Purpose: To develop and validate a fully balanced steady-state free precession (bSSFP) MRI pulse sequence with parallel imaging and compressed sensing (PICS) for post-implant prostate cancer brachytherapy MRI without an endorectal coil (ERC).
Methods: All studies were conducted on a Siemens 3T Prisma MRI scanner. Variable density Poisson-disk sampling was implemented into a bSSFP pulse sequence (CISS). Two phantoms were scanned with the new pulse sequence and with increasing acceleration factors (R) to assess different aspects of image quality: (1) the ACR MRI phantom was scanned to assess spatial resolution and (2) a prostate phantom with implanted stranded dummy brachytherapy seeds and positive-MRI contrast seed markers (Sirius, C4 Imaging) was scanned (with/without a 2-channel rigid ERC) to assess SNR (using the NEMA definition) and seed marker contrast. The missing phase encoding lines were recovered using the L1-ESPIRiT algorithm. The reconstructed k-space data were written to synthetic Siemens raw data files and processed offline in the Siemens Image Calculation Environment to create DICOM images.
Results: Slight blurring of the ACR resolution patterns were observed for higher acceleration factors. However, the 0.9mm grid pattern was still identifiable at acceleration factors of 5 and 6. Increasing NEX improved SNR in the images acquired without the ERC. With the same total scan time, the images acquired with NEX=R=5 and without an ERC had higher SNR than the images with NEX=R=1 and with an ERC. The average seed marker contrast was 293%Â±131% and 212%Â±166% for the NEX=R=1 image (with ERC) and NEX=R=5 image (without ERC), respectively.
Conclusion: We successfully implemented PICS into a bSSFP pulse sequence. Phantom test results showed that the new sequence produced increased SNR at the same total scan time compared to the conventional bSSFP sequence and has the potential to enable post-implant MRI for prostate cancer brachytherapy without an ERC.
Funding Support, Disclosures, and Conflict of Interest: Steven Frank is a cofounder of C4 Imaging, has ownership interests in the company, and is an inventor of US and international patents on the C4 seed marker technology. Jingfei Ma is an inventor of US patents that are licensed to Siemens Healthineers and is a consultant for C4 Imaging.