Rahul S. Singh

A Review of Tissue Substitutes for Ultrasound Imaging

The characterization and calibration of ultrasound imaging systems requires tissue-mimicking phantoms with known acoustic properties, dimensions and internal features. Tissue phantoms are available commercially for a range of medical applications. However, commercial phantoms may not be suitable in ultrasound system design or for evaluation of novel imaging techniques. It is often desirable to have the ability to tailor acoustic properties and phantom configurations for specific applications. A multitude of tissue-mimicking materials and phantoms are described in the literature that have been created using a variety of materials and preparation techniques and that have modeled a range of biological systems. This paper reviews ultrasound tissue-mimicking materials and phantom fabrication techniques that have been developed over the past four decades, and describes the benefits and disadvantages of the processes. Both soft tissue and hard tissue substitutes are explored.

THz Medical Imaging: in vivo Hydration Sensing

The application of THz to medical imaging is experiencing a surge in both interest and federal funding. A brief overview of the field is provided along with promising and emerging applications and ongoing research. THz imaging phenomenology is discussed and tradeoffs are identified. A THz medical imaging system, operating at ~525 GHz center frequency with ~125 GHz of response normalized bandwidth is introduced and details regarding principles of operation are provided. Two promising medical applications of THz imaging are presented: skin burns and cornea. For burns, images of second degree, partial thickness burns were obtained in rat models in vivo over an 8 hour period. These images clearly show the formation and progression of edema in and around the burn wound area. For cornea, experimental data measuring the hydration of ex vivo porcine cornea under drying is presented demonstrating utility in ophthalmologic applications.

Terahertz sensing in corneal tissues.

This work introduces the potential application of terahertz (THz) sensing to the field of ophthalmology, where it is uniquely suited due to its nonionizing photon energy and high sensitivity to water content. Reflective THz imaging and spectrometry data are reported on ex-vivo porcine corneas prepared with uniform water concentrations using polyethylene glycol (PEG) solutions. At 79% water concentration by mass, the measured reflectivity of the cornea was 20.4%, 14.7%, 11.7%, 9.6%, and 7.4% at 0.2, 0.4, 0.6, 0.8, and 1 THz, respectively. Comparison of nine corneas hydrated from 79.1% to 91.5% concentration by mass demonstrated an approximately linear relationship between THz reflectivity and water concentration, with a monotonically decreasing slope as the frequency increases. The THz-corneal tissue interaction is simulated with a Bruggeman model with excellent agreement. THz applications to corneal dystrophy, graft rejection, and refractive surgery are examined from the context of these measurements.

Ultra-Wideband Multifunctional Communications/Radar System

We have designed, simulated, fabricated, and tested an ultra-wideband (UWB) multifunctional communication and radar system utilizing a single shared transmitting antenna aperture. Two surface acoustic wave bandpass chirp filters were used to modulate the radar and communications pulses, generating linear frequency modulation waveforms with opposite slope factors. The system operates at a center frequency of 750 MHz with 500 MHz of instantaneous bandwidth. The measured range resolution is 63 cm (25 in) using targets with a radar cross section of 2.7 m2. The probability of detection was measured to be 99%, and the probability of false alarm was 7% with the communication and radar systems operating simultaneously. The bit error rate for simultaneous communication at 1 Mb/s, and radar at 150 kHz pulse repetition frequency and 1.5-ns pulsewidth is 2e - 3. Our UWB multifunctional system demonstrates the ability to simultaneously interrogate the environment and communicate through a shared transmitting antenna aperture, while realizing a simple system architecture with low output power and not employing time-division multiplexing.

Imaging of human tooth enamel using ultrasound

This paper reports the results of a complete circumferential scan of a human tooth and its underlying dentino-enamel junction using ultrasound at frequencies in the 10-MHz range. The imagery shows clearly a two-dimensional contour of the dentino-enamel junction with a depth and lateral resolution of approximately 100 /spl mu/m and 750 /spl mu/m, respectively. The resulting sonograph is compared with an optical micrograph of the same tooth to verify the accuracy of the ultrasonic technique. The results are a significant step toward the biolocation of submillimeter size features within the tooth volume.

In vivo terahertz imaging of rat skin burns

A reflective, pulsed terahertz (THz) imaging system was used to acquire high-resolution (d(10-90)/λ~1.925) images of deep, partial thickness burns in a live rat. The rats abdomen was burned with a brass brand heated to ~220°C and pressed against the skin with contact pressure for ~10 sec. The burn injury was imaged beneath a Mylar window every 15 to 30 min for up to 7 h. Initial images display an increase in local water concentration of the burned skin as evidenced by a marked increase in THz reflectivity, and this likely correlates to the post-injury inflammatory response. After ~1 h the area of increased reflectivity consolidated to the region of skin that had direct contact with the brand. Additionally, a low reflecting ring of tissue could be observed surrounding the highly reflective burned tissue. We hypothesize that these regions of increased and decreased reflectivity correlate to the zones of coagulation and stasis that are the classic foundation of burn wound histopathology. While further investigations are necessary to confirm this hypothesis, if true, it likely represents the first in vivo THz images of these pathologic zones and may represent a significant step forward in clinical application of THz technology.

