As MRI scanning technology becomes more accessible to research neuroscientists, there has been an increase in interest in studying the functional effects of electrical stimulation on the human brain.  For example, researchers may be interested in examining the effects of peripheral nerve activation on specific areas of the brain, employing noxious and non-noxious electrical stimuli delivered by a Digitimer stimulator.

Due to the presence of strong magnetic fields and RF interference, electrical stimulation within the fMRI environment is technically challenging in order to assure the safety of the patient/subject and operator.  As well as the obvious physical harzard of using devices containing ferrous material in a strong magnetic field, users also have to ensure electrodes and cables used within the scanner will not induce artefacts in the scans or cause RF burns.

There are a number of MRI compatible electrical stimulators available, but they are typically low voltage devices that are not capable of delivering high current levels, especially when used with small surface area and high impedance electrodes.

Digitimer does not manufacture any electrical stimulators or supply accessories that are designated as MRI safe or MRI compatible, BUT this does not prevent use of our stimulators in fMRI based research studies.  Below we provide some guidance for those embarking on this type of study.

Correct Placement of the Stimulator

Our isolated electrical stimulators contain significant quantities of ferrous material, particularly inside the mains transformer, so they cannot be used within the reach of the powerful magnetic field present in an MRI scanner.  This means that when using our stimulators in conjunction with MRI, the stimulator must be placed in a location remote from the scanner and typically the scanner control room is the optimal choice.  The control room will normally contain a power socket for the stimulator and associated equipment, such as laptop or desktop PC.

Control of the stimulation settings can be managed by an operator located in the control room, either manually using front panel controls or via software running on a PC in the room.

Choice of Stimulator

In respect of human research, we are aware of our DS7A/AH/R, DS5, DS8R and D185 stimulators being used alongside fMRI, so there is no MRI specific reason to choose one over another.  As a result, stimulator selection is primarily down to your individual requirements in terms of stimulation parameters and the preferred control method.  In situations where you are happy to manually adjust stimulation intensity and pulse duration, our DS7 range generally meet this need as they suit a “set and forget” protocol that might only be adjusted between subjects.  However if your stimulaiton protocol needs more active intervention by the operator, or you want to introduce a software controlled aspect, then our DS5 or DS8R would be recommended.

As the DS5 was launched in 2007, it has been used in more fMRI studies than the DS8R, but it does have a more complex and technically demanding method of control, which may make it less appealing to some users.  The DS5 operates like a voltage to current amplifier, so unlike many of our other devices, including the DS8R, it is not triggered.  Instead, you need to apply a variable voltage waveform that defines your stimulus shape and the DS5 converts this to an isolated constant current stimulus.  As this stimulator requires an analogue voltage to drive it, it is normally used in conjunction with a PC-based DAQ system and software.

The DS8R is a more conventional triggered stimulator, however, unlike the DS7 range, it adds external control capabilities that mean it can be fully controlled by software via a USB connection.  For situations where higher stimulation frequencies of >10Hz are needed, the DS8R features a hardware trigger-in allowing other devices to trigger it with a standard TTL/digital trigger pulse.

Cabling from the Stimulator to the Scanner

Conventional electrode cabling may be used to carry the stimulator output to the external wall of the scanner, so we would generally recommend that our D185-HB4 output extension cable is used at this stage.  Available in lengths from 2m to 12m, the D185-HB4 features a moulded 4mm connector at the stimulator end and a pair of 1.5mm DIN42802 connectors at the distal end.

MRI scanners must provide an effective radio frequency shield to prevent incoming and outgoing RF signals.  This means that pipes, tubing and fibre optic cables must be passed through a narrow aperture in the wall called a waveguide.  In contrast, electrical cabling, including stimulator cables must penetrate the walls of the scanner via a patch-panel that incoporates an RF-bandstop filter.  This is a passive electronic assembly consisting of inductive and capacitive components which is placed in series with the electrical connection that penetrates the wall.

The 1.5mm DIN 42802 connectors fitted to our D185-HB4 cables are not compatible with MRI patch panels, but if the cable is severed at a convenient point along its length, the two copper conductors within the cable can be wired to plugs compatible with the patch panel connections e.g. standard BNC connectors.

Stimulator Cabling within the Scanner

The internal connections of the patch panel allow a stimulus to be safely carried into the scanner and applied to the subject, however, any cabling connections within the scanner need to be very carefully chosen and approved by the manager of the facility.  Where our D185-HB4 has been used for the external connections from the stimulator to the patch panel, we understand from customer feedback that it may be feasible to use the distal section internally up to a pre-defined distance from the scanner bore, but we have not carried out any of our own testing and each MRI facility will have unique safety requirements that need to be adhered to.

Electrodes and Electrode Leads

All electrodes and leads within the vicinity of the scanner bore must be MR-conditional or MR-safe for your application.  To avoid RF burns, leads need to be appropriately routed without loops and taped down.  For stimulation leads, they must have non-ferrous conductors (e.g. carbon fibre) and MR-rated insulation.

As Digitimer does not supply MR-safe electrodes or leads we recommend these are sourced from an established supplier of MRI accessories, with the appropriate certification.