Performance of ultrasonic speckle tracking in various tissues

Musculoskeletal ultrasound: a technical and historical perspective

During the past four decades, musculoskeletal ultrasound has become popular as an imaging modality due to its low cost, accessibility, and lack of ionizing radiation. The development of ultrasound technology was possible in large part due to concomitant advances in both solid-state electronics and signal processing. The invention of the transistor and digital computer in the late 1940s was integral in its development. Moore's prediction that the number of microprocessors on a chip would grow exponentially, resulting in progressive miniaturization in chip design and therefore increased computational power, added to these capabilities. The development of musculoskeletal ultrasound has paralleled technical advances in diagnostic ultrasound. The appearance of a large variety of transducer capabilities and rapid image processing along with the ability to assess vascularity and tissue properties has expanded and continues to expand the role of musculoskeletal ultrasound. It should also be noted that these developments have in large part been due to a number of individuals who had the insight to see the potential applications of this developing technology to a host of relevant clinical musculoskeletal problems. Exquisite high-resolution images of both deep and small superficial musculoskeletal anatomy, assessment of vascularity on a capillary level and tissue mechanical properties can be obtained. Ultrasound has also been recognized as the method of choice to perform a large variety of interventional procedures. A brief review of these technical developments, the timeline over which these improvements occurred, and the impact on musculoskeletal ultrasound is presented below.

Optimum Speckle Tracking Based on Ultrafast Ultrasound for Improving Blood Flow Velocimetry

Speckle tracking using optimum comparison frames (STO) is proposed to improve the blood flow velocity profile (BFVP) estimation based on ultrafast ultrasound with coherent plane-wave compounding. The optimum comparison frames are as far as possible from the reference frame image while possessing a speckle correlation above a given threshold. The correlation thresholds for different kernel sizes are determined via an experiment based on a vascular-mimicking phantom. In in vitro experiments with different peak velocities of the flow ranging from 0.38 to 1.18 m/s, the proposed STO method with three kernel sizes ( 0.46 × 0.46 , 0.31 × 0.69 , and 0.92 × 0.92 mm²) is used for the BFVP estimations. The normalized root mean square errors (NRMSEs) between the estimated and theoretical BFVPs are calculated and compared with the results based on the speckle tracking using adjacent-frame images. For the three kernel sizes, the mean relative decrements in the STO-based NRMSEs are 46.6%, 44.7%, and 52.9%, and the standard deviations are 36.8%, 37.6%, and 35.9%, respectively. The STO method is also validated by in vivo experiments using rabbit iliac arteries with contrast agents. With parabolic curves fitting to the mean velocity estimates, the average relative increments for the STO-based R² (coefficients of determination) are 7.22% and 6.25% for kernel sizes of 0.46 × 0.46 and 0.31 × 0.69 mm², respectively. In conclusion, the STO method improves the BFVP measurement accuracy, whereby accurate diagnosis information can be acquired for clinical applications.

Sagittal Measurement of Tongue Movement During Respiration: Comparison Between Ultrasonography and Magnetic Resonance Imaging

The tongue makes up the anterior pharyngeal wall and is critical for airway patency. Magnetic resonance imaging (MRI) is commonly used to study pharyngeal muscle function in pharyngeal disorders such as obstructive sleep apnoea. Tagged MRI and ultrasound studies have separately revealed ∼1 mm of anterior tongue movement during inspiration in healthy patients, but these modalities have not been directly compared. In the study described here, agreement between ultrasound and MRI in measuring regional tongue displacement in 21 healthy patients and 21 patients with obstructive sleep apnoea was evaluated. We found good consistency and agreement between the two techniques, with an intra-class correlation coefficient of 0.79 (95% confidence interval: 0.75-0.82) for anteroposterior tongue motion during inspiration. Ultrasound measurements of posterior tongue displacement were 0.24 ± 0.64 mm greater than MRI measurements (95% limits of agreement: 1.03 to -1.49). This may reflect the higher spatial and temporal resolution of the ultrasound technique. This study confirms that ultrasound is a suitable method for quantifying inspiratory tongue movement.

Ultrasound Strain Imaging to Assess the Biceps Brachii Muscle in Chronic Poststroke Spasticity

OBJECTIVES

The aim of the study was to assess the feasibility of ultrasound strain imaging in characterizing the biceps brachii muscle in chronic poststroke spasticity.

