Previously, magnetic resonance imaging (MRI) was surpassed by CT in the investigation of lobar collapse largely due to the superior spatial resolution of lung parenchyma in the latter, but also due to cost, availability and speed of acquisition. In particular, endobronchial tumours and smaller bronchi were less well demonstrated by MRI than CT. CT is still the imaging investigation of choice but some studies have investigated the ability of MRI to differentiate a tumour mass from postobstructive collapse by utilising differences in signal characteristics.
Sometimes the distinction can be made on T1-weighted images, but it is generally accepted that T2-weighted images are superior as the tumour is of lower signal intensity than the obstructed lung which has higher water content.
On ultrasound of a large pleural effusion, the underlying collapsed lung is often visible as a hyperechoic wedgeshaped area within hypoechoic or anechoic fluid (Fig.1). In practice, the main utility of ultrasound is to readily distinguish pleural effusion from a collapsed and consolidated lung when radiographic appearances are equivocal, but ultrasound may also be used to guide biopsy of a peripheral lesion within a collapsed lobe (Fig. 2C).
On positron emission tomography (PET), a collapsed lobe demonstrates less uptake of fluorodeoxyglucose than tumour. CT PET may provide more accurate delineation of tumour from postobstructive collapsed lung than CT alone, which may be useful for guiding biopsy and treatment, e.g. radiotherapy (Fig. 2).
FIGURE 1 ■ Ultrasound demonstrating a linear collapsed lower lobe with a large pleural effusion. The asterisks demonstrate the distance between the collapsed lung and hemidiaphragm.
FIGURE 2 ■ Intravenous contrast-enhanced CT demonstrating right middle lobe collapse (A). Image from a CT PET study at the same level (B) shows increased uptake of radioisotope within the collapse. A targeted ultrasound-guided biopsy was performed (C), and bronchogenic carcinoma confirmed.
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