2D ultrasound thermometry during thermal ablation with high-intensity focused ultrasound.

The clinical use of high intensity focused ultrasound (HIFU) therapy for noninvasive tissue ablation has recently gained momentum. Guidance is provided by either magnetic resonance imaging (MRI) or conventional B-mode ultrasound imaging, each with its own advantages and disadvantages. The main limitation of ultrasound imaging is its inability to provide temperature measurements over the ranges corresponding to the target temperatures during ablative thermal therapies (between 55 °C and 70 °C). Here, variations in ultrasound backscattered energy (ΔBSE) were used to monitor temperature increases in liver tissue up to an absolute value of 90 °C during and after HIFU treatment. In vitro experimental measurements were performed in 47 bovine liver samples using a toroidal HIFU transducer operating at 2.5 MHz to increase the temperature of tissues. An ultrasound imaging probe working at 7.5 MHz was placed in the center of the HIFU transducer to monitor the backscattered signals. The free-field acoustic power was set to 9 W, 12 W or 16 W in the different experiments. HIFU sonications were performed for 240 s using a duty cycle of 83 % to allow ultrasound imaging and raw radiofrequency data acquisition during exposures. Measurements showed a linear relationship between ΔBSE (in dB) and temperature (r = 0.94, p < 0.001) over a temperature range from 37 °C to 90 °C, with a high reliability of temperature measurements below 75 °C. Monitoring can be performed at the frame rate of ultrasound imaging scanners with an accuracy within an acceptable threshold of 5 °C, given the temperatures targeted during thermal ablations. If the maximum temperature reached is below 70 °C, ΔBSE is also a reliable approach for estimating the temperature during cooling. Histological analysis shown the impact of the treatment on the spatial arrangement of cells that can explain the observed variation of ΔBSE. These results demonstrate the ability of ΔBSE measurements to estimate temperature in ultrasound images within an effective therapeutic range. This method can be implemented clinically and potentially applied to other thermal-based therapies.

Post-mortem Tissue Degassing Using Positive Pressure Is Superior to Negative Pressure.

OBJECTIVE: To compare the effectiveness of positive pressure (PP) and negative pressure (NP) for reducing gas inclusions in biological tissues in preparation for acoustic imaging. METHODS: Eighteen pieces of porcine liver in degassed saline were included in this study. For the PP group (n = 9 samples), a wristwatch waterproof tester was used to pressurize samples to 0.41 MPa (59 psi) for 10 min. For the NP group (n = 9 samples), a desiccator at -0.08 MPa (-12 psi) was used for 30 min. Backscatter coefficients (BSCs) were calculated over the central frequency range of the backscattered spectra and paired-samples t-tests were performed. RESULTS: Utilization of PP resulted in a decrease in BSC for all samples, indicating less gas post-PP (pre-PP -13.0 ± 4.3 dB [mean ± SD], post-PP -18.9 ± 5.0 dB, p = .001). Utilization of NP resulted in an increase in BSC for the majority of samples (pre-NP -14.6 ± 6.0 dB, post-NP -13.1 ± 5.3 dB, p = .177). CONCLUSION: Utilization of a simple PP chamber consistently resulted in a decrease in tissue gas, at lower pressures than previously reported. The vacuum method is ineffective, may result in a paradoxical increase in tissue gas, and may not be recommended for tissue degassing.

Ultrasound attenuation of cortical bone correlates with biomechanical, microstructural, and compositional properties.

BACKGROUND: We investigated the relationship of two commonly used quantitative ultrasound (QUS) parameters, speed of sound (SoS) and attenuation coefficient (α), with water and macromolecular contents of bovine cortical bone strips as measured with ultrashort echo time (UTE) magnetic resonance imaging (MRI). METHODS: SoS and α were measured in 36 bovine cortical bone strips utilizing a single-element transducer with nominal 5 MHz center frequency based on the time of flight principles after accommodating for reflection losses. Specimens were then scanned using UTE MRI to measure total, bound, and pore water proton density (TWPD, BWPD, and PWPD) as well as macromolecular proton fraction and macromolecular transverse relaxation time (T2-MM). Specimens were also scanned using microcomputed tomography (µCT) at 9-µm isometric voxel size to measure bone mineral density (BMD), porosity, and pore size. The elastic modulus (E) of each specimen was measured using a 4-point bending test. RESULTS: α demonstrated significant positive Spearman correlations with E (R = 0.69) and BMD (R = 0.44) while showing significant negative correlations with porosity (R = -0.41), T2-MM (R = -0.47), TWPD (R = -0.68), BWPD (R = -0.67), and PWPD (R = -0.45). CONCLUSIONS: The negative correlation between α and T2-MM is likely indicating the relationship between QUS and collagen matrix organization. The higher correlations of α with BWPD than with PWPD may indicate that water organized in finer structure (bound to matrix) provides lower acoustic impedance than water in larger pores, which is yet to be investigated thoroughly. RELEVANCE STATEMENT: This study highlights the importance of future investigations exploring the relationship between QUS measures and all major components of the bone, including the collagenous matrix and water. Investigating the full potential of QUS and its validation facilitates a more affordable and accessible tool for bone health monitoring in clinics. KEY POINTS: • Ultrasound attenuation demonstrated significant positive correlations with bone mechanics and mineral density. • Ultrasound attenuation demonstrated significant negative correlations with porosity and bone water contents. • This study highlights the importance of future investigations exploring the relationship between QUS measures and all major components of the bone.

