Variable Flip Angle Series for Angiographic MRI Signal Stability
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Solution Overview
Problem
Existing contrast agent-free dynamic magnetic resonance angiography methods suffer from a rapid loss of signal over time due to continuous scanning, limiting the acquisition of vessels that fill late with marked blood, particularly in examinations with unclear flow relationships.
Innovation Solution
A method involving a series of temporally successive angiographic magnetic resonance images is generated using a varying sequence of flip angles, starting from small to large, combined with selective and non-selective spin markings, to maintain a consistent signal difference between marked and unmarked blood over time, allowing for improved vessel depiction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If continuous scanning with constant flip angle is used to acquire temporally successive angiographic images, then time resolution and workflow coverage are improved, but the marking signal is lost over time and late-filling vessels are not adequately depicted
Solution Approach 1:
The patent applies dynamics by making the flip angle variable over time rather than constant. The flip angle increases progressively during the continuous scanning process, allowing the system to adapt to the changing magnetization state of inflowing blood. This dynamic adjustment maintains the marking signal throughout the entire observation period, enabling both high time resolution and reliable vessel depiction including late-filling vessels.
Solution Approach 2:
The patent changes the parameter of flip angle from a constant value to a time-varying parameter that increases during the scanning sequence. This parameter change compensates for the natural decay of the marking signal over time, maintaining optimal signal contrast for angiographic imaging throughout the entire acquisition window and enabling reliable detection of vessels at all filling stages.
2Measurement precision
If the marking edge is placed near the vessels of interest to maximize inflow blood acquisition, then early-filling vessels are well depicted, but the acquisition volume is strongly limited and late-filling vessels are missed
Solution Approach 1:
The patent extends the effective acquisition volume dynamically by using progressively increasing flip angles during continuous scanning. As the flip angle increases over time, the detection sensitivity extends deeper into the imaging volume, allowing late-filling vessels in distant regions to be captured with sufficient signal strength, thereby expanding the effective acquisition volume beyond what would be possible with a fixed marking edge position.
3Measurement precision
If a large number of flip angles are used in the series to improve image resolution, then the continuous curve of flip angles provides high resolution, but the acquisition time and data complexity increase
Solution Approach 1:
The patent maintains continuous useful action by performing uninterrupted continuous scanning throughout the observation period. The flip angle series progresses continuously without interruption, and the system continuously acquires data at each flip angle step. This continuous approach efficiently utilizes the available time to capture the complete inflow process with high temporal and spatial resolution, minimizing total acquisition time while maximizing image quality.
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 visibility of late-filling vessels by maintaining a higher signal level and extending the 'marking' duration, providing better representation of flow relationships without the need for contrast agents.
Implementation Method 1
In these methods, blood flowing in is marked with the aid of what is known as a spin labeling method and imaged during the inflow into an examination volume of interest (VOI).
Implementation Method 2
In the FAIR method a first measurement is implemented in a first measurement after a selective inversion (marking) of the VOI and a second measurement is implemented after a non-selective inversion of VOI and the surrounding region.
Implementation Method 3
Gradient echo methods can be used for this, for example FLASH imaging (Fast Low Angle Shot) or FISP or, respectively, TrueFISP imaging (Fast Imaging with Steady State Precession, also known as b-SSFP (Balanced Steady State Free Precession)).
Implementation Method 4
Each component is acquired according to a gradient echo method with a flip angle which exists at a position in the series of flip angles, which position corresponds to the component.
Data Source
AI summary
In a magnetic resonance system and operating method, a series with a predetermined number of flip angles is established, that begins with a first flip angle that is smaller than the last flip angle of the series, and an arbitrary flip angle is greater than or equal to the preceding flip angle in the series. Respective predetermined regions of k-space are acquired that form first and second magnetic resonance data. Using a gradient echo sequence, each region is acquired during time interval as a series of components respectively with different flip angles in the series of flip angles. Each of the angiography magnetic resonance images has a flip angle range assigned thereto, and is formed by combining respective regions of k-space filled with components of the first and second magnetic resonance data that were acquired in that range.