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Chronic PE
- 1. RadioGraphics 2009; 29:31–53 CT Diagnosis of Chronic Pulmonary Thromboembolism Castañer et al : Department of Radiology, Universitari Parc Taul ํ -UAB, Barcelona Present by Ekkasit S.
- 13. Pathogenesis “ In more than 90% of patients includes total resolution or resolution with minimal residual and restoration of normal pulmonary hemodynamics within 30 days after treatment”
- 16. Various possible results of disturbed resolution of a thrombus
- 20. CT Technique 100 700 Pulmonary thromboembolism-specific window 40 350 Mediastinum window -600 1500 Lung window window level window width
- 37. Main pulmonary artery diameter more than 29 mm
- 38. Chronic pulmonary thromboembolism and pulmonary hypertension in a 42-year-old man. Axial contrast-enhanced CT scan.
- 68. Differential Diagnosis Mean attenuation in chronic PE (87 HU ± 30) is higher than acute PE (33 HU ± 15). Dilated bronchial arteries. Nonobstructive eccentric filling defect forms obtuse angles with the vessel wall. Nonobstructive eccentric filling defect forms acute angles with the vessel wall. Decreased Diameter of pulmonary artery. Increased Diameter of pulmonary artery. Chronic PE Acute PE
Notes de l'éditeur
- In the presence of chronic thromboembolic disease, the bronchial and nonbronchial systemic circulation is markedly increased as a result of the development of systemic-to-pulmonary anastomoses, which help to maintain pulmonary blood flow in the presence of vessel obstruction .
- Chronic pulmonary thromboembolism is often identified during the diagnostic work-up in patients with unexplained pulmonary hypertension. At our institution, most cases of chronic pulmonary thromboembolism are discovered at CT PA performed to rule out acute pulmonary thromboembolism. We perform CT PA with a MD CT scanner (Sensation 16; Siemens, Erlangen, Germany) (120 kV, 70–120 mAs, 0.5 second scanning time, 0.75 mm detector width, pitch of 1.5). Images are reconstructed with a 1-mm section thickness at a 0.7-mm interval. Patients receive 100 mL of contrast material (iopromide, Ultravist 300; Schering, Berlin, Germany) at an injection rate of 4 mL/sec. When chronic pulmonary embolism is suspected, we modify the CT pulmonary angiography protocol: The intravenous administration of the contrast material bolus is timed so that both the pulmonary and the systemic circulation are opacified. The bronchial circulation, which usually originates from the descending aorta, is markedly increased in patients with chronic pulmonary thromboembolism, and the enhancement of bronchial vessels may aid in the differential diagnosis. Bolus timing also allows assessment of all cardiac chambers. The desired opacification of the pulmonary and systemic circulation can be achieved by using a longer delay from contrast material injection to image acquisition; we use a higher trigger threshold, 200 HU (21) (our threshold for evaluation of acute pulmonary embolism is 120 HU) with a circular region of interest centered on the main pulmonary artery.
- We perform scanning in the caudal-cranial direction because most pulmonary emboli are found in the lower lung lobes (17) and, if the patient is unable to sustain breath holding throughout image acquisition, the lower lobes are imaged in the initial seconds of the breath hold. Because some signs of chronic thromboembolism (eg, bands) may be overlooked with the highcontrast settings, we view the images by using three different gray scales for interpretation: a lung window (window width, 1500 HU; window level, -600 HU), a mediastinal window (window width, 350 HU; window level, 40 HU), and a pulmonary thromboembolism–specific window (window width, 700 HU; window level, 100 HU) (22).
- Multiplanar reformatted images and maximum intensity projection images that provide longitudinal views of vessels may help clarify confusing or questionable findings and may better depict obstructions, stenoses, and flattened peripheral thrombotic material (Fig 2) that otherwise might be overlooked (23).
- We classify the CT features of chronic pulmonary thromboembolism as vascular signs or parenchymal signs. The vascular signs include direct pulmonary artery signs (results of thrombus organization), signs due to pulmonary hypertension (results of the sustained increase in pulmonary vascular resistance), and signs due to systemiccollateral supply (results of decreased pulmonary artery flow). The parenchymal signs include scars, a mosaic perfusion pattern, focal groundglass opacities, and bronchial dilatation. The vascular and parenchymal signs are described in greater detail in the next sections.
- Complete Obstruction. —At angiography, complete vessel cutoff results in a convex margin of the contrast material bolus, a feature that has been described as a “pouch defect”. At CT, this feature is difficult to see, and the additional findings of an abrupt decrease in vessel diameter and absence of contrast material in the vessel segment distal to the total obstruction are easier to identify (Fig 3a). The reduction in vessel diameter is persistent and is caused by contraction of the thrombus . CT scans viewed at lung window settings depict segmental and subsegmental vessels that are abnormally small in comparison with the accompanying bronchi.
