Synopsis

Keywords: Image acquisition: Fast imaging, Cardiovascular: Cardiac, Image acquisition: Sequences

MRI is an extremely valuable imaging technique, but it is inherently slow. This results in long acquisition times and sensitivity to motion. This makes imaging of the heart particularly challenging due to both cardiac and respiratory motion. There has been a lot of work on speeding up acquisition of data – many of these rely on efficient trajectories, as well as reducing the amount of data collected (including undersampling). This presentation will cover the main methods for speeding up cardiac MRI and the main advantages and disadvantages of each method.

Introduction

MRI is an extremely valuable imaging technique, but it is inherently slow. This results in long acquisition times and sensitivity to motion. This makes imaging of the heart particularly challenging, due to both cardiac and respiratory motion, which result in image artefacts. There are many ways to perform cardiac MRI to compensate for this motion, including cardiac gating and accelerated imaging.

Methods

It is possible to use an external ECG signal to trigger the acquisition of data to only a certain part of the cardiac cycle. As the acquisition of data is slow, it is possible to just collect a subset of k-space lines in each heartbeat (a segment). As cardiac motion is periodic it is possible to combine data from multiple cardiac cycles, to form the final images. This can still be slow, and there is a lot of work on speeding up acquisition of MRI data. There are two main ways to reduce scan times; collecting less data, and collecting the data more efficiently. Simple methods to reduce the amount of data collected include rectangular field-of-view and partial-Fourier acquisition. However, the speed ups achievable are relatively small. Alternatively, data undersampling can significantly reduce scan times, however this results in image artefacts, and must be paired with a suitable reconstruction (parallel imaging, temporal encoding, compressed sensing or machine learning). Efficient k-space sampling offers an alternative, or complementary, way to speed up the acquisition of data, where a larger portion of k-space is covered after each excitation; echo planar imaging (EPI) and spiral trajectories. Combined, these schemes can enable shorter Cardiac gating techniques, or may enable single-shot, or real-time imaging.

Conclusion

Cardiac MRI is challenging due to cardiac and respiratory motion. Cardiac gating is often combined breath-hold imaging to achieve good quality images. However, the breath-hold times can be very long, and difficult for patients. Therefore, speeding up imaging can enable reduced breath-hold times, or even alleviate the need for breath-holds completely. There are many ways to achieve rapid cardiac MRI, which makes the scans easier for patients, improve image quality, improve diagnostic readability and reduce scan times.

Acknowledgements

Supported by UKRI Future Leaders Fellowship (MR/S032290/1)