While the `Reslice' window
extracts data on a
The manifold is defined in the `Reslice' window. First, select a slice which shows the curvilinear structure you wish to visualise. The example below shows a bifurcation of the hepatic veins, where we wish to examine the two branches meeting at an acute angle.
Next, trace along the curvilinear structure using the left mouse button. This fully defines the manifold, which is constructed by sweeping the green curve through 3D space along a path normal to the slice plane. In other words, the green curve in the `Reslice' window shows an end-on view of the manifold.
If you prefer, you can trace along the curvilinear structure in the `Reslice' window one vertex at a time. To do this, switch on the keyboard's `Caps Lock' and then click with the left mouse button at the first vertex position: a green cross appears where you clicked (with some operating systems, it is necessary to hold down the `Caps Lock' key while clicking with the mouse). You can now define subsequent vertices by clicking with the left mouse button. Click with the right mouse button to define the final vertex. Remember to switch off the `Caps Lock' when you want to revert to the continuous curve definition mode.
If necessary, you can edit the green curve using the middle mouse button. The edit procedure is similar to that described elsewhere for segmentation contours. Clicking near the green curve with the right mouse button deletes the manifold.
Edit operations can also be carried out one vertex at a time. The first vertex is defined by clicking the middle mouse button while holding down the control key. Click with the middle mouse button to define subsequent vertices, and the right mouse button for the final vertex.
Now open the `Manifold' window using the `Visualisation' menu. Data is extracted along the manifold and flattened out onto a plane, producing the image below: note how the selected veins are now straight.
The flattening process used to produce the manifold image is not a projection. If you think of the manifold as a piece of curved sheet metal, the image is produced by "hammering" the sheet of metal flat. For this reason, distances measured in the manifold window do not correspond to real anatomical distances.
A `Save' button is provided which enables the manifold image to be saved to a ppm file. 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 manifold image. Note that setting the width too high does not affect the manifold image (except beyond the edges of the recorded area), but does slow down the interpolation algorithm.
The manifold also shows up in the `Outline' window as a shaded green surface (unless you have switched it off using the `Mfold' toggle button, or asked to see the image itself, instead of the green surface, using the `Texture' toggle button). In the example below, the outline window has been configured to show the manifold and segmentation contours only. Notice how the manifold passes through the bifurcation and also some other segmented vessels.
The manifold also shows up in the `Review' window as a green curve (unless you have switched it off using the `Reslice' toggle button). The curve shows the intersection of the manifold surface with the current B-scan. In the example below, the review window has been configured to show the intersection with the reslice plane and manifold surface (green line and curve respectively), and the segmentation contours (cyan curves).
Here's another example, showing how the `Manifold' window can be used to obtain a frontal view of a 16-week foetus' leg. The leg was bent at the knee and ankle, but has been unrolled by the `Manifold' window.
Another obstetrics application is to visualise the foetal spine. The example below shows a 22-week foetus. The contour in the `Reslice' window follows the spine, with the resulting non-planar reslice displayed in the `Manifold' window below.
The slider and buttons at the bottom of the `Manifold' window can be used to construct `thick' manifolds, as described in the documentation for thick reslices. Here's the foetal spine again, this time using maximum intensity compounding with a 3mm manifold thickness. Note the improved bone visibility: by giving the manifold a little thickness, it is not necessary to position the surface so precisely in order to see the desired anatomical structure.
The extent of the thick manifold is apparent in both the `Reslice' and `Outline' windows. Note that the `Outline' window in this example is also displaying the reslice plane and user-defined segmentation contours.
Here's another example: a 7mm thick manifold is defined in the `Reslice' window below.
Here's the resulting thick non-planar reslice, which shows an almost complete picture of the unrolled skeleton, including the spine, ribcage, pelvis, both femurs, both fibulas and tibulas and both feet. A small section of umbilical cord is visible between the femurs, though neither arm is contained within the reslice volume. Maximum intensity compounding has been used to highlight strong reflectors like bone.
If the `Surface' window is open and displaying a valid surface segmentation, then everything inside the manifold image will be cropped to that segmentation. This is particularly useful for thick non-planar reslices, where the surface segmentation can be used to eliminate distracting background clutter. Have a look at the `Reslice' window documentation for a planar example.
If the manifold 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 `Manifold' 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.