- Partial Filling Defects. —An organized thrombus may cause vessel narrowing, intimal irregularities, bands, and webs. Abrupt vessel narrowing is caused by recanalization within a large thrombus or by stenosis due to an organized thrombus that lines the arterial wall. In the presence of recanalization, contrast material is seen flowing through thickened and often smaller arteries (Fig 2). The organized thrombus runs parallel to the arterial lumen and appears as a thickening of the artery wall, sometimes producing an irregular contour of the intimal surface (Fig 4a). A chronic thrombus in an artery with a course that is transverse to the scanning plane has the appearance of a peripheral, crescent-shaped intraluminal defect that forms obtuse angles with the vessel wall(Fig 4b). Poststenotic dilatation or aneurysm may be observed (Figs 4a, 5). A band is defined as a linear structure that is anchored at both ends to the vessel wall and has a free, unattached midportion. A band generally has a length of 0.3–2 cm and width of 0.1–0.3 cm. It is often oriented in the direction of blood flow, along the long axis of the vessel. A web consists of multiple bands that have branches forming a network. At CT angiography, bands and webs are depicted as thin lines surrounded by contrast material. These features most frequently are found in lobar or segmental arteries and rarely are seen in the main pulmonary artery. Calcifications within chronic thrombi are seen in a small number of patients. On contrast enhanced CT images with the usual mediastinal window settings, calcified thrombi may be obscured by surrounding contrast material. Selection of a wider window setting or creation of maximum intensity projections are helpful for visualizing calcification. Calcified thrombi in subsegmental arteries are often indistinguishable from calcified lung nodules. However, their tubular shape and location at the site of arterial branching may aid in their differentiation.
- Axial contrast-enhanced CT scan shows bilateral eccentric chronic thrombi producing irregular contours of the intimal surface of both main pulmonary arteries (arrows) and poststenotic dilatation (arrowheads) in the posterior segmental artery of the right upper lobe.
- an eccentrically located thrombus with a broad base forming obtuse angles with the vessel wall in the left lower lobe pulmonary artery
- (a) Oblique coronal 30-mm-thick maximum intensity projection CT image shows aneurysms in the posterior segment of the right upper lobe pulmonary artery ( * ) and aneurysmal dilatation of the right lower lobe pulmonary artery (arrows) distal to a band (arrowhead).
- (b) Oblique coronal 10-mmthick maximum intensity projection CT image provides a closer view of the band (arrowhead) and the poststenotic dilatation. The marked increase in diameter and the tortuosity of the pulmonary arteries (arrows in b ) are indicative of pulmonary hypertension.
- Residual band from a pulmonary thrombus in an 83-year-old woman with dyspnea. (a) Axial contrast-enhanced CT image shows a linear structure anchored to the vessel wall in the left lower lobe pulmonary artery (arrow).
- (b) Coronal 10-mm-thick maximum intensity projection CT image shows the attachment of the band to the vessel wall in more detail (arrows).
- viewed with a wide window setting (width, 1100 HU; level, 100 HU) to facilitate calcium detection shows a partially calcified thrombus in the right ulmonary artery.
- We classify the CT features of chronic pulmonary thromboembolism as vascular signs or parenchymal signs. The vascular signs include direct pulmonary artery signs (results of thrombus organization), signs due to pulmonary hypertension (results of the sustained increase in pulmonary vascular resistance), and signs due to systemiccollateral supply (results of decreased pulmonary artery flow). The parenchymal signs include scars, a mosaic perfusion pattern, focal groundglass opacities, and bronchial dilatation. The vascular and parenchymal signs are described in greater detail in the next sections.
- Increased vascular resistance due to the obstructed vascular bed leads to dilatation of the central pulmonary arteries. Enlargement of the main pulmonary artery to a diameter of more than 29 mm may occur in the presence of pulmonary hypertension, regardless of the cause; such enlargement is a common finding in patients with chronic thromboembolic pulmonary hypertension(Figs 4a, 5, 7). The CT diameter of the main pulmonary artery is measured in the scanning plane of its bifurcation, at a right angle to its long axis and just lateral to the ascending aorta (Fig 8). When the ratio of the diameter of the main pulmonary artery to the diameter of the aorta measured on CT scans is greater than 1:1, there is a strong correlation with elevated pulmonary artery pressure, especially in patients younger than 50 years (31) (Fig 8). In contrast to the symmetric pulmonary enlargement typically seen in nonthromboembolic pulmonary hypertension, central pulmonary arteries in patients with chronic thromboembolic pulmonary hypertension often are asymmetric in size(Figs 4a, 8). The walls of the pulmonary arteries may show atherosclerotic calcification. Tortuous pulmonary vessels have been described in patients with pulmonary hypertension and are seen also in patients with chronic thromboembolic pulmonary hypertension (33) (Fig 5b).
- enlarged pulmonary trunk with a maximum diameter of 39 mm (line) near its bifurcation and asymmetric enlargement of the right pulmonary artery secondary to an extensive thrombus ( * ). Atherosclerotic calcification of the left pulmonary artery also is visible (arrows).
- tortuosity of the pulmonary arteries (arrows in b ) are indicative of pulmonary hypertension.
- Right heart disease is a common and expected finding secondary to pulmonary hypertension: The increased workload borne by the right heart results in right ventricular enlargement and hypertrophy (right ventricular myocardial thickness greater than 4 mm) (25) (Fig 9a). Overtime, right ventricular function deteriorates, even in the absence of recurrent embolism, presumably because of the development of hypertensive vascular lesions in the nonobstructed pulmonary artery bed and of vasculopathy in vessels distal to obstructed arteries. Dilatation of the right ventricle is considered present when the ratio of the diameter of the right ventricle to that of the left ventricle is greater than 1:1 and there is bowing of the interventricular septum toward the left ventricle. At CT, these signs can be evaluated even without electrocardiographic gating. The minor axes of the right and left ventricular chambers can be measured in the axial plane at their widest points, in diastole, between the inner surface of the free wall and the surface of the interventricular septum (Fig 9a). The diastolic maxima of the right and left ventricles may be reached at slightly different levels. Right ventricular enlargement may be accompanied by dilatation of the tricuspid valve annulus and resultant tricuspid valve regurgitation (Fig 9b). Patients with severe pulmonary hypertension may present with mild pericardial thickening or a small pericardial effusion. The presence of pericardial effusion implies a worse prognosis. Patients with chronic thromboembolic pulmonary hypertension may have enlarged lymph nodes. At histologic examination of these enlarged nodes, a vascular transformation of the lymph node sinus may be seen, often in association with sclerosis of varying degrees. Similar histologic features may be observed also in lymph nodes from patients with pulmonary hypertension due to other causes.
- Right heart abnormalities secondary to chronic thromboembolic pulmonary hypertension in a 47-year-old man (a) Axial contrast-enhanced CT scan shows dilatation of the right ventricle (RV) , with a ratio of more than 1:1 between the right and left ventricle (LV) diameters (lines); leftward septal bowing (arrowheads); thickening of the free right ventricular wall (arrows); and dilatation of the right atrium (RA) . (b) Axial contrast-enhanced CT scan at a lower level shows opacification of the inferior vena cava and suprahepatic veins because of retrograde flow of contrast material, which is often seen in patients with elevated right atrial and right ventricular pressures.
- (b) Axial contrast-enhanced CT scan at a lower level shows opacification of the inferior vena cava and suprahepatic veins because of retrograde flow of contrast material, which is often seen in patients with elevated right atrial and right ventricular pressures.
- We classify the CT features of chronic pulmonary thromboembolism as vascular signs or parenchymal signs. The vascular signs include direct pulmonary artery signs (results of thrombus organization), signs due to pulmonary hypertension (results of the sustained increase in pulmonary vascular resistance), and signs due to systemiccollateral supply (results of decreased pulmonary artery flow). The parenchymal signs include scars, a mosaic perfusion pattern, focal groundglass opacities, and bronchial dilatation. The vascular and parenchymal signs are described in greater detail in the next sections.
- chronic obstruction of the pulmonary arteries in patients with chronic thromboembolic pulmonary hypertension. In addition, transpleural systemic collateral vessels (eg, intercostal arteries) may develop (20,38). Normally, the bronchial arteries only supply nutrition to the bronchi and do not take part in gas exchange. However, in pathologic conditions that diminish pulmonary artery circulation, flow through the bronchial arteries increases, and they participate in blood oxygenation. Systemic hypervascularization is a nonspecific response to stimuli such as reduced pulmonary artery flow, hypoxemia, fibrosis, chronic inflammation, and chronic infection. The normal bronchial arterial blood flow is of the order of 1%–2% of the cardiac output. In patients with chronic thromboembolic pulmonary hypertension, bronchial flow may represent almost 30% of the systemic blood flow. To fill pulmonary arteries downstream, systemic-to-pulmonary arterial anastomoses develop beyond the level of obstruction (41). The bronchial arteries usually arise from the descending aorta at the level of the carina. Abnormal dilatation of the proximal portion of the bronchial arteries (diameter of more than 2 mm) and arterial tortuosity (Figs 2, 10) are CT findings indicative of bronchial artery hypervascularization. In a recent study (20), abnormally enlarged bronchial and nonbronchial systemic arteries were found more frequently in patients with chronic thromboembolic pulmonary hypertension (73%) than in patients with idiopathic pulmonary hypertension (14%); these findings could help distinguish between these two entities. In this study, the most frequently depicted abnormal nonbronchial systemic arteries were the inferior phrenic, intercostal (Fig 11), and internal mammary arteries. Acute pulmonary embolism does not appear to cause dilatation of the bronchial arteries; in patients in whom the distinction between acute and chronic or recurrent pulmonary embolism at CT angiography is unclear, the presence of dilated bronchial arteries should favor the diagnosis of chronic or recurrent pulmonary embolism. Another important finding is that dilated bronchial arteries are positively correlated with a lower mortality rate after pulmonary thromboendarterectomy (41). Development of systemic hypervascularization may also be responsible for hemoptysis in these patients (42).
- chronic obstruction of the pulmonary arteries in patients with chronic thromboembolic pulmonary hypertension. In addition, transpleural systemic collateral vessels (eg, intercostal arteries) may develop (20,38). Normally, the bronchial arteries only supply nutrition to the bronchi and do not take part in gas exchange. However, in pathologic conditions that diminish pulmonary artery circulation, flow through the bronchial arteries increases, and they participate in blood oxygenation. Systemic hypervascularization is a nonspecific response to stimuli such as reduced pulmonary artery flow, hypoxemia, fibrosis, chronic inflammation, and chronic infection. The normal bronchial arterial blood flow is of the order of 1%–2% of the cardiac output. In patients with chronic thromboembolic pulmonary hypertension, bronchial flow may represent almost 30% of the systemic blood flow. To fill pulmonary arteries downstream, systemic-to-pulmonary arterial anastomoses develop beyond the level of obstruction (41). The bronchial arteries usually arise from the descending aorta at the level of the carina. Abnormal dilatation of the proximal portion of the bronchial arteries (diameter of more than 2 mm) and arterial tortuosity (Figs 2, 10) are CT findings indicative of bronchial artery hypervascularization. In a recent study (20), abnormally enlarged bronchial and nonbronchial systemic arteries were found more frequently in patients with chronic thromboembolic pulmonary hypertension (73%) than in patients with idiopathic pulmonary hypertension (14%); these findings could help distinguish between these two entities. In this study, the most frequently depicted abnormal nonbronchial systemic arteries were the inferior phrenic, intercostal (Fig 11), and internal mammary arteries. Acute pulmonary embolism does not appear to cause dilatation of the bronchial arteries; in patients in whom the distinction between acute and chronic or recurrent pulmonary embolism at CT angiography is unclear, the presence of dilated bronchial arteries should favor the diagnosis of chronic or recurrent pulmonary embolism. Another important finding is that dilated bronchial arteries are positively correlated with a lower mortality rate after pulmonary thromboendarterectomy (41). Development of systemic hypervascularization may also be responsible for hemoptysis in these patients (42).
