SQUID sensors a successful complementary electrophysiological instrument for

SQUID sensors a successful complementary electrophysiological instrument for imaging brain and nerve activities Zvonko Trontelj Physics Dept. , University of Ljubljana and Institute for Mathematics, Physics and Mechanics, Ljubljana

Participating Researchers University of Ljubljana: Vojko Jazbinsek, Matjaz Slibar, Ales Stampfl, Robert Zorec PTB Berlin: Sergio Erné, Lutz Trahms, Martin Burghoff Wolfgang Mueller, Gerd Wuebbeler. FU Berlin: Gabriel Curio, Peter Aust TU Darmstadt: Gerhard Thiel, Michael Wacke Vanderbilt University: Franz Baudenbacher, Luis Fong, Jenny Holzer, John Wikswo Producers of instrumentation: Neuromag-Electa, CTF

Outline of talk § § § § Introduction On SQUID sensor Measuring systems Basic steps in data analysis and modeling of sources of bioelectric activity Examples: a)Peripheral nerve studies b) Some examples of brain studies Conclusions

Objectives: § To apply SQUID(s) in order to obtain the noninvasive information on: a) Ionic currents in electrically stimulated peripheral nerve medianus. b) Localizations of epileptical focus. c) Localization and functional information on some parts of brain cortex. § To demonstrate the pre-diagnostic capabilities of SQUID(s) § To model the intracellular curent (m. field) and its relation to AP

SQUID sensors 1. What is SQUID? 2. What they offer to us? 3. Where we can use them? 4. Why SQUID sensors in electrophysiology?

Ad 1 and Ad 2 § Superconducting QUantum Interference Device § Magnetic flux-to-voltage convertor (the most sensitive sensor for quasi dc magnet. fields m. ) § Measured m. field; via Amper’s law the source § Based on 3 facts described by QM - superconductivity with Cooper pairs - C. pair tunneling - m. flux quantization

From Josephson jct. to closed sc. circuit

Dc SQUID configuration

Outer magnetic field is present at the SQUID

Ad 3 M. flux has to be transp. to SQUID and ad Ad 4 - SQUID has to be in m. shielded env. High sensitivity and spatio. temp. r.

Multichannel SQUID system for brain studies

Multichannel SQUID system for brain staudies

Part of SQUID microscope and C. c. internodal cell holder § We measure: § Electric AP § K+ anesthesia technique § Magnetic field § SQUID Microscope § Both measurements are simultaneous

SQUID microscope prepared for the C. c. inernodal cell studies (schematically)

Basic steps in analysis and modelling of current sources in living matter § § Distribution of ionic currents in tissue. Complicated Direct and inverse problem The direct problem – a unique solution F T = ZT E F E § The inverse problem is ill-posed problem § F E = ZET FT § Simple geometry – analyt. solutions, otherweise modeling

Simple geometry § Single cylindrically shaped cell (1 D case) § DF = 0 § Bound. cond. : Fm(z) = Fi (a, z) – Fe (a, z) § n. Ji (a, z) = n. Je(a, z)

Simple geometry (contin. ) § From Ampere law: § Bi = Integr. [G(r, a, z – z’)Ji(a, z’)]dz § Applying the Fourier and the inverse Fourier transformations one can come from potential to mag. field and v. a. v.

Some methods in modeling of current sources § Current multipole expansion § Current distribution with the minimum norm estimation § Covariance method (to extract the dc component of the measured modulated magnetic field data)

Part of an axon or(C. Corallina intern. Cell): stimulus location and measuring points; intra-, extracell. curr. ; m. field

The time evolution of magnetic field (vert. comp. ) measured in 37 points above the C. corallina

Examples: § Electrically stimulated peripheral nerve medianus. Simultaneus electrical and magnetic measurements.

Sketch of experimental setup

Mag. field after stimulation at t=0, x=0: a) propagation, traces at y=30 mm, x=285 mm, 335 mm, 385 mm b) polarity reversal: x=335 mm, y= -30 mm and 30 mm

Linear scan of the magnetic recordings along the y-axis (a) Elec. pot. rec. simult. at y=0 and x=335 mm (b)

Isofield pattern in the x’-y plane with 20 f. T steps between two isofield lines. Crosses indicates the positins of input data points. The calculated equivalent current dipol is shown.

