RESUMEN
Recently, theoretical investigations of the beamforming capability of two-dimensional (2-D) transducer arrays have characterized the array parameters required to steer a symmetrically focused ultrasound beam up to 45 degrees off-axis. These investigations have also shown that the number of elements in a steered 2-D array can be dramatically reduced by using a sparse set of elements, randomly distributed throughout the aperture of the transducer. The penalty paid for the use of a sparse array is the development of a "pedestal" sidelobe in the beam profile, the amplitude of which increases as the number of elements in the array decreases. In this paper the potential of 2-D arrays for medical imaging is assessed by simulating B-scan images of spherical lesions, both cystic and scattering, embedded in a large random scattering volume. Similar contrast characteristics over a range of cyst sizes are demonstrated for a dense 2-D array and a sparse array with 1/8th the number of elements, both operating at 5 MHz. A 32nd order sparse array is shown to perform at a reduced level, producing unacceptable artifactual echoes within images of cysts. The 8th order sparse array pattern has been fabricated on a fixed-focus poly(vinylidene difluoride) transducer using photolithographic techniques. Experimental images from this transducer are used to verify some of the theoretical predictions made in this paper. Comparisons between simulated B-scan images from linear and 2-D phased arrays are presented in a companion paper.
Asunto(s)
Ultrasonografía/métodos , Artefactos , Quistes/diagnóstico por imagen , Humanos , Aumento de la Imagen , Modelos Estructurales , Tecnología Radiológica , TransductoresRESUMEN
The use of annular array transducers in diagnostic ultrasound applications is growing. The development of this equipment raises a number of questions concerning both the role these systems will play in the clinic and how the annular array can be best implemented in an ultrasound scanner. In this paper, we will build on the results of the previous companion paper to describe the development of a 12 element 30 mm diameter 4.5 MHz laboratory prototype scanner. The mechanical probe and probe acoustics are discussed and a unique digital receive beamformer (DRB) and signal processor are described. Focusing down to an f-number (focal length/diameter) of 0.9 is demonstrated. Measured angular beamwidths of 0.62 degrees at -6 dB and 2.8 degrees at -50 dB are in good agreement with theoretical predictions. Images of phantoms and normal volunteers showed exceptional resolution with little variation in the fine speckle texture as a function of depth. The effect of the number of transmit focal zones on image quality is demonstrated.