Abstract
Purpose
To design and implement a magnetic resonance imaging (MRI) pulse sequence capable of performing three‐dimensional T~1ρ~‐weighted MRI on a 1.5‐T clinical scanner, and determine the optimal sequence parameters, both theoretically and experimentally, so that the energy deposition by the radiofrequency pulses in the sequence, measured as the specific absorption rate (SAR), does not exceed safety guidelines for imaging human subjects.
Materials and Methods
A three‐pulse cluster was pre‐encoded to a three‐dimensional gradient‐echo imaging sequence to create a three‐dimensional, T~1ρ~‐weighted MRI pulse sequence. Imaging experiments were performed on a GE clinical scanner with a custom‐built knee‐coil. We validated the performance of this sequence by imaging articular cartilage of a bovine patella and comparing T~1ρ~ values measured by this sequence to those obtained with a previously tested two‐dimensional imaging sequence. Using a previously developed model for SAR calculation, the imaging parameters were adjusted such that the energy deposition by the radiofrequency pulses in the sequence did not exceed safety guidelines for imaging human subjects. The actual temperature increase due to the sequence was measured in a phantom by a MRI‐based temperature mapping technique. Following these experiments, the performance of this sequence was demonstrated in vivo by obtaining T~1ρ~‐weighted images of the knee joint of a healthy individual.
Results
Calculated T~1ρ~ of articular cartilage in the specimen was similar for both and three‐dimensional and two‐dimensional methods (84 ± 2 msec and 80 ± 3 msec, respectively). The temperature increase in the phantom resulting from the sequence was 0.015°C, which is well below the established safety guidelines. Images of the human knee joint in vivo demonstrate a clear delineation of cartilage from surrounding tissues.
Conclusion
We developed and implemented a three‐dimensional T~1ρ~‐weighted pulse sequence on a 1.5‐T clinical scanner. J. Magn. Reson. Imaging 2003;17:730–736. © 2003 Wiley‐Liss, Inc.