Abdominal aortic aneurysm follow-up by shear wave elasticity imaging after endovascular repair in a canine model

OBJECTIVES

To investigate if shear wave imaging (SWI) can detect endoleaks and characterize thrombus organization in abdominal aortic aneurysms (AAAs) after endovascular aneurysm repair.

METHODS

Stent grafts (SGs) were implanted in 18 dogs after surgical creation of type I endoleaks (four AAAs), type II endoleaks (13 AAAs) and no endoleaks (one AAA). Color flow Doppler ultrasonography (DUS) and SWI were performed before SG implantation (baseline), on days 7, 30 and 90 after SG implantation, and on the day of the sacrifice (day 180). Angiography, CT scans and macroscopic tissue sections obtained on day 180 were evaluated for the presence, size and type of endoleaks, and thrombi were characterized as fresh or organized. Endoleak areas in aneurysm sacs were identified on SWI by two readers and compared with their appearance on DUS, CT scans and macroscopic examination. Elasticity moduli were calculated in different regions (endoleaks, and fresh and organized thrombi).

RESULTS

All 17 endoleaks (100 %) were identified by reader 1, whereas 16 of 17 (94 %) were detected by reader 2. Elasticity moduli in endoleaks, and in areas of organized thrombi and fresh thrombi were 0.2 ± 0.4, 90.0 ± 48.2 and 13.6 ± 4.5 kPa, respectively (P < 0.001 between groups). SWI detected endoleaks while DUS (three endoleaks) and CT (one endoleak) did not.

CONCLUSIONS

SWI has the potential to detect endoleaks and evaluate thrombus organization based on the measurement of elasticity.

KEY POINTS

• SWI has the potential to detect endoleaks in post-EVAR follow-up. • SWI has the potential to characterize thrombus organization in post-EVAR follow-up. • SWI may be combined with DUS in post-EVAR surveillance of endoleak.

High-Frame-Rate Echocardiography Using Coherent Compounding With Doppler-Based Motion-Compensation

High-frame-rate ultrasonography based on coherent compounding of unfocused beams can potentially transform the assessment of cardiac function. As it requires successive waves to be combined coherently, this approach is sensitive to high-velocity tissue motion. We investigated coherent compounding of tilted diverging waves, emitted from a 2.5 MHz clinical phased array transducer. To cope with high myocardial velocities, a triangle transmit sequence of diverging waves is proposed, combined with tissue Doppler imaging to perform motion compensation (MoCo). The compound sequence with integrated MoCo was adjusted from simulations and was tested in vitro and in vivo. Realistic myocardial velocities were analyzed in an in vitro spinning disk with anechoic cysts. While a 8 dB decrease (no motion versus high motion) was observed without MoCo, the contrast-to-noise ratio of the cysts was preserved with the MoCo approach. With this method, we could provide high-quality in vivo B-mode cardiac images with tissue Doppler at 250 frames per second. Although the septum and the anterior mitral leaflet were poorly apparent without MoCo, they became well perceptible and well contrasted with MoCo. The septal and lateral mitral annulus velocities determined by tissue Doppler were concordant with those measured by pulsed-wave Doppler with a clinical scanner (r(2)=0.7,y=0.9 x+0.5,N=60) . To conclude, high-contrast echo cardiographic B-mode and tissue Doppler images can be obtained with diverging beams when motion compensation is integrated in the coherent compounding process.

Staggered Multiple-PRF Ultrafast Color Doppler

Color Doppler imaging is an established pulsed ultrasound technique to visualize blood flow non-invasively. High-frame-rate (ultrafast) color Doppler, by emissions of plane or circular wavefronts, allows severalfold increase in frame rates. Conventional and ultrafast color Doppler are both limited by the range-velocity dilemma, which may result in velocity folding (aliasing) for large depths and/or large velocities. We investigated multiple pulse-repetition-frequency (PRF) emissions arranged in a series of staggered intervals to remove aliasing in ultrafast color Doppler. Staggered PRF is an emission process where time delays between successive pulse transmissions change in an alternating way. We tested staggered dual- and triple-PRF ultrafast color Doppler, 1) in vitro in a spinning disc and a free jet flow, and 2) in vivo in a human left ventricle. The in vitro results showed that the Nyquist velocity could be extended to up to 6 times the conventional limit. We found coefficients of determination r(2) ≥ 0.98 between the de-aliased and ground-truth velocities. Consistent de-aliased Doppler images were also obtained in the human left heart. Our results demonstrate that staggered multiple-PRF ultrafast color Doppler is efficient for high-velocity high-frame-rate blood flow imaging. This is particularly relevant for new developments in ultrasound imaging relying on accurate velocity measurements.

