In vivo liver tracking with a high volume rate 4 D 1 ultrasound scanner and a 2 D matrix array probe

The effectiveness of Intensity Modulated Radiation Therapy (IMRT) is 9 compromised by involuntary motion (e.g. respiration, cardiac activity). The feasibility 10 of processing ultrasound echo data to automatically estimate 3D liver motion for real11 time IMRT guidance was previously demonstrated, but performance was limited by an 12 acquisition speed of 2 volumes per second due to hardware restrictions of a mechanical 13 linear array probe. Utilizing a 2D matrix array probe with parallel receive beamforming 14 offered increased acquisition speeds and an opportunity to investigate the benefits of 15 higher volume rates. In vivo livers of three volunteers were scanned with and without 16 respiratory motion at volume rates of 24 and 48 Hz, respectively. Respiration was 17 suspended via voluntary breath hold. Correlation-based, phase-sensitive 3D speckle 18 tracking was applied to consecutively-acquired volumes of echo data. Volumes were 19 omitted at fixed intervals and 3D speckle tracking was reapplied to study the effect 20 of lower scan rates. Results revealed periodic motion that corresponded with the 21 heart rate or breathing cycle, in the absence or presence of respiration, respectively. 22 For cardiac-induced motion, volume rates for adequate tracking ranged from 8-12 Hz 23 and was limited by frequency discrepancies between tracking estimates from higher24 and lower-frequency scan rates. Thus, the scan rate of volume data acquired without 25 respiration was limited by the need to sample the frequency induced by the beating 26 heart. In respiratory-dominated motion, volume rate limits ranged from 4-12 Hz, 27 interpretable from the root mean squared deviation (RMSD) from tracking estimates 28 at 24 Hz. While higher volume rates yielded RMSD values less than 1 mm in most 29 cases, lower volume rates yielded RMSD values on the order of 2-6 mm. 30