- Figure 10. Chronic pulmonary thromboembolism in an 80-year-old woman with a history of acute pulmonary thromboembolism (same patient as in Fig 4). (a) Axial contrast-enhanced CT scan shows enlargement of the proximal portion of the right bronchial artery (arrowhead) and aneurysmal dilatation of an upper lobe artery ( * ). (b) Oblique coronal 20-mm-thick maximum intensity projection CT image better depicts the course of the right bronchial artery (arrowheads) and shows a large eccentric thrombus in the main and left lower lobe pulmonary arteries ( * ). The bronchial circulation appears as fine hyperattenuating lines inside the thrombus.
- Figure 10. Chronic pulmonary thromboembolism in an 80-year-old woman with a history of acute pulmonary thromboembolism (same patient as in Fig 4). (a) Axial contrast-enhanced CT scan shows enlargement of the proximal portion of the right bronchial artery (arrowhead) and aneurysmal dilatation of an upper lobe artery ( * ). (b) Oblique coronal 20-mm-thick maximum intensity projection CT image better depicts the course of the right bronchial artery (arrowheads) and shows a large eccentric thrombus in the main and left lower lobe pulmonary arteries ( * ). The bronchial circulation appears as fine hyperattenuating lines inside the thrombus.
- Figure 11. Chronic pulmonary thromboembolism in a 47-year-old man with multiple episodes of acute pulmonary thromboembolism. (a) Coronal 30-mm-thick maximum intensity projection CT image shows marked enlargement of branches of the right and left inferior phrenic arteries (straight arrows), right and left bronchial arteries (arrowheads), and an intercostal artery (curved arrow). (b) Coronal 30-mm-thick maximum intensity projection image shows enlargement of intercostal arteries on the right side compared with those on the left.
- We classify the CT features of chronic pulmonary thromboembolism as vascular signs or parenchymal signs. The vascular signs include direct pulmonary artery signs (results of thrombus organization), signs due to pulmonary hypertension (results of the sustained increase in pulmonary vascular resistance), and signs due to systemiccollateral supply (results of decreased pulmonary artery flow). The parenchymal signs include scars, a mosaic perfusion pattern, focal groundglass opacities, and bronchial dilatation. The vascular and parenchymal signs are described in greater detail in the next sections.
- Scars from prior pulmonary infarctions are commonly depicted in areas of decreased lung attenuation on CT scans obtained in patients with chronic thromboembolic pulmonary hypertension. These scars may appear as parenchymal bands, wedge-shaped opacities,peripheral nodules (12) , cavities (Fig 3b), or irregular peripheral linear opacities (Fig 13). The appearance most suggestive of scar tissue from infarction is a wedge-shaped pleura-based opacity; however, an infarct may constrict with age and take on the more linear shape of a parenchymal band (Fig 14). Parenchymal scars often occur in multiples, generally are found in the lower lobes, and often are accompanied by pleural thickening (20). A mosaic pattern of perfusion also is seen at CT in the presence of chronic thromboembolic pulmonary hypertension. This pattern appears as sharply demarcated regions of decreased and increased attenuation because of irregular perfusion (Figs 12, 14). Low attenuation is due either to hypoperfusion in areas distal to occluded vessels or to distal vasculopathy in nonoccluded areas, and increased attenuation has been related to the redistribution of blood flow to the patent arterial bed (12,32). These assumptions can be confirmed by several findings: larger vessels in regions of increased attenuation, stronger enhancement of the hyperattenuating areas after contrast material administration (1,44), and correlation between areas of low attenuation on CT images and areas of hypoperfusion depicted at single photon emission computed tomography (32).
- Figure 12. Chronic pulmonary thromboembolism. CT scan (lung window) obtained in an 85-year-old woman 2 years after an episode of massive acute thromboembolism shows a subpleural wedge-shaped area of consolidation (arrow) in a region of decreased attenuation, a feature indicative of pulmonary infarction. The region did not enhance after contrast material was administered.
- Axial CT scan (lung window) shows segmental and subsegmental vessels in the right lower lobe that are abnormally small compared with their accompanying bronchi. Peripheral nodular opacities (arrowheads) in the right lower and right middle lobes are secondary to previous infarction.
- Figure 13. Chronic pulmonary thromboembolism. CT scan (lung window) obtained in a 48-year-old man shows multiple peripheral linear opacities in both lower lung lobes, features that represent old infarcts.
- Figure 14. Chronic pulmonary thromboembolism in a 65-year-old man. CT scan (lung window) shows a mosaic perfusion pattern with marked regional variations in attenuation of the lung parenchyma and disparity in the size of the segmental vessels, with larger-diameter vessels in regions of increased attenuation (arrows). A peripheral parenchymal band or scar (arrowhead) from infarction also is depicted.
