How I do it: feasibility of a new ultrasound probe fixator to facilitate high quality stress echocardiography

An Optimization Approach for Creating Application-specific Ultrasound Speckle Tracking Algorithms

OBJECTIVE

Ultrasound speckle tracking enables in vivo measurement of soft tissue deformation or strain, providing a non-invasive diagnostic tool to quantify tissue health. However, adoption into new fields is challenging since algorithms need to be tuned with gold-standard reference data that are expensive or impractical to acquire. Here, we present a novel optimization approach that only requires repeated measurements, which can be acquired for new applications where reference data might not be readily available or difficult to get hold of.

METHODS

Soft tissue motion was captured using ultrasound for the medial collateral ligament (MCL) of three quasi-statically loaded porcine stifle joints, and medial ligamentous structures of a dynamically loaded human cadaveric knee joint. Using a training subset, custom speckle tracking algorithms were created for the porcine and human ligaments using surrogate optimization, which aimed to maximize repeatability by minimizing the normalized standard deviation of calculated strain maps for repeat measurements. An unseen test subset was then used to validate the tuned algorithms by comparing the ultrasound strains to digital image correlation (DIC) surface strains (porcine specimens) and length change values of the optically tracked ligament attachments (human specimens).

RESULTS

After 1500 iterations, the optimization routine based on the porcine and human training data converged to similar values of normalized standard deviations of repeat strain maps (porcine: 0.19, human: 0.26). Ultrasound strains calculated for the independent test sets using the tuned algorithms closely matched the DIC measurements for the porcine quasi-static measurements (R > 0.99, RMSE < 0.59%) and the length change between the tracked ligament attachments for the dynamic human dataset (RMSE < 6.28%). Furthermore, strains in the medial ligamentous structures of the human specimen during flexion showed a strong correlation with anterior/posterior position on the ligaments (R > 0.91).

CONCLUSION

Adjusting ultrasound speckle tracking algorithms using an optimization routine based on repeatability led to robust and reliable results with low RMSE for the medial ligamentous structures of the knee. This tool may be equally beneficial in other soft-tissue displacement or strain measurement applications and can assist in the development of novel ultrasonic diagnostic tools to assess soft tissue biomechanics.

High frame rate multi-perspective cardiac ultrasound imaging using phased array probes

Ultrasound (US) imaging is used to assess cardiac disease by assessing the geometry and function of the heart utilizing its high spatial and temporal resolution. However, because of physical constraints, drawbacks of US include limited field-of-view, refraction, resolution and contrast anisotropy. These issues cannot be resolved when using a single probe. Here, an interleaved multi-perspective 2-D US imaging system was introduced, aiming at improved imaging of the left ventricle (LV) of the heart by acquiring US data from two separate phased array probes simultaneously at a high frame rate. In an ex-vivo experiment of a beating porcine heart, parasternal long-axis and apical views of the left ventricle were acquired using two phased array probes. Interleaved multi-probe US data were acquired at a frame rate of 170 frames per second (FPS) using diverging wave imaging under 11 angles. Image registration and fusion algorithms were developed to align and fuse the US images from two different probes. First- and second-order speckle statistics were computed to characterize the resulting probability distribution function and point spread function of the multi-probe image data. First-order speckle analysis showed less overlap of the histograms (reduction of 34.4%) and higher contrast-to-noise ratio (CNR, increase of 27.3%) between endocardium and myocardium in the fused images. Autocorrelation results showed an improved and more isotropic resolution for the multi-perspective images (single-perspective: 0.59 mm × 0.21 mm, multi-perspective: 0.35 mm × 0.18 mm). Moreover, mean gradient (MG) (increase of 74.4%) and entropy (increase of 23.1%) results indicated that image details of the myocardial tissue can be better observed after fusion. To conclude, interleaved multi-perspective high frame rate US imaging was developed and demonstrated in an ex-vivo experimental setup, revealing enlarged field-of-view, and improved image contrast and resolution of cardiac images.

Continuous hands-free monitoring of echocardiographic exercise test using probe fixation device

BACKGROUND

Stress echocardiography has been widely used in clinical practice for decades and has recently gained even more importance in diagnostic approaches to ischemic heart disease. However, it still has numerous limitations. Despite advantages of physical exercise as most physiologic stressor, it is impossible to continuously monitor the cardiac function during treadmill test and difficult to maintain an optimal acoustic window during cycle ergometer exercise tests. The aim herein, is to assess the feasibility of probe fixation for use during exercise echocardiography.

