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MRI Movies

Brian Hargreaves, Ph.D.

All material on these pages is available free of charge for teaching purposes.
Acknowledgment is appreciated where appropriate.


The resonance of nuclear magnetic resonance is shown here. Nuclei in a magnetic field will precess around the axis of the magnetic field, at the Larmor frequency, which is proportional to the static field strength. resonance.mpg

Relaxation and Precession

The component of magnetization parallel to the static field will recover to its polarized state. This recovery is exponential with a time constant called T1.

The component of magnetization perpendicular to the static field will decay exponentially to zero, with a time constant T2. This transvers magnetization is what precesses about the field. The precession of transverse magnetization induces the MR signal in the receive coil.

This animation shows T1 and T2 relaxation as well as precession. T1 and T2 vary in biological tissue, and are the main sources of image contrast in MRI.


An RF field, oriented in a transverse plane, alters the net magnetic field. The magnetization will precess about this net field. The RF field amplitude is much smaller than the static field, so the net field is almost unchanged. However, by rotating the RF field direction (yellow arrow) at the Larmor frequency, magnetization can be tipped arbitrarily into the transverse plane. The resulting transverse magnetization gives an MR signal.

This animation shows the rotating Rf (B1) field, and the path that the magnetization takes as it is excited into the transverse plane. nonrotb1tip.mpg

If a "rotating coordinate system" is used, we see that we can arbitrarily tip magnetization about a transverse axis. rotb1tip.mpg

This series of animations shows the effect if the RF (B1) is tuned to different frequencies. The numbers represent the ratio of B1 frequency to Larmor frequency. Note that at 1.0, B1 is tuned to the Larmor frequency, and the magnetization is tipped a full 90 degrees.

0.0 | 0.5 | 0.8 | 0.9 | 1.0 | 1.1 | 1.2 | 1.5 | 2.0

Gradients and Imaging

Here are shown two spins, at different positions. With a gradient on, the red spin precesses at a faster frequency than the blue. (The red also has a higher magnetization.) The following animation shows the induced signals from the red and blue spins, and the total signal. signal.mpg

Although the total signal seems unrecognizable, the Fourier transform gives the two peaks corresponding to the spin densities, here.


Any image can be written as a weighted sum of spatial harmonics.
Each point in k-space represents the weight that corresponds to that harmonic.
This movie shows the harmonics that correspond to selected points in k-space.

The following show how the image forms as k-space is filled in 3 common ways:

Spin Echoes

Static field inhomogeneities (and other effects) can result in spins having different precession frequencies. A 180 degree "refocosing" pulse can refocus the spins to a spin echo. This animation shows the dephasing due to different frequencies, and the refocusing effect of a 180 pulse. spinecho.mpgSteady-State Imaging These still to come! Several different movies showing spins in steady state, with and without alternating the RF are available here. The most popular is probably one showing multiple frequencies, and alternating RF, here.
All code on these pages is available free of charge for any use.
Acknowledgment is certainly appreciated where appropriate.

I am very happy to discuss possible applications and/or extensions of the information provided.

Please send comments / questions / fixes(!) to Brian Hargreaves.