Related Experiment Video
Updated: Jun 25, 2026
09:30
Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease
Published on: December 18, 2016
Altering MRI rotating frame relaxations by changing the truncation level of Hyperbolic Secant pulse
Sara Ponticorvo1, Lin Wu1, Andrej Lasica2
1Department of Radiology, Center for Magnetic Resonance Research (CMRR), University of Minnesota, Minneapolis, MN, USA.
Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|June 23, 2026
Summary
RETRO (RElaxation dependent on TRuncatiOn) alters MRI rotating frame relaxations by adjusting Hyperbolic Secant pulse truncation. This novel strategy enables flexible MRI contrasts for probing fast-relaxing components inaccessible with conventional methods.
Area of Science:
- Magnetic Resonance Imaging (MRI)
- Pulse Sequence Design
- Relaxation Physics
Background:
- Conventional Hyperbolic Secant (HS) pulses for adiabatic inversion have limitations in controlling relaxation pathways.
- The initial B1 field in typical HS pulses is negligible, simplifying relaxation characterization to T1ρ,1(t) or T2ρ,1(t).
- Non-zero truncation levels in HS pulses introduce an initial B1 field, enabling simultaneous T1ρ,1(t) and T2ρ,1(t) relaxation pathways.
Purpose of the Study:
- Introduce the RETRO (RElaxation dependent on TRuncatiOn) strategy to manipulate rotating frame relaxations in MRI.
- Investigate the theoretical basis of relaxation pathways under the RETRO strategy, including dipolar and exchange-induced relaxations.
- Evaluate the in vivo performance of RETRO contrasts in rodent brains and assess its potential for probing fast-relaxing components.
Main Methods:
- Developed the RETRO strategy by modifying the truncation level of amplitude and frequency modulation functions in HS radiofrequency pulses.
- Provided theoretical descriptions for homonuclear dipolar relaxations in a weak collision regime and exchange-induced relaxations between spin populations.
- Acquired in vivo RETRO contrasts in rodent brains using varying pulse durations and truncation levels.
Main Results:
- Non-zero truncation levels in HS pulses create an initial B1 field, influencing relaxation through both T1ρ,1(t) and T2ρ,1(t) pathways.
- High truncation levels in RETRO allow for shorter pulse durations while maintaining suitable RF amplitudes for in vivo imaging.
- RETRO strategy demonstrated flexible and robust MRI contrasts, successfully probing fast-relaxing components in vivo.
Conclusions:
- The RETRO strategy offers a versatile method for controlling rotating frame relaxations in MRI by adjusting HS pulse truncation.
- This approach enables the generation of flexible and robust MRI contrasts, expanding the scope of detectable biological signals.
- RETRO facilitates the investigation of fast-relaxing components in vivo, overcoming limitations of conventional adiabatic T1,2ρ techniques.
Related Concept Videos
01:17NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences
A pulse is a short burst of radio waves distributed over a range of frequencies that simultaneously excites all the nuclei in the sample. Upon passing a radio frequency pulse along the x-axis, the nuclei absorb energy corresponding to their Larmor frequencies and achieve resonance. This shifts the net magnetization vector from the z-axis toward the transverse plane. This angle of rotation of the magnetization vector, or the flip angle, is proportional to the duration and intensity of the pulse.
01:14NMR Spectrometers: Resolution and Error Correction
When magnetic nuclei in a sample achieve resonance and undergo relaxation, the signal detected in NMR is an approximately exponential free induction decay. Fourier transform of an exponential decay yields a Lorentzian peak in the frequency domain. Lorentzian peaks in an NMR spectrum are defined by their amplitude, full width at half maximum, and position, where the peak width is governed by the spin-spin relaxation time alone. In real experiments, however, the applied magnetic field is rendered...