Purpose: To quantify and to improve the measurement accuracy of the “MRI scanner field inhomogeneity evaluation” for the “ACR MRI system Acceptance Test” and the “ACR MRI System Annual Performance Test”.
Methods: A spherical phantom of 25cm diameter (which is MRI site facility dependent, normally larger) at the isocenter of the MRI scanner bore was scanned with a 3D dual gradient echo sequence with TE1=7ms and TE2=21.7ms (which are scanner field strength dependent). A slob of 25-slices (which is phantom size dependent) was scanned and a total of 2x25 amplitude plus 2x25 phase images were acquired. An appropriate 3D phase unwrapping method was developed and applied to the phase images. The field variation map was then generated according to the guidance in the “ACR 2015 Quality Control Manual Medical Physicist Section” (Chapter IV, Section A-4) and the “ACR Acceptance Testing and Quality Assurance Procedures for MRI Facilities” (Chapter III, Section E-1) based on these two unwrapped 3D-phase images. The field at the isocenter was set as the reference point.
Results: The result shows that each 3D-phase image is unwrapped perfectly as one unit and the 3D-field map generated based on these 3D-phase images is smooth and intuitive. It can be viewed with any 3D viewing tools from any viewing angle. The result shows that for this particular scanner, up to a diameter of 25cm, the crossing field variation is about 0.3ppm in the transverse plan while the field variation reaches over 1.0ppm in the longitudinal direction.
Conclusion: A semi-automated tool to generate the MRI scanner field variation map (2D and 3D) is developed based on this method and it provides a convenient, efficient and accurate mean for physicists to perform the MRI scanner field inhomogeneity evaluation for the “ACR MRI system Acceptance Test” and the “MRI System Annual Performance Test”.