High Frequency Ultrasound Imaging Research Articles

Digital High Frequency Coded Imaging System

Coded transmission is an approach to solve the inherent compromise between penetration and resolution required in ultrasound imaging. It is universally acknowledged that this technique gives major improvement in SNR. A novel high frequency (20-35 MHz) ultrasound real-time imaging system for research and evaluation was developed. The digital programmable coder-digitizer module based on the field programmable gate array (FPGA) supports arbitrary waveform coded transmission and RF echoes sampling up to 200 MSPS, as well as real-time streaming of digitized RF data via a high speed USB interface to the PC. All RF and image data processing were implemented in the software. A novel balanced software architecture using CPU and GPU processing supports real-time processing and display at rates up to 30 frames/sec. The system was used to acquire data for sine burst and 16-bit Golay code excitation using a single element scanning head with thick film focused spherical transducer 25 MHz center frequency and 56% system bandwidth. SNR gain for the Golay codes (referenced to single burst) of 15 dB for 20 MHz and 16 dB for 35 MHz were obtained. In water the axial resolution for both single burst and the Golay codes was the same FWMH=20 ns at 20 MHz and FWMH=15 ns at 35 MHz.

High Frequency Ultrasound Imaging Detects Cardiac Dyssynchrony in Noninfarcted Regions of the Murine Left Ventricle Late After Reperfused Myocardial Infarction

Cardiac dyssynchrony in the left ventricles of murine hearts late (≥28 d) after reperfused myocardial infarction (post-MI) was assessed using high frequency 30 MHz B-mode ultrasound imaging. Nine post-MI and six normal C57Bl/6 mice were studied in both short- and long-axis views. Regional time to peak displacement (T peak_d) and time to peak strain (T peak_s) were calculated in 36 sectors along the myocardial circumference; then their standard deviations (SD_T peak_d and SD_T peak_s) were computed among noninfarcted myocardial regions for each mouse and were compared between the normal and post-MI mouse groups with Student's t-test. The comparison revealed that SD_T peak_d and SD_T peak_s were significantly larger in the post-MI hearts than in the normal hearts. The displacement uniformity ratio was determined to be 0.97 ± 0.01 and 0.85 ± 0.07 for radial and circumferential displacements in the normal hearts, respectively; and 0.59 ± 0.17 and 0.64 ± 0.24 in the post-MI hearts. In conclusion, this high resolution ultrasound image tracking method provides for the detection of cardiac dyssynchrony in the noninfarcted regions in the murine left ventricles late after MI by identifying the temporal and spatial disparity of regional myocardial contraction. (E-mail: jh7fj@virginia.edu)

Cholinergic stimulation induces asynchrony between the circular and longitudinal muscle contraction during esophageal peristalsis

In healthy subjects, a close temporal correlation exists between contractions of the circular muscle (CM) and longitudinal muscle (LM) layers of the esophagus. Patients with nutcracker esophagus show disassociation between the peak of contractions of the CM and LM layers and the peak of contraction 1-3 s apart (Jung HY, Puckett JL, Bhalla V, Rojas-Feria M, Bhargava V, Liu J, Mittal RK. Gastroenterology 128: 1179-1186, 2005). The purpose of the present study was to evaluate the effect of acetylcholinesterase inhibitor (edrophonium) and acetylcholine receptor antagonist (atropine) on human esophageal peristalsis in normal subjects. High-frequency intraluminal ultrasound imaging and manometry were performed simultaneously during swallow-induced peristalsis in ten normal subjects. Standardized 5-ml water swallows were recorded 2 cm above the lower esophageal sphincter under three study conditions: control, edrophonium (80 microg/kg iv), and atropine (10 microg/kg iv). A close temporal correlation exists between the peak pressure and peak wall thickness during the control period. The mean time lag between the peak LM and peak CM contraction was 0.03 s. After edrophonium administration, the mean contraction amplitude increased from 101 +/- 9 mmHg to 150 +/- 20 mmHg (P < 0.05) and mean peak muscle thickness increased from 3.0 +/- 0.2 mm to 3.6 +/- 0.3 mm (P < 0.01), and duration of both CM and LM contractions were also increased. Furthermore, the mean time difference between the peak LM and CM was increased to 1.1 s, (ranging 0.2 to 3.4 s) (P < 0.0001). We conclude that cholinomimetic agent induces discoordination between the two muscle layers of the esophagus.

Noninvasive real-time imaging of intima thickness after rat carotid artery balloon injury using ultrasound biomicroscopy

Objectives High frequency ultrasound imaging for small animal research (ultrasound biomicroscopy, UBM) has recently become available. Here, we evaluated the possibility to determine intima thickness in the rat carotid artery after balloon injury and to monitor intimal hyperplasia formation by UBM during pharmacological treatment. Methods Balloon injury of the left carotid artery was performed on Sprague-Dawley rats. Carotid arteries of all animals were examined by Vevo 770 UBM (VisualSonics Inc.) using 55 MHz probe at day 1, 7, 14 and 21 after the injury. Whole vessel wall, intima and media thicknesses as well as lumen diameter were measured at different levels. Histomorphometric analyses were performed on day 14 and 21. A group of animals were treated with picropodophyllin, an insulin-like growth factor-1 receptor inhibitor. Results Ex-vivo comparison of UBM and histology demonstrated an excellent correspondence of intimal tears, and the internal and external elastic membranes could be identified. We found also an agreement (Bland–Altman test) between histological measurements and UBM delineations of the rat carotid artery wall layers, with a significant correlation for intima–media thickness ( r = 0.97; p < 0.0001) and intima measurements. We were able to follow changes in the vessel wall structure and vessel diameter as a response to balloon injury in real time. Furthermore, the therapeutic effect of picropodophyllin could be assessed using UBM. Conclusions UBM provides a reliable noninvasive, in-vivo visualization of rat vasculature. It allows us to perform longitudinal studies of intimal thickness progression and regression as well as lumen changes in individual animals.

Polymer Microring Resonators for High-Frequency Ultrasound Detection and Imaging

Polymer microring resonators fabricated by nanoimprinting are presented as a means of ultrasound detection. Acoustic waves impinging on a ring-shaped optical resonator cause strain in the ring dimensions, modulating optical output. Basic acoustic and optical characteristics of the microring sensor are presented. Measurements at several frequencies show a high sensitivity and low noise-equivalent pressure. The angular response is determined by sensing the optoacoustic excitation of a 49 μm polyester microsphere and shows wide-angle sensitivity. A 1-D array consisting of 4 microrings is demonstrated using wavelength multiplexing for addressing each element. The high sensitivity, bandwidth, and angular response make it a potentially useful sensor platform for many applications including high-frequency ultrasonic and photoacoustic imaging.

A Novel Envelope Detector for High-Frame Rate, High-Frequency Ultrasound Imaging

This paper proposes a novel design of envelope detectors capable of supporting a small animal cardiac imaging system requiring a temporal resolution of more than 150 frames per second. The proposed envelope detector adopts the quadrature demodulation and the lookup table (LUT) method to compute the magnitude of the complex baseband components of received echo signals. Because the direct use of the LUT method for a square root function is not feasible due to a large memory size, this paper presents a new LUT strategy dramatically reducing its size by using binary logarithmic number system (BLNS). Due to the nature of BLNS, the proposed design does not require an individual LOG-compression functional block. In the implementation using a field programmable gate array (FPGA), a total of 166.56 Kbytes memories were used for computing the magnitude of 16-bit in-phase and quadrature components instead of 4 Gbytes in the case of the direct use of the LUT method. The experimental results show that the proposed envelope detector is capable of generating LOG-compressed envelope data at every clock cycle after 32 clock cycle latency, and its maximum error is less than 0.5 (i.e., within the rounding error), compared with the arithmetic results of square root function and LOG compression.

Usefulness of High-Frequency Vascular Ultrasound Imaging and Serum Inflammatory Markers to Predict Plaque Rupture in Patients With Stable and Unstable Angina Pectoris

