Development of a thermal test object for the measurement of ultrasound intracavity transducer self-heating

The elevated surface temperature of diagnostic ultrasound transducers imposes an important limitation to their safe use in clinical situations. Moreover, particular care should be taken if transvaginal transducers are to be used during routine scans in the first few weeks of pregnancy as the transducer surface can be very close to embryonic/fetal tissues. Published results have shown that the heating of tissue due to transducer self-heating can equal and often exceed the acoustic heating contribution. In this article, we report the development of a portable self contained thermal test object (TTO) capable of assessing the self-heating of intracavity diagnostic ultrasound transducers. The thermal conductivity and volumetric heat capacity of the tissue mimicking material (TMM) used in the TTO were measured, yielding values of (0.56 +/- 0.01) W m(-1) K(-1) and (3.5 +/- 0.8) MJ m(-3) K(-1). The speed of sound of the TMM was measured as 1540 m s(-1) and the attenuation over a frequency range of 2 to 10 MHz was found to be (0.50 +/- 0.01) dB cm(-1) MHz(-1). These results are in excellent agreement with the International Electrotechnical Commission (IEC 60601-2-37) requirements and the previously published properties of biological soft tissue. The temperature stability and uniformity, and suitability of the TTO for the measurement of transducer self-heating were tested and found to be satisfactory. The TTO reached a stable temperature of 37 degrees C in 3 h and the spatial variation in temperature was less than +/- 0.2 degrees C. Lastly, transducer self-heating measurements from a transvaginal transducer exceeded the IEC temperature limit of 43 degrees C in less than 5 min and the temperature reached after 30 min was 47.3 degrees C.     

Particle image-resolution test object

A blur definition and measurement technique for high-subject-contrast image resolution is proposed. A new test object utilizing the blur principle has been developed and feasibility tested. In developing this new test object, tests of various sizes and concentrations of small plastic particles in a gelatin base were performed. Optimum particle sizes and concentrations were selected for the popular clinical transducer frequencies. Test images were obtained with static and real-time transducers of various focal properties. In all cases the proposed test object clearly indicated the zone (focal zone) of optimum high-subject-contrast image resolution. The advantages of the proposed blur definition and measurement technique of high-subject-contrast image resolution include: test object simplicity, ease of performing and interpreting the measurement, usefulness for all types of real-time scanners, and the need for only one scan for both axial and lateral resolution information at all image depths.     

Design of ultrasound transducers using new piezoelectric ceramic materials

Three parameters α, DR and D_n(A), i.e. the efficiency, the dynamic range and the normalized axial resolution are used to evaluate the performance of ultrasound transducers for diagnostic imaging equipment. These parameters determine optimum conditions for use in the design of high frequency array transducers. A newly-developed piezoelectric ceramic material composed of (Pb,Sr)(Ti,Zr)O₃ or Pb[(Mg_{$\text{1}\text{3}$}, Nb_{$\text{2}\text{3}$}), Ti,Zr]O₃ which satisfies the above-mentioned conditions was used to fabricate a co-phase array ultrasound transducer. The transducer showed good electrical and acoustic performance in accordance with theoretical considerations.     

Generation of separate density and compressibility images in tissue

A method is suggested for reconstructing separate images of the variations in density and compressibility in the same tissue sample. The two long, rectangular transducer elements. As in diffraction tomography, 180 degrees access around the region of interest is required. This approach differs from conventional diffraction tomography, however, in that no transducer arrays are required and broadband illumination is used. A flat transducer, assumed long relative to the extent of the object, is used as a source of broadband, plane-wave illumination, and as a receiver of the backscattered sound. A second transducer, oriented at a different angle with respect to the first, issued as a receiver only. The two transducers are rotated together 180 degrees around the object, and the scattered sound is recorded at the plane-wave spectrum of the object directly, and provides sufficient information to reconstruct independent images of the variations in both the density and compressibility of the scattering medium. Image resolution is limited by the bandwidth of the illuminating sound.     

