Lower-Dose CT: Effect on Reader Evaluation in Detecting Pulmonary Embolism
Lower-Dose CT: Effect on Reader Evaluation in Detecting Pulmonary Embolism
Objective: The purpose of this study was to evaluate the effect of reduction in radiation dose on CT detection of pulmonary embolism.
Subjects and Methods: Emergency department patients were evaluated for pulmonary embolism with standard and simulated reduced-dose CT angiography. Simulated lower-dose CT angiograms obtained at 90, 45, 22, and 10 mAseff were reconstructed by mathematical addition of noise to the standard dose (180 mAseff) data from the images of 18 patients with and 20 patients without pulmonary embolism. Four radiologists blinded to the study parameters separately interpreted each CT angiogram. Dose trends for subjective measures (diagnostic certainty, image quality, and perceived technical limitations) were evaluated, test characteristics for the detection of pulmonary embolism were computed, and clot burden was measured.
Results: Readers indicated significant reductions in diagnostic certainty (p < 0.02) and image quality (p < 0.02) and an increase in perceived technical limitations (p < 0.01) as the simulated radiation dose was decreased. These subjective measures also showed significant adverse dose trends when the mAseff was reduced (p < 0.001). At reduced radiation doses, the sensitivity and positive predictive value for detection of pulmonary embolism diminished significantly. The sensitivity was 0.94 (lower bound of 0.95 CI, 0.92); specificity, 0.99 (lower bound of 0.95 CI, 0.98); positive predictive value, 0.95 (lower bound of 0.95 CI, 0.92); and negative predictive value, 0.99 (lower bound of 0.95 CI, 0.97). All patients had a low to moderate clot burden.
Conclusion: Reduction in dose for CT angiography in the detection of pulmonary embolism has a significant adverse effect on readers' subjective assessment of diagnostic confidence and image quality. Detection of pulmonary embolism also decreases as the tube current dose is reduced.
The role of MDCT angiography in the primary evaluation of acute pulmonary embolism (PE) is well established. With a well-defined role and increasing availability, the use of CT for PE detection has steadily increased, particularly in emergency departments. CT angiography has a great advantage over other techniques (e.g., ventilation-perfusion imaging and conventional pulmonary angiography) because it can be used to evaluate both mediastinal and parenchymal structures and to visualize clot in the pulmonary arteries. Furthermore, as many as two thirds of patients in whom PE is initially suspected receive other diagnoses based on CT findings, including potentially life-threatening conditions such as aortic dissection, pneumonia, lung cancer, and pneumothorax. Although it is a powerful and versatile imaging technique, CT inherently entails substantial radiation exposure.
From an epidemiologic perspective, an increased rate of radiation-induced malignancy is the cost of increased radiation exposure of the population, particularly the pediatric population. In comparison with other radiologic examinations of adults, chest CT delivers substantial amounts of radiation to the breasts. Ionizing radiation is a well-established cause of breast cancer, the highest risk occurring among young women. Dosimetry measurements for emergency CT examinations of the cervical spine have shown that CT delivers increased radiation to the thyroid. Visualization of the pulmonary artery at least to the subsegmental level has been the major emphasis in optimizing CT protocols for the detection of PE. The aim of this study was to contribute to the growing body of literature focused on optimizing pulmonary CT angiographic protocols for PE detection through an examination of the effects of simulated dose reduction according to the as low as reasonably achievable (ALARA) principle.
Various dose-reduction strategies include increasing pitch, online modulation of tube current, and lowering tube current-time product (milliampere-second) presets. Studying dose reduction in CT angiography is challenging because there is only one chance at timing the correct radiation dose with the IV contrast bolus. It also would be unacceptable to study the effect of dose on image quality by assembling a patient population and obtaining multiple CT angiograms of each patient. To compensate for these practical limitations, we simulated the effect of lower-dose CT protocols by mathematically adding gaussian-distributed quantum noise to the raw CT data, effectively generating CT images equivalent to images acquired at lower levels of radiation exposure.
This study was designed to assess reader evaluation of lower-dose CT protocols for detecting acute PE. By adding specific levels of noise to raw CT data, we explored the effects of dose reduction on image quality, diagnostic certainty, and PE detection levels. We tested the hypothesis that in comparison with standard-radiation-dose protocols, simulated reduced-dose CT protocols have diminished diagnostic certainty, imaging quality, and accuracy in detection of PE.
