Cardiac perfusion imaging using blood flow saturation in simultaneous multi-slice cine MRI

Abstract

Cardiac perfusion magnetic resonance imagining (MRI) is often used for ischemic diagnostics. However, the currently most popular technique uses an intravenous contrast agent: gadoliniumchelate. As this injection can cause problems in patients with renal problems, which are more prevalent in cardiac patients, a non-contrast cardiac perfusion technique would be ideal. Arterial spin labelling (ASL) is such a non-contrast technique. However, ASL requires using separate labelling pulses, complicating timing and sequence design. Concurrent labelling and imaging would solve this, which is why in this project simultaneous multi-slice (SMS) ASL was investigated. SMS is an imaging method during which multiple slices are excited at once and after receiving their signals the images are separated. If SMS were to be applied to two slices that contain the same arteries, the upstream slice would be functioning as a labelling and imaging site at the same time, as the blood in the upstream slice inevitably gets saturated during imaging. This is a way around the aforementioned downsides of ASL. Since blood flow in the arteries is highly pulsatile, SMS-cine was used, a dynamic SMS method which is more robust to motion. To isolate the saturated blood from background tissue and to compensate for bias from magnetization transfer between boound pools, the subtraction between upstream blood saturation and downstream blood saturation was taken. The hypothesis of this report is that by continuously labelling and imaging at the same time with SMS-cine, a signal can be produced directly related to the dynamic cardiac perfusion of the arteries. The viability of the proposed technique is investigated and optimized using numerical models and phantom experiments. From the phantom experiments and model predictions, it follows that for SMSASL a large flip angle and large slice thickness are optimal. The relative flow enhancement seen in the phantom experiments, using tubes with flowing water, responded to the varying of these parameters as predicted by the simulations. Yet, the relative flow enhancement was much higher than predicted by the model. This is probably caused by the fact that the tubes and water have a different T1 than blood and tissue. The model predicts relative flow enhancements of more than 90 % caused by pulsatile flow, regardless of the dicretization grid in z used, which indicates that it would be fruitful to further explore the potential of SMS-ASL in vivo. It is recommended to use an SPGR sequence and both a large flip angle and large slice thickness for this. Unbalancing the flip angle and slice thickness could also be explored using an expanded version of the numerical model and later phantom and in vivo experiments. Finally, a large potential improvement could be made to the numerical model by applying a strict conservation of total magnetization when moving spins to simulate flow, which is currently not guaranteed by the interpolation embedded in flow simulation.