Improving the robustness of perfusion imaging with arterial spin labeling magnetic resonance imaging.

摘要

Tissue function depends heavily on perfusion, a process which brings the blood supply to the tissue through the arterial system and removes the metabolic by-products via the veins. Abnormalities and disruptions in this process can have profound effects. Cerebral blood flow (CBF), a quantitative perfusion measurement in the brain in units of milliliters of blood per 100 grams of tissue per minute, can be used to understand complex neurophysiology and reveal neuropathology. Cerebral perfusion is conventionally measured with the use of an exogenous contrast agent that is invasive and has limited repeatability. A new class of technique, known as arterial spin labeling (ASL), uses the water molecules in the arterial blood as an endogenous tracer to measure CBF via magnetic resonance imaging. ASL is therefore completely non-invasive and has been widely adapted in both clinical and research environments.;The principle of ASL is based on acquiring a control image where the inflowing blood is fully relaxed, and a label image where the inflowing blood is inverted. When the label image is subtracted from the control image, the remaining signal is proportional to the blood that has perfused into the brain after the static tissue signal cancels out. The ASL technique has two components; the spin preparation that tags that arterial blood by magnetic inversion or saturation, and the image acquisition that collects the imaging data. In this dissertation work, we have developed novel techniques to improve the robustness of ASL from both aspects.;ASL technique was employed in the clinical environment at Wake Forest University Baptist Medical Center. With the initial experience of a large clinical population, ASL was found to be susceptible to hardware instability and patient motion. A post-processing filtering algorithm was developed to improve the overall perfusion image quality, including cases that were deemed uninterpretable. The filtering algorithm improved image quality in nearly 40% of the total population, and salvaged over 1000 previously unusable cases.;In research, a novel image acquisition method, 3D GRASE PROPELLER (3DGP) was developed to address issues in ASL learned from the clinical experience. By combining a single-shot 3D acquisition method and a unique rotational trajectory, 3DGP improved the image resolution by 60% while reducing scan time by 50% compared to the clinical protocol. In addition to superior image quality, 3DGP was insensitive to patient motion and demonstrated higher perfusion sensitivity. Improvement was also made from the aspect of spin preparation. Pseudo-continuous ASL (PCASL) was a recently invented labeling technique that has shown optimal perfusion sensitivity. Not only PCASL has demonstrated improved perfusion sensitivity in human, it also provided a suitable labeling scheme for perfusion imaging in non-human primates (NHP). CBF was difficult to measure in NHP using conventional ASL method due to the small brain size. With PCASL, NHP perfusion images with good quality can be obtained without requiring special hardware.