MRI Thermometry Water-Fat Signal Separation
Solution Overview
Problem
Current methods for non-invasive temperature mapping in tissues containing a mix of water and lipids, such as breast tissue, are confounded by the temperature insensitivity of lipids, leading to inaccurate temperature measurements in hyperthermia treatments.
Innovation Solution
The method involves acquiring MRI images at multiple echo times, separating water and fat signals, and using the phase difference between water and fat signals to measure temperature, which is self-referenced and not affected by external factors like magnetic field drift, allowing for accurate temperature mapping in tissues with both water and lipids.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If standard frequency shift thermometry is used to measure temperature in tissues, then temperature measurement is possible, but the presence of lipids confounds the measurement and reduces accuracy
Solution Approach 1:
The patent segments the tissue signal into separate water and lipid components using IDEAL (Iterative Decomposition of water and fat with Echo Asymmetry and Least-squares estimation) processing. By acquiring multiple echo times and iteratively solving for water and fat signal contributions, the method isolates the water signal which carries the temperature-dependent frequency shift information, eliminating lipid interference from the temperature measurement.
2Measurement precision
If water signal phase is used for temperature measurement, then temperature sensitivity is achieved, but magnetic field drift affects the measurement accuracy
Solution Approach 1:
The patent uses lipid signal phase as an intermediary reference that is insensitive to temperature changes. By comparing the water signal phase (which is temperature-sensitive) against the lipid signal phase (which is temperature-insensitive), the method creates a differential measurement that cancels out common-mode disturbances such as magnetic field drift, leaving only the temperature-dependent phase difference.
3Measurement precision
If multiple echo times are acquired for water-fat separation, then accurate temperature measurement is possible, but scan time increases
Solution Approach 1:
The patent employs IDEAL processing which uses the asymmetric echo times and the known chemical shift difference between water and fat to separate their signals. By optimizing the echo time spacing and using least-squares estimation, the method achieves accurate water-fat separation with a minimal number of echo times, balancing measurement accuracy with acceptable scan duration.
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 provides a more accurate and non-invasive temperature mapping technique that is not affected by external factors, improving the control of temperature therapy distribution in hyperthermia treatments and enabling precise temperature measurements within tumors and surrounding tissues.
Implementation Method 1
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
Implementation Method 2
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, Mz, may be rotated, or 'tipped', into the x-y plane to produce a net transverse magnetic moment Mt
Implementation Method 3
Previous work has shown the value of using the temperature sensitivity of the tissue water proton resonant frequency shift (PUS) or the apparent diffusion coefficient (ADC) to measure temperature change
Implementation Method 4
the IDEAL method employs pulse sequences to acquire multiple images at different echo times (TE) and an iterative, linear least squares approach to estimate the separate water and fat signal components
Data Source
AI summary
The in vivo measurement of tissue temperature is performed during a medical procedure using an MRI system. Fat and Water images are acquired at each temperature measurement time and corresponding phase images are produced. A temperature map is produced by subtracting the phase at each Fat image pixel from the corresponding pixel in the Water phase image to improve measurement accuracy in tissues with fat/water mixtures.