Multiecho sequences with variable refocusing flip angles: optimization of signal behavior using smooth transitions...

Area of Science:

• Magnetic Resonance Imaging
• Pulse Sequence Design
• Medical Physics

Background:

• Rapid Acquisition with Relaxation Enhancement (RARE), also known as Turbo Spin-Echo (TSE) or Fast Spin-Echo (FSE), is a widely used MRI technique.
• Conventional RARE/TSE/FSE sequences often require significant radiofrequency power (RFP).
• Optimizing signal acquisition while minimizing RFP is crucial for advanced MRI applications and patient safety.

Purpose of the Study:

• To present a modified RARE sequence utilizing variable flip angles.
• To demonstrate the feasibility of maintaining magnetization close to the static pseudo steady state (PSS) with dynamic flip angle adjustments.
• To achieve significant radiofrequency power reduction without sacrificing signal intensity or image quality.

Main Methods:

• Implementation of a RARE sequence with variable flip angles during the echo train.
• Initial preparation of magnetization into the static pseudo steady state (PSS) using a low refocusing flip angle.
• Application of high refocusing flip angles only for echoes encoding the center of k-space.

Main Results:

• Achieved a reduction in radiofrequency power (RFP) by a factor of 2.5-6.
• Maintained 100% of the attainable signal intensity by applying high flip angles to central k-space echoes.
• Observed a reduction in effective echo time (TE) due to stimulated echo pathways, enabling longer echo trains.

Conclusions:

• The proposed variable flip angle RARE technique offers substantial RFP savings.
• This method allows for enhanced imaging efficiency and potentially improved T2 contrast.
• The strategy enables longer echo trains for acquiring high-quality MR images with reduced power deposition.