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 and sized to allow display of all the image data even after rotation. 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.
The orientation of the reslice can be changed by dragging the tracker-ball, in which case the rotations are applied relative to the current view in the 3D window. Alternatively, you can click and drag in the reslice window with the rotation tool selected. 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 cursor keys can also be used over these grids or the actual window to move the reslice by a specific increment.
The red line shows the intersection of the reslice with the current frame, and can be disabled using the display controls on the toolbar. If you need to look at orthogonal views of the current reslice, the 'Rotate' buttons rotate the reslice by 90 degrees about each of the reslice axes. Pressing these buttons twice will result in a reslice which is flipped (so you can see the other side) or rotated in-plane (so you can see it the other way up).
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 'Auto centre and size' button, which will adjust the size and location of the reslice so that all the data can be seen, though it does not change the reslice plane offset. Alternatively, the 'Locate to current frame' button 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 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. This and the 'Auto centre and resize' button are particularly useful if the reslice is being used to export voxels to a file.
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 cropping to '(all)', which uses all 3D surfaces, or '(none)', which results in the whole of the reslice being shown. The 'show data outside surface' option displays data outside of the current crop selection, rather than inside.
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. It is also possible to use a script to save a movie of the reslice being animated, for instance rotated. This can be particularly useful in conjunction with very thick 'slab rendering', described below.
The size, orientation and position of the reslice is also saved when the current data file is saved, so that it stays the same the next time the data set is loaded. It can also be saved to a settings file by including the 'general display settings'. This allows you to save multiple different reslice locations (and 3D window viewpoints, and various other parameters) for easy access later by re-loading the saved settings file.
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. However, it is more precise than volume rendering in the 3D window, though also slower since it runs on the CPU rather than the GPU.
If the 'Max' button is selected, the image is constructe 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 and can also be used to mimic X-rays from CT data.
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 reslice rotates the reslice around vertical and horizontal axes.
Two sliders at the bottom of the reslice visualisation page can be used to change the width and height of the reslice plane.