Breast imaging with a conical transducer/annular array hybrid scanner

Biomedical ultrasound beam forming

The principles of biomedical ultrasound beam forming control the quality of diagnostic imaging. Beam parameters associated with imaging quality are: (1) lateral and axial resolutions; (2) depth of field; (3) contrast and (4) frame rate. In this paper, we review some of the current beam forming techniques and their principles. We focus on trade-offs among the above four aspects of beam forming and relate them to system parameters such as aperture size, f-number (the ratio between focal length and aperture diameter), central frequency (wavelength), system bandwidth and sidelobes. Methods for steering conventional and limited diffraction beams with array transducers are also reviewed.

Ultimate limits in ultrasonic imaging resolution

According to elementary theory, the resolution of an ultrasonic imaging system increases with the ultrasonic frequency. However, frequency is limited by frequency-dependent attenuation. For imaging at any required depth, resolution improvement beyond the limit imposed by ultrasonic frequency can be obtained by increasing the ultrasonic intensity. This is itself, however, dependent on safety considerations and the effects of nonlinearity. In homogeneous media, image resolution increases with decreasing f-number. Particularly at low f-numbers, however, tissue inhomogeneity leads to a deterioration in image quality. Inhomogeneity may also be considered in terms of phase aberration. It has been found that for a given aperture, image degradation due to phase aberration is worse at higher frequencies. Schemes have been proposed for correction of this problem, but so far model systems do not lend themselves to clinical application. Deconvolution is unsatisfactory, speed correction is impracticable and synthetic aperture scanning and holography are virtually useless in biological tissues. Ultrasound-computed tomography has had only limited success. Speckle reduction can improve target detectability, but at the expense of resolution. Time-frequency control provides a useful partial solution to the problem of resolution reduction resulting from attenuation. It is clear that improved resolution would result in significant clinical benefits. An optimisation system for aperture size and ultrasonic frequency is proposed with signal averaging for resolution enhancement of a defined object area. This would have a compact ultrasonic beam and would allow frame rate to be traded for resolution, by means of signal averaging.

Ultrasound imaging

Modern ultrasonic transducers mainly employ lead zirconate titanate (PZT) but vinylidene fluoride trifluoroethylene copolymer (P (VDF-TrPE)) is becoming more competitive. The static scanner is now largely replaced by mechanical or electronically controlled array real time systems; the speed of scanning is limited by the speed of sound and the resolution depends on the wavelength and so, ultimately, on the attenuation in tissue. Tissue inhomogeneities degrade the resolution. Intraoperative and intracavitary scanners have advantages in some anatomical situations and ultrasonic imaging can guide extracorporeal shock wave lithotripsy. Inexpensive battery powered scanners will soon become available. Duplex scanners are used to localize the acquisition of Doppler signals; blood flow volume rate can be estimated from measurements of blood velocity and vessel cross-sectional area, or by the attenuation-compensated technique which avoids the main sources of error. Colour flow mapping combines real time imaging with Doppler information, but has limited scanning speed. Computed tomography and acoustical microscopy are feasible. Speckle arises from the coherent nature of ultrasound and can be suppressed by summing uncorrelated images or by filtering. Image manipulation and display techniques are being developed to cope with three dimensional scan data and the approach is compatible with picture archiving and communication systems (PACS). Tissue characterization based on the measurement of properties has been disappointing but blood flow analysis and contrast agents are promising. Quality assurance programmes are crucial; ultrasonic diagnosis appears to be free from hazard and prudent use is determined by cost-benefit considerations.

The improvement and quantitative assessment of B-mode images produced by an annular array/cone hybrid

Hybrid ultrasound imaging systems, which combine spherical focusing on transmit with axicon focusing on receive, provide excellent resolution over a useful depth of field. This paper presents a new hybrid design with improved sensitivity, in which the axicon focusing is achieved by two conical mirrors and a PZT 5A disk out into 8 sectors. We have investigated two methods of processing the signals from the 8 sectors. In the first, phase insensitive sector addition (PISA), the B-scan is formed from the sum of the 8 demodulated signals. In the second, multiplicative processing (MP), the 8 rf waveforms are multiplied and the resultant is demodulated to form the image. Both techniques result in smoothed speckle but degraded lateral resolution. As well, MP decreases the off-axis sensitivity of the system and artifacts characteristic of axicon focusing. Quantitative assessment of the effects of PISA and MP was performed using a new approach called contrast-to-speckle ratio (CSR). The CSR data, which is a measure of the image contrast of cylindrical voids in a random scattering medium relative to contrast fluctuations due to speckle, shows the superiority of PISA and MP. This conclusion is supported by images of in vitro human breast tissue.