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**QUANTIFICATION OF PERFUSION CHANGES DURING A MOTOR TASK USING ASL.**

P. VILELA (1), M. PIMENTEL (2), I. SOUSA(3), P. FIGUEIREDO(3) 1 – NEURORADIOLOGY - IMAGING DEPARTMENT, HOSPITAL DA LUZ, LISBON, PORTUGAL; 2 - FACULDADE DE CIÊNCIAS E TECNOLOGIA, UNIVERSIDADE NOVA, LISBON, PORTUGAL; 3 - INSTITUTO SUPERIOR TÉCNICO, TECHNICAL UNIVERSITY, LISBON, PORTUGAL.

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**rest CBF; activation CBF; Δ CBF; % Δ CBF**

Objectives: Quantification of the CBF variation induced by the neural activity during a common motor task (finger tapping). rest CBF; activation CBF; Δ CBF; % Δ CBF CMRO2

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Material & Methods Subjects: 15 healthy volunteers (6F/9M, mean age 25.6) Stimulus: finger tapping by sequential thumb-digit opposition of the right hand Acquisition: 3.0T MRI system (Siemens Magnetom Verio) Paradigm Protocol: #1 ASL: 1 cycles rest/task (total acquisition time 3min51secX2). #2 BOLD-ASL: 5 cycles rest/task, 25 sec blocks (total acquisition time 4min12.5sec). 3.51 min 25 sec

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Material & Methods Subjects: 15 healthy volunteers (6F/9M, mean age 25.6) Stimulus: finger tapping by sequential thumb-digit opposition of the right hand Acquisition: 3.0T MRI system (Siemens Magnetom Verio) ASL imaging: TR/TE = 2500ms/25ms 9 contiguous slices; 8mm slice thickness; 91x2 volumes, matrix 64x64; voxel size 3x3x8 mm3 (pulsed ASL sequence: PICORE Q2TIPS TI1 = 700 ms, TI1s = 1600 ms and TI2 = 1800 ms ) EPI BOLD-ASL imaging: TR/TE = 2500ms/11ms 9 contiguous slices; 8mm slice thickness; 101 volumes, matrix 64x64; voxel size 3x3x6 mm3 (pulsed ASL sequence: PICORE Q2TIPS TI1/TI2=700ms/1800ms)

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**Material & Methods Cluster of activation: Z map Threshold: Z > 2.5**

Analysis: Standard General Linear Model (GLM) approach using FEAT from FSL [ Pre-processing: motion correction, spatial smoothing FWHM = 5 mm, high-pass temporal filtering (f = 100ms) Protocol #1 Concatenation of the rest and activation scans in one single time-series data with the elimination of the first volume of each time-series, creating a single time-series dataset with 180 volumes (first 90 volumes – rest state and the other 90 volumes - activation state). Analysis: Cluster of activation: Z map Threshold: Z > 2.5 Cluster significance threshold: p < 0.05 Protocol #1 Concatenation of the rest and activation scans in one single time-series data with the elimination of the first volume of each time-series, creating a single time-series dataset with 180 volumes (first 90 volumes – rest state and the other 90 volumes - activation state). Protocol #2: concatenate the images of the 5 blocks in rest and images of the 5 five blocks in activation, in order to produce also two quantitative maps for both conditions. For each subject and for each protocol, the respective CBF activation clusters given by statistical analysis were used to mask the respective quantitative maps, and the mean value within the cluster was calculated.

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Material & Methods The respective CBF activation clusters given by statistical analysis were used to mask the respective quantitative maps, and the mean value within the cluster was calculated. Clusters of activation: CBF Quantification (ml / 100g / min) CBF rest; CBF activation Δ CBF = CBFact – CBFrest % Δ CBF = 100 x (CBFact – CBFrest) / CBFrest

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**Clusters of activation: CMRO2 Quantification**

Material & Methods The respective CBF activation clusters given by statistical analysis were used to mask the respective quantitative maps, and the mean value within the cluster was calculated. Clusters of activation: CMRO2 Quantification fractional BOLD signal change: ∆S/S0 (Davis et al. 1998). α = 0.38 (Grubb et al.1974; Mandeville et al. 1998). β = 1.3 (Bulte et al. 2009) M value= 4.3 (Chiarelli et al. 2007)

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**Results: Rest CBF Analysis**

Protocol #1 mean rest CBF: 61.0 ml/100g/min Protocol #2 mean rest CBF: 69.4 ml/100g/min The statistical analyses were performed by means of the MATLAB software and the Statistical Package for the Social Sciences (SPSS) Version For all statistical tests described below, significance was accepted if p<0.05. Values Mean value

