Saturday, February 16, 2008

Magnetic Susceptibility

Radiologists are fond of throwing around the term "susceptibility artifact", lexicon which can confuse rather than clarify. My teenage son is susceptible to the magnetic attraction of the opposite sex, but here we aim to demystify the radiologic term "magnetic susceptibility" .

"Magnetic susceptibility" refers to the internal magnetization of a substance when it is placed in a magnetic field. When two materials with different susceptibilites are placed next to one another, there will be a distortion in the local magnetic field. Since MRI scanners need a homogeneous magnetic field to create accurate images, any distortion of the magnetic field will lead to an artifact.

That, in a nutshell, is susceptibility artifact.

The main culprits in susceptibility artifact are metal and air.

This has real-world implications. Here is a patient who underwent prior surgery for osteomyelitis of the thumb. She had recurrent swelling of her thumb. Here is a sagittal gradient-echo image from her MRI:

The orthopedic surgeon called me and wanted to know what the black areas (yellow arrows) in the soft tissues represented. These could be areas of calcification (calcium can be dark on MRI), but the answer is revealed when we look at two additional images:

Note that these dark areas are prominent on the gradient echo image (green arrow), but are much less apparent on the T1 spin echo image (yellow arrow). The dark spots "bloom" on the gradient echo image. This is characteristic of susceptibility artifact- it is best seen on gradient echo (vs. spin echo).

The surgeon then told me that he did not use any instruments that would have left behind any metal, so how could this be metal (susceptibility) artifact? Well, there is a trail of metal debris left behind during surgery. One can almost always see some metal artifact, if gradient echo sequences are used, no matter how meticulous the surgical technique was.

The other major substance that can cause susceptibility artifact is air. Here is an example of a shoulder MRI arthrogram, with iatrogenic air (yellow arrows) in the subscapularis, introduced during the course of the joint injection. Note that the air cannot be seen on the axial T2 fatsat spin echo image, since spin echo images are much less prone to susceptibility artifact:

Whether you are a surgeon or radiologist, susceptibility artifact will reveal your tracks!

Vic David MD


Anna said...

short, sweet and to the point - thank you Anna

Alison said...

If the gas bubble was due to recent subluxation, would it be darker and show up in the T2 Fat Suppressed image too?