Synopsis

Keywords: Contrast mechanisms: Diffusion, Contrast mechanisms: Perfusion, Cross-organ: Tissue characterisation

Intravoxel Incoherent Motion (IVIM) presents a compelling approach to studying tissue (micro)circulation and perfusion. In this talk, we explore why IVIM is a valuable tool in medical imaging and how it differs from other perfusion measures. By dissecting the principles behind IVIM, we uncover its ability to differentiate between diffusion and (microvascular) perfusion in tissue, offering insights into various pathologies. Moreover, we will touch upon the different mechanisms and assumptions behind the IVIM signal model and discuss when they hold and when not.

Intravoxel Incoherent Motion (IVIM) offers a fascinating window into tissue perfusion and microcirculation¹. This educational presentation introduces IVIM, highlighting its importance and practical applications in clinical settings.

We start by unraveling the fundamental principles of IVIM, emphasizing its distinction from other perfusion measures and its relevance in understanding tissue microstructure. Although multiple techniques measure "perfusion", due to their different mechanisms, they can measure different aspects of perfusion. Particularly, IVIM measures diffusion (D), pseudo-diffusion (D*) and the pseudo-diffusion signal fraction (f), where conventionally, the pseudo-diffusion is interpreted as blood microcirculation. However, in practice, IVIM is sensitive to all intravoxel incoherent motion². This could include blood in larger vessels due to laminar flow profiles, other flowing fluids such as CSF, or tissue deformation (cardiac and potentially respiratory motion).

Besides the mechanisms being different², IVIM has also several practical advantages. In particular, in contrast to dynamic contrast-enhanced (DCE) MRI, IVIM imaging eliminates the need for contrast injections, offering a non-invasive alternative for assessing tissue perfusion. Unlike arterial spin labeling (ASL), which requires meticulous planning of label planes, IVIM simplifies the process with its global encoding approach. Moreover, IVIM probes the microvasculature, providing valuable insights into tissue microcirculation that are not achievable with black blood imaging techniques. Additionally, IVIM analysis provides information about free diffusion within tissues, further enhancing its utility for comprehensive perfusion assessment.

Finally, we discuss some advanced IVIM acquisition and modeling techniques that try to separate different sources of incoherent motions. Particularly, tri-exponential fitting may split fast and slow intravoxel incoherent motions in the liver³, brains⁴ and kidneys^5,6. This could be a tool to separate laminar flow from slower microperfusion. Moreover, the diffusion gradients can be manipulated such that they are motion- and flow-compensated⁷ to probe certain aspects of the incoherent motion.

Acknowledgements

The KWF Dutch Cancer Society supports dr. Gurney-Champion under grant No. KWF-UvA 2021.13785 and NWO supports dr. Gurney-Champion under grant number VIDI-2022-20367