Abstract
Purpose
To develop a T1ρ‐prepared, balanced gradient echo (b‐GRE) pulse sequence for rapid three‐dimensional (3D) T1ρ relaxation mapping within the time constraints of a clinical exam (<10 minutes), examine the effect of acquisition on the measured T1ρ relaxation time and optimize 3D T1ρ pulse sequences for the knee joint and spine.
Materials and Methods
A pulse sequence consisting of inversion recovery–prepared, fat saturation, T1ρ‐preparation, and b‐GRE image acquisition was used to obtain 3D volume coverage of the patellofemoral and tibiofemoral cartilage and lower lumbar spine. Multiple T1ρ‐weighted images at various contrast times (spin‐lock pulse duration [TSL]) were used to construct a T1ρ relaxation map in both phantoms and in the knee joint and spine in vivo. The transient signal decay during b‐GRE image acquisition was corrected using a k‐space filter. The T1ρ‐prepared b‐GRE sequence was compared to a standard T1ρ‐prepared spin echo (SE) sequence and pulse sequence parameters were optimized numerically using the Bloch equations.
Results
The b‐GRE transient signal decay was found to depend on the initial T1ρ‐preparation and the corresponding T1ρ map was altered by variations in the point spread function with TSL. In a two compartment phantom, the steady state response was found to elevate T1ρ from 91.4 ± 6.5 to 293.8 ± 31 and 66.9 ± 3.5 to 661 ± 207 with no change in the goodness‐of‐fit parameter R^2^. Phase encoding along the longest cartilage dimension and a transient signal decay k‐space filter retained T1ρ contrast. Measurement of T1ρ using the T1ρ‐prepared b‐GRE sequence matches standard T1ρ‐prepared SE in the medial patellar and lateral patellar cartilage compartments. T1ρ‐preparedb‐GRE T1ρ was found to have low interscan variability between four separate scans. Mean patellar cartilage T1ρ was elevated compared to femoral and tibial cartilage T1ρ.
Conclusion
The T1ρ‐prepared b‐GRE acquisition rapidly and reliably accelerates T1ρ quantification of tissues offset partially by a TSL‐dependent point spread function. J. Magn. Reson. Imaging 2008;28:744–754. © 2008 Wiley‐Liss, Inc.