Where do the two differ?
As previously mentioned, the most obvious difference between TMS and TPS is that TMS uses magnetic fields to influence the brain, whereas the TPS uses high-frequency acoustic pulses.
Type of stimulation
The second difference between the treatments is what is meant by “stimulation”. TMS causes neurons to fire by altering transmembrane currents. By increasing the rate at which neurons in that specific area of the brain are firing during treatment the therapeutic effect is achieved.
TPS does not cause neurons to fire. The momentum of the waves create slight movements in the cells they pass, stretching and deforming them and temporarily increasing their permeability to ions and certain molecules, as well as inducing the release of growth factors and other beneficial chemicals. The therapeutic effect of TPS is thought to be due to the release of these chemicals as well as increases in cerebral blood flow.
TMS causes a small amount of tissue heating when used. This is typically less than 1°C, so is unlikely to cause damage to brain tissue, but it is a factor that the practitioner needs to consider when providing the treatment. Brain implants such as aneurysm clips and stimulation electrodes can heat up quickly during TMS.
Because of the nature of the energy emitted during TPS, tissue heating is not a concern. Any heating caused would be almost immeasurable and will have no effect on brain tissue or the patient.
Treatment depth & focal point
The maximum treatment depth of TMS depends on how deep the magnetic field of the device penetrates when held against the skull. This is typically around 5cm. A type of TMS called deep TMS, which is less commonly used, can reach deeper areas of the brain. The treatment area is also typically broad in TMS treatments, as any neuron within the magnetic field will be influenced.
TPS can reach slightly deeper into the cortex, up to 8cm. Similar to focused shockwave therapy, the strength of the acoustic waves in TPS are not strongest at the source, but rather at the treatment area. Because of this the energy can be focused with millimetre accuracy at a targeted brain area and minimise effects on surrounding tissue.