1. fMRI: Biological Basis and Experiment Design Lecture 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?

2. 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)

3. 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.

4. dB/dt 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. 3T 3T + 50cm*2G/cm = 3T + 1000G = 3.01T 3T - 50cm*2G/cm = 3T - 1000G = 2.99T +G z -G z  B = 0.02T EPI sequence,  t  1ms, so dB/dt = 0.02T/0.001s = 20T/s

5. 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 !!!!!!!!!!