The correlation value of a specific model of speckle known as Fully Developed Speckle (FDS) can be used to estimate the out-of-plane displacement between ultrasound images in sensorless 3D freehand systems. This method requires the B-scans to contain mostly FDS pattern but this kind of pattern is rare in scans of real tissue. One way around this problem is to quantify the amount of coherency at different regions of images and adapt decorrelation curves based on the amount of non-FDS regions in the image. The novelty of this thesis is firstly adjusting the method to be applicable on B-mode
ultrasound images rather than RF ultrasound data. Secondly, the experiment setup is truly freehand and the motion of the probe is not constrained. The method is shown to work quite remarkable (accuracy of around 5%) for the elevational distance estimation for both test phantoms and real human tissue data sets.