Purpose: T2‐weighted MR provides excellent delineation of malignant liver lesions due to its superior tumor‐to‐tissue contrast. This study aims to develop a novel T2‐weighted retrospective 4D‐MRI technique for imaging organ/tumor respiratory motion with improved soft‐tissue contrast. Method and Materials: Determine the number of repeated scans (NR) required obtaining sufficient phase information for each slice is the critical component in developing this technique and needs substantial testing with many samples. To do that, computer simulations using RPM respiratory signals of 29 cancer patients were performed to derive the relationships between NR and the following factors: number of slice to scanned (Ns), number of respiratory phases of the 4D‐MRI (Np), and starting phase at image acquisition (P0). Assuming T2‐w HASTE/SSFSE MR sequence to be used to acquire raw images for 4D‐MRI, frame rate of 2 frames/s was used in the simulation. To validate our technique, 4D‐MRI acquisition and reconstruction were simulated on a 4D digital human phantom using parameters derived from the above studies. Retrospective sorting of 4D‐MRI was achieved using a novel phase and amplitude hybrid sorting algorithm by effectively utilizing redundant images. Result: Percentage of complete acquisition of all required phases (Cp) increased as NR increased in an inverse‐exponential (Cp=100*[1‐exp(−0.11*NR)],when Ns=50,Np=10) fashion. NR to achieve 95% completion (Cp=95%) of all required phases, defined as the NR needed for 4D‐MRI, is linearly proportional to Np (Nr∼2.86*Np, r=1.0) but independent of NS and P0. Simulated 4D‐MRI on the digital phantom showed clear pattern of respiratory motion. Tumor motion trajectories measured on 4D‐MRI were comparable to the average input signal, with a mean relative difference in motion amplitude of 16%, presumably due to breathing irregularity. Conclusions: A novel T2‐weighted 4D‐MRI technique based on HASTE/SSFSE sequence have been developed and validated. Future evaluation on patients is desired. This work is partly supported by funding from NIH (1R21CA165384‐01A1) and a research grant from the Golfers Against Cancer (GAC) Foundation.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging