Caliper-based precise positioning of the target (CALIPPOT) for transcranial magnetic stimulation without neuronavigation system

Abstract

To the editor: Transcranial magnetic stimulation (TMS) is a non-invasive brain modulation technique. One important usage of TMS is the transient interruption of cognitive brain function (also named virtual lesion) for investigating precisely where and when a specific cortical region contributes to a specific cognitive function.1 A more important usage of TMS is the treatment of brain disorders by repetitive TMS (rTMS). The spatial accuracy of the ‘Figure-8’ coil could be up to 3 mm with a TMS robot.2 Functional magnetic resonance imaging (fMRI) has been used to guide neuronavigation systems for precise positioning of TMS targets.3–6 While rTMS is a routine treatment approach in many hospitals, few practitioners are using neuronavigation systems. One major reason is the expense of neuronavigation systems which is usually more than CN¥350 000 (US$50 000) and more expensive than the TMS machine itself. Another reason is the complexity of its usage. Here, we proposed a simple, precise and cheap method, named Caliper-based precise positioning of the target (CALIPPOT) for TMS without a neuronavigation system. After MRI scanning with two or more imageable marks, experimenters use two outside callipers to precisely locate the stimulation target on the scalp. Each outside calliper costs about CN¥200 (US$28) and is reusable. The imageable marks are disposable and cost about CN¥0.6 (US$0.08) for each participant. The positioning duration is less than 10 min. Two experimenters tested the accuracy in 10 participants. The mean error was 2.32 mm. All participants signed informed consent before scanning. The following introduces the positioning steps and then the verification experiment. ### Step 1. Marks on the scalp MRI imageable marks were purchased from an e-shop (https://shop196017839.taobao.com)—this is a kind of anti-collision silica gel (round, 10 mm diameter, 2 mm thickness) with glue on one side. Before putting the marks on the scalp, we drew a point at the centre …