Spin Phase Effects and Gradient Moment Nulling, MRA | MR angiography | MRI Physics Course #25

Understanding Blood Vessel Imaging Techniques

Overview of Signal in Blood Vessels

• The discussion begins with the impact of high velocity signal loss and turbulence on blood vessel imaging, leading to hypointensity in images.

• Introduction to flow-related enhancement, where unsaturated blood traveling into a slice can produce high signal intensity, particularly in time-of-flight MR angiography.

• Focus shifts to the spin phase effect, explaining how moving spins during gradient application can cause hypointensity in images.

Key Concepts: Gradient Moment Nulling

• Explanation of gradient moment nulling as a method to compensate for phase loss due to moving spins across gradients.

• Description of pulse sequences involving magnetic gradients applied across dimensions for slice selection and frequency encoding.

Phase Differences and Their Implications

• When applying gradients, frequency differences arise based on location within the gradient, affecting processional frequencies of spins.

• Upon switching off the gradient, induced phase differences lead to spins being out of phase with each other based on their locations.

Importance of Phase Encoding Gradient

• In phase encoding gradients, maintaining phase differences is crucial for determining signal origin along the Y-axis of the slice.

• The application of a phase encoding gradient induces frequency differences that result in varying phases across the slice.

Analyzing Spin Behavior Across Gradients

• Spins are represented using phase data; faster processing spins accumulate positive phase changes while slower ones accumulate negative changes.

• Linear incremental units of phase change occur during any applied gradient; equal and opposite gradients allow for rephasing if spins remain stationary.

Effects of Moving Spins During Gradient Application

• If spins move while a gradient is applied (e.g., blood vessels), they will accumulate non-linear (exponential) phases based on their velocity relative to the gradient strength.

Understanding Phase Changes in MRI: The Role of Gradient Strength and Spin Velocity

Assumptions in Phase Change Calculation

• The formula for calculating phase change assumes that the gradient strength remains constant and that the spin moves at a constant velocity parallel to the gradient.

• The phase change can be calculated using a constant determined by blood velocity, gradient strength, and duration of gradient application. Longer application results in greater phase shift.

Behavior of Stationary vs. Moving Spins

• Stationary spins gain phase linearly with time when a constant gradient is applied; each unit of time corresponds to one unit of phase gained.

• In contrast, moving spins experience exponential phase changes due to their velocity along the gradient; initial gains are followed by increasingly larger increments as time progresses.

Exponential Phase Gain in Moving Spins

• For moving spins, after one unit of time, they gain one unit of phase; after two units, they gain four units (2^2), leading to an exponential increase in total phase change over time.

• As spins continue moving along the gradient, they accumulate more significant phases—one unit initially, three additional units next (totaling five), demonstrating cumulative effects on processional frequency.

Signal Loss Due to Phase Incoherence

• Moving spins experience greater defocusing or incoherence compared to stationary spins due to their continuous movement across gradients during imaging processes, resulting in signal loss in MRI images.

• This loss occurs because stationary spins can be rephased effectively with equal and opposite gradients while moving spins cannot regain coherence as easily under similar conditions.

Compensating for Phase Changes During Imaging

• Rephasing loops used for stationary spins do not work effectively for moving spins; these still lose coherence despite attempts at rephasing through standard pulse sequences applied during imaging sessions.

Gradient Moment Nulling in MRI

Understanding Gradient Moment Nulling

• Gradient moment nulling compensates for phase loss experienced by both stationary and moving spins as they traverse through a slice.

• Initially, the same gradient is applied across the slice, leading to both stationary and moving spins gaining one unit of phase during this first period.

• A second gradient is introduced, which is twice as strong but in the opposite direction. This results in stationary spins losing two units of phase due to increased field strength.

• Moving spins experience a more significant effect; instead of losing three units of phase, they lose six units because their velocity remains constant while traversing a steeper gradient.

Rephasing Mechanism

• After applying the stronger opposite gradient, when the initial gradient is reapplied for the same duration, stationary spins regain one unit of phase and rephase with each other.

• For moving spins at constant velocity along this gradient, they gain five units of phase change during this final period due to their movement across the positive direction of the gradient.

• The process allows moving spins to rephase effectively despite their varying positions within the slice throughout the sequence.

Importance and Applications

• Gradient moment nulling enables signal acquisition from moving spins without interference from stationary ones, provided that their velocities are constant.

• Understanding spin phase effects and gradient moment nulling is crucial for grasping advanced concepts like Phase Contrast MR Angiography (MRA).