Neural Imaging II: Imaging Brain Function

ANA 516:

February 13, 2007

Jane E. Joseph, PhD

Outline

• Physiological Basis of functional MRI (fMRI)

• Experimental Design and Data Analysis Issues

• Brief survey of fMRI studies in animals (mostly)

What is fMRI?

One of a number of brain imaging techniques that reveal some dynamic, in vivo aspect of brain function.

What is fMRI?S

patia

l res

olut

ion

Temporal resolution

What is fMRI?

• An INDIRECT measure of neural activity

• Measures relative concentrations of de-oxygenated blood

• Don’t need any contrast agents or radiation

Brief History of MRI / fMRI

• 1890 -- Roy and Sherrington postulated that changes in activity associated with brain function would lead to increases in blood flow in those regions

• 1945 – Bloch and Purcell share the Nobel prize for their work with magnetic resonance

Brief History of fMRI

• 1990 – Ogawa, et al. showed that MR images could be used to detect changes in blood oxygenation in vivo (in mouse brain)

• 1991 – First human fMRI studies

fMRI vs Other techniquesNon-invasive which makes it ideal for:

(1) developmental studies (kids can go in MRI scanner)

(2) longitudinal studies (kids / people can go in multiple

times)(3) aging studies

Widely available (c.f. PET, TMS)

No known health risks (c.f. PET)

fMRI vs Other techniques

fMRI is NOT a good choice in certain situations:

• Many surgical implants cannot go in MRI scanner

• Presence of metal in body (or on body – tattoos, makeup)

• To measure neurotransmitters (use spectroscopy or PET)

fMRI vs Other techniques

fMRI is NOT a good choice in certain situations:

• Claustrophobia• To study gross motor behavior• Certain patient populations with

movement disorders (e.g PD)

Structural v. Functional MRI

POSTERIORPOSTERIOR

What is the physiological process or event that contributes to contrast in a functional image? POSTERIOR

Want to measure some aspect of neural activity, but fMRI does not do this directly

Instead, fMRI is based on changes in oxygen consumption and blood flow, which are indirectly related to neural activity

Excitatory post-synaptic potentials (EPSPs), Inhibitory post-synaptic potentials (IPSPs), and Action potentials (APs) are not very metabolically demanding

But the return to resting state does require energy!

How is blood flow related to neural activity?

Increased neural activity causes release of vasoactive substances, which cause vessels (arterioles) to dilate – these effects can occur locally and upstream from the activity

How are these physiological changes measured with MRI?

Deoxygenated hemoglobin (dHb) is paramagnetic (e.g. has unpaired electrons) due to losing O2 molecules

dHb disrupts the magnetic field locally (dephasing spins shorten T2*)

More dHb loss of MR signal

Pure O2 (100%)

20% O2

oxygenated blood

de-oxygenated blood

oxygenated blood

de-oxygenated blood

magnetic field gradient

De-oxy hemoglobin disrupts magnetic field and causes a signal loss

oxygenated blood

de-oxygenated blood

magnetic field gradient

Surplus of oxy hemogolobin relatively lower concentration of deoxy hemoglobin in active regions

oxygenated blood

de-oxygenated blood

magnetic field gradient

Less decrement in signal (OR small increments in signal) at activated sites

Blood Oxygenation Level Dependent Contrast (BOLD):

1. dHb is paramagnetic

2. Surplus of oxygenated blood at sites of activation

Early fMRI studies (Belliveau, et al. 1991)

Look at blank screen

Look at flashing checkerboard pattern

This study not based on BOLD but used contrast !!!!

Subtract Image A from Image B

First BOLD fMRI studies (Kwong, et al. 1992)

First BOLD fMRI studies (Kwong, et al. 1992)

First BOLD fMRI studies (Blamire, et al. 1992)

Blamire, et al. (1992) showed that the BOLD response is delayed relative to the neural events associated with stimulus presentation (hemodynamic lag)

we are not measuring neural processes directly

BOLD response is related to neural activity (even though it is delayed)

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

-5 0 5 10 15 20 25 30 35 40

Time since stimulus onset (s)

BO

LD

sig

na

l ch

an

ge

(%

)

A typical hemodynamic response (HDR):

y-axis:

Usually express in % change from baseline

POSTERIOR

A typical functional brain activation map

Talairach Atlas (Talairach & Tournoux, 1988)

-y

anterior

posterior

superior

inferior

left

right

+y +x

-x

+z

-z Anterior commissure

Posterior commissure

Brodmann’s Areas (Brodmann, 1909)Lateral Surface Primary Sensory:

- Visual (17)

- Auditory (41)

- Motor (4)

- Somatosensory (1,2,3)

Secondary:

- Visual (18)

- Auditory (42)

Association:

- Visual (19, 37)

- Auditory (22)

- Somatosensory (5)

Brodmann’s Areas (Brodmann, 1909)Medial Surface

Primary Sensory:

- Olfactory (28, 38)

Association:

- Limbic (23, 24, 25, 26, 27, 29, 30, 31, 32, 33)

Experimental Design and Data Analysis Issues

• Technique constraints

• Limitations on experimental designs

• Advantages of fMRI

Technique Constraints

• BOLD signal ~= neuronal activity

• Poor temporal resolution

• No information about temporal order of events

• Spatial resolution

Limitations on Experimental Design

• No ferrous magnetic materials in scanner• Use tasks that minimize amount of movement

(gross motor tasks, speaking?)• Auditory stimulus presentation may be

masked by scanner noise• Limits on length of experiments (hardware

limitations and subject comfort)• Subtraction Technique

Limitations on Experimental Design

• Subtraction Technique:

– Must have a baseline condition with which to compare an experimental condition

– May also have a control condition (or several)

– What is a good baseline condition?

Advantages of fMRI

• Can do repeated measures learning, practice, intervention, recovery of function

• Can test pediatric populations developmental, longitudinal and aging studies

• Can look at individual-subject activation patterns

Some applications

• Imaging brain function in animals

Zhang et al. (2006). NeuroImage, 33, 636-643

Pharmacological MRI (phMRI) in MPTP lesioned rhesus monkeys

Functional MRI (fMRI) in alert, unanesthetized rhesus monkeys

Joseph et al. (2006). Journal of Neuroscience Methods, 157, 10-24

Both monkeys showed novelty detection effects in the amygdala (only 1 monkey shown here)

BOLD imaging of spinal cord in rats

Lilja et al. (2006). The Journal of Neuroscience, 26, 6330-6336

Hindlimb electrical stimulation dorsal column activity on the ipsilateral side

BOLD imaging of spinal cord in humans

Stracke et al. (2005). Neuroradiology, 47, 127-143

Stimulate different fingers dorsal column activity in different dermatomes

MRI of insect brains!