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W. RooneyMay 20, 2004

Imaging the Awake AnimalMRI Efforts Overview

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MRI EnvironmentMRI Environment

I. Static Magnetic Field

II. Radiofrequency Field

III. Magnetic Field Gradients

Fre

qu

ency

en

cod

e

Phase encode

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Motion through magnetic field gradients

Larmor relation:

= B

magnetic field gradients render B is spatially dependent

Fre

qu

ency

en

cod

e

Phase encode

kx

ky

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Motion Induced RMotion Induced R22** changes changes

ΔR2* (Hz)

-3.0

+2.6-2.8-0.5

+7.5+0.3

ΔR2* (Hz)

+1.3

+1.4-0.3+0.5

-1.8+1.9

5.8 degrees

1.5 degrees

• Rotations as small as 1.5° may cause R2

*

changes similar to those during brain activation

• Lateral slices show larger R2

* changes.

• R2* changes increase for

larger angles.

ΔR2* change during

brain activation: ~ 2-3Hz

Caparelli, et al 2003

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Quantitative MRI - DifficultiesQuantitative MRI - Difficulties

Signal changes are often small (<5%)

Subject motion causes signal changes of similar magnitude, due to:

Rigid body transformation (translations & rotations)

Magnetic field changes– geometric distortions & susceptibility

Even the most advanced retrospective motion correction algorithms

fail if motion is excessive

Therefore, it is very difficult or impossible to perform MRI in awake animals sick patients and children

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Overall Goal - MRIOverall Goal - MRI

Increase 5 to 10-fold the range of

motion acceptable for susceptibility-

based functional MRI techniques.

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MRI ProjectsMRI Projects

I. Prospective motion correction in MRI

(Lead: Rooney, BNL Chemistry)

II. Dynamic prospective adjustment of main

magnetic field during MRI

(Lead: Wanderer, BNL Magnet Division)

III. Retrospective motion correction

(Lead: Ernst, BNL Medical)

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W. Rooney, X. Li, J. Mead, R. WangMay 20, 2004

Imaging the Awake AnimalProspective Motion Correction in MRI

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Prospective Motion Correction in MRIProspective Motion Correction in MRI

- MRI is extremely motion sensitive

- Post-acquisition corrections have limitations

- Dynamic adjustment of MRI acquisition possible

Specific Aims:

- develop and validate an MRI compatible motion sensing device

- design and construct electronic module to synthesize sensor output

- integrate motion sensor and electronic module into MRI instrument

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time

labframe

“brain” frame

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1 2 3

MRI is Motion SensitiveMRI is Motion Sensitive

Y

Z

X

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spatial realfrequency space

labframe

“brain” frame

lab frame

“brain” frame

Image Quality RestorationImage Quality Restoration

motion corrupted

hybrid corrected

2

4

4

2

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Laser PSD2

TR

PSD1

ADC-DSP

Motion Tracking Compensation

Circuit

X

INPUT YZ

RFAcquisition parameters

X' Y' Z' RF

PSDPSD33

Dynamic Adjustment of MRI Acquisitions

Position sensingdetectors (PSDs)

Output to MRI instrument

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• Detector array performs superbly in MRI environment:

4 T B0, gradients (B0/ t = 30 T/s), and RF (170MHz)

• 20 μm vibration “noise” output during MRI operation

Detector Validation in MRIDetector Validation in MRIY

Positio

n (

mm

)

0

1

2

3

B0 = ~0

B0 = 4 T

beam flexion (mechanical resonance)

vibration “noise”

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- LabView System (2 ADC/DAC boards, 2.4 GHz PC)

- PSD sensor array

- Inputs

3 gradient waveforms

6 PSD sensor signals

2 trigger signals

-Outputs

3 modified gradient waveforms

RF modifying signals

Motion Detection/Compensation System

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Motion Compensation System Integration

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Motion Compensation Control Panel

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Baseline MRI

5-compartment phantom

4 T whole-body MRI (GRE sequence)

Scan acquired with MRI instrument in normal configuration

Baseline 0

Test Phantom

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Baseline MRI

5-compartment phantom

4 T whole-body MRI

(GRE sequence)

Scan acquired with motion detection & compensation unit in-line

Detector in-line

Detector System Integration

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Detector in-line & analog filter & digital filter

Detector System Integration

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Baseline 0 Rotate 90ºz & invert

z

y

x

Gradient Mixing

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8º Wobble Rotation 8º Wobble w/Correction

z

y

x

Rotational Image Correction

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D. Schlyer, C. Woody, P. Vaska, W. RooneyMay 20, 2004

PET-MRI Instrumentation

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PET Detector System Test

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Detector Test in MRI Environment

Benchtop 4 T, RF dB0/dt

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PET/MRI Detector Configurations

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W. Rooney, X. Li, J. Mead, R. WangMay 20, 2004

Animal Position Tracking

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Position Sensing Detectors (PSDs)Position Sensing Detectors (PSDs)

PSDs are silicon photodiodes

Sensitive to 400-1100 nm light

Analog signal output proportional to position of light spot

Excellent linearity, resolution and response time

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Rigid Body Transformation AlgorithmRigid Body Transformation Algorithm

3 translations & 3 rotations

t = 0 t > 0

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Rotation Matrix - Accuracy (%)

Translation Accuracy (%)

0.0016 0.0003 0.0002

0.00002 0.0027 0.019

0.00004 0.00002 1.938

0.00012 1.954 0.00003

Angular accuracy < 0.1°

Accuracy < 1μm

y

xz

Computer-controlled precision motion platform (R.Ferrieri)

Precision: 0.8 m and 0.0005º

Programmable movements at 200 μm/s

Provides motion “gold standard”

Rigid Body Transformation AlgorithmRigid Body Transformation Algorithm

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1

2

3

4

5

6

Rigid-Body Motion SensorRigid-Body Motion Sensor

Sensor array Sensor output

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Sensor Input Algorithm Output

X

Y

Z

1

2

3

4

5

6

Rotations

Translations

Motion Tracking AlgorithmMotion Tracking Algorithm