Contactless remote induction of shear waves in soft tissues using a transcranial magnetic stimulation device

This study presents the first observation of shear waves induced remotely within soft tissues. It was performed through the combination of a transcranial magnetic stimulation device and a permanent magnet. A physical model based on Maxwell and Navier equations was developed. Experiments were performed on a cryogel phantom and a chicken breast sample. Using an ultrafast ultrasound scanner, shear waves of respective amplitudes of 5 and 0.5 μm were observed. Experimental and numerical results were in good agreement. This study constitutes the framework of an alternative shear wave elastography method.

The Imaging Modulography Technique Revisited for High-Definition Intravascular Ultrasound: Theoretical Framework

Mechanical characterization of atherosclerotic lesions remains an essential step for the detection of vulnerable plaques (VPs). Recently, an intravascular ultrasound (IVUS) elasticity reconstruction method (iMOD) has been tested in vivo by our group. The major limitation of iMOD is the need to estimate the strain field in the entire VP despite attenuated depth penetration signals when using high-definition (HD) IVUS systems. Therefore, an extended iMOD approach (E-iMOD) was designed and applied to coronary lesions of patients imaged in vivo with IVUS. The E-iMOD method (i) quantified necrotic core areas with a mean absolute relative error of 3.5 ± 3.5% and (ii) identified Young's moduli of the necrotic cores and fibrous regions with mean values of 5.7 ± 0.8 kPa and 794.5 ± 22.0 kPa instead of 5 kPa and 800 kPa, respectively. This study demonstrates the potential of the improved HD-IVUS modulography technique E-iMOD to characterize coronary VPs.

Unifying Concepts of Statistical and Spectral Quantitative Ultrasound Techniques

Quantitative ultrasound (QUS) techniques using radiofrequency (RF) backscattered signals have been used for tissue characterization of numerous organ systems. One approach is to use the magnitude and frequency dependence of backscatter echoes to quantify tissue structures. Another approach is to use first-order statistical properties of the echo envelope as a signature of the tissue microstructure. We propose a unification of these QUS concepts. For this purpose, a mixture of homodyned K-distributions is introduced to model the echo envelope, together with an estimation method and a physical interpretation of its parameters based on the echo signal spectrum. In particular, the total, coherent and diffuse signal powers related to the proposed mixture model are expressed explicitly in terms of the structure factor previously studied to describe the backscatter coefficient (BSC). Then, this approach is illustrated in the context of red blood cell (RBC) aggregation. It is experimentally shown that the total, coherent and diffuse signal powers are determined by a structural parameter of the spectral Structure Factor Size and Attenuation Estimator. A two-way repeated measures ANOVA test showed that attenuation (p-value of 0.077) and attenuation compensation (p-value of 0.527) had no significant effect on the diffuse to total power ratio. These results constitute a further step in understanding the physical meaning of first-order statistics of ultrasound images and their relations to QUS techniques. The proposed unifying concepts should be applicable to other biological tissues than blood considering that the structure factor can theoretically model any spatial distribution of scatterers.

A Robotic Ultrasound Scanner for Automatic Vessel Tracking and Three-Dimensional Reconstruction of B-Mode Images

Locating and evaluating the length and severity of a stenosis is very important for planning adequate treatment of peripheral arterial disease (PAD). Conventional ultrasound (US) examination cannot provide maps of entire lower limb arteries in 3-D. We propose a prototype 3D-US robotic system with B-mode images, which is nonionizing, noninvasive, and is able to track and reconstruct a continuous segment of the lower limb arterial tree between the groin and the knee. From an initialized cross-sectional view of the vessel, automatic tracking was conducted followed by 3D-US reconstructions evaluated using Hausdorff distance, cross-sectional area, and stenosis severity in comparison with 3-D reconstructions with computed tomography angiography (CTA). A mean Hausdorff distance of 0.97 ± 0.46 mm was found in vitro for 3D-US compared with 3D-CTA vessel representations. To evaluate the stenosis severity in vitro, 3D-US reconstructions gave errors of 3%-6% when compared with designed dimensions of the phantom, which are comparable to 3D-CTA reconstructions, with 4%-13% errors. The in vivo system's feasibility to reconstruct a normal femoral artery segment of a volunteer was also investigated. These results encourage further ergonomic developments to increase the robot's capacity to represent lower limb vessels in the clinical context.

