|Hemodynamic Stress & Metabolic Measure|
Caffeine Does Not Affect Regional Vascular Reactivity
Yufen Chen1, 2, Todd B. Parrish1
1Northwestern University, Chicago, Illinois, USA
Caffeine is a widely consumed methylxanthine that acts as an antagonist to adenosine and decreases cerebral blood flow through vasoconstriction. Previous work using ultrasound has shown that there are no differences in vasoreactivity in the middle cerebral arteries; however, detailed regional analysis has not been done to date. In this study, we use a combination of arterial spin labeling and BOLD to demonstrate that caffeine does not alter vascular reactivity to CO2 in motor and visual cortices.
Negative BOLD Signal Under 6% Hypercapnia Reflects
Solely Oxygen Extraction from the Blood
Anne-Catherin Zappe1, Kamil Uludag1, Nikos K. Logothetis1, 2
1Max-Planck-Institute for biological Cybernetics, Tuebingen, Germany; 2University of Manchester, Manchester, UK
The BOLD signal is sensitive to cerebral blood flow (CBF), blood volume (CBV) and oxygen extraction. In the anesthetized monkey, we observe a vasodilatory ceiling effect during inhalation of 6% CO2 where CBF or CBV are not further increased by visual stimulation. In contrast, simultaneously measured local field potential responds to the stimulation as strong as during normocapnia. As a consequence, the stimulus-induced fMRI response during 6% hypercapnia has been found to be negative reflecting only the oxygen extraction from the blood. With this method, oxygen extraction can be imaged by means of fMRI without injection of an exogenous drug.
Validation of Oxygen Extraction Fraction
Measurement by QBOLD Technique
Xiang He1, Mingming Zhu1, Dmitriy A. Yablonskiy1
1Washington University in St Louis, School of Medicine, St. Louis, USA
Measurement of brain tissue oxygen extraction fraction (OEF) can provide important information on brain functioning in health and disease. The recently proposed quantitative BOLD (qBOLD) model allows for separation of contributions to the BOLD signal from OEF and deoxygenated blood volume (DBV). The objective of this study is to validate qBOLD OEF measurements in a rat model. The qBOLD OEF results averaged across entire brain were compared with the oxygenation of venous blood samples drawn directly from the superior sagittal sinus and measured with a blood gas analyzer. The results demonstrate a very good agreement between qBOLD and direct measurements.
Non-Invasive Quantification of Venous Oxygenation and
Cerebral Metabolic Rate of Oxygen in Humans
Feng Xu1, Hanzhang Lu1
1University of Texas Southwestern Medical Center, Dallas, USA
T2-Relaxation-Under-Spin-Tagging (TRUST) MRI, a recently developed technique to quantify cerebral venous oxygenation (Yv) non-invasively, is applied to measure internal jugular veins (IJV) Yv. The blood T2 values in the IJVs were 62.8±10.6ms, corresponding to a Yv of 64.3±7.2%. Compared with sagittal sinus (SS), whose T2 is 57.9±9.1ms and Yv was 62.2±6.1%, no significant difference is found by pair t-test (p=0.13, n=12). The IJV Yv is further combined with venous flux (52.7±11.1ml/100g brain/min) acquired by phase contrast (PC) scan for eight subjects, giving whole-brain cerebral metabolic rate of oxygen (CMRO2) of 1.46± 0.20 µmol/g/min.
Estimation of the Regional Cerebral
Metabolic Rate of Oxygen Consumption with MRI During the First 60
Seconds of 17O2 Inhalation in Swine
Eric Albert Mellon1, Reddy Shashank Beesam1, James E. Baumgardner1, Arijitt Borthakur1, Walter R. Witschey1, Ravinder Reddy1
1University of Pennsylvania, Philadelphia, Pennsylvania, USA
The non-radioactive, naturally-occurring, NMR active isotope 17O can be used for the non-invasive, non-toxic, and widely-employable quantification of tissue metabolism. We demonstrate measurement of CMRO2 in pigs before recirculation. A custom circuit for precise 17O2 delivery provides a step change of mouth gas concentration. A simple model of lung mixing is presented. Analysis of arterial H217O shows the time until recirculated H217O starts to be 60-80 seconds. Indirect and direct imaging is correlated to estimate CMRO2 non-invasively during that time. Only one breath of 17O2 is used, reducing cost significantly. For indirect imaging, only a clinical 1.5T scanner is required.
