Lowering the voltage to 100 kV is an effective method of reducing the radiation of coronary computed tomographic angiography (CTA). It is unknown, however, whether one could use a 100-kV CTA protocol with overweight or obese patients. We, thus, evaluated the effect of increasing body mass index (BMI) on various image quality parameters of 100-kV CTA. We also compared the radiation dose and diagnostic accuracy of 100-kV CTA with CTA performed at 120 kV. Three different protocols were studied: 120 kV, retrospective; 100 kV, retrospective; and 100 kV, prospective. The image quality and radiation doses were analyzed for each protocol. The effect of increasing BMI was also examined. A worsening of the noise, contrast-to-noise, and signal-to-noise ratios occurred with increasing BMI and decreasing voltages. The radiation exposure was significantly lowered with the 100-kV protocol and with prospective gating. Despite this image degradation, however, diagnostic images were obtained with 100-kV CTA, even in overweight and many obese subjects. Of the 66 subjects referred for invasive angiography because of the findings from CTA, 55 were correctly characterized (overall positive predictive value [PPV] of 83.3%). This PPV remained reasonable, irrespective of the voltage, until a BMI of 35 kg/m 2 was reached (PPV for 100-kV protocol 90.0% [27 of 30]; PPV for a BMI of ≥25 kg/m 2 but <30 kg/m 2 84.4% [27 of 32]; and PPV for a BMI of ≥30 kg/m 2 but <35 kg/m 2 81.8% [18 of 22]). In conclusion, 100-kV coronary CTA is feasible in overweight and many obese subjects.
Coronary multidetector computed tomographic angiography (CTA) has generated considerable enthusiasm among cardiologists and radiologists. Although the greater spatial resolution of multidetector CTA has resulted in significant improvement in the diagnostic accuracy of obstructive stenoses, this advance has also resulted in an undesirable increase in radiation exposure. Lowering the voltage from 120 to 100 kV would be an effective method of lowering the radiation exposure of CTA. Studies using this dose-reducing strategy have successfully reduced the exposure of CTA by ≥50%. This strategy, however, has generally been recommended for nonobese and nonoverweight patients. Lowering the voltage to 100 kV increases the image noise and could ultimately result in unacceptable image degradation. It is unknown whether one could use a 100-kV CTA protocol with overweight or obese patients. We, thus, evaluated the effect of increasing body mass index (BMI) on the image quality of 100-kV CTA. We also compared the radiation dose and diagnostic accuracy of 100-kV CTA with CTA performed at 120 kV.
Methods
The present study included 914 patients who had been referred for coronary CTA (302 patients were included in the 120-kV retrospective protocol, 253 in the 100-kV retrospective protocol, and 359 in the 100-kV prospective protocol). We excluded those with atrial fibrillation, significant renal insufficiency, coronary artery bypass grafting, or a significant contrast allergy. All participants had given previous written consent. The local institutional review board had approved the study. All participants underwent questioning regarding the presence of cardiovascular risk factors. Diabetes was defined as a previously documented fasting glucose level of ≥126 mg/dl or treatment with antidiabetic medications. Hypertension was either self-reported or defined as a previously documented systolic blood pressure of ≥140 mm Hg or diastolic blood pressure of ≥90 mm Hg, or both. Hyperlipidemia was either self-reported or defined as a previously documented total cholesterol level of ≥200 mg/dl. Moreover, all patients were questioned regarding current tobacco use.
Contrast-enhanced CTA was performed with a 64-slice scanner (Lightspeed VCT, GE Healthcare, Princeton, New Jersey). Unless clinically contraindicated, intravenous metoprolol and/or verapamil was given to achieve a heart rate of 45 to 65 beats/min. All patients underwent an initial noncontrast-enhanced computed tomographic scan (100 kV, 200 mA) for coronary artery calcium scoring. The total calcium burden was quantified using the Agatston method. In addition, a low-dose, automatic timing, bolus protocol (100 kV, 50 mA, 20 ml contrast [5 ml/s] followed by a 20-ml saline chaser [5 ml/s] using a twin injector) was used to optimize the delay time from the start of injection to the start of scanning at the level of the left main origin. Approximately 90 to 100 ml of iodinated contrast (Visipaque, GE Healthcare, or Omnipaque, GE Healthcare) was administered at a flow rate of 5 to 6 ml/s for the computed tomographic angiographic images.
