Reslice

A reslice is an image calculated from the pixels in the original 3D data that are closest to a particular plane. The plane is initially positioned to intersect the image currently shown in the image window. The reslice is shown flat in the bottom window, 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 reslice

The orientation of the reslice can be changed by dragging the tracker-ball. The rotations are applied relative to the current view in the 3D window.

The position of the reslice within the plane can be changed using the tracker-grid labelled 'Translate in plane'. The translations are applied relative to the orientation of the reslice shown in the bottom window. The reslice can be moved perpendicular to its plane using the tracker-strip labelled 'Translate normal to plane'.

The red line shows the intersection of the reslice with the current frame. If you would rather view the slice plane from the other side, press the 'Flip' button. Alternatively, if you would rather view the slice upside down (rotated through 180 degrees), press the 'Rotate 180' button.

If you cannot see the reslice plane in the 3D window and the reslice image is blank, this is probably because the plane is positioned a long way from your data. This can be fixed quickly by pressing the 'Locate to current frame' button, which moves the reslice plane so that it intersects the current frame (selected using the slider that runs across the bottom of the application window).

It is also possible to determine the location of the reslice image using landmarks. First select three landmarks on the Landmarks task page. When you press the 'Locate to selected landmarks' the plane will be set to pass through the landmarks. It will be centred on landmark A, with landmark B above.

The 'Locate for minimum voxels' button is useful if the reslice is being used to export voxels to a file. Pressing this button will automatically locate the reslice at an orientation which minimises the size of the output voxel data, and with a width and height which will ensure that all the data is exported.

It is possible to crop the reslice to the boundary defined by the intersection with any 3D surface defined on the draw task page. A selector box towards the bottom of the 'Reslice' visualisation page, labelled 'Crop to surface', 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 the reslice being shown.

When you have got an image you like, you can save it in a variety of formats (Windows bitmap, PNG, JPEG, TIFF) using the 'Save' button at the bottom of the task page. Alternatively, the 'Save movie' button will save a sequence of 72 reslice images with the same choice of formats. During this sequence, the reslice is gradually rotated about its central vertical axis. The sequence can be used to create movies, particular useful in conjunction with very thick 'slab rendering', described below.

Slab rendering

The slider and buttons at the bottom of the 'Reslice' 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.

With the rotation tool selected, clicking and dragging on any slab rendering rotates the slab around vertical and horizontal axes, giving a useful indication of the 3D anatomy.

Other controls

Two sliders at the bottom of the reslice visualisation page can be used to change the width and height of the reslice plane. There is also a checkbox and a slider for controlling partitioning when using multiple sweeps.