Objective: The purpose of this study was to evaluate the effect of reduction in radiation dose on CT detection of pulmonary embolism.
Subjects and Methods: Emergency department patients were evaluated for pulmonary embolism with standard and simulated reduced-dose CT angiography. Simulated lower-dose CT angiograms obtained at 90, 45, 22, and 10 mAseff were reconstructed by mathematical addition of noise to the standard dose (180 mAseff) data from the images of 18 patients with and 20 patients without pulmonary embolism. Four radiologists blinded to the study parameters separately interpreted each CT angiogram. Dose trends for subjective measures (diagnostic certainty, image quality, and perceived technical limitations) were evaluated, test characteristics for the detection of pulmonary embolism were computed, and clot burden was measured.
Results: Readers indicated significant reductions in diagnostic certainty (p < 0.02) and image quality (p < 0.02) and an increase in perceived technical limitations (p < 0.01) as the simulated radiation dose was decreased. These subjective measures also showed significant adverse dose trends when the mAseff was reduced (p < 0.001). At reduced radiation doses, the sensitivity and positive predictive value for detection of pulmonary embolism diminished significantly. The sensitivity was 0.94 (lower bound of 0.95 CI, 0.92); specificity, 0.99 (lower bound of 0.95 CI, 0.98); positive predictive value, 0.95 (lower bound of 0.95 CI, 0.92); and negative predictive value, 0.99 (lower bound of 0.95 CI, 0.97). All patients had a low to moderate clot burden.
Conclusion: Reduction in dose for CT angiography in the detection of pulmonary embolism has a significant adverse effect on readers' subjective assessment of diagnostic confidence and image quality. Detection of pulmonary embolism also decreases as the tube current dose is reduced.
The role of MDCT angiography in the primary evaluation of acute pulmonary embolism (PE) is well established. With a well-defined role and increasing availability, the use of CT for PE detection has steadily increased, particularly in emergency departments. CT angiography has a great advantage over other techniques (e.g., ventilation-perfusion imaging and conventional pulmonary angiography) because it can be used to evaluate both mediastinal and parenchymal structures and to visualize clot in the pulmonary arteries. Furthermore, as many as two thirds of patients in whom PE is initially suspected receive other diagnoses based on CT findings, including potentially life-threatening conditions such as aortic dissection, pneumonia, lung cancer, and pneumothorax. Although it is a powerful and versatile imaging technique, CT inherently entails substantial radiation exposure.
From an epidemiologic perspective, an increased rate of radiation-induced malignancy is the cost of increased radiation exposure of the population, particularly the pediatric population. In comparison with other radiologic examinations of adults, chest CT delivers substantial amounts of radiation to the breasts. Ionizing radiation is a well-established cause of breast cancer, the highest risk occurring among young women. Dosimetry measurements for emergency CT examinations of the cervical spine have shown that CT delivers increased radiation to the thyroid. Visualization of the pulmonary artery at least to the subsegmental level has been the major emphasis in optimizing CT protocols for the detection of PE. The aim of this study was to contribute to the growing body of literature focused on optimizing pulmonary CT angiographic protocols for PE detection through an examination of the effects of simulated dose reduction according to the as low as reasonably achievable (ALARA) principle.
Various dose-reduction strategies include increasing pitch, online modulation of tube current, and lowering tube current-time product (milliampere-second) presets. Studying dose reduction in CT angiography is challenging because there is only one chance at timing the correct radiation dose with the IV contrast bolus. It also would be unacceptable to study the effect of dose on image quality by assembling a patient population and obtaining multiple CT angiograms of each patient. To compensate for these practical limitations, we simulated the effect of lower-dose CT protocols by mathematically adding gaussian-distributed quantum noise to the raw CT data, effectively generating CT images equivalent to images acquired at lower levels of radiation exposure.
This study was designed to assess reader evaluation of lower-dose CT protocols for detecting acute PE. By adding specific levels of noise to raw CT data, we explored the effects of dose reduction on image quality, diagnostic certainty, and PE detection levels. We tested the hypothesis that in comparison with standard-radiation-dose protocols, simulated reduced-dose CT protocols have diminished diagnostic certainty, imaging quality, and accuracy in detection of PE.