METHODS

We prospectively analyzed strain imaging data from bilateral biceps brachii muscles in 8 healthy volunteers and 7 patients with poststroke chronic spasticity. Axial deformations of the biceps brachii muscle and overlying subcutaneous tissue were produced by external compression using a sandbag (1.0 kg) attached to a transducer. The lengthening and shortening of the biceps brachii muscle and subcutaneous tissue were produced by manual passive elbow extension (from 90° to 0°) and flexion (from 0° to 90°), respectively. We used offline 2-dimensional speckle tracking to estimate axial and longitudinal strain ratios (biceps brachii strain/subcutaneous tissue strain), and the longitudinal tissue velocity of the biceps brachii muscle. Statistical analyses included analysis of variance for testing differences in strain imaging parameters among healthy, nonspastic, and spastic biceps brachii muscles, the Bonferroni correction for further testing differences in US strain imaging among paired groups (healthy versus spastic, nonspastic versus spastic, and healthy versus nonspastic), and the Pearson correlation coefficient for assessing the intraobserver reliability of performing strain imaging in stroke survivors.

RESULTS

The differences in strain imaging parameters between healthy and spastic and between nonspastic and spastic biceps brachii muscles were significant at both 90° elbow flexion and maximal elbow extension (P < .01). There was no significant difference in axial strain ratios at 90° of elbow flexion or longitudinal tissue velocities between healthy and nonspastic muscles (P > .05). The intraobserver reliability of performing strain imaging in stroke survivors was good (r = 0.85; P < .01).

CONCLUSIONS

Ultrasound strain imaging seems to be feasible for characterizing the biceps brachii muscle in chronic poststroke spasticity.

Ultrasound Strain Imaging in Assessment of Biceps Muscle Stiffness and Dynamic Motion in Healthy Adults

We prospectively evaluated the feasibility of using ultrasound strain imaging (USI) to assess biceps brachii muscle (BBM) stiffness and dynamic motion in 10 healthy adults. The BBM axial deformation was produced by external compression with a sandbag (1.0 kg) tied onto the transducer. The BBM lateral movement was produced by manual passive elbow flexion and extension. By use of 2-D speckle tracking, captured 5-s real-time ultrasound data of BBM were processed to estimate axial strain, representing muscle stiffness, and lateral strain and tissue velocity, representing muscle dynamic motion. Axial (lateral) strain ratio was defined as BBM strain divided by subcutaneous soft tissue strain. There was no significant difference in lateral strain or tissue velocity between the left and right BBM (lateral strain ratio: 4.69 ± 0.07 vs. 4.51 ± 0.08 for extension, 4.82 ± 0.09 vs. 4.69 ± 0.11 for flexion; tissue velocity: 1.58 ± 0.32 cm/s vs. 1.78 ± 0.85 cm/s for extension, -2.03 ± 0.63 vs. -2.03 ± 0.59 for flexion; all p values > 0.05) or between men and women (lateral strain ratio: 4.52 ± 0.06 vs. 4.67 ± 0.1 for extension, 4.71 ± 0.11 vs. 4.83 ± 0.09 for flexion; tissue velocity, cm/s: 1.76 ± 0.76 vs. 1.66 ± 0.65 for extension, -2.21 ± 0.65 vs. -1.88 ± 0.52 for flexion, all p values > 0.05). The difference in axial stain between men and women was significant (axial strain ratio: 3.09 ± 0.43 vs. 3.52 ± 0.26, p = 0.02). Inter- and intra-observer reliability in performing USI of the BBM was good (all intra-class correlation coefficients [ICCs] >0.75). Our results suggest that USI seems to be feasible for and reproducible in estimating BBM mechanical properties and motion dynamics in healthy adults.

A novel ultrasound technique to measure genioglossus movement in vivo

Upper airway muscles are important in maintaining airway patency. Visualization of their dynamic motion should allow measurement, comparison, and further understanding of their roles in healthy subjects and those with upper airway disorders. Currently, there are few clinically feasible real-time imaging methods. Methods such as tagged magnetic resonance imaging have documented movement of genioglossus (GG), the largest upper airway dilator. Inspiratory movement was largest in the posterior region of GG. This study aimed to develop a novel ultrasound (US) method to measure GG movement in real time. We tested 20 healthy, awake subjects (21–38 yr) breathing quietly in the supine posture with the head in a neutral position. US images were collected using a transducer positioned submentally. Image correlation analysis measured regional displacement of GG within a grid of points in the midsagittal plane throughout the respiratory cycle. Typically, motion began before inspiratory flow in an anteroinferior direction and peaked in midinspiration. Average peak displacements of the anterior, posterior, superior, and inferior grid points were 0.44 ± 0.23 (mean ± SD), 0.57 ± 0.35, 0.38 ± 0.20, and 0.62 ± 0.41 mm, respectively. Largest displacements occurred in the most inferoposterior part (0.70 ± 0.48 mm). This method had good intrarater repeatability within the same testing session, as well as across sessions. We have devised a simple noninvasive US method, which should be a useful tool to assess GG movement in normal subjects and those with sleep-disordered breathing.

