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UW-Madison PHYSICS 208 - Biomagnetism

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1BiomagnetismRon WakaiProfessorDepartment of Medical PhysicsMagnetism in Medicine: A bad beginningFranz Mesmer (1734-1815)• “discoverer” of “animalmagnetism”Some current “applications” of magnetism:Current Medical Applications of MagnetismMagnetic resonance imaging (MRI)Transcranial magnetic stimulation (TMS)Magnetoencephalography (MEG) Nerve and muscle cells act like tiny batteries that drive ionic currents • current dipole, Q (current source) • volume current, JV ! magnetic field, B Ionic currents produce electric and magnetic signals:• electrocardiogram (ECG) and magnetocardiogram (MCG)• electroencephalogram (EEG) and magnetoencephalogram (MEG)**MEG provides improved source localizationOrigin of Surface Electric vs. Magnetic SignalsExtracellular current (volume current)  surface electric potentials• topography is distorted by inhomogenous conductivityIntracellular current (primary current)  surface magnetic fields• topography of magnetic signals is distorted much less, allowing accurate source localizationNeuromagnetism• Dendritic activity gives rise to EEG/MEG• Neurons organized in columnar arrangement• Need >1000 neurons to get detectable signal• Focal activity produces dipolar spatial pattern• Source lies below phase inversion2! geofield! typical urban magnetic noise! magnetic field of magnetic contaminants in lung! magnetocardiogram! fetal magnetocardiogram! spontaneous magnetoencephalogram! evoked magnetic brain responsesInstrumentation Requirements1. extremely sensitivity detector: SQUID(superconducting quantum interference device)2. noise rejection: magnetically shielded room (high-magnetic permeability alloy, aluminum)Whole-head system148 channelsDual-system system74 channelsSQUID (Superconducting Quantum Interference Device)Superconducting loopinterrupted by two “weak-links” (Josephson tunneljunctions)Current-voltage characteristic• current at zero voltage (supercurrent, Ic)Modulation of I-V Characteristic by Magnetic Signal•supercurrent show interferencepattern Ehrenberg-Siday-Aharonov-Bohm EffectSQUID1. particle beam divides2. paths enclose region of flux3. vector potential, A, alters phase(B= ∇xA)phase advancesphase retards3Current-Biased SQUID: Periodic V vs. ΦHigh Magnetic Permeability Shieldingmu-metalEddy-Current ShieldingApproximate weight: 7,000 kgApproximate cost: $350,000Magnetically ShieldedRoomtime-varying magneticinterferenceeddy-currentsEssential for low frequency shieldingHighly cost-effectiveMagnetic source imaging (MSI)= MEG+MRIwhole-head mapping ofauditory evoked responseisofield contourmapdipole overlayNakasato et al., 1995Statement of problem: To compute the distributionof brain currents based on measurement ofexternal magnetic field**Has no unique solution**Modeling Assumptions• Dipole approximation: signal due to collection ofcurrent dipoles—localized current elements—thatlie on convoluted cortical surface• Homogeneous sphere model: model head ashomogenous conducting sphereInverse ProblemPositron Emission Tomography (PET)Functional Magnetic Resonance Imaging (fMRI)Functional brain imaging techniques spatial temporal brainmodality resolution resolution region cost1. positron emission 1 cm minutes whole $3Mtomography (PET) brain2. functional magnetic 1 cm seconds whole $2Mresonance imaging (fMRI) brain3. electric source imaging >1cm msec cortex $100k4. magnetic source imaging 1 cm msec cortex $2M(MEG + MRI)MSI: records cortical activity directly and with high temporal resolution, butsuffers from an ill-posed inverse problem4Clinical applications• presurgical functional mapping– somatosensory, auditory– language• epilepsy– localization of epileptogenic foci– most useful for neocortical epilepsyapproved for clinical use in 2002Magnetic Localization of Somatosensory Cortexsites of mechanical stimulationsites of cortical responseNakamura et al., Neuroimage 7(4 Pt 1):377 (1998)Presurgical mapping of central sulcusCentral sulcus, containing somatosensory cortex, wasat A, not B, allowing surgical resection of tumorOrrison et al., Perspect Neurol Surg 4:141 (1993)Language Mapping: Response to Visually Presented WordsSimos et al., J Clin Exp Neuropsych 1998; 5:706 language activity sensory responseSensory andlanguage responsesare temporallydistinctValidated viaintraoperativemappingMagnetoencephalographic mapping of theMagnetoencephalographic mapping of thelanguage-specific cortex.language-specific cortex.Papanicolaou, SimosPapanicolaou, Simos et al. J et al. J NeurosciNeurosci 1997 1997Language LateralizationNRNLNRNLLI+!=Picture NamingSalmelin et al., Nature 368:463-8 (1994)5Application of MEG to Epilepsy• Incidence of about 1%• 10-20% of epilepsy is intractable to drug therapyPresurgical evaluation• Focal or generalized?• Number and relative timing of foci (primary vs.secondary focus)• Location with respect to eloquent cortexMirror focusPrimary focusTransmission of electric and magnetic fetal signalsprimary currents→ fMCG,fMEGvolume currents → fECG,fEEG•fECG is attenuated strongly by thepoor conductivity of vernix caseosa•fMCG is affected much lessFetal Magnetocardiography (fMCG)6FMCG FROM 17-38 WEEKS FROM SAME SUBJECTInformation Content of fMCGRR interval fetal heart rate (FHR), FHR variabilityQRS amplitude variations fetal (trunk) movementP-, QRS-, T-wave timing and morphology fetal rhythm•RRRQRSampPRQTQRSFetal Heart Rate (FHR) Monitoring• Primary clinical method• Specific for hypoxiaReassuring FHR patterns• reactivity: FHR accelerations associated with movement• normal FHR variabilityOminous FHR patterns• decelerations that are late or variable with respect touterine contractions• absent variabilityMovement-related Signal Amplitude Variation:fMCG Actography• fetal trunk movement results in prominent movement artifact• instantaneous QRS amplitude serves as actogram tracingAssessing fetal activity from the fMCG:fMCG Actocardiography• 2 pumps: left (systemic), right (pulmonary)• 4 chambers: 2 atria, 2 ventricles valves at inlet and outlet of ventriclesThe Heart7Unique Electrical Properties of Cardiac Tissue!threshold1. rhythmic self-excitation - pacemaker tissue3. continuous conducting surface- myocardial syncytium2. long refractory period- 250 ms (vs. 2 ms for neurons)repolarizationdepolarizationrefractory periodMechanisms of


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UW-Madison PHYSICS 208 - Biomagnetism

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