Cortical depth dependence of the BOLD initial dip and poststimulus undershoot in human visual cortex at 7 Tesla

PurposeOwing to variability in vascular dynamics across cerebral cortex, blood-oxygenation-level-dependent (BOLD) spatial and temporal characteristics should vary as a function of cortical-depth. Here, the positive response, initial dip (ID), and post-stimulus undershoot (PSU) of the BOLD response in human visual cortex are investigated as a function of cortical depth and stimulus duration at 7 Tesla (T). MethodsGradient-echo echo-planar-imaging BOLD fMRI with high spatial and temporal resolution was performed in 7 healthy volunteers and measurements of the ID, PSU, and positive BOLD response were made as a function of cortical depth and stimulus duration (0.5-8 s). Exploratory analyses were applied to understand whether functional mapping could be achieved using the ID, rather than positive, BOLD signal characteristics ResultsThe ID was largest in outer cortical layers, consistent with previously reported upstream propagation of vasodilation along the diving arterioles in animals. The positive BOLD signal and PSU showed different relationships across the cortical depth with respect to stimulus duration. ConclusionThe ID and PSU were measured in humans at 7T and exhibited similar trends to those recently reported in animals. Furthermore, while evidence is provided for the ID being a potentially useful feature for better understanding BOLD signal dynamics, such as laminar neurovascular coupling, functional mapping based on the ID is extremely difficult. Magn Reson Med 73:2283-2295, 2015. (c) 2014 Wiley Periodicals, Inc

Ultra-high field MRI : Advancing systems neuroscience towards mesoscopic human brain function

Human MRI scanners at ultra-high magnetic field strengths of 7 T and higher are increasingly available to the neuroscience community. A key advantage brought by ultra-high field MRI is the possibility to increase the spatial resolution at which data is acquired, with little reduction in image quality. This opens a new set of opportunities for neuroscience, allowing investigators to map the human cortex at an unprecedented level of detail. In this review, we present recent work that capitalizes on the increased signal-to-noise ratio available at ultra-high field and discuss the theoretical advances with a focus on sensory and motor systems neuroscience. Further, we review research performed at sub-millimeter spatial resolution and discuss the limits and the potential of ultra-high field imaging for structural and functional imaging in human cortex. The increased spatial resolution achievable at ultra-high field has the potential to unveil the fundamental computations performed within a given cortical area, ultimately allowing the visualization of the mesoscopic organization of human cortex at the functional and structural level

Quantitative T1 mapping under precisely controlled graded hyperoxia at 7T

Increasing the concentration of oxygen dissolved in water is known to increase the recovery rate (R1 = 1/T1) of longitudinal magnetization (T1 relaxation). Direct T1 changes in response to precise hyperoxic gas challenges have not yet been quantified and the actual effect of increasing arterial oxygen concentration on the T1 of brain parenchyma remains unclear. The aim of this work was to use quantitative T1 mapping to measure tissue T1 changes in response to precisely targeted hyperoxic respiratory challenges ranging from baseline end-tidal oxygen (PetO2) to approximately 500 mmHg. We did not observe measureable T1 changes in either gray matter or white matter parenchymal tissue. The T1 of peripheral cerebrospinal fluid located within the sulci, however, was reduced as a function of PetO2 No significant T1 changes were observed in the ventricular cerebrospinal fluid under hyperoxia. Our results indicate that care should be taken to distinguish actual T1 changes from those which may be related to partial volume effects with cerebrospinal fluid, or regions with increased fluid content such as edema when examining hyperoxia-induced changes in T1 using methods based on T1-weighted imaging

Quantitative T1 mapping under precisely controlled graded hyperoxia at 7T

Increasing the concentration of oxygen dissolved in water is known to increase the recovery rate (R1 = 1/T1) of longitudinal magnetization (T1 relaxation). Direct T1 changes in response to precise hyperoxic gas challenges have not yet been quantified and the actual effect of increasing arterial oxygen concentration on the T1 of brain parenchyma remains unclear. The aim of this work was to use quantitative T1 mapping to measure tissue T1 changes in response to precisely targeted hyperoxic respiratory challenges ranging from baseline end-tidal oxygen (PetO2) to approximately 500 mmHg. We did not observe measureable T1 changes in either gray matter or white matter parenchymal tissue. The T1 of peripheral cerebrospinal fluid located within the sulci, however, was reduced as a function of PetO2 No significant T1 changes were observed in the ventricular cerebrospinal fluid under hyperoxia. Our results indicate that care should be taken to distinguish actual T1 changes from those which may be related to partial volume effects with cerebrospinal fluid, or regions with increased fluid content such as edema when examining hyperoxia-induced changes in T1 using methods based on T1-weighted imaging

Is there any difference in Amide and NOE CEST effects between white and gray matter at 7T?

