Quantitative susceptibility mapping‐based cerebral metabolic rate of oxygen mapping with minimum local variance
Signal phase can be used through quantitative susceptibility mapping (QSM) processing to generate tissue magnetic susceptibility 9. Unlike R2 and JOURNAL/mrim/04.02/01445475-201801000-00017/math_17MM2/v/2017-12-21T175206Z/r/image-png , the relationship between [dH] and tissue susceptibility is linear in its concentration and independent of imaging parameters 11. Hence, QSM can be used to quantify oxygenation in large veins 14 and in tissue 19. The [dH] in tissue can be determined from QSM by compensating for contribution from non‐heme iron, such as those stored in ferritin. A single vascular challenge to modify blood flow, either vasoconstrictive or vasodilative, has been used to separate the susceptibility contribution from blood and nonblood tissue 19. However, even the requirement of a single challenge may limit its practical utility.
In this study, a new algorithm called the minimal local variance (MLV) is proposed to obtain CMRO2 maps without blood flow challenge. Minimal local variance assumes the constant CMRO2 and nonblood tissue susceptibility ( JOURNAL/mrim/04.02/01445475-201801000-00017/math_17MM3/v/2017-12-21T175206Z/r/image-png ) within each tissue type (gray and white matter) across small regions of the brain. This added prior information mitigates the ill‐posed nature of the optimization problem. The results in healthy subjects are compared with those obtained using a caffeine challenge.