High-frequency Quantitative Ultrasound Imaging of Human Rotator Cuff Muscles: Assessment of Repeatability and Reproducibility.

This study evaluated the repeatability and reproducibility of using high-frequency quantitative ultrasound (QUS) measurement of backscatter coefficient (BSC), grayscale analysis, and gray-level co-occurrence matrix (GLCM) textural analysis, to characterize human rotator cuff muscles. The effects of varying scanner settings across two different operators and two US systems were investigated in a healthy volunteer with normal rotator cuff muscles and a patient with chronic massive rotator cuff injury and substantial muscle degeneration. The results suggest that BSC is a promising method for assessing rotator cuff muscles in both control and pathological subjects, even when operators were free to adjust system settings (depth, level of focus, and time-gain compensation). Measurements were repeatable and reproducible across the different operators and ultrasound imaging platforms. In contrast, grayscale and GLCM analyses were found to be less reliable in this setting, with significant measurement variability. Overall, the repeatability and reproducibility measurements of BSC indicate its potential as a diagnostic tool for rotator cuff muscle evaluation.

Quantitative evaluation of rat sciatic nerve degeneration using high-frequency ultrasound.

In this study, we evaluated the utility of using high-frequency ultrasound to non-invasively track the degenerative process in a rat model of peripheral nerve injury. Primary analyses explored spatial and temporal changes in quantitative backscatter coefficient (BSC) spectrum-based outcomes and B-mode textural outcomes, using gray level co-occurrence matrices (GLCMs), during the progressive transition from acute to chronic injury. As secondary analyses, correlations among GLCM and BSC spectrum-based parameters were evaluated, and immunohistochemistry were used to suggest a structural basis for ultrasound outcomes. Both mean BSC spectrum-based and mean GLCM-based measures exhibited significant spatial differences across presurgical and 1-month/2-month time points, distal stumps enclosed proximity to the injury site being particularly affected. The two sets of parameters sensitively detected peripheral nerve degeneration at 1-month and 2-month post-injury, with area under the receiver operating charactersitic curve > 0.8 for most parameters. The results also indicated that the many BSC spectrum-based and GLCM-based parameters significantly correlate with each other, and suggested a common structural basis for a diverse set of quantitative ultrasound parameters. The findings of this study suggest that BSC spectrum-based and GLCM-based analysis are promising non-invasive techniques for diagnosing peripheral nerve degeneration.

Skeletal Muscle Assessment Using Quantitative Ultrasound: A Narrative Review.

Ultrasound (US) is an important imaging tool for skeletal muscle analysis. The advantages of US include point-of-care access, real-time imaging, cost-effectiveness, and absence of ionizing radiation. However, US can be highly dependent on the operator and/or US system, and a portion of the potentially useful information carried by raw sonographic data is discarded in image formation for routine qualitative US. Quantitative ultrasound (QUS) methods provide analysis of the raw or post-processed data, revealing additional information about normal tissue structure and disease status. There are four QUS categories that can be used on muscle and are important to review. First, quantitative data derived from B-mode images can help determine the macrostructural anatomy and microstructural morphology of muscle tissues. Second, US elastography can provide information about muscle elasticity or stiffness through strain elastography or shear wave elastography (SWE). Strain elastography measures the induced tissue strain caused either by internal or external compression by tracking tissue displacement with detectable speckle in B-mode images of the examined tissue. SWE measures the speed of induced shear waves traveling through the tissue to estimate the tissue elasticity. These shear waves may be produced using external mechanical vibrations or internal "push pulse" ultrasound stimuli. Third, raw radiofrequency signal analyses provide estimates of fundamental tissue parameters, such as the speed of sound, attenuation coefficient, and backscatter coefficient, which correspond to information about muscle tissue microstructure and composition. Lastly, envelope statistical analyses apply various probability distributions to estimate the number density of scatterers and quantify coherent to incoherent signals, thus providing information about microstructural properties of muscle tissue. This review will examine these QUS techniques, published results on QUS evaluation of skeletal muscles, and the strengths and limitations of QUS in skeletal muscle analysis.

Quantitative Ultrasound Techniques Used for Peripheral Nerve Assessment.

AIM: This review article describes quantitative ultrasound (QUS) techniques and summarizes their strengths and limitations when applied to peripheral nerves. METHODS: A systematic review was conducted on publications after 1990 in Google Scholar, Scopus, and PubMed databases. The search terms "peripheral nerve", "quantitative ultrