Two-dimensional spatial compounding with warping

Spatial Compounding of 3-D Fetal Brain Ultrasound Using Probabilistic Maps

A new method to address the problem of shadowing in fetal brain ultrasound volumes is presented. The proposed approach is based on the spatial composition of multiple 3-D fetal head projections using the weighted Euclidean norm as an operator. A support vector machine, which is trained with optimal textural features, was used to assign weighting according to the posterior probabilities of brain tissue and shadows. Both phantom and real fetal head ultrasound volumes were compounded using previously reported operators and compared with the proposed composition method to validate it. The quantitative evaluations revealed increases in signal-to-noise ratio ≤35% and in contrast-to-noise ratio ≤135% using real data. Qualitative comparisons made by obstetricians indicated that this novel method adequately recovers brain tissue and improves the visibility of the main cerebral structures. This may prove useful both for fetal monitoring and in the diagnosis of brain defects. Overall this new approach outperforms spatial composition methods previously reported.

Minimum variance beamforming applied to ultrasound imaging with a partially shaded aperture

Shadowing of an imaging aperture occurs when ultrasound beams are partially obstructed by an acoustically hard tissue, e.g., bone tissue. This effect leads to reduced resolution and, in some cases, geometrical distortion. In this paper, we initially introduce a binary apodization model to simulate effects of the shadowing on the point scatterers located close to a bone structure. Further, in a simulation study and an in vitro experiment, the minimum variance (MV) beamforming method is employed to image scatterers partly located in the shadow of bone. We show that the MV beamformer can result in a distorted image when the imaging aperture is highly obstructed by the bone structure. This distortion can be seen as an apparent lateral shift of the point spread function and a decrease in the sensitivity. Based on the signal power across the aperture, we adaptively determine the shadowed elements and discard their corresponding data from the covariance matrix to improve the MV beamformer performance. This modified MV beamformer can retain the resolution and compensate for the apparent lateral shifting and signal attenuation for the shadowed point scatterers.

Alignment and calibration of dual ultrasound transducers using a wedge phantom

We present a novel method of aligning two orthogonal ultrasound transducers into a coincident scan plane. A wedge phantom design provides visual feedback to the user to facilitate alignment. Calibration provides the transformation from one transducer to the other as well as a measure of the residual error in alignment. Mean alignment error is shown to be under 1° in the rotation axes and 1 mm in translation after repeated manual alignments. The repeatability of wedge based calibration has similar results compared with N-fiducial based calibration. The accuracy of the calibration for mapping points from one transducer to the other is found to have a mean error of 1.6 mm. The dual transducer system is well suited to imaging anatomy such as the breast and may be used for spatial compounding for improving B-mode images and motion estimation compounding for improving elastography results.

Automatic measurement of human subcutaneous fat with ultrasound

This paper presents an approach to measure human subcutaneous fat thickness automatically using ultrasound radio frequency (RF) signals. We propose using spatially compounded spectrum properties extracted from the RF signals of ultrasound for the purpose of fat boundary detection. Our fat detection framework consists of 4 main steps. The first step is to capture RF data from 11 ultrasound beam angles and at 4 different focal positions. Second, spectrum dispersion is calculated from the local spectrum of RF data using the short-time Fourier transform and moment analysis. The values of the spectrum dispersion are encoded as gray-scale parametric images. Third, averaging is used to reduce speckle noise in the parametric image and improve the visualization of the subcutaneous fat layer. Finally, we apply Rosin's thresholding and random sample consensus boundary detection to extract the fat boundary. Our method was applied on 36 samples obtained in vivo at the suprailiac, thigh, and triceps of 9 human participants. In our study, high correlations between the manual and automatic ultrasound measurements (r > 0.7 at all body sites), and between the skinfold caliper and automatic ultrasound measurements (r > 0.7 at all body sites) were observed.

Adaptive ultrasound imaging of the lumbar spine for guidance of epidural anesthesia

Ultrasound imaging can help in choosing the needle trajectory for epidural anesthesia but anatomical features are not always clear. Spatial compounding can emphasize structures; however, features in the beam-steered images are not aligned due to varying speeds of sound. A non-rigid registration method, called warping, shifts pixels of the beam-steered images to best match the reference image. Linear prediction is used to find the warping vectors and decrease computational cost. An adaptive median-based combination technique for compounding is also investigated. The algorithms are tested on a spine phantom and human subjects. The results show a significant improvement in quality when using warping with adaptive median-based compounding.

