Dr Ali Anjum - Stretch it or not: a quantitative analysis of relationship between stretched- and bi-exponential forms for parametric modeling of diffusion MRI data

Webinar: Thursday 29 April 2021 10:00 pm Brisbane. Register here.

Abstract: MRI relaxation and diffusion measurements are often quantified using parametric models based on functional representation of data. In vogue are the bi-exponential and stretched exponential forms which often show improved fits in biological tissue systems. In the case of relaxation, the bi-exponential and stretched exponential models are considered useful for capturing water motional properties in complex tissue environments. In the case of diffusion MRI, the intravoxel incoherent motion model and the fractional Fickian diffusion model imply different physical pictures where the former assumes two distinct pools of water while the latter assumes superdiffusion in a heterogeneous medium. Prognostic value of parameters quantified from both models have been studied for discriminating between healthy and pathological processes in different tissue systems. However, the quantitative relationship of the parameters of the two models has not been examined. This work presents a mathematical framework for obtaining direct transformation of the parameters of the stretched exponential model to those of the bi-exponential model. Results will be compared with bi-exponential estimates obtained from the non-linear least squares fitting of stretched exponential data generated from parameters observed in vivo. Behaviour of the bi-exponential model poles in z-plane will be discussed, and the error of the bi-component fit will be analysed. Fitting error analysis reveals that for values of stretching exponent ranging between 0.75 and 1 (range observed in muscle), the autocorrelation sequence of the fitting error approaches that of white noise, making stretched exponential model output indistinguishable from the respective bi-exponential fit.

Bio: Muhammad Ali Raza Anjum received his bachelor’s degree in electrical engineering in 2004 from University of Engineering & Technology, Taxila, Pakistan, and master’s degree in telecommunications engineering in 2007 from NWFP University of Engineering & Technology, Peshawar, Pakistan. From 2016 to 2019, he was a PhD student at School of Engineering and Computer Science, Victoria University of Wellington, New Zealand, where he worked on development of analytical and statistical signal processing methods for high-resolution parametric estimation of one- and two-dimensional NMR spectroscopy and relaxometry data. Since April 2019, he is a postdoctoral fellow at Reiter Lab, Emory University School of Medicine, USA, where he is responsible for development of analytical and computational tools for investigating tissue microarchitecture and function using MR imaging, relaxometry and spectroscopy techniques.