FAQ7 ECT IMAGE FORMAT

The permittivity distribution of a mixture of two fluids is often displayed as a series of normalised pixels located on either a (32 x 32) or (64 x 64) square pixel grid, using an appropriate colour scale to indicate the normalised pixel permittivity as shown, for example, in the figure below. This uses a graduated blue/green/red colour scale, where pixel values corresponding to the lower permittivity material used for calibration have the value zero and are shown in blue, while pixels corresponding to the higher permittivity material have the value 1 and are shown in red. The normalised permittivity distribution corresponds to the fractional concentration distribution of the higher permittivity material. Back to top

Typical ECT image illustrating permittivity colour scale

If the sensor cross-section is circular, this circular contour must be projected onto the square grid containing typically 1024 pixels. Some of the pixels will lie outside the vessel circumference and the image is therefore formed from those pixels that lie inside the vessel. A typical arrangement which is commonly used is shown in the figure below, where the circular image is constructed using 812 of the available 1024 pixels. Back to top

Circular image pixel grid

The resolution of an ECT permittivity image is limited by the number of independent measurements that can be made and this relationship can be considered to be an example of spatial filtering, as shown in the figure below. The resolution limit is difficult to define mathematically, but a simple engineering estimate can be made by assuming that the number of independent measurements M corresponds to a similar number of discrete regions inside the sensor. If we assume that the angular resolution is equal to the number of electrodes E, then the radial resolution will equal M/E. For protocol 1 and a 12 electrode sensor, this gives a radial resolution limit of 5.5. For protocol 2 and 24 electrodes, this figure increases to 10.5. Back to top

Effect of image reconstruction algorithm on image resolution

It is not possible to obtain a unique solution for each image pixel when the number of pixels in the image exceeds the number of capacitance measurements. Furthermore, image distortion can occur because ECT is an inherently soft-field imaging method (the electric field is distorted by the material distribution inside the sensor). Back to top

Last updated 17-05-2002

Copyright 2001 PTL