Ultrasound detection of submerged dental implants through soft tissue in a porcine model

STATEMENT OF PROBLEM Current methods of measuring soft tissue thickness over potential dental implant sites and locating submerged implants may be imprecise or invasive. PURPOSE The purpose of this study was to develop and demonstrate proof of the concept of a customized ultrasound imaging system in locating and measuring the depth of implants submerged beneath soft tissue. MATERIAL AND METHODS A complete ultrasound system, including a customized soft tissue-matched transducer, transceiver, and digital signal processing algorithms, was created for the specific application of detecting dental implants anchored in bone beneath soft tissue. The system was used to locate implants placed in cancellous bone and measure overlying soft tissue depth in a porcine model. Ten measurements were made on each porcine model by manually moving the transducer laterally over the soft tissue surface. Data were analyzed with descriptive statistics. RESULTS The mean signal-to-noise ratio, SNR (standard deviation), from the bone surfaces, was 19.1 (4.6) dB, and the mean SNR from the implant surfaces was 36.6 (2.2) dB, resulting in a mean difference of 17.5 dB, or x56.2, in average signal power between the bone and implant surfaces. Consequently, implants were easily and accurately (+/-0.2 mm) located beneath at least 5 mm of soft tissue. Likewise, soft tissue depths over bone and implants were accurately measured and were within the corresponding caliper tissue measurement error (+/-0.5 mm). CONCLUSIONS The specialized ultrasound imaging system located and measured the depth of implants placed in bone submerged beneath soft tissue in a porcine model.

A compact 30 GHz low loss balanced hybrid coupler fabricated using micromachined integrated coax

Recent years have seen increased interest in the K and Ka frequency bands due to continued growth of satellite communications, particularly the USA Air Force Wideband Gapfiller Satellite (WGS) programme. This work presents a 4-port 30-GHz quadrature hybrid coupler fabricated using micromachined integrated coaxial line. The best electrical performance was measured at 31.6 GHz, where the amplitude and phase difference between the through and coupled ports are within 0.23 dB and 1.5/spl deg/, respectively, of the nominal -3 dB, 90/spl deg/ design specification. The excess losses for the through and coupled port at 31.6 GHz are 0.31 dB and 0.54 dB. The overall footprint of the component is /spl lambda//10 by /spl lambda//5 at 30 GHz (1.0 /spl times/ 1.7 mm), making this hybrid extremely small compared to alternative waveguide or coaxial designs. We believe this is the smallest coaxial hybrid coupler ever fabricated for 30-GHz.

THz Imaging Based on Water-Concentration Contrast

Terahertz medical imaging has emerged as a promising new field because of its non-ionizing photon energy and its acute sensitivity to water concentration. To better understand the primary contrast mechanism in THz imaging of tissues, the reflectivity of varying water concentrations was measured. Using a pulsed THz reflective imaging system, a 0.3 mm thin paper sample with varying water concentrations was probed and from the measured data a noise equivalent delta water concentration (NEΔWC) of 0.054% was derived. The system is based on a photoconductive pulsed source and time-gated waveguide-mounted Schottky diode receiver. It operates at a center frequency of 500 GHz with 125 GHz of noise-equivalent bandwidth and at a standoff of 4 cm, the imaging system achieved a spot size of 2.2 mm. The high water sensitivity of this system was exploited to image burned porcine (pig) skin models in reflection using differences in water content of burned and unburned skin as the contrast mechanism. The obtained images of the porcine skin burns are a step towards the ability to quantify burn injuries using THz radiation.

P3D-6 Simulation, Fabrication, and Characterization of a Novel Flexible, Conformal Ultrasound Transducer Array

A flexible, conformal ultrasound transducer array has been developed for use in medical imaging. The transducer has been designed such that it can wrap conformally around curved body surfaces and provide volumetric imagery of soft tissue and hard tissue structures without mechanical scanning. PZFlextrade, finite element time domain piezoelectric simulation software, was used to model and simulate the curved transducer array in two dimensions. A flexible, conformal transducer array prototype was fabricated, and characterization of the array elements yielded a 15.5 MHz fundamental resonance, a 35% instantaneous bandwidth, and a 300 ns pulse width, resulting in high axial (220 mum in water) resolution. The design and fabrication process allows for scaling of the array to fit various applications, including imaging of the extremities, abdominal imaging, and ultrasound guidance of procedures.