Temperature Rise due to Medical Implants · Temperature Rise due to Medical Implants ICNIRP Thermal Workshop, Istanbul Turkey, May 26-28, 2015 Hypotheses • a signiﬁcant portion

Temperature Rise due to Medical ImplantsNiels KusterEarl Zastrow

Thursday, May 28, 15

Temperature Rise due to Medical Implants ICNIRP Thermal Workshop, Istanbul Turkey, May 26-28, 2015

RF Coupling Mechanisms w/ External Fields



RF Coupling Mechanisms w/ External Fields

in silico in vivo

From J. M. Henderson, et al., "Permanent Neurological Deficit Related to Magnetic Resonance Imaging in a Patient with Implanted Deep Brain Stimulation Electrodes for Parkinson's Disease: Case Report", Neurosurg., 57(5), 2005



RF Coupling Mechanisms: Simple Wires



RF Coupling Mechanisms: Simple Wire in Tissues



RF Coupling Mechanisms: Simple Wire in Tissues

ξ=0 rad/mξ=4 rad/mξ=9 rad/mξ=15 rad/m



Content• IT’IS activities on implant safety• hypotheses• exposure limits & FDA safety considerations• localized heating due to generic implants• demonstration of MRI-safe implants• temperature rise during MRI exposure & risk management• conclusions



IT’IS Activities: Virtual Population (ViP)

ELLA female adult 1.60m, 58kg

FATSobese male 1.78m, 120kg

THELONIOUSboy 6y1.17m, 20kg

EARTHAgirl 8y1.35m, 30kg

ROBERTAgirl 5y1.09m, 16kgDUKE

male adult 1.74m, 70kg

BILLIEgirl 11y1.46m, 36kg

LOUISboy 14y1.65m, 50kg

DIZZYboy 8y 1.40m, 26kg

GLENN male elderly1.73m, 65kg



IT’IS Activities: Multi-Physics Solvers



IT’IS Activities: Testing Systems



IT’IS Activities: V&V



Hypotheses• a significant portion of the population (people w/ passive or active medical

implants) is not protected from serious health hazards below 100 MHz (e.g., MRI, wireless power transfer)

• spatial averaging mass of 10g is too large for localized exposure• risk-benefit considerations needed for implants in MRI



Exposure Safety Limits• temperature Limits

- FDA: no special consideration needed for <+2ºC temperature increase- ICNIRP: <+1ºC whole-body temperature increase- MRI: <39ºC/40ºC (normal/first level)

• EM/SAR Limits- ICNIRP Basic Restrictions: wbSAR 0.08/0.4 W/kg; psSAR10g: 2/10 W/kg - ICNIRP Derived Limits: E(S) & H(S) as function of frequency- IEC60601-2-33 (MRI): wbSAR: 2/4 W/kg; psSAR10g: not defined for body coil- ISO/TS10974 (Implant-MRI): follows IEC60601-2-33



ICNIRP Derived Limits (>100 kHz)



Generic Implant Study• local deposition & ΔT in vicinity of implant

- 100 - 800 mm- 10MHz - 1GHz

• evaluation at ICNIRP limits• averaging mass for local absorption

‣ a significant portion of the population (people w/ passive or active medical implants) is not protected from serious health hazards below 100 MHz (e.g., MRI, wireless power transfer)

‣ spatial averaging mass of 10g is too large for localized exposure



Generic Implant StudypsSAR0.1g / ΔT = 55.5 W/kg / 1K



Generic Implant Study






Generic Implant Studyoccupational limituncontrolled limitderived limit for implant from study

occupational limituncontrolled limitderived limit for implant from study



Wire Length: 100mm Wire Length: 200mm

Wire Length: 500mm Wire Length: 800mm



Generic Implant Studymass (g):20.3. Power (%):99.0

(mm)

(mm

)

-30 -20 -10 0 10 20 30

-10

0

10

mass (g):10.0. Power (%):96.0

(mm)

(mm

)

-30 -20 -10 0 10 20 30

-10

0

10

mass (g):1.0. Power (%):78.6

(mm)

(mm

)

-30 -20 -10 0 10 20 30

-10

0

10

mass (g):0.1. Power (%):39.4

(mm)

(mm

)

-30 -20 -10 0 10 20 30

-10

0

10



Generic Implant Studymass (g):10.0. Power (%):96.0

(mm)

(mm

)

