MRI Fat Suppression via Pre-Pulse Timing and Phase Encoding
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Solution Overview
Problem
In multi-slice MRI imaging, the frequent application of CHESS pulses for fat suppression extends imaging time and increases Specific Absorption Rate (SAR), with the effect of fat suppression decreasing over time as longitudinal magnetization of fat protons recovers.
Innovation Solution
Divide k-space into segments and vary the measurement order so that low spatial frequency areas are measured at the timing with the highest pre-pulse effect, applying the pre-pulse less frequently than the number of slices, ensuring consistent fat suppression across all slices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a CHESS pulse is applied just before each exciting pulse of actual imaging in multi-slice imaging, then excellent fat suppression effect is obtained for all slices, but the repeat time TR is lengthened and imaging time extends
Solution Approach 1:
A CHESS pulse is applied in advance before the imaging sequence starts, preparing the fat proton magnetization state beforehand. This preliminary action eliminates the need to apply CHESS pulses repeatedly before each slice excitation, thereby reducing the repeat time TR and shortening total imaging time while maintaining fat suppression effectiveness.
Solution Approach 2:
Instead of applying CHESS pulses continuously before each excitation, the patent uses periodic action by applying a single CHESS pulse at the beginning and relying on the longitudinal relaxation characteristics of fat protons to maintain suppression effect throughout the imaging sequence, thus reducing the frequency of pre-pulse application.
2Reliability
If a CHESS pulse is applied just before each exciting pulse of actual imaging in multi-slice imaging, then excellent fat suppression effect is obtained for all slices, but SAR is increased
Solution Approach 1:
The CHESS pulse is applied once in advance before imaging begins, rather than repeatedly before each slice. This preliminary application reduces the total number of RF pulses, thereby lowering the Specific Absorption Rate (SAR) while still achieving effective fat suppression through the longitudinal relaxation properties of fat protons.
Solution Approach 2:
The patent reduces the periodic application of CHESS pulses by using a single initial application followed by the imaging sequence without intermediate CHESS pulses, thus decreasing the cumulative RF energy deposition and reducing SAR.
3Productivity
If the frequency of pre-pulse application is reduced, then imaging time is shortened and SAR is lowered, but the pre-pulse effect may be insufficient for all slices
Solution Approach 1:
The patent applies different phase encode amounts to different slices in a specific order, ensuring that each slice receives the appropriate amount of phase encoding at the optimal time after the CHESS pulse. This local optimization maintains consistent fat suppression effect across all slices even with reduced pre-pulse application frequency.
Solution Approach 2:
The imaging sequence dynamically adjusts the phase encode amounts for different slices based on their position in the imaging sequence and the timing relative to the CHESS pulse application. This dynamic adjustment ensures that each slice is imaged at the optimal moment when the fat suppression effect is still effective, maintaining consistency across all slices.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach maintains excellent fat suppression for all slices while reducing the frequency of pre-pulse application, shortening imaging time and lowering SAR, thus improving imaging efficiency and quality.
Implementation Method 1
the magnetic moment of atomic nuclear spins of tissue composition molecules makes a precession movement around the BO direction at a resonance frequency inherent to each spin
Implementation Method 2
When these spins are exposed to magnetic field (irradiation radio-frequency magnetic field B1) having a frequency near to the resonance frequency from a direction perpendicular to the BO direction, a net magnetic moment M is rotated (excited) toward x-y plane
Implementation Method 3
transverse magnetization of the fat protons which are selectively excited by the CHESS pulse are subjected to phase dispersion to vanish the magnetization of the fat protons
Implementation Method 4
when the irradiation radio-frequency magnetic field B1 is turned off, the excited magnetic moment is returned (relaxed) to the original state while emitting energy (NMR signal)
Data Source
AI summary
A magnetic resonance imaging apparatus for imaging a plurality of different slice planes having pre-pulse applying means configured to apply a pre-pulse for affecting in-plane magnetization of all slices of a measurement target, measuring means configured to make a measurement for applying one phase encode amount for one slice plane to obtain an echo signal and dispose the echo signal in a k space, and control means configured to control operations of the pre-pulse applying means and the measuring means. The control means has first control means configured to control to repeat an operation of executing the measurement once according to a predetermined order for all slice planes of the measurement target after a first pre-pulse is applied while a phase encode amount is varied in a predetermined order until all k spaces of all slices are filled, and second control means configured to control the pre-pulse applying means so that the pre-pulse is applied every time the measurement is executed at a frequency which is different from the number of the slices of the measurement target and a multiple number of the number of the slices, and the first control means sets an initial phase encode amount of each slice so as to measure a low spatial frequency area of the k space at a timing having a large effect of the pre-pulse.