Ultrasonic edge shadowing around cylindrical cavities with and without walls.

In B-mode ultrasound images, the speckle distal to the edges of cavities of contrasting speed of sound (SOS) can be imprinted with a pattern characteristic of the cavity. This pattern, termed edge shadowing, is likely to involve alteration of both the speckle amplitude and its correlation length. Using the acoustic field calculated from the exact solution to the wave equation, we have simulated the ensemble-averaged speckle amplitude in B-mode images of cylindrical cavities both with and without walls, and compared the results to a simpler ray-based model. The simulations show that edge shadowing is caused predominantly by contrasts of the SOS, rather than of the density. The shadows on both walled and wall-less cylinders, for a focused incident beam, grow darker as the magnitude of the SOS contrast increases over a range up to +/- 10%. Extra shadows, caused by the inner wall boundary, appear on images of walled cylinders. The ray-based model agrees well with the wave model except in the shadow regions, within which the complexity of the phenomena seems to require the wave model.

Identifying radiotherapy target volumes in brain cancer by image analysis.

To establish the optimal radiotherapy fields for treating brain cancer patients, the tumour volume is often outlined on magnetic resonance (MR) images, where the tumour is clearly visible, and mapped onto computerised tomography images used for radiotherapy planning. This process requires considerable clinical experience and is time consuming, which will continue to increase as more complex image sequences are used in this process. Here, the potential of image analysis techniques for automatically identifying the radiation target volume on MR images, and thereby assisting clinicians with this difficult task, was investigated. A gradient-based level set approach was applied on the MR images of five patients with grades II, III and IV malignant cerebral glioma. The relationship between the target volumes produced by image analysis and those produced by a radiation oncologist was also investigated. The contours produced by image analysis were compared with the contours produced by an oncologist and used for treatment. In 93% of cases, the Dice similarity coefficient was found to be between 60 and 80%. This feasibility study demonstrates that image analysis has the potential for automatic outlining in the management of brain cancer patients, however, more testing and validation on a much larger patient cohort is required.

Origins of the edge shadowing artefact in medical ultrasound imaging.

Ultrasound (US) B-mode images distal to smooth, rounded cavities, such as cysts, containing a fluid with a speed of sound mismatch to the surrounding tissue, often exhibit a "refractile" edge shadowing artefact. This usually appears as narrow, hypoechoic, shadow lines extending a significant distance distal to the lateral edges of the fluid cavity and parallel to the US beam. The true reasons for this artefact are likely to be complex and to vary from case to case, with many different explanations found in the literature. However, we present a simplified theoretical model for the phenomenon based on a pulsed, finite-beam solution of US scattering from circular fluid-filled cylinders that suggests that "edge" shadows can occur distal not only to edges but also to points where the incident beam intersects the cavity near to the critical angle. Both mechanisms support the view that edge shadows can arise from the combination of unusually high wavefront spreading and the speckle-generating nature of the surrounding tissue. In vitro data from a tissue-mimicking phantom confirms that the edge shadow structure depends on the sign of the speed of sound contrast between the cylinder fluid and the surrounding medium.

Intraluminal ultrasound intensity distribution and backscattered Doppler power.

Ultrasound (US) incident obliquely on a cylindrical vessel is redistributed in space when the propagation path includes walls with acoustic impedance different from that of the surrounding media. We investigated this using low-density polyethylene (PE) as the vessel wall material. Both simulations and experiments were carried out. Direct hydrophone measurements of the acoustic field were made within a half section of the PE tube, and the distribution of backscattered Doppler power along a scan line was obtained using a range-Doppler instrument. Both simulation and hydrophone results demonstrate lateral shadow regions within the lumen. In every one of various Doppler flow experiments conducted, the backscattered Doppler power, compensated for on-axis transducer behaviour, increased with depth. Simulation results for an incident continuous-wave (CW) plane wave show that it tends to be focused by the curvature of the PE tube walls. The wall interactions are, however, angle-dependent and so the behaviour of a focused US beam depends on the beam as well as the walls. This study demonstrates alterations in the spatial distribution of US within a cylindrical vessel as a result of known vessel wall properties. It also provides evidence that local intensity variations within the lumen affect the relative Doppler power backscattered from small sample volumes.

Angle-independent estimation of maximum velocity through stenoses using vector Doppler ultrasound.

Categorisation for arterial stenoses treatment is determined primarily by the degree of occlusion, which is often estimated ultrasonically from blood velocity measurements. In current single-beam ultrasound (US) systems, this estimate can suffer from gross errors due to angle-dependence. The purpose of this study was to find out if an experimental dual-beam US system could reduce the angle-dependence of the velocity estimates. We compared four dual-beam velocity estimation algorithms on both a string phantom and straight tube wall-less flow phantoms incorporating symmetrical and asymmetrical stenoses from 0% to 91% by area. The estimated maximum velocity varied, on average, by 7.6% for beam-vessel angles from 40 degrees to 80 degrees. The fluctuation in the magnitude estimate was reduced by a factor of 2.6 using a hybrid single-dual-beam algorithm. We conclude that, when the true velocity lies in the scan plane, the dual-beam system reduces the angle-dependence and, thus, has the potential to improve categorisation of patients with arterial stenoses.

Angle-dependence and reproducibility of dual-beam vector doppler ultrasound in the common carotid arteries of normal volunteers.

Dual-beam vector Doppler has the potential to improve peak systolic blood velocity measurement accuracy by automatically correcting for the beam-flow Doppler angle. Using a modified linear-array system with a split receive aperture, we have assessed the angle-dependence over Doppler angles of 40 degrees -70 degrees and the reproducibility of the dual-beam blood maximum velocity estimate measured in the common carotid arteries (CCA) 1 to 2 cm prior to the bifurcation of 9 presumed-healthy volunteers. The velocity magnitude estimate was reduced by approximately 7.9% as the angle between the transmit beam and the vessel axis was increased from 40 degrees to 70 degrees. With repeat measurements made, on average, approximately 6 weeks apart, the 95% velocity magnitude limits of agreement were as follows: Intraobserver -41.3 to +45.2 cm/s; interobserver -29.6 to +46.8 cm/s. There was an 8.6 cm/s interobserver bias in velocity magnitude. We conclude that the dual-beam vector Doppler system can measure blood velocity within its scan plane with low dependence on angle and with similar reproducibility to that of single-beam systems.

Error propagation bounds in dual and triple beam vector Doppler ultrasound.

Measurement of the velocity components along two (three) directions enables the two (three)-dimensional velocity vector to be estimated exactly. However, in practical systems employing such multiple beam techniques, there will usually be errors in the measured velocity components along each beam, which will lead to errors in the estimated velocity magnitude and direction. This error propagation problem is analyzed in both two and three dimensions by decomposition of the velocity estimation matrix, and exact upper and lower bounds are derived for both the magnitude and angle bias as a function of the angle between the beams. The bias in triple beam systems is shown to have identical bounds to that in dual beam systems with an equivalent interbeam angle. It is found that small errors in the individual beam velocity components can be magnified in the final determination of velocity magnitude and angle. Plots are presented to assist system designers to specify the interbeam angle(s) to avoid gross velocity estimation errors.

Velocity bias and fluctuation in the standard dual beam Doppler reconstruction algorithm.

Bias and fluctuation of the standard velocity reconstruction algorithm for dual beam vector Doppler velocity estimation systems are analyzed; both magnitude and angle properties are considered. Bias can arise from any of the error sources known to affect single beam systems in addition to both translation and angle misregistration between the two sample volumes; standard deviation is the result of random temporal fluctuations in Doppler frequency estimates in each beam. Approximate closed-form expressions for both biases and standard deviations of the velocity estimates are derived, and the performance of a typical practical dual beam system is discussed as an illustration of the theory.

Lumen pressure within obliquely insonated absorbent solid cylindrical shells with application to Doppler flow phantoms.

Flow phantoms used in medical ultrasound usually employ a plastic tube as a blood vessel mimic. These tubes often have acoustic properties differing significantly from the tissue and blood-mimicking media, which results in distortion of the acoustic pressure field within the tubes and, hence, of the Doppler flow spectra. Previous analyses of this problem have used some form of the infinite plate transmission coefficient, although at least one ray-based analysis has considered a cylindrical interface but with zero wall thickness. In this paper, we compare these approximate pressure fields with the exact solution for oblique incidence on a viscoelastic cylindrical shell at 5 MHz to find for which materials the plate approximation is valid. The shell has water both inside and outside, but it can be modified to use a different fluid inside and also to include absorption in either fluid. We find the plate approximation is reasonable for soft tubes such as the copolymer Cflex (Cole-Palmer, Niles, IL) but much less so for hard tubes such as polymethylmethacrylate (PMMA).

Sample volume misregistration in linear array-based dual beam Doppler ultrasound systems.

Large velocity estimation errors can occur in dual beam Doppler ultrasound velocity measurement systems when there is left/right sample volume misregistration, particularly when the interbeam angle is small. Such misregistration will occur when there is tissue inhomogeneity. This is investigated for a typical type of inhomogeneity--a layer of fat--by calculating the amount of both angle and translation misregistration occurring in such a system realized using a single linear array transducer. The complex sample volume sensitivity is calculated using a modified time domain approach, combining the spatial impulse response method with ray tracing. The effects on these misregistrations of altering the aperture sizes and their relative positions on the array is then investigated to derive an improved aperture configuration for dual beam velocity estimation. Arrangements with transmit apertures wider than the receive apertures are shown to be preferable in this context.

A simulation study of sample volume sensitivity for oblique pulsed finite beam insonation of Doppler ultrasound flow phantom cylindrical vessels.

Our previous analysis of the lumen pressure in Doppler ultrasound flow phantoms subject to continuous wave, infinite beam excitation is extended here to consider the pressure and Doppler sample volume complex sensitivity within a range of solid absorbent tubes typical of those used in Doppler ultrasound flow phantoms insonated with a focussed pulsed ultrasound beam. The beam may be incident on the cylindrical shell from any angle and with any offset from the shell axis. The examples considered are of a 5 MHz beam with a 6 dB lateral fullwidth of 1 mm at the focus and a transducer surface acceleration pulse with standard deviation of 1 micros propagating through 10 mm outer diameter, 8 mm inner diameter, Cflex, low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polymethylmethacrylate (PMMA) shells surrounded by water at various beam-vessel angles. Our results confirm earlier analyses suggesting that PMMA, being less well matched to the surrounding media, causes much greater distortion of the sample volume sensitivity than Cflex.

Velocity fluctuation reduction in vector Doppler ultrasound using a hybrid single/dual-beam algorithm.

In order to reduce the fluctuations in the velocity magnitude estimate, we propose a modification to the standard algorithm for reconstructing the (two component) vector velocity from the measured Doppler shifts in two directions. This uses the standard dual-beam algorithm, combined with temporal smoothing, to find only the velocity angle, then uses the single-beam algorithm to estimate the velocity magnitude. We present initial data showing the significant reduction in velocity estimate fluctuation that this hybrid method achieves compared to the standard algorithm.