Diagnostic identification and troubleshooting of scan failures. High-yield for CBCMR exams.
Need-to-know for clinical practice
Board Lab 2.0: Artifact Diagnosis
High-Fidelity Signal Processing Simulator
Select Board Scenario
Field of View (FOV)320mm
Matrix Size256px
Receiver Bandwidth32kHz
Voxel size
1.25 mm
Rel. SNR
2.0x
Quick Challenges
1. Aliasing (Wrap-around)
Aliasing happens when the object being imaged is larger than the Field of View. The frequencies outside the FOV are "aliased" back into the image.
Board Fact: Phase vs Freq
Aliasing usually only occurs in the phase-encoding direction. Why? Because frequency-encoding uses an analog filter to cut off frequencies outside the FOV, but phase-encoding relies on discrete steps that cannot be filtered.
The Fix:
Increase FOV (reduces resolution unless matrix is also increased).
No-Phase-Wrap (oversampling): Collecting extra lines of k-space but only showing the center.
Swap Phase/Freq: Change the orientation so the short axis of the body is in the phase direction.
2. Chemical Shift (1st Kind)
Fat and water protons don't precess at exactly same rate. Fat is "shielded" by electrons, causing it to precess ~3.5 ppm slower.
Calculated Shift
At 1.5T, the shift is 220 Hz. If your receiver bandwidth is 32 kHz (125 Hz per pixel), the fat will be shifted by nearly 2 pixels!
Clinical Correlation
Epicardial Fat Identification
This artifact creates a dark band on one side of a fat-water interface and a bright band on the other. It is often useful for identifying epicardial fat or fat in myocardial tumors.To reduce it, increase the Receiver Bandwidth.
3. Gibbs (Truncation) Artifact
This appears as fine "ringing" or parallel lines at sharp boundaries (like the spinal cord/CSF or heart/lung interface).
It occurs because we are trying to represent a sharp step-function (the interface) with a finite number of sine waves (Fourier components).The fix: Increase the Matrix Size (collect more high-frequencies).
Fellows' Board Review Quiz
These questions are modeled after the CBCMR sub-exams.
Knowledge Check1 / 3
You see 'ghost' images of the chest wall repeating across the heart. What is the most likely cause?
