A Statistical Model for the Quantification of Microbubbles in Destructive Imaging:

Assessing Vessel Reconstruction in Ultrasound Localization Microscopy by Maximum Likelihood Estimation of a Zero-Inflated Poisson Model

In clinical applications of super-resolution ultrasound imaging, it is often not possible to achieve a full reconstruction of the microvasculature within a limited measurement time. This makes the comparison of studies and quantitative parameters of vascular morphology and perfusion difficult. Therefore, saturation models were proposed to predict adequate measurement times and estimate the degree of vessel reconstruction. Here, we derive a statistical model for the microbubble counts in super-resolution voxels by a zero-inflated Poisson (ZIP) process. In this model, voxels either belong to vessels with probability P_v and count events with Poisson rate Λ , or they are empty and remain zero. In this model, P_v represents the vessel voxel density in the super-resolution image after infinite measurement time. For the parameters P_v and Λ , we give Cramér-Rao lower bounds (CRLBs) for the estimation variance and derive maximum likelihood estimators (MLEs) in a novel closed-form solution. These can be calculated with knowledge of only the counts at the end of the acquisition time. The estimators are applied to preclinical and clinical data and the MLE outperforms alternative estimators proposed before. The estimated degree of reconstruction lies between 38% and 74% after less than 90 s. Vessel probability P_v ranged from 4% to 20%. The rate parameter Λ was estimated in the range of 0.5-1.3 microbubbles/voxel. For these parameter ranges, the CRLB gives standard deviations of less than 2%, which supports that the parameters can be estimated with good precision already for limited acquisition times.

Poisson Statistical Model of Ultrasound Super-Resolution Imaging Acquisition Time

A number of acoustic super-resolution techniques have recently been developed to visualize microvascular structure and flow beyond the diffraction limit. A crucial aspect of all ultrasound (US) super-resolution (SR) methods using single microbubble localization is time-efficient detection of individual bubble signals. Due to the need for bubbles to circulate through the vasculature during acquisition, slow flows associated with the microcirculation limit the minimum acquisition time needed to obtain adequate spatial information. Here, a model is developed to investigate the combined effects of imaging parameters, bubble signal density, and vascular flow on SR image acquisition time. We find that the estimated minimum time needed for SR increases for slower blood velocities and greater resolution improvement. To improve SR from a resolution of λ /10 to λ /20 while imaging the microvasculature structure modeled here, the estimated minimum acquisition time increases by a factor of 14. The maximum useful imaging frame rate to provide new spatial information in each image is set by the bubble velocity at low blood flows (<150 mm/s for a depth of 5 cm) and by the acoustic wave velocity at higher bubble velocities. Furthermore, the image acquisition procedure, transmit frequency, localization precision, and desired super-resolved image contrast together determine the optimal acquisition time achievable for fixed flow velocity. Exploring the effects of both system parameters and details of the target vasculature can allow a better choice of acquisition settings and provide improved understanding of the completeness of SR information.

A Temporal and Spatial Analysis Approach to Automated Segmentation of Microbubble Signals in Contrast-Enhanced Ultrasound Images: Application to Quantification of Active Vascular Density in Human Lower Limbs

Contrast-enhanced ultrasound (CEUS) using microbubble contrast agents has shown great promise in visualising and quantifying active vascular density. Most existing approaches for vascular density quantification using CEUS are calculated based on image intensity and are susceptible to confounding factors and imaging artefact. Poor reproducibility is a key challenge to clinical translation. In this study, a new automated temporal and spatial signal analysis approach is developed for reproducible microbubble segmentation and quantification of contrast enhancement in human lower limbs. The approach is evaluated in vitro on phantoms and in vivo in lower limbs of healthy volunteers before and after physical exercise. In this approach, vascular density is quantified based on the relative areas microbubbles occupy instead of their image intensity. Temporal features of the CEUS image sequences are used to identify pixels that contain microbubble signals. A microbubble track density (MTD) measure, the ratio of the segmented microbubble area to the whole tissue area, is calculated as a surrogate for active capillary density. In vitro results reveal a good correlation (r² = 0.89) between the calculated MTD measure and the known bubble concentration. For in vivo results, a significant increase (129% in average) in the MTD measure is found in lower limbs of healthy volunteers after exercise, with excellent repeatability over a series of days (intra-class correlation coefficient = 0.96). This compares to the existing state-of-the-art approach of destruction and replenishment analysis on the same patients (intra-class correlation coefficient ≤0.78). The proposed new approach shows great potential as an accurate and highly reproducible clinical tool for quantification of active vascular density.

