Rationale and Objectives
To assess image quality, radiation dose, and diagnostic accuracy of 70-kVp high-pitch computed tomography pulmonary angiography (CTPA) using 40 mL contrast agent and sinogram affirmed iterative reconstruction (SAFIRE) compared to 100-kVp CTPA using 60 mL contrast agent and filtered back projection.
Materials and Methods
Eighty patients underwent CTPA at either 70 kVp (group A, n = 40; 3.2 pitch, 40 mL contrast medium, and SAFIRE) or 100 kVp (group B, n = 40; 1.2 pitch, 60 mL contrast medium, and filtered back projection). Signal-to-noise ratio and contrast-to-noise ratio were calculated. Subjective image quality was evaluated using a five-grade scale, and diagnostic accuracy was assessed. Radiation doses were compared.
Results
Computed tomography values, signal-to-noise ratio, and contrast-to-noise ratio of pulmonary arteries were higher in group A compared to group B (all P < 0.001). Subjective image quality showed no difference between the two groups ( P = 0.559) with good interobserver agreement (κ = 0.647). No difference was found regarding diagnostic accuracy between the two groups ( P > 0.05). The effective dose for group A was lower by 80% compared to group B ( P < 0.001).
Conclusions
70-kVp high-pitch CTPA with reduced contrast media and SAFIRE provides comparable image quality and substantial radiation dose savings compared to a routine CTPA protocol.
Introduction
Pulmonary embolism (PE) is considered the third most common acute cardiovascular disease after myocardial infarction and stroke . The prompt diagnosis and early initiation of appropriate medical treatment can reduce mortality of acute PE . Therefore, it is imperative to correctly diagnose PE in a timely fashion. Currently, computed tomography pulmonary angiography (CTPA) is established as the first-line imaging procedure to exclude PE . Wide availability and overuse of CTPA may result in substantial radiation exposure to patients, especially those requiring multiple follow-up CTPA studies for the evaluation of PE. Thus, increasing attention is now being paid to the optimization of radiation exposure from CTPA examinations . An additional concern is the contrast-induced nephropathy related to the administration of large doses of iodinated contrast media . In recent years, usage of reduced contrast agent volume has been advocated for CTPA .
Multiple strategies have been developed to reduce radiation dose and contrast volume of computed tomography angiography (CTA), including tube current modulation techniques, reduced tube voltage, automatic tube potential selection, high-pitch acquisition, iterative reconstruction algorithms, and combinations of these techniques . Among these dose-saving techniques, reduced tube voltage acquisition has been shown to be a powerful tool to lower radiation dose and contrast volume. A tube voltage of 80 kVp has been confirmed to be feasible for CTPA, with substantial reduction of radiation dose . Recently, Wichmann et al. demonstrated the feasibility of 70-kVp CTPA with 1.2 pitch and 70 mL contrast agent, and the lowest effective dose (ED) reported was 0.94 ± 0.17 mSv for single-source CTPA . By combining reduced tube voltage, high-pitch acquisition, and iterative reconstruction techniques, radiation dose and contrast agent volume can be substantially lowered. Lu et al. obtained sufficient image quality and a mean ED as low as 0.9 mSv in patients with suspected PE who underwent 80-kVp high-pitch CTPA with iterative reconstruction .
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Materials and Methods
Patients
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Computed Tomography (CT) Scanning Protocol
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Image Quality Evaluation
Objective Image Quality Evaluation
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SNR=CT valuepulmonary artery/background noise SNR
=
CT value
pulmonary artery
/
background noise
CNR=(CT valuepulmonary artery−CT valuemuscle)/background noise CNR
=
(
CT value
pulmonary artery
−
CT value
muscle
)
/
background noise
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Subjective Image Quality Evaluation
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Detection of Pulmonary Emboli
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Radiation Dose Estimation
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Statistical Analysis
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Results
Subjects
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Image Quality Evaluation
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TABLE 1
Objective Image Quality Measurements in the Two CTPA Groups
Location Group A Group B_P_ Value MPA CT number (HU) 712.2 ± 129.3 401.4 ± 81.6 <0.001 SNR 41.8 ± 12.5 29.1 ± 14.3 <0.001 CNR 36.9 ± 11.7 25.4 ± 13.0 <0.001 RPA CT number (HU) 682.1 ± 105.8 384.6 ± 74.1 <0.001 SNR 40.1 ± 11.7 28.0 ± 14.3 <0.001 CNR 35.2 ± 10.8 24.3 ± 13.0 <0.001 LPA CT number (HU) 678.0 ± 107.4 379.4 ± 79.5 <0.001 SNR 39.9 ± 11.8 27.6 ± 14.1 <0.001 CNR 35.0 ± 11.0 23.9 ± 12.8 <0.001 RULA CT number (HU) 650.7 ± 117.8 359.6 ± 88.0 <0.001 SNR 38.3 ± 12.3 26.0 ± 12.1 <0.001 CNR 33.4 ± 11.5 22.2 ± 11.0 <0.001 RILA CT number (HU) 633.3 ± 85.1 350.3 ± 77.0 <0.001 SNR 37.3 ± 10.8 25.6 ± 13.8 <0.001 CNR 32.4 ± 9.9 21.8 ± 12.5 <0.001 LULA CT number (HU) 653.1 ± 99.2 345.2 ± 102.1 <0.001 SNR 38.6 ± 12.3 25.1 ± 13.9 <0.001 CNR 33.7 ± 11.4 21.4 ± 12.7 <0.001 LILA CT number (HU) 677.3 ± 83.3 347.5 ± 77 <0.001 SNR 39.8 ± 11.2 25.4 ± 13.6 <0.001 CNR 34.9 ± 10.3 21.7 ± 12.3 <0.001 Muscle CT number (HU) 84.0 ± 21.3 52.0 ± 8.6 <0.001 Noise (HU) 17.9 ± 4.2 14.8 ± 2.9 <0.001
CNR, contrast-to-noise ratio; CT, computed tomography; CTPA, computed tomography pulmonary angiography; HU, Hounsfield units; LILA, left inferior lobe pulmonary artery; LPA, left pulmonary artery; LULA, left upper lobe pulmonary artery; MPA, main pulmonary artery; RILA, right inferior lobe pulmonary artery; RPA, right pulmonary artery; RULA, right upper lobe pulmonary artery; SNR, signal-to-noise ratio.
