Characterization of Ultrasound Elevation Beamwidth Artefacts for Brachytherapy Needle Insertion

Abstract

Ultrasound elevation beamwidth is the out of plane thickness causing image artefacts normally appearing around anechoic areas in the medium. These artefacts could also cause uncertainties in localizing objects (such as a surgical needle) in the ultrasound image slices. This thesis studies the clinical significance of elevation beamwidth artefacts in needle insertion procedures. A new measurement device was constructed to measure the transrectal ultrasound elevation beamwidth. The beam profiles of various lateral and axial distances to the transducer were generated. It is shown that the ultrasound elevation beamwidth converges to a point within its focal zone close to the transducer. This means that the transrectal ultrasound images have the best resolution within the focal zone of the ultrasound close to the transducer. It is also shown that the ultrasound device settings have a considerable impact on the amount of beamwidth artefacts. Needle tip localization error was examined for a curvilinear transrectal ultrasound transducer. Beveled prostate brachytherapy needles were inserted through all holes of a grid template orthogonal to the axial beam axis. The effects of device imaging parameters were also investigated on the amount of localization error. Based on the developed results, it was found that the imaging parameters of an ultrasound device have direct impact on the amount of object localization error from 0.5 mm to 4 mm. The smallest localization error occurs laterally close to the center of the grid template, and axially within the beam’s focal zone. Similarly, the largest localization error occurs laterally around both sides of the grid template, and axially within the beam’s far field. Using the ultrasound device with appropriate imaging settings could minimize the effects of these artefacts. We suggest to reduce the gain setting of the ultrasound device. This will reduce the energies assigned to the off-axis beams and as a result, the elevation beamwidth artefacts are minimized.