Spin-Echo Sequence for Multi-Slice MR Data Acquisition
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Solution Overview
Problem
Current magnetic resonance imaging techniques face challenges in acquiring MR data for multiple slices within a reduced field of view in two spatial directions without saturating longitudinal magnetization in adjacent slices, leading to reduced signal amplitude and longer acquisition times.
Innovation Solution
A spin echo sequence method is employed, involving selective excitation and refocusing pulses with specific gradient orientations and crusher gradients to invert spins in adjacent slices, ensuring minimal saturation and rapid recovery of longitudinal magnetization, allowing for quicker acquisition of MR data across multiple slices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If selective refocusing is applied along a direction orthogonal to the slice selection direction to reduce field of view in two spatial directions, then acquisition time is reduced, but longitudinal magnetization in adjacent slices is saturated leading to reduced signal amplitude
Solution Approach 1:
The refocusing process is segmented into two separate gradient applications: a first gradient along the slice selection direction and a second gradient along an orthogonal direction. This segmentation allows selective refocusing of spins in the region of interest while avoiding saturation in adjacent slices, thus maintaining signal amplitude while reducing acquisition time.
Solution Approach 2:
The magnetic field gradients are designed to create localized refocusing conditions specifically in the region of interest (intersection of the excited slice and the slab defined by the second gradient). This local quality ensures that only the desired volume element is refocused, preventing unwanted saturation in adjacent slices while maintaining high signal amplitude in the target region.
2Reliability
If conventional spin echo sequence is used to acquire MR data in multiple slices, then complete coverage is achieved, but acquisition time increases due to saturation of longitudinal magnetization in adjacent slices
Solution Approach 1:
A preliminary selective refocusing action is applied using the second gradient along the orthogonal direction before the standard refocusing pulse. This preliminary action prepares the spin system by partially refocusing spins in the region of interest, allowing subsequent rapid acquisition of multiple slices without causing saturation in adjacent slices, thus reducing total acquisition time while maintaining complete coverage.
Solution Approach 2:
The refocusing approach is extended from one dimension (slice selection direction only) to two dimensions by adding a gradient along an orthogonal direction. This dimensional extension creates a more selective refocusing volume that intersects only the desired region, enabling faster multi-slice acquisition without sacrificing signal coverage in adjacent slices.
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 method enables the rapid acquisition of MR data in multiple slices with reduced signal loss and faster recovery of longitudinal magnetization, overcoming the limitations of previous techniques by minimizing saturation and allowing for efficient data collection across parallel slices.
Implementation Method 1
A first magnetic field gradient is applied as a slice selection gradient along a first direction. An RF excitation pulse is radiated for the selective excitation of a specified slice
Implementation Method 2
A refocusing pulse is radiated while the second gradient is applied. By the emission of the refocusing pulse, spins in a further slice are inverted
Implementation Method 3
the acquisition of MR data with a magnetic resonance scanner and the entry of the acquired MR data into k-space is based on the condition that the spacing between k-space points in each direction fulfils the Nyquist sampling conditions
Implementation Method 4
Through the combination of the refocusing pulse and the further RF-pulse, the spins in the further slice, which are located outside the specified slice, are flipped back into their starting position by the double inversion
Data Source
AI summary
In a method and apparatus for acquiring magnetic resonance (MR) data from multiple slices of a subject, a spin echo sequence is executed wherein a radio frequency (RF) excitation pulse, a refocusing pulse, and a further RF pulse are radiated, together with the application of a number of gradients, in order to acquire MR data from an intersection volume of a selected slice with a further slice. The MR data are entered into at least one k-space line in a memory organized as k-space.