3D Free-breathing multichannel absolute Mapping in the human body at 7T.

Area of Science:

• Magnetic Resonance Imaging
• Physics
• Medical Imaging

Background:

• Ultrahigh field (UHF) MRI offers enhanced signal-to-noise ratio but faces challenges with homogenous flip angle (FA) distribution.
• Accurate FA mapping is essential for quantitative imaging and optimizing pulse sequences at UHF.

Purpose of the Study:

• To develop and validate a free-breathing, three-dimensional (3D) method for absolute flip angle (FA) mapping of the human body at ultrahigh field (UHF).
• To enable the generation of homogenous FA distributions crucial for advanced UHF MRI applications.

Main Methods:

• A 3D relative FA mapping sequence using a radial phase-encoding (RPE) trajectory was developed and tested in 11 healthy subjects at 7T.
• Absolute FA maps were generated by calibrating relative maps using a dedicated B1 shim setting.
• In vivo validation involved comparison with a 2D Cartesian method and evaluation of motion artifacts and respiration effects.

Main Results:

• Phantom validation showed good agreement between dynamic and static B1 map acquisitions.
• The 3D method successfully produced motion-artifact-free absolute FA maps in vivo across a range of body mass indexes.
• Respiration-resolved maps revealed significant FA variations (up to 24%) due to breathing, highlighting the need for motion compensation.

Conclusions:

• The developed method provides respiration-resolved absolute 3D FA maps at UHF, crucial for improving image quality.
• This technique facilitates the development of 3D B1 shimming and parallel transmission methods for enhanced UHF body imaging.