Microbubble Axial Localization Errors in Ultrasound Super-Resolution Imaging.

Acoustic super-resolution imaging has allowed the visualization of microvascular structure and flow beyond the diffraction limit using standard clinical ultrasound systems through the localization of many spatially isolated microbubble signals. The determination of each microbubble position is typically performed by calculating the centroid, finding a local maximum, or finding the peak of a 2-D Gaussian function fit to the signal. However, the backscattered signal from a microbubble depends not only on diffraction characteristics of the waveform, but also on the microbubble behavior in the acoustic field. Here, we propose a new axial localization method by identifying the onset of the backscattered signal. We compare the accuracy of localization methods using in vitro experiments performed at 7-cm depth and 2.3-MHz center frequency. We corroborate these findings with simulation results based on the Marmottant model. We show experimentally and in simulations that detecting the onset of the returning signal provides considerably increased accuracy for super-resolution. Resulting experimental cross-sectional profiles in super-resolution images demonstrate at least 5.8 times improvement in contrast ratio and more than 1.8 times reduction in spatial spread (provided by 90% of the localizations) for the onset method over centroiding, peak detection, and 2-D Gaussian fitting methods. Simulations estimate that these latter methods could create errors in relative bubble positions as high as at these experimental settings, while the onset method reduced the interquartile range of these errors by a factor of over 2.2. Detecting the signal onset is, therefore, expected to considerably improve the accuracy of super-resolution.

Urinary catheters: history, current status, adverse events and research agenda.

For more than 3500 years, urinary catheters have been used to drain the bladder when it fails to empty. For people with impaired bladder function and for whom the method is feasible, clean intermittent self-catheterization is the optimal procedure. For those who require an indwelling catheter, whether short- or long-term, the self-retaining Foley catheter is invariably used, as it has been since its introduction nearly 80 years ago, despite the fact that this catheter can cause bacterial colonization, recurrent and chronic infections, bladder stones and septicaemia, damage to the kidneys, the bladder and the urethra, and contribute to the development of antibiotic resistance. In terms of medical, social and economic resources, the burden of urinary retention and incontinence, aggravated by the use of the Foley catheter, is huge. In the UK, the harm resulting from the use of the Foley catheter costs the National Health Service between £1.0-2.5 billion and accounts for ∼2100 deaths per year. Therefore, there is an urgent need for the development of an alternative indwelling catheter system. The research agenda is for the new catheter to be easy and safe to insert, either urethrally or suprapubically, to be retained reliably in the bladder and to be withdrawn easily and safely when necessary, to mimic natural physiology by filling at low pressure and emptying completely without damage to the bladder, and to have control mechanisms appropriate for all users.

Quantitative ultrasonic computed tomography using phase-insensitive pyroelectric detectors.

The principle of using ultrasonic computed tomography (UCT) clinically for mapping tissue acoustic properties was suggested almost 40 years ago. Despite strong research activity, UCT been unable to rival its x-ray counterpart in terms of the ability to distinguish tissue pathologies. Conventional piezoelectric detectors deployed in UCT are termed phase-sensitive (PS) and it is well established that this property can lead to artefacts related to refraction and phase-cancellation that mask true tissue structure, particularly for reconstructions involving attenuation. Equally, it has long been known that phase-insensitive (PI) detectors are more immune to this effect, although sufficiently sensitive devices for clinical use have not been available. This paper explores the application of novel PI detectors to UCT. Their operating principle is based on exploiting the pyroelectric properties of the piezoelectric polymer polyvinylidene difluoride. An important detector performance characteristic which makes it particularly suited to UCT, is the lack of directionality of the PI response, relative to the PS detector mode of operation. The performance of the detectors is compared to conventional PS detection methods, for quantitatively assessing the attenuation distribution within various test objects, including a two-phase polyurethane phantom. UCT images are presented for a range of single detector apertures; tomographic reconstruction images being compared with the known structure of phantoms containing inserts as small as 3 mm, which were readily imaged. For larger diameter inserts (>10 mm), the transmitter-detector combination was able to establish the attenuation coefficient of the insert to within ±10% of values determined separately from plane-wave measurements on representative material plaques. The research has demonstrated that the new PI detectors are significantly less susceptible to refraction and phase-cancellation artefacts, generating realistic images in situations where conventionally-employed through-transmission PS detection techniques were unable to do so. The implications of the study to the potential screening of breast disease are discussed.

Noninvasive measurement of local arterial pulse wave velocity in humans by ultrasound.

