Combination of tissue harmonic sonography, real-time spatial compound sonography and adaptive image processing technique for the detection of carotid plaques and intima-medial thickness

Effects of ultrasound technology advances on measurement of carotid intima-media thickness: A review

In this review, we describe how technological advances in ultrasound imaging related to transducer construction and image processing fundamentally alter generation of ultrasound images to produce better quality images with higher resolution. However, carotid intima-media thickness (IMT) measurements made from images acquired on modern ultrasound systems are not comparable to historical population nomograms that were used to determine wall thickness thresholds that inform atherosclerotic cardiovascular disease risk. Because it is nearly impossible to replicate instrumentation settings that were used to create the reference carotid IMT nomograms and to place an individual's carotid IMT value in or above a clinically relevant percentile, carotid IMT measurements have a very limited role in clinical medicine, but remain a useful research tool when instrumentation, presets, image acquisition, and measurements can be standardized. In addition to new validation studies, it would be useful for the ultrasound imaging community to reach a consensus regarding technical aspects of ultrasound imaging acquisition, processing, and display for blood vessels so standard presets and imaging approaches could reliably yield the same measurements.

Experimental evaluation of ultrasound higher-order harmonic imaging with Filtered-Delay Multiply And Sum (F-DMAS) non-linear beamforming

Tissue Harmonic Imaging (THI) mode is currently one of the preferred choices by the clinicians for its ability to provide enhanced ultrasound images, thanks to the use of the second harmonic component of backscattered echoes. This paper aims at investigating whether the combination of THI with Filtered-Delay Multiply And Sum (F-DMAS) beamforming can provide further improvements in image quality. F-DMAS is a new non-linear beamformer, which, similarly to THI, is based on the use of the second harmonics of beamformed signals and is known to increase image contrast resolution and noise rejection. Thus, we have first compared the images obtained by using F-DMAS and the standard Delay And Sum (DAS) beamformers when only the second harmonics of the received signals was selected. Moreover, possible improvements brought about by other harmonic components generated by the combined use of the fundamental plus second harmonics and F-DMAS beamforming have been explored. Experimental results demonstrate that, as compared to standard harmonic imaging with DAS, THI and F-DMAS can be joined to improve the -20 dB lateral resolution up to 1 mm, the contrast ratio up to 12 dB on a cyst-phantom and up to 9 dB on in vivo images.

An Adaptive Synthetic Aperture Method Applied to Ultrasound Tissue Harmonic Imaging

In recent years, the minimum variance (MV) beamformer has been highly regarded since it provides high resolution and contrast in B-mode ultrasound imaging compared with nonadaptive delay-and-sum (DAS) beamformer. However, the performance of MV beamformer is degraded in the presence of the noise due to inaccurate estimation of the covariance matrix resulting in low-quality images. The conventional tissue harmonic imaging (THI) offers multiple advantages over conventional pulse-echo ultrasound imaging, including enhanced contrast resolution and improved axial and lateral resolutions, but low signal-to-noise ratio (SNR) is a major problem facing this imaging method, which uses a fixed transmit focus and dynamic receive focusing (DRF). In this paper, a synthetic aperture method based on the virtual source, namely, bidirectional pixel-based focusing (BiPBF), has been combined with the MV beamformer and then applied to second-harmonic ultrasound imaging. The main objective is suppressing the noise level to enhance the performance of the MV beamformer in the harmonic imaging, especially in lower and deeper depths where the SNR is low. In addition, combining the BiPBF and MV weighting results in simultaneous improvement in imaging resolution and contrast, in comparison with the conventional methods: DRF (DAS), BiPBF (DAS), and DRF (MV). The performance of the proposed method is evaluated on simulated and experimental RF data. The THI is achieved using the pulse-inversion technique. The results of the simulated wire phantom demonstrate that the proposed beamformer can achieve the best lateral resolution, along different depths, compared with DRF (DAS), BiPBF (DAS), and DRF (MV) methods. The results of the simulated and experimental cyst phantoms show that the new beamformer improves the contrast ratio (CR) and contrast-to-noise ratio (CNR) of the resulting images. In results of simulated cyst phantom, in average, the new beamformer improves the CR and CNR of the cyst about (7.4 dB, 49%), (3.2 dB, 16%), and (5 dB, 26%) compared with DRF (DAS), BiPBF (DAS), and DRF (MV), respectively. In results of experimental cyst phantom, these relative improvements are about (4.2 dB, 22%), (1.7 dB, 7%), and (2.6 dB, 15%). In addition, BiPBF (MV) method offers improved edge definition of cysts in comparison with the other methods.

Methods for reverberation suppression utilizing dual frequency band imaging

Reverberations impair the contrast resolution of diagnostic ultrasound images. Tissue harmonic imaging is a common method to reduce these artifacts, but does not remove all reverberations. Dual frequency band imaging (DBI), utilizing a low frequency pulse which manipulates propagation of the high frequency imaging pulse, has been proposed earlier for reverberation suppression. This article adds two different methods for reverberation suppression with DBI: the delay corrected subtraction (DCS) and the first order content weighting (FOCW) method. Both methods utilize the propagation delay of the imaging pulse of two transmissions with alternating manipulation pressure to extract information about its depth of first scattering. FOCW further utilizes this information to estimate the content of first order scattering in the received signal. Initial evaluation is presented where both methods are applied to simulated and in vivo data. Both methods yield visual and measurable substantial improvement in image contrast. Comparing DCS with FOCW, DCS produces sharper images and retains more details while FOCW achieves best suppression levels and, thus, highest image contrast. The measured improvement in contrast ranges from 8 to 27 dB for DCS and from 4 dB up to the dynamic range for FOCW.

Evaluation of the accuracy of articular cartilage thickness measurement by B-mode ultrasonography with conventional imaging and real-time spatial compound ultrasonography imaging

The present study aimed to quantify the thickness of articular cartilage (Tc) in vitro using both conventional and real-time spatial compound B-mode ultrasonography (US) with a clinically used transducer and to evaluate the accuracy of measurement by comparing the results with values obtained microscopically. Femoral condyle samples were obtained from a 6-month-old pig and a 3-year-old pig. B-mode US images with conventional imaging and real-time spatial compound imaging (RTSCI) of osteochondral blocks were acquired. Tc determined using US (Tc-US) was measured from line data parallel to US beam direction acquired from B-mode images with an objective method for determining cartilage surface and bone-cartilage interfaces at the peak brightness values. Tc was also determined under microscopy (Tc-optical) using the corresponding points from US measurement. Tc-US was compared with Tc-optical to assess accuracy. Tc-US correlated significantly with Tc in both conventional imaging and RTSCI (r = 0.961, 0.976, respectively). Bland-Altman plots showed mean differences between Tc-optical and Tc-US were -0.0073 mm and 0.0139 mm with standard deviations of 0.171 mm and 0.131 mm for conventional imaging and RTSCI, respectively. Our results show that Tc-US measurement using B-mode US allows accurate measurement of Tc. Considering correlation coefficients between Tc-US and Tc-optical, RTSCI US may offer higher accuracy for measuring Tc than conventional methods when an objective tissue border determination algorithm is used, even though both showed good accuracy in our study.

Transmit beamforming for optimal second-harmonic generation

A simulation study of transmit ultrasound beams from several transducer configurations is conducted to compare second-harmonic imaging at 3.5 MHz and 11 MHz. Second- harmonic generation and the ability to suppress near field echoes are compared. Each transducer configuration is defined by a chosen f-number and focal depth, and the transmit pressure is estimated to not exceed a mechanical index of 1.2. The medium resembles homogeneous muscle tissue with nonlinear elasticity and power-law attenuation. To improve computational efficiency, the KZK equation is utilized, and all transducers are circular-symmetric. Previous literature shows that second-harmonic generation is proportional to the square of the transmit pressure, and that transducer configurations with different transmit frequencies, but equal aperture and focal depth in terms of wavelengths, generate identical second-harmonic fields in terms of shape. Results verify this for a medium with attenuation f1. For attenuation f1.1, deviations are found, and the high frequency subsequently performs worse than the low frequency. The results suggest that high frequencies are less able to suppress near-field echoes in the presence of a heterogeneous body wall than low frequencies.

Noninvasive imaging of carotid arteries in stroke: emerging value of real-time high-resolution sonography in carotid occlusion due to cardiac embolism

OBJECTIVE

Comprehension of the pathophysiologic characteristics of atherosclerosis has focused its attention on the study of dynamic and metabolic processes involving the vessel wall as possible causes of stroke. When compared with conventional radiologic techniques, sonography has the main advantage of being a real-time imaging modality. We report 2 acute stroke cases in which carotid sonography showed some dynamic features that could not be identified with computed tomography (CT) and magnetic resonance angiography (MRA).

METHODS

Carotid sonography with high-resolution probes (9-14 MHz) was compared with CT and MRA findings showing carotid axis occlusion in 2 patients with acute stroke.

RESULTS

In case 1, the internal carotid artery occlusion observed on CT and MRA was interpreted as a dissection on a clinical basis, but sonography showed a mobile embolus originating from the heart in the internal carotid artery. In case 2, the occlusion of the whole carotid axis observed on CT and MRA was instead related to a heart-originating embolus floating in the common carotid artery.

CONCLUSIONS

The evaluation of dynamic aspects of atherosclerosis is fundamental to understanding the pathophysiologic characteristics of stroke. Sonography is fundamental in carotid artery imaging for its possibility of showing dynamic processes that could be misdiagnosed with "static" imaging. The correct identification of the pathophysiologic characteristics of stroke in these cases could have led to different diagnostic and therapeutic algorithms.