2D and 3D strain imaging

Stradwin offers four different ways to acquire and visualise strain data. Common to them all is the need to record RF data: it is not possible to calculate strain images from rasterised B-mode images.

2D strain imaging

2D strain images are calculated by varying the probe contact pressure while tracking the relative displacements of windows of sampled RF data in sequential frames, differentiating and filtering this displacement, then mapping both strain and quality values to a display colour. The contact pressure does not need to vary much: relative strains of between 0.1 and 1 percent are ideal for the strain algorithms in Stradwin.

Stradwin displays strain images in real-time. There are many parameters that you can vary, but as a quick-start guide, you should: (a) configure a suitable RF image source (either a Gage Card, or an interface to a Terason or Sonix ultrasound machine); (b) use the RF display dialog to tell Stradiwn to display strain images and B-scans side-by-side; (c) enable live mode in the main tool bar; (d) use the selection tool to select a suitable region of interest for strain imaging. Stradwin will then display the strain image in real-time, with low precision strain estimates obscured behind a colour wash: this facility is especially useful when learning how to acquire good quality strain data. The strain task page provides a simple interface for visualising strain data, and the advanced RF strain settings dialog exposes many of the strain imaging algorithm's low-level parameters.

Tracked freehand 3D strain imaging

If you have a calibrated position sensor, you can record a volume of freehand 3D ultrasound data, varying the probe contact pressure a little as you go. As long as the frames are close together (within the transducer's elevational decorrelation range), it is possible to estimate the axial strain between neighbouring frames and construct a 2D strain image for each pair of adjacent frames. Moreover, since Stradwin knows the relative locations of the 2D strain images, it can reslice the data just as with any other 3D data set. Selecting the 'Ortho' or 'Reslice' visualisation option causes Stradwin to run a smart 3D filter over the strain data to produce a 3D data set, which can then be resliced in the usual way. The 3D filtering strength is controlled by the 'filter range' slider on the RF strain settings dialog.

Mechanically-swept inter-volume 3D strain imaging

If you have a mechanically-swept 3D probe, another way to acquire 3D strain images is to record a volume of data, change the contact pressure a little, then record another volume of data. Stradwin can then use its unique, fully-3D strain imaging algorithm to compare the two volumes and construct a 3D axial strain image. To do this, the motor control options should be set to 'Repeated sweeps' and 'Volume at once'. Also, set the 'Video rate' in the record task page to 'Buffered' and 'Display' in the RF Display dialog to 'Strain'. When you start recording, Stradwin will display (in real-time) only the central strain image from each volume pair while recording whole volumes. The probe must be held stationary while recording each volume and the pressure adjusted slightly in the short pause between volumes while the data is transferred to the PC. When there is at least one pair of volumes in the record buffer providing good strain data, the recording can be stopped and the 'Export volumes' option in the File menu used to save this pair of volumes.

The strain between the two volumes can then be visualised by selecting the 'Ortho' or 'Reslice' visualisation option, which causes a 3D strain volume to be calculated, according to the various settings in the RF strain dialog. Note that the top-left display canvas can show either a conventional 2D strain image between corresponding B-scans in the two volumes, or a slice through the 3D strain volume calculated using Stradwin's unique 3D strain algorithm. With the frame slider within the range of the first volume, the former is displayed. With the frame slider within the range of the second volume, the latter is displayed.

Mechanically-swept inter-frame 3D strain imaging

With a mechanically-swept probe, you can also record inter-frame strain data in a mechanically controlled sweep. To do this set the motor control options to 'Repeated sweeps' and 'Multiple frames'. Set the video rate in the record task page to 'Full speed' and 'Display' in the RF Display dialog to 'Strain'. While recording, you should continually vary the probe contact pressure, as in 2D strain imaging. The motor will advance gradually through the sweep and Stradwin will acquire multiple images (set by the 'Volumes' slider in the probe motor controls) at each motor step. Only the best strain image from each step will be displayed (in real-time). Recording will stop automatically once acquisition is complete and the 'Export volumes' option in the File menu option can be used to extract the pair of frames giving the best strain image at each location. Selecting the 'Ortho' or 'Reslice' visualisation option generates 3D strain data views in the same way as for tracked freehand 3D strain data.