- A mosaic pattern of perfusion also is seen at CT in the presence of chronic thromboembolic pulmonary hypertension. This pattern appears as sharply demarcated regions of decreased and increased attenuation because of irregular perfusion (Figs 12, 14). Low attenuation is due either to hypoperfusion in areas distal to occluded vessels or to distal vasculopathy in nonoccluded areas, and increased attenuation has been related to the redistribution of blood flow to the patent arterial bed (12,32). These assumptions can be confirmed by several findings: larger vessels in regions of increased attenuation, stronger enhancement of the hyperattenuating areas after contrast material administration (1,44), and correlation between areas of low attenuation on CT images and areas of hypoperfusion depicted at single photon emission computed tomography. Mosaic lung attenuation is nonspecific and is seen more often in patients with pulmonary hypertension due to vascular disease than in those with pulmonary hypertension due to cardiac or lung disease. Mosaic perfusion is seen much more commonly in patients with chronic thromboembolic pulmonary hypertension than in patients with idiopathic pulmonary hypertension. ขขขขขขขขขขขขขขขขขขขขขขขขขขขขขขขขขขขขขขขชชชชชชชชชชชช Systemic perfusion of the peripheral pulmonary arterial bed accounts for the presence of isolated focal areas of ground-glass attenuation(Fig 15).
- Figure 12. Chronic pulmonary thromboembolism. CT scan (lung window) obtained in an 85-year-old woman 2 years after an episode of massive acute thromboembolism shows a subpleural wedge-shaped area of consolidation (arrow) in a region of decreased attenuation, a feature indicative of pulmonary infarction. The region did not enhance after contrast material was administered.
- Figure 14. Chronic pulmonary thromboembolism in a 65-year-old man. CT scan (lung window) shows a mosaic perfusion pattern with marked regional variations in attenuation of the lung parenchyma and disparity in the size of the segmental vessels, with larger-diameter vessels in regions of increased attenuation (arrows). A peripheral parenchymal band or scar (arrowhead) from infarction also is depicted.
- Figure 15. Chronic pulmonary thromboembolism in an 80-year-old woman with a history of acute pulmonary thromboembolism (same patient as in Figs 4 and 10). Coronal 20-mm-thick maximum intensity projection CT image shows foci of ground-glass attenuation in the right upper lobe (arrows) secondary to systemic perfusion in peripheral areas, as well as bilateral enlargement of the bronchial arteries (arrowheads).
- Arakawa et al (44) found direct evidence of airway obstruction—air trapping on expiratory CT images—in patients with chronic pulmonary thromboembolism. Air trapping is commonly seen in areas of hypoperfusion due to chronic embolism (Fig 16). Arakawa and colleagues found significant associations between the appearance of air trapping and the presence of a proximal arterial stenosis or clot and between the extent of air trapping and the degree of impairment of pulmonary function of the small airways in these patients (44). Cylindrical bronchial airway dilatation is seen in two thirds of patients with chronic thromboembolic pulmonary hypertension (47). It occurs at the level of segmental and subsegmental bronchi, adjacent to severely stenosed or completely obstructed and retracted pulmonary arteries (Fig 17). Two different hypotheses might explain this phenomenon. First, findings by Wetzel et al (48) in an investigation of the airway response to hypoxia in a pig model are suggestive of hypoxic bronchodilatation. Another hypothesis involves the bronchial circulation, which supplies nutrients to the bronchial walls: If the pulmonary arteries are chronically obstructed, there is an increased demand on the bronchial circulation to provide pulmonary parenchymal perfusion, possibly robbing the bronchial walls of nutrients and weakening them, which leads to airway dilatation(47). Although parenchymal findings (eg, mosaic attenuation with asymmetric artery size, pulmonary infarcts) are nonspecific, in the appropriate clinical setting they may be regarded as supportive of the diagnosis of chronic thromboembolic pulmonary hypertension.
- Figure 16. Chronic pulmonary thromboembolism in an 82-year-old woman. (a) Axial inspiratory CT scan (lung window) shows chronic thromboembolism of the left lower lobe pulmonary arteries. Note the mosaic perfusion pattern and the diminished size of vessels in this lobe compared with the right lower lobe. (b) Axial expiratory CT scan (lung window) at the same level as a shows evidence of air trapping in areas of lower attenuation. Air trapping is not specific to airway disease and may be a sign of chronic thromboembolism.
- Figure 17. Chronic pulmonary thromboembolism in an 82-year-old woman. (a) Axial CT scan (lung window) shows increased bronchial diameters and an absence of normal distal tapering of the segmental and subsegmental bronchi of the left lower lobe (arrow). Note the small arterial segments (arrowhead) at the lateral border of each dilated bronchus. (b) Axial contrast-enhanced CT scan at a level slightly higher than a shows a marked reduction of arterial perfusion in the left lower lobe.
- We classify the CT features of chronic pulmonary thromboembolism as vascular signs or parenchymal signs. The vascular signs include direct pulmonary artery signs (results of thrombus organization), signs due to pulmonary hypertension (results of the sustained increase in pulmonary vascular resistance), and signs due to systemiccollateral supply (results of decreased pulmonary artery flow). The parenchymal signs include scars, a mosaic perfusion pattern, focal groundglass opacities, and bronchial dilatation. The vascular and parenchymal signs are described in greater detail in the next sections.
- Chronic thromboembolic pulmonary hypertension is often misdiagnosed, and the initial thromboembolic event in most patients is asymptomatic or overlooked. Both congenital and acquired conditions may cause pulmonary hypertension or obstruction of the pulmonary arteries and mimic chronic thromboembolic pulmonary hypertension.
