Non-invasive Small Vessel Imaging of Human Thyroid Using Motion-Corrected Spatiotemporal Clutter Filtering

Abstract

Reliable assessment of small vessel blood flow in the thyroid, without using any contrast agents, can be challenging because of increased physiological motion resulting from its proximity to the pulsating carotid artery. In this study, we hypothesized that correction of tissue motion prior to singular value decomposition (SVD)-based clutter filtering can improve the coherency of the tissue components and, thus, may allow better clutter suppression and visualization of small vessels in the thyroid. We corroborated this hypothesis by conducting phantom and in vivo studies using a clinical ultrasound scanner implemented with compounded plane wave imaging. The phantom studies were conducted using a homogeneous tissue-mimicking phantom to study the impact of motion on the covariance of the spatiotemporal Doppler data, in the absence of blood activity. The non-invasive in vivo study was conducted on a 74-y-old woman with a thyroid nodule suspicious of malignancy. A rigid body-based motion correction was performed using tissue displacements obtained from 2-D normalized cross-correlationÀbased speckle tracking. Subsequently, the power Doppler images were computed using SVD-based spatiotemporal clutter filtering. The results from the phantom study revealed that motion can considerably reduce the covariance of the spatiotemporal data and, thus, increase the rank of the tissue components. When the phantom was subjected to a total translation displacement of 6 pixels over the entire ensemble, in each direction (axial and lateral), the covariance dropped by more than 25%. The results obtained from the non-invasive in vivo study indicated that visualization of small vessel blood flow improved with motion correction of the power Doppler ensemble. The contrast-to-noise ratio of the blood signal in motion-corrected power Doppler images was considerably higher (8.17 and 8.32 dB), compared with that obtained using the standard SVD approach at an optimal threshold (0.87 and 4.33 dB) and a lower singular value threshold (1.92 and 3.05 dB). Further, the covariance of the in vivo thyroid spatiotemporal data increased by approximately 10% with motion correction. These preliminary results indicate that motion correction can be used to improve the visualization of small vessel blood flow in the thyroid, without using any contrast agents. The results of this feasibility study were encouraging, and warrant further development and more in vivo validation in moving tissues and organs.

Impact of imaging cross-section on visualization of thyroid microvessels using ultrasound: Pilot study

“…Further, the signature of carotid pulsation was visible in the lateral displacement estimates (a), which was expected given the anatomical orientation of the thyroid relative to carotid artery. These observations were similar to our previous report on feasibility of motion correction in thyroid nodules 14,34 . Lateral motion of the thyroid lesion was noticeably higher in the longitudinal view, compared to transverse.…”

“…Specifically, in the transverse view magnitude of thyroid motion in the axial and lateral directions were comparable, whereas, in the longitudinal view, lateral motion predominated. Further, the www.nature.com/scientificreports www.nature.com/scientificreports/ estimated displacements (b,d) also displayed presence of translational motion, which could be due to the slipping of the nodule or the probe during scanning 14,34 . Figure 5 displays the mean cross-correlation coefficient of the entire Doppler ensemble with respect the to the reference frame (i.e the first frame in the ensemble), across both longitudinal (blue) and transverse (red) views.…”

“…3 and 4, as described in 14,34 , with and without the correction of thyroid motion. The reported error-bars were computed from three blood vessel ROIs, which intrinsically varied in flow intensity, size and depth 14,34 . The absolute SNR and CNR varied across the patient population and the imaging views, as presented in supplementary data.…”

“…The mean axial and lateral displacements estimated from each frame, averaged over the demarcated tumor nodule, were used to correct first-order rigid-body motion 14 . Since we expected that transverse plane may experience increased out of plane motion, performing motion correction prior to clutter filtering could have hampered reliable tissue suppression in the transverse plane compared to longitudinal plane 34 . Therefore, motion correction prior to clutter-filtering was avoided to prevent the negative impact of erroneous motion tracking and correction on the performance of clutter filtering.…”

Author Correction: Impact of imaging cross-section on visualization of thyroid microvessels using ultrasound: Pilot study

“…This is based on the anatomical orientation of the thyroid with respect to the rigid trachea and the pulsating carotid artery, and its surrounding tissue. With recent developments in motion-robust clutter-filtering 1 , 4 – 6 , a suitable investigation to assess the impact of imaging cross-section on feasibility of thyroid microvasculature imaging can be useful in successfully translating it to the clinic for non-invasive diagnosis. An important question is: What are the disastrous consequences of imaging thyroid microvasculature with increased out-of-plane motion?…”