Phase contrast magnetic resonance imaging

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. It also provides a method of magnetic resonance angiography. Since modern PC-MRI is typically time-resolved, it provides a means of 4D imaging (three spatial dimensions plus time). 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. It also provides a method of magnetic resonance angiography. Since modern PC-MRI is typically time-resolved, it provides a means of 4D imaging (three spatial dimensions plus time). 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 ( ϕ ) {displaystyle (phi )} of a spin is a function of the gradient field G ( t ) {displaystyle mathbf {G} (t)} : where γ {displaystyle gamma } is the Gyromagnetic ratio and r {displaystyle mathbf {r} } is defined as: r 0 {displaystyle mathbf {r} _{0}} is the initial position of the spin, v r {displaystyle mathbf {v} _{r}} is the spin velocity, and a r {displaystyle mathbf {a} _{r}} is the spin acceleration.