Muscle thickness and echo intensity measurements of the rectus femoris muscle of healthy subjects: Intra and interrater reliability of transducer tilt during ultrasound

This study aimed to assess the intra and interrater reliability of transducer tilt during the ultrasound (US) measurements of the muscle thickness and the echo intensity of the rectus femoris muscle (RF). Fourteen healthy male subjects (20.8 ± 0.8 years) participated in this study. The transducer tilt was measured using a digital angle gauge (°) during US. Two experimenters took two images to measure the muscle thickness (mm) and the echo intensity (a.u.: arbitrary unit). The intra and interclass correlation coefficient (ICC), standard error of measurement (SEM), and minimal detectable change (MDC) were also calculated. These measurements were immediately repeated. The ICC for the intrarater reliability for the transducer tilt, muscle thickness, and echo intensity were 0.96 (SEM: 0.9°, MDC: 2.6°), 0.99 (SEM: 0.4 mm, MDC: 0.1 mm), and 0.97 (SEM: 0.6 a.u., MDC: 1.7 a.u.), respectively. The ICC for the interrater reliability for the transducer tilt, muscle thickness, and echo intensity were 0.40 (SEM: 4.0°, MDC: 11.1°), 0.96 (SEM: 0.7 mm, MDC: 2.0 mm), and 0.95 (SEM: 0.9 a.u., MDC: 2.4 a.u.), respectively. The intrarater reliability of the transducer tilt was reliable, but the interrater reliability was questionable. Meanwhile, both the intra- and interrater reliability of the muscle thickness and the echo intensity were reliable.     

一次性换能器在机械通气病人套囊压力测量中的应用

目的：通过与机械通气病人套囊压力专用测压表所测压力的比较,探讨一次性换能器在套囊压力测定方面的应用价值。方法：将96例重症监护室（ICU）机械通气病人随机分成两组,分别用专用测压表及换能器测量患者套囊压力及套囊容积进行比较。结果：两组呼吸机检查均不漏气,一次性换能器组套囊充气容积（7.51±1.15）mL,测得压力值（21.09±1.97）mmHg（1 mmHg=0.133 kPa）;压力表组套囊充气容积（7.25±1.15）mL,测得压力值（21.32±2.58）mmHg,套囊充气容积及测得压力值两组间比较差异（P〉0.05）均无统计学意义。结论：一次性换能器测压法是一种简单易行,准确可靠、成本低廉的科学、理想的方法,可以应用于临床测量机械通气病人套囊压力。     

Effect of scan format on refraction artifacts

Refraction artifacts occur when the ultrasound beam is bent from its original direction as it passes through a boundary between tissues having different sound speeds. Refraction artifacts result in both the improper positioning and the improper brightness of echoes displayed in clinical sonograms. The effect of scan format on the sonographic appearance of several refraction artifacts due to a circular object possessing a sonic speed differing from its surrounding tissue was studied using a mathematical model and computer generated images. A quantitative index of artifact prominence was developed to compare the differences between the rectilinear and the sector scan formats. Theoretically predicted results were compared with actual sonograms of a specially designed phantom. Results included the finding that the retrolenticular afterglow was more prominent in the sector scan format, and that differences were most prominent when the refracting object was close to the transducer.     

Microbubbles induce renal hemorrhage when exposed to diagnostic ultrasound in anesthetized rats

The generation of ultrasound (US) bioeffects using a clinical imaging system is controversial. We tested the hypothesis that the presence of microbubbles in the US field of a medical imager induces biologic effects. Both kidneys of anesthetized rats were insonified for 5 min using a medical imaging system after the administration of microbubbles. One kidney was insonified using a continuous mode (30 Hz) and the opposite kidney was insonified using an intermittent (1 Hz) technique. The microbubbles were exposed to three different transducer frequencies and four transducer output powers. After insonification, the animals were euthanized, the kidneys were removed and their gross appearance scored under "blinded" conditions using a defined scale. After the administration of microbubbles, US imaging of the kidney caused hemorrhage in the renal tissue. The severity and area of hemorrhage increased with an increase in the transducer power and a decrease in the transducer frequency. Intermittent insonification in the presence of microbubbles produced a greater degree of renal hemorrhage than continuous imaging techniques.     

