Rohit Nayak

Noninvasive vascular elastography using plane-wave and sparse-array imaging

Stroke may occur when an atherosclerotic plaque ruptures in the carotid artery. Noninvasive vascular elastography (NIVE) visualizes the strain distribution within the carotid artery, which is related to its mechanical properties that govern plaque rupture. Strain elastograms obtained from the transverse plane of the carotid artery are difficult to interpret, because strain is estimated in Cartesian coordinates. Sparsearray (SA) elastography overcomes this problem by transforming shear and normal strain to polar coordinates. However, the SAs transmit power may be too weak to produce useful elastograms in the clinical setting. Consequently, we are exploring other imaging methods to solve this potential problem. This study evaluated the quality of elastograms produced with SA imaging, plane-wave (PW) imaging, and compounded-plane-wave (CPW) imaging. We performed studies on simulated and physical vessel phantoms, and the carotid artery of a healthy volunteer. All echo imaging was performed with a linear transducer array that contained 128 elements, operating at 5 MHz. In SA imaging, 7 elements were fired during transmission, but all 128 elements were active during reception. In PW imaging, all 128 elements were active during both transmission and reception. We created CPW images by steering the acoustic beam within the range of -15° to 15° in increments of 5°. SA radial and circumferential strain elastograms were comparable to those produced using PW and CPW imaging. Additionally, side-lobe levels incurred during SA imaging were 20 dB lower than those produced during PW imaging, and 10 dB lower than those computed using CPW imaging. Overall, SA imaging performs well in vivo; therefore, we plan to improve the technique and perform preclinical studies.

Principal Strain Vascular Elastography: Simulation and Preliminary Clinical Evaluation

It is difficult to produce reliable polar strain elastograms (radial and circumferential) because the center of the carotid artery is typically unknown. Principal strain imaging can overcome this limitation, but suboptimal lateral displacement estimates make this an impractical approach for visualizing mechanical properties within the carotid artery. We hypothesized that compounded plane wave imaging can minimize this problem. To test this hypothesis, we performed (i) simulations with vessels of varying morphology and mechanical behavior (i.e., isotropic and transversely isotropic), and (ii) a pilot study with 10 healthy volunteers. The accuracy of principal and polar strain (computed using knowledge of the precise vessel center) elastograms varied between 7% and 17%. In both types of elastograms, strain concentrated at the junction between the fibrous cap and the vessel wall, and the strain magnitude decreased with increasing fibrous cap thickness. Elastograms of healthy volunteers were consistent with those of transversely isotropic homogeneous vessels; they were spatially asymmetric, a trend that was common to both principal and polar strains. No significant differences were observed in the mean strain recovered from principal and polar strains (p > 0.05). This investigation indicates that principal strain elastograms measured with compounding plane wave imaging overcome the problems incurred when polar strain elastograms are computed with imprecise estimates of the vessel center.

Quantitative sparse array vascular elastography: the impact of tissue attenuation and modulus contrast on performance

Abstract. Quantitative sparse array vascular elastography visualizes the shear modulus distribution within vascular tissues, information that clinicans could use to reduce the number of strokes each year. However, the low transmit power sparse array (SA) imaging could hamper the clinical usefulness of the resulting elastograms. In this study, we evaluated the performance of modulus elastograms recovered from simulated and physical vessel phantoms with varying attenuation coefficients (0.6, 1.5, and 3.5  cm−1) and modulus contrasts (−12.04, −6.02, and −2.5  dB) using SA imaging relative to those obtained with conventional linear array (CLA) and plane-wave (PW) imaging techniques. Plaques were visible in all modulus elastograms, but those produced using SA and PW contained less artifacts. The modulus contrast-to-noise ratio decreased rapidly with increasing modulus contrast and attenuation coefficient, but more quickly when SA imaging was performed than for CLA or PW. The errors incurred varied from 10.9% to 24% (CLA), 1.8% to 12% (SA), and ≈4% (PW). Modulus elastograms produced with SA and PW imagings were not significantly different (p>0.05). Despite the low transmit power, SA imaging can produce useful modulus elastograms in superficial organs, such as the carotid artery.

Noninvasive carotid artery elastography using multielement synthetic aperture imaging: Phantom and in vivo evaluation

Purpose: Vascular elastography can visualize the strain distribution in the carotid artery, which could be useful in assessing the propensity of advanced plaques to rupture. In our previous studies, we demonstrated that sparse synthetic aperture (SA) imaging can produce high quality vascular strain elastograms. However, the low output power of SA imaging may hamper its clinical utility. In this study, we hypothesize that multi‐element defocused emissions can overcome this limitation and improve the quality of the vascular strain elastograms. Methods: To assess the impact of attenuation on the elastographic performance of SA and (multi‐element synthetic aperture) MSA imaging, we conducted experiments using heterogeneous vessel phantoms with ideal (0.1 dB cm−1 MHz−1) and realistic (0.75 dB cm−1 MHz−1) attenuation. Further, we validated the results of the phantom study in vivo, on a healthy male volunteer. All echo imaging was performed at a transmit frequency of 5 MHz, using a commercially available ultrasound scanner (Sonix RP, Ultrasonix Medical Corp., Richmond, BC, Canada). Results: The results from the phantom results demonstrated that plaques were visible in all strain elastograms, but those produced using MSA imaging had less artifacts. MSA imaging improved the elastographic contrast to noise ratio (CNRe) of the vascular elastograms by 14.58 dB relative to SA imaging, and 9.1 dB relative to compounded plane wave (CPW) imaging. Further, the results demonstrated that the elastographic performance of MSA imaging improved with increase in (a) the number of transmit‐receive events and (b) the size of the transmit sub‐aperture, up to 13 elements. Using larger sub‐apertures degraded the elastographic performance. The results from the in vivo study were in good agreement with the phantom results. Specifically, using a defocused multi‐element transmit sub‐aperture for SA imaging improved the performance of vascular elastography. Conclusions: The results suggested that MSA imaging can produce reliable vascular stain elastograms. Future studies will involve using coded excitations to improve the CNRe and frame‐rate of the proposed technique for vascular elastography.

