title: Investigating Intravoxel Incoherent Motion Magnetic Resonance Imaging of the Liver and Kidney: Dependence on Acquisition Parameters and Techniques
Creators: name: Loh, Martin
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2025

description: Intravoxel incoherent motion (IVIM) is a concept in diffusion-weighted magnetic resonance imaging that enables the distinction between water diffusion and perfusion-related motion within biological tissues. There are still several mechanisms and influences of acquisition parameters that, to date, are not yet fully understood, which particularly limit the reproducibility and comparability of IVIM studies. This work placed a special focus on measurements in the liver and kidney. All measurements were supported and validated by in-silico investigations. The first project aimed to investigate the influence of simultaneous multislice (SMS) acquisition, slice properties and repetition time (TR) on biexponential IVIM parameters in the liver. To investigate this research question, volunteer measurements were performed using conventional slice excitation (linear order) and SMS acquisition — both at a short TR (1,300 ms) and a long TR (4,500 ms) with 5 mm slice thickness. A subset of the volunteers was additionally measured with a thicker slice thickness of 10 mm at both short and long TR. The measurements, as expected, showed no difference in the biexponential IVIM parameters between linear slice order and SMS. Additionally, TR did not have an impact on the measured IVIM parameters in the 5 mm slices, but the influence on the perfusion fraction was more pronounced in the 10 mm thick slices. Simulations accounting for blood flow and T1 relaxation suggested that the perfusion fraction considerably depends on TR for slice settings with no slice gaps. Additionally, using an interleaved slice order instead of linear can influence the measured perfusion fraction for certain slice settings. Therefore, it is recommended to report slice thickness, size of slice gaps, and slice order in future IVIM publications. The second project focused on the kidney to explore the contributions of blood and primary urine to the IVIM effect. To this end, the different T2 times of blood and primary urine were utilized, and images were acquired at four different echo times (TEs) (45 ms, 60 ms, 75 ms, and 90 ms). Biexponential IVIM parameters were then compared. Only the diffusion coefficient exhibited an increase at TE = 90 ms, while the perfusion fraction and the pseudodiffusion coefficeint remained constant across all TEs. Simulations using a multi-compartment model showed that the TE dependency of the perfusion fraction becomes virtually negligible when the volume fraction of primary urine does not exceed that of blood. They further show that, as the tubular fluid volume increases, the TE dependence of the perfusion fraction becomes increasingly apparent. The sudden increase in the diffusion coefficient at TE = 90 ms might be either attributed to the presence of primary urine with small flow velocity, or it could be considered a mere artifact. In summary, the contribution of primary urine to the IVIM effect could neither be confirmed nor refuted so far. However, it was found that the biexponential IVIM parameters are comparable across different TEs.