3D Liver tracking using a matrix array: Implications for ultrasonic guidance of IMRT

The effectiveness of Intensity Modulated Radiation Therapy (IMRT) is compromised by involuntary motion (e.g. respiration, cardiac activity). Feasibility of processing ultrasound echo data to automatically estimate 3D liver motion for real-time IMRT guidance was previously demonstrated with an acquisition speed limit of 2 volumes per second, due to hardware restrictions of a mechanical linear array probe. Utilizing a 2D matrix array probe with parallel receive beamforming offered increased acquisition speeds, and the contributions of higher volume rates were investigated. In vivo livers of three volunteers were scanned with and without respiratory motion, at volume rates of 24 and 48 Hz, respectively. Correlation-based, phase-sensitive 3D speckle tracking was applied to consecutively-acquired volumes. Volumes were omitted at fixed intervals and 3D speckle tracking was reapplied to study the effect of lower scan rates. Results revealed periodic motion that corresponded with the heart rate or breathing cycle, in the absence or presence of respiration, respectively. For cardiac-induced motion, volume rate limits ranged from 8-12 Hz and were limited by the frequency of the beating heart. In respiratory-dominated motion, volume rate limits ranged from 4-12 Hz and were limited by the accuracy of tracking estimates.