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**Results: Activation CBF Analysis**

Protocol #1 mean activation CBF: ml /100g/min Protocol #2 mean activation CBF: ml/100g/min Values Mean value

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**Results: Δ CBF Analysis**

Protocol #1 Δ CBF: 43.7 ml/100g/min Protocol #2 Δ CBF: 40.5 ml/100g/min Values Mean value

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**Results: %Δ CBF Analysis**

Protocol #1 %Δ CBF: 73±6 % - Protocol #2 %Δ CBF: 62±7 % Values Mean value

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**Results: summary Protocol #1 Protocol #2 mean rest CBF mL/100g/min 61**

69.4 mean activation CBF mL/100g/min 104.8 109.9 Δ CBF mL/100g/min 43.7 40.5 %Δ CBF 73.6% 62.7%

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**Results: summary ∆CBF = rCBFact – rCBFrest GM WM**

Tissue type: p < 0.001; Correction: p = 0.031; Segmentation method: p < 0.001 GM WM

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**Results: CMRO2 Cerebral metabolic rate of oxygen (CMRO2). Evaluation**

Protocol #2 9 volunteers Mean %GM: 49% Mean %CBF: 62.49% (SE:8.52%) Mean BOLD SC: 0.71(SE:006%) Mean %CMRO2:22.56% (SE:5.48%) fractional BOLD signal change: ∆S/S0 (Davis et al. 1998). α = (Grubb et al.1974; Mandeville et al. 1998). β = 1.3 (Bulte et al. 2009) M value= 4.3 (Chiarelli et al. 2007) CMRO2 which have been shown to be tightly linked with changes in neural activity. BOLD signal calibration method for estimating changes in CMRO2, was first proposed by (Davis et al. 1998). This method is based on the measurement of BOLD and CBF responses to a hypercapnia which triggers cerebral vasodilatation so that CMRO2 levels remain constant (Moseley et al. 1992). The hypercapnic challenge is a physiologic means of manipulating CBF independent of CMRO2, and allows the computation of a calibration constant, M as described in equation 1 M Value review of the literature: varies between where the subscript HC means hypercapnic challenge, the subscript 0 denotes baseline values, the constant β reflects the influence of the deoxyhemoglobin concentration on the signal and the constant α translates the relation between CBV and CBF changes. Davis et al., found the value 1.5 for the constant β at 1.5T, this value was scaled (Bulte et al. 2009) for its field dependence assuming the value 1.3 at 3T. The value of α was calculated by (Grubb JR et al. 1974) and validated using contrast agents. This calibration of the BOLD signal against CBF allows evaluating variations across the brain in functionally induced changes in CRMO2, using any functional task. The CMRO2 changes are calculated as described in equation 2. The parameter M is the proportionality constant that reflects baseline deoxyhemoglobin content and defines the maximum possible BOLD signal change for that region. In the context of the model this parameter is proportional to the baseline blood volume fraction and O2 extraction fraction. The parameter α is the exponent in an assumed power law relationship between cerebral blood flow and cerebral blood volume, and is taken to be α = 0.38 (Grubb et al. 1974; Mandeville et al. 1998). The parameter β was introduced as an empirical description of the signal changes found in Monte Carlo simulation studies of spins diffusing near magnetized cylinders, a model for the vascular system (Boxerman et al. 1995b). In our studies the value is taken to be β = 1.5 (Boxerman et al. 1995a; Davis et al. 1998). The parameters α and β are assumed to be global properties with the same values in each subject, and all calculations are based on assumed values. The parameter M is a local parameter estimated from the hypercapnia experiment using Equation [1] with the assumption that mild hypercapnia does not alter CMRO2. Specifically, the ratio CMRO2/CMRO2o is assumed to equal one, and the measured changes in CBF and BOLD with hypercapnia within a specified region of interest (ROI) are combined with Equation [1] to calculate M. The derived value of M for each ROI was then applied to the activation data to calculate the stimulus-evoked change in O2 metabolic rate CMRO2/ CMRO2o. In addition to this primary use of the hypercapnia experiment to calibrate the BOLD effect, this data also provides a direct measure of the local vascular responsiveness, measured as the percent change in CBF divided by the change in end-tidal CO2 (mm Hg), which we define as the CBF Response to CO2 (CRC). Aerobic (oxidative) metabolism

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**Results: CMRO2 Cerebral metabolic rate of oxygen (CMRO2). Evaluation**

