Neuromagnetic instrumentation.

In considering what type of MEG system is needed, there are four main considerations: 1. Ambient magnetic noise at the intend site(s). The need to reject external noise will determine the need for a shielded room. Here, manufacturer's claims can be compared in terms of magnetic field sensitivities (BN/square root Hz) at the measurement site. There are several tradeoffs to consider. A relatively quiet environment may allow use of a less expensive eddy-current-shielded room combined with second-derivative gradiometer coils. A more harsh environment might need an MSR, but a gain in sensitivity may be afforded by using first-derivative gradient coils. A truly hostile environment could require multiple eddy-current shields combined with an MSR using three or more layers of mu-metal. 2. Head coverage and spatial resolution. The number of channels will roughly determine the number of times the Dewar(s) must be moved to cover the entire region of interest. Until MEG systems are available that can cover the entire head, coverage will be an important factor. Spatial resolutions (related to the diameter of the pickup coils and their spacing) should be adequate for all intended measurements. 3. Required sensitivity. Since the objects to be studied are current dipoles, magnetic field sensitivities (in fT/square root Hz) are not appropriate. This should be in terms of sensitivity to a current dipole measured in ampere-meters (Eq. 10) as a function of depth below the bottom of the Dewar tail. 4. Data acquisition systems and system software. Major considerations include the following: At what rate is data to be gathered? What is the total amount of data to be gathered in a single session? Must the data be processed real-time? How is the data to be interpreted? How is the data to be displayed? By examining these factors, it should be possible to compare available systems for neuromagnetic measurements and determine which system is appropriate for your needs.