Journal of Magnetic Resonance Imaging | 2021
Editorial for “Quantification of Regional Cerebral Blood Flow Using Diffusion Imaging With Phase‐Contrast”
Abstract
Editorial for “Quantification of Regional Cerebral Blood Flow Using Diffusion Imaging With Phase-Contrast” A well-known saying dictates that “what goes in, must come out,” thereby referring to one of the fundamental laws of classical physics stating that mass cannot be generated nor destroyed. The principle of conservation of mass can be used to quantify flow in a way which otherwise might be challenging or impossible. In the context of human physiology, the blood circulation can be considered as such a closed system. In the paper, the concept of a mass balance is adopted by the authors to quantitatively measure regional cerebral blood flow (rCBF) in the human brain. Their work builds upon more than three decades of research and development to either manipulate or measure the phase of the MR signal. This has enabled measuring physiological principles at different time scales and lengths, namely blood flow, perfusion, and diffusion of water molecules. Intravoxel incoherent motion (IVIM) imaging was introduced for jointly modeling diffusion and perfusion parameters using a bi-exponential function. This requires diffusion MRI measurements at a large range of diffusion weightings, or b-values. Follow-up work proposed to expand to a tri-exponential model, estimating perfusion-related diffusion, fast-free diffusion, and slow-restricted diffusion. By using a more accurate diffusion modeling, perfusion-related information could also be obtained with lower error, despite still as a relative measure. This work, coined phase contrast (PC) adjusted diffusion imaging (DPC), proposes to quantify rCBF through an additional phase contrast MRI (PC-MRI) measurement. From a PC-MRI scan, the total incoming flow can be computed in the four feeding arteries of the brain and converted into total CBF (tCBF), with the use of an additional brain mask. Using this tCBF-estimate, the perfusion-related diffusion estimate obtained from diffusion MRI can be converted into an rCBF estimate. DPC can become an alternative to arterial spin labeling (ASL), which labels incoming blood as endogenous contrast agent for quantitative rCBF measurements. In comparison, the authors showed that their estimate using DPC correlated well with rCBF in gray matter as obtained from ASL in healthy volunteers. Potentially, DPC is of added benefit in situations where ASL has limitations, such as in case of variation in arterial transit time, as can occur in patients with vascular disease. The authors show that their estimates of rCBF fall within the reported range of the reference rCBF-method O H2O position emission tomography (PET). To address the limitation of the required high signal-tonoise ratio for triexponential fitting, the authors applied denoising for more accurate and precise perfusion estimation. Further improvement might be possible using model precision assessment and advanced parameter estimation techniques exploiting the correlation in parameter values between adjacent voxels. Recent work on deep learning for IVIM showed low variability and error, while being much faster in parameter estimation. Together with state of the art acceleration techniques for single-shot Echo Planar Imaging (e.g. simultaneous multislice or multiband imaging), this might be exploited to include the additionally required PC-MRI scan in the examination without at least a further increase and possibly a reduction in scanning time. On the road toward clinical implementation and use, it is worth emphasizing that the proposed quantitative rCBF metric is estimated in conjunction with diffusion metrics. Coming from the same sequence, these metrics are already principally aligned, thus requiring no coregistration. In the context of stroke, joint diffusion and perfusion assessment is pivotal to assess onset and treatment, and predict outcome for patients. The necessity and timeliness of this research cannot be understated in this time, where the use of exogenous contrast agents is more and more debated. In clinical practice, dynamic susceptibility contrast (DSC) MRI is long established and de facto the reference method for assessing CBF, requiring gadolinium administration during the examination. Though DPC is still in its infancy, it may join ASL in offering an gadoliniumfree alternative to the assessment of CBF. In conclusion, this work demonstrates that building on the fundamental laws of physics helps us to advance in quantifying cerebral perfusion without contrast administration. Because what does not go in, also gives no need for wash out.