Endovascular Repair of Abdominal Aortic Aneurysm: Follow-up with Noninvasive Vascular Elastography in a Canine Model

PURPOSE

To assess the ability of noninvasive vascular elastography (NIVE) to help characterize endoleaks and thrombus organization in a canine model of abdominal aortic aneurysm after endovascular aneurysm repair with stent-grafts, in comparison with computed tomography (CT) and pathologic examination findings.

MATERIALS AND METHODS

All protocols were approved by the Animal Care Committee in accordance with the guidelines of the Canadian Council of Animal Care. Stent-grafts were implanted in a group of 18 dogs with aneurysms created in the abdominal aorta. Type I endoleak was created in four aneurysms; type II endoleak, in 13 aneurysms; and no endoleak, in one aneurysm. Doppler ultrasonography and NIVE examinations were performed at baseline and at 1-week, 1-month, 3-month, and 6-month follow-up. Angiography, CT, and macroscopic tissue examination were performed at sacrifice. Strain values were computed by using the Lagrangian speckle model estimator. Areas of endoleak, solid organized thrombus, and fresh thrombus were identified and segmented by comparing the results of CT and macroscopic tissue examination. Strain values were compared by using the Wilcoxon rank-sum and Kruskal-Wallis tests.

RESULTS

All stent-grafts were successfully deployed, and endoleaks were clearly depicted in the last follow-up elastography examinations. Maximal axial strains over consecutive heart cycles in endoleak, organized thrombus, and fresh thrombus areas were 0.78% ± 0.22, 0.23% ± 0.02, 0.10% ± 0.04, respectively. Strain values were significantly different between endoleak and organized or fresh thrombus areas (P < .000) and between organized and fresh thrombus areas (P < .0002). No correlation was found between strain values and type of endoleak, sac pressure, endoleak size, and aneurysm size.

CONCLUSION

NIVE may be able to help characterize endoleak and thrombus organization, regardless of the size, pressure, and type of endoleak.

Noninvasive Vascular Elastography With Plane Strain Incompressibility Assumption Using Ultrafast Coherent Compound Plane Wave Imaging

Plane strain tensor estimation using non-invasive vascular ultrasound elastography (NIVE) can be difficult to achieve using conventional focus beamforming due to limited lateral resolution and frame rate. Recent developments in compound plane wave (CPW) imaging have led to high speed and high resolution imaging. In this study, we present the performance of NIVE using coherent CPW. We show the impact of CPW beamforming on strain estimates compared to conventional focus sequences. To overcome the inherent variability of lateral strains, associated with the low lateral resolution of linear array transducers, we use the plane strain incompressibility to constrain the estimator. Taking advantage of the approximate tenfold increase in frame rate of CPW compared with conventional focus imaging, we introduce a time-ensemble estimation approach to further improve the elastogram quality. By combining CPW imaging with the constrained Lagrangian speckle model estimator, we observe an increase in elastography quality (∼ 10 dB both in signal-to-noise and contrast-to-noise ratios) over a wide range of applied strains (0.02 to 3.2%).

Automatic detection of selective arterial devices for advanced visualization during abdominal aortic aneurysm endovascular repair

Here we address the automatic segmentation of endovascular devices used in the endovascular repair (EVAR) of abdominal aortic aneurysms (AAA) that deform vascular tissues. Using this approach, the vascular structure is automatically reshaped solving the issue of misregistration observed on 2D/3D image fusion for EVAR guidance. The endovascular devices we considered are the graduated pigtail catheter (PC) used for contrast injection and the stent-graft delivery device (DD). The segmentation of the DD was enhanced using an asymmetric Frangi filter. The segmented geometries were then analysed using their specific features to remove artefacts. The radiopaque markers of the PC were enhanced using a fusion of Hessian and newly introduced gradient norm shift filters. Extensive experiments were performed using a database of images taken during 28 AAA-EVAR interventions. This dataset was divided into two parts: the first half was used to optimize parameters and the second to compile performances using optimal values obtained. The radiopaque markers of the PC were detected with a sensitivity of 88.3% and a positive predictive value (PPV) of 96%. The PC can therefore be positioned with a majority of its markers localized while the artefacts were all located inside the vessel lumen. The major parts of the DD, the dilatator tip and the pusher surfaces, were detected accurately with a sensitivity of 85.9% and a PPV of 88.7%. The less visible part of the DD, the stent enclosed within the sheath, was segmented with a sensitivity of 63.4% because the radiopacity of this region is low and uneven. The centreline of the DD in this stent region was alternatively traced within a 0.74 mm mean error. The automatic segmentation of endovascular devices during EVAR is feasible and accurate; it could be useful to perform elastic registration of the vascular lumen during endovascular repair.

