Post on 29-Sep-2020
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