Room: Exhibit Hall | Forum 8
Purpose: Image non-uniformity is a well-known problem for imaging at 7T MRI, and poses a problem for translating standard test phantoms, such as the American College of Radiology (ACR) quality control (QC) phantom, from lower field to 7T scanners. Problems such as inhomogeneous Bâ‚?+ fields and radiofrequency (RF) coil loading at 7T contribute to this problem. The aim of this work was to investigate a more suitable QC test procedure for a new 7T clinical MRI scanner, involving modifications to the phantom and analysis method to better suit the 7T imaging conditions.
Methods: The ACR released a small QC phantom with 100mm inside length and diameter for the testing of extremity coils, together with suggested image acquisition and analysis procedures. This phantom fits into the only two RF coils (head and knee) available on the FDA-cleared 7T clinical MRI scanner (Terra, Siemens, Germany). The standard NiCl2 based solution in the phantom was replaced with Marcol 82 oil. Care was taken to ensure the oil penetrated all structures in the phantom, including the 0.7mm diameter resolution targets, with no air bubbles. A dedicated holder was manufactured for reproducible positioning of the phantom within the head coil. Changes to the base scan acquisition protocol involved: spatial resolution 0.5x0.5x5mmÂ³, field-of-view 14cm, TE=6.0ms, TR=700ms.
Results: The modified phantom achieved a uniformity of 83.7%, compared to 12.5% for the original phantom. The low contrast detectability test also achieved a spoke count of 20 (0 for the original phantom). All other analyzed tests passed with both phantom solutions. An increased FOV of 14cm allowed better placement of ghosting ROIs at the ACR specified length-to-width ratio of 4:1, and a total area of 300 to 500 mmÂ².
Conclusion: The modified 7T test procedure allows for improved standardized ACR phantom quantitative testing and ongoing system performance testing.