Implemented coil geometries¶
This section describes the coil geometries currently implemented in MNE. The coil types fall in two general categories:
Axial gradiometers and planar gradiometers and
Planar magnetometers.
For axial sensors, the z axis of the local coordinate system is parallel to the field component detected, i.e., normal to the coil plane.For circular coils, the orientation of the x and y axes on the plane normal to the z axis is irrelevant. In the square coils employed in the Vectorview (TM) system the x axis is chosen to be parallel to one of the sides of the magnetometer coil. For planar sensors, the z axis is likewise normal to the coil plane and the x axis passes through the centerpoints of the two coil loops so that the detector gives a positive signal when the normal field component increases along the x axis.
Normal coil descriptions. lists the parameters of the normal coil geometry descriptions Accurate coil descriptions lists the accurate descriptions. For simple accuracy, please consult the coil definition file, see The coil definition file. The columns of the tables contain the following data:
The number identifying the coil id. This number is used in the coil descriptions found in the FIF files.
Description of the coil.
Number of integration points used
The locations of the integration points in sensor coordinates.
Weights assigned to the field values at the integration points. Some formulas are listed instead of the numerical values to demonstrate the principle of the calculation. For example, in the normal coil descriptions of the planar gradiometers the weights are inverses of the baseline of the gradiometer to show that the output is in T/m.
Note
The coil geometry information is stored in the file mne/data/coil_def.dat
Id |
Description |
n |
r/mm |
w |
---|---|---|---|---|
2 |
Neuromag-122 planar gradiometer |
2 |
(+/-8.1, 0, 0) mm |
+/-1 ⁄ 16.2mm |
2000 |
A point magnetometer |
1 |
(0, 0, 0)mm |
1 |
3012 |
Vectorview type 1 planar gradiometer |
2 |
(+/-8.4, 0, 0.3) mm |
+/-1 ⁄ 16.8mm |
3013 |
Vectorview type 2 planar gradiometer |
2 |
(+/-8.4, 0, 0.3) mm |
+/-1 ⁄ 16.8mm |
3022 |
Vectorview type 1 magnetometer |
4 |
(+/-6.45, +/-6.45, 0.3)mm |
1/4 |
3023 |
Vectorview type 2 magnetometer |
4 |
(+/-6.45, +/-6.45, 0.3)mm |
1/4 |
3024 |
Vectorview type 3 magnetometer |
4 |
(+/-5.25, +/-5.25, 0.3)mm |
1/4 |
2000 |
An ideal point magnetometer |
1 |
(0.0, 0.0, 0.0)mm |
1 |
4001 |
Magnes WH magnetometer |
4 |
(+/-5.75, +/-5.75, 0.0)mm |
1/4 |
4002 |
Magnes WH 3600 axial gradiometer |
8 |
(+/-4.5, +/-4.5, 0.0)mm (+/-4.5, +/-4.5, 50.0)mm |
1/4 -1/4 |
4003 |
Magnes reference magnetometer |
4 |
(+/-7.5, +/-7.5, 0.0)mm |
1/4 |
4004 |
Magnes reference gradiometer measuring diagonal gradients |
8 |
(+/-20, +/-20, 0.0)mm (+/-20, +/-20, 135)mm |
1/4 -1/4 |
4005 |
Magnes reference gradiometer measuring off-diagonal gradients |
8 |
(87.5, +/-20, 0.0)mm (47.5, +/-20, 0.0)mm (-87.5, +/-20, 0.0)mm (-47.5, +/-20, 0.0)mm |
1/4 -1/4 1/4 -1/4 |
5001 |
CTF 275 axial gradiometer |
8 |
(+/-4.5, +/-4.5, 0.0)mm (+/-4.5, +/-4.5, 50.0)mm |
1/4 -1/4 |
5002 |
CTF reference magnetometer |
4 |
(+/-4, +/-4, 0.0)mm |
1/4 |
5003 |
CTF reference gradiometer measuring diagonal gradients |
8 |
(+/-8.6, +/-8.6, 0.0)mm (+/-8.6, +/-8.6, 78.6)mm |
1/4 -1/4 |
Note
If a plus-minus sign occurs in several coordinates, all possible combinations have to be included.
