Phase retrieval-based phase-contrast CT for vascular imaging with microbubble contrast agent.

Abstract

The introduction of microbubble contrast agent into tissues can create significant phase shifts. Phase retrieval (PR)-based phase-contrast computed tomography (PCCT) is an imaging method for retrieving and reconstructing the phase shifts within an object. This study aimed to evaluate the feasibility of PR-based PCCT with microbubble contrast agent for vascular imaging. Projection phase-contrast images of individual microbubbles and a cluster of microbubbles were captured and compared. Contrast enhancement from microbubbles was evaluated by comparing to the gold standard iodine-based contrast agent invitro. The arterial systems of 14 Sprague-Dawley rats were perfused with microbubbles or saline. The rat hearts and the arterial systems were excised and imaged exvivo. CT imaging was performed at the energy of 22keV. PR was performed using the phase-attenuation duality (PAD) method with different δ/β values (PAD property). The contrast-to-noise ratio (CNR) was used for quantitatively assessing the contrast enhancement. Individual microbubbles functioned as a lens to focus the x rays, whereas, a cluster of microbubbles scattered the x rays. In the invitro experiment, the contrast enhancement from iodine was significantly greater than that from microbubbles (P<0.05). In the heart samples, the CNRs for microbubbles on PR-based PCCT were significantly greater than those on absorption-contrast CT (ACCT) and PR-free PCCT (both P<0.001). The CNRs for microbubbles were also significantly greater than those for saline on PR-based PCCT in the samples (P<0.001). Although they provided weaker contrast enhancement than that from iodine, microbubbles could still provide sufficient contrast enhancement to clearly show the 3D architecture of rat aortas and their main branches. The imaging modality can currently be used as a complement or alternative to absorption-based microCT for imaging vessels in biological samples.