The Relation Between BOLD Amplitude and Baseline
Cerebral Blood Flow Depends on the Analysis Scale
Joy Liau1, Joanna Perthen1, Thomas T. Liu1
1University of California at San Diego, La Jolla, California , USA
The baseline cerebral blood flow (CBF) may influence the BOLD response amplitude through: 1) changes in the maximum BOLD response and 2) changes in the functional CBF response. In this study, we show that changes in the functional CBF response dominate BOLD variability in subject-wise measurements while changes in the maximum BOLD response dominate in voxel-wise measurements. These results are consistent with the theory that the oxygen extraction fraction is inversely related to resting blood flow across subjects but is relatively independent of baseline blood flow across voxels.
Normalization of FMRI Signal with Basal
Physiologic State Improves Sensitivity in Differentiating Subject Groups
Hanzhang Lu1, Uma Yezhuvath1, Kelly Lewis-Amezcua1
1University of Texas Southwestern Medical Center, Dallas, USA
We have previously shown that cerebral venous oxygenation can be quantified with TRUST MRI and that stimulation-evoked fMRI signal is inversely correlated with baseline oxygenation across subjects. Here we aim to test the utility of TRUST MRI in distinguishing subject groups. Checkerboards flashing at 8Hz and 4Hz were presented to two healthy control groups, respectively. Comparison between the groups with and without using TRUST MRI showed that the inter-subject variation was substantially reduced and group separation was more significant after the normalization. These results suggest that TRUST MRI may be beneficial for fMRI studies of psychiatric and neurological disorders.
Reproducibility of BOLD Signal Change Induced by
Stefano Magon1, Gianpaolo Basso2, Paolo Farace1, Giuseppe K. Ricciardi3, Andrea Sbarbati1, Alberto Beltramello3
1University of Verona, Verona, Italy; 2University of Trento, Mattarello, Italy; 3Borgo Trento Hospital, University of Verona, Italy
The aim of the present study was to investigate the changes and reproducibility of BOLD signal, within and across subjects, in response to different Breath Holding (BH) durations (9, 15 or 21 seconds). The parameters of the BOLD BH-induced response (Percent Signal Change, Time To Peak, Integral of Subtended Area and the number of activated voxels) were significantly different among the three BH durations. The reproducibility of the signal is directly dependent on the duration of the BH, being better for longer BH, possibly because subjects brain vascular reactivity is pushed toward its maximum
Hypercapnia-Based Calibration Techniques for
Measurement of Cerebral Oxygen Metabolism with MRI
Knut Drescher1, Peter Jezzard2, Daniel Bulte2
1University of Cambridge, Cambridge, UK; 2University of Oxford, Oxford, UK
Three different methods of obtaining a hypercapnic state were compared for their applicability as calibration steps for functional MRI. Breathing carbon dioxide in air, carbon dioxide in oxygen and breath holding were investigated in healthy volunteer subjects to determine the effects that each technique had on physiology, flow, BOLD signal and relaxation times. Of the three methods, breathing an increased fraction of CO2 in air proved to be the preferred option, having the fewest complications which limit the effectiveness of the models in regards to changes in arterial oxygen saturation and apparent changes in metabolism.
Measuring the Effect of Hyperoxia and Hypercapnia on
R2* and the Balanced SSFP Signal at 3T
Stefanie Winkelmann1, Hannes Dahnke1, Sebastian Flacke2, Martin Söhle2, Ingobert Wenningmann2, Attila Kovacs2, Frank Träber2, Juergen Gieseke3, Hans Heinz Schild2, Petra Mürtz2
1Philips Research Europe, Hamburg, Germany; 2University Hospital Bonn, Bonn, Germany; 3Philips Medical Systems, Hamburg, Germany
The effect of hypercapnia (elevated level of CO2) and hyperoxia (elevated level of O2) on the MRI signal is of major diagnostic value in the field of oncology as it gives insight into a wide range of tumor parameters, e.g. the vascular growth, hemodynamic changes in the response to treatments, vasoreactivity, vascular function and vessel maturation. This work compares the MR response of R2* and the balanced SSFP signal to hyperoxia and hypercapnia in terms of time course and sensitivity, using respiratory challenges in 5 healthy volunteers during MR imaging.