From the Archives of the AFIP: Intralobar Sequestration

Radiologic-Pathologic Correlation

Abstract Introduction Postulated Pathogenesis Clinical Characteristics

Pathologic Features Histologic Features Radiologic Appearance

Therapy And Prognosis Conclusions Source Information References

Radiologic Appearance

Radiographic Findings

Uncomplicated intralobar sequestration typically manifests radiographically as a homogeneous consolidation with irregular margins (Figure 9a, Figure 10a) or as a uniformly dense mass with smooth or lobulated contours, located in the posterior basal portion of a lower lobe (Figure 9b, Figure 9c, Figure 10c, Figure 11, Figure 12) (9,19,22,25,26,44). Focal bronchiectasis, subsegmental atelectasis, decreased lung volume, mediastinal shift, and prominence of the pulmonary hilum have also been described (11,33,45). Pleural effusions rarely (less than 4% of cases) occur (1). Intralobar sequestration is a diagnosis that should always be considered in the setting of recurrent or persistent pneumonia localized to the lower lobe (22,33). After antibiotic treatment, the radiographic abnormality may diminish or alter significantly, and a small irregular, opaque area or a multicystic lesion may remain (Figure 9, Figure 10) (5,36,46). Although many intralobarsequestrations manifest as parenchymal consolidations as described above, a large number of these lesions contain air (26).

Cavitation is defined as a gas-filled space, with or without a fluid level, that is located within a pulmonary consolidation or mass (44). This radiographic manifestation of intralobar sequestration is not uncommon, although interestingly it does not uniformly represent infection (44,47). The proportion of consolidation that remains after the onset of cavitation is variable.

Intralobar sequestration complicated by advanced chronic inflammation may evolve into a predominantly cystic lesion, which typically manifests as closely related "ring" shadows (22,25,26,33,44). In one study, at least half of all cases of intralobar sequestration demonstrated this radiographic feature (1). There is a considerable spectrum of appearances, ranging from a single large cyst to a collection of multiple cysts of variablesize (8). Approximately one-third of all cystic intralobar sequestrations demonstrate fluid levels on chest radiographs (Figure 13a, Figure 14a) (1). When cysts contain air-fluid levels or air alone, the cysts have at least partial communication with the tracheobronchial tree (22,44,47). The thin walls of the cysts may become indistinct, and the cysts may even become obscured by adjacent consolidation during periods of active inflammation (Figure 14b) (8,31,32,44).

In unusual cases, an intralobar sequestration may manifest only as a subtle, tubular area of opacity within the lower lobe; this finding may represent the systemic feeding vessel or draining vein associated with the lesion (19,22,26,36). An unusual radiographic manifestation of intralobar sequestration is localized emphysema without an associated consolidation or mass (26,47). In rare cases, punctate or peripheral linear calcifications may be seen within an intralobar sequestration, but this finding is more readily detected with CT (38,41,44,48).

When intralobar sequestration appears as a mass at the inferior paravertebral aspect of the thorax, the differential diagnosis may include neurogenic tumor, lateral thoracic meningocele, extramedullary hematopoiesis, and pleural tumor (44). A cavitary intralobar sequestration can mimic the appearance of a lung abscess, necrotizing pneumonia, fungal or mycobacterial pneumonia, cavitating neoplasm, and empyema (44). The predominantly cystic form of intralobar sequestration has a broad differential diagnosis that includes pulmonary abscess, empyema, bronchiectasis, emphysema, bronchogenic cyst, foregut cyst, pericardial cyst, even- tration of the diaphragm, and congenital cystic adenomatoid malformation (1,31).

CT Findings

CT not only recapitulates the radiographic features of intralobar sequestration but also further reveals the complex character of the lesion. Intralobar sequestration typically manifests as a homogeneous or heterogeneous soft-tissue mass in the lower lobe or as a heterogeneous consolidation that shares an irregular border with the adjacent lung parenchyma (Figure 15a, Figure 16a) (12,26,49,50). The lesion is usually located in the posteromedial portion of the lower lobe. Part of the consolidation may be replaced by areas of cavitation containing air or air-fluid levels (Figure 13b) (43,47,49,51). The lesion may partially and heterogeneously enhance when contrast material is administered; in some cases, CT has demonstrated hypervascularity and the presence of dilated vessels (Figure 15a) (26,52). Similar to its appearance on radiographs, an intralobar sequestration may be seen on CT scans as an almost purely cystic mass or as a confluence of multiple thin-walled cysts containing fluid, air-fluid levels, or air alone (Figure s 16a, Figure 17a) (15,26). The multicystic form can achieve a considerable size (up to at least 10 cm) (15).

Other interesting features of intralobar sequestration may be observed at CT. The bronchovascular bundles of the remaining functional lung may be peripherally displaced by the lesion (8,51). Emphysematous changes surrounding the lesion are often seen, a finding interpreted as "air trapping" within the transition zone between the sequestration and normal lung parenchyma (26,47,49,53). A dense branching pattern, consistent in configuration with mucus-impacted ectatic bronchi, is a rare finding in intralobar sequestration (Figure 18) (49). Calcification is an unusual feature, may be diffuse or peripheral, and has been observed within the sequestration and in the anomalous systemic artery (26,33,38,39,41,48,54). In rare cases, pleural effusion is seen (36).

