stimulus presentation and image acquisition

Stimulus Presentation and Image Acquisition

Valentina Petre

How do we Design an Experiment

Event related

Block design

Block design

* Very good statistical power* Good for “all or none” phenomena (example visual, or acoustic stim)* Good for an initial experimental probe – pilot study (exceptions)* The optimal block length 14 to 20 s * Do not depend much on the assumption of a certain hemodynamic function* Subject might anticipate the stimulus

Event related

* Multiple trials presentation in one run * Optimum time 16s * Close trials 4s –still can be separated * You must assume linearity * Depend on the model of the hemodynamic function * Reduced in sensitivity – vary the inter trial interval

Stim on

Slice i

Frame aq

Event Related stimulus presentation desynchronized from scanner acquisition

Protocols available 128 x 128 64 x 64 Mosaic

Resolution

Signal to Noise

Time betw. Acq.

Max # of slices for 128 frames

Time to acquire one slice

Delay time

Excitation order

high

2.3 x 2.3 mm

low

long (~10 min)

16 slices with 3.5s TR

120-130ms

140-150ms

low

5 x 5 mm

high

short

27 slices with 3 s

TR

100ms

140ms

ProtocolProp

Interleaved

Ascending

Small voxel volume, low signal Large voxel, high signal

Air tissue interface

Air

Tissue

The field inhomogeneity is large, thus it will cause large phase dispersion across the voxel. The signal is reduced.

How to avoid intravoxel dephasing

• Reduce the slice thickness. The longest dimension of your voxel should not be parallel with the direction of field variation

• Orient the slices oblique

• For spin echo sequences, the effect is smaller

Bo

Equipment available• 1.5 T Vision Siemens scanner

head coil surface coil

• Head holder device ear protection nose piece bite bar

• Mirror and mirror stand

• Mouse & joy stick

• Trigger box

• Projector cables RGB VGA screen

• Physiological monitoring

Accessories

MR compatible mouse

Outside the scanner room Inside the scanner room

Penetration panel

Screens

Types of stimulation used at the MNI

• Visual

• Acoustic

• Pain

• Somatosensory

• EEG

Programs used for stimulus presentation

• GLstim available on the SGI (R. Hoge )• Media Control Function on PC (Windows) Pierre

Ahad• SuperLab http://www.superlab.com• Make your own.

Parameters for the protocol to be used

Artifacts

Patient wearing a metal studded belt Braces

http://www1.stpaulshosp.bc.ca/stpaulsstuff/MRartifacts.html

RF Artifacts

Safety issues

• Implants• Pacemaker• Metallic part left after surgery, or metallic

fragments in their eyes• Belts• Tools (screw drivers, scissors, pens…)• Credit cards, bus pass.• Certain type of makeup, tattoos• Avoid loops

Safety issues

Safety issues contd..