Adiabatic T2 preparation sequence for magnetic resonance imaging with reduced B1 sensitivity
Inventors
Nezafat, Reza • Ouwerkerk, Ronald • Stuber, Matthias
Assignees
Johns Hopkins University • US Department of Health and Human Services
Publication Number
US-7787930-B2
Publication Date
2010-08-31
Expiration Date
2025-06-06
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Abstract
Adiabatic pulses that define an amplitude modulation and a frequency modulation are applied in a sequence of pulses to obtain a T2 weighted magnetic resonance image. Such an adiabatic T2 prep sequence typically includes a first 90° pulse, an even number of adiabatic pulses, and a second 90° pulse. Adiabatic pulses can be selected based on function pairs, or can be defined numerically. A magnetic resonance imaging (MRI) system includes a library of adiabatic pulse waveforms, and is configured to select a waveform and apply an RF magnetic field based on the selected pulse waveform.
Core Innovation
The invention describes methods and apparatus that provide T2-weighted magnetic resonance images by applying pulse sequences that include one or more adiabatic pulses. These adiabatic pulses combine amplitude and frequency modulation of the RF field designed to achieve magnetization rotation that is independent of RF field strength variations over a substantial range. Typically, these adiabatic pulses substitute conventional 180° pulses, resulting in images superior to those obtained with only amplitude modulated or conventional hard RF pulses having time-invariant amplitude.
The problem being addressed arises from limitations in conventional T2 preparation (T2 prep) sequences used in MRI, especially for coronary magnetic resonance angiography (MRA). Conventional sequences use MLEV composite pulses to compensate for RF field (B1) imperfections and flow, but they increase the specific absorption rate (SAR), limiting their effectiveness, particularly at high magnetic fields (B0). As a result, MLEV pulse-based T2 prep sequences produce artifacts and inadequate robustness, leading to poor contrast and image quality, especially in 3D coronary MRA applications where contrast between coronary blood and myocardium is low. Therefore, there is a need for improved T2 prep methods and apparatus that reduce sensitivity to B1 inhomogeneities, minimize artifacts, and control SAR.
Claims Coverage
The patent contains three independent claims covering a magnetic resonance imaging method, a computer readable medium with instructions, and a magnetic resonance imaging system, with additional independent claims related to methods of reducing image sensitivity.
T2-weighted imaging method using two adiabatic RF pulses
A method applying an initial RF pulse to rotate longitudinal magnetization into the transverse plane; applying two sequential adiabatic RF pulses with amplitude and frequency modulation spaced by approximately TE/4 and TE/2 intervals to invert components of transverse magnetization; followed by a final RF pulse rotating the transverse magnetization back to a stored T2-weighted longitudinal magnetization; and obtaining a specimen image based on this magnetization.
Selection and common waveform of adiabatic pulses
The first and second adiabatic RF pulses have a common amplitude and frequency modulation, which may be defined by mathematical functions such as hyperbolic secant and tangent or by numerical optimization.
Use of composite, adiabatic half passage, or pseudo-adiabatic pulses for excitation
The method includes performing the initial rotation of longitudinal magnetization into the transverse plane with either composite 90° pulses, adiabatic half passage pulses, or pseudo-adiabatic half passage pulses.
Magnetic resonance imaging system configured for adiabatic T2 prep
A system controller defines a pulse sequence including first and second 90° pulses and two instances of an adiabatic pulse with temporal separations of TE/4, TE/2, and TE/4; an RF coil produces the defined sequence producing T2-weighted longitudinal magnetization; and a receiver coil detects signals based on this magnetization for image processing.
Incorporation of user interface and pulse selection
The system controller includes a user interface to receive user input to select among adiabatic pulse definitions stored in a computer readable medium, enabling selection of pulse waveforms used in the sequence.
Reducing image sensitivity to B0 and B1 variations
A method applying a first 90° pulse, two adiabatic pulses spaced by intervals of about TE/4 and TE/2, and a second 90° pulse spaced TE/4 after the second adiabatic pulse, producing stored T2-weighted longitudinal magnetization less sensitive to magnetic field inhomogeneities and RF penetration effects.
Use in imaging cardiac systems triggered by ECG
Application of the method to specimens including cardiac regions with pulses applied in synchronization with electrocardiogram signals to produce images within a single cardiac cycle.
The independent claims cover inventive methods employing sequences of adiabatic pulses with specific timing relative to echo time to produce T2-weighted images with reduced B1 sensitivity, system architectures for implementing such methods including pulse selection options, and methods explicitly targeting reduction of image sensitivity to magnetic field inhomogeneities, particularly in cardiac imaging applications.
Stated Advantages
Adiabatic T2 prep sequences provide images with reduced sensitivity to B1 inhomogeneities compared to conventional MLEV composite pulses.
Improved image contrast between coronary blood and myocardium with fewer artifacts associated with field inhomogeneity.
The sequences achieve comparable or superior T2 weighting without excessive increases in specific absorption rate (SAR).
Adiabatic pulses enable robust magnetization rotations over a range of B1 strengths and frequency offsets, improving image quality in challenging magnetic environments.
The methods can be synchronized with cardiac triggers to obtain images within a single cardiac cycle enhancing clinical utility.
Documented Applications
Coronary magnetic resonance angiography (MRA) to image the coronary artery tree with improved signal-to-noise ratio (SNR) and contrast between coronary blood and myocardium.
Acquisition of T2-weighted magnetic resonance images of cardiac systems, including portions of the heart, with gating based on electrocardiogram signals.
Application in three-dimensional coronary MRA for visualization of tortuous coronary vessels and distal branches with reduced artifacts.
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