The effect of electronically steering a phased array ultrasound transducer on near‐field tissue heating

Abstract

Purpose: This study presents the results obtained from both simulation and experimental techniques that show the effect of mechanically or electronically steering a phased array transducer on proximal tissue heating. Methods: The thermal response of a nine-position raster and a 16-mm diameter circle scanning trajectory executed through both electronic and mechanical scanning was evaluated in computer simulations and experimentally in a homogeneous tissue-mimicking phantom. Simulations were performed using power deposition maps obtained from the hybrid angular spectrum (HAS) method and applying a finite-difference approximation of the Pennes' bioheat transfer equation for the experimentally used transducer and also for a fully sampled transducer to demonstrate the effect of acoustic window, ultrasound beam overlap and grating lobe clutter on near-field heating. Results: Both simulation and experimental results show that electronically steering the ultrasound beam for the two trajectories using the 256-element phased array significantly increases the thermal dose deposited in the near-field tissues when compared with the same treatment executed through mechanical steering only. In addition, the individual contributions of both beam overlap and grating lobe clutter to the near-field thermal effects were determined through comparing the simulated ultrasound beam patterns and resulting temperature fields from mechanically and electronically steered trajectories using the 256-randomized element phased array transducer to an electronically steered trajectory using a fully sampled transducer with 40 401 phase-adjusted sample points. Conclusions: Three distinctly different three distinctly different transducers were simulated to analyze the tradeoffs of selected transducer design parameters on near-field heating. Careful consideration of design tradeoffs and accurate patient treatment planning combined with thorough monitoring of the near-field tissue temperature will help to ensure patient safety during an MRgHIFU treatment.

Design and characterization of a laterally mounted phased-array transducer breast-specific MRgHIFU device with integrated 11-channel receiver array

“…While tilting the transducer at the most extreme angles toward vertical will place the chest wall partially in the far-field region of the ultrasound beam, this is still a significant improvement when compared to devices that propagate the ultrasound beam vertically from below the breast. During ablation, while some heating and subsequent ablation does occur in the near field of the ultrasound beam due to absorption and the electronic steering of the beam, 33 there is minimal thermal dose that accumulates in the far field of the beam, as seen in Fig. 8.…”

Predicting variation in subject thermal response during transcranial magnetic resonance guided focused ultrasound surgery: Comparison in seventeen subject datasets

“…For example, in a patient with low experimental skull efficiency, the loss of power with electronic steering may be too large to allow for treatment far from the geometric focus of the transducer near the center of the brain, whereas in a patient with a high experimental skull efficiency, the focus may be electronically steered 35 to treat regions further away from the center of the brain. This may be important in selecting patients with Parkinson disease in whom the target may be the globus pallidus interna.…”

Effects of MRTI sampling characteristics on estimation of HIFU SAR and tissue thermal diffusivity

“…A key component to this goal is accurate modeling of the bioheat transfer equation (BHTE) (Pennes 1948), for which investigators need accurate knowledge of the magnitude and spatial distribution of the ultrasonic specific absorption rate (SAR) and of tissue thermal properties (Roemer 1999). However, due to variations among and within different organs, variability across individuals, and limitations of treatment hardware or prediction software (Mahoney et al 2001, Payne et al 2011), the modeled SAR values and table thermal parameters used in the BHTE may cause significant errors in temperature calculations used to predict thermal dose.…”