Measurement of Chemical Exchange Saturation Transfer (CEST) is providing tissue physiology dependent contrast, e.g. by looking at Amide and NOE (Nuclear Overhauser Enhancement) effects. CEST is unique in providing quantitative metabolite information at high imaging resolution. However, direct comparison of Amide and NOE effects between different tissues may result in wrong conclusions on the metabolite concentration due to the additional contributors to the observed CEST contrast, such as water content (WC) and water T1 relaxation (T1w). For instance, there are multiple contradictory reports in the literature on Amide and NOE effects in white matter (WM) and gray matter (GM) at 7T. This study shows that at 7T, tissue water T1 relaxation is a stronger contributor to CEST contrasts than WC. After water T1 correction, there was no difference in Amide effects between WM and GM, whereas WM/GM contrast was enhanced for NOE effects

In vivo quantification of hyperoxic arterial blood water T-1

Normocapnic hyperoxic and hypercapnic hyperoxic gas challenges are increasingly being used in cerebrovascular reactivity (CVR) and calibrated functional MRI experiments. The longitudinal arterial blood water relaxation time (T-1a) change with hyperoxia will influence signal quantification through mechanisms relating to elevated partial pressure of plasma-dissolved O-2 (pO(2)) and increased oxygen bound to hemoglobin in arteries (Y-a) and veins (Y-v). The dependence of T-1a on Y-a and Y-v has been elegantly characterized ex vivo; however, the combined influence of pO(2), Y-a and Y-v on T(1a)in vivo under normal ventilation has not been reported. Here, T-1a is calculated during hyperoxia in vivo by a heuristic approach that evaluates T-1-dependent arterial spin labeling (ASL) signal changes to varying gas stimuli. Healthy volunteers (n=14; age, 31.5 +/- 7.2years) were scanned using pseudo-continuous ASL in combination with room air (RA; 21% O-2/79% N-2), hypercapnic normoxic (HN; 5% CO2/21% O-2/74% N-2) and hypercapnic hyperoxic (HH; 5% CO2/95% O-2) gas administration. HH T-1a was calculated by requiring that the HN and HH cerebral blood flow (CBF) change be identical. The HH protocol was then repeated in patients (n=10; age, 61.4 +/- 13.3years) with intracranial stenosis to assess whether an HH T-1a decrease prohibited ASL from being performed in subjects with known delayed blood arrival times. Arterial blood T-1a decreased from 1.65s at baseline to 1.49 +/- 0.07s during HH. In patients, CBF values in the affected flow territory for the HH condition were increased relative to baseline CBF values and were within the physiological range (RA CBF=36.6 +/- 8.2mL/100g/min; HH CBF=45.2 +/- 13.9mL/100g/min). It can be concluded that hyperoxic (95% O-2) 3-T arterial blood T-1aHH=1.49 +/- 0.07s relative to a normoxic T-1a of 1.65s. Copyright (c) 2015 John Wiley & Sons, Ltd

Is there any difference in Amide and NOE CEST effects between white and gray matter at 7T?

Measurement of Chemical Exchange Saturation Transfer (CEST) is providing tissue physiology dependent contrast, e.g. by looking at Amide and NOE (Nuclear Overhauser Enhancement) effects. CEST is unique in providing quantitative metabolite information at high imaging resolution. However, direct comparison of Amide and NOE effects between different tissues may result in wrong conclusions on the metabolite concentration due to the additional contributors to the observed CEST contrast, such as water content (WC) and water T1 relaxation (T1w). For instance, there are multiple contradictory reports in the literature on Amide and NOE effects in white matter (WM) and gray matter (GM) at 7T. This study shows that at 7T, tissue water T1 relaxation is a stronger contributor to CEST contrasts than WC. After water T1 correction, there was no difference in Amide effects between WM and GM, whereas WM/GM contrast was enhanced for NOE effects

In vivo quantification of hyperoxic arterial blood water T-1

Normocapnic hyperoxic and hypercapnic hyperoxic gas challenges are increasingly being used in cerebrovascular reactivity (CVR) and calibrated functional MRI experiments. The longitudinal arterial blood water relaxation time (T-1a) change with hyperoxia will influence signal quantification through mechanisms relating to elevated partial pressure of plasma-dissolved O-2 (pO(2)) and increased oxygen bound to hemoglobin in arteries (Y-a) and veins (Y-v). The dependence of T-1a on Y-a and Y-v has been elegantly characterized ex vivo; however, the combined influence of pO(2), Y-a and Y-v on T(1a)in vivo under normal ventilation has not been reported. Here, T-1a is calculated during hyperoxia in vivo by a heuristic approach that evaluates T-1-dependent arterial spin labeling (ASL) signal changes to varying gas stimuli. Healthy volunteers (n=14; age, 31.5 +/- 7.2years) were scanned using pseudo-continuous ASL in combination with room air (RA; 21% O-2/79% N-2), hypercapnic normoxic (HN; 5% CO2/21% O-2/74% N-2) and hypercapnic hyperoxic (HH; 5% CO2/95% O-2) gas administration. HH T-1a was calculated by requiring that the HN and HH cerebral blood flow (CBF) change be identical. The HH protocol was then repeated in patients (n=10; age, 61.4 +/- 13.3years) with intracranial stenosis to assess whether an HH T-1a decrease prohibited ASL from being performed in subjects with known delayed blood arrival times. Arterial blood T-1a decreased from 1.65s at baseline to 1.49 +/- 0.07s during HH. In patients, CBF values in the affected flow territory for the HH condition were increased relative to baseline CBF values and were within the physiological range (RA CBF=36.6 +/- 8.2mL/100g/min; HH CBF=45.2 +/- 13.9mL/100g/min). It can be concluded that hyperoxic (95% O-2) 3-T arterial blood T-1aHH=1.49 +/- 0.07s relative to a normoxic T-1a of 1.65s. Copyright (c) 2015 John Wiley & Sons, Ltd