rTMS therapy aims to induce depolarization of the cortex located just below the stimulation coil thanks to a magnetic field.2 The intensity of the stimulation to deliver to the DLPFC is determined by measuring the MT. The MT is defined by the electric charge required to induce a muscle response.1
But the distance from the scalp to the DLPFC may be greater than the distance from the scalp to the motor cortex as in the present case (Figure 1). Since the magnetic field decreases dramatically with distance from the source, we were unable to confirm that DLPFC neurons were activated in this case.3
FIGURE 1.Parasagittal MRI of the Right Cerebral Hemisphere: Difference Between the Depth of the Motor Cortex and the Dorsolateral Prefrontal Cortex
By way of illustration, we used an electric charge of 94 A/ms to stimulate the DLPFC (120% MT) in our patient, in whom the distance from the scalp to the motor cortex was 1.8 cm whereas that to the DLPFC was 2.2 cm. When we placed the coil over the motor cortex at a distance of 2.2 cm with electric charge at 94 A/ms, we found that we were unable to induce a motor response in the contralateral muscles. In fact, the electric charge required to reach the MT in our patient at this distance from the motor cortex was 125 A/ms. This value is so high that it was impossible to treat the patient using 120% MT, because it would have required 100.8% of the stimulator’s power.
Brain anatomy including variations in the depth of the cortex may be an important factor for the efficacy of TMS therapy.