Medical Ultrasound Imaging is widely used clinically because of its relatively low cost, portability, lack of
ionizing radiation, and real-time nature. However, even with these advantages ultrasound has failed to
permeate the broad array of clinical applications where its use could be of value. A prime example of this untapped potential is the routine use of ultrasound to guide intravenous access. In this particular application existing systems lack the required portability, low cost, and ease-of-use required for widespread acceptance.
Our team has been working for a number of years to develop an extremely low-cost, pocket-sized, and
intuitive ultrasound imaging system that we refer to as the "Sonic Window." We have previously described
the first generation Sonic Window prototype that was a bench-top device using a 1024 element, fully
populated array operating at a center frequency of 3.3 MHz. Through a high degree of custom front-end
integration combined with multiplexing down to a 2 channel PC based digitizer this system acquired a full
set of RF data over a course of 512 transmit events. While initial results were encouraging, this system
exhibited limitations resulting from low SNR, relatively coarse array sampling, and relatively slow data acquisition.
We have recently begun assembling a second-generation Sonic Window system. This system uses a 3600 element fully sampled array operating at 5.0 MHz with a 300 micron element pitch. This system extends the
integration of the first generation system to include front-end protection, pre-amplification, a programmable
bandpass filter, four sample and holds, and four A/D converters for all 3600 channels in a set of custom
integrated circuits with a combined area smaller than the 1.8 x 1.8 cm footprint of the transducer array. We
present initial results from this front-end and present benchmark results from a software beamformer
implemented on the Analog Devices BF-561 DSP. We discuss our immediate plans for further integration
and testing. This second prototype represents a major reduction in size and forms the foundation of a fully
functional, fully integrated, pocket sized prototype.
We describe a very low cost handheld ultrasound system that we are currently developing for routine applications such as image guided needle insertion. We provide a system overview and focus discussion on our beamforming strategy, direct sampled I/Q (DSIQ) beamforming. DSIQ beamforming is a low cost approach that relies on phase rotation of in-phase/quadrature (I/Q) data to implement focusing. The I/Q data are generated by directly sampling the received radio frequency (RF) signal, rather than through conventional baseband demodulation. We describe our efficient hardware implementation of the beamformer, which results in significant reductions in beamformer size and cost. We also present the results of experiments and simulations that compare the DSIQ beamformer to more conventional approaches, namely time delay beamforming and traditional complex demodulated I/Q beamforming. Results that show the effect of an error in the direct sampling process, as well as dependence on signal bandwidth and system f number (f#) are presented. These results indicate that the image quality and robustness of the DSIQ beamformer are adequate for routine applications.
Angular scatter offers a new source of tissue contrast and an opportunity for tissue characterization in ultrasound imaging. We have previously described the application of the translating apertures algorithm (TAA) to coherently acquire angular scatter data over a range of scattering angles. While this approach works well at the focus, it suffers from poor depth of field (DOF) due to a finite aperture size. Furthermore, application of the TAA with large focused apertures entails a tradeoff between spatial resolution and scattering angle resolution. While large multielement apertures improve spatial resolution, they encompass many permutations of transmit/receive element pairs. This results in the simultaneous interrogation of multiple scattering angles, limiting angular resolution. We propose a synthetic aperture imaging scheme that achieves both high spatial resolution and high angular resolution. In backscatter acquisition mode, we transmit successively from single transducer elements, while receiving on the same element. Other scattering angles are interrogated by successively transmitting and receiving on different single elements chosen with the appropriate spatial separation between them. Thus any given image is formed using only transmit/receive element pairs at a single separation. This synthetic aperture approach minimizes averaging across scattering angles, and yields excellent angular resolution. Likewise, synthetic aperture methods allow us to build large effective apertures to maintain a high spatial resolution. Synthetic dynamic focusing and dynamic apodization are applied to further improve spatial resolution and DOF. We present simulation results and experimental results obtained using a GE Logiq 700MR system modified to obtain synthetic aperture TAA data. Images of wire targets exhibit high DOF and spatial resolution. We also present a novel approach for combining angular scatter data to effectively reduce grating lobes. With this approach we have been able to push the grating lobes below -50 dB in simulation and effectively eliminate their presence in the experimental wire target images.
Conventional techniques used to design transducer apertures for medical ultrasound are generally iterative and ad-hoc. They do not guarantee optimization of parameters such as mainlobe width and sidelobe levels. We propose a dynamic aperture weighting technique, called the Minimum Sum Squared Error (MSSE) technique, that can be applied in arbitrary system geometries to design apertures optimizing these parameters. The MSSE technique utilizes a linear algebra formulation of the Sum Squared Error (SSE) between the point spread function (psf) of the system, and a goal or desired psf. We have developed a closed form expression for the aperture weightings that minimize this error and optimize the psf at any range. We present analysis for Continuous Wave (CW) and broadband systems, and present simulations that illustrate the flexibility of the technique.