Transcript Human subjects SAR, dB/dt

fMRI: Biological Basis and Experiment Design
Lecture 18: Physical practicalities
• Digression: analysis
• ICE9: Example for WA8
• Safety limits
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dB/dt
SAR
Acoustic noise
Implants, etc.
• Human subjects
– Informed consent
– Abnormalities
1 light year = 5,913,000,000,000 miles?
Safety considerations
• Implants and sundry scary things
• Acoustic noise
– Rapid switching in gradient coils results in mechanical motion
– FDA limit: ____ dB
• dB/dt
– Changing field (gradients) induces currents in body tissue
– FDA limit: ____ T/s
• SAR: specific absorption rate
– Tissue heating by microwave energy from RF coil
– FDA limit: ___ W/kg, averaged over time. (depending on tissue)
Description of SAR taken from http://www.mr-tip.com
The Specific Absorption Rate is defined as the RF power absorbed per unit of mass of
an object, and is measured in watts per kilogram (W/kg).
The SAR describes the potential for heating of the patient's tissue due to the application of the RF energy necessary to
produce the MR signal. Inhomogeneity of the RF fields leads to a local exposure where most of the power that is absorbed is
applied to one body region rather than the entire person, leading to the concept of a local SAR. Hot spots may occur in the
exposed tissue, to avoid or at least minimize effects of such theoretical complications, the frequency and the power of the RF
irradiation should be kept at the lowest possible level. Averaging over the whole body leads to the global SAR.
It increases with field strength, radio frequency power and duty cycle, transmitter-coil type and body size. In high and ultrahigh
fields, some of the multiple echo, multiple-slice pulse sequences may create a higher SAR than recommended by the
agencies.
Normally no threatening increase in temperature could be shown. Even in high magnetic fields, the local temperature
increases not more than 1°C. 2.1°C is the highest measured increase in skin temperature. Eddy currents may heat up
implants and thus may cause local heating.
FDA SAR limits:
4 W/kg averaged over the whole body for any 15-minute period
3 W/kg averaged over the head for any 10-minute period; or
8 W/kg in any gram of tissue in the extremities for any period of 5 minutes.
SAR limits of some European countries:
Level 0 (normal operating mode): Less than or equal 1.5 W/kg
Level I (first level controlled operating mode): Greater than 1.5 W/kg but less than 4 W/kg
Level II (second level controlled operating mode): Greater than 4 W/kg.
In some European countries the systems are limited to a maximum SAR of 4 W/kg, so scanning in level II is impossible.
For Level I, in addition to routine monitoring, particular caution must be exercised for patients who are sensitive to temperature
increases or to FR energy.
For Japan different SAR limits are valid.
dB/dt
-Gz
3T + 50cm*2G/cm = 3T + 1000G = 3.01T
B = 0.02T
3T
+Gz
3T - 50cm*2G/cm = 3T - 1000G = 2.99T
EPI sequence, t  1ms, so
dB/dt = 0.02T/0.001s = 20T/s
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4.2 Pulsed Gradient Magnetic Fields (exerpt from http://www.fda.gov/cdrh/ode/primerf6.html)
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Another component of the MR environment is a pulsed gradient magnetic field that is used for
signal localization. When this gradient magnetic field is applied, the magnetic field intensity
changes rapidly, giving rise to a time-varying magnetic field. During the rise time of the magnetic
field, a voltage is induced in an electrical conductor, even when it is stationary in the field.
However, in most MRI systems, the currents induced by the pulsed magnetic gradient field are
about 1,000 times smaller than those induced by the pulsed RF component and are therefore not
of great concern with regard to thermal injuries. Major concerns with the pulsed gradient fields are
biological effects including electrical nerve stimulation and the generation of light flashes
(magnetophosphenes) that may result from a slight torque exerted on the retinal cones. Current
FDA guidance limits the Time Rate of Change of Magnetic Field (dB/dt) to levels which do not
result in painful peripheral nerve stimulation.
Human Subjects
• Institutional Review Board must approve every study
– Ahead of time
– For a limited number of subjects
– For the exact task
• Informed consent
– Subjects need to know what's going on!
• Subject comfort !!!!!!!!!!