RF Pulse Design for MRI Magnetization Transfer Reduction

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Solution Overview

Problem

Magnetization transfer effects, which occur due to the exchange of magnetization between bound and free protons, often reduce image quality and signal-to-noise ratio in magnetic resonance imaging (MRI), especially when imaging tissues with many bound protons, such as white matter, and can be exacerbated by multi-slice acquisitions, leading to poor signal quality and increased specific absorption rate (SAR).

Innovation Solution

Designing RF pulses with optimized profiles that include low and high amplitude portions, where the high amplitude portions are played out more times than the low amplitude portions, and using a computer to determine a k-space trajectory that iteratively optimizes the RF pulse profile to minimize magnetization transfer while maintaining a given flip angle and alternating slice-select gradient, thereby reducing off-resonance magnetization transfer saturation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Area of stationary object

If conventional RF pulses are used for multi-slice acquisitions, then image coverage is improved, but magnetization transfer saturation increases and signal quality deteriorates

Engineering Contradiction:

Improveimaged areaVSAvoidsignal quality

Core Design Contradiction:

Area of stationary objectVSReliability

Solution Approach 1:

The RF pulse is segmented into multiple amplitude portions (low amplitude and high amplitude portions) that are played out different numbers of times. The high amplitude portions are played out more times than the low amplitude portions, creating a non-uniform distribution that reduces cumulative magnetization transfer saturation while maintaining adequate signal quality across multi-slice acquisitions.

Inventive Principle:

Principle #1Segmentation

2Use of energy by stationary object

If VERSE pulses are used to reduce peak power, then SAR is reduced, but magnetization transfer effects spread over a wider region increasing cumulative MT

Engineering Contradiction:

ImproveSARVSAvoidcumulative magnetization transfer

Core Design Contradiction:

Use of energy by stationary objectVSObject-generated harmful factors

Solution Approach 1:

Different portions of the RF pulse are assigned different amplitudes and repetition frequencies. The high amplitude portions are played out more times to maintain signal quality in regions where it matters most, while low amplitude portions are played out fewer times to reduce cumulative magnetization transfer. This local differentiation resolves the contradiction between reducing SAR and minimizing cumulative MT effects.

Inventive Principle:

Principle #3Local quality

3Reliability

If RF pulse amplitude is increased to improve signal quality, then SNR is improved, but magnetization transfer saturation increases

Engineering Contradiction:

Improvesignal qualityVSAvoidmagnetization transfer saturation

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The RF pulse employs periodic alternation between low amplitude and high amplitude portions, with the high amplitude portions played out more times than the low amplitude portions. This periodic structure allows the system to accumulate sufficient signal for good SNR while the interspersed low amplitude portions provide recovery time that reduces cumulative magnetization transfer saturation.

Inventive Principle:

Principle #19Periodic action

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 effectively limits magnetization transfer saturation, maintaining high signal quality and acceptable SAR, even in multi-slice acquisitions, by distributing the flip angle over multiple mainlobe portions of the RF pulse, reducing the impact of magnetization transfer on image quality and improving contrast.

Implementation Method 1

When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or 'longitudinal magnetization', MZ, may be rotated, or 'tipped ', into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.

Methodology Applied

Scientific EffectNuclear Magnetic Resonance:

Implementation Method 2

Magnetization transfer is the exchange of magnetization between macromolecular or 'bound' protons and bulk or 'free' protons, most prevalent in water. This exchange can occur, for example, by dipolar interaction or chemical exchange between different tissue types and molecules.

Methodology Applied

Scientific EffectMagnetization Transfer:

Data Source

PatentUS8022702B2MR imaging with an RF pulse producing reduced magnetization transfer

Publication Date: 2011.09.20 DIGNITY HEALTH

AI summary

A system and method are provided herein for designing and transmitting RF pulses which cause a reduced off-resonance magnetization transfer saturation. An RF pulse shape may be optimized according to a set of Bloch solutions defining a desired magnetization profile. An RF pulse may be transmitted according to this optimized shape according to a k-space trajectory which traverses a high amplitude portion of the RF pulse more times than one or more low amplitude portions. In addition, a generally alternating slice select gradient may be applied during transmission of the RF pulse.