Stimulated Echo MR Temperature Mapping
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Solution Overview
Problem
Current MR-based temperature mapping methods, such as the proton resonance frequency shift (PRF) method, face challenges including long echo times, image artifacts, motion-induced errors, fat contamination, and flow-related phase errors, making real-time temperature monitoring in MR-HIFU and other applications inefficient and inaccurate.
Innovation Solution
A stimulated echo sequence is used for PRF shift measurements, allowing for fast and robust gradient echo pulse trains with short repetition times, flexible main magnetic field adaptation, and reduced flow-sensitivity, enabling real-time temperature mapping with automatic error reduction through low flow-sensitivity and optional fat signal suppression techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the proton resonance frequency shift (PRF) method is used for temperature mapping, then temperature measurement sensitivity is improved, but echo time becomes long and motion-induced errors increase
Solution Approach 1:
The imaging sequence is divided into a preparation period with stimulated echo preparation and an acquisition period with fast gradient echo pulse train. This segmentation allows the PRF shift encoding to be completed during the preparation period, enabling fast readout during acquisition with short repetition times, thereby reducing total echo time while maintaining temperature measurement sensitivity
Solution Approach 2:
The stimulated echo preparation pulses are applied in advance during the preparation period to encode the PRF shift information before the actual temperature measurement acquisition. This preliminary encoding allows the subsequent readout to be performed rapidly without compromising measurement precision
2Measurement precision
If the PRF method is used for temperature mapping, then temperature monitoring capability is improved, but image artifacts and motion-induced errors increase
Solution Approach 1:
By separating the PRF encoding step (preparation period) from the readout step (acquisition period), the method reduces the time during which motion can introduce errors, while still capturing the temperature information with high precision. The short repetition time in the acquisition period minimizes motion artifacts
Solution Approach 2:
The use of periodic gradient echo pulse trains with short repetition times during the acquisition period allows for rapid sampling of the encoded PRF information, reducing the window for motion-induced errors while maintaining temperature monitoring capability
3Loss of information
If conventional MR sequences are used for temperature mapping, then comprehensive tissue information is obtained, but flow-related phase errors and fat contamination occur
Solution Approach 1:
The stimulated echo preparation sequence selectively extracts and encodes only the water proton resonance frequency information while suppressing fat signals. This extraction of the desired water signal separates it from contaminating fat signals and flow-related phase errors that affect conventional sequences
4Productivity
If fast imaging is implemented for real-time temperature monitoring, then monitoring speed is improved, but temperature mapping accuracy decreases
Solution Approach 1:
The sequence is segmented into preparation and acquisition phases, where the preparation phase performs the temperature-sensitive PRF encoding and the acquisition phase performs rapid readout. This allows fast imaging with short repetition times during acquisition while maintaining temperature mapping accuracy through the prior encoding step
Solution Approach 2:
The PRF shift encoding is performed in advance during the preparation period before the fast readout. This preliminary encoding ensures that temperature information is captured with high precision before the rapid acquisition begins, enabling both fast imaging and accurate temperature mapping
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 stimulated echo-based method enables real-time temperature mapping with improved robustness against motion and flow-induced errors, allowing for accurate temperature monitoring in MR-HIFU and other applications by acquiring temperature maps in a single shot and separating water and fat signals to avoid contamination.
Implementation Method 1
The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which 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 2
The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which 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). From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicular to the z-axis, so that the magnetization performs a precessional motion about the z-axis
Implementation Method 3
After termination of the RF pulse, 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), and 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 4
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
Implementation Method 5
One of the most sensitive MR-based temperature mapping approaches is the known proton resonance frequency shift (PRF) method. The magnetic resonance frequency of water protons changes as a function of temperature
Data Source
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AI summary
The invention relates to a method of MR imaging of an object (10) placed in the examination volume of a MR device (1). It is the object of the invention to provide an improved MR-based temperature mapping method. The method of the invention comprises the steps of: subjecting the object (10) to an imaging sequence of RF pulses and switched magnetic field gradients, which imaging sequence is a stimulated echo sequence including: a) at least two preparation RF pulses (a) radiated toward the object (10) during a preparation period (21), and b) one or more reading RF pulses (β) radiated toward the object (10) during an acquisition period (22) temporally subsequent to the preparation period (21); acquiring at least two MR signals during the acquisition period (22), wherein the two MR signals are either (i) a FID signal (I1, FID) and a stimulated echo signal (I2) or (ii) two stimulated echo signals (STE, STE*); and deriving a temperature map indicating the spatial distribution of the temperature within the object (10) from the at least two acquired MR signals. Moreover, the invention relates to a MR device (1) and to a computer program for a MR device (1).