An ultrasonic imaging speckle-suppression and contrast-enhancement technique by means of frequency compounding and coded excitation

A method for improving the contrast resolution of B-mode images is proposed by combining the speckle-reduction technique of frequency compounding (FC) and the coded excitation and pulse-compression technique called resolution enhancement compression (REC). FC suppresses speckle but at the expense of a reduction in axial resolution. Using REC, the axial resolution and bandwidth of the imaging system was doubled. Therefore, by combining REC with FC (REC-FC), the tradeoff between axial resolution and contrast enhancement was extended significantly. Simulations and experimental measurements were conducted with a single-element transducer (f/2.66) having a center frequency of 2.25 MHz and a -3-dB bandwidth of 50%. Simulations and measurements of hyperechoic (+6 dB) tissue-mimicking targets were imaged. Four FC cases were evaluated: full-, half-, third-, and fourth-width of the true impulse response bandwidth. The image quality metrics used to compare REC-FC to conventional pulsing (CP) and CP-FC were contrast-to-noise ratio (CNR), speckle signal-to-noise ratio, histogram pixel intensity, and lesion signal-to-noise ratio. Increases in CNR of 121%, 231%, 302%, and 391% were obtained in experiments when comparing REC-FC for the full-, half-, third-, and fourth-width cases to CP. Furthermore, smaller increases in CNR of 112%, 233%, and 309% were obtained in experiments when comparing CP-FC for the half-, third-, and fourth-width cases to CP. Improved lesion detectability was observed by using REC-FC.

A motion compounding technique for speckle reduction in ultrasound images

The quality of ultrasound images is usually influenced by speckle noise and the temporal decorrelation of the speckle patterns. To reduce the speckle noise, compounding techniques have been widely applied. Partially correlated images scanned on the same subject cross-section are combined to generate a compound image with improved image quality. However, the compounding technique might introduce image blurring if the transducer or the target moves too fast. This blurring effect becomes especially critical when assessing tissue deformation in clinical motion examinations. In this paper, an ultrasound motion compounding system is proposed to improve the quality of ultrasound motion sequences. The proposed motion compounding technique uses a hierarchical adaptive feature weighted motion estimation method to realign the frames before compounding. Each frame is first registered and warped to the reference frame before being compounded to reduce the speckle noise. Experimental results showed that the motion could be assessed accurately and better visualization could be achieved for the compound images, with improved signal-to-noise and contrast-to-noise ratios.

Ultrasound compounding with automatic attenuation compensation using paired angle scans

Variations in attenuation in tissue can result in shadowing and enhancement in ultrasound images. Angular compounding of ultrasound images by lateral beam-steering can be used to improve delineation of structures, but causes such shadows and enhancements to appear in less recognisable forms. We present an algorithm which uses lateral beam-steering to produce compounded images with significantly reduced artefacts, by considering the response from equal and opposite angles. This is compared to several other alternative algorithms for attenuation estimation, some of them embedded for the first time in a multi-angle framework. Algorithms are tested on simulated and in vitro data in 2D and 3D contexts. Gain variations across all observed shadows and enhancements are reduced to below 5 dB. The new algorithm is as good as the best alternative on all data sets tested, and is straightforward to implement. We end by discussing further work required to relax the necessary assumptions in order to achieve a similar level of performance on in vivo data.

Ultrasound image compounding based on motion compensation

The quality of ultrasonic images is usually influenced by speckle noises. To reduce the speckle noise, compounding techniques have been widely applied to improve image quality through signal averaging. In this paper, a 2-D ultrasonic motion compounding system is proposed that uses a hierarchical adaptive feature weighted motion estimation method to realign the frames before compounding. Each frame is first registered to the reference frame and then compounded to reduce the speckle noises. Several compounding strategies are then used to remove the unwanted degradation. Performance has been tested on both synthetic and in vivo clinical ultrasonic images. The experimental results show that the motion can be assessed accurately and the compounded images achieve good signal-to-noise ratio for improving the quality of ultrasonic images.

Increasing frame rate in ultrasound imaging by temporal morphing using tissue Doppler

The diagnostic value of echocardiographic images seems to diminish when the frame rate is low. In this work, morphing based on velocity information was used to improve the perceived smoothness of B-mode cine-loops with low frame rate. Based on an estimate of the velocity field calculated from B-mode speckle tracking and tissue Doppler measurements, morphed cine-loops with arbitrarily high frame rate were created. Morphing was applied to cardiac ultrasound cine-loops with apical insonation. The quality of the morphed data was evaluated by removing frames from duplex B-mode and tissue Doppler recordings, then replacing the removed B-mode frames with morphed ones. The decimated and morphed sequences were compared to the original ones. Wall motion scoring, a subjective evaluation technique for regional viability of the myocardium, was applied to data from 20 patients with varying pathology. Sixty cine-loops were scored twice, first with original data and later with morphed data. The results were compared for each recording, and the scorings were identical in the two cases for 94% of the segments. We conclude that much of the diagnostic value is retained in recordings with 15 frames per second when temporal morphing is applied.

Three-dimensional extended field-of-view ultrasound

Three-dimensional (3-D) extended field-of-view ultrasound creates a mosaic view from a set of volumes acquired from a dedicated 3-D ultrasound machine combined with a position tracker. A simple compounding technique can be used to combine the volumes together using only the position measurements, but some misalignment remains. Two different registration methods were developed to correct these errors in the overlapping regions. The first method divides the overlap into smaller blocks and warps the blocks to best align the features. The second method is similar, but uses rigid body registration of the blocks. Experiments in vitro and in vivo showed that block-based registration with warping produced the most reproducible results and the greatest increase in similarity among the overlapping regions. It also produced the best reconstruction accuracy, with a mean distance error of 0.4 mm measured across 101.78 mm in a phantom, representing 0.4% error.