Transient acoustic holography is a useful technique for characterization of ultrasound transducers. It involves hydrophone measurements of the 2-D distribution of acoustic pressure waveforms in a transverse plane in front of the transducer—a hologram—and subsequent numerical forward projection (FP) or backward projection of the ultrasound field. This approach enables full spatiotemporal reconstruction of the acoustic field, including the vibrational velocity at the transducer surface. This allows identification of transducer defects as well as structural details of the radiated acoustic field such as sidelobes and hot spots. However, numerical projections may be time-consuming ( 10¹⁰ – 10¹¹ operations with complex exponents). Moreover, backprojection from the measurement plane to the transducer surface is sensitive to misalignment between the axes of the positioning system and the axes associated with the transducer. This article presents an open-access transducer characterization toolbox for use in MATLAB or Octave on Windows computers (https://github.com/pavrosni/xDDx/releases). The core algorithm is based on the Rayleigh integral implemented in C++ executables for graphics and central processing units (GPUs and CPUs). The toolbox includes an automated procedure for correcting axes misalignments to optimize the visualization of transducer surface vibrations. Beyond using measured holograms, the toolbox can also simulate the fields radiated by user-defined transducers. Measurements from two focused 1.25-MHz 12-element sector transducers (apertures of 87 mm and focal distances of 65 and 87 mm) were used with the toolbox for demonstration purposes. Simulation speed tests for different computational devices showed a range of 0.2 s – 3 min for GPUs and 1.6 s – 57 min for CPUs.

BACKGROUND
The liver is the most common organ injured in blunt abdominal trauma and makes up roughly 5% of all trauma admissions. Current treatments are invasive and resource intensive, which may delay care. We aim to develop and validate a contrast-enhanced ultrasound (CEUS)–guided noninvasive tool to treat liver lacerations at the bedside.

METHODS
Two 1.8-MHz high-intensity focused ultrasound (HIFU) elements were coupled to a C1-6 diagnostic ultrasound probe and a Logiq E10 scanner (GE HealthCare, Waukesha, WI) using a custom enclosure for coregistered imaging and ablation. A phantom was created from polyacrylamide gel combined with thermochromic ink whose color changes above biological ablative temperatures (60°C). The HIFU wave was focused approximately 0.5 cm below the surface using a 50% duty cycle generating 11.9 MPa for 20, 30, 40, 50, and 60 seconds. Experiments were repeated on ex vivo chicken livers in a water bath. Finally, the livers of four live swine underwent up to six CEUS-guided treatments using parameters optimized from in vitro work.

RESULTS
Treatment of the phantom between 20 and 60 seconds produced ablation sizes from 0.016 to 0.4 cm³. The relationship between time and size was exponential (R² = 0.992). Ablation areas were also well visualized on with ultrasound imaging. The ex vivo liver ablation size at 20 seconds was 0.37 cm³, at 30 seconds was 0.66 cm³, and at 100 seconds was 5.0 cm³. For the in vivo swine experiments, the average ablation area measured 2.0 x 0.75 cm with a maximum of 3.5 x 1.5 cm. Contrast-enhanced ultrasound was used with the contrast agent Definity (Lantheus Medical Imaging, North Billerica, MA) for identification of lacerations and immediate postoperative evaluation of therapy.

CONCLUSION
These experiments demonstrate the feasibility of CEUS-guided transdermal HIFU ablation and the time-dependent size of ablation. This work warrants future investigations into using ultrasound to detect active bleeding and HIFU to coagulate grades III and IV liver laceration.

Large abscesses are walled-off collections of pus and bacteria that often do not respond to antibiotic therapy. Standard of care involves percutaneous placement of indwelling catheter(s) for drainage, a long and uncomfortable process with high rehospitalization rates. The long-term goal of this work is to develop therapeutic ultrasound approaches to eradicate bacteria within abscesses as a noninvasive therapeutic alternative. Inertial cavitation induced by short pulses of focused ultrasound (histotripsy) is known to generate lethal mechanical damage in bacteria. Prior studies with Escherichia coli (E. coli) in suspension demonstrated that bactericidal effects increase with increasing peak negative amplitude, treatment time, and duty cycle. The current study investigated correlates of bactericidal activity with histotripsy cavitation cloud size. Histotripsy was applied to E. coli suspensions in 10-mL sample vials at 810 kHz, 1.2 MHz, or 3.25 MHz for 40 min. The cavitation activity in the sample vials was separately observed with high-speed photography. The cavitation cloud area was quantified from those images. A linear relationship was observed between bacterial inactivation and cavitation cloud size ( R² = 0.98 ), regardless of the acoustic parameters (specifically frequency, pulse duration, and power) used to produce the cloud.