltage (peak kilovolt, kvp). Greater slice thickness (mm).
How to reduce radiation dose Higher table advance per gantry rotation expressed as a fraction of the combined width of all slices acquired simultaneously (also referred to as pitch, which is dimensionless). A higher value of pitch indicates faster table advance and, hence, less overlap of irradiation bt between successive gantry rotations tti. Lower patient body mass (kg).
Radiation Reduction Techniques Reduce KV if BMI< 30 ( 120 to 100 50 % ) ECG pulsing 20 % Step and shot sequential ilvs. spiral limaging i 68% Uniform beta blocker preparation
Coronary Computed Tomography Angiography With Coronary Computed Tomography Angiography With Low Radiation Exposure Curved MPR of RCA (A), LAD (B), and LCX (C) obtained by contrast enhanced dual source computed tomography with prospective triggering and with a tube voltage of 100 kvp. Estimated effective radiation dose for this scan was 1.8 msv.
Conclusion This study is the first national study to explore the radiation dose from cardiac CT The radiation dose from CCT in our patients is comparable to that published internationally Overweight patients and post CABG patients are exposed to significantly higher radiation dose during CCT. Measures have to be applied to minimize the radiation dose during CCT.
For a normal population, this would translate into an estimated average lifetime risk of pproximately 0.05% 05% to 0.08% 08% of dying from a malignancy resulting from a typical coronary CT angiogram with an E of 10 msv. This risk is superimposed on the 21% intrinsic population averaged averaged lifetime risk in the United States of dying of a malignancy
TheSiemens single source source 64 was the best followed by the Phillips 64 and the Siemens dual source 64. The worst two studied were the Toshiba 64 and the GE 64.
1.Use of the lowest settings of tube current and tube voltage consistent with diagnostic image quality In the PROTECTION I study, reduced tube voltage from greater than or equal to 120 to 100 kv was used in only 5% of subjects but was asassociated ated with an estimated 46% relative ereduction in radiation adat dose 2. Use of ECG controlled tube current modulation to reduce tube current during the portions of the cardiac cycleunlikely to be used for image reconstruction (typically systole). Increasing the length of time during the cardiac cycle during which the tube current is reduced is more feasible in scanners with higher temporal resolution. In the PROTECTION I study, ECG controlled dtube current modulation was used in 73% of patients t who had spiral CT data acquisition and was associated with a 25% relative reduction in estimated radiation dose
Scan length minimization Uniform beta blocker preparation kvp reduction from 120 to 100 for patients with a body mass index less than 30 Narrow window retrospective gating (70 x 70 window) or prospective gating Use of tube current modulation
3.Prospective triggering g or step and shoot mode with radiation output only during predetermined portions of the cardiac cycle (also called sequential scanning).. In the PROTECTION I study, this method was used in only 6% of patients and was associated with a 78% relative reduction in estimated radiation dose 4. Heart rate dependent increase of pitch (181), or eliminating slice overlap altogether (with area detectors or scanners that can cover the entire length of the heart in the z direction with 1 gantry rotation).