INTERLACED LINEAR ARRAYS FOR VIBRO-ACOUSTOGRAPHY: A NUMERICAL SIMULATION STUDY Glauber T. Silva , James F. Greenleaf, and Mostafa Fatemi Mayo Clinic and Foundation, Rochester, MN, USA. Email: silva.glauber@mayo.edu Abstract In this work, we study the beamforming of interlaced linear arrays for vibro-acoustography systems. The aim of this research is to model and design a system with a contact probe for in vivo applications. Two transduc- ers with two identical linear arrays are studied: inline and fully interlaced arrays. The system point-spread function is given in terms of the dynamic ultrasound radiation force produced by the interference of the ul- trasound beams generated by each array. A computer program simulates the beamforming of the arrays based on the spatial impulse method. Simulations using array parameters similar to conventional ultrasound systems present the resolution cells of both transducers focused at 50 mm of about mm in azimuth, elevation, and range. Sidelobes can be reduced to less than dB through apodization. The interlaced ar- rays exhibit transverse resolution acceptable for medi- cal imaging applications. Introduction Vibro-acoustography is an imaging technique that produces a map (image) of the mechanical properties of an object by applying ultrasound dynamic radiation force on the object [1]. The radiation force is gener- ated by two harmonic ultrasound focused beams driven at slightly different frequencies. The beams interfere in the focal region of the system producing a modulated ultrasound beam. The resulting beam generates the dy- namic radiation force that has a component at the modu- lation frequency. This force causes the object (or region of interest) to vibrate emitting an acoustic field (acous- tic emission) that can be detected by a hydrophone or microphone. The detected signal is used to synthesize an image of the object. Clinical applications of ultrasound imaging systems require electronic beam focusing and steering. Linear array transducers are widely used for this purpose [2], because they can focus and steer the ultrasound beam laterally by electronically delaying the signals of the array elements. Linear array beamforming for vibro- acoustography shares similarities with its counterpart in conventional ultrasound (B-mode). The goals of beam- forming for vibro-acoustography remain to achieve nar- row beams with low sidelobes and minor effects of grat- ing lobes. An imaging system is characterized by its point- spread function (PSF). In vibro-acoustography, the PSF is given in terms of the dynamic radiation force at the modulation frequency. Thus, we shall call the vibro- acoustography PSF as the radiation force point-spread function (RFPSF). The dynamic radiation force de- pends, among other factors, on how the generated ul- trasound beams overlap. This is related to the relative position of transducers that generate the beams. Conse- quently, the system resolution cell, defined as the vol- ume enclosed by the RFPSF at dB, depends on the relative position of the transducers. Here, we study the beamforming of interlaced lin- ear arrays for vibro-acoustography through numerical simulations based on the spatial impulse method [3]. Two transducers formed by two identical linear arrays are analyzed: inline and fully interlaced arrays. In the former, the elements of both arrays alternate along az- imuth. In the latter, the alternation of the elements of both arrays take place in azimuth and elevation. In a re- cent work [4], transducers based on separated linear ar- rays were studied for vibro-acoustography applications. In these transducers, two linear arrays are placed side- by-side along azimuth (inline separated array) or ele- vation (parallel separated array). One advantage of in- terlaced over separated arrays is that in the former the array elements spans the entire transducer aperture. Re- sults show that the resolution cell of both interlaced ar- rays focused at mm is about mm in azimuth, elevation, and range. Sidelobes are reduced to less than dB through apodization. The trans- verse resolution of the interlaced arrays are acceptable for medical imaging applications. The superiority of in- terlaced over separated arrays in vibro-acoustography is discussed. Theory Dynamic ultrasound radiation force Consider two collimated ultrasound beams with fre- quencies and propagating in an ideal fluid. The quantities and are the center and the difference frequencies, respectively. In vibro- acoustography applications . The beams overlap in a region of the space which is defined as the focal zone of the system. This is referred here as dual beam mode (see Fig. 1). The resulting beam in the focal zone is a modulated ultrasound wave that is described WCU 2003, Paris, september 7-10, 2003 201