Ortho

This visualisation shows two orthogonal reslices: these are images calculated from the pixels in the original 3D data that are closest to a particular plane. The planes are initially positioned to intersect the image currently shown in the image window. The reslices are shown flat in the bottom windows, and displayed texture-mapped in the 3D window. The default interpolation is linear, however faster nearest-neighbour interpolation can be enabled by clicking the 'Faster, less accurate reslice' checkbox.

Positioning and saving the reslices

The position of the reslices within their planes can be changed using the tracker-grids labelled 'Translate'. The translations are applied relative to the orientations of the reslices shown in the bottom windows. The reslices can be moved perpendicular to their planes using the tracker-strips labelled 'Offset'.

The red lines show the intersections of the reslices with the current frame.

If you cannot see the reslice planes in the 3D window and the images are blank, this is probably because the two orthogonal reslice planes are positioned a long way from your data. This can be fixed quickly by pressing the 'Centre on current frame' button, which will move the reslice planes to the frame that is currently selected (using the slider that runs across the bottom of the application window).

It is possible to crop the reslices to the boundaries defined by the intersection with any 3D surface defined on the draw task page. A selector box towards the bottom of the 'Ortho' visualisation page can be used to select which object to use for the cropping. This selector also has an option for no cropping, which results in the whole of both of the reslices being shown.

When you have got images you like, you can save them in a variety of formats (Windows bitmap, PNG, JPEG, TIFF) using the two buttons at the bottom of the task page.

Slab rendering

The slider and buttons at the bottom of the 'Ortho' visualisation page can be used to construct 'thick' reslices, where the reslice plane has a finite thickness. In effect, when the 'Reslice thick' slider is not set to zero, a number of reslice images are computed for a stack of parallel reslice planes, and then compounded together before being displayed on the screen. The thickness of the stack is set by the 'Reslice thick' slider, while the three buttons control the nature of the compounding. Another way of thinking about the thick reslice facility is as a form of volume rendering.

Be warned that thick reslices can take a long time to compute. To avoid multiple computations, use the middle mouse button to locate the slider with a single click, rather than dragging it with the left button.

If the 'Max' button is selected, the image is constructed using maximum intensity compounding: only the brightest pixels in the stack are displayed. Maximum intensity compounding is good for highlighting strong reflectors like bone.

If the 'Min' button is selected, the image is constructed using minimum intensity compounding: only the darkest pixels in the stack are displayed. Minimum intensity compounding is good for highlighting fluid-filled cavities and blood vessels.

If the 'Ave' button is selected, the image is constructed using average compounding: the pixels in the stack are averaged together to construct the reslice image. Average compounding is good for revealing out-of-plane structure while reducing speckle noise.

With Doppler data, we need to define what we mean by the 'brightest' or 'darkest' pixel. For the purpose of thick reslices, we consider any coloured pixel (red or blue) to be brighter than any grey pixel. Where we need to choose between red and blue pixels of the same intensity, we (arbitrarily) consider the red pixel to be brighter.