Tracking in high-frame-rate imaging

Speckle tracking has been used for motion estimation in ultrasound imaging. Unlike conventional Doppler techniques, which are angle-dependent, speckle tracking can be utilized to estimate velocity vectors. However, the accuracy of speckle-tracking methods is limited by speckle decorrelation, which is related to the displacement between two consecutive images, and, hence, combining high-frame-rate imaging and speckle tracking could potentially increase the accuracy of motion estimation. However, the lack of transmit focusing may also affect the tracking results and the high computational requirement may be problematic. This study therefore assessed the performance of high-frame-rate speckle tracking and compared it with conventional focusing. The effects of the signal-to-noise ratio (SNR), bulk motion, and velocity gradients were investigated in both experiments and simulations. The results show that high-frame-rate speckle tracking can achieve high accuracy if the SNR is sufficiently high. In addition, its computational complexity is acceptable because smaller search windows can be used due to the displacements between frames generally being smaller during high-frame-rate imaging. Speckle decor-relation resulting from velocity gradients within a sample volume is also not as significant during high-frame-rate imaging.

Preliminary in vivo atherosclerotic carotid plaque characterization using the accumulated axial strain and relative lateral shift strain indices

In this paper, we explore two parameters or strain indices related to plaque deformation during the cardiac cycle, namely, the maximum accumulated axial strain in plaque and the relative lateral shifts between plaque and vessel wall under in vivo clinical ultrasound imaging conditions for possible identification of vulnerable plaque. These strain indices enable differentiation between calcified and lipidic plaque tissue utilizing a new perspective based on the stiffness and mobility of the plaque. In addition, they also provide the ability to distinguish between softer plaques that undergo large deformations during the cardiac cycle when compared to stiffer plaque tissue. Soft plaques that undergo large deformations over the cardiac cycle are more prone to rupture and to release micro-emboli into the cerebral bloodstream. The ability to identify vulnerable plaque, prone to rupture, would significantly enhance the clinical utility of this method for screening patients. We present preliminary in vivo results obtained from ultrasound radio frequency data collected over 16 atherosclerotic plaque patients before these patients undergo a carotid endarterectomy procedure. Our preliminary in vivo results indicate that the maximum accumulated axial strain over a cardiac cycle and the maximum relative lateral shift or displacement of the plaque are useful strain indices that provide differentiation between soft and calcified plaques.

Radio-frequency ablation electrode displacement elastography: a phantom study

This article describes the evaluation of a novel method of tissue displacement for use in the elastographic visualization of radio-frequency (rf) ablation-induced lesions. The method involves use of the radio-frequency ablation electrode as a displacement device, which provides localized compression in the region of interest. This displacement mechanism offers the advantage of easy in vivo implementation since problems such as excessive lateral and elevational displacements present when using external compression are reduced with this approach. The method was tested on a single-inclusion tissue-mimicking phantom containing a radio-frequency ablation electrode rigidly attached to the inclusion center. Full-frame rf echo signals were acquired from the phantom before and after electrode displacements ranging from 0.05 to 0.2 mm. One-dimensional cross-correlation analysis between pre- and postcompression signals was used to measure tissue displacements, and strains were determined by computing the gradient of the displacement. The strain contrast, contrast-to-noise ratio, and signal-to-noise ratio were estimated from the resulting strain images. Comparisons are drawn between the elastographically measured dimensions and those known a priori for the single-inclusion phantom. Electrode displacement elastography was found to slightly underestimate the inclusion dimensions. The method was also tested on a second tissue-mimicking phantom and on in vitro rf-ablated lesions in canine liver tissue. The results validate previous in vivo findings that electrode displacement elastography is an effective method for monitoring rf ablation.

Automated tracking of the mitral valve annulus motion in apical echocardiographic images using multidimensional dynamic programming

We developed a semiautomatic method for tracking the mitral valve annulus (MVA) in echocardiographic images, in particular, tracking the septal and the lateral mitral valve hinge points. The algorithm is based on multidimensional dynamic programming combined with apodized block matching. The method was tested on single-beat apical four chamber image sequences of 20 patients with acute myocardial infarction. The automated tracking results were evaluated by comparing them with the average manual tracking results of two experts. The mitral valve hinge point displacements and the total mitral excursions obtained by the automatic technique agreed well with those obtained manually and outperformed two commonly used tracking methods (forward tracking and minimum tracking). In conclusion, this novel semiautomatic tracking method is clinically valuable and capable of tracking the MVA motion within the limits of interobserver variability. The technique is robust, even in low frame rate, redigitized VCR images of clinical quality.

