Multislice Computed Tomography for Pulmonary Embolism — A Technological Marvel
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《新英格兰医药杂志》
Rapid and accurate diagnosis of pulmonary embolism will improve survival and quality of life because treatment decreases mortality and the likelihood that thromboembolic pulmonary hypertension or the post-thrombotic syndrome will develop. The paramount challenge is to consider pulmonary embolism as a diagnostic possibility and therefore order the appropriate diagnostic tests and institute timely and effective therapy.
Computed tomographic (CT) scanning of the chest has revolutionized our diagnostic approach to suspected pulmonary embolism. Ventilation–perfusion lung scanning used to be the pivotal imaging test, but the lung scan was problematic because it rarely provided a definitive "high probability" or "normal" result. Its ambiguity led to an array of murky descriptions, such as "intermediate probability," "indeterminate probability," "moderate probability," "low end of moderate probability," or "moderately high probability." Clinicians responded to this confusion by summoning expert subspecialists for urgent consultation to declare whether pulmonary embolism was present and whether invasive pulmonary angiography was warranted to make a definitive diagnosis.
Selective pulmonary angiography, which was then the "gold standard," is uncomfortable for the patient but safe when performed by properly trained staff. Still, passage of a catheter through the right ventricle in patients prone to arrhythmia can provoke nonsustained ventricular tachycardia. After positioning and securing the catheter, the staff coaches patients with dyspnea to hold their breath for about 30 seconds. Each lung requires at least two separate angiographic views and, therefore, at least two injections. Aside from the hot, flushed feeling produced by the angiographic dye, some patients have transient hypotension and coughing spasms from the bolus of contrast material delivered by the automated power injector. Coughing renders angiographic images blurry and often necessitates repeating the injection. Even when successfully completed, these studies are often difficult to interpret beyond third-order vessels. I can attest to the disagreement among angiographic readers on the basis of the many angiogram-scoring sessions that I have chaired for clinical trials.
The advent of chest CT scanning for the diagnosis of pulmonary embolism was hailed as an improvement, even before rigorous studies were undertaken. By 2001, CT scanning was being used more often than lung scanning to investigate suspected pulmonary embolism.1 There are now multiple generations of CT scanners, but even first-generation machines delivered images that were dramatic in clarity, rapidly acquired, and accurate in delineating the proximal pulmonary arterial tree. This noninvasive technology has evolved rapidly. A 16-slice multidetector-row CT scanner can image the entire chest with submillimeter resolution and requires a breath-hold of less than 10 seconds.2 Contemporary scanners can provide information about massive pulmonary embolism that can be helpful in planning emergency surgery, catheter embolectomy, or thrombolysis (Figure 1A). The latest generation of scanners can diagnose tiny submillimeter pulmonary embolism in sixth-order vessels (Figure 1B). These thrombi are so small that their clinical significance is uncertain.3
Figure 1. Two Cases of Pulmonary Embolism as Shown on Contrast-Enhanced 16-Slice Multidetector-Row Computed Tomography.
Panel A is an anterior view of the chest of a 72-year-old man, showing extensive, acute central pulmonary embolism with a "saddle embolus" (arrows) extending into the left and right central pulmonary arteries. Panel B is a maximum-intensity projection in the coronal plane (seen from the posterior view) of a 64-year-old man with a history of vague pleuritic chest pain. The scan shows bilateral isolated emboli (arrows) in subsegmental arteries of the right and left lower lobes. (Courtesy of Drs. U. Joseph Schoepf and Douglas R. Lake, Medical University of South Carolina.)
The diagnosis of pulmonary embolism is only one aspect of this technological breakthrough. The CT examination can image the legs, pelvis, and chest in a single study, often detecting the source of the clot.4 Regardless of whether pulmonary embolism is present, the information obtained from examining the lung parenchyma can identify small new lung cancers or pneumonia not visualized on chest radiography.
After the diagnosis of pulmonary embolism is made, CT can help identify patients at high risk for death or major in-hospital complications. The prognosis can be assessed by detecting right ventricular enlargement on reconstructed four-chamber views. Right ventricular enlargement, defined as a ratio of the diameter of the right ventricle to the left ventricle that is more than 0.9, is a powerful predictor of outcome. In a series of 431 patients with pulmonary embolism at Brigham and Women's Hospital in Boston, right ventricular enlargement independently heralded an increase in mortality by a factor of five.5 Rapid risk stratification may help clinicians select the most appropriate candidates for thrombolysis or embolectomy.