- Both chronic thromboembolic pulmonary hypertension and idiopathic pulmonary hypertension manifest clinically with exertional dyspnea, pulmonary hypertension, and signs of right heart failure. Although the diagnosis of chronic thromboembolic disease relies on precise vascular features, its differentiation may be difficult because in situ pulmonary artery thrombosis in patients with idiopathic pulmonary hypertension may mimic pulmonary artery occlusions caused by thrombotic material of embolic origin (14). Enlargement of bronchial and nonbronchial systemic arteries is seen in 73% of patients with chronic pulmonary thromboembolism but in only 14% of patients with idiopathic pulmonary hypertension(20). The combination of mosaic lung attenuation and marked regional variation in the size of segmental vessels is seen frequently in patients with chronic thromboembolic pulmonary hypertension and is hardly ever seen in those with idiopathic pulmonary hypertension (43,45). The peripheral opacities typically produced by infarcts are rarely seen in patients with idiopathic pulmonary hypertension (43).
- Chronic pulmonary thromboembolism often is discovered during CT pulmonary angiography performed to rule out acute pulmonary thromboembolism in a patient who presents with dyspnea. Acute and chronic thromboembolism commonly coexist. In cases of acute complete obstruction, the diameter of the pulmonary artery may be increased because of impaction of the thrombus by pulsatile flow (49) (Fig 18a). Conversely, in chronic thromboembolic disease, the diameter of the vessel distal to a complete obstruction is markedly decreased (Fig 18b). An acute nonobstructive filling defect may be central or eccentric in location. In acute thromboembolism, a nonobstructive eccentric filling defect forms acute angles with the vessel wall (Fig 19a). Conversely, partially obstructive chronic thromboembolism appears as a peripheral crescent-shaped defect that forms obtuse angles with the vessel wall (Fig 4b). An acute nonobstructive central defect appears surrounded by contrast-enhanced blood (Fig 19b) (50). If the distinction between acute and chronic or recurrent pulmonary thromboembolism is unclear, the presence of dilated bronchial arteries supports a diagnosis of recurrent or chronic pulmonary thromboembolism (38). Wittram et al (51) defined the attenuation values of acute and chronic pulmonary thromboembolism. The mean attenuation (± standard deviation) in the presence of chronic thromboembolism (87 HU ± 30) is significantly higher than that in acute thromboembolism (33 HU ± 15). The higher mean attenuation in the presence of chronic pulmonary thromboembolism is likely related to enhancement of the organizing thrombus, retraction of the thrombus with its concentrations of hemoglobin and iron, and, possibly, calcium deposition (Fig 7) (51). Acute embolic obstruction of a significant amount of the pulmonary circulation (usually esti- mated as more than 30%) increases pulmonary vascular resistance and leads to acute pulmonary hypertension and, in some cases, right ventricular dysfunction and dilatation (52). However, since pulmonary hypertension is not firmly established in cases of acute obstruction, right ventricular hypertrophy has not yet developed.
- Chronic pulmonary thromboembolism often is discovered during CT pulmonary angiography performed to rule out acute pulmonary thromboembolism in a patient who presents with dyspnea. Acute and chronic thromboembolism commonly coexist. In cases of acute complete obstruction, the diameter of the pulmonary artery may be increased because of impaction of the thrombus by pulsatile flow (49) (Fig 18a). Conversely, in chronic thromboembolic disease, the diameter of the vessel distal to a complete obstruction is markedly decreased (Fig 18b). An acute nonobstructive filling defect may be central or eccentric in location. In acute thromboembolism, a nonobstructive eccentric filling defect forms acute angles with the vessel wall (Fig 19a). Conversely, partially obstructive chronic thromboembolism appears as a peripheral crescent-shaped defect that forms obtuse angles with the vessel wall (Fig 4b). An acute nonobstructive central defect appears surrounded by contrast-enhanced blood (Fig 19b) (50). If the distinction between acute and chronic or recurrent pulmonary thromboembolism is unclear, the presence of dilated bronchial arteries supports a diagnosis of recurrent or chronic pulmonary thromboembolism (38). Wittram et al (51) defined the attenuation values of acute and chronic pulmonary thromboembolism. The mean attenuation (± standard deviation) in the presence of chronic thromboembolism (87 HU ± 30) is significantly higher than that in acute t hromboembolism (33 HU ± 15). The higher mean attenuation in the presence of chronic pulmonary thromboembolism is likely related to enhancement of the organizing thrombus, retraction of the thrombus with its concentrations of hemoglobin and iron, and, possibly, calcium deposition (Fig 7) (51). Acute embolic obstruction of a significant amount of the pulmonary circulation (usually esti-mated as more than 30%) increases pulmonary vascular resistance and leads to acute pulmonary hypertension and, in some cases, right ventricular dysfunction and dilatation (52). However, since pulmonary hypertension is not firmly established in cases of acute obstruction, right ventricular hypertrophy has not yet developed.
- Figure 18. Evolution of chronic occlusive pulmonary thromboembolism from acute embolism in a 40-year-old man. (a) Axial contrast-enhanced CT scan shows acute embolism in the left lower lobe, with increased arterial diameters (arrows) due to impacted thrombi. (b) Axial contrast-enhanced CT scan obtained at the same level as a , 1 year later, when the patient presented with dyspnea, shows a permanent reduction in the diameters of the left lower lobe arteries (arrows) because of thrombus organization and retraction, findings indicative of chronic thromboembolism.
