Rad225/Bioe225

Ultrasound

Fall 2019MR Acoustic Radiation Force Imaging and MR Thermometry

Rad225/Bioe225

Ultrasound

Fall 2019

MR Acoustic Radiation Force Imaging

Rad225/Bioe225

Ultrasound

Fall 2019

US

MR-ARFI1.0

0.8

0.6

0.4

0.2

0.0

-0.2

12080400RF1.0

0.8

0.6

0.4

0.2

0.0

-0.2

12080400

Gde

• displacement or motion encoding gradients

ω = 2πγ B

ΔB = GΔx

J. Chen, MRM 2010

Rad225/Bioe225

Ultrasound

Fall 2019

MR Acoustic Radiation Force Imaging

Rad225/Bioe225

Ultrasound

Fall 2019

Focal Spot Localization

- spin echo imaging, flexible contrast- good image quality- temperature effects below FDA limits

1 cm

E. Kaye, ISMRM 2010

Rad225/Bioe225

Ultrasound

Fall 2019

Focal Spot Localization

A. Holbrook, Med Phys 2011

Rad225/Bioe225

Ultrasound

Fall 2019

Tissue Changes During Treatment

R. Bitton, JMRI 2012

Pre-ablation Post-ablation

Dis

plac

emen

t

Rad225/Bioe225

Ultrasound

Fall 2019

MR Thermometry

Rad225/Bioe225

Ultrasound

Fall 2019

Proton Resonance Frequency

rotation rate is very specific

42.57 MHz/T

ω = γ B0

Rad225/Bioe225

Ultrasound

Fall 2019

Proton Resonance Frequency

rotation rate is very specific

42.57 MHz/T

ω = γ B0

Rad225/Bioe225

Ultrasound

Fall 2019

Proton Resonance FrequencyShift Thermometry

As the temperature rises, the hydrogen bonds stretch, bend, and break.

The protons are more shielded from the field, decreasing the resonant frequency.

ω = γ B0

Rad225/Bioe225

Ultrasound

Fall 2019Phase Difference

Known values: α, γ, B0, TENothing that is tissue type dependent here.Linear over the temperature range of interest.No hysteresis.

ΔT = Δφαγ B0TE

φ = γ BTEω = γ B

α = −0.01ppm / °CΔϕ =αΔTγ B0TE

Rad225/Bioe225

Ultrasound

Fall 2019

α=-.01ppm/°Call aqueous tissue

Rad225/Bioe225

Ultrasound

Fall 2019

InSightec α=-.00909ppm/°C Phillips α=-.0094ppm/°C

Rad225/Bioe225

Ultrasound

Fall 2019

Image Phase

Reasons for phase:• poor field homogeneity (poor shim)• poor field homogeneity (perturbations from the body)• temperature• velocity• other stuff

Need to extract the temperature dependent part

Rad225/Bioe225

Ultrasound

Fall 2019

PRF Shift Thermometry

Phase image during heating

Background phase we need to separate out

Temperature rise we want to extract

Rad225/Bioe225

Ultrasound

Fall 2019

PRF Shift Thermometry

Pre-heat image“Baseline Image”

PhaseDifference

_ =Phase image

during heating

Rad225/Bioe225

Ultrasound

Fall 2019

PRF Shift Thermometry

Pre-heat image“Baseline Image”

PhaseDifference

_ =

Δω =αΔTγ B0

Phase image during heating

Rad225/Bioe225

Ultrasound

Fall 2019

PRF Shift Thermometry

_ =

Δφ =αΔTγ B0TE

Pre-heat image“Baseline Image”

PhaseDifference

Phase image during heating

Rad225/Bioe225

Ultrasound

Fall 2019

PRF Shift Thermometry

PhaseDifference

_ =

T, Thermal Dose

ΔT = Δφαγ B0TE

Pre-heat image“Baseline Image”

Phase image during heating

Rad225/Bioe225

Ultrasound

Fall 2019

Prostate Ablation

• central heat fixed area• surrounded by coagulative necrosis

CE TTC

Multiple Tubular Radiators (8 MHz)180o or more directional output5-15 watts, typically over 15 minutes

Rad225/Bioe225

Ultrasound

Fall 2019

Temperature MappingTemperature maps appear every 3 seconds.

0

1

2

σT40°C45°C50°C

V. Rieke, JMRI 2013

Rad225/Bioe225

Ultrasound

Fall 2019

PRF Thermometry Limitation: Fat

no changes in water hydrogen bondingno PRF shift thermometry in fat

Rad225/Bioe225

Ultrasound

Fall 2019

Image during heating

Pre-heat image Difference

Phase Difference

_ =

T, Thermal Dose

Motion Corrupts!

f0 = gB0ΔT = Δφ

αγ B0TE

Rad225/Bioe225

Ultrasound

Fall 2019Magnetic Susceptibility of the Lung Changes the Magnetic Field

Long Range EffectTemporally Varying

Rad225/Bioe225

Ultrasound

Fall 2019

Δω =αΔTγ B0

Long Range EffectTemporally Varying

Magnetic Susceptibility of the Lung Changes the Magnetic Field

Rad225/Bioe225

Ultrasound

Fall 2019

Long Range EffectTemporally Varying

Δω =αΔTγΔB0

Magnetic Susceptibility of the Lung Changes the Magnetic Field

Rad225/Bioe225

Ultrasound

Fall 2019

Long Range EffectTemporally Varying

Problem for abdomen, pelvis, brain

Δω =αΔTγΔB0

Magnetic Susceptibility of the Lung Changes the Magnetic Field

Rad225/Bioe225

Ultrasound

Fall 2019

Image Registration is not sufficient

The effect is huge.1 mm shift in acquisition looks like a 20°C rise

4.4 mm3.3 mm1.0 mm0.0 mm

Temperature images were shifted and registered to the baselineDifferent lung fillings during acquisition creates large errors.

Baseline-Temperature Image Acquisition Discrepancy

27oC 77oC

Rad225/Bioe225

Ultrasound

Fall 2019MR-ARFI Displacement and MR Temperature Increase with Power

E. Kaye, MRM 2013