Steady-State Spin Echo MR Imaging Sequence
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Solution Overview
Problem
Current MR imaging techniques face challenges with magnetic field inhomogeneities, leading to signal loss and artifacts, particularly in fast gradient echo sequences, which are sensitive to field inhomogeneities and result in banding artifacts or signal loss.
Innovation Solution
A rapid, steady-state spin echo-based MR imaging method using an imaging sequence with alternating units of excitation and refocusing RF pulses and switched magnetic field gradients, allowing for high repetition rates without temporal delay, thereby reducing sensitivity to main magnetic field inhomogeneities and enabling fast imaging with improved signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If fast gradient echo sequences are used to achieve high imaging speed, then productivity is improved, but the images show banding artifacts or signal loss due to sensitivity to field inhomogeneity
Solution Approach 1:
The sequence is divided into repeated acquisition blocks, each containing multiple spin echo signals acquired at different echo times. This segmentation allows rapid sampling of k-space while maintaining spin echo robustness against field inhomogeneity, resolving the contradiction between speed and image quality
Solution Approach 2:
The imaging sequence uses periodic repetition of acquisition blocks with multiple spin echo signals acquired at regularly spaced echo times. This periodic structure enables fast imaging through efficient k-space sampling while the spin echo refocusing pulses continuously compensate for field inhomogeneity, achieving both high productivity and reliable image quality
2Reliability
If conventional spin echo sequences are used to achieve insensitivity to field inhomogeneity, then reliability is improved, but scan times are longer and productivity is reduced
Solution Approach 1:
The sequence maintains continuous useful action by acquiring multiple spin echo signals within each acquisition block and rapidly repeating these blocks. This continuous sampling approach fills k-space efficiently, reducing total scan time while maintaining the field inhomogeneity resistance inherent to spin echo methods
Solution Approach 2:
The invention changes the temporal parameters of the spin echo sequence by acquiring multiple echoes per acquisition block at different echo times and rapidly repeating blocks. This parameter optimization reduces scan time while preserving the T2 weighting and field inhomogeneity resistance of spin echo imaging
3Productivity
If multiple spin echo signals are acquired per acquisition block to improve imaging speed, then productivity is improved, but the sequence complexity increases
Solution Approach 1:
The acquisition block structure serves multiple functions simultaneously: it acquires multiple spin echo signals for fast sampling, provides T2 weighting through variable echo times, and maintains field inhomogeneity resistance. This multi-functionality achieves high productivity without proportionally increasing sequence complexity
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
The method achieves robust, fast MR imaging that is insensitive to magnetic field inhomogeneities, enabling high-resolution 3D and dynamic imaging with reduced RF energy deposition and the ability to generate multiple spin echoes for varied contrast properties and quantitative T1/T2 mapping.
Implementation Method 1
the body of the patient to be examined is arranged in a strong, uniform magnetic field (B0 field) whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength
Implementation Method 2
Transitions between these energy levels can be excited (spin resonance) by application of an electromagnetic alternating field (RF field, also referred to as B1 field) of defined frequency (so-called Larmor frequency, or MR frequency)
Implementation Method 3
the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle
Implementation Method 4
the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time)
Implementation Method 5
the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time)
Implementation Method 6
The variation of the magnetization can be detected by means of one or more receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis
Implementation Method 7
The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils
Implementation Method 8
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency
Data Source
AI summary
MR imaging comprising the steps of: subjecting an object (10) to an imaging sequence of RF pulses and switched magnetic field gradients (GS, GP, GM), which imaging sequence is a steady state sequence comprising a plurality of repeatedly applied acquisition blocks (21), wherein each acquisition block (21) comprises two units (22, 23) in immediate succession, namely: i) a first unit (22) starting with an excitation RF pulse radiated toward the object (10), with the duration of the first unit being an integer multiple of a given time interval T, and ii) a second unit (23) starting with a refocusing RF pulse radiated toward the object (10) and comprising a readout magnetic field gradient (GM) and a phase encoding magnetic field gradient (GP), with the duration of the second unit (23) being an integer multiple of the time interval T, acquiring one or more phase-encoded spin echo signals (31, 32) in a sequence of acquisition blocks (21), and reconstructing one or more MR images from the acquired spin echo signals (31, 32). Moreover, the invention relates to a MR device (1) and to a computer program for a MR device (1).