Optimal sparsifying bases for frequency-domain optical-coherence tomography

We address the reconstruction problem in frequency-domain optical-coherence tomography (FDOCT) from undersampled measurements within the framework of compressed sensing (CS). Specifically, we propose optimal sparsifying bases for accurate reconstruction by analyzing the backscattered signal model. Although one might expect Fourier bases to be optimal for the FDOCT reconstruction problem, it turns out that the optimal sparsifying bases are windowed cosine functions where the window is the magnitude spectrum of the laser source. Further, the windowed cosine bases can be phase locked, which allows one to obtain higher accuracy in reconstruction. We present experimental validations on real data. The findings reported in this Letter are useful for optimal dictionary design within the framework of CS-FDOCT.

Visualizing Angle-Independent Principal Strains in the Longitudinal View of the Carotid Artery: Phantom and In Vivo Evaluation

Arterial stiffness is a strong indicator of atherosclerosis, and can be evaluated by computing the arterial strain in the carotid artery using ultrasound elastography. Researchers have used axial strain to visualize the radial motion in the longitudinal plane of the carotid artery. However, axial strains can be influenced by the geometry of the vessel (Fig. 1 a, b), and thus could limit the scope of vascular elastography in the clinic. In this study we hypothesize that principal strain elastograms computed using compounded plane wave (CPW) imaging could reliably estimate angle-independent vascular strains in the carotid artery.

Non-invasive carotid artery elastography using multi-element synthetic aperture and plane wave imaging: Phantom and in vivo evaluation

Vascular elastography can visualize the strain distribution in the carotid artery, which governs plaque rupture. In this study, we hypothesize that multi-element synthetic aperture (MSA) imaging, which produces divergent transmit beams can produce high quality vascular strain elastograms, relative to those obtained using compounded plane wave (CPW) imaging.

Elastographic imaging of the carotid arteries of HIV infected patients with and without cardiovascular disease: A pilot study

Individuals infected with the human immunodeficiency virus (HIV) are prone to develop cardiovascular disease (CVD) because the HIV infection and combination antiretroviral therapy (cART) produces atherosclerosis. Clinicians need sensitive tests to monitor interventions designed to prevent CVD in patients with HIV on long term cART. We hypothesize that vascular elastography can fulfill this role. To corroborate this hypothesis, we performed vascular elastography on 60 volunteers (mean age of 55.5 y), which we divided into four groups: (a) HIV positive with CVD (HIV++), (b) HIV positive without CVD (HIV+−), (c) HIV negative with CVD (HIV-+), and (c) HIV negative without CVD (HIV−−).

Visualizing angle-independent principal strains in the longitudinal view of the carotid artery: Phantom and in vivo evaluation

Arterial stiffness is a strong indicator of atherosclerosis, and can be evaluated by computing the arterial strain in the carotid artery using ultrasound elastography. Researchers have used axial strain to visualize the radial motion in the longitudinal plane of the carotid artery. However, axial strains can be influenced by the geometry of the vessel (Fig. 1 a, b), and thus could limit the scope of vascular elastography in the clinic. In this study we hypothesize that principal strain elastograms computed using compounded plane wave (CPW) imaging could reliably estimate angle-independent vascular strains in the carotid artery.

Principal strain vascular elastography using compounded plane wave imaging

Principal strain imaging could enhance the diagnostic performance of vascular elastography, but conventional ultrasound system cannot measure lateral displacements precisely, information needed to compute principal strain. The goal of this study is to investigate the feasibility of using compounded plane wave imaging to visualize principal strain within the carotid artery. To compare the quality of principal strain relative to radial and circumferential (polar) strain elastogram, we performed a simulation study. We synthesized radio-frequency (RF) echo frames of homogeneous and heterogeneous vessels. The simulated ultrasound scanner acquired plane wave images from transmission angles of -14 to 14 in steps of 2 using a linear transducer array operating at 5. The accuracy of principal and polar strains were comparable (both varied between 7 % to 12 %), but principal strain offered considerably higher elastographic contrast-to-noise ratio (4 - 14 dB). The results also revealed that the elastographic contrast-to-noise ratios of principal strain elastograms were significantly larger than those of polar (p <; 0.05). The results suggest that compounding plane wave vascular elastography can produce useful principal strain elastograms.