Contrast-to-noise ratio of multiple slice spin lock technique: prospects for liver imaging

Survey of water proton longitudinal relaxation in liver in vivo

Objective

To determine the variability, and preferred values, for normal liver longitudinal water proton relaxation rate R₁ in the published literature.

Methods

Values of mean R₁ and between-subject variance were obtained from literature searching. Weighted means were fitted to a heuristic and to a model.

Results

After exclusions, 116 publications (143 studies) remained, representing apparently normal liver in 3392 humans, 99 mice and 249 rats. Seventeen field strengths were included between 0.04 T and 9.4 T. Older studies tended to report higher between-subject coefficients of variation (CoV), but for studies published since 1992, the median between-subject CoV was 7.4%, and in half of those studies, measured R₁ deviated from model by 8.0% or less.

Discussion

The within-study between-subject CoV incorporates repeatability error and true between-subject variation. Between-study variation also incorporates between-population variation, together with bias from interactions between methodology and physiology. While quantitative relaxometry ultimately requires validation with phantoms and analysis of propagation of errors, this survey allows investigators to compare their own R₁ and variability values with the range of existing literature.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10334-021-00928-x.

Quantification of T(1ρ) relaxation by using rotary echo spin-lock pulses in the presence of B(0) inhomogeneity

T(1ρ) relaxation is traditionally described as a mono-exponential signal decay with spin-lock time. However, T(1ρ) quantification by fitting to the mono-exponential model can be substantially compromised in the presence of field inhomogeneities, especially for low spin-lock frequencies. The normal approach to address this issue involves the development of dedicated composite spin-lock pulses for artifact reduction while still using the mono-exponential model for T(1ρ) fitting. In this work, we propose an alternative approach for improved T(1ρ) quantification with the widely-used rotary echo spin-lock pulses in the presence of B(0) inhomogeneities by fitting to a modified theoretical model which is derived to reveal the dependence of T(1ρ)-prepared magnetization on T(1ρ), T(2ρ), spin-lock time, spin-lock frequency and off-resonance, without involving complicated spin-lock pulse design. It has potentials for T(1ρ) quantification improvement at low spin-lock frequencies. Improved T(1ρ) mapping was demonstrated on phantom and in vivo rat spin-lock imaging at 3 T compared to the mapping using the mono-exponential model.

Image Compression Based on Generalized Principal Components Analysis and Simulated Annealing

The authors propose a new data dimensionality reduction method that is formulated as an optimization problem solved in two stages. In the first stage, Generalized Principal Component Analysis (GPCA) is used to find a solution with local maximum (local solution) whereas the algorithm Simulated Annealing (SA) is performed, in the second stage, to converge the local solution to the optimal solution. The performance of GPCA and GPCA with Simulated Annealing (GPCA-SA) as images compressors was evaluated in terms of the Compression Peak Signal-to-Noise Rate (CPSNR), memory size necessary to store the resulting compressed image and Contrast-to-Noise ratio. The results show that GPCA and GPCA-SA requires the same amount of memory to store compressed data, but GPCA-SA provides better CPSNR than GPCA. They also compared the performance of our designed method with a wavelet-based compression technique widely used in medical imaging, known as Lifting, to demonstrate the efficiency of GPCA-SA in clinical application.