When single-detector helical CT was first introduced, Perrier et al. in Geneva studied 299 patients who presented with suspected pulmonary embolism because the investigators were skeptical about the accuracy of the scanning technique. They found that first-generation scanners missed 30 percent of patients with pulmonary embolism, and they concluded that this imaging test should not be used alone for suspected pulmonary embolism.6 Subsequently, Dutch investigators showed that the use of first-generation CT plus three serial venous ultrasonographic examinations of the leg on days 1, 3, and 7 for those with normal CT scans was a safe management strategy.7 Though safe, this approach was impractical and expensive.
As reported in this issue of the Journal, Perrier et al. postulated that multislice CT, with its improved resolution, could serve as a stand-alone imaging test.8 They designed a prospective three-hospital study among patients suspected of having pulmonary embolism to determine whether leg ultrasonography was still necessary despite the advances in CT technology. They found, contrary to their experience with first-generation CT, that among patients with negative multislice CT scans, ultrasonography yielded an incremental rate of detection of venous thrombosis of only 0.9 percent. Thus, ultrasonography seems unnecessary in patients with negative multislice CT scans.
The authors ruled out pulmonary embolism in 232 of their 674 patients who did not have a high clinical probability of pulmonary embolism and had normal measures of D-dimer on enzyme-linked immunosorbent assay (34 percent). Thus, one third of their patients who were suspected of having pulmonary embolism did not require CT. This approach is well validated9 and can help prevent multislice CT from becoming a runaway technology. We can marvel at contemporary CT, but we should not order scans of every patient presenting with dyspnea or pleurisy. Ruling out pulmonary embolism by the measurement of D-dimer, which is sensitive but not specific, is a logical step in managing health care costs.
Although multislice CT scanning excluded pulmonary embolism and did not require ultrasonography as a complementary diagnostic test, ultrasonography itself cannot serve reliably as a surrogate for chest CT. In patients with suspected pulmonary embolism and elevated D-dimer levels, negative ultrasonography as a stand-alone test is inadequate to rule out pulmonary embolism. In the Geneva study, fewer than half of the patients with pulmonary embolism as confirmed by CT had ultrasonographic results that were positive for deep venous thrombosis, probably because the clot had embolized to the lungs.
The thought-provoking article by Perrier et al. raises other fascinating questions. Thromboembolism developed in fewer than 2 percent of the patients with negative CT scans during the 90-day follow-up period. Of those scans, 89 percent were 4-slice and the rest were 16-slice. Yet, at our hospital, we are now installing a 64-slice scanner. Though our new scanner is a technological marvel, is it more of a frill than a necessity? In addition, since the current study showed that concomitant leg ultrasonography is unnecessary, do we really need CT venography of the legs and pelvis when we order CT to rule out pulmonary embolism? At some hospitals, the imaging of pelvic and leg veins is done automatically, without consulting the clinician. Are the additional radiation exposure and expense justified?
In summary, the Geneva group has validated a strategy of using a clinical probability assessment, D-dimer screening, and multislice chest CT scanning, without the need for venous ultrasonography in patients whose CT scans are negative. This approach should streamline and expedite diagnosis, decrease the delay before the institution of therapy, and ultimately reduce the rates of death, chronic thromboembolic pulmonary hypertension, and the post-thrombotic syndrome.
Source Information
From the Cardiovascular Division, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston.
References
Stein PD, Kayali F, Olson RE. Trends in the use of diagnostic imaging in patients hospitalized with acute pulmonary embolism. Am J Cardiol 2004;93:1316-1317.
Schoepf UJ, Goldhaber SZ, Costello P. Spiral computed tomography for acute pulmonary embolism. Circulation 2004;109:2160-2167.
Ravenel JG, Kipfmueller F, Schoepf UJ. CT angiography with multidetector-row CT for detection of acute pulmonary embolus. Semin Roentgenol 2005;40:11-19.
Loud PA, Katz DS, Belfi L, Grossman ZD. Imaging of deep venous thrombosis in suspected pulmonary embolism. Semin Roentgenol 2005;40:33-40.
Schoepf UJ, Kucher N, Kipfmueller F, Quiroz R, Costello P, Goldhaber SZ. Right ventricular enlargement on chest computed tomography: a predictor of early death in acute pulmonary embolism. Circulation 2004;110:3276-3280.
Perrier A, Howarth N, Didier D, et al. Performance of helical computed tomography in unselected outpatients with suspected pulmonary embolism. Ann Intern Med 2001;135:88-97.
van Strijen MJ, de Monye W, Schiereck J, et al. Single-detector helical computed tomography as the primary diagnostic test in suspected pulmonary embolism: a multicenter clinical management study of 510 patients. Ann Intern Med 2003;138:307-314.
Perrier A, Roy P-M, Sanchez O, et al. Multidetector-row computed tomography in suspected pulmonary embolism. N Engl J Med 2005;352:1760-1768.