Id |
Description |
n |
r/mm |
w |
---|---|---|---|---|
2 |
Neuromag-122 planar gradiometer |
8 |
+/-(8.1, 0, 0) mm |
+/-1 ⁄ 16.2mm |
2000 |
A point magnetometer |
1 |
(0, 0, 0) mm |
1 |
3012 |
Vectorview type 1 planar gradiometer |
2 |
(+/-8.4, 0, 0.3) mm |
+/-1 ⁄ 16.8mm |
3013 |
Vectorview type 2 planar gradiometer |
2 |
(+/-8.4, 0, 0.3) mm |
+/-1 ⁄ 16.8mm |
3022 |
Vectorview type 1 magnetometer |
4 |
(+/-6.45, +/-6.45, 0.3)mm |
1/4 |
3023 |
Vectorview type 2 magnetometer |
4 |
(+/-6.45, +/-6.45, 0.3)mm |
1/4 |
3024 |
Vectorview type 3 magnetometer |
4 |
(+/-5.25, +/-5.25, 0.3)mm |
1/4 |
4001 |
Magnes WH magnetometer |
4 |
(+/-5.75, +/-5.75, 0.0)mm |
1/4 |
4002 |
Magnes WH 3600 axial gradiometer |
4 |
(+/-4.5, +/-4.5, 0.0)mm (+/-4.5, +/-4.5, 0.0)mm |
1/4 -1/4 |
4004 |
Magnes reference gradiometer measuring diagonal gradients |
8 |
(+/-20, +/-20, 0.0)mm (+/-20, +/-20, 135)mm |
1/4 -1/4 |
4005 |
Magnes reference gradiometer measuring off-diagonal gradients |
8 |
(87.5, +/-20, 0.0)mm (47.5, +/-20, 0.0)mm (-87.5, +/-20, 0.0)mm (-47.5, +/-20, 0.0)mm |
1/4 -1/4 1/4 -1/4 |
5001 |
CTF 275 axial gradiometer |
8 |
(+/-4.5, +/-4.5, 0.0)mm (+/-4.5, +/-4.5, 50.0)mm |
1/4 -1/4 |
5002 |
CTF reference magnetometer |
4 |
(+/-4, +/-4, 0.0)mm |
1/4 |
5003 |
CTF 275 reference gradiometer measuring diagonal gradients |
8 |
(+/-8.6, +/-8.6, 0.0)mm (+/-8.6, +/-8.6, 78.6)mm |
1/4 -1/4 |
5004 |
CTF 275 reference gradiometer measuring off-diagonal gradients |
8 |
(47.8, +/-8.5, 0.0)mm (30.8, +/-8.5, 0.0)mm (-47.8, +/-8.5, 0.0)mm (-30.8, +/-8.5, 0.0)mm |
1/4 -1/4 1/4 -1/4 |
6001 |
MIT KIT system axial gradiometer |
8 |
(+/-3.875, +/-3.875, 0.0)mm (+/-3.875, +/-3.875, 0.0)mm |
1/4 -1/4 |
The coil definition file¶
The coil geometry information is stored in the text file
mne/data/coil_def.dat
. In this file, any lines starting
with the pound sign (#) are comments. A coil definition starts with
a description line containing the following fields:
** <class>**
This is a number indicating class of this coil. Possible values are listed in Coil class values.
** <id>**
Coil id value. This value is listed in the first column of Tables Normal coil descriptions. and Accurate coil descriptions.
** <accuracy>**
The coil representation accuracy. Possible values and their meanings are listed in Coil representation accuracies..
** <np>**
Number of integration points in this representation.
** <size/m>**
The size of the coil. For circular coils this is the diameter of the coil and for square ones the side length of the square. This information is mainly included to facilitate drawing of the coil geometry. It should not be employed to infer a coil approximation for the forward calculations.
** <baseline/m>**
The baseline of a this kind of a coil. This will be zero for magnetometer coils. This information is mainly included to facilitate drawing of the coil geometry. It should not be employed to infer a coil approximation for the forward calculations.
** <description>**
Short description of this kind of a coil. If the description contains several words, it is enclosed in quotes.
Value |
Meaning |
---|---|
1 |
magnetometer |
2 |
first-order axial gradiometer |
3 |
planar gradiometer |
4 |
second-order axial gradiometer |
1000 |
an EEG electrode (used internally in software only). |
Value |
Meaning |
---|---|
1 |
The simplest representation available |
2 |
The standard or normal representation (see Normal coil descriptions.) |
3 |
The most accurate representation available (see Accurate coil descriptions) |
Each coil description line is followed by one or more integration point lines, consisting of seven numbers:
** <weight>**
Gives the weight for this integration point (last column in Tables Normal coil descriptions. and Accurate coil descriptions).
** <x/m> <y/m> <z/m>**
Indicates the location of the integration point (fourth column in Tables Normal coil descriptions. and Accurate coil descriptions).
** <nx> <ny> <nz>**
Components of a unit vector indicating the field component to be selected. Note that listing a separate unit vector for each integration points allows the implementation of curved coils and coils with the gradiometer loops tilted with respect to each other.