It remains unclear what kind of morphologic and biochemical features best predict plaque rupture in patients with angina pectoris (AP). This study aimed to investigate whether combined high-frequency vascular ultrasound imaging and measurements of serum inflammatory biomarkers can predict coronary plaque ruptures in patients with AP. The study population consisted of 20 patients with stable AP and 40 patients with unstable AP. High-frequency vascular ultrasound imaging was performed in the 2 groups to measure intima-media thickness, the plaque acoustic density of the common carotid arteries, and the flow-mediated dilation of the brachial arteries. Serum lipid profile and inflammatory biomarkers were measured in all patients. Using intravascular ultrasound, a list of coronary imaging parameters was obtained. A multivariate logistic regression model was applied to calculate the odds ratio of each parameter to predict coronary plaque ruptures detected by intravascular ultrasound. Of 139 coronary artery plaques identified by intravascular ultrasound, 48 plaques (9 in stable AP and 39 in unstable AP) developed ruptures. Among measured parameters, the values of carotid intima-media thickness, coronary external elastic membrane area, plaque area, plaque burden, plaque eccentric index and remodeling index, serum high-sensitivity C-reactive protein, soluble intercellular adhesion molecule-1, and soluble vascular cell adhesion molecule-1 were significantly higher in unstable AP than in stable AP (p <0.05 to 0.01). Of these parameters, carotid intima-media thickness, serum high-sensitivity C-reactive protein, and the coronary remodeling index were found to be significant predictors of coronary plaque rupture, with odds ratios of 9.51 (95% confidence interval 1.29 to 21.81), 3.02 (95% confidence interval 1.01 to 7.65), and 0.01 (95% confidence interval 0.00 to 0.34), respectively. In conclusion, combined high-frequency ultrasound imaging of coronary and carotid arteries and measurements of serum inflammatory markers are able to predict coronary plaque ruptures in patients with AP.

Nail thickness measurements using optical coherence tomography and 20-MHz ultrasonography

Nail diseases are often troubling to the patient and may present a diagnostic challenge to the dermatologist. Biopsies from the nail may be required although often perceived uncomfortable by the patient and potentially scarring. Noninvasive technologies are therefore of particular interest in the study of nails. Optical coherence tomography (OCT) is an optical imaging modality which may provide improved data. This study evaluates nail morphology and thickness in OCT images in comparison with high-frequency ultrasound (HFUS) imaging of the nail. Ten healthy volunteers were recruited for imaging and nail measurements; OCT and HFUS images were compared qualitatively. Nail thickness measurements with four different techniques were compared: ultrasound, OCT, polarization-sensitive (PS) OCT and callipers. The OCT system was developed at Risoe National Laboratory, Denmark. A commercially available 20-MHz ultrasound system (Dermascan; Cortex Technology, Hadsund, Denmark) was used. In standard OCT the nail plate appeared as a layered structure containing a varying number of horizontal homogeneous bands of varying intensity and thickness. PS-OCT images of the nail plate also showed a layered structure. The refractive index of the nail was 1.47 +/- 0.09. OCT and PS-OCT had low coefficients of variation, 6.31 and 6.53, respectively, compared with other methods: HFUS 12.70 and callipers 14.03. PS-OCT has to our knowledge not been applied to OCT analysis of the nail, and offers some advantages in separation of the nail bed from the nail plate.

High-Frequency Ultrasound of Extranodal Limbal Rosai-Dorfman Disease

To correlate the clinical, high-frequency ultrasound, and pathology characteristics of an epibulbar Rosai-Dorfman tumor. We report a case of a steroid-resistant yellow perilimbal epibulbar tumor referred for ophthalmic oncology evaluation. It was documented by slit-lamp photography and evaluated by high-frequency ultrasound. A hematology-oncology evaluation and excisional biopsy were performed. Ophthalmic examination revealed a solitary yellow perilimbal epibulbar tumor. High-frequency ultrasound imaging revealed low internal reflectivity and partial-thickness scleral and corneal invasion with no extension into the anterior segment. Primary excision was performed. Although histopathology revealed large atypical histiocytes, immunochemistry found them to be both S-100 positive and CD1a negative (diagnostic of Rosai-Dorfman disease). Hematology-oncology evaluation revealed no systemic disease or links to human herpesvirus. Local control required cryotherapy and sub-Tenon steroid injection. Epibulbar Rosai-Dorfman tumors can invade the sclera and are often treated by surgical excision. High-frequency ultrasound imaging should be used to determine the presence or extent of invasion before surgery.

In Vivo Ultrasound Biomicroscopy of Skin: Spectral System Characteristics and Inverse Filtering Optimization

High-frequency ultrasound (HFUS) in the 20 MHz to 100 MHz range has to meet the opposite requirements of good spatial resolution and of high penetration depth for in vivo ultrasound biomicroscopy (UBM) of skin. The attenuation of water, which serves as sound propagation medium between utilized single element transducers and the skin, becomes very eminent with increasing frequency. Furthermore, the spectra of acquired radio frequency (rf) echo signals change over depth because of the diffracted sound field characteristics. The reduction of the system's center frequency and bandwidth causes a significant loss of spatial resolution over depth. In this paper, the spectral characteristics of HFUS imaging systems and the potential of inverse echo signal filtering for the optimization of pulse-echo measurements is analyzed and validated. A Gaussian model of the system's transfer function, which takes into account the frequency-dependent attenuation of the water path, was developed. Predictions of system performance are derived from this model and compared with measurement results. The design of a HFUS skin imaging system with a 100 MHz range transducer and a broadband driving electronics is discussed. A time-variant filter for inverse rf echo signal filtering was designed to compensate the system's depth-dependent imaging properties. Results of in vivo measurements are shown and discussed.