CT cross-section of the right upper arm at x=335 mm as seen in the distal direction. The encircled dot at the edge of median nerve is the position of equivalent cur. dipole.

The model calculation of the cmpound action current

Results of peripheral nerve study: § The point-like current dipole is a suitable model for a simple geometry as it is in this case. § The localization of the nerve was within 2 mm (based on the CT).

Examples: § Determination of epileptical focus in the case of focal epilepsy

§ The flowchart of the current approach to localizing epileptic focus. A: Time domain waveforms showing epileptic spikes. B: Spectrogram showing focal increases of spectral power. C: Magnetic source image (MSI) showing an epileptic focus.

Spread non-normal (epileptic) activity

The location of epileptical discharges

Examples: § Study of functional stimulation

FUNCTIONAL IMAGING: Evoked response to median nerve stimulation (not clear in the average of MEG sensors (the top overlay). The earliest peak is from the ACG. Followed by the events in the CS. The cerebellum response is seen as well.

Conclusions § Magnetic measurements offer also in the world of living state valuable noninvasive information. § Both, multichannel SQUID system and SQUID microscope can be applied. The last option offers good spatial resolution. § Results from magnetic measurements can be considered as complementary to the existing electric measurements in many cases. They can be combined with different imaging modalities. § SQUID measurements provide direct information on the behavior of ionic currents. § The highest spatial resolution.

Thank you for your attention!

Examples from the world of plants: § a) Simple plant cell – Internodal cell of green algae Chara corallina

Our cell culture

Chara corallina internodal cell

Multi-SQUID measuring configuration (37 channels) schematically

The isofield lines representation § 3 particular time values (1. 3 s, 1. 6 s, 1. 9 s) after the stimulus

The isof. representation (at 1. 4 s, 2. 5 s, 3. 6 s); model. calc. of current dipol and current density along the C. c. intern. c.

Measured and calculated AP and B

Some results § Spreading of excitation along the cell: v ~ 4 cm/s § Conductivity: si = 1. 2 W -1 m -1 , sex = 0. 025 W -1 m -1 § Length of the depolarized area: l ~ 50 mm § Maximal intracellular current: Ii = 1 m. A

Examples from the world of plants: b) the influence of visible light on AP and on B in Chara corallina: The chemical nature of AP obtained from the noninvasive observation (by SQUID microscope) of the intracellular current under the influence of light

Protocol of the C. c. experiment with white light illumination • Light OFF reference • Light ON 10 min • Light ON 20 min • Light ON 60 min AP as function of light exposure

The influence of ilumination on the measured B and AP of electrically stimulated C. c. internodal cell

Model which explains the illumination experiment in the context of 2 nd messenger system §[Ca 2+]c is altered under the influence of light/dark transitions (Miller@Sanders 1987) §AP can be described by an electrically stimulated release of Ca 2+ from internal store: -a) the voltage depend. synthesis/breakdown of the 2 nd mesenger IP 3. -b) the joint action of IP 3 and Ca 2+ on the gating of the receptor channels, which conduct Ca 2+ release from internal stores. -c) modification: cells move excess Ca 2+ from the cytoplasm back into internal stores by an endogeneous Ca 2+ pump system (described by the Hill function. - Quantitative evaluation follows the Othmer model.

Simulated [Ca+2]c transients in response to a single electrical stimulation

Some results § Assuming that the activation of the Cl- channels, that cause the depolarization, is the direct consequence of the change in [Ca 2+]c , the measurements quantitavely agree well with the model.

Examples from the world of plants: c) the injury induced ionic currents in the plant organs - leaves in the higher developed plant Vicia faba, detected magnetically by the multichannel SQUID system.

Measuring setup

The position of injury (panel B cut)

Time evolution of magnetic field in all channels: panel A 15 min. before injury, panel B 1 -16 min after injury, panel C time evolution of field RMS value, panels D end E isofield maps 10 min before and 1. 5 min after injury.

Some results § All measured injured Vicia f. plants responded to injuries with detectable quasi-d. c. magnetic signals. § Large injury leads to easily localizable current source of dipolar pattern. The characteristic time delay is about 10 min. § No long-distance spreading of electrical activity was generally observed.

§ The flowchart of the current approach to localizing epileptic focus. A: Time domain waveforms showing epileptic spikes. B: Spectrogram showing focal increases of spectral power. C: Magnetic source image (MSI) showing an epileptic focus.