The participants underwent CTA with the following scan parameters: 64 × 0.6 mm collimation, rotation time 350 ms, pitch 0.18 to 0.22, and tube current 300 to 450 mA. The tube current was selected as a function of BMI (BMI <25 kg/m 2 , 300 mA; BMI ≥25 kg/m 2 but <30 kg/m 2 , 350 mA; BMI ≥30 kg/m 2 but <40 kg/m 2 , 400 mA; and BMI ≥40 kg/m 2 , 450 mA). In addition, an electrocardiographic dose modulation program was used in the studies that included retrospective gating.
The source image data sets were loaded to reconstruct both the thin-slab maximum intensity projections and the curved multiplanar reconstructions. Transaxial images were reconstructed at a 0.6-mm slice width. For those undergoing retrospective CTA, multiphase reconstructions at 10% increments were performed from 5% to 95% of the RR interval for ejection fraction calculation and 5% increments from 70% to 80% for the assessment of stenoses. For those undergoing prospective CTA, the reconstructions were performed in mid-diastole at 75% of the RR interval. Pertinent scan parameters were collected and analyzed.
Semiquantitative indexes of image quality were collected, as previously reported by Hausleiter et al. Specifically, image noise was defined as the standard deviation of the Hounsfield density values within a 4.5 to 5.5 mm 2 region of interest in the mid-left ventricle. The contrast-to-noise ratio (CNR) was defined as the difference between the mean density of the mid-left ventricular chamber and the mean density of the mid-left ventricular septum, which was then divided by the image noise. The signal-to-noise ratio (SNR) was calculated as the mean density of the contrasted left main lumen divided by the standard deviation of this mean value. No region of interest included any calcification.
All computed tomographic angiographic studies were interpreted by 2 experienced readers independently. Significant stenoses were detected using axial slices, curved multiplanar reconstructions, and maximum intensity projections. Significant stenoses were defined as a ≥50% luminal narrowing of the left main artery and/or a ≥70% luminal narrowing elsewhere. Patients with significant stenoses as defined were referred for invasive coronary angiography. These angiograms were performed by independent cardiologists according to standard techniques. The results of these invasive coronary angiograms were correlated with the corresponding coronary findings from CTA to determine the positive predictive value (PPV) of each protocol.
The effective dose was calculated as the product of the total scan dose-length product and an accepted chest conversion coefficient (k = 0.017 mSv × mGy −1 × cm −1 ). The total scan dose-length product was defined as the sum of the dose-length products of the calcium scoring scan, timing bolus scan, and the actual contrast-enhanced computed tomographic angiographic scan. The dose-length product averages the radiation dose in the x-, y-, and z-axes and was obtained according to the manufacturer’s specifications of each study. The effective dose of coronary CTA was estimated using a method proposed by the European Working Group for Guidelines on Quality Criteria in Computed Tomography.
Continuous data are expressed as the mean ± SD or median with the interquartile range. Categorical data are reported as percentages. Clinical characteristics are reported as a function of the CTA protocols used. Finally, we also reported the image quality parameters as a function of protocol and BMI category.
Results
Table 1 lists the characteristics of the study participants. The study patients were primarily older, overweight men. Most were asymptomatic, with ≤1% exhibiting angina and <13% reporting of atypical chest pain. The atherosclerosis burden was severe, with a median coronary artery calcium score of 469. A high frequency of cardiovascular risk factors was present; the most prevalent conditions were hypertension (53.2%), a history of tobacco use (43.6%), and hyperlipidemia (83.7%). A low prevalence of diabetes (7.2%) was noted. Other referral requests included abnormal/equivocal stress test findings (26.9%) or abnormal/equivocal perfusion study findings (4.9%). No significant differences were found among the various protocols regarding the frequency of angina, abnormal/equivocal stress test findings, or abnormal/equivocal perfusion study findings. In addition, no significant differences were noted with the median BMI or the frequency of hypertension, hyperlipidemia, or history of tobacco use. During the latter portion of the study, as the negative predictive value of coronary CTA became more evident, an increasing number of patients had been referred to rule out significant occlusive coronary disease. Thus, in the 100-kV prospective group, the median coronary artery calcium score was significantly lower median, a greater number of patients had diabetes, and a greater number had an atypical chest pain presentation.
