Data acquisition speed is one of the major limitations of MRI, as the data points are acquired sequentially in MRI. At the same time, many clinical applications require fast scanning either due to physiological processes they are trying to depict (e.g., cardiac motion), the need to capture the first pass of a contrast agent, or the requirement of patient breath-hold to reduce motion. We will begin by demonstrating effects of faster sampling through reduction of the number of acquired k-space lines, such as loss of resolution or aliasing artifacts. Parallel MRI allows accelerating acquisition without sacrificing spatial resolution. Parallel MRI techniques exploit the fact that each coil in multi-channel receiver obtains a signal of the object modulated by spatially varying coil sensitivities. Such multiple views of the object allow compensating for the missing data points either by unfolding the aliasing in image space or by approximating the unsampled signal in k-space^1-4. In this talk, we will discuss in detail two representative parallel imaging approaches.
Accelerations achievable with parallel MRI depend on the number of coils and coil geometry, and generally are limited to a factor of 2-3 per encoding dimension. The main limitation of parallel MRI lies in spatially non-uniform noise amplification, which grows rapidly with acceleration level. Therefore, we will pay special attention to noise properties of parallel MRI. The reduction in the acquired data and the reconstruction lead to noise increase, which is two-fold. First, there is a fundamental loss of signal-to-noise ratio (SNR) that is proportional to the square root of reduction factor and is common for all techniques. Next, there is SNR loss during the image reconstruction process traditionally described by a spatially varying geometry factor, or g-factor, which depends on a number of factors, such as coil geometry, scan orientation, number of coils, and overall reduction factor¹. We will discuss the limitations that SNR loss places on parallel imaging acceleration as well as other practical considerations in the usage of parallel MRI techniques. We will also consider simultaneous multi-slice (SMS) imaging as a special case of parallel MRI, which exploits variations of coil sensitivities in slice direction, and discuss SMS sampling and reconstruction strategies^5-6.
Parallel MRI has demonstrated utility in a number of clinical applications and can be used to increase spatial resolution in dynamic imaging while preserving temporal resolution and scan time, to reduce breath-hold duration, to improve spatial and/or temporal resolution in functional MRI, and to reduce geometric distortions/susceptibility artifacts by shortening readout in EPI imaging. We will finish with examples of some pulse sequence types and clinical applications that can benefit from the use of parallel MRI.

Acknowledgements

No acknowledgement found.