Ultrasound imaging is a widely available and highly portable real-time, ionizing radiation-free imaging modality used as first-line in multiple areas. Conventionally, its use is focused on providing 2D imaging using ultrasound probes. However, with the introduction of matrix probes, the acquisition of 3D scans became possible enabling the sonographer to accurately interpret the scanned anatomy without needing to build a possibly error prone 3D mental map and thus, decreasing operator-dependency. Although with this 3D imaging, volume estimation accuracy increases and surveillance becomes less difficult, matrix probes are expensive, have a limited field-of-view and produce a low contrast image. This necessitates the sonographer to still build a mental map whenever a portion of an anatomy is not fully visible in a single volumetric scan, leading to more laborious surveillance. Ultrasound stitching is investigated as a potential method to overcome these limitations by tracking multiple ultrasound scans and merging them using their position and orientation coordinates. To this end, this work uses a robotic arm as the selected tracking system for the stitching of 2D xplane ultrasound images. First, the initial clinical application for the system was derived using a combination of literature study and interviews with medical experts which led to the decision of utilizing the robotic arm for liver imaging and biopsy needle guidance. The suitability of this application was validated with the evaluation of the three required stages namely anatomy reconstruction, needle path planning and needle insertion. Two phantoms mimicking lesions and surrounding vessels were fabricated for this purpose. They were firstly reconstructed by manually segmenting the elements of interest from multiple scans, placing these segmented scans in their respective 3D positions and finally applying an alpha-shapes reconstruction algorithm. Once that the centroid of the lesions was calculated and together with the position information about the surface of the phantom obtained with the robotic arm, it was possible to evaluate all the paths that connect the surface with the centroid and subsequently select only the ones that did not pass next to a vessel within a specific distance. Finally and as an extension of using the robotic arm to track the ultrasound probe, the combined tracking of the probe with a needle holder was investigated for the needle insertion stage. The needle holder was used to maintain the needle always in plane with the ultrasound beam and it was placed at a preset distance with respect to the probe. The holder also provided insertion depth information that facilitated the representation of the needle via a line in the visualization environment whose end indicated the position of the needle tip. This position was updated in real-time with the information received from the needle holder while the needle was being inserted. The position of the needle tip was verified by overlapping real-time ultrasound images with the visualized line. Ultimately, to further illustrate the suitability of the robotic arm in a clinical setting, an ex-vivo and a clinical experiment for vessel tracking and reconstruction was performed.

Ultrasound gives the opportunity to look at muscles and observe their change in length. This tool has increased the knowledge about muscle-tendon dynamics and sometimes revealed surprising muscle stretch behaviour. System identification experiments use robots to disturb the ankle and measure its torque and angle. Muscle movement is derived from the measured joint angle and their assumed relationship. This study uses plane-wave ultrasound imaging to investigate the relation between ankle angle and muscle length during system identification experiments. The first goal is to determine the feasibility of using ultrasound measurements for system identification.  The second goal is to investigate the validity of the assumption that muscle stretch is proportional to ankle angle and the effect of this assumption on the prediction of reflex size. Transient and continuous disturbances were applied to the ankle, while images of the soleus and gastrocnemius were recorded with ultrasound and processed with an image tracking algorithm. For small (1° SD) continuous perturbations ankle angle and muscle length can be assumed proportional during a relax task. However, a conclusion cannot be drawn for the position task due to the low coherence of the muscle length measurements. For transient perturbations with a high velocity (>90°/s) the muscle length showed oscillations that were not present in the ankle angle, demonstrating a non-proportional relationship. The gastrocnemius velocity predicted the size of the short latency reflex better than the perturbation velocity of the ankle robot. Using plane-wave ultrasound imaging for system identification experiments was feasible. 

In medical ultrasound imaging phased array transducers are used for non-invasive imaging. For 3D intra-cardiac-echography technical borders are reached since blood vessel size limits the dimensions of the array. This dimensional limitation lowers lateral resolution. To solve part of the problem, non conventional ways of lateral resolution improvements are needed. For linear arrays R. Reeg [1] proposed null subtraction imaging (NSI), which resulted at best in 30 dB lower side lobes and a width reduction of up to 25 times for the main lobe. This is a non-linear image processing technique based on implementing multiple apodizations in post-processing. Three images are made from which one has a sharp drop to zero in the middle which can be exploited by subtracting it from the other images. This results in a improvement over a regular apodization scheme.
To verify the technique for 2D phased array imaging, simulations are done using Field-II [2, 3] while taking array dimension into account. Measurements take place on an artificial phantom with evenly spaced line scatter targets. For the experiment a P4-l phased array transducer was used in combination with a Verasonics Vantage 256TMwhere a sub-aperture of 30 elements is used. The experiment is done on a CIRS 040GSE Phantom, where evenly spaced line scatter targets and lateral resolution targets are looked at. To form an image a simple delay-and-sum (DAS) algorithm was used. The effect of NSI on the speckle in an image is also measured, where for normal developed speckle a signal to noise ratio (SNR) of 1.91 is expected. Results for simulation were as expected with an average beamwidth reduction of 22.5 times and on average higher main lobe to side lobe ratio (MSR) of 33.4 dB for NSI imaging opposed to using a rectangular apodization. Experimentally an average beamwidth reduction of 5 times and an on average higher MSR of 20.5 dB are realised. The beamwidth reduction is lower then expected, this is due to rounding errors in the DAS algorithm, where data can only be delayed by integer elements. For the lateral scatter targets no extra targets can be distinguished when using NSI imaging opposed to rectangular apodization imaging, only the targets that where already visible show reduction in beamwidth and side lobe levels. When using NSI speckle SNR is reduced from 1.78 to on average 0.50 and seems to scale with energy present in the image. An hybrid image between the NSI image and rectangular apodization image is made to solve this. It results in reduced beamwidth, restored speckle SNR but the MSR is reduced. It can be concluded that the method works for phased array imaging under perfect conditions, the simulation. To get the full potential of the technique more research is needed towards the effects of improper sampling. Which in this case resulted in less then ideal ’sharp’ drops, lowering the reduction in beamwidth. Also the reduced speckle needs further research to understand it well. Once this is all achieved the next step to 3D imaging can be made.