MATERIALS CHANGE THE FIELD
Susceptibility describes how a material responds to the magnetic field.
Different materials become magnetized by different amounts and directions inside B0. Where materials with different susceptibility meet, the local magnetic field becomes less uniform.
Nearby spins then precess at slightly different frequencies. Their phases spread apart, transverse signal cancels, and the area may become dark on a T2*-sensitive image.
FROM FIELD CHANGE TO SIGNAL LOSS
One local disturbance creates a chain reaction.
Blood products, iron, calcium, air, and metal can differ magnetically from nearby tissue.
The susceptibility difference perturbs the local magnetic field.
Spins within a voxel precess at different rates and lose phase coherence.
T2* signal loss and distortion become visible on sensitive acquisitions.
Longer TE usually gives spins more time to accumulate phase differences, increasing susceptibility contrast and artifact. Field strength, voxel size, orientation, sequence design, and the material also influence the appearance.
GRE, T2 STAR, AND SWI
Gradient echo reveals the effect; SWI strengthens the contrast.
Gradient echo does not use a 180° RF refocusing pulse, so local field-related dephasing remains visible as signal loss.
Susceptibility-weighted imaging processes phase information and combines it with magnitude data to emphasize susceptibility differences.
Quantitative susceptibility mapping uses additional processing to estimate local tissue susceptibility rather than only displaying blooming contrast.
Blooming means a small susceptibility source may look larger than its physical size. This increases sensitivity, but the dark area should not be used as a direct measurement of the source.
An artifact does not determine whether an implant or object is MR safe.
Image appearance and MR safety are separate questions. Always verify the exact device or object using approved labeling, conditions of use, and institutional safety procedures.
THE WRINKLED SHEET ANALOGY
A small object can disturb a much larger area.
Place a small object beneath a smooth sheet. The object is small, but the wrinkle spreads beyond it. A susceptibility source can disturb the local field beyond its true boundary, creating a larger dark bloom.
Remember: Local field change → dephasing → T2* signal loss → blooming.
CHECK YOUR UNDERSTANDING
Connect magnetic differences to image appearance.
Why is gradient echo sensitive to susceptibility?
Without a 180° RF refocusing pulse, dephasing from local field differences remains part of the measured T2* signal.
What does blooming mean?
A susceptibility source creates signal loss that extends beyond its true physical boundary, making it appear larger on the image.
How does SWI differ from ordinary GRE magnitude imaging?
SWI processes phase information and combines it with magnitude data to increase susceptibility contrast. Exact processing and phase appearance vary by system.
LESSON 19 COMPLETE
You understand how susceptibility differences create T2* signal loss and blooming.Educational references
This lesson introduces susceptibility principles. Signal loss is not specific to one material or diagnosis, and phase conventions vary by scanner and processing method.