Protocol #2 9 volunteers Mean %GM: 49% Mean %CBF: 62.49% (SE:8.52%) Mean BOLD SC: 0.71(SE:006%) Mean %CMRO2:22.56% (SE:5.48%) CMRO2 / CBF: 0.33 (normal range ) Perfusion /CMRO2 Coupling CBF CMRO2 which have been shown to be tightly linked with changes in neural activity. BOLD signal calibration method for estimating changes in CMRO2, was first proposed by (Davis et al. 1998). This method is based on the measurement of BOLD and CBF responses to a hypercapnia which triggers cerebral vasodilatation so that CMRO2 levels remain constant (Moseley et al. 1992). The hypercapnic challenge is a physiologic means of manipulating CBF independent of CMRO2, and allows the computation of a calibration constant, M as described in equation 1 M Value review of the literature: varies between where the subscript HC means hypercapnic challenge, the subscript 0 denotes baseline values, the constant β reflects the influence of the deoxyhemoglobin concentration on the signal and the constant α translates the relation between CBV and CBF changes. Davis et al., found the value 1.5 for the constant β at 1.5T, this value was scaled (Bulte et al. 2009) for its field dependence assuming the value 1.3 at 3T. The value of α was calculated by (Grubb JR et al. 1974) and validated using contrast agents. This calibration of the BOLD signal against CBF allows evaluating variations across the brain in functionally induced changes in CRMO2, using any functional task. The CMRO2 changes are calculated as described in equation 2. The parameter M is the proportionality constant that reflects baseline deoxyhemoglobin content and defines the maximum possible BOLD signal change for that region. In the context of the model this parameter is proportional to the baseline blood volume fraction and O2 extraction fraction. The parameter α is the exponent in an assumed power law relationship between cerebral blood flow and cerebral blood volume, and is taken to be α = 0.38 (Grubb et al. 1974; Mandeville et al. 1998). The parameter β was introduced as an empirical description of the signal changes found in Monte Carlo simulation studies of spins diffusing near magnetized cylinders, a model for the vascular system (Boxerman et al. 1995b). In our studies the value is taken to be β = 1.5 (Boxerman et al. 1995a; Davis et al. 1998). The parameters α and β are assumed to be global properties with the same values in each subject, and all calculations are based on assumed values. The parameter M is a local parameter estimated from the hypercapnia experiment using Equation [1] with the assumption that mild hypercapnia does not alter CMRO2. Specifically, the ratio CMRO2/CMRO2o is assumed to equal one, and the measured changes in CBF and BOLD with hypercapnia within a specified region of interest (ROI) are combined with Equation [1] to calculate M. The derived value of M for each ROI was then applied to the activation data to calculate the stimulus-evoked change in O2 metabolic rate CMRO2/ CMRO2o. In addition to this primary use of the hypercapnia experiment to calibrate the BOLD effect, this data also provides a direct measure of the local vascular responsiveness, measured as the percent change in CBF divided by the change in end-tidal CO2 (mm Hg), which we define as the CBF Response to CO2 (CRC). CMRO2

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Conclusions These results show that both activation vs rest (protocol #1) and block design (protocol #2) functional protocols were capable to detect consistent variations in perfusion associated with a simple motor task. The block design has the advantages of requiring shorter acquisitions and allowing the acquisition of simultaneous BOLD contrast information, being the preferable approach for the evaluation of perfusion changes to endogenous stimuli. In fact, the BOLD activation results extracted from ASL protocol #1 aren’t consistently with the results given by the other experiments, as we can see in Figure 5.10; they don’t point the activation region near the HMC on the left side of the brain.

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Conclusions Perfusion ASL is a reliable method for the quantification (CBF) of the hemodynamic brain response to brain activation, and by combining the BOLD and CBF is able to estimate the cerebral metabolic rate of oxygen (CMRO2). This combined information (CBF & CMRO2) widens the scope of the ASL-fMRI applications as a non-invasive and reliable imaging approach to the study of the brain hemodynamic responses and metabolism activity.

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**Acknowledgments Authors: Technologists: Marco Pimentel Inês Sousa**

Patrícia Figueiredo Technologists: Ana Cristina Santos Cidália Martins Fernando Gonçalves Ruben Teixeira

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**QUANTIFICATION OF PERFUSION CHANGES DURING A MOTOR TASK USING ASL.**

P. VILELA (1), M. PIMENTEL (2), I. SOUSA(3), P. FIGUEIREDO(3) 1 – NEURORADIOLOGY - IMAGING DEPARTMENT, HOSPITAL DA LUZ, LISBON, PORTUGAL; 2 - FACULDADE DE CIÊNCIAS E TECNOLOGIA, UNIVERSIDADE NOVA, LISBON, PORTUGAL; 3 - INSTITUTO SUPERIOR TÉCNICO, TECHNICAL UNIVERSITY, LISBON, PORTUGAL.

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