# Presentation: 066992

## Peter A. Bandettini

Unit on Functional Imaging Methods
Laboratory of Brain and Cognition
&
Functional MRI Core Facility

- Unit on Functional Imaging Methods
- Laboratory of Brain and Cognition
- &
- Functional MRI Core Facility

**Neuronal Current Imaging**

## Primary People Involved

**Jerzy Bodurka**
**Natalia Petridou**
**Frank Ye**
**Rasmus Birn**

## The Basic Idea...

**J.P. Wikswo Jr et al.*****  J Clin Neuronphy *****8(2): 170-188, 1991**

- 100 fT at on surface of skull**  **

## Derivation of B field generated in an MRI 

- voxel by a current dipole

- Single dendritic tree having a diameter d, and length L behaves like a conductor with conductivity . Resistance is R=V/I, where R=4L/(d2 ). From Biot-Savart:

**r****2**

- by substituting d = 4m,   0.25 -1 m-1, V = 10mV and
- r = 4cm (measurement distance when using MEG) the resulting value is:  **B********0.002 fT**

- Because **B****MEG****=100fT** is measured by MEG on the scalp**, **(0.002 fT x 50,000 = 100 fT), must coherently act to generate such field. These bundles of neurons produce, within a typical voxel, 1 mm x 1 mm x 1 mm, a field of order:

**B****MRI** ******0.2nT**

**J. Bodurka, P. A. Bandettini. ***Toward direct mapping of neuronal activity: MRI detection of ultra weak transient magnetic field changes.*** Magn. Reson. Med. ****47: 1052-1058, (2002****).**

## Some background...

**J. Bodurka, P. A. Bandettini. ***Toward direct mapping of neuronal activity: MRI detection of ultra weak transient magnetic field changes.*** Magn. Reson. Med. ****47: 1052-1058, (2002****).**

**M. Singh, ***Sensitivity of MR phase shift to detect evoked neuromagnetic fields inside the head.*
**IEEE Transactions on Nuclear Science. ****41: 349-351, (1994)****.**

**D. Konn, P. Gowland, R. Bowtell, ***MRI detection of weak magnetic fields due to an extended current dipole in a conducting sphere: a model for direct detection of neuronal currents in the brain.*** Magn. Reson. Med. 50: 40-49, (2003).**

**J. Xiong, P. T. Fox, J.-H. Gao, ***Direct MRI Mapping of neuronal activity.*** Human Brain Mapping, 20: 41-49, (2003)**

**G. C. Scott, M. L. Joy, R. L. Armstrong, R. M. Henkelman, ***RF current density imaging homogeneous media. ***Magn. Reson. Med. ****28: 186-201, (1992)****.**

**H. Kamei, J, Iramina, K. Yoshikawa, S. Ueno, ***Neuronal current distribution imaging using MR.*** IEEE Trans. On Magnetics, 35: 4109-4111, (1999)**

## wire

**wire**

**wire**

- X

- Z

- Current Phantom Experiment

## 

**calculated |*********B******c*****|  ||** ***B******0***

**Measurement**

**Correlation image**

**Spectral image**

**Simulation**

**Single shot GE EPI**

- ****

- 70 A current

****** ****20****0**

**J. Bodurka, P. A. Bandettini. Toward direct mapping of neuronal activity: MRI detection of ultra weak transient magnetic field changes, Magn. Reson. Med. ****47: 1052-1058, (2002****). **

## J. Bodurka, P. A. Bandettini. Toward direct mapping of neuronal activity: MRI detection of ultra weak transient magnetic field changes, Magn. Reson. Med. 47: 1052-1058, (2002). 

**J. Bodurka, P. A. Bandettini. Toward direct mapping of neuronal activity: MRI detection of ultra weak transient magnetic field changes, Magn. Reson. Med. ****47: 1052-1058, (2002****). **

## Human Respiration

- 00

- 400

- 200

**B**

**B****0**

- 00

- 400

- 200

**D**

**B****0**

- BR10 nT

- BR7 nT

## The use of spin-echo to “tune” to transients..

**M. Singh, ***Sensitivity of MR phase shift to detect evoked neuromagnetic fields inside the head.*
**IEEE Transactions on Nuclear Science. ****41: 349-351, (1994)****.**

**The use of spin-echo to “tune” to transients..**

## Phase vs. Magnitude...

**0.1 to 0.3 Deg.**

## in vitro model

**100 ********m**

**in vitro model**

- Patch electrode recording

- coronal sections of newborn-rat brains ; in-plane:~1mm2, thickness: ~60-100 m
- Neuronal Population: 10,000-50,000

-   Spontaneous synchronized activity ; current: ~ 180nA-2A,
-    B: ~ 60pT-0.5nT

- Plenz, D. and S.T. Kital. Nature, 1999. **400**: p. 677-682.

**Cortex**

**Striatum**

**Subthalamic nucleus**

**Globus Pallidus**

## Imaging

-  3T, Surface coil receive
-  FSE structural images (256x256)
-  SE EPI single shot, TE: 60ms, TR:1s, flip angle: 900,
-    FOV: 18cm, matrix: 64x64, 4 slices (3mm)

**methods - *****imaging ***

## Six Experiments

**two conditions per experiment**

**methods - *****imaging ***

_**Active**_**    **
**600 images**
**neuronal activity present**

_**Inactive**_** **
**600 images **
**neuronal activity terminated**
**via TTX administration**

## methods - analysis

**methods - *****analysis***

***Phase images*****  **
**  ****Spectrum for each voxel**
**  ****Two voxel groups (all slices)**

**Principal  Component Analysis**
***of the Spectrum***
***per group***

## results

**results**

**A: 0.15 Hz activity, on/off frequency**
**B: activity**
**C: scanner noise (cooling-pump)**

_**Active**_** condition: black line**
_**Inactive**_** condition: red line**

**Notes:**
Figure with in/out voxel 

## A

**results**

**Hz**

_***Culture***_

**A: 0.19 Hz activity**
**C: scanner cooling-pump**

_**Active**_** condition: black line **_**Inactive**_** condition: red line**

**0.19Hz map**

**Echo Planar Image**

**A**

**C**

**Notes:**
Figure with in voxel but epi image and all maps

## Strategies for Detection

**Time shifted sampling**
**Under sampling**

## Time shifted sampling

**Time shifted sampling**

**M. Singh, ***Sensitivity of MR phase shift to detect evoked neuromagnetic fields inside the head.*
**IEEE Transactions on Nuclear Science. ****41: 349-351, (1994)****.**

## Undersampling

**Photodiode**

**MEG**

**8 Hz alternating **
**checkerboard**

**Undersampling**

## Undersampling

**TR**

**Alternating Checkerboard Frequency**

**Undersampling**

## Power spectra

- 0.5 Hz

- 0.5 Hz

**Eyes closed**

**Eyes open**

- Phase 0.12Hz

- Closed

- Open

- Magnitude 0.12 Hz

## Caution, Despair, Hope...

**Need to rule out BOLD or other mechanisms**
**Noise is larger than effect**
**MR sampling rate is slow**
**Neuronal activation timing is variable and unspecified**
**Models describing spatial distribution and locally induced magnetic fields remain relatively uncharacterized...therefore could be off by up to an order of magnitude.**
**Well characterized stimuli**
**“****Transient-tuned” pulse sequences (spin-echo, multi-echo)**
**Sensitivity and/or resolution improvements**
**Simultaneous electrophysiology – animal models?**
**Synchronization improvements.**