- Interruption of the left pulmonary artery is usually associated with a congenital cardiovascular anomaly, most commonly tetralogy of Fallot. Interruption of the right pulmonary artery is more common and is an isolated finding in most instances. The pulmonary artery ends blindly at the hilum, and blood is supplied through collateral systemic vessels, mainly bronchial arteries. Unlike chronic pulmonary embolism, proximal interruption of the pulmonary artery is characterized by smooth, abrupt tapering of the pulmonary artery, without endoluminal changes. A helpful diagnostic clue in most cases of chronic pulmonary thromboembolism is the presence of multiple bilateral arterial abnormalities. Occlusion of one main pulmonary artery, mimicking proximal interruption, is rarely seen and has been reported in only 3% of cases (55).
- Figure 20. Unilateral proximal interruption of the right pulmonary artery in a 52-year-old woman with progressive dyspnea. Axial contrast-enhanced CT scan shows only the proximal portion of the right pulmonary artery (arrowhead), with enlargement of the main and left pulmonary arteries secondary to pulmonary hypertension. No endoluminal or periluminal changes are depicted. (Reprinted, with permission, from reference54.)
- Takayasu arteritis is an idiopathic arteritis that mainly affects the elastic arteries. It frequently affects the aorta and its major branches; pulmonary artery involvement occurs in 50%–80% of patients and is a manifestation of late-stage disease (56). The most characteristic findings are stenosis and occlusion, mainly of the segmental and subsegmental arteries and less commonly of the lobar or main pulmonary arteries (56). In the presence of Takayasu arteritis, CT scans may depict concentric inflammatory mural thickening in affected vessels. In addition, findings of wall thickening in the aorta and aortic branches and the absence of intraluminal thrombi in the pulmonary arteries are diagnostic (57). Unilateral pulmonary artery occlusion may occur at an advanced stage of the disease (Fig 21a), and late-stage Takayasu arteritis should be considered in cases of chronic pulmonary artery obstruction of unknown origin (56). As in other conditions involving decreased pulmonary flow, collateral vessels may develop (Fig 21).
- Figure 21. Late-stage Takayasu arteritis with right pulmonary artery involvement in a 63-year-old woman. Axial contrast-enhanced CT scan shows right pulmonary artery occlusion (straight arrow), enlarged bronchial arteries (curved arrow) in the right hilum, and an enlarged mammary artery (arrowhead). Axial contrast-enhanced CT scan at the level of the supra-aortic trunks shows soft tissue that surrounds the brachiocephalic trunk (curved arrows), occlusion of the left carotid artery (straight arrow), poor visibility of vessels in the right lung because of right pulmonary artery involvement, and development of collateral vessels from intercostal arteries (arrowheads). (Reprinted, with permission, from reference 54.)
- Primary sarcoma of the pulmonary artery is rare. Undifferentiated sarcoma and leiomyosarcoma are the types of sarcoma that most frequently affect the pulmonary arteries. The main or proximal pulmonary arteries are most frequently involved. The clinical manifestations may mimic those of acute or chronic pulmonary embolism. Contrast-enhanced CT scans show the tumor as an intraluminal filling defect that resembles a thromboembolus. The filling defect frequently spans the entire luminal diameter of the main or proximal pulmonary artery (Fig 22), a finding that is unusual in pulmonary thromboembolism. Other findings that may be helpful for distinguishing a pulmonary artery sarcoma from pulmonary thromboembolism include extension of the lesion into the lung parenchyma or mediastinum and delayed enhancement at CT angiography (58). Chong et al (59) reported a case of pulmonary artery sarcoma that showed positive uptake of fluorine 18 fluorodeoxyglucose at positron emission tomography integrated with CT; this feature may be helpful in differentiating a pulmonary artery sarcoma from pulmonary thromboembolism.
- Figure 22. Pulmonary artery sarcoma in a 70-yearold man with dyspnea. Axial contrast-enhanced CT scan shows filling defects in the main, left, and right pulmonary arteries and the right interlobar pulmonary artery. The arterial lumina are expanded, and extravascular mediastinal invasion is seen.
- Bronchial dilatation is a well-known hallmark of chronic obstructive pulmonary disease (COPD). In this setting, mucus-filled dilated bronchi, pulmonary infiltrates, or both are usually present. From a clinical standpoint, bronchiectasis in patients with COPD is related to sputum production. CT findings of bronchial dilatation in patients without clinical and functional evidence of COPD should arouse suspicion about the possibility of airway involvement in chronic thromboembolic disease (47) (Fig 17).
- When chronic thromboembolic pulmonary hypertension is suspected, an extensive diagnostic work-up is undertaken. Major goals are to determine whether thromboembolic disease is present, quantify the degree of pulmonary hypertension, and identify the cause or contributing factors. Echocardiography Transthoracic echocardiography allows diagnosis of pulmonary hypertension by showing the pressures in the right atrium and the degree of tricuspid regurgitation. Echocardiography also may help exclude other cardiac causes of pulmonary hypertension (eg, cardiac shunts) (15). Ventilation-Perfusion Scintigraphy ecently, Tunariu et al (60) reported that ventilation-perfusion (V/Q) scintigraphy has a higher sensitivity than CT pulmonary angiography for detecting chronic thromboembolic pulmonary hypertension. Normal findings at V/Q scintigraphy practically rule out the presence of chronic thromboembolic pulmonary hypertension. By that are unmatched by findings on ventilation scintigrams make chronic thromboembolic pulmonary hypertension the most likely diagnosis, although other conditions, including pulmonary veno-occlusive disease, may result in similar findings (60). However, V/Q scintigraphy does not allow determination of the magnitude, location, or proximal extent of disease and thus cannot predict its surgical operability. V/Q scintigraphy also does not help identify other causes of pulmonary hypertension.
