Blood flow is a measure that allows us to determine the rate at which oxygen and nutrients are supplied to a tissue and metabolic by-products can be disposed. Many organs are critically dependent on a steady supply of nutrients not only for function but also for their own survival. Some organs are more dependent upon it than others, but at any rate, cellular death is inevitable if blood flow drops below a certain threshold and for a prolonged period of time.

Organ function can be perturbed by even small reductions in basal blood flow, but irreversible damage to cells may be averted if blood flow is normalized. Autoregulatory mechanisms attempt to normalize blood flow when there is a reduction. In some chronic conditions, these measures may already be at maximal capacity to allow the organ to maintain normal or slightly reduced blood flow. Any further reduction of blood flow can result in inadequate perfusion, resulting in cellular dysfunction, and if the decrease is severe or prolonged enough, it can result in cell death. Two typical examples where the degree of autoregulation of blood flow is examined are cerebrovascular reserve testing and fractional myocardial flow reserve testing. Conversely, in conditions where local metabolic activity is in “overdrive” (e.g., neoplasms and seizures), higher than normal blood flow values can be observed to sustain the increased metabolic demand.

This raises the questions “What is normal blood flow?” When researching the literature, it turns out that there can be a large variability in blood flow values that are reported as “normal.” Such values often depend on the modality used to measure blood flow, ranging from true microsphere experiments to nuclear medicine tracers, nitrous oxide, positron emission tomography (PET), xenon computed tomography (CT), bolus perfusion techniques, such as CT and magnetic resonance imaging dynamic susceptibility contrast (MRI-DSC) perfusion, and arterial spin labeling (ASL). Aside from intertechnique differences, there is also variability within a technique from imaging equipment, as a result of the use of different models and assumptions (e.g., partition coefficients, small vessel hematocrit, processing algorithms, and relaxation times of blood and tissue). Because of the complexity of the organ’s vascular supply (e.g., dual blood supply of the liver via the portal vein and hepatic artery), organ size (e.g., bowel), and organ motion (e.g., heart, lung), blood flow may be difficult to measure. Consequently, not only do reported values vary widely, but there is also a paucity of reported reference values from large normative cohorts.

It can be expected that blood flow changes with age, but again because of the paucity of published values, this hypothesis cannot be backed with data for each organ. An exception is the brain, where blood flow has been extensively studied not only in pathologic conditions but also in normal subjects. In pediatric brain, normal blood flow is considerably higher, particularly in gray matter (GM).¹ This comes as no surprise to those who have looked at time-of-flight angiograms of a child, and this is most likely a result of the higher metabolic demand. Leenders et al.² reported that in a cohort of 22- to 82-year-old subjects studied with ¹⁵O PET, cerebral blood flow (CBF) decreases at a rate of 0.5% per year of age. Biagi et al.³ found in a recent ASL study that in a

cohort of 44 subjects ranging in age between 4 and 78 years that CBF values decreased with age: 97 ± 5 mL/100 g/min in GM and 26 ± 1 mL/100 g/min in white matter (WM) for the children, GM 79 ± 3 mL/100 g/min, and WM 22 ± 1 mL/100 g/min for the teenagers, and GM 58 ± 4 mL/100 g/min, WM 20 ± 1 mL/100 g/min for the adults. Their quantitative results suggest a rapid drop rather than a gradual decrease in cerebral perfusion between children and adult subjects, especially in the GM. This decrease in CBF occurred during adolescence, at approximately the 16 years of age. A significant decrease in the ratio of gray-matter to white-matter perfusion was found by Parkes et al.⁴ with increasing age (0.79% per year; P < .0005) in their ASL perfusion study of 34 healthy subjects. They attributed this mainly to a reduction in gray-matter perfusion, which they found to decrease by 0.45% per year (P = .04). Regional analysis suggested that the GM age-related changes were predominantly localized to the frontal cortex. Interestingly, whole brain perfusion was 13% higher (P = .02) in females compared to males.