$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
Critical Steps Within the Protocol
The most important component of any dTMS protocol is the correct measurement of the MT. The MT determines the individualized dosage or stimulator intensity necessary and safe to treat the patient. If a patient's MT is incorrectly measured at higher than their actual MT, they will end up getting a higher intensity treatment, increasing the patient's seizure risk. Similarly, if the patient receives too low of a dose (e.g., 110% of the MT instead of 120% during treatment for depression), they will not go into remission. It is also imperative that the component of the coil that is being used is positioned on the head over the region one is trying to stimulate. When stimulating the left PFC, the wires from the left front half of the helmet should be touching the skull overlying the left PFC; there may be several centimeters of space between the right side of the helmet and the skull. When stimulating the right PFC, the right front half of the helmet should be touching the skull overlying the right PFC, and there will probably be a space between the left side of the helmet and the skull. When stimulating the mPFC, the front of the helmet should be pushed down onto the top of the forehead. The sides of the coil can be brought closer together by tightening a drawstring in the back of the coil.
Modifications and troubleshooting
The most common modifications in clinical practice are adjustments to the tilt of the coil while it is over the PFC, due to comfort, and differences in the distance of the coil from the MC, caused by variations in head size. If a patient feels too much right temporal stimulation during the left PFC protocol for depression, the helmet can be tilted towards the symmetrical position. Additionally, if advancing the coil 6 cm from the MC puts the front of the helmet below the patient's eyebrows, the helmet should be adjusted posteriorly. If there is difficulty in finding the resting MT, the first step should be to find the active MT, which is always lower.
Limitations of the technique
The stimulation protocols listed in Table 1, with the exception of major depression, are far from final. Even the depression protocol may not be optimal. These are potential protocols that were conceived according to knowledge available at the time of the specific experiment, and when they were utilized over those anatomical regions, they were successful. As time goes by, protocols can be improved due to accumulation of knowledge with regard to the brain network that is involved in the specific neuropathology, dTMS field distribution, mechanism of action, optimal parameters, safety data, device durability data, and publication of more and larger case series. In addition, if one wanted to stimulate a very focal, specific target, this would not be an appropriate coil. For such a goal, the figure-8 coil, which stimulates very focal and superficial regions on the cortex surface, would be better suited. However, since stimulation by the figure-8 coil is so focal, it can easily miss important DLPFC structures relevant for mood disorders. Indeed, with the simple 5-cm rule, the figure-8 may even be located outside the PFC1,29. Moreover, recent studies suggest that stimulation of prefrontal cortical regions with extensive connections to the subgenual cingulate may be crucial for the antidepressant action of standard rTMS2,3,30. Since the exact location of these cortex regions varies greatly between individuals3, optimal stimulation targets may be easily missed with a figure-8 coil. In order to remedy this problem, the physician must send the patient to have an fMRI and must use neuro-navigation. All these problems do not arise with the H1, since its broad field stimulates all the relevant PFC targets.
Significance of the technique with respect to existing/alternative methods
The H1 dTMS coil is the newest coil to enter the rTMS arena. It has been widely adopted by psychiatrists due to its high efficacy and tolerability for patients with treatment-resistant depression, its short treatment time, and its ease in determining the MT. All of these are functions of the ability of the H1 to stimulate a much deeper and larger volume of neuronal tissue than figure-8 coils. However, the fact the coil is in a helmet and is not visible to the eye makes the idea of moving the coil from its intended target almost heretical. Additionally, the hard external helmet causes clinicians to forget that a key aspect of the H-coils is their design with soft, bendable copper wires. The base of the coil is meant to be adjacent to the skull near the neuronal fibers that one wants to stimulate. It is conceptually difficult for clinicians who have not taken math and physics in many years to comprehend the design of the dTMS coils.
Figure-8 coils are easier to understand, completely visible, and their effects are very focal. Clinicians are much more comfortable moving them from location to location. Additionally, they have been in use for many more years, and there are more publications describing their use for off-label conditions. However, this should not discourage the application of the H1 coil to targets outside the DLPFC in accordance with the protocols that were reviewed here or in a novel fashion.
Regarding the electric field diagrams as a measurement of the potential effects of the device, electric field diagrams measured from a saline solution-filled head model have advantages over alternative methods. Some investigators have calculated or modeled the induced fields using a spherical head model, which is less accurate31,32,33,34. Measuring the induced field of the real coil in a realistically-shaped head model filled with saline is more representative than any mathematical model, but it is not completely accurate35. Recently, investigators have modeled the electric fields in anatomically-correct virtual tissue34,36,37,38. More accurate electric field diagrams could be obtained from cadavers implanted with multiple recording electrodes, but this experiment has not yet been done.
Future applications or directions after mastering this technique
After understanding the concept of reviewing the coil diagram and the electric field diagram to apply the coil to different anatomical targets, use the same procedure for different H-coils and disorders based on what is already known in the literature with regard to possible targets and stimulation parameters. For example, the H7 coil is designed to be placed over the mPFC and anterior cingulate cortex (ACC) for the treatment of OCD. The H7 coil can be placed over the medial MC for the treatment of diabetic neuropathy of the feet and over the posterior parietal cortex (PPC) for stimulation of the precuneus in mild cognitive impairment.