国产多晶阵心腔内超声成像换能器性能的初步测试

目的 检验自行设计研发的多晶阵心腔内超声成像换能器性能,为多功能心腔内超声导管系统的研制提供基础.方法 通过超声体模检测该心腔内超声成像换能器的分辨率.观察离体及在体心腔内结构并在犬急性心肌梗死模型中(n=6)测试其对室壁运动的评估能力.结果 此心腔内超声成像换能器为32晶阵侧向分布,导管化后外径为8F,其横向分辨率和纵向分辨率均为1.0 mm.离体及在体条件下均显示清晰的心腔内结构.在急性心肌梗死模型中观察到左前降支结扎2 h后,左心室前壁心肌收缩末期厚度变薄(1.03±0.29 cm vs 0.51±0.22 cm)、增厚率降低(56.31%±10.18% vs9.23%±1.52%)、最大搏幅下降[(0.58±0.08)cm vs (0.13±0.03)cm](P均<0.01).结论 此多晶阵心腔内超声成像换能器达到介入尺寸要求,其成像性能初步满足设计需求.     

Acceptance Criteria for Reprocessed AcuNav® Catheters: Comparison Between Functionality Testing and Clinical Image Assessment

The AcuNav^®-catheter is a vector-phased array ultrasound catheter that has shown great utility for both diagnosis and electrophysiological interventions. To test the feasibility of limited catheter reuse and to ensure that reprocessed catheters would produce acceptable clinical images, the present study compared the 2-D and Doppler image quality, as determined by clinical assessment, with the catheter's functional status as determined by the FirstCall 2000™ transducer tester. Reprocessed catheters from four functional categories, two acceptable and two unacceptable, were used to collect images, 2-D and Doppler, from a porcine heart. The images were blinded and then rated by clinical evaluation. The study found that catheter images from all functional categories were found to be clinically acceptable except for those from the lowest unacceptable category. In addition, examination of tip deflection characteristics showed no significant difference between new and reprocessed catheters. We conclude that reprocessed AcuNav^® catheters that pass functional tests are able to produce clinical images, 2-D and Doppler, which are equivalent to their new counterparts. (E-mail: blester@sterilmed.com)     

Real-time ultrasound transducer localization in fluoroscopy images by transfer learning from synthetic training data

The fusion of image data from trans-esophageal echography (TEE) and X-ray fluoroscopy is attracting increasing interest in minimally-invasive treatment of structural heart disease. In order to calculate the needed transformation between both imaging systems, we employ a discriminative learning (DL) based approach to localize the TEE transducer in X-ray images. The successful application of DL methods is strongly dependent on the available training data, which entails three challenges: (1) the transducer can move with six degrees of freedom meaning it requires a large number of images to represent its appearance, (2) manual labeling is time consuming, and (3) manual labeling has inherent errors.This paper proposes to generate the required training data automatically from a single volumetric image of the transducer. In order to adapt this system to real X-ray data, we use unlabeled fluoroscopy images to estimate differences in feature space density and correct covariate shift by instance weighting. Two approaches for instance weighting, probabilistic classification and Kullback–Leibler importance estimation (KLIEP), are evaluated for different stages of the proposed DL pipeline. An analysis on more than 1900 images reveals that our approach reduces detection failures from 7.3% in cross validation on the test set to zero and improves the localization error from 1.5 to 0.8 mm. Due to the automatic generation of training data, the proposed system is highly flexible and can be adapted to any medical device with minimal efforts.     

