Approach to Thoracic Imaging

Approach to Thoracic Imaging

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Functional imaging of lung perfusion and ventilation (V/Q) has been performed for over 40 years, its advent a medical breakthrough in that it replaced invasive pulmonary angiography for the diagnosis of pulmonary embolism. V/Q scans remain an important part of the clinical nuclear medicine practice despite the advent of CT angiography, particularly in efforts to reduce radiation dose. Evolving interpretation algorithms have made it easier for readers to interpret exams and for clinicians to understand the interpretation for their clinical practice. Thoracic surgeons rely on quantitative lung perfusion analysis for patients with borderline lung function who may undergo lobectomy or pneumonectomy, ensuring that patients will have sufficient postsurgical lung capacity.

Once a robust indication, the characterization of atypical, opportunistic, and neoplastic pulmonary diseases in immunocompromised hosts using Ga-67 and Th-201 scans has largely been replaced with high-resolution CT and bronchoscopy. However, characterization of active vs. inactive granulomatous disease using F-18 FDG PET/CT continues to support clinical therapeutic decision making.

Ventilation-Perfusion Scans for Diagnosis of Pulmonary Embolism

When first introduced in the 1970s, the V/Q scan replaced invasive pulmonary angiography for the diagnosis of pulmonary embolism. By the 1990s, a large, prospective clinical trial of mostly inpatients, called the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED), was published, giving physicians an interpretation algorithm fraught with confusing terms and indeterminate findings. It is understandable that with the advent of CT angiogram (CTA) in the 1990s, V/Q scans for the diagnosis of pulmonary embolism fell out of favor as CTA was more readily available and V/Q scan interpretations could be difficult to apply clinically.

Over the years, reanalysis of the valuable PIOPED data and further studies have refined V/Q interpretation algorithms, making interpretation easier. As the data have been reexamined, CTA and V/Q scan have been shown to have similar diagnostic accuracy. In one study, emergency department patients with suspected pulmonary embolism were triaged based on chest x-ray. If normal, the patient underwent V/Q instead of CTA. The number of positive V/Q scans was ~ 4%, and the number of positive CTAs was 13%. The false-negative rates on these studies were the same at 1%. The clinical value of V/Q scans becomes apparent in light of radiation dose received for a study with similar diagnostic accuracy as CTA.

A simplified trinary interpretation algorithm for V/Q scans in which conclusions are reported as pulmonary embolism present, absent, or nondiagnostic are helping clarify interpretations for referring clinicians. Training readers in this interpretation algorithm may also improve ordering clinician confidence in V/Q interpretations. A binary interpretation for V/Q scan using SPECT or SPECT/CT is also being utilized.

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May 7, 2023 | Posted by in CARDIOVASCULAR IMAGING | Comments Off on Approach to Thoracic Imaging

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