Velocity-Selective ASL Pulse Sequence for MRI Transit Delay Error Reduction
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Solution Overview
Problem
The existing slice-selective ASL (Arterial Spin Labeling) technique faces challenges with low sensitivity and image quality due to transit delay time errors, unbalanced spoiling of transverse magnetization, and extended imaging time, particularly when labeling blood flows with varying velocities.
Innovation Solution
The proposed MRI apparatus employs a velocity-selective pulse sequence that reverses the polarity of transverse magnetization in both control and tag modes, allowing for improved labeling sensitivity and reduced transit delay time errors, while also enabling labeling of blood flows across a broader velocity range and compensating for spatial phase distributions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If slice-selective ASL technique is used to label upstream artery outside the imaged slab, then the imaging can be performed non-invasively without contrast medium, but transit delay time errors occur and sensitivity is reduced
Solution Approach 1:
The patent divides the labeling process into two separate operations: a slice-selective pulse to label blood in a specific slab, and a velocity-selective pulse to label blood based on flow velocity. This segmentation allows independent optimization of each labeling mechanism, reducing transit delay errors while maintaining non-invasive capability.
Solution Approach 2:
The patent adds velocity selection as an additional dimension to the traditional slice-selective labeling approach. By introducing velocity-selective pulses alongside slice-selective pulses, the system can distinguish and label blood flows based on both spatial location and flow velocity, thereby reducing transit delay time errors.
2Measurement precision
If velocity-selective pulse is applied to label blood flows with varying velocities, then labeling sensitivity improves, but unbalanced spoiling of transverse magnetization occurs
Solution Approach 1:
The patent applies preliminary phase compensation by introducing a phase shift to the velocity-selective pulse before it is applied. This preliminary action counteracts the unbalanced spoiling of transverse magnetization that would otherwise occur, thereby maintaining labeling sensitivity while eliminating the harmful effect.
Solution Approach 2:
The patent modifies the parameters of the velocity-selective pulse by introducing a phase shift component. This parameter change allows the pulse to selectively label blood flows while compensating for the spoiling effect, thereby achieving both high sensitivity and reduced harmful effects.
3Reliability
If conventional ASL imaging is performed with slice-selective pulsing, then imaging time is extended to allow blood flow to reach tissue of interest, but transit delay time errors increase
Solution Approach 1:
The patent performs preliminary labeling of blood flows using velocity-selective pulses before the blood reaches the tissue of interest. This preliminary action allows the system to account for transit delay effects and adjust imaging parameters accordingly, thereby reducing imaging time while maintaining labeling accuracy.
Solution Approach 2:
The patent incorporates feedback mechanisms to monitor and adjust the imaging process based on the observed blood flow characteristics. By using velocity-selective pulses to identify flow characteristics in real-time, the system can dynamically adjust imaging parameters to compensate for transit delay effects, thereby reducing overall imaging time.
4Measurement precision
If velocity-selective pulse sequence is used to reverse polarity of transverse magnetization, then sensitivity matches conventional SS-ASL, but imaging time is shortened
Solution Approach 1:
The patent merges the velocity-selective pulsing with the slice-selective pulsing in a combined pulse sequence. This merging allows the system to achieve both velocity-based labeling and spatial selection simultaneously, thereby maintaining high sensitivity while reducing imaging time through more efficient use of the imaging cycle.
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 sensitivity of ASL imaging to match conventional SS-ASL techniques, improves image quality by reducing transit delay time-induced errors, and allows for efficient labeling of blood flows across a wide velocity range, thereby shortening imaging time and improving visualization capabilities.
Implementation Method 1
Magnetic resonance imaging is a technique involving magnetically exciting nuclear spins in a subject laid in a static magnetic field using a high frequency signal at the Larmor frequency
Implementation Method 2
a velocity-selective pulse that selectively excites magnetization spins in a fluid passing through the region to be imaged and having a constant velocity range for the spins to undergo transition to transverse magnetization
Implementation Method 3
the velocity-selective pulse is formed in such a manner that the longitudinal magnetization excited in each of the control mode and the tag mode is reversed in polarity upon velocity-selective excitation by the velocity-selective pulse
Implementation Method 4
signals from the stationary tissue are generally cancelled and signals from the spins flowing into the capillary bed are obtained as a blood flow image
Implementation Method 5
The image data obtained in each of the tag mode and the control mode is subjected to a pixel-by-pixel difference operation between these two images
Data Source
AI summary
An MRI apparatus obtains an ASL (Arterial Spin Labeling) image of a region to be imaged in a subject by performing a scan to the region to be imaged independently in a control mode and in a tag mode according to a pulse sequence based on an ASL technique. The pulse sequence includes a velocity-selective pulse, BVS (Band-limited Velocity-Selective)-pulse, that selectively excites magnetization spins in a blood flow passing through the region to be imaged and having a constant velocity range for the spins to undergo transition to transverse magnetization, and then performs excitation to cause the transverse magnetization to flip back to longitudinal magnetization. The velocity-selective pulse is formed in such a manner that the transverse magnetization excited in each of the control mode and the tag mode gives rise to a phase shift in an opposite polarity upon velocity-selective excitation.