The anomalous systemic artery that supplies the lesion is visualized in up to 80% of cases after contrast material administration and may be seen in cross section (Figure 19) or as an enhancing linear structure adjacent to the aorta in the inferior pulmonary ligament (Figure 20a) (13,26,49,51,55). Use of dynamic scanning technique, thin-section CT, and helical (spiral) CT have substantially improved delineation of the origin and course of the anomalous systemic artery (26,55,56). However, failure to visualize the artery does not exclude the diagnosis of pulmonary sequestration (12). Aneurysm or thrombosis of the systemic artery has been identified at CT (48). A recent limited study demonstrated consistent enlargement of the azygos system (as determined on the basis of strict CT criteria), a finding that the authors attributed to long-standing pulmonary infection (20).

MR Imaging Findings

With its multiplanar capability, MR imaging can successfully demonstrate the systemic feeding vessels of an intralobar sequestration (Figure 13c, Figure 16b) (26,28,57,58). MR imaging can also depict the pulmonary venous return of the lesion and the relationship of the draining vein to the cardiac chambers (59).

In addition to conventional spin-echo cardiac-gated protocols, newer gradient-echo techniques can be used to image vascular structures optimally. These techniques, including gradient-recalled acquisition steady state (Figure 13c) and fast low-angle shot sequences, have improved delineation of the aberrant artery arising from the aorta and identification of draining veins (52,60). MR imaging is an excellent noninvasive diagnostic tool, particularly for patients in whom angiography is contraindicated (33,47,52). A notable limitation of MR imaging is that small anomalous vessels may not be detected (25).

MR imaging can also reveal the cystic nature of many intralobar sequestrations (Figure 21), as well as the variable solid, fluid, hemorrhagic, and mucus-containing components (8,33). Mucus-impacted bronchi may be seen as linear and ectatic branching structures of high signal intensity on multiecho images (57). Spontaneous hemorrhage within the lesion has been described as an area of high signal intensity on both T1- and T2-weighted spin-echo images (61).

Angiographic Findings

Angiography is performed to support the diagnosis of intralobar sequestration. Use of aortography allows the anomalous systemic arterial supply to be identified (Figure 20b). The feeding artery arises from the thoracic aorta in 73% of cases, the abdominal aorta or celiac axis in 20%, and the intercostal arteries in 4% (2). Multiple arteries supply the lesion in 16% of cases (2). In rare cases, the lesion is supplied by anomalous branches of the coronary arteries or may have a combined systemic and pulmonary arterial supply (19,62,63). Surgical reports confirm that systemic branches that arise from the descending thoracic aorta are found within the leaves of the inferior pulmonary ligament, and that feeding vessels that originate from the abdominal aorta pierce the hemidiaphragm or traverse the aortic or esophageal hiatus (1,5,9). A single aortic branch may divide into several tributaries before entering the sequestration (1,64).

Selective injection angiography or digital subtraction angiography (Figure 13d, Figure 13e) can demonstrate the characteristic pulmonary venous drainage of intralobar sequestrations (33). In the vast majority of cases (95%), venous drainage occurs through the pulmonary veins of the parent lobe into the left atrium (1,2). The remaining 5% of intralobar sequestrations drain into the systemic circulation through the azygos system, hemiazygos system, intercostal veins, or superior vena cava toward the right atrium (1,2). In rare cases, the lesion may drain into a branch of the pulmonary artery, joining antegrade arterial flow into normal lung parenchyma (19,63,65). Pulmonary angiograms that have been obtained in unsuspected cases of intralobar sequestration reveal a sparsity of vessels supplying the affected segments of the lower lobe (66).

According to Turk and Lindskog (45), a vessel of 3 mm or less in diameter is more likely to be one of multiple vessels supplying the lesion. The mean diameter of the feeding artery is 6.3 6.6 mm, but the caliber may approach 2.5 cm (1,66). When an intralobar sequestration manifests in the neonatal period, the aberrant vessel often approaches the caliber of the neighboring aorta (16,67). Atherosclerosis of the feeding artery is common, as discussed, and aneurysms are unusual but do occur (1,35,37,68).

Findings from Other Imaging Studies

Sonography is ideally suited for evaluating the prenatal and postnatal chest (69). At sonography, intralobar sequestration typically manifests as a solid, echogenic intrathoracic mass, with or without small cystic areas, and is best evalu-ated in the absence of interposed aerated lung (37,69,73). The sonographic appearance of intralobar sequestration is nonspecific, and the differential diagnosis includes congenital diaphragmatic hernia, cystic adenomatoid malformation, cystic mediastinal teratoma, and bronchogenic and enteric cysts (74). When color and duplex Doppler sonography are used, the pulsatile flow of the anomalous artery may be discovered originating from the aorta or celiac axis and internal vessels may be visualized (67,72,74,75). Venous drainage into the left atrium may be detected by using spectral waveform analysis and Doppler guidance (72,75,76).

Radionuclide imaging with ventilation (xenon-133) and perfusion (technetium-99m macroaggregated albumin) studies characteristically demonstrates a matched segmental or subsegmental defect (11,27,77,78). In one report, the entry of Xe-133 gas into the lesion was delayed and the gas was retained during the washout phase; these findings were attributed to collateral ventilation (79). Radionuclide angiography may demonstrate absence of perfusion to the lesion during the pulmonary phase, followed by rapid perfusion during the early systemic phase. In accordance with the systemic arterial supply of intralobar sequestration, the radionuclide must first pass through the pulmonary circulation and enter the aorta to reach the feeding vessel (80).

Abstract Introduction Postulated Pathogenesis Clinical Characteristics

Pathologic Features Histologic Features Radiologic Appearance

Therapy And Prognosis Conclusions Source Information References