Effect of microbubble contrast on intracranial blood flow velocity assessed by transcranial Doppler

PURPOSE

Ultrasound contrast agents (UCA) salvage a considerable number of transcranial Doppler (TCD) exams which would have failed because of poor bone window. UCA bolus injection causes an undesirable increase in measured blood flow velocity (BFV). The effect of UCA continuous infusion on measured BFV has not been investigated, and some in vitro experiments suggest that gain reduction during UCA administration may also influence measured BFV. This study aimed to investigate the effect of UCA continuous infusion on BFV measured by TCD and the influence of gain reduction on these measurements in a clinical setting.

METHODS

The right middle cerebral artery of ten patients with optimal bone window was insonated using a 2 MHz probe. UCA were administered using an infusion pump. BFV was measured (1) at baseline, (2) during UCA infusion, (3) during UCA infusion with gain reduction, and (4) after UCA wash-out phase. Gain reduction was based on the agreement between two neurosonographers on the degree of gain reduction necessary to restore baseline Doppler signal intensity (DSI). Actual DSI was estimated offline by analysis of raw data.

RESULTS

BFV measured during UCA infusion with no gain adjustment was significantly higher than baseline BFV [peak systolic velocity (PSV): 85.1 ± 19.7 vs. 74.4 ± 19.7 cm/s, p < 0.0001; Mean velocity (MV): 56.5 ± 11.8 vs. 50.2 ± 12.3 cm/s, p < 0.0001]. BFV measured during UCA infusion with gain reduction was not significantly higher than baseline BFV (PSV: 74.3 ± 18.9 vs. 74.4 ± 19.4 cm/s, p = 0.8; MV: 49.4 ± 11.0 vs. 50.2 ± 12.3 cm/s, p = 0.8). Actual DSI during UCA infusion with gain reduction was not significantly higher than baseline DSI (13 ± 1 vs. 13 ± 1 dB).

CONCLUSION

This study shows that UCA continuous infusion leads to an increase in measured BFV which may be counteracted by reducing Doppler gain thus restoring pre-contrast DSI.

Phase shift variance imaging - a new technique for destructive microbubble imaging

The detection of microbubble contrast agents with ultrasound imaging techniques is the subject of ongoing research. Commonly, the nonlinear response of the agent is employed for detection. The performance of these techniques is, however, affected by nonlinear sound propagation. As an alternative, the change in echo response resulting from microbubble destruction can be employed to detect the agent. In this work, we propose a novel criterion for microbubble destruction detection that allows the rejection of tissue at a defined significance level even for highly echogenic structures in the presence of nonlinear propagation. Most clinical systems provide the hardware requirements for acquisitions consisting of multiple pulses transmitted at the same position, as used in Doppler imaging. Therefore, we develop a processing strategy that distinguishes contrast agent from other stationary or moving structures using these sequences. The proposed criterion is based on the variance of the phase shift of consecutive echoes in the sequence, which, in addition to tissue rejection, permits the distinction of motion from agent disruption. Phantom experiments are conducted to show the validity of the criterion and demonstrate the performance of the new method for contrast detection. Each detection series consists of 20 identical pulses at 9.5 MHz (4.7 MPa peak negative pressure) transmitted at a pulse repetition frequency of 5 kHz. The sequence is applied to phantoms under varied motion and flow conditions. As a first step toward molecular imaging, the technique is applied to microbubbles targeted to vascular endothelial growth factor receptor 2 (VEGFR2) in vitro. The results show a uniform rejection of the background signal while maintaining a contrast enhancement by more than 40 dB. The area under the receiver operating characteristics (ROC) curve is used as the performance metric for the separation of contrast agent and tissue signals, and values larger than 97% demonstrate that an excellent separation was achieved.

Advanced characterization and refinement of poly N-butyl cyanoacrylate microbubbles for ultrasound imaging

We aimed to develop and characterize poly n-butylcyanoacrylate (PBCA) microbubbles (MBs) with a narrow size distribution. MBs were synthesized by established emulsion polymerization techniques, size-isolated by centrifugation and functionalized for molecular imaging by coating their surface with streptavidin. The physical and acoustic properties of the parent solution, different-size isolated populations and functionalized MBs were measured and compared. As expected from negative zeta potentials at pH 7, cryo scanning electron microscopy showed no aggregates. In phantoms MBs were destructible at high mechanical indices and showed a frequency-dependent attenuation and backscattering. The MBs were stable in solution for more than 14 weeks and could be lyophilized without major damage. However, for injection, small needle diameters and high injection rates are shown to be critical because both lead to MB destruction. In summary, when being handled correctly, size-isolated PBCA MBs are promising candidates for preclinical functional and molecular ultrasound imaging.