METHODS

Forty-eight subjects (47 men, mean age 42 ± 17 years, 25 healthy volunteers, 23 patients with suspected coronary artery disease) were included in this study. All subjects underwent exercise stress test on treadmill (32 cases) or cycle ergometer (16 cases). Both sector and matrix probes were used (in 17 and 31 tests, respectively). The semi-quantitative quality of acquired apical views were assessed at each stage using a four-point grading system.

RESULTS

The mean time required for probe fixation was 9 ± 2 min. At baseline, 10 patients had at least one apical window of quality precluding reliable analysis. Twenty-five patients required probe repositioning during exercise (more often on a treadmill). During peak exercise quality of images in all views declined, but for diagnostic purposes it remained sufficient in 29 patients. Thus, 76% of performed tests (60% study population) had sufficient image quality.

CONCLUSIONS

Probe fixation offers the possibility of continuous acquisition of echocardiographic images during physical exercise. The device is suitable almost exclusively for male patients and in some patients requires repositioning.

Motion-capture technique-based interface screen displaying real-time probe position and angle in kidney ultrasonography

Professional skill is required to reproduce ultrasound images of the kidney as an optimal cross-section is easily lost with slight deviation in scanning location or angle of the probe. We developed a motion-capture technique-based interface screen that displays the real-time probe position and angle to overlap those provided beforehand. When a professional operator captured the approximate kidney image, our system recorded the relative spatial relationship between the subject and the probe. Next, an amateur operator who had no experience of clinical practice manipulated the probe only with the aid of the interface until the probe position and angle coincided with the professional ones. Eventually, amateur operators could place the probe with a deviation of distance of (x = 2.7 ± 1.2 mm, y = 3.0 ± 1.7 mm, z = 6.6 ± 1.8 mm) and angle of (Rx = 1.5 ± 0.3 degrees, Ry = 2.6 ± 1.1 degrees, Rz = 1.1 ± 0.3 degrees) from the professional goal to produce very similar cross-sectional kidney images (N = 8). Also, motion-capture technique-based evaluation of relative locations of the probe and subject body revealed difficulty in reproducing those without the interface screen navigation. In summary, our motion-capture technique-based ultrasound guide system provides operators with the opportunity to handle the probe just as another operator would beforehand. This could help in medical procedures wherein the same cross-sectional image should be repeatedly obtained. Moreover, it requires no conventional probe training for beginners and could even shift the paradigm for ultrasound probe handling.

Dynamic handgrip exercise for the evaluation of mitral valve regurgitation: an echocardiographic study to identify exertion induced severe mitral regurgitation

Handgrip exercise (HG) has been occasionally used as a stress test in echocardiography. The effect of HG on mitral regurgitation (MR) is not well known. This study aims to evaluate this effect and the possible role of HG in the echocardiographic evaluation of MR. 722 patients with MR were included (18% primary, 82% secondary disease). We calculated effective regurgitant orifice area (EROA) and regurgitant volume (RVOL) at rest and during dynamic HG. Increase in MR was defined as any increase in EROA or RVOL. We analyzed the data to identify possible associations between clinical or echocardiographic parameters and the effect of HG on MR. MR increased during dynamic HG in 390 of 722 patients (54%) (∆EROA = 25%, ∆RVOL = 27%). Increase of regurgitation occurred in 66 of 132 patients with primary MR (50%) and in 324 of 580 patients with secondary MR (55%). This increase was associated with larger baseline EROA and RVOL, but it was independent from other clinical or echocardiographic parameters. In secondary MR, dynamic HG led to a reclassification of regurgitation severity from non-severe at rest to severe MR during HG in 104 of 375 patients (28%). There was a significant association between this upgrade in MR classification and higher New York Heart Association (NYHA) class (OR 1.486, 95%-CI 1.138-1.940, p = 0.004). Dynamic HG exercise increases MR in about half of patients independent of the etiology. Dynamic HG may be used to identify symptomatic patients with non-severe secondary MR at rest but severe MR during exercise.

A demonstration of high field-of-view stability in hands-free echocardiography

Background

Exercise stress echocardiography is clinically used to assess cardiovascular diseases. For accurate cardiac evaluation, a stable field-of-view is required. However, transducer orientation and position are difficult to preserve. Hands-free acquisitions might provide more consistent and reproducible results. In this study, the field-of-view stability and variability of hands-free acquisitions are objectively quantified in a comparison with manually obtained images, based on image structural and feature similarities. In addition, the feasibility and consistency of hands-free strain imaging is assessed.

Methods

In twelve healthy males, apical and parasternal images were acquired hands-free, using a fixation device, and manually, during semi-supine exercise sessions. In the final ten seconds of every exercise period, the image structural similarity and cardiac feature consistency were computed using a steerable pyramid employing complex, oriented wavelets. An algorithm discarding images displaying lung artifacts was created. Hands-free strain consistency was analyzed.

Results

Hands-free acquisitions were possible in 9 of the 12 subjects, whereas manually 10 out of 12 could be imaged. The image structural similarity was significantly improved in the hands-free apical window acquisitions (0.91 versus 0.82), and at least equally good in the parasternal window (0.90 versus 0.82). The change in curvature and orientation of the interventricular septum also appeared to be lower in the hands-free acquisitions. The variability in field-of-view was similar in both acquisitions. Longitudinal, septal strain was shown to be at least as consistent when obtained hands-free compared to manual acquisitions.

Conclusions

The field-of-view was shown to be more or equally stable and consistent in the hands-free data in comparison to manually obtained images. The variability was similar, thus respiration- and exercise-induced motions were comparable for manual and hands-free acquisitions. Additionally, the feasibility of hands-free strain has been demonstrated. Furthermore, the results suggest the hands-free measurements to be more reproducible, though further analysis is required.

The use of a probe stabilizer to reduce musculoskeletal overload of ultrasound operators in routine diagnostic echocardiographic imaging

Introduction: Echocardiography is essential in the evaluation of patients with cardiovascular disease. Repetitive movements, however, expose ultrasound operators to a high risk of musculoskeletal strain injuries. Aim: We investigated to what extent a probe stabilizer could reduce repetitive movements. Materials and methods: The study population consisted of 31 male patients referred for routine transthoracic echocardiography. A good apical acoustic window was prerequisite for inclusion. Standard apical views and measurements were first recorded without using the probe stabilizer. Afterwards, the same apical views and measurements were acquired with utilization of the probe stabilizer. During the entire procedure, shoulder abduction and muscle activity of right forearm flexor and extensor muscles were recorded. To this purpose, an EMG-sensor was attached to the right lower arm and a gyroscope to the right shoulder blade. Results: Extreme right arm abduction (>30˚) occurred in 58% of the time with use of the stabilizer and in 98% of the time without (p <0.01). Activity of right forearm extensor muscles was 42% with and 60% without stabilizer (p = 0.04). For the flexor muscles these percentages were 47% and 87%, respectively (p <0.01). Use of the stabilizer did not affect the time needed for image acquisition (308s versus 309s, respectively, p = 0.46). Conclusions: This study demonstrated that the use of a stabilizer during acquisition of apical views in routine transthoracic echocardiography reduces the total time of shoulder abduction and the use of the right forearm muscles, while acquisition time was not affected.

The use of an external ultrasound fixator (Probefix) on intensive care patients: a feasibility study

Background

In critical care medicine, the use of transthoracic echo (TTE) is expanding. TTE can be used to measure dynamic parameters such as cardiac output (CO). An important asset of TTE is that it is a non-invasive technique. The Probefix is an external ultrasound holder strapped to the patient which makes it possible to measure CO using TTE in a fixed position possibly making the CO measurements more accurate compared to separate TTE CO measurements. The feasibility of the use of the Probefix to measure CO before and after a passive leg raising test (PLR) was studied. Intensive care patients were included after detection of hypovolemia using Flotrac. Endpoints were the possibility to use Probefix. Also CO measurements with and without the use of Probefix, before and after a PLR were compared to the CO measurements using Flotrac. Side effects in terms of skin alterations after the use of Probefix and patient’s comments on (dis)comfort were evaluated.

Results

Ten patients were included; in eight patients, sufficient recordings with the use of Probefix could be obtained. Using Bland–Altman plots, no difference was found in accuracy of measurements of CO with or without the use of Probefix before and after a PLR compared to Flotrac generated CO. There were only mild and temporary skin effects of the use of Probefix.

Conclusions

In this small feasibility study, the Probefix could be used in eight out of ten intensive care patients. The use of Probefix did not result in more or less accurate CO measurements compared to manually recorded TTE CO measurements. We suggest that larger studies on the use of Probefix in intensive care patients are needed.