A lead such as used for pacemaking or neural stimulation has a current block IC which blocks current when RF is detected. In this way problems of tissue heating are reduced or eliminated when the patient carrying such a lead is given an MRI. The lead may also detect localized temperature, to be reported to equipment external to the lead. This may permit more judicious use of the RF energy, for example during an MRI session. Localized and spurious tissue heating nearby to an electrode of a lead, due to RF energy from an MRI, is reduced or eliminated by detecting the RF field, deriving energy from the field, and actively driving a node that is coupled to such an electrode so as to minimize voltage excursions at the electrode. A state is maintained or set within electronics nearby to an electrode of a lead, so that a can which connects with the lead is able to learn whether an MRI event has occurred since the last time that the can had sent commands to the electronics to set particular internal states. For a lead that delivers pacing pulses, a pacing detector is provided in addition to an MRI detector, and the blocking action is carried out most of the time but is disabled briefly for each pacing pulse. During a time of presence of RF energy, a bucking circuit attempts, to the extent possible, to drive a node toward blood potential, the node capacitively coupled with electrodes of the lead.
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