Technical note: Unexpected external markers artifact in 3D k-space based parallel imaging turbo spin-echo magnetic...

Area of Science:

• Medical Imaging
• Radiotherapy Physics
• Radiology

Background:

• Accurate spatial referencing is crucial for MRI-guided radiotherapy planning.
• Immobilization devices and external markers are commonly used for spatial referencing.
• Image artifacts can compromise the accuracy of radiotherapy planning.

Purpose of the Study:

• To investigate a challenging artifact in MRI for radiotherapy planning.
• To identify the cause of artifacts resembling metastases when combining specific MRI techniques and markers.
• To evaluate the impact of marker proximity and imaging parameters on artifact generation.

Main Methods:

• Utilized a 3D variable flip angle Turbo Spin Echo sequence on 1.5T and 3T MRI scanners.
• Tested two types of markers (Liquimark LM1, Spee-D-Mark) with flexible and head/neck coils.
• Employed a silicone oil phantom and 3D Fourier transforms to analyze artifact origins, comparing CAIPIRINHA and compressed sensing acceleration.

Main Results:

• Markers can appear in unexpected locations, not aligned with original or reconstructed slices, particularly with CAIPIRINHA acceleration and 3D distortion correction.
• Artifact occurrence probability increases with marker-patient distance (13% at <=2mm, 80% at >2mm).
• Artifacts were observed post-contrast, mimicking metastases, and did not occur with compressed sensing acceleration.

Conclusions:

• External markers used in radiotherapy MRI can generate artifacts misinterpreted as metastases.
• The combination of CAIPIRINHA parallel imaging and 3D distortion correction increases artifact risk.
• High signal intensity surface structures, like ears, can also cause similar diagnostic errors.