Double-click your test image so it is loaded into a viewport.

Once the image is loaded, click the 'Annotations' icon to bring up the annotations window.

Click the option for 'Rectangle', which is usually under the 'Measure' tab.

Click and drag a rectangle on a section of the test image.

Once the rectangle has been drawn on the test image, click and select the rectangle.

Within the annotations window, select the 'Tools' tab.

Under the 'Tools' tab, click the button for 'Calculate average grey value'.

The greyscale value will appear within the 'Average gray value' box above the 'Calculate average grey value' button.

This value will be in a 16-bit format so the value will range from 0–65535.