2.8 Bioelectric Field Quantities and Units

The quantities of interest in bioelectric field problems are conductivity, potentials, electric field, dipole source, current density, and current source density. These quantities and their associated units are listed in table 2.1.

Table 2.1: SCIRun bioelectric field quantities and units

Quantity	Unit
Conductivity	amps/volts/meter mathend000# ( siemens/meter mathend000#)
Potential	volts mathend000#
Electric Field	volts/meter mathend000#
Dipole Source	amps/meter mathend000#
Current Density	amps/meter2 mathend000#
Current Source Density	amps/meter3 mathend000#

SCIRun doesn't keep track of unit scale factors (e.g. centi-, milli-, etc). SCIRun assumes a dipole source is in units of amps/meter mathend000#, conductivities are in siemens/meter mathend000#, model geometry is in meters mathend000#; results are therefore in units of volts mathend000#. If this isn't true (e.g. you know that the model is in mm mathend000#), then you have to carry those scale factors through your computation (e.g. know that the results you get out are in kv mathend000#).

The rule of thumb is that if we increase the conductivity, then we decrease the potentials by that same scale factor. Conversely, if we increase the dipole source moment, then we increase the potentials by that same scale factor. i.e. V = I/sigma mathend000#. Similarly, if we hold everything else constant, but increase the size of the domain, then we decrease the potentials by that same scale factor.

For example, the SCIRun Utah Torso model has a length scale of centimeters ( 10^-2meters mathend000#) and conductivity units of siemens/meter mathend000#, so our output potentials are in hectovolts mathend000# ( 10²volts mathend000#).