Phase contrast magnetic resonance imaging
Phase contrast magnetic resonance imaging | |
---|---|
dissections of the celiac artery (upper) and the superior mesenteric artery (lower). Laminar flow is present in the true lumen (closed arrow) and helical flow is present in the false lumen (open arrow).[1] | |
Purpose | method of magnetic resonance angiograph |
Phase contrast magnetic resonance imaging (PC-MRI) is a specific type of
How it Works
All of these spins have a phase that is dependent on the atom's velocity.
where is the Gyromagnetic ratio and is defined as:
- ,
is the initial position of the spin, is the spin velocity, and is the spin acceleration.
If we only consider static spins and spins in the x-direction, we can rewrite equation for phase shift as:
We then assume that acceleration and higher order terms are negligible to simplify the expression for phase to:
where is the zeroth moment of the x-gradient and is the first moment of the x gradient.
If we take two different acquisitions with applied magnetic gradients that are the opposite of each other (bipolar gradients), we can add the results of the two acquisitions together to calculate a change in phase that is dependent on gradient:
The phase shift is measured and converted to a velocity according to the following equation:
where is the maximum velocity that can be recorded and is the recorded phase shift.
The choice of defines range of velocities visible, known as the ‘dynamic range’. A choice of below the maximum velocity in the slice will induce aliasing in the image where a velocity just greater than will be incorrectly calculated as moving in the opposite direction. However, there is a direct trade-off between the maximum velocity that can be encoded and the signal-to-noise ratio of the velocity measurements. This can be described by:
where is the signal-to-noise ratio of the image (which depends on the magnetic field of the scanner, the voxel volume, and the acquisition time of the scan).
For an example, setting a ‘low’ (below the maximum velocity expected in the scan) will allow for better visualization of slower velocities (better SNR), but any higher velocities will alias to an incorrect value. Setting a ‘high’ (above the maximum velocity expected in the scan) will allow for the proper velocity quantification, but the larger dynamic range will obscure the smaller velocity features as well as decrease SNR. Therefore, the setting of will be application dependent and care must be exercised in the selection. In order to further allow for proper velocity quantification, especially in clinical applications where the velocity dynamic range of flow is high (e.g. blood flow velocities in vessels across the thoracoabdominal cavity), a dual-echo PC-MRI (DEPC) method with dual velocity encoding in the same repetition time has been developed.[5] The DEPC method does not only allow for proper velocity quantification, but also reduces the total acquisition time (especially when applied to 4D flow imaging) compared to a single-echo single- PC-MRI acquisition carried out at two separate values.
To allow for more flexibility in selecting ,
Encoding Methods
When each dimension of velocity is calculated based on acquisitions from oppositely applied gradients, this is known as a six-point method. However, more efficient methods are also used. Two are described here:
Simple Four-point Method
Four sets of encoding gradients are used. The first is a reference and applies a negative moment in ,, and . The next applies a positive moment in , and a negative moment in and . The third applies a positive moment in , and a negative moment in and . And the last applies a positive moment in , and a negative moment in and .[7] Then, the velocities can be solved based on the phase information from the corresponding phase encodes as follows:
Balanced Four-Point Method
The balanced four-point method also includes four sets of encoding gradients. The first is the same as in the simple four-point method with negative gradients applied in all directions. The second has a negative moment in , and a positive moment in and . The third has a negative moment in , and a positive moment in and . The last has a negative moment in and a positive moment in and .[8] This gives us the following system of equations:
Then, the velocities can be calculated:
Retrospective Cardiac and Respiratory Gating
For
Applications
Phase contrast MRI is one of the main techniques for
Limitations
In particular, a few limitations of PC-MRI are of importance for the measured velocities:
- Partial volume effects (when a voxel contains the boundary between static and moving materials) can overestimate phase leading to inaccurate velocities at the interface between materials or tissues.
- Intravoxel phase dispersion (when velocities within a pixel are heterogeneous or in areas of turbulent flow) can produce a resultant phase that does not resolve the flow features accurately.
- Assuming that acceleration and higher orders of motion are negligible can be inaccurate depending on the flow field.
- Displacement artifacts (also known as misregistration and oblique flow artifacts) occur when there is a time difference between the phase and frequency encoding. These artifacts are highest when the flow direction is within the slice plane (most prominent in the heart and aorta for biological flows)[10]
Vastly undersampled Isotropic Projection Reconstruction (VIPR)
A Vastly undersampled Isotropic Projection Reconstruction (VIPR) is a radially acquired MRI sequence which results in high-resolution MRA with significantly reduced scan times, and without the need for breath-holding.[11]