Wells PS, Anderson DR, Rodger M, et al. Excluding pulmonary embolism at the bedside without diagnostic imaging: management of patients with suspected pulmonary embolism presenting to the emergency department by using a simple clinical model and D-dimer. Ann Intern Med 2001;135:98-107.(Samuel Z. Goldhaber, M.D.)
Computed tomographic (CT) scanning of the chest has revolutionized our diagnostic approach to suspected pulmonary embolism. Ventilation–perfusion lung scanning used to be the pivotal imaging test, but the lung scan was problematic because it rarely provided a definitive "high probability" or "normal" result. Its ambiguity led to an array of murky descriptions, such as "intermediate probability," "indeterminate probability," "moderate probability," "low end of moderate probability," or "moderately high probability." Clinicians responded to this confusion by summoning expert subspecialists for urgent consultation to declare whether pulmonary embolism was present and whether invasive pulmonary angiography was warranted to make a definitive diagnosis.
Selective pulmonary angiography, which was then the "gold standard," is uncomfortable for the patient but safe when performed by properly trained staff. Still, passage of a catheter through the right ventricle in patients prone to arrhythmia can provoke nonsustained ventricular tachycardia. After positioning and securing the catheter, the staff coaches patients with dyspnea to hold their breath for about 30 seconds. Each lung requires at least two separate angiographic views and, therefore, at least two injections. Aside from the hot, flushed feeling produced by the angiographic dye, some patients have transient hypotension and coughing spasms from the bolus of contrast material delivered by the automated power injector. Coughing renders angiographic images blurry and often necessitates repeating the injection. Even when successfully completed, these studies are often difficult to interpret beyond third-order vessels. I can attest to the disagreement among angiographic readers on the basis of the many angiogram-scoring sessions that I have chaired for clinical trials.
The advent of chest CT scanning for the diagnosis of pulmonary embolism was hailed as an improvement, even before rigorous studies were undertaken. By 2001, CT scanning was being used more often than lung scanning to investigate suspected pulmonary embolism.1 There are now multiple generations of CT scanners, but even first-generation machines delivered images that were dramatic in clarity, rapidly acquired, and accurate in delineating the proximal pulmonary arterial tree. This noninvasive technology has evolved rapidly. A 16-slice multidetector-row CT scanner can image the entire chest with submillimeter resolution and requires a breath-hold of less than 10 seconds.2 Contemporary scanners can provide information about massive pulmonary embolism that can be helpful in planning emergency surgery, catheter embolectomy, or thrombolysis (Figure 1A). The latest generation of scanners can diagnose tiny submillimeter pulmonary embolism in sixth-order vessels (Figure 1B). These thrombi are so small that their clinical significance is uncertain.3
Figure 1. Two Cases of Pulmonary Embolism as Shown on Contrast-Enhanced 16-Slice Multidetector-Row Computed Tomography.
Panel A is an anterior view of the chest of a 72-year-old man, showing extensive, acute central pulmonary embolism with a "saddle embolus" (arrows) extending into the left and right central pulmonary arteries. Panel B is a maximum-intensity projection in the coronal plane (seen from the posterior view) of a 64-year-old man with a history of vague pleuritic chest pain. The scan shows bilateral isolated emboli (arrows) in subsegmental arteries of the right and left lower lobes. (Courtesy of Drs. U. Joseph Schoepf and Douglas R. Lake, Medical University of South Carolina.)
The diagnosis of pulmonary embolism is only one aspect of this technological breakthrough. The CT examination can image the legs, pelvis, and chest in a single study, often detecting the source of the clot.4 Regardless of whether pulmonary embolism is present, the information obtained from examining the lung parenchyma can identify small new lung cancers or pneumonia not visualized on chest radiography.
After the diagnosis of pulmonary embolism is made, CT can help identify patients at high risk for death or major in-hospital complications. The prognosis can be assessed by detecting right ventricular enlargement on reconstructed four-chamber views. Right ventricular enlargement, defined as a ratio of the diameter of the right ventricle to the left ventricle that is more than 0.9, is a powerful predictor of outcome. In a series of 431 patients with pulmonary embolism at Brigham and Women's Hospital in Boston, right ventricular enlargement independently heralded an increase in mortality by a factor of five.5 Rapid risk stratification may help clinicians select the most appropriate candidates for thrombolysis or embolectomy.
When single-detector helical CT was first introduced, Perrier et al. in Geneva studied 299 patients who presented with suspected pulmonary embolism because the investigators were skeptical about the accuracy of the scanning technique. They found that first-generation scanners missed 30 percent of patients with pulmonary embolism, and they concluded that this imaging test should not be used alone for suspected pulmonary embolism.6 Subsequently, Dutch investigators showed that the use of first-generation CT plus three serial venous ultrasonographic examinations of the leg on days 1, 3, and 7 for those with normal CT scans was a safe management strategy.7 Though safe, this approach was impractical and expensive.
