Imaging and Detection with Frequency-Separated Unfocused Ultrasound

Presently, we are examining new ultrasound methods that can operate at reduced frequencies and are thusly suited for transcranial propagation. A frequency-separation method uses a reconstruction approach designed to resolve two-dimensional radial and axial information from a single stationary transmitter and a single receiver. Like traditional ultrasound backscatter imaging, the emitter is produces an acoustic field that contains a large bandwidth. However, this field that is not time localized and does not require a high center frequency to achieve radial resolution. The transmitted field is not focused, but rather achieves spatial resolution by producing a unique complex spectral pattern at each point over a region of interest (ROI). Likewise, the receiver is not focused. Reflections from objects that may be within the ROI are recorded by a single point-like receiver, and the entire ROI is reconstructed by interpretation of a single waveform. The approach creates an acoustic field whose frequency content is spatially dependent, giving a unique – or nearly unique – amplitude and phase spectrum for each point in the ROI.  This spatially dependent signature carries spatial information that is superimposed into the time-domain recorded signal. A cross-correlation method is then used to reconstruct the two-dimensional ROI, using the knowledge of the complex spectrum at each point in the ROI.

Feasibility of the approach has been established through the design, construction, and testing of a preliminary 2-D B-mode imaging system. The transducer geometry is that of a hexahedral right prism having two nonparallel surfaces, one of these being the radiating surface. The transducer is polarized normal to this surface resulting in a frequency response that varies as a function of position. The field produced by an impulsive driving potential then has the required spatially dependent amplitude and phase spectrum, which has been coupled with a broadband point-like detector to reconstruct the location of scatters within the ROI.