Experimental application of ultrafast imaging to spectral tissue characterization

Ultrasound ultrafast imaging (UI) allows acquisition of thousands of frames per second with a sustained image quality at any depth in the field of view. Therefore, it would be ideally suited to obtain good statistical sampling of fast-moving tissues using spectral-based techniques to derive the backscatter coefficient (BSC) and associated quantitative parameters. In UI, an image is formed by insonifying the medium with plane waves steered at different angles, beamforming them and compounding the resulting radiofrequency images. We aimed at validating, experimentally, the effect of these beamforming protocols on the BSC, under both isotropic and anisotropic conditions. Using UI techniques with a linear array transducer (5-14 MHz), we estimated the BSCs of tissue-mimicking phantoms and flowing porcine blood at depths up to 35 mm with a frame rate reaching 514 Hz. UI-based data were compared with those obtained using single-element transducers and conventional focusing imaging. Results revealed that UI compounded images can produce valid estimates of BSCs and effective scatterer size (errors less than 2.2 ± 0.8 and 0.26 ± 0.2 dB for blood and phantom experiments, respectively). This work also describes the use of pre-compounded UI images (i.e., steered images) to assess the angular dependency of circulating red blood cells. We have concluded that UI data sets can be used for BSC spectral tissue analysis and anisotropy characterization.

Frequency adaptation for enhanced radiation force amplitude in dynamic elastography

In remote dynamic elastography, the amplitude of the generated displacement field is directly related to the amplitude of the radiation force. Therefore, displacement improvement for better tissue characterization requires the optimization of the radiation force amplitude by increasing the push duration and/or the excitation amplitude applied on the transducer. The main problem of these approaches is that the Food and Drug Administration (FDA) thresholds for medical applications and transducer limitations may be easily exceeded. In the present study, the effect of the frequency used for the generation of the radiation force on the amplitude of the displacement field was investigated. We found that amplitudes of displacements generated by adapted radiation force sequences were greater than those generated by standard nonadapted ones (i.e., single push acoustic radiation force impulse and supersonic shear imaging). Gains in magnitude were between 20 to 158% for in vitro measurements on agar-gelatin phantoms, and 170 to 336% for ex vivo measurements on a human breast sample, depending on focus depths and attenuations of tested samples. The signal-to-noise ratio was also improved more than 4-fold with adapted sequences. We conclude that frequency adaptation is a complementary technique that is efficient for the optimization of displacement amplitudes. This technique can be used safely to optimize the deposited local acoustic energy without increasing the risk of damaging tissues and transducer elements.

Cost-utility analysis of nonalcoholic steatohepatitis screening

OBJECTIVES

Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease in Western countries. No studies have examined the cost-effectiveness of screening its advanced form, nonalcoholic steatohepatitis (NASH).

METHODS

We performed a cost-utility analysis of annual noninvasive screening strategies using third-party payer perspective in a general population in comparison to screening a high-risk obese or diabetic population. Screening algorithms involved well-studied techniques, including NAFLD fibrosis score, transient elastography (TE), and acoustic radiation force impulse (ARFI) imaging for detecting advanced fibrosis (≥ F3); and plasma cytokeratin (CK)-18 for NASH detection. Liver biopsy and magnetic resonance elastography (MRE) were compared as confirmation methods. Canadian dollar (CAD or C$) costs were adjusted for inflation and discounted at 5%. Incremental cost-effectiveness ratio (ICER) of ≤C$ 50,000 was considered cost-effective.

RESULTS

Compared with no screening, screening with NAFLD fibrosis score/TE/CK-18 algorithm with MRE as confirmation for advanced fibrosis had an ICER of C$ 26,143 per quality-adjusted life year (QALY) gained. Screening in high-risk obese or diabetic populations was more cost-effective, with an ICER of C$ 9,051 and C$ 7,991 per quality-adjusted life-year (QALY) gained, respectively. Liver biopsy confirmation was not found to be cost-effective.

CONCLUSIONS

Our model suggests that annual NASH screening in high-risk obese or diabetic populations can be cost-effective.

KEY POINTS

• This cost-utility analysis suggests that screening for nonalcoholic steatohepatitis may be cost-effective. • In particular, screening of high-risk obese or diabetic populations is more cost-effective. • Magnetic resonance elastography was more cost-effective to confirm disease compared to biopsy. • More studies are needed to determine quality of life in nonalcoholic steatohepatitis. • More management strategies for nonalcoholic steatohepatitis are also needed.