| Variable | 120 kV (n = 302) | 100-kV Retrospective (n = 253) | 100-kV Prospective (n = 359) |
|---|---|---|---|
| Age (years) | 62.4 ± 9.7 | 62.4 ± 8.9 | 61.1 ± 9.4 |
| Men | 268 (89.0%) | 218 (87.2%) | 208 (85.6%) |
| Body mass index (kg/m 2 ) | 27.6 ± 4.5 | 27.4 ± 4.3 | 27.9 ± 4.3 |
| Coronary artery calcium score (Agatston) | 521 (162, 1,084) | 557 (225, 1,297) | 369 (76, 842) |
| Angina pectoris | 3 (1.0%) | 1 (0.4%) | 3 (0.8%) |
| Chest pain, atypical | 27 (8.9%) | 22 (8.7%) | 65 (18.1%) |
| Abnormal/equivocal stress test finding | 87 (28.8%) | 62 (24.5%) | 97 (27.0%) |
| Abnormal/equivocal perfusion study finding | 18 (6.0%) | 13 (5.1%) | 14 (3.9%) |
| Hypertension | 161 (53.3%) | 141 (55.7%) | 184 (51.2%) |
| Diabetes mellitus | 17 (5.6%) | 13 (5.1%) | 36 (10.0%) |
| Hyperlipidemia | 244 (80.8%) | 220 (87.0%) | 301 (83.8%) |
| History of tobacco use | 133 (44.0%) | 109 (43.1%) | 157 (43.7%) |
Table 2 lists the scan parameters and radiation doses of the 3 different protocols. No clinically meaningful differences in the systolic blood pressure, diastolic blood pressure, pitch, heart rate, or scan length were noted. As expected, a dramatic lowering of the estimated radiation dose occurred from 18.0 ± 5.9 mSv with the 120-kV coronary CTA protocol to 5.0 ± 1.0 mSv with the 100-kV retrospective protocol. Additional lowering of the radiation dose to 1.8 ± 0.2 mSv occurred with 100-kV prospective gating.
| Variable | 120 kV (n = 302) | 100-kV Retrospective (n = 235) | 100-kV Prospective (n = 359) |
|---|---|---|---|
| Systolic blood pressure (mm Hg) | 125.5 ± 14.7 | 125.2 ± 15.1 | 127.2 ± 16.9 |
| Diastolic blood pressure (mm Hg) | 67.5 ± 11.3 | 70.9 ± 7.9 | 74.2 ± 8.4 |
| Heart rate (beats/min) | 64.4 ± 13.3 | 68.5 ± 12.1 | 58.9 ± 6.5 |
| Scan length (mm) | 130.0 ± 25.1 | 127.1 ± 32.5 | 139.1 ± 4.1 |
| Pitch | 0.2 ± 0.02 | 0.2 ± 0.02 | 0.2 ± 0.02 |
| Dose estimate (mSv) | 18.0 ± 5.9 | 5.0 ± 1.0 | 1.8 ± 0.2 |
Table 3 lists the image quality indexes of the 3 different scan protocols. As the tube voltage was lowered, a worsening of the image noise, CNR, and SNR occurred. Also, some mild worsening of the various image quality parameters resulted when using prospective gating.
| Variable | 120 kV (n = 302) | 100-kV Retrospective (n = 253) | 100-kV Prospective (n = 359) |
|---|---|---|---|
| Image noise (Hounsfield units) | 32.9 ± 12.1 | 52.9 ± 16.3 | 59.0 ± 20.0 |
| Contrast-to-noise ratio | 8.0 ± 4.0 | 5.8 ± 2.9 | 5.6 ± 2.7 |
| Signal-to-noise ratio | 13.8 ± 5.6 | 9.9 ± 3.9 | 8.2 ± 3.3 |
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