- When chronic thromboembolic pulmonary hypertension is suspected, an extensive diagnostic work-up is undertaken. Major goals are to determine whether thromboembolic disease is present, quantify the degree of pulmonary hypertension, and identify the cause or contributing factors. Echocardiography Transthoracic echocardiography allows diagnosis of pulmonary hypertension by showing the pressures in the right atrium and the degree of tricuspid regurgitation. Echocardiography also may help exclude other cardiac causes of pulmonary hypertension (eg, cardiac shunts) (15). Ventilation-Perfusion Scintigraphy ecently, Tunariu et al (60) reported that ventilation-perfusion (V/Q) scintigraphy has a higher sensitivity than CT pulmonary angiography for detecting chronic thromboembolic pulmonary hypertension. Normal findings at V/Q scintigraphy practically rule out the presence of chronic thromboembolic pulmonary hypertension. By that are unmatched by findings on ventilation scintigrams make chronic thromboembolic pulmonary hypertension the most likely diagnosis, although other conditions, including pulmonary veno-occlusive disease, may result in similar findings (60). However, V/Q scintigraphy does not allow determination of the magnitude, location, or proximal extent of disease and thus cannot predict its surgical operability. V/Q scintigraphy also does not help identify other causes of pulmonary hypertension.
- Combined right heart catheterization (to determine the severity of pulmonary hypertension) and selective pulmonary angiography (to determine whether thromboembolic disease is present) is considered the reference standard for diagnosis of chronic thromboembolic pulmonary hypertension. At present, right ventricular catheterization is the best method for determining the mean pulmonary artery pressure and pulmonary vascular resistance, essential data for quantification of the disease severity and determination of the postoperative prognosis (15). Conventional pulmonary angiography is the traditional cornerstone for evaluation of chronic thromboembolic pulmonary hypertension. It helps confirm the diagnosis and gives an indication of surgical operability (24,33). However, in the future, pulmonary angiography probably will be performed only when an adequate surgical roadmap has not been provided by CT and magnetic resonance (MR) imaging .
- CT Angiography CT angiography is a useful alternative to conventional angiography not only for diagnosing chronic thromboembolic pulmonary hypertension but also for determining surgical operability and confirming technical success postoperatively (1,27,33). CT angiography is reported to be more sensitive than conventional angiography in depicting the presence of central thrombotic disease (27) and qually sensitive to MR angiography in depicting the disease at the segmental level. CT angiography is superior to MR angiography for the depiction of patent subsegmental arteries and intraluminal webs and for the direct demonstration of thrombotic wall thickening CT angiography also may provide evidence pointing toward an alternative diagnosis or a different cause of pulmonary hypertension.
- Although CT is highly sensitive for the detection of proximal and subsegmental thrombi, it does not yield sufficient quantitative information about the severity of functional impairment. MR imaging does enable sufficient characterization of the impairment of function in the right side of the heart (64). In addition, MR imaging permits accurate estimation of flow in the bronchial arteries in patients with chronic thromboembolic pulmonary hypertension (41). It also may play an important role in postoperative follow-up. However, MR imaging cannot take the place of conventional angiography and right heart catheterization for the preoperative determination of pulmonary vascular resistance and mean pulmonary artery pressure. MR imaging certainly has the potential to play a central role in the diagnosis, differential diagnosis, treatment planning, and assessment of postoperative outcome in patients with chronic thromboembolic pulmonary hypertension (65).
- In chronic thromboembolic pulmonary hypertension, the thromboembolic material is endothelialized and incorporated into the vessel wall; therefore—unlike the situation in acute thromboembolism—anticoagulation therapy is not effective. Nevertheless, lifelong anticoagulant therapy is recommended to avoid recurrent thromboembolism or in situ growth of existing obstructions of pulmonary arteries (14) The primary treatment for chronic thromboembolic pulmonary hypertension is surgical pulmonary thromboendarterectomy, which leads to a permanent improvement in pulmonary hemodynamics (66). In this surgical procedure, the thrombus and the adjacent medial layer are carefully dissected. The reported mortality rate ranges from 4% to 14% (67,68). Pulmonary thromboendarterectomy is considered for symptomatic patients who have hemodynamic or ventilatory impairment at rest or with exercise; it is also considered for patients who have normal or nearly normal pulmonary hemodynamics at rest but in whom marked pulmonary hypertension develops during exercise. The location and extent of the proximal thromboembolic obstruction are the most critical determinants of surgical operability: Occluding thrombi must involve the main, lobar, or proximal segmental arteries (10,67). If the hemodynamic impairment derives mainly from more distal, surgically inaccessible disease or from the resistance conferred by secondary small-vessel arteriopathy, then pulmonary hypertension will persist postoperatively and may have adverse short-term and longterm consequences. Some preoperative CT angiographic features are considered predictive of a good response to pulmonary thromboendarterectomy. For example, evidence of extensive central vessel disease and limited small-vessel involvement is considered positive (69,70). Dilated bronchial arteries are positively correlated with a lower mortality rate after the surgical procedure (41). Heinrich et al (71) suggested that patients without dilated bronchial arteries have more severe distal vascular disease (either thromboembolism or secondary small-vessel disease) leading to higher postoperative pulmonary vascular resistance than that in patients with dilated bronchial arteries. Placement of a filter in the inferior vena cava is recommended before surgery in all patients except those with a clearly defined source of emboli other than deep veins in the legs. Lifelong anticoagulant therapy is strongly recommended to prevent recurrent thrombosis after pulmonary thromboendarterectomy (14).