Value of C-Arm Computed Tomography to Evaluate Stent Deployment During Femoro-Popliteal Revascularization

PURPOSE

To compare the accuracy of C-arm computed tomography (CT) and digital subtraction angiography (DSA) in detecting incomplete stent expansion (ISE) after superficial femoral artery (SFA) stenting using intravascular ultrasound (IVUS) as a gold standard.

MATERIALS

Fifty patients with symptomatic SFA occlusive disease requiring angioplasty were prospectively included. Once technical success (<30 % residual stenosis) was obtained on post-procedural DSA, C-arm CT and IVUS were acquired. DSA and C-arm CT examinations were reviewed by 2 investigators and correlated with IVUS. C-arm CT image quality was rated on a four-point scale. Doppler ultrasound was performed at 1-year follow-up.

RESULTS

The ankle-brachial index was 0.69 ± 0.10 and 0.99 ± 0.40, respectively, pre- and post-procedure. C-arm CT imaging quality was rated as good or excellent in 80%. In-stent minimal luminal diameter (MLD) was evaluated at 4.71 ± 0.7 mm on DSA, 3.39 ± 0.6 mm on IVUS, and 3.12 ± 0.9 mm on C-arm CT. Compared to IVUS, DSA demonstrated an overestimation of MLD (p = 0.0001), an underestimation of ISE (DSA = 18.8% ± 7.6; IVUS = 29.8% ± 9) (p < 0.0001), and a poor inter-technique intra-class correlation coefficient (ICC = 0.24). No difference was observed between IVUS and C-arm CT in ISE as calculated by diameter (29.8 ± 9 vs. 28.2 ± 12.5%, p = 0.5) and area (30.2 ± 8.4 vs. 33.3 ± 9.5%, p = 0.2). Inter-technique ICC between C-arm CT and IVUS was 0.72 [95%CI 0.49; 0.85] for MLA measurements. The inter-observer ICC for MLD and MLA measurements on C-arm CT were, respectively, estimated at 0.75 [95% CI 0.40, 0.89] and 0.77 [95% CI 0.43, 0.90)].

CONCLUSIONS

C-arm CT presents a better correlation with IVUS than DSA to determine lumen diameter and ISE immediately after percutaneous revascularization.

In vivo venous assessment of red blood cell aggregate sizes in diabetic patients with a quantitative cellular ultrasound imaging method: proof of concept

Background

Diabetic patients present higher level of red blood cell (RBC) aggregation contributing to the development of vascular complications. While it has been suggested that this hematology/rheology parameter could bring additional prognostic information for the management of those patients, RBC aggregation screening is not included as a clinical practice. Most medical centers are not equipped to measure properly this parameter, although sedimentation tests can bring some indication. Here, we aimed at evaluating the feasibility of using ultrasound to assess in-vivo hyper-aggregation in type 2 diabetic patients.

Research design and methods

Seventeen diabetic patients and 15 control subjects underwent ultrasound measurements of RBC aggregation in both cephalic and great saphenous veins. Non-invasive in-vivo ultrasound measurements were performed using a newly developed cellular imaging technique, the structure factor size and attenuation estimator (SFSAE). Comparisons with an ex-vivo gold standard rheometry technique were done, along with measurements of pro-aggregating plasma molecule concentrations.

Results

In-vivo RBC aggregation was significantly higher in diabetic patients compared with controls for cephalic vein measurements, while a trend (p = 0.055) was noticed in the great saphenous vein. SFSAE measurements were correlated with gold standard in-vitro measures, fibrinogen and C-reactive protein plasma concentrations.

Conclusion

RBC aggregation can be measured in-vivo in diabetic patients using ultrasound. Prospective studies are needed to determine whether the SFSAE method could help clinicians in the early management of vascular complications in this patient population.

Development of a photoacoustic, ultrasound and fluorescence imaging catheter for the study of atherosclerotic plaque

Atherosclerotic cardiovascular diseases are a major cause of death in industrialized countries. Molecular imaging modalities are increasingly recognized to be a promising avenue towards improved diagnosis and for the evaluation of new drug therapies. In this work, we present an acquisition system and associated catheter enabling simultaneous photoacoustic, ultrasound and fluorescence imaging of arteries designed for in vivo imaging. The catheter performance is evaluated in tissue-mimicking phantoms. Simultaneous imaging with three modalities is demonstrated at frame rates of 30 images per second for ultrasound and fluorescence and 1 image per 13 seconds for photoacoustic. Acquired radio-frequency ultrasound data could be processed to obtain radial strain elastograms. With motorized pullback, 3D imaging of phantoms was performed using the three modalities.