Combined MR proton lung perfusion/angiography and helium ventilation: potential for detecting pulmonary emboli and ventilation defects

Abstract

Three-dimensional (3D) perfusion imaging allows the assessment of pulmonary blood flow in parenchyma and main pulmonary arteries simultaneously. MRI using laser-polarized (3)He gas clearly shows the ventilation distribution with high signal-to-noise ratio (SNR). In this report, the feasibility of combined lung MR angiography, perfusion, and ventilation imaging is demonstrated in a porcine model. Ultrafast gradient-echo sequences have been used for 3D perfusion and angiographic imaging, in conjunction with the use of contrast agent injections. 2D multiple-section (3)He imaging was performed subsequently by inhalation of 450 ml of hyperpolarized (3)He gas. The MR techniques were examined in a series of porcine models with externally delivered pulmonary emboli and/or airway occlusions. With emboli, perfusion deficits without ventilation defects were observed; airway occlusion resulted in matched deficits in perfusion and ventilation. High-resolution MR angiography can unambiguously reveal the location and size of the blood emboli. The combination of the three imaging methods may provide complementary information on abnormal lung anatomy and function.

FIG. 1

Schematic diagram of time-line of imaging procedures for the integration of MR scans. There was a 1-hr delay between the preemboli and postemboli studies to ensure residual contrast agent would not significantly interfere with the postemboli imaging.

FIG. 2

Images from lungs of a pig pre-injection (top row) and postinjection (bottom row) of pulmonary emboli in the form of blood clots. Perfusion images a and c are reconstructed from a 3D dataset using MIP. The two long arrows in image c indicate multiple parenchymal perfusion abnormalities induced by the two emboli, as indicated by arrowheads. Main and both right and left lobar pulmonary arteries are also visualized. Images b and d are high-resolution MR angiography (MIP), clearly showing the smaller subsegmental and even subsubsegmental vessels. In d, emboli are marked by arrows. Note two short arrows in d point to two subsegmental emboli which are not seen in perfusion image c (short arrow). Image e is a single slice from ³He and shows normal ventilation distribution of the lung postemboli with high SNR.

FIG. 3

Images from lungs of a pig before (top row) and after (bottom row) creation of a ventilation defect. Images a and e are one slice from a 3D perfusion dataset. Images b and f are maximum intensity images (SI_max) calculated from the dynamic perfusion datasets. Images c and g are time delay maps, as defined in the text, with hyperintensity representing longer delays. Image d schematically illustrates the definitions of the parameters in the calculated parametric maps. Image h was acquired postventilation-defect using ³He. The arrow in e, g, and h points to the region of defective ventilation.