DSC, Oxygenation & Reactivity
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Tuesday 8 May 2012
Room 202  10:00 - 12:00 Moderators: Fernando Calamante, Greg Zaharchuk

10:00 0195.   
Non-Parametric Quantification of Cerebral Haemodynamics from Dynamic Susceptibility Contrast MRI
Amit Mehndiratta1, Bradley J MacIntosh2, David E Crane2, Stephen J Payne1, and Michael A Chappell1
1Institute of Biomedical Engineering, University of Oxford, Oxford, Oxfordshire, United Kingdom, 2Medical Biophysics, University of Toronto, Toronto, ON, Canada

DSC-MRI analysis is an ill-posed inverse problem involving deconvolution of the observed MR signal with an AIF. The current standard SVD methods underestimate perfusion with an oscillatory residue function whereas the alternative vascular model-based (VM) approach permits only predefined shapes that may not be appropriate in pathology. Here we propose a deconvolution method that can estimate perfusion along with a physiological plausible residue function without any bias to a specific class of functional shapes. Our results showed the perfusion estimates were comparable to VM and the method formulation ensures physiologically realistic smooth functions.

10:12 0196.   
Steady-State and Dynamic Susceptibility Contrast using USPIOs in Humans
Thomas Christen1, Deqiang Qiu1, Wendy Wei Ni1, Heiko Schmiedeskamp1, Roland Bammer1, Michael Moseley1, and Greg Zaharchuk1
1Department of Radiology, Stanford University, Stanford, California, United States

In this study, we acquired both steady-state and dynamic susceptibility CBV maps using ferumoxytol (an FDA-approved ultra-small paramagnetic iron oxide (USPIO) compound) in 4 volunteers and compared the quantitative values at different doses and spatial resolutions. The results show similar patterns between all the maps and average blood volumes that are consistent with prior literature values. The study suggests that high-resolution quantitative CBV maps can be obtained with the proposed steady-state approach and could be used to detect smaller lesions.

10:24 0197.   Dynamic susceptibility Contrast MRI: Compromising Perfusion Accuracy for a Better Discrimination of Hypoperfused Tissue
Birgitte Fuglsang Kjølby1, Søren Christensen2, Irene Klærke Mikkelsen1, Kim Mouriden1, Peter Gall3, Valerij G Kiselev3, and Leif Østergaard1
1CFIN, Department of Neuroradiology, Aarhus University Hospital, Aarhus, Denmark, 2Department of Neurology and Radiology, University of Melbourne, Melbourne, Australia, 3Department of Diagnostic Radiology, Medical Physics, University Hospital Freiburg, Freiburg, Germany

In perfusion DSC-MRI, the precision (random error) and accuracy (systematic bias) of perfusion estimates rely critically on the noise regularization used in the deconvolution process. Existing methods are commonly optimized for the best reproducibility of ’true’ perfusion values. We show that this accuracy is obtained at the expense of precision, which negatively impacts the ability to identify critical hypoperfusion thresholds. We propose a frequency-domain optimized regularization favoring precision. This approach reveals that optimal regularization depends critically on signal to noise ratio, sampling rate and AIF shape. Application of this method to simulated data improves discrimination of hypoperfused tissue.

10:36 0198.   
Characterizing the Susceptibility Calibration Factor in Heterogeneous Vascular Networks
Natenael B Semmineh1, Junzhong Xu2, and C Chad Quarles3
1institute of imaging science, Vanderbilt University, nashville, TN, United States, 2Vanderbilt University, 3institute of Imaging science, Vanderbilt University, Nasville, TN

The use of DSC-MRI in tumors can be confounded by the assumption that a linear relationship, with a spatially uniform rate constant termed the vascular susceptibility calibration factor (kp), exists between the contrast agent (CA) concentration and the measured transverse relaxation rate change. Using simulations we demonstrate that vascular susceptibility calibration factors found in tumor-like vessel trees are significantly different than those found in normal tissue.

10:48 0199.   In vivo imaging of vessel diameter, size and density: a comparative study between MRI and histology
Benjamin Lemasson1,2, Samuel Valable1,3, Régine Farion1,2, Alexandre Krainik1,2, Chantal Rémy1,2, and Emmanuel L Barbier1,2
1U836, Inserm, Grenoble, France, 2Grenoble Institut des Neurosciences, Université Joseph Fourier, Grenoble, France, 3UMR 6232, CNRS, Caen, France

The purpose of the study was to compare MRI and histological estimates of the mean vessel diameter (mVD), the vessel density (Density), and the vessel size index (VSI) obtained in the same tumor-bearing animals (C6 and RG2 models). MRI and histology differed by -15 to 26%. A positive correlation was found between MRI and histology for mVD, Density, and VSI counterparts (R²=0.62, 0.50, 0.73, respectively; p<0.001 in all cases). As Density and mVD or VSI provide complementary information, it is worth computing them to characterize angiogenesis beyond blood volume fraction.