An improved method for noninvasive measurement of the local velocity of arterial pulse wave propagation by an echo-tracking-based ultrasound system is described. A data acquisition image interface was programmed in the ultrasound machine simultaneously to record M-mode ultrasound signals at two locations of a given distance apart along an artery. The selections of measurement sites, separation, and time resolution were performed on the control interface. The temporal sampling frequency could be as high as 10 kHz. The displacements of the blood vessel wall along the time axis were calculated from the M-mode signals by cross-correlation of the radio-frequency data and the distension waveforms were obtained. The temporal separation of the feet of the distension curves from the two measurement locations was derived to give the travel time of the pulse wave. Measurements were made in vivo on human carotid arteries. The pulse wave velocities measured from four volunteers were from 4.1 to 7.2 m/s with coefficients of variation from 5.9 to 29.5%. Some of the factors contributing to the variation in measured values of the velocity are discussed. The method is simple to implement and should be suitable for clinical research into local pulse wave velocity.

Ultrasonic imaging technologies in perspective.

Ultrasonic imaging is a mature and widely used medical diagnostic technology but it is also a field of intense research activity. Innovations are viewed with differing perspectives by the stakeholders- users, industrialists, regulators, and researchers and research funders. The more important recent developments include advances in transducers, scanning schemes, coded excitation, three-dimensional, high-resolution and high-speed imaging, contrast agents, harmonic, elasticity and strain imaging, point-of-care devices, computed tomography, thermoacoustic, photoacoustic, acousto-optic and Hall effect imaging. Viewed from diverse perspectives, the assessment of ultrasonic imaging technologies is intellectually challenging. This is a general problem, which demands a multidisciplinary approach. An emerging, integrated, context for such assessment is presented. Given the straitened economies around the world, the need to articulate value for each and all stakeholders is becoming increasingly important.

Medical ultrasound: imaging of soft tissue strain and elasticity.

After X-radiography, ultrasound is now the most common of all the medical imaging technologies. For millennia, manual palpation has been used to assist in diagnosis, but it is subjective and restricted to larger and more superficial structures. Following an introduction to the subject of elasticity, the elasticity of biological soft tissues is discussed and published data are presented. The basic physical principles of pulse-echo and Doppler ultrasonic techniques are explained. The history of ultrasonic imaging of soft tissue strain and elasticity is summarized, together with a brief critique of previously published reviews. The relevant techniques-low-frequency vibration, step, freehand and physiological displacement, and radiation force (displacement, impulse, shear wave and acoustic emission)-are described. Tissue-mimicking materials are indispensible for the assessment of these techniques and their characteristics are reported. Emerging clinical applications in breast disease, cardiology, dermatology, gastroenterology, gynaecology, minimally invasive surgery, musculoskeletal studies, radiotherapy, tissue engineering, urology and vascular disease are critically discussed. It is concluded that ultrasonic imaging of soft tissue strain and elasticity is now sufficiently well developed to have clinical utility. The potential for further research is examined and it is anticipated that the technology will become a powerful mainstream investigative tool.

Coherent ultrasonic Doppler tomography.

Ultrasonic imaging based on the pulse-echo principle is widely used throughout the world, particularly in medical applications. However, its spatial resolution is poor (around 2 times the wavelength, or 200 µm at 15 MHz), limiting its ability to detect small but clinically important lesions (such as microcalcifications in breast cancer). The work presented here is different from the traditional approach. Continuous-wave ultrasound is transmitted to insonate a rotating object, then the amplitude and phase of the returned signals are coherently processed to reconstruct a Doppler tomographic image of the object's backscatter field. It is demonstrated numerically that the spatial resolution is up to 0.19 wavelengths and the sampling requirement and image formation method are given. To show the performance of the method, we present the results obtained by applying the new technique in simulation and experiment. A string phantom consisting of very thin copper wires and two cylindrical phantoms constructed by tissue-mimicking-material were scanned. It is demonstrated that the copper wires were located very accurately with very high spatial resolution, and good shape approximation for the cylindrical phantoms was achieved.

Continuous wave ultrasonic Doppler tomography.

In continuous wave ultrasonic Doppler tomography (DT), the ultrasonic beam moves relative to the scanned object to acquire Doppler-shifted frequency spectra which correspond to cross-range projections of the scattering and reflecting structures within the object. The relative motion can be circular or linear. These data are then backprojected to reconstruct the two-dimensional image of the object cross section. By using coherent processing, the spatial resolution of ultrasonic DT is close to an order of magnitude better than that of traditional pulse-echo imaging at the same ultrasound frequency.

Temperature measurement by thermal strain imaging with diagnostic power ultrasound, with potential for thermal index determination.