High frequency ultrasound imaging of a single-species biofilm

Objective This study evaluated the feasibility of a high frequency ultrasound scan to examine the 3D morphology of Streptococcus mutans biofilms grown in vitro. Methods Six 2-day S. mutans biofilms and six 7-day biofilms were grown on tissue culture membranes and on bovine dentine discs. A sterile growth medium on the membrane and disc were used as controls. Surfaces were rinsed and then immersed in sterile saline. High-frequency ultrasound imaging system was used to scan these surfaces at 55 MHz, and a computer program calculated the average thickness of the biofilm layer from the 3D images. Results 3D pictures of the biofilm layers were obtained. Different cross-sections and plains are easily demonstrated. The average thickness of the 7-day biofilm was significantly bigger than the 2-day on both the membranes and dentinal discs. No structures were observed on the sterile membrane or disc. Conclusion Three-dimensional structural imaging in situ is possible without damaging the biofilm layer in a quick and easy manner and can therefore be used to evaluate biofilms longitudinally as a function of time.

High Frequency Ultrasound Imaging of the Growth and Development of the Normal Chick Embryo

The purpose of this study is to delineate with high frequency ultrasound imaging the normal growth and development of the chick embryo throughout its incubation period. White Leghorn chick embryos were imaged through an opening in the egg air cell from incubation day 0–19 (Hamburger & Hamilton stage 1–45) using a 13 MHz clinical high frequency linear small parts transducer. Multiple anatomic growth parameters were measured. Normal growth was confirmed with Hamburger and Hamilton staging. A timeline was constructed showing when each anatomic growth parameter could be visualized. Means and standard deviations of each parameter were plotted against incubation days studied to create nomograms and numerical tables of normal growth and development of the chick embryo. With this set of data, abnormal growth and development of the chick embryo can now be assessed. (E-mail: maschellpfeffer@gmail.com)

Ultrasonic Characterization of Whole Cells and Isolated Nuclei

High frequency ultrasound imaging (20 to 60 MHz) is increasingly being used in small animal imaging, molecular imaging and for the detection of structural changes during cell and tissue death. Ultrasonic tissue characterization techniques were used to measure the speed of sound, attenuation coefficient and integrated backscatter coefficient for (a) acute myeloid leukemia cells and corresponding isolated nuclei, (b) human epithelial kidney cells and corresponding isolated nuclei, (c) multinucleated human epithelial kidney cells and d) human breast cancer cells. The speed of sound for cells varied from 1522 to 1535 m/s, while values for nuclei were lower, ranging from 1493 to 1514 m/s. The attenuation coefficient slopes ranged from 0.0798 to 0.1073 dB mm −1 MHz −1 for cells and 0.0408 to 0.0530 dB mm −1 MHz −1 for nuclei. Integrated backscatter coefficient values for cells and isolated nuclei showed much greater variation and increased from 1.71 × 10 −4 Sr −1 mm −1 for the smallest nuclei to 26.47 × 10 −4 Sr −1 mm −1 for the cells with the largest nuclei. The findings suggest that integrated backscatter coefficient values, but not attenuation or speed of sound, are correlated with the size of the nuclei. (E-mail: mkolios@ryerson.ca)

Staging of Esophageal Cancer Using High-Frequency Ultrasound (HFUS)

