Once a plane has been defined using the `Outline', `Review' or `Preview' window, the slice through a recorded data set can be visualised using the `Reslice' window.
Here, the `Outline' window shows the position of the slice plane (the shaded green rectangle) going across a data set of recorded scans.
The `Review' window also shows the intersection with the slice plane as a green line.
This is the `Reslice' window itself, showing a nice coronal view of a human kidney. It can be resized to produce larger or smaller pictures, as required.
There is a slider (labeled `B-scan thick') to control the width of gaps that are filled-in perpendicular to each B-scan plane. The default setting is zero, in which case the width is calculated automatically, ensuring that no gaps remain unfilled-in between the B-scans. By manually adjusting this slider, you can get a feel for how far apart the B-scans are and how much interpolation is required to generate the reslice image. Note that setting the width too high does not affect the reslice image (except beyond the edges of the recorded area), but does slow down the interpolation algorithm.
The red line shows the intersection of the slice with the current B-scan (the one displayed in the `Review' window). A `Save' button is provided which enables the reslice image to be saved to a ppm file. 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' button. Note that the reslice image itself, as opposed to a shaded green surface, can be displayed in the `Outline' window by selecting `Texture'.
The 'Voxels' button is an alternative way to open the Voxel Controls window, which can be used to save an entire resliced data set in voxel format, optionally also registered to external data using fiducial points.
The slice plane can also be set with reference to segmentation contours. Here's the `Outline' window, showing a slice plane defined to cut through the bifurcation in a hepatic blood vessel.
Here's the `Review' window, showing the intersection of the slice plane with the current scan (just past the bifurcation).
And here's the `Reslice' window itself, showing the bifurcation quite nicely.
The slider and buttons at the bottom of the `Reslice' window 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.
If the `Max' button is selected, then 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. The example below shows a 15mm thick reslice of a 22-weeks foetus' arm, constructed using maximum intensity compounding.
The thickness of the reslice is evident in the `Review' window. Note how all the fingers (visible in cross-section to the right of the head) are contained within the thick reslice.
For comparison, here is the thin reslice generated by setting the `Reslice thick' slider to zero. Not all the bones in the hand are visible.
If the `Min' button is selected, then 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. The example below shows a 20mm thick reslice through the hepatic blood vessels, constructed using minimum intensity compounding. Compared with the thin reslice further up this page, we can see more, better defined vessels.
If the `Ave' button is selected, then 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. The example below shows a 13mm thick reslice through a 22-week foetus, constructed using average compounding.
For comparison, here is the thin reslice generated by setting the `Reslice thick' slider to zero.
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. Here's an example of a 20mm thick reslice through a bifurcation in the popliteal artery, constructed using maximum intensity compounding.
Note the artefact due to the Doppler region of interest displayed on the ultrasound machine's screen. An automatic segmentation of the artery, displayed in the `Outline' window, shows how the thick reslice encompasses the entire bifurcation.
When average compounding is applied to Doppler data, and at least one of the pixels to be averaged is coloured, then the averaging is applied to the underlying fluid velocities and not the pixel intensities. So grey, very dark red and very dark blue pixels are all associated with zero velocity fluid, bright red pixels are associated with high, positive velocity fluid, and bright blue pixels are associated with high, negative velocity fluid. The velocities are averaged and then converted back to an appropriate colour for display on the screen.
A particularly good way to appreciate 3D
anatomical structure is to animate a thick reslice by interactively
changing the orientation of the reslice plane. You can do this by
clicking on the `Anim' toggle button. Stradx will then calculate
twenty extra thick reslices, by rotating the original thick reslice
around horizontal and vertical axes. Don't worry, this doesn't take
twenty times as long as the original calculation: typically it takes
only about twice as long. When Stradx has finished calculating, hold
down the shift key and then click and drag with the left mouse button
to display the rotated views in real time. If you like one of the
rotated views and wish to save it, use the `Save' button as usual, but
hold down the shift key while pressing the button: this causes the
last displayed rotated view to be saved, instead of the usual centre
view. When you've finished animating, it's a good idea to deselect
the `Anim' toggle button, so that reslicing reverts to its original,
fast speed.
If the `Surface' window is open and displaying
a valid surface segmentation, then everything inside the reslice image
will be cropped to that segmentation. This is particularly useful for
thick reslices, where the surface segmentation can be used to
eliminate distracting background clutter. For example, here's an
uncropped thick reslice showing a foetus in the womb.
Here's the same reslice cropped to a segmentation drawn around the
region of interest.
If the segmentation involves multiple
objects, then the reslice image is cropped to the currently
selected object only. Selecting a different object in the object selection panel causes the reslice
window to redraw.
If the reslice image appears to be corrupted, this may be because
you are trying to visualise a large structure that you have scanned
with multiple sweeps of the ultrasound probe. If this is the case,
then you can improve the image by introducing dividing planes to partition space between
the sweeps.
Calibrated rulers appear
along the sides of the `Reslice' window whenever the `Measurements' window is open.
Note that if image registration
has been applied, this window will show the registered data, not the
original data.