As reported in this issue of the Journal, Perrier et al. postulated that multislice CT, with its improved resolution, could serve as a stand-alone imaging test.8 They designed a prospective three-hospital study among patients suspected of having pulmonary embolism to determine whether leg ultrasonography was still necessary despite the advances in CT technology. They found, contrary to their experience with first-generation CT, that among patients with negative multislice CT scans, ultrasonography yielded an incremental rate of detection of venous thrombosis of only 0.9 percent. Thus, ultrasonography seems unnecessary in patients with negative multislice CT scans.
The authors ruled out pulmonary embolism in 232 of their 674 patients who did not have a high clinical probability of pulmonary embolism and had normal measures of D-dimer on enzyme-linked immunosorbent assay (34 percent). Thus, one third of their patients who were suspected of having pulmonary embolism did not require CT. This approach is well validated9 and can help prevent multislice CT from becoming a runaway technology. We can marvel at contemporary CT, but we should not order scans of every patient presenting with dyspnea or pleurisy. Ruling out pulmonary embolism by the measurement of D-dimer, which is sensitive but not specific, is a logical step in managing health care costs.
Although multislice CT scanning excluded pulmonary embolism and did not require ultrasonography as a complementary diagnostic test, ultrasonography itself cannot serve reliably as a surrogate for chest CT. In patients with suspected pulmonary embolism and elevated D-dimer levels, negative ultrasonography as a stand-alone test is inadequate to rule out pulmonary embolism. In the Geneva study, fewer than half of the patients with pulmonary embolism as confirmed by CT had ultrasonographic results that were positive for deep venous thrombosis, probably because the clot had embolized to the lungs.
The thought-provoking article by Perrier et al. raises other fascinating questions. Thromboembolism developed in fewer than 2 percent of the patients with negative CT scans during the 90-day follow-up period. Of those scans, 89 percent were 4-slice and the rest were 16-slice. Yet, at our hospital, we are now installing a 64-slice scanner. Though our new scanner is a technological marvel, is it more of a frill than a necessity? In addition, since the current study showed that concomitant leg ultrasonography is unnecessary, do we really need CT venography of the legs and pelvis when we order CT to rule out pulmonary embolism? At some hospitals, the imaging of pelvic and leg veins is done automatically, without consulting the clinician. Are the additional radiation exposure and expense justified?
In summary, the Geneva group has validated a strategy of using a clinical probability assessment, D-dimer screening, and multislice chest CT scanning, without the need for venous ultrasonography in patients whose CT scans are negative. This approach should streamline and expedite diagnosis, decrease the delay before the institution of therapy, and ultimately reduce the rates of death, chronic thromboembolic pulmonary hypertension, and the post-thrombotic syndrome.
Source Information
From the Cardiovascular Division, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston.
References
Stein PD, Kayali F, Olson RE. Trends in the use of diagnostic imaging in patients hospitalized with acute pulmonary embolism. Am J Cardiol 2004;93:1316-1317.
Schoepf UJ, Goldhaber SZ, Costello P. Spiral computed tomography for acute pulmonary embolism. Circulation 2004;109:2160-2167.
Ravenel JG, Kipfmueller F, Schoepf UJ. CT angiography with multidetector-row CT for detection of acute pulmonary embolus. Semin Roentgenol 2005;40:11-19.
Loud PA, Katz DS, Belfi L, Grossman ZD. Imaging of deep venous thrombosis in suspected pulmonary embolism. Semin Roentgenol 2005;40:33-40.
Schoepf UJ, Kucher N, Kipfmueller F, Quiroz R, Costello P, Goldhaber SZ. Right ventricular enlargement on chest computed tomography: a predictor of early death in acute pulmonary embolism. Circulation 2004;110:3276-3280.
Perrier A, Howarth N, Didier D, et al. Performance of helical computed tomography in unselected outpatients with suspected pulmonary embolism. Ann Intern Med 2001;135:88-97.
van Strijen MJ, de Monye W, Schiereck J, et al. Single-detector helical computed tomography as the primary diagnostic test in suspected pulmonary embolism: a multicenter clinical management study of 510 patients. Ann Intern Med 2003;138:307-314.
Perrier A, Roy P-M, Sanchez O, et al. Multidetector-row computed tomography in suspected pulmonary embolism. N Engl J Med 2005;352:1760-1768.
Wells PS, Anderson DR, Rodger M, et al. Excluding pulmonary embolism at the bedside without diagnostic imaging: management of patients with suspected pulmonary embolism presenting to the emergency department by using a simple clinical model and D-dimer. Ann Intern Med 2001;135:98-107.(Samuel Z. Goldhaber, M.D.)