Purpose: Standard echoendoscopic evaluation of superficial esophageal tumors using frequencies of 7.5MHz and 12MHz has limited ability to optimally visualize the mucosal and submucosal layers. High-frequency catheter-based miniprobes operating at 20MHz permit improved imaging of the superficial layers of the esophagus. The diagnostic accuracy of HFUS staging in superficial esophageal tumors has not been definitively established. The aim of this study was to assess our institution's staging accuracy for superficial esophageal cancers using HFUS, and to determine whether the use of HFUS improves patient management. Methods: Between December 1999 and December 2004, 42 patients with esophageal cancer underwent preoperative EUS staging without neoadjuvant chemoradiotherapy. Within this group, five patients underwent staging with HFUS, and make up the study subset. Two patients were referred for Barrett's esophagus with high-grade dysplasia (HGD) on prior endoscopic biopsy, and three patients were referred for Barrett's esophagus with esophageal nodules found to be at least intramucosal adenocarcinoma on prior endoscopic biopsy. Overall staging accuracy, rate of complications, and impact on clinical management were assessed. Results: Four of the five patients (80%) had technically adequate HFUS imaging. By surgical pathology, all four patients had T1 disease (confined to the mucosa or submucosa). The accuracy of HFUS T staging in this group was 100% (95% CI 40–98%). No complications occurred as a result of HFUS examination. In both patients referred with HGD in the setting of Barrett's esophagus, demonstration of at least intramucosal adenocarcinoma changed management, prompting esophagectomy instead of endoscopic ablation with photodynamic therapy. Conclusions: 1. Use of HFUS at 20MHz frequency appears to be highly accurate in T staging of superficial esophageal cancers. However, larger studies will now be needed to confirm these results. 2. Use of HFUS appears to be safe. 3. HFUS staging has the potential to significantly impact patient management, particularly in its ability to detect intramucosal or more deeply invasive adenocarcinoma in patients with Barrett's esophagus with known HGD.

75 MHz Ultrasound Biomicroscopy of Anterior Segment of Eye

Very high frequency ultrasound (35-50 MHz) has had a significant impact upon clinical imaging of the anterior segment of the eye, offering an axial resolution as small as 30 microm. Higher frequencies, while potentially offering even finer resolution, are more affected by absorption in ocular tissues and even in the fluid coupling medium. Our aim was to develop and apply improved transducer technology utilizing frequencies beyond those routinely used for ultrasound biomicroscopy of the eye. A 75-MHz lithium niobate transducer with 2 mm aperture and 6 mm focal length was fabricated. We scanned the ciliary body and cornea of a human eye six years post-LASIK. Spectral parameter images were produced from the midband fit to local calibrated power spectra. Images were compared with those produced using a 35 MHz lithium niobate transducer of similar fractional bandwidth and focal ratio. The 75-MHz transducer was found to have a fractional bandwidth (-6 dB) of 61%. Images of the post-LASIK cornea showed higher stromal backscatter at 75 MHz than at 35 MHz. The improved lateral resolution resulted in better visualization of discontinuities in Bowman's layer, indicative of microfolds or breaks occurring at the time of surgery. The LASIK surface was evident as a discontinuity in stromal backscatter between the stromal component of the flap and the residual stroma. The iris and ciliary body were visualized despite attenuation by the overlying sclera. Very high frequency ultrasound imaging of the anterior segment of the eye has been restricted to the 35-50 MHz band for over a decade. We showed that higher frequencies can be used in vivo to image the cornea and anterior segment. This improvement in resolution and high sensitivity to backscatter from the corneal stroma will provide benefits in clinical diagnostic imaging of the anterior segment.

Estimation methods for flow imaging with high frequency ultrasound

This article proposes to estimate slow blood flow with high frequency ultrasound imaging. The proposed technique combines 2 methods. First, a statistical method, called Speckle Flow Imaging (SFI) based on the analysis of changes in the speckle pattern along time, gives an index directly related to the total velocity vector. Secondly, a block matching approach estimates the in-plane velocity components. Results on calibrated flow sequences of blood mimicking fluid have shown good agreement with the statistical model. The quantification of flow is achieved with pulsed flow and is also angle independent when the flow is perpendicular to the ultrasound beam. Speckle Tracking has been evaluated on the same data and has shown good estimation of the in-plane velocity vector when the component of velocity perpendicular to the imaging plane is inferior to 1 mm/s. The results of these two methods permit the evaluation of the total 3D velocity field and the orthogonal velocity component relative to the imaging plane. This allows the quantification of blood flow (volumetric per time unit across the sequence).

Surface roughness imaging using the acoustic emission induced by the dynamic radiation force of ultrasound

High frequency ultrasound imaging systems are traditionally used in industry to obtain high resolution images of solid surfaces, and in some cases, to evaluate surface roughness. In this study we show that imaging fine surface roughness in the order of few microns is achievable at audio range frequencies (kilohertz range) by using the vibroacoustography (VA) technique. This technique allows one to image surface roughness on the basis of the ultrasound radiation force stimulated acoustic emission in a standing ultrasound wave field. Images obtained here demonstrate that VA may be used as a powerful tool for nondestructive inspection and imaging of surface roughness.