11:00 0200.   
Is the T2* relaxivity of gadolinium in brain microvasculature linear with concentration?
Vishal Patil1, and Glyn Johnson1
1Radiology, NYU School of Medicine, New York, New York, United States

Generally, linearity is assumed between T2* relaxation and gadolinium concentration, but it is well known that compartmentalization and secondary magnetic field perturbations generate deviations from linearity in vivo. In this study we test the reliability of both linear and non-linear relaxivity expressions estimating cerebral blood volume in grey and white matter at different field strengths and echo times. Results show that the non-linear expression yields remarkable agreement between tissue measurements while the linear expression systematically over and under estimates blood volume depending on imaging parameters, emphasizing the problem in finding a single linear relaxation relationship that fits multiple field strengths.

11:12 0201.   Vessel Size Index and Cerebral Blood Volume Maps using Hypercapnic Contrast at 3T
Thomas Christen1, Georges Hankov1, Heiko Schmiedeskamp1, Roland Bammer1, and Greg Zaharchuk1
1Department of Radiology, Stanford University, Stanford, California, United States

The objective of this study was to use the variations in R2 and R2* relaxation times induced by the inhalation of carbogen (95% O2, 5% CO2) to create parametric maps of the Vessel Size Index and Cerebral Blood Volume. The use of a gradient and spin echo EPI sequence allowed the acquisition of VSI and CBV values simultaneously with a high temporal resolution. This enabled the determination of a stable period during which the estimates are accurate. The measurements were performed in 5 healthy volunteers at 3.0T and the values obtained were in accord with past studies.

11:24 0202.   Assessment of Regional Rates of Change in CBF in Response to Changes in PaCO2: A Combined ASL and Phase Contrast Study
Noam Alperin1, Ahmet M Bagci1, Jessica Schmidtman1, Andreas Pomschar2, Birgit Ertl-Wagner2, and Clinton Wright1
1University of Miami, Miami, FL, United States, 2University of Munich, Munich, Germany

A method for assessment of regional cerebral blood perfusion in response to changes in PaCO2 is presented. The method combines PASL in conjunction with anatomical T1W imaging for assessment of regional CBF values at different states of PaCO2 and phase contrast based measurements of total CBF. Since it is well established that global CBF is linearly related to PaCO2, the phase contrast based tCBF measurements eliminates the need for direct measurements of PaCO2, which are invasive. Preliminary results in healthy subjects demonstrate differences in auto regulation between brain regions supplied by the anterior and posterior circulation.

11:36 0203.   
Calibration and Implementation of Quantitative Blood Oxygenation Measurement at 7T
Lisa C. Krishnamurthy1,2, Jinsoo Uh1, Ivan Dimitrov1,3, Feng Xu1, Peiying Liu1, Kim Kangasniemi1, and Hanzhang Lu1
1Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, TX, United States, 2Dept. of Biomedical Engineering, UT Arlington, Arlington, TX, United States, 3Philips Medical Systems, Cleveland, OH, United States

Quantification of cerebral venous oxygenation have demonstrated potential utility in normalization of fMRI signals, evaluation of brain metabolism, and understanding brain disorders. To date, all such studies have been performed at the field strength of 3T or lower. Given the field dependence of deoxyhemoglobin susceptibility effects, it is reasonable to expect that 7T may provide an advantage in improving the sensitivity of these techniques. We have established a calibration plot between blood T2 and oxygenation at 7T for various hematocrit levels. We have also implemented a recently developed TRUST MRI technique at 7T and determined venous blood T2 in vivo.

11:48 0204.   
Cerebrovascular reactivity in the brain white matter: magnitude, temporal delays, and age effects
Binu P. Thomas1,2, Peiying Liu1, Denise Park3, Matthias J.P. van Osch4, and Hanzhang Lu1
1Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, TX, United States, 2Bioengineering, University of Texas Southwestern Medical Center/University of Texas at Arlington, Arlington, TX, United States, 3Center for Vital Longevity, University of Texas at Dallas, Dallas, TX, United States, 4Radiology, Leiden University Medical Center, Leiden, Netherlands

Vascular properties of brain’s white matter (WM) were examined. Cerbrovascular reactivity (CVR), baseline cerebral blood flow (CBF) and structural MPRAGE scans were obtained from healthy volunteers: 15 young (27±5) and 15 elderly (75±7) years. Temporal delay in BOLD CVR response from gray matter (GM) to WM was calculated. Response was slower in young (20.8 sec) and faster in older subjects (12.26 sec), (p=0.003). The response amplitude (%BOLD/mmHgCO2) is higher in WM (p=0.048) and lower in GM (p=0.005) in older subjects compared to young. Baseline CBF was higher in WM (p=0.072) and lower in GM (p=0.001) in old compared to young.