Incremental Phase Refocusing Pulses for NMR Artifact Suppression
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Solution Overview
Problem
In nuclear magnetic resonance (NMR) measurements, especially in well logging, existing pulse sequences face challenges in effectively removing unwanted resonances and artifacts caused by phase changes, which can obscure desired resonance peaks and complicate data interpretation, particularly in environments with strong magnetic gradients.
Innovation Solution
The implementation of single-transient phase cycling using a variant of the Carr Purcell Meiboom Gill (CPMG) pulse sequence with an excitation pulse and a train of refocusing pulses, where each refocusing pulse has a phase increment, allowing for the separation of NMR signals by phase coherence and subsequent filtering to remove artifacts, thereby isolating the desired signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional CPMG pulse sequences are used, then NMR measurements can be performed in strong magnetic gradients, but unwanted resonances and artifacts remain that obscure desired resonance peaks
Solution Approach 1:
The patent applies periodic phase cycling to the refocusing pulses in the CPMG sequence. By systematically varying the phase of refocusing pulses in a periodic pattern (e.g., 0°, 90°, 180°, 270° cycles), the method enables selective reinforcement of desired echo signals while causing unwanted artifacts to cancel out through destructive interference, thereby improving signal clarity without requiring multiple separate transients
Solution Approach 2:
The patent modifies the phase parameter of refocusing pulses dynamically throughout the echo train. Instead of using constant phase refocusing pulses, the phase is changed incrementally or periodically across successive pulses, which alters the coherence pathways and allows separation of desired signals from artifacts through phase filtering in the frequency domain
2Measurement precision
If multiple transients or moving averages are used to remove artifacts, then signal accuracy improves, but measurement time increases
Solution Approach 1:
By implementing phase cycling within a single transient, the patent achieves artifact suppression without requiring multiple repeated measurements. The periodic phase modulation allows desired signals to constructively interfere while artifacts destructively interfere within the same echo train, eliminating the time penalty associated with acquiring and averaging multiple transients
Solution Approach 2:
The patent maintains continuous signal acquisition throughout the echo train by applying phase-cycled refocusing pulses without interrupting the measurement sequence. This continuous approach preserves all available signal information while systematically suppressing artifacts, whereas traditional methods requiring multiple transients would have gaps and repetitions that increase total measurement time
3Measurement precision
If phase cycling is applied to remove artifacts, then resolution improves, but pulse sequence complexity increases
Solution Approach 1:
The patent employs a systematic periodic phase cycling pattern that, while adding a layer of complexity to the pulse sequence, follows a regular and predictable structure. This periodic approach allows for automated implementation and post-processing phase filtering, which manages the complexity in a structured way rather than requiring complex real-time control or multiple independent measurement sequences
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 enhances the resolution and accuracy of NMR measurements by effectively eliminating unwanted artifacts, allowing for precise characterization of subsurface layers without the need for multiple transients or moving averages, and is robust to pulse errors and magnetic field gradients.
Implementation Method 1
separation of NMR signals by phase coherence
Implementation Method 2
nuclear magnetic resonance (NMR) measurements
Implementation Method 3
variant of the Carr Purcell Meiboom Gill (CPMG) pulse sequence
Data Source
AI summary
An example pulse sequence for performing phase coherence order selection within a single transient acquisition includes an excitation pulse with a tip angle of 90° and phase ϕA, followed by a train of N refocusing pulses with tip angles of 180°, with the center of the first refocusing pulse occurring time τ after the center of the excitation pulse, and the center of the nth refocusing pulse occurring at time (2n+1)τ after the center of the excitation pulse. This causes a train of echoes to form at times 2nt after the center of the excitation pulse. In this example, the first refocusing pulse has phase ϕB, where \ϕB−ϕA\=90°, and each successive refocusing pulse (304) has a phase ϕδ greater than the last refocusing pulse. This incremental change in pulse phase over the course of the echo train has the effect of aiabatically “dragging” the echo phase around the unit circle in a predictable manner corresponding to the phase coherence order of the relevant signals.