Are we ready to image the incoherent molecular motion in our minds?

Abstract

The increasing radiological awareness about intravoxel incoherent motion imaging (IVIM) is reflected in this month’s issue of Neuroradiology. Hauser et al. [1] following hard on the heels of previous work [2] contribute substantially to the accumulating evidence for the utility of IVIM in tissue characterization. Several studies in abdominal imaging, where the theoretical principles of IVIM are verified in vivo in a satisfactory way [3], spearheaded the translation of IVIM into clinical applications after some time of hibernation. In human tissue, diffusion-weighted imaging (DWI) detects the molecular motion, which includes the molecular diffusion of water as well as the blood microcirculation in the capillary network. The cardinal DWIderived parameter, apparent diffusion coefficient, has been adapted over time to investigate the tissue-ofinterest but is dependent on the choice of b values and is endowed with a lumped and nonspecific character. The separation of pure diffusion from flow-related molecules motion demands complex models to describe the signal decay in DWI after application of increasing “motion probing” gradient intensities. The parameters D (tissue diffusivity or pure diffusion coefficient), D* (pseudodiffusion coefficient or perfusion-related incoherent microcirculation coefficient), and f (microvascular volume or perfusion fraction), which were dubbed by Le Bihan et al. [4] as the hallmarks of IVIM-sensitized DWI, seem to adequately resolve the perfusion (microcirculation or net flow) from the true diffusion given a sufficient b value sampling and a biexponential curve fit analysis. Almost simultaneous to the premier presentation of the IVIM technique, considerable skepticism was voiced concerning the suitability of f, D*, and fD* to measure perfusion in the “classical” sense of the term [5]. The debate remains ongoing and probably will not be terminated since it all seems to be a matter of definition. Actually, the rather loosely used term perfusion implies the efficiency of blood to saturate the tissue with oxygen and nutrients as well as to desaturate it from waste products. The monitoring of exogenous tracers (gadolinium compounds) by MRI provides the well-known blood flow parameter, which is conceptually related to perfusion. Diverse applied models rely on several assumptions that oversimplify the physiological background, resulting in an apparently meaningful determination of blood volume, which is not perfusion or blood flow. In contrast, IVIM devotees, by measuring the phase effects of flowing blood (endogenous marker), provide new insights into the capillary microcirculation, which may be conceptually related to “classical” perfusion under several assumptions about capillary network structure, too [6, 7]. Remarkably, the cellular shape/structuring may cause pseudo-perfusion like “guided flow” (for example in the kidney medulla or in the white brain matter). The apparent “overestimation” of perfusion fraction f compared to dynamic contrast-enhanced MR perfusion is inevitably influenced by the choice of motion-probing gradient directions versus the directionality of the neurons in the white matter. The bottom line is that the terms should bemeticulously chosen to avoid confusing imprecision that only serve to please our ears. Thus, before any further work clarifies these issues, it would be advisable to denominate f strictly as fractional volume of capillary flow instead of perfusion fraction. Whether IVIM launch means that “the end is nigh” for the contrast-enhanced perfusion-weighted MRI, the answer is probably not. The compartmental water exchange in the DWI experiments is not adequately captured due to the short evolution time (≈100 ms) compared to the dynamic contrastenhanced MRI, which tracks the intraand extravascular as This is a Comment to Original Article doi:10.1007/s00234-013-1154-9