Real-time ultrasound transducer localization in fluoroscopy images by transfer learning from synthetic training data

The fusion of image data from trans-esophageal echography (TEE) and X-ray fluoroscopy is attracting increasing interest in minimally-invasive treatment of structural heart disease. In order to calculate the needed transformation between both imaging systems, we employ a discriminative learning (DL) based approach to localize the TEE transducer in X-ray images. The successful application of DL methods is strongly dependent on the available training data, which entails three challenges: (1) the transducer can move with six degrees of freedom meaning it requires a large number of images to represent its appearance, (2) manual labeling is time consuming, and (3) manual labeling has inherent errors.This paper proposes to generate the required training data automatically from a single volumetric image of the transducer. In order to adapt this system to real X-ray data, we use unlabeled fluoroscopy images to estimate differences in feature space density and correct covariate shift by instance weighting. Two approaches for instance weighting, probabilistic classification and Kullback–Leibler importance estimation (KLIEP), are evaluated for different stages of the proposed DL pipeline. An analysis on more than 1900 images reveals that our approach reduces detection failures from 7.3% in cross validation on the test set to zero and improves the localization error from 1.5 to 0.8 mm. Due to the automatic generation of training data, the proposed system is highly flexible and can be adapted to any medical device with minimal efforts.     

Radiolucent holder for echocardiographic recordings during fluoroscopy and exercise.

The design and function of a device that holds an ultrasound transducer against the chest wall in a stationary position is described. This holder is useful for obtaining echocardiograms during cinefluoroscopic procedures, various positional maneuvers, and exercise testing.     

探头频率和声波能量对心肌背向散射积分测值的影响

目的　观察超声背向散射积分心动周期变化幅度 (CVIB)测值与探头频率及发射声波能量 (AP)的关系。方法　用HP 5 5 0 0型超声诊断仪 ,取胸骨旁乳头肌短轴观 ,测量了 3 2例正常人在不同探头频率和AP设置条件下 ,左室后壁心肌组织的CVIB值。结果　探头频率为 2 .0MHz时 ,所测CVIB值较探头频率为 3 .0MHz和 4.0MHz时降低。AP过高 ( >3 5dB)或过低 ( <2 0dB)均使CVIB测值明显降低。结论　CVIB测值受探头频率和AP设置的影响 ,测量局部心肌组织的实际最大CVIB值需要选择恰当的探头频率和合理的AP设置。     

Objective performance testing and quality assurance of medical ultrasound equipment

There is an urgent need for a measurement protocol and software analysis for objective testing of the imaging performance of medical ultrasound equipment from a user's point of view. Methods for testing of imaging performance were developed. Simple test objects were used, which have a long life expectancy. First, the elevational focus (slice thickness) of the transducer was estimated and the in-plane transmit focus was positioned at the same depth. Next, the postprocessing look-up-table (LUT) was measured and linearized. The tests performed were echo level dynamic range (dB), contrast resolution (i.e., gamma of display, number of gray levels/dB) and sensitivity, overall system sensitivity, lateral sensitivity profile, dead zone, spatial resolution and geometric conformity of display. The concept of a computational observer was used to define the lesion signal-to-noise ratio, SNR(L) (or Mahalanobis distance), as a measure for contrast sensitivity. All the measurements were made using digitized images and quantified by objective means, i.e., by image analysis. The whole performance measurement protocol, as well as the quantitative measurements, have been implemented in software. An extensive data-base browser was implemented from which analysis of the images can be started and reports generated. These reports contain all the information about the measurements, such as graphs, images and numbers. The approach of calibrating the gamma by using a linearized LUT was validated by processing simultaneously acquired rf data. The contrast resolution and echo level of the rf data had to be compressed by a factor of two and amplified by a gain factor corresponding to 12 dB. This resulted in contrast curves that were practically identical to those obtained from DICOM image data. The effects of changing the transducer center frequency on the spatial resolution and contrast sensitivity were estimated to illustrate the practical usefulness of the developed approach of quality assurance by measuring objective performance characteristics. The developed methods might be considered as a minimum set of objective quality assurance measures. This set might be used to predict clinical performance of medical ultrasound equipment, taking into account the performance at a unique point in space i.e., the coinciding depths of the elevation and in-plane (azimuth) foci. Furthermore, it should be investigated whether the approach might be used to compare objectively various brands of equipment and to evaluate the performance specifications given by the manufacturer. Last but not least, the developed approach can be used to monitor, in a hospital environment, the medical ultrasound equipment during its life cycle. The software package may be viewed and downloaded at the website http://www.qa4us.eu.     