Over the years, there has been a substantial increase in acoustic exposure in diagnostic ultrasound as new imaging modalities with higher intensities and frame rates have been introduced; and more electronic components have been packed into the probe head, so that there is a tendency for it to become hotter. With respect to potential thermal effects, including those which may be hazardous occurring during ultrasound scanning, there is a correspondingly growing need for in vivo techniques to guide the operator as to the actual temperature rise occurring in the examined tissues. Therefore, an in vivo temperature estimator would be of considerable practical value. The commonly-used method of tissue thermal index (TI) measurement with a hydrophone in water could underestimate the actual value of TI (in one report by as much as 2.9 times). To obtain meaningful results, it is necessary to map the temperature elevation in 2-D (or 3-D) space. We present methodology, results and validation of a 2-D spatial and temporal thermal strain ultrasound temperature estimation technique in phantoms, and its apparently novel application in tracking the evolution of heat deposition at diagnostic exposure levels. The same ultrasound probe is used for both transmission and reception. The displacement and thermal strain estimation methods are similar to those used in high-intensity focused ultrasound thermal monitoring. The use of radiofrequency signals permits the application of cross correlation as a similarity measurement for tracking feature displacement. The displacement is used to calculate the thermal strain directly related to the temperature rise. Good agreement was observed between the temperature rise and the ultrasound power and scan duration. Thermal strain up to 1.4% was observed during 4000-s scan. Based on the results obtained for the temperature range studied in this work, the technique demonstrates potential for applicability in phantom (and possibly in vivo tissue) temperature measurement for the determination of TI.

Attenuation correction in ultrasound contrast agent imaging: elementary theory and preliminary experimental evaluation.

Progress in imaging and quantification of tissue perfusion using ultrasound (US) and microbubble contrast agents has been undermined by the lack of an effective automatic attenuation correction technique. In this article, an elementary model of the US attenuation processes for microbubble contrast enhanced imaging is developed. In the model, factors such as nonlinear bubble scattering, nonlinear attenuation, attenuation to both fundamental and harmonic and the US beam profile are considered. Methods are proposed for fast formation of images with automatic attenuation correction based on the model. In the proposed method, linear tissue echoes are extracted and filtered and then used to compensate for the attenuation in nonlinear bubble echoes at the same location to produce quantities that are a truer representation of bubble concentration. The technique does not require additional measurements and can be implemented in real time. Preliminary experiments on laboratory phantoms consisting of bubbles and tissue-mimicking materials are presented and the effectiveness of the proposed method is supported by improvements in image quality compared with unprocessed data. This development is an important step towards real-time quantitative contrast US imaging.

Lord Rayleigh: John William Strutt, third Baron Rayleigh.

John William Strutt, first son of the second Baron Rayleigh, was born on November 12, 1842. He was a sickly boy, so his schooling was sporadic. Nevertheless, he graduated first in his year at Cambridge and subsequently was a Fellow of Trinity College until his marriage in 1871. His father died in 1873, and he succeeded to the title third Baron Rayleigh. He converted the stable block of his country house, Terling Place, into a laboratory. In 1879, he moved back to Cambridge as Professor of Experimental Physics, but he returned to Terling in 1884. He published The Theory of Sound in 1877/1878 and, in his lifetime, 466 scientific articles. He received the 1904 Nobel Prize in Physics for the discovery of argon and made numerous seminal contributions to scientific progress. In the field of acoustics, he studied scattering, the diffraction limit, surface waves, resonance phenomena, reciprocity, streaming, radiation force, cavitation, relaxation, and binaural perception. He received many honors, was President of the Royal Society, one of the founding members of the Order of Merit, and Chancellor of Cambridge University. He also was interested in psychical research. Lord Rayleigh died on June 30, 1919.

Incoherent imaging using continuous wave ultrasound. A preliminary study using bovine intervertebral discs.

As an object rotates with respect to a stationary planar ultrasonic beam, the scattering centres within the object return echoes that are Doppler-shifted in frequency by amounts depending on the velocities of the individual scatterers. The backscattered echo amplitude at any particular frequency is the line integral of the scattered radiation at the cross-range corresponding to that frequency. The amplitude as a function of frequency can be interpreted as a tomographic projection. A tomographic reconstruction algorithm can produce an image of the distribution of scattering centres in the insonified object from these projections. This paper describes the development and characterisation of a microscanner to investigate the approach of using continuous wave ultrasound for three-dimensional cross-sectional imaging. The results of preliminary tissue investigation, conducted using bovine coccygeal intervertebral discs, are described. The radial imaging resolution improves as the Doppler frequency resolution improves but the circumferential resolution degrades proportionally. As the number of projections increases, there is a finite increase in image quality. Two- and three-dimensional images of the intervertebral disc reveal an alternate light and dark banding pattern that is characteristic of the laminar structure of the annulus fibrosus.