Phase contrast magnetic resonance imaging (PC-MRI) is a specific type of magnetic resonance imaging used primarily to determine flow velocities. PC-MRI can be considered a method of Magnetic Resonance Velocimetry. Since modern PC-MRI is typically time-resolved, it can also be referred to as 4D imaging (three spatial dimensions plus time).
Contents
How it Works
Atoms with an odd number of protons or neutrons have a randomly aligned angular spin momentum. When placed in a strong magnetic field, some of these spins align with the axis of the external field, which causes a net 'longitudinal' magnetization. These spins precess about the axis of the external field at a frequency proportional to the strength of that field. Then, energy is added to the system through a Radio frequency (RF) pulse to 'excite' the spins, changing the axis that the spins precess about. These spins can then be observed by receiver coils (Radiofrequency coils) using Faraday's law of induction. Different tissues respond to the added energy in different ways, and imaging parameters can be adjusted to highlight desired tissues.
All of these spins have a phase that is dependent on the atom's velocity. Phase shift
where
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
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:
where
The phase shift is measured and converted to a velocity according to the following equation:
where
The choice of
where
For an example, setting a ‘low’
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
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
Then, the velocities can be calculated:
Retrospective Cardiac and Respiratory Gating
For medical imaging, in order to get highly resolved scans in 3D space and time without motion artifacts from the heart or lungs, retrospective cardiac gating and respiratory compensation are employed. Beginning with cardiac gating, the patient’s ECG signal is recorded throughout the imaging process. Similarly, the patient’s respiratory patterns can be tracked throughout the scan. After the scan, the continuously collected data in k-space (temporary image space) can be assigned accordingly to match-up with the timing of the heart beat and lung motion of the patient. This means that these scans are cardiac-averaged so the measured blood velocities are an average over multiple cardiac cycles.
Limitations
In particular, a few limitations of PC-MRI are of importance for the measured velocities: