**6. TMS systems for OCD**

#### **6.1. Coil types**

Transcranial magnetic stimulation (TMS) uses magnetic pulses to induce electrical current in the underlying neuronal tissue. There are a variety of different TMS coils on the market, and they differ primarily with the orientation and flexibility of the wire windings in the coil. Several types of coils have been used in sham-controlled OCD studies. The first is a rigid circular coil. These are typically large and very non-focal as the induced current is identical in intensity anywhere under the wire. The orientation of the current in relation to the neurons is important. The coil can be reoriented 180° to switch the direction of the current under one side of the coil, but the other side of the coil will still have an effect. The second is a figure-8 or butterfly shaped coil. This is a rigid coil, usually in a 180° plane, which induces the strongest current beneath the center of the two circular coils. However, the field decays relatively rapidly. The manufacturer for most of the research figure-8 systems is Magstim, but Magventure (formerly Medtronic) makes a double-blind figure-8 system as well. Approximately 20 companies make active figure-8 coils. An option not yet used is a bent or double cone coil, which is also a rigid coil, but it is larger and bent at a 120° fixed angle. It is capable of reaching a greater depth than a figure-8 coil. To date, it has never been used in an OCD clinical trial, but it is made by many companies similarly to the figure-8 coil. The newest coil on the market is the H7 or HAC-coil. It has flexible windings that run along the skull and sum at depth, and these windings are tightened to the head. It can reach 3 cm beneath the cortex at high frequency and high intensity; additionally, the magnetic field decays slowly. The H-coil has gaps between the central groups, which make the field different from the fixed denser distribution of the bent or double cone coil. The H7/ HAC coil had the largest and only multicenter sham-controlled OCD study to date.

#### **6.2. Cooling systems**

When TMS is done repetitively (rTMS), especially at high frequencies for long periods of time, the coils will heat up. To prevent this from occurring, the coils are cooled using one of three methods; fan cooled with room temperature air (Magstim), liquid cooled (Magventure or Mag and More), or fan cooled with a forced cooled air system (Brainsway). The figure-8 coil and bent coil are available liquid cooled or fan cooled. The H-coil uses a forced air conditioning system.

#### **6.3. Blinding system**

ignored. These regions include the dorsolateral prefrontal cortex (DLPFC), orbitofrontal cortex (OFC), medial prefrontal cortex (mPFC), cingulate cortex, caudate nucleus, striatum and thalamus. An abnormality in the functioning of this pathway results in impulsivity, compulsivity, obsessivity, uncertainty, deficits in attentional allocation, sensory-motor gating, modu-

The greatest evidence for OCD as a circuit disorder comes from the success of circuit interventions at various locations along the pathway. Specifically, circuit interventions have demonstrated efficacy at the striatum, globus pallidus interna, substantia nigra, thalamus, subthalamic nucleus, anterior cingulate cortex (ACC), OFC and anterior capsulotomy [5].

Transcranial magnetic stimulation (TMS) uses magnetic pulses to induce electrical current in the underlying neuronal tissue. There are a variety of different TMS coils on the market, and they differ primarily with the orientation and flexibility of the wire windings in the coil. Several types of coils have been used in sham-controlled OCD studies. The first is a rigid circular coil. These are typically large and very non-focal as the induced current is identical in intensity anywhere under the wire. The orientation of the current in relation to the neurons is important. The coil can be reoriented 180° to switch the direction of the current under one side of the coil, but the other side of the coil will still have an effect. The second is a figure-8 or butterfly shaped coil. This is a rigid coil, usually in a 180° plane, which induces the strongest current beneath the center of the two circular coils. However, the field decays relatively rapidly. The manufacturer for most of the research figure-8 systems is Magstim, but Magventure (formerly Medtronic) makes a double-blind figure-8 system as well. Approximately 20 companies make active figure-8 coils. An option not yet used is a bent or double cone coil, which is also a rigid coil, but it is larger and bent at a 120° fixed angle. It is capable of reaching a greater depth than a figure-8 coil. To date, it has never been used in an OCD clinical trial, but it is made by many companies similarly to the figure-8 coil. The newest coil on the market is the H7 or HAC-coil. It has flexible windings that run along the skull and sum at depth, and these windings are tightened to the head. It can reach 3 cm beneath the cortex at high frequency and high intensity; additionally, the magnetic field decays slowly. The H-coil has gaps between the central groups, which make the field different from the fixed denser distribution of the bent or double cone coil. The H7/

HAC coil had the largest and only multicenter sham-controlled OCD study to date.

When TMS is done repetitively (rTMS), especially at high frequencies for long periods of time, the coils will heat up. To prevent this from occurring, the coils are cooled using one of three methods; fan cooled with room temperature air (Magstim), liquid cooled (Magventure or Mag and More), or fan cooled with a forced cooled air system (Brainsway). The figure-8 coil and bent coil are available liquid cooled or fan cooled. The H-coil uses a forced air conditioning system.

lation of motor activity and more [6, 7].

92 Transcranial Magnetic Stimulation in Neuropsychiatry

**6. TMS systems for OCD**

**6.1. Coil types**

**6.2. Cooling systems**

Clinical trials with TMS ideally should include blinding for the patient, rater and operator. However, in order for the operator of the TMS device to be blinded a unique research TMS system is required. Because of this many TMS studies do not have a blinded TMS operator and the sham arm has the operator rotate the coil 90° against the scalp delivering cutaneous stimulation with the identical sound. Magventure manufactures a double-blind coil, which is in one device; then, the computer tells the operator which side of the coil to use for the patient. Magstim has separate active and sham coils. When conducting a clinical trial, one can use coil A or coil B, as well as a third coil to determine the motor threshold. Both the Magstim and Magventure, require a cutaneous nerve stimulator to induce a superficial sensation during the sham train. Some single-blind studies do not even create a cutaneous sensation at all. Brainsway has the most practical approach, since the H-coil is in a helmet both the active and sham coil is in the same helmet. The subject is assigned a card that interacts with the stimulator and coil through an interface module. The motor threshold is determined with an operator card; then the coil is advanced to the treatment position, and the subject card is inserted, which selects whether the sham or active coil is activated. The sham coil is made of conical windings that do not penetrate the cortex; so, the identical sound and a superficial sensation are felt, but no neuronal stimulation is induced.
