1!Bioengineering 280A"Principles of Biomedical Imaging""Fall Quarter 2010"MRI Lecture 7"!http://www.humanconnectomeproject.org/overview/!fMRI!MRI studies brain anatomy. Functional MRI (fMRI) studies brain function. http://defiant.ssc.uwo.ca/Jody_web/fmri4dummies.htm!fMRI Setup!http://defiant.ssc.uwo.ca/Jody_web/fmri4dummies.htm!2!fMRI Acquisition!High spatial resolution High temporal resolution!MP-RAGE!Voxel volume: 1 mm3!Imaging time: 6 min!EPI!Voxel volume: 45 mm3!Imaging time: 60 msec!Buxton 2002!History of Functional MRI!Source: Ogawa et al., 1992 Finger Tapping Task!Functional MRI!3!Cabernet and fMRI!Hemoglobin and Field Inhomogeneities!Oxygen binds to the iron atoms to form oxyhemoglobin HbO2Release of O2 to tissue results in deoxyhemoglobin dHBO2http://www.people.virginia.edu/~rjh9u/hemoglob.html!B0Field MapsMore dHB Less dHB Signal Decay!time0! TESome dHB, Some dephasingMore dHB, More dephasing, Decrease in MR signal"Blood Flow and Oxygen Metabolism!Cerebral Blood Flow (CBF) measures delivery of blood to brain tissue (units of ml/(g-min))!Cerebral Metabolic Rate of (CMRO2) is the rate of oxygen consumption (units of µmol/(g-min))!CBF [O2]arterial!CMRO2!Oxygen extraction fraction (E)!CMRO2= E CBF [O2]arterial!4!"Deoxyhemoglobin!= CMRO2 / 4CBFCMRO2 [dHB]venous !CBF[dHB]venous !O2![dHB]venous ≈ E [O2]arterial / 4!Thomas Liu, BE280A, UCSD, Fall 2008!fMRI: Spatial Temporal Dynamics!arteriole venule capillary bed CMRO2 CBF oxyHb deoxyHb Initial dip Positive BOLD Post-stimulus Response Neural activity CMRO2 CBV CBF dHb CMRO2 CBV CBF dHb CMRO2 CBV CBF dHb Task-Based fMRI!Fox and Raichle; Nature Rev. Neuro, 2007!Resting-State fMRI!Fox and Raichle; Nature Rev. Neuro, 2007!5!Bullmore and Sporns 2009!GraphsVertex/Node!Edge/Connection!Number of edges connected to a node is the degree k of the vertex!GraphsBearman et al, J. Sociology, 2004!GraphsChristakis and Fowler, NEJM, 2007!6!Graph Metrics• Degree: Number of edges connected to a vertex. • Characteristic Length (L): Number of edges in shortest path between two vertices, averaged over all vertices.• Clustering Coefficient (C): A measure of the extent to which neighboring nodes are also directly connected. (i.e. a measure of how many of your friends are also friends of each other; cliquishness). Measure of resilience to random error – if vertex is lost, its neighbors still stay connected. Wang and Chen, 2003!Shortest Path AlgorithmClusteringLow Clustering! High Clustering!7!Wang and Chen, 2003!Each vertex connected to its nearest neighbors. High C and High L. !Small fraction of edges randomly rewired. High C and low L. !Low C and low L!Small World NetworksWatts and Strogatz 1998!Regular! Small World!Random!SynchronizabilityWang and Chen, 2003!Stam and Reijneveld 2007!! "SW=C /CRandomL /LRandom8!J. Clarke, UC Berkeley!J. Clarke, UC Berkeley!9!J. Clarke, UC Berkeley!Seeley et al, JMR 2004!Seeley et al, JMR 2004!Compressed Sensing!FT ↓ Randomly throw away 83% of samples * E.J. Candes, J. Romberg and T. Tao. Slide Credit: http://www.stanford.edu/~mlustig/!10!Compressed Sensing!FT ↓ Minimum - norm conventional linear reconstruction!Min. Total Variation (TV)"A convex non-linear reconstruction!* E.J. Candes, J. Romberg and T. Tao. Slide Credit:
View Full Document
Unlocking...