Annular Transducer Research Articles

Correction of phase aberrations for sectored annular array ultrasound transducers

Two methods for correction of unknown phase aberrations induced by inhomogeneous acoustic velocities in tissues are explored for the two dimensional geometry of a sectored annular array system. The methods employed are adaptations of a cross correlation technique and a speckle brightness maximization technique. The methods correct phase distortions via the introduction of phase shifts in the timing sequence at the beamformer stage of a sectored annular array transducer. The techniques are investigated employing software models and a computer controlled automated scanning system. A 65-element sectored annular array is modelled via a rotating 5 element transducer. Tissue equivalent materials were moulded into a double layer aberrating medium to simulate phase distortions encountered in the rectus abdominis muscle in vivo. A comparison of the effectiveness of the two correction methods is presented. Contrast of an anechoic region is increased from 0.34 ± 0.08 to 0.48 ± 0.06 for the cross correlation technique, and up to 0.62 ± 0.05 for the speckle brightness maximization method. The performance of these correction techniques on target phantoms suggests that considerable improvements in image quality should be possible for clinical systems.

Ultrasonic NDE using a concentric laser/EMAT system

A pulsed laser source, together with a concentric annular electromagnet acoustic transducer (EMAT) as detector, have been investigated for use as a noncontact ultrasonic inspection system. The characteristics of this hybrid transduction method, and its application to various problems in nondestructive evaluation have been studied. Results are presented for applications in imaging, the testing of diffusion and adhesive bonding, and thickness estimation.

Ultrasonic nondiffracting transducer for medical imaging

The nondiffracting J(0) Bessel beam is evaluated, and its application to medical imaging is suggested. Computer simulations and experimental results for a ten-ring annular Bessel shaded transducer are described. Both continuous-wave (CW) and pulse-wave (PW) excitations are shown and compared to conventional Gaussian beams. The nondiffracting beam has about 1.27-nm radius main lobe with a 20-cm depth of field compared to the Gaussian transducer of the same size with a 1.27-mm radius main lobe at a focus of 12 cm and 2x4-cm depth of field. The side lobes of the nondiffracting beam are the same as the J(0) Bessel function. The effects of heterogeneity due to tissue on the nondiffracting beam and on the focused Gaussian beam are also reported.

Transesophageal Examination of the Left Coronary Artery With a 7.5 MHz Annular Array Two-dimensional Color Flow Doppler Transducer

To determine the value of a recently developed 7.5 MHz annular array two-dimensional and color flow Doppler transducer for examination of the proximal parts of the left coronary artery, 25 patients were studied immediately after coronary artery bypass surgery, and the findings were compared with preoperative coronary angiograms. With two-dimensional imaging, the left main coronary artery was visualized in 22 patients (88%), left anterior descending in 13 patients (52%), and circumflex in 22 patients (88%). Stenoses were correctly identified in four of four left main coronary arteries, in five of seven left anterior descending arteries, and in none of four left circumflex arteries. Adding color flow to two-dimensional imaging in 20 patients, we found a disturbed flow signal, as a sign of stenosis, in five of five left anterior descending artery and two of four left circumflex artery stenoses. We concluded that these initial findings with this new 7.5 MHz annular array two-dimensional and color flow Doppler transducer, are promising in imaging and detection of stenoses in the proximal parts of the left coronary artery.

The initial clinical evaluation of a transesophageal system with pulsed Doppler, continuous wave Doppler, and color flow imaging based on an annular array technology.

The application of transesophageal echocardiography (TEE) offers access to a great deal of important clinical information regarding cardiovascular anatomy and physiology. Two applications which have not been reported and would appear to be of interest are continuous wave Doppler capabilities and the implementation of higher frequency transducers. A TEE system designed at the Institute of Biomedical Engineering in Trondheim, which is based on an annular array technology, offers these capabilities. We evaluated this instrument in the clinical setting in a series of 30 patients to test the probe function in terms of the tissue and flow imaging quality with a 7.5 MHz carrier frequency, and to report on the implementation of a continuous wave Doppler modality in a TEE probe. We found that the annular array method permitted the use of high frequency probes for tissue and flow imaging which resulted in excellent image resolution, and that shifting the carrier frequency of the transducer to a lower frequency permitted the optimization of the Doppler sensitivity. The continuous wave Doppler was used to measure abnormal blood flow velocities in excess of 5.0 m/s, and was particularly useful in the operating room as velocity measurements could be obtained without compromising the sterile field. The results of our evaluation indicate that high imaging frequencies and continuous wave Doppler can be applied by an annular array TEE transducer.

Characterization of modified lead titanate piezoceramics. Application to the design of array transducers

Due to their high electromechanical anisotropy, lead titanate piezoelectric ceramics of modified composition are very interesting for designing either linear or annular ultrasonic transducer arrays used in medical diagnosis and in non-destructive testing. In this paper results of the characterization of three varieties of this type of material are presented. The characterization includes the measurement of the resonance frequency dispersion diagram of thin parallelepipedic elements and the measurement of the characteristics in the longitudinal thickness mode and the planar mode. In view of the application to the design of transducers, several types of structures are presented.

Response of an edge‐supported, annular, electret earphone

In many electret transducers, the diaphragm is held above a metalized backplate by circular ridges machined into the backplate. Such transducers may be thought of as a series of annular electret transducer segments, each of which has an edge‐supported diaphragm. In the treatment presented here the behavior of an edge‐supported, annular, electret earphone is studied analytically in a two‐step procedure. First, the static diaphragm deflection is determined. A stability criterion is developed which physically corresponds to a limit in the maximum stable static deflection. Second, the diaphragm dynamic motion is determined and the resulting sound pressure in the ear canal calculated. Using this method the linear response and distortion response may be separately analyzed. Both tend to increase in level as the transducer approaches the point of static instability. Experimental results agree well with the predictions of the theory. [Work done in part at Bell Laboratories, Murray Hill, NJ 07974.]

Radiation field calculations of pulsed ultrasonic transducers: Part 1: Planar circular, square and annular transducers

The exact equation to describe the propagation of ultrasonic pressure waves in a lossless, homogeneous medium was evaluated assuming pulse-excited radiators. Acoustical field patterns were studied using the dimensions and shape of the transducer, the ultrasonic frequency and number of cycles within the pulse as parameters. The results emphasize the influence of the number of cycles and piston shape on interference phenomena within the near-field and on beam-narrowing effects in the intermediate range between near- and far-field. The following transducer shapes are studied: planar circular, planar square and planar annular.

Focusing of an ultrasonic field upon transmission using an annular transducer

(1978). Focusing of an ultrasonic field upon transmission using an annular transducer. Journal of Medical Engineering & Technology: Vol. 2, No. 2, pp. 77-78.