Whilst learning MRI theory I occasionally came across statements which, whilst describing sources of error in MRI images, ended with

“…and other off-resonance effects.”

It turns out, off-resonance effects are not a black art or higher plane of MR knowledge after all. Here’s a quick recap. What are off-resonance effects? Off-resonance effects are any signal which has a frequency different from that expected for a nucleus in an idealised system with perfectly uniform static magnetic field (B₀) throughout the sample, and perfect linear gradients. Even if you had a perfect magnet, you’d still get signals which aren’t at the expected (Larmor) precessional frequency.

Since MRI relies on the pulse sequence to relate spatial position in the scanner to precessional frequency, a number of things can occur:

• spatial distortion (images can be distorted globally, or spatial offsets such as chemical shift artefact)
• signal loss (in which net magnetisation vectors within a voxel become out of phase and destructively interfere)
• ghosts
• blurring (in certain k-space trajectories)
• local signal artefacts (dark lines, high signal hot spots).

So you have your perfect magnet, costing you a bazillion groats, or whatever. Proud as Punch, you put a patient in the scanner bore. What happens? The differring magnetic susceptibilities of tissues (think “magnetisability”) causes magnetic field gradients at the interfaces between tissues, and between tissues and free space. These are magnetic field gradients which are not part of our pulse sequence, and cause signal loss, and signals at the “wrong” resonant frequency (off-resonance). These static susceptibility effects are the most common source of off-resonance effects.

Other sources of off-resonance effects are:

• Eddy currents. These arise in conducting structures in the MR scanner when changes in the amplitude of the applied magnetic field gradients occur (i.e. when you switch gradients on and off). Eddy currents decay away, and so produce time-varying off-resonance effects.
• Concomitant gradients (Maxwell terms). Magnetic field changes orthogonal to an applied magnetic field gradient occur due to Maxwell’s equations. So for example, if you apply a z gradient, spatial variations in B_x and B_y also occur. These cause spatial variations in the Larmor frequency and produce off-resonance effects.

Use of spin echoes reduces off-resonant effects. Higher performance hardware can help reduce off-resonance effects, as can the use of parallel imaging in some circumstances. Other strategies include phase correction (using a measured or inferred field map), slice-by-slice shimming, and post-processing corrections such as image registration or image consistency checks.

Thanks to Prof. Jo Hajnal, who gave an excellent introduction to off-resonance effects at ISMRM 2001 (Glasgow).

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