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TABLE 2
Subjective Image Quality Evaluation in the Two CTPA Groups
Readers Group A Group B_P_ Value Reader 1 1.05 ± 0.22 1.08 ± 0.27 0.646 Reader 2 1.03 ± 0.16 1.08 ± 0.27 0.308 Both readers 1.03 ± 0.16 1.05 ± 0.22 0.559 Kappa value 0.655 0.640 —
CTPA, computed tomography pulmonary angiography.
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Detection of Pulmonary Emboli
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TABLE 3
Pulmonary Embolism Detected by Two Readers
True Positive False Negative False Positive True Negative Group A Group B Group A Group B Group A Group B Group A Group B Patients Reader 1 7 7 0 0 0 0 33 33 Reader 2 6 7 1 0 0 0 33 33 Central PE Reader 1 15 8 0 0 0 0 345 352 Reader 2 15 8 0 0 0 0 345 352 Peripheral PE Reader 1 23 29 1 3 2 3 774 765 Reader 2 21 27 3 5 1 2 775 766
PE, pulmonary embolism.
The data for the central arteries refer to the number of affected main and lobar pulmonary arteries. The data for the peripheral arteries refer to the number of affected pulmonary segmental and subsegmental arteries.
TABLE 4
Diagnostic Accuracy Comparison of the Two CTPA Protocols
Sensitivity Specificity PPV NPV Accuracy_P_ Value Group A Group B Group A Group B Group A Group B Group A Group B Group A Group B Patients Reader 1 100
(59.0–100) 100
(59.0–100) 100
(89.4–100) 100
(89.4–100) 100
(59.0–100) 100
(59.0–100) 100
(89.4–100) 100
(89.4–100) 100
(71.4–100) 100
(71.4–100) >0.99 Reader 2 85.7
(42.1–99.6) 100
(59.0–100) 100
(89.4–100) 100
(89.4–100) 100
(54.1–100) 100
(59.0–100) 100
(84.7–99.9) 100
(89.4–100) 97.5
(69.3–100) 100
(71.4–100) 0.91 Central PE Reader 1 100
(78.2–100) 100
(63.1–100) 100
(98.9–100) 100
(99.0–100) 100
(78.2–100) 100
(63.1–100) 100
(98.9–100) 100
(99.0–100) 100
(89.9–100) 100
(89.9–100) >0.99 Reader 2 100
(78.2–100) 100
(63.1–100) 100
(98.9–100) 100
(99.0–100) 100
(78.2–100) 100
(63.1–100) 100
(98.9–100) 100
(99.0–100) 100
(89.9–100) 100
(89.9–100) >0.99 Peripheral PE Reader 1 95.8
(78.9–99.9) 90.6
(75.0–98.0) 99.7
(99.1–100) 99.6
(98.9–99.9) 92.0
(74.0–99.0) 90.6
(75.0–98) 99.9
(99.3–100) 99.6
(98.9–99.9) 99.6
(92.8–100) 99.3
(92.5–100) 0.94 Reader 2 87.5
(67.6–97.3) 84.4
(67.2–94.7) 99.9
(99.3–100) 99.8
(99.1–100) 95.5
(77.2–99.9) 93.1
(77.2–99.2) 99.6
(98.9–99.9) 99.4
(98.5–99.8) 99.5
(92.7–100) 99.1
(92.3–100) 0.94
CTPA, computed tomography pulmonary angiography; NPV, negative predictive value; PPV, positive predictive value; PE, pulmonary embolism.
Data in parentheses indicate a 95% confidence interval.
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Radiation Dose Comparison
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TABLE 5
Radiation Dose Estimation and Comparison in This Study
Parameters Group A Group B_P_ Value CTDIvol (mGy) 0.8 ± 0.1 4.3 ± 0.9 <0.001 DLP (mGy × cm) 28.6 ± 8.3 142.7 ± 27.1 <0.001 ED (mSv) 0.4 ± 0.1 2.0 ± 0.4 <0.001
CTDIvol, volume CT dose index; DLP, dose–length product; ED, effective dose.
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Discussion
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