MHz Phased Array Transducer Research Articles

Real-time angular scatter imaging system for improved tissue contrast in diagnostic ultrasound images

A new type of real-time ultrasound imaging system has been developed. In contrast to conventional systems, which process only echoes scattered directly back from tissue to form an image, this system images tissue by displaying energy scattered at other angles. In its present form, the system uses one 32 element, 2.4 MHz phased array transducer in transmit and a second, spatially separate 32 element, 2.4 MHz phased array transducer in receive, to detect sound which is scattered away from the transmit transducer. In order to form an image line, the transmit transducer sends into the body a steered pulse, which is tracked dynamically from the side by the receive transducer. The signal detected by the receive transducer is processed in the same manner as in a standard B-mode phased array system. The final display format is a gray scale sector originating from the transmit transducer. Real-time angular scatter images of phantom and in vivo targets have been formed and compared to standard backscatter B-mode images of the same targets. >

The relationship between “Normal” transesophageal color-flow doppler-defined tricuspid regurgitation and thermodilution right ventricular ejection fraction measurements

Twenty coronary artery revascularization patients, aged 58 ± 15 years, were studied intraoperatively to define the impact of Doppler-defined tricuspid regurgitation on measurement of thermodilution right ventricular ejection fraction (50 msec response pulmonary artery catheter). Right ventricular function was also estimated using a measurement technique independent of flow patterns across the tricuspid valve (transesophageal two-dimensional echocardiographic 5.0 MHz phased-array transducer). Measurements included transverse plane long- and short-axis planimetered area ratio, respectively, and tricuspid annular plane systolic excursion ratio (ratio = end-diastolic minus end-systolic value divided by end-diastolic value). Data were expressed as thermodilution-echocardiographic gradients, ie, thermodilution ejection fraction minus long-axis planimetered area ratio, short-axis planimetered area ratio, and tricuspid annular plane systolic excursion ratio, respectively. Tricuspid regurgitation was quantified by color-flow Doppler perimetry of maximal regurgitation jet area and analysis of transduced right atrial pressure waveform. Doppler estimates were expressed as absolute values and as a function of corresponding atrial area (tricuspid regurgitation index = planimetered jet area divided by right atrial area). Data were obtained following endotracheal intubation, sternotomy, pericardiotomy, cardiopulmonary bypass, and chest closure. Data were evaluated by regression analysis, with separate analyses performed for each time period. Profiles were unassociated with right atrial pressure waveform abnormalities. There was no significant relationship between thermodilution ejection fraction variance values and tricuspid regurgitation jet area or regurgitation index, respectively. In each measurement period, thermodilution-echocardiographic gradients were also unrelated to the tricuspid regurgitation estimates. These findings suggest that transesophageal color-flow Doppler evidence of tricuspid regurgitation, without atrial pressure waveform abnormalities, does not invalidate thermodilution right ventricular ejection fraction data.

Quantification of ultrasonic anisotropy in normal myocardium with lateral gain compensation of two-dimensional integrated backscatter images

Anisotropy of ultrasonic scattering and attenuation in heart tissue depends on the specific orientation of myofibers with respect to angle of insonification. We used lateral gain compensation (LGC) to correct two-dimensional cardiac images for physiologic anisotropy. Normal hearts excised from three dogs and five pigs were insonified in a water tank with both 2.5 and 5.0 MHz phased-array transducers. Integrated backscatter was measured from a short-axis approach in the anterior wall perpendicular to the principal fiber axis, and in the septum parallel to the fiber axis. The gain in a vertical sector encompassing the septum was adjusted to compensate the image for anisotropy by matching the intensity of scattering from septal and anterior regions. The average gain required to compensate the septum for anisotropy was 16 dB at 2.5 MHz, and 20 dB at 5.0 MHz. Five healthy volunteers underwent imaging with a 2.5 MHz transducer from a parasternal short-axis view. The LGC required in vivo was approximately 16 dB at 2.5 MHz and was equivalent to that required for correction of septal anisotropy in excised hearts. Thus, normal myocardium exhibits substantial ultrasonic anisotropy that can be quantified and compensated for with clinically applicable tissue characterization techniques.

Comparison of various agents in contrast enhancement of color doppler flow images: An in vitro study

The commercially manufactured contrast agents, Echovist and Albunex, were compared with sonicated conventional agents, indocyanine green, 29% renografin-60, 0.9% normal saline and 25% mannitol in their ability to enhance color Doppler flow signals. In a pneumatically regulated pulsatile flow system, a glycerine, saline (0.9%) and sand (5 μm particle size) solution was imaged using a 2.5 MHz phased-array transducer. Four different flow velocities (0.40, 0.35, 0.30 and 0.25 m/s) as measured by color Doppler guided pulse Doppler were utilized. All color Doppler settings were kept constant throughout the study. Utilizing a power injector, four different volumes (1.0, 1.5, 2.0 and 2.5 mL) of each contrast agent were injected into the flow medium at various transducer angles (20, 30, 40 and 60°) and various distances from the transducer (3.38, 5.5, 6.76 cm). For Echovist and Albunex, several concentrations varying from 2% to 100% were used. Keeping instrument settings constant, color flow areas obtained before and after each contrast injection were planimeterized and the percent increase in the color flow area computed and compared. At full (100%) concentration, 20° transducer angle and a flow velocity of 0.40 m/s, the maximum increase in the color flow area was 568%, 251%, 180%, 110%, 71%, and 38% for Echovist, Albunex, sonicated indocyanine green, renografin, normal saline and mannitol, respectively. However, a significant reduction in the degree of color flow enhancement was observed, with decreases in the concentration of these agents, and increases in the Doppler beam incident angle or distance from the transducer. Increasing the flow velocity of the medium into which contrast was injected did not produce significant changes in the contrast enhancement effect for all agents except Echovist. Increasing the injection volume significantly increased the color flow area for sonicated agents but not for Echovist or Albunex. This preliminary in vitro study shows that the commercially manufactured contrast agents, Echovist and Albunex, are much superior to sonicated conventional contrast agents in the enhancement of color Doppler flow signals. Of the sonicated agents, indocyanine green had the best enhancement capability.

Accuracy of colour Doppler ultrasound velocity measurements in small vessels

Colour Doppler ultrasound offers the possibility of imaging small vessels not visible by B-mode alone. The colour Doppler image of velocities allows the course of small vessels to be imaged in the X-Y plane of the scan provided the Doppler frequency shift is of sufficient magnitude. This permits alignments of the Doppler cursor, allowing angle correction to provide true velocity measurements from the Doppler shift obtained. Before attempting to make velocity measurements, however, it is essential to be aware of the possible error in the Z plane caused by the thickness of the Doppler sample volume. To quantify this source of error, hydrophone and flow-rig measurements were performed on an Acuson 128 colour Doppler scanner with both 5 MHz linear-array and 3.5 MHz phased-array transducers. Measurements of the transmitted pulses using a point hydrophone showed that both probes employ approximately 3.5 MHz Doppler pulses (in both colour and pulsed Doppler modes). The two transducers have the same axial resolution. In colour Doppler mode the axial length of the sample volume increases automatically with depth by up to 0.5 mm. Measurements of colour and pulsed Doppler signal strength were obtained in a controlled flow rig. Both transducers produced accurate colour flow images of the phantom at their optimum depths; flow velocity errors due to Z-plane thickness are < 5%. There was, however, substantial error outside these optimum conditions (up to 20%).

Evaluation of a new finger-type intraoperative epicardial echocardiographic probe for infants and small children

A new finger-type epicardial echocardiographic probe was evaluated in 26 children with complex cardiac anomalies during open heart surgery. The probe is a small 3.75 MHz phased array transducer for 2-dimensional echo and color Doppler imaging. The tip is slightly angled to facilitate the apical approach through a median sternotomy. The patients included 4 neonates and 8 infants, and the major cardiac lesions were complete atrioventricular (AV) canal, transposition of the great arteries, double outlet right ventricle, and others. The intraoperative study was possible in most of the patients facilitating various approaches for the assessment of left-sided AV valve function, left ventricular outflow obstruction, residual shunt and intraatrial anatomy. Four patients received subsequent operative procedures after the assessment. This transducer appears to be useful in the intraoperative study during surgery for complex cardiac anomalies in neonates and small infants.

Immediate improvement of dysfunctional myocardial segments after coronary revascularization: Detection by intraoperative transesophageal echocardiography

To ascertain the immediate effects of coronary artery bypass grafting on regional myocardial function, intraoperative transesophageal two-dimensional echocardiograms were obtained in 20 patients using a 3.5 MHz phased array transducer at the tip of a flexible gastroscope. Cross-sectional images of the left ventricle were obtained at multiple levels before skin incision and were repeated serially before and immediately after cardiopulmonary bypass. Using a computer-aided contouring system, percent systolic wall thickening was determined for eight anatomic segments in each patient at similar loading conditions (four each at mitral and papillary muscle levels). Of the 152 segments analyzed, systolic wall thickening improved from a prerevascularization mean value (+/- SEM) of 42.7 +/- 2.9% to a postrevascularization mean value of 51.6 +/- 2.6% (p less than 0.001). Thickening improved most in those segments with the worst preoperative function (p less than 0.001). Chest wall echocardiograms obtained 8.4 +/- 2.3 days after operation showed no deterioration or further improvement in segmental motion compared with transesophageal echocardiograms obtained after revascularization. Thus: regional myocardial function frequently improves immediately after bypass grafting, with increases in regional thickening being most marked in those segments demonstrating the most severe preoperative dysfunction, and this improvement appears to be sustained; and in some patients, chronic subclinical ischemic dysfunction is present which can be improved by revascularization.