Simultaneous Axial Multifocal Imaging Using a Single Acoustical Transmission: A Practical Implementation

Optimized Simultaneous Axial Multifocal Imaging via Frequency Multiplexed Focusing

Simultaneous axial multifocal imaging (SAMI) using a single acoustical transmission was developed to enhance the depth of field. This technique transmits a superposition of axial multifoci waveforms in a single transmission, thus increasing the frame rate. However, since all the waveforms are transmitted at a constant center frequency, there is a tradeoff between attenuation and lateral resolution when choosing a constant frequency for all the axial depths. In this work, we developed an optimized SAMI method by adding frequency dependence to each axial multifocus. By gradually increasing the frequency as a function of the focal depth, this method makes it possible to compensate for the gradually increasing F-number in order to achieve constant lateral resolution across the entire field of view. Alternatively, by gradually decreasing the axial multifoci frequencies as a function of depth, enhanced penetration depth and contrast are obtained. This method, termed frequency multiplexed SAMI (FM-SAMI), is described analytically and validated by resolution and contrast experiments performed on resolution targets, tissue-mimicking phantoms, and ex vivo biological samples. This is the first real-time implementation of a frequency multiplexing approach for axial multifoci imaging that facilitates high-quality imaging at an increased frame rate.

Spatio-temporal ultrasound beam modulation to sequentially achieve multiple foci with a single planar monofocal lens

Ultrasound focusing is a hot topic due to its multiple applications in many fields, including biomedical imaging, thermal ablation of cancerous tissues, and non destructive testing in industrial environments. In such applications, the ability to control the focal distance of the ultrasound device in real-time is a key advantage over conventional devices with fixed focal parameters. Here, we present a method to achieve multiple time-modulated ultrasound foci using a single planar monofocal Fresnel Zone Plate. The method takes advantage of the focal distance linear dependence on the operating frequency of this kind of lenses to design a sequence of contiguous modulated rectangular pulses that achieve different focal distances and intensities as a function of time. Both numerical simulations and experimental results are presented, demonstrating the feasibility and potential of this technique.

Imaging the Activation of Low-Boiling-Point Phase-Change Contrast Agents in the Presence of Tissue Motion Using Ultrafast Inter-frame Activation Ultrasound Imaging

Nanoscale phase-change contrast agents (PCCAs) have been found to have great potential in non-invasive extravascular imaging and therapeutic delivery. However, the contrast-to-tissue ratio (CTR) of PCCA images is usually limited because of either physiological motion or incomplete cancelation of tissue signal. Therefore, to improve the CTR of PCCA images in the presence of physiological motion, a new imaging technique, ultrafast inter-frame activation ultrasound (UIAU) imaging, is proposed and validated. Results of studies with controlled motion in tissue-mimicking phantoms indicate UIAU could provide significantly higher CTRs (maximum: 17.3 ± 0.9 dB) relative to conventional pulse inversion imaging (maximum CTR: 3.4 ± 1.4 dB). UIAU has CTRs up to 16.1 ± 1.0 dB relative to 3.9 ± 2.3 dB for differential imaging in the presence of physiological motion at 20 mm/s. In vivo imaging of PCCAs in the rat liver also reveals the ability of UIAU to enhance PCCA image contrast in the presence of physiological motion.

Improved Depth-of-Field Photoacoustic Microscopy with a Multifocal Point Transducer for Biomedical Imaging

In this study, a photoacoustic microscopy (PAM) system based on a multifocal point (MFP) transducer was fabricated to produce a large depth-of-field tissue image. The customized MFP transducer has seven focal points, distributed along with the transducer’s axis, fabricated by separate spherically-focused surfaces. These surfaces generate distinct focal zones that are overlapped to extend the depth-of-field. This design allows extending the focal zone of 10 mm for the 11 MHz MFP transducer, which is a great improvement over the 0.48 mm focal zone of the 11 MHz single focal point (SFP) transducer. The PAM image penetration depths of a chicken-hemoglobin phantom using SFP and MFP transducers were measured as 5 mm and 8 mm, respectively. The significant increase in the PAM image-based penetration depth of the chicken-hemoglobin phantom was a result of using the customized MFP transducer.

Multiplexed ultrasound beam summation for side lobe reduction

Two-way focusing, which relies on sweeping a focused beam across a field of view, is the conventional method for performing high-quality ultrasound imaging. Side lobes resulting from diffraction reduce the image contrast, thus degrade the image quality. In this paper, we present a new method for beam shaping the transmitted ultrasound waveform in order to reduce side lobes and improve image quality. The beam shaping is achieved by transmitting two different waveforms that are interlaced between the odd and even elements. One waveform generates a standard diffraction-limited single focus, and the second waveform generates two foci at the same focal depth as the single focus. The distance between the two foci is selected such that they will destructively interfere with the first order side lobes of the single focus, effectively eliminating these side lobes. A 7.5 dB side lobe reduction was measured experimentally at a depth of 60 mm, using a phased array transducer with a center frequency of 3 MHz. This real-time method utilizes standard receive beamforming, identical to traditional two-way focusing, and does not require post-processing. The method can be implemented with conventional ultrasound systems, without the need for additional components. The proposed method is described analytically, optimized via numerical simulation, and validated by experiments using wire targets, tissue-mimicking phantoms, and in vivo imaging of the rat bladder.