The magnetic stimulation (MS) coil arrangement having a housing 8a, 8b for receiving one or more windings of a conductive element and a coolant flow path defined between the housing and windings, and a leak detection arrangement comprising first 2 and second 4 electrodes, associated with potential coolant leak flow pathways, such as join 24, wherein a change of impedance between the electrodes is indicative of a coolant leak. There may be a control arrangement to monitor the impedance. Electrode pairs may be nested, ring shaped, split rings, or elongate with constant separation. There may be coolant inlets 10a, 10b and outlet 14 may which may also be potential coolant leak flow pathways with associated electrodes 2, 4.