SU‐FF‐I‐87: High Frequency Ultrasound To Monitor Murine Orthotopic Bladder Cancer Model : An Alternative To Magnetic Resonance Imaging

Purpose: The study of human bladder cancer is often performed using subcutaneous xenograft models. But, a more relevant model is the orthotopic approach where tumors are grown in their native host environment. Studying the growth kinetics of these intraabdominal tumors is much more difficult as they are not accessible for caliper measurements. The purpose of this study is to demonstrate the feasibility of high frequency ultrasound (US) imaging as an efficient and time saving alternative to magnetic resonance (MR) in longitudinal monitoring of an orthotopic bladder cancer model. Method and Materials: Eleven athymic nude mice underwent orthotopic injection of 10 6 253‐J B‐V cells into their bladder walls. MR imaging was performed weekly on a 4.7 T small animal MR scanner (Bruker Biospin). Axial T1 weighted spin‐echo and T2 weighted fast spin‐echo acquisitions (TR: 700ms, TE: 8.5ms, FOV: 4.0cm × 3.0cm, Matrix:256 × 192,duration:45–50 minutes) were performed. 3D US data (step size:0.25mm, range:12mm, duration:5–10 minutes) was collected using a Vevo 660 (Visualsonics) system operating at 40 MHz. Tumor size at necropsy was measured with a caliper, and volume calculated with the formula , where a, b, c are the tumor dimensions. Results: The MR volume measurements were made from the fused T1 and T2 images, as done in previous experiments. Two independent observers made the MR and US measurements. There was very good correlation between the MR and 3D US volume measurements with Pearson's correlation coefficient of 0.987 (p&lt;0.05). The correlation between the US volume and the specimen measurements was 0.793 (p&lt;0.05). Conclusion: High frequency ultrasound can effectively monitor murine intraabdominal tumor growth. It is a cheaper and faster modality than CT and MR. Extensions of this technology in the future could include orthotopic injections under US guidance, Power Doppler and harmonic imaging with microbubbles for evaluation of tumor vascularity.

WE‐C‐ValB‐03: Functional Analysis of Tumor Vasculature Post Antiangiogenic Intervention with Argeted Anti‐VEGFR2 Therapy

High frequency ultrasound biomicroscopy (UBM) permits in vivo assessment of anatomical and physiological parameters at high resolution. The ability to image the mouse noninvasively facilitates longitudinal studies with repeated measurements in the same animal over time, to follow normal development or disease processes as well as response to therapeutic intervention. Anatomical images are obtained using B mode, giving a cross‐section of the structure of interest. Anatomical changes can now be quantitated using 3D volumetric B mode imaging, and rendered to give an accurate contour of the structure of interest. 3D UBM may be used to measure organ growth through normal development, or to compare treated versus untreated populations in mouse models of human disease. High frequency ultrasound Doppler permits measurement of blood flow velocity in selected vessels of interest. Recent advances in blood flow analysis, including speckle variance analysis, permit investigation of blood flow velocity of flow velocities down to 100 microns per second, as well as perfusion alterations.High frequency ultrasound imaging modes, imaging strategies, and post‐processing methods used to generate anatomical and physiological data will be discussed.Educational Objectives:1. Understand the concepts behind high frequency ultrasound imaging modes.2. Understand the application of high frequency ultrasound to image development and disease processes in the mouse, including experimental design, data acquisition and data processing.3. Understand therapeutic study design, data acquisition and analysis using UBM.

High-frequency ultrasound annular array imaging. Part II: digital beamformer design and imaging

This is the second part of a two-paper series reporting a recent effort in the development of a high-frequency annular array ultrasound imaging system. In this paper an imaging system composed of a six-element, 43 MHz annular array transducer, a six-channel analog front-end, a field programmable gate array (FPGA)-based beamformer, and a digital signal processor (DSP) microprocessor-based scan converter will be described. A computer is used as the interface for image display. The beamformer that applies delays to the echoes for each channel is implemented with the strategy of combining the coarse and fine delays. The coarse delays that are integer multiples of the clock periods are achieved by using a first-in-first-out (FIFO) structure, and the fine delays are obtained with a fractional delay (FD) filter. Using this principle, dynamic receiving focusing is achieved. The image from a wire phantom obtained with the imaging system was compared to that from a prototype ultrasonic backscatter microscope with a 45 MHz single-element transducer. The improved lateral resolution and depth of field from the wire phantom image were observed. Images from an excised rabbit eye sample also were obtained, and fine anatomical structures were discerned.