Evaluation of a quarter wavelength matching layer transducer in abdominal scanning.

A focused ultrasonic transducer employing quarter wavelength matching technology is compared to one of conventional design. The effect of the 6 db improvement in signal-to-noise ratio is demonstrated.     

B-scan transducer peak frequency measurement by diffraction grating spectroscopy.

Making use of technology developed for optics, transmission diffraction grating spectroscopy is proposed as a method of measuring ultrasound transducer frequencies. Diffraction grating theory is presented and extended to the case of a new test object proposed to measure a B-scan transducer's peak frequency. A prototype test object was built and its feasibility tested. The experimental results indicate that the test object performed well and has high measurement precision. The question of measurement accuracy was not resolved by the feasibility test. This complex point is discussed in detail. Recent improvements of the test object and the author's ongoing studies of diffraction grating characteristics are presented.     

Development of an ultra-broadband ultrasonic imaging system: prototype mechanical sector device

Purpose We constructed an ultra-broadband ultrasonic transducer in a mechanical sector device, and prototyped a system to generate real-time images with higher harmonics. The potential of the system to reduce speckle was also studied. Methods To efficiently detect the higher harmonic components in echo signals, in addition to the transmitting transducer, another transducer only for broadband reception is necessary. We constructed a receiving transducer by bonding a polyvinylidene fluoride (PVDF) high-polymer piezofilm to the radiation surface of the transmitting lead zirconate titanate (PZT) transducer. By building this PZT-PVDF bi-layered ultrasonic probe into a mechanical sector scanner of an ultrasonic diagnosis system, an ultra-broadband ultrasonic imaging system capable of real-time imaging was prototyped. Results Using images of phantoms in water acquired using the prototype system, the acquisition of higher harmonic images with less noise up to the fourth order was demonstrated. In addition, we confirmed that the logarithmic summation of harmonic components from the fundamental to the fourth harmonic effectively reduces speckle noise in the images. Conclusions By obtaining an echo signal from phantoms using a PZT-PVDF bi-layered ultrasonic mechanical sector probe, real-time imaging was carried out, and the effectiveness of its higher harmonic components from the fundamental to the fourth harmonic was confirmed with respect to speckle reduction.     

The split image artifact in pelvic ultrasonography: the anatomy and physics

Transverse ultrasound scans of the pelvis often demonstrate double images or an apparent widening of the structures deep to the bladder. The phenomenon occurs when the real-time transducer is placed on the midline of the abdominal wall. Most of the widening effect or split image is caused by refraction of sound at the interface between the abdominal wall muscle and the collection of fat deep to the muscle. Using Snell's Law, it is possible to calculate the amount of splitting for a given anatomic configuration. The phenomenon is most exaggerated when the object is furthest away from the transducer and when the curvature between muscle and fat is largest.     

Synthetic B-scan images by numerical solution of a wave equation

We describe an efficient method for obtaining model sector-scan images by direct solution of a linear wave equation characterizing pulse scattering from spatially varying bulk modulus and density distributions. Using a pseudospectral approach, the wave equation is solved numerically for each transducer orientation to obtain a set of A-line signals. After preprocessing the raw data, sector-scan images are constructed and displayed. Examples are given for several scattering object configurations, illustrating both specular and diffractive scattering in inhomogeneous media and demonstrating the utility of the model.