The Colours

There are a number of factors that must be considered when viewing a thermal image; the colour map, the temperature base, and the temperature range. Most important though is to understand what the colours themselves mean, so we will start there.

The Colours - It is important to understand that a particular colour is not assigned to a particular temperature in all thermal images.

The colours are simply a visual aid to understanding the temperature gradients involved in each image. Colours are assigned arbitrarily according to the particular colour map used for an image (see below).

To assume that a particular colour means warmer, cooler etc. is to make a dangerous assumption. The Australian Thermology Association set out a default standard which is explained below, but can also be viewed in its Code of Practice.

The Colour Bar is the display on the left of each of the pictures which shows the available colours relative to the range. The portion called Cursor showing a temperature is used only in the imaging program and is not relevant to this discussion.

The Colour Map - From the simple concept of a greyscale image versus a colour image, an understanding of colour maps can be gained. Essentially, colour maps define the number of colours that will be attributed to a temperature band in a particular image.

To the right are three breast images of a lady who has a number of cysts in the left upper breast, with the top image in a standard 16 colour "colour map", the middle in greyscale "colour map", and the bottom in red/green/blue "colour map".

The Temperature Base

 - The temperature base is displaying an 8°C spread (range) between 27°C and 35°C. The displayed colours all associate with specific temperature bands.

Looking at the this image in a Standard 16 colour "colour map", there are sixteen colours available for assignment in the image from white to black.

Each colour in this colour map is assigned a half degree temperature band. In this image dark red may be assigned to any temperature between 34.5°C and 35°C. This is also the standard default colour map used in all Thermal Images.

Lowering the Temperature Base

To understand the principle of temperature base adjustment, it is easier to see what happens to the same images if we keep the 8° temperature range, but lower the temperature base 1.5 degrees to represent a temperature base of 25.5°C to 33.5°C degrees (from 27-35).

As you can see, we have lost a great deal of definition "out of the top' of our colour scale.

In other words, the temperatures of the subject lie within a certain range, and the base selected for the displaying of those temperatures is inappropriate, failing to represent the subject fully.

Raising the Temperature Base

Likewise, if we raise the represented temperature base 1.5°C to represent an 8° temperature range from 28.5°C - 36.5°C, you can see that we have lost detail "out of the bottom" of the represented temperatures.

Thus, choosing the right temperature base is critical to the images being useful in a clinical sense.

The Temperature Range

Now you have seen the changes that setting the temperature base can make, alteration of the temperature range will be examined.

The temperature range is the actual band of temperature to which colours are applied.

We will continue to use the top breast image in our examples, but these principles hold for all of the colour maps. To recap so far, using the standard 16 colours, the images have been assigned sixteen colours over a band of eight degrees.

This equates to a half degree step between each colour (8/16 = 0.5). By decreasing the temperature range, the bands of temperature represented by each colour also decreases correspondingly. 

In the images to the right, the range has been narrowed to 4°C. There are still the same colours available (this is the colour map), but the colours are now assigned to a spread of 4° instead of 8°.

In the standard colour map each colour now represents a 0.25 (1/4) degree step between each colour (4/16 = 0.25).

You can see that narrowing the range can actually increase the definition of a porion of the image, but it also loses a lot of information over the rest of the image.

A narrowed temperature range will be used as an additional image (to the standard) to better define a particular phenomena.

Standardised Images and Modified Images

The Australian Thermology Association has set a range of 8 degrees and a colour map of 16 colours as the default settings that its members will commonly save an image in. Should there be the need to deviate from this standard, a second image will be saved with the new settings.

This means that in every study, there will be a standard format image AND an additional special format image as well. There should never be a case where there is no standard format image.

When one body part, say an outside thigh, is imaged, it must be saved using the same settings as its contra-lateral (other side) partner. By doing the scans this way, when you look at a left and right outside thigh, you know the colours mean the very same thing for both images. The images below illustrate this principle, and the one described in the previous paragraph.

The above image is the outside left thigh and knee, the centre is the outside right thigh and knee, with both images being captured in the same temperature base and range in the standard colour map.

As you can see, there is a large hot line on the outside thigh in the centre image, with most of the details of the actual phenomena beyond the top end of the temperature base (white).

The bottom image is a modified version of the centre picture showing greater detail of the inflamed varicose vein that was causing the symptoms. This is also a good example of contra-lateral comparison.

Summary 

• The colour map determines how many colours and what colours are assigned to specific temperature ranges.
• The temperature base is the top and bottom temperature to which a colour is assigned by the colour map.
• The temperature range determines what spread of temperatures will be examined in a particular image.
• All images, unless labelled otherwise, will be saved in sixteen colours representing a range of eight degrees.
• All contra-lateral (other side) images must be saved in identical colour map, base and range.