-30 -20 -10 0 10 20 30

-10

0

10

mass (g):1.0. Power (%):78.6

(mm)

(mm

)

-30 -20 -10 0 10 20 30

-10

0

10

mass (g):0.1. Power (%):39.4

(mm)

(mm

)

-30 -20 -10 0 10 20 30

-10

0

10

mass (g):20.3. Power (%):99.0

(mm)

(mm

)

-30 -20 -10 0 10 20 30

-10

0

10




0.01g0.1g1g10g




0.1$

1$

10$

100$

0.000001$ 0.00001$ 0.0001$ 0.001$ 0.01$ 0.1$ 1$

ΔT"[K

]"

Exposure"Radius"[m]"

Muscle$

Fat$

1g1mg 10g



Conclusions: Safety of People w/ Implants• the hypothesis that “a significant portion of the population (people w/ passive or

active medical implants) is not protected from serious health hazards below 100 MHz (e.g., MRI, wireless power transfer)” is true

‣ revision of limits or statement in standard “people w/ implant are not protected” needed

• the hypothesis that “spatial averaging mass of 10g is too large for localized exposure” is true for implant safety consideration

‣ reduced averaging mass needed for conservative ΔT estimation

‣ restriction to contiguous tissue is important



Demonstration of MRI Safety of Patients w/ Implants



Evaluation of Implant in MRI: Concept• severity of RF-heating hazard: DBS, CI, pacemaker, SCS, etc.• magnitude of RF-heating relates to in vivo incident conditions

- MRI system: f(RF coil, power calculation & reporting)- implant: f(built,routing)- patient imaging condition: f(anatomy, posture, position)‣ in vivo incident conditions cannot be experimentally evaluated‣ need in silico approach to handle a multitude of variables



Implant Safety during MRI: RF-Heating Hazard• IEC 60601-2-33: RF exposure limits‣ whole-body, head, partial-body SAR

• ISO/TS 10974: hazard evaluation‣ tissue deposition @ RF exposure limits

(60601-2-33)‣ tissue deposition --> hazard/risk

induced current and magnetic fields

induced electric fields



Implant Safety during MRI: RF-Heating Hazard

BILLIE THELONIOUSELLADUKE

head imaging

thorax imaging

pelvis imaging




BILLIE THELONIOUSELLADUKE

head imaging

thorax imaging

pelvis imaging






Implant in vivo Incident Conditions during MRI

Head Thoracic



ΔT during MRI

DBS (R) DBS (L) Pacemaker (R)

Pacemaker (L)

ps SAR0.1g (W/kg) 8.97E+03 8.71E+03 9.77E+01 5.26E+02

ΔT w/ implant (K) 92.5 96.4 1.3 6.3

ΔT w/o implant (K) <1 <1 <1 <1

ps SAR0.1g (W/kg) 1.11E+03 1.26E+03 1.01E+03 1.82E+03

ΔT w/ implant (K) 11.0 22.7 7.9 11.0

ΔT w/o implant (K) <1 <1 <1 <1

head

thor

acic



Risk-Benefit Consideration for Implants• <+2ºC temperature limit

- would require new lead designs• pacing threshold limit (pacemaker): not yet standardized

- determined in animal studies- ΔT estimate of +10ºC- additional scar tissue around electrode considered to be acceptable

• RF-heating of brain tissue (DBS)- potentially very acute at current exposure limit- ΔT estimate of +100ºC from a generic implant

‣ the hypothesis “risk-benefit considerations for implants in MRI needed” is true



Summary• the hypothesis that “a significant portion of the population (people w/ passive or

active medical implants) is not protected from serious health hazards below 100 MHz (e.g., MRI, wireless power transfer)” is true.

‣ revision of limits or statement in standard “people w/ implant are not protected” needed

• the hypothesis that “spatial averaging mass of 10g is too large for localized exposure” is true for implant safety consideration.

‣ reduced averaging mass needed for conservative ΔT estimation

‣ restriction to contiguous tissue is important• most elongated implants in MR are not intrinsically safe

‣ risk-benefit considerations for implants in MRI are needed



Acknowledgments• CO-ME, Swiss National Foundation• KTI, Commission of Innovations & Technology• Boston Scientific Corporation


Documents

Temperature Rise due to Medical Implants · Temperature Rise due to Medical Implants ICNIRP Thermal Workshop, Istanbul Turkey, May 26-28, 2015 Hypotheses • a signiﬁcant portion