fMRI: Biological Basis and Experiment Design Lecture 18: Physical practicalities

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fMRI Biological Basis and Experiment
DesignLecture 18 Physical practicalities

• Digression analysis
• ICE9 Example for WA8
• Safety limits
• dB/dt
• SAR
• Acoustic noise
• Implants, etc.
• Human subjects
• Informed consent
• Abnormalities

1 light year 5,913,000,000,000 miles?
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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)

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Description of SAR taken from http//www.mr-tip.co
m
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 1C. 2.1C 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.
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dB/dt
?B 0.02T
EPI sequence, ?t ? 1ms, so dB/dt 0.02T/0.001s
20T/s

• 4.2 Pulsed Gradient Magnetic Fields (exerpt from
  http//www.fda.gov/cdrh/ode/primerf6.html)
• 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.

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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 !!!!!!!!!!