Rehabilitative ultrasound imaging: understanding the technology and its applications

The use of ultrasound imaging by physical therapists is growing in popularity. This commentary has 2 aims. The first is to introduce the concept of rehabilitative ultrasound imaging (RUSI), provide a definition of the scope of this emerging tool in regard to the physical therapy profession, and describe how this relates to the larger field of medical ultrasound imaging. The second aim is to provide an overview of basic ultrasound imaging and instrumentation principles, including an understanding of the various modes and applications of the technology with respect to neuromusculoskeletal rehabilitation and in relation to other common imaging modalities.

Two-dimensional strain imaging: a new echocardiographic advance with research and clinical applications

Over the past two decades the quest for quantitative evaluation of left ventricular function and regional wall motion has escalated, allowing several aspects of myocardial contractile patterns to be quantified, both during stress echocardiography and in the assessment of dyssynchrony. Most of the literature to date has used Tissue Doppler Imaging (TDI) techniques to assess essentially long-axis function due to the angle dependency of Doppler based techniques. This brief review introduces the early development, validation and potential clinical applications of a new technique of quantifying two-dimensional (radial and circumferential) strains and strain rates through tracking myocardial "speckles". In-vivo and in-vitro validation of this 2D-strain imaging technique has been undertaken and reached a point where it is considered ready for more widespread investigations into clinical utility. One important advantage over TDI techniques is that it is not limited by dependency on the angle of insonation. Several recent studies looking at ventricular function in specific groups of patients have reported practical ability to distinguish the abnormally from the normally contracting regions of ventricular walls. It provides new and complementary quantitative information about ventricular dyssynchrony and regional wall motion abnormalities. More research studies are needed to determine the sensitivity and specificity of the measurements obtained using this technique and define its strengths and limitations. In particular, whether the measured values correlate well with clinical outcomes will need to be established in longitudinal interventional studies. The clinical utilities of this technique over the coming years are likely to expand rapidly.

Motion compensation for intravascular ultrasound palpography

Rupture of vulnerable plaques in coronary arteries is the major cause of acute coronary syndromes. Most vulnerable plaques consist of a thin fibrous cap covering an atheromous core. These plaques can be identified using intravascular ultrasound (IVUS) palpography, which measures radial strain by cross-correlating RF signals at different intraluminal pressures. Multiple strain images (i.e., partial palpograms) are averaged per heart cycle to produce a more robust compounded palpogram. However, catheter motion due to cardiac activity causes misalignment of the RF signals and thus of the partial palpograms, resulting in less valid strain estimates. To compensate for in-plane catheter rotation and translation, we devised four methods based on block matching. The global rotation block matching (GRBM) and contour mapping (CMAP) methods measure catheter rotation, and local block matching (LBM) and catheter rotation and translation (CRT) estimate displacements of local tissue regions. These methods were applied to nine in vivo pullback acquisitions, made with a 20 MHz phased-array transducer. We found that all these methods significantly increase the number of valid strain estimates in the partial and compounded palpograms (P < 0.008). The best method, LBM, attained an average increase of 17% and 15%, respectively. Implementation of this method should improve the information coming from IVUS palpography, leading to better vulnerable plaque detection.

Towards ultrasound cardiac image segmentation based on the radiofrequency signal

In echocardiography, the radio-frequency (RF) image is a rich source of information about the investigated tissues. Nevertheless, very few works are dedicated to boundary detection based on the RF image, as opposed to envelope image. In this paper, we investigate the feasibility and limitations of boundary detection in echocardiographic images based on the RF signal. We introduce two types of RF-derived parameters: spectral autoregressive parameters and velocity-based parameters, and we propose a discontinuity adaptive framework to perform the detection task. In classical echographic cardiac acquisitions, we show that it is possible to use the spectral contents for boundary detection, and that improvement can be expected with respect to traditional methods. Using the system approach, we study on simulations how the spectral contents can be used for boundary detection. We subsequently perform boundary detection in high frame rate simulated and in vivo cardiac sequences using the variance of velocity, obtaining very promising results. Our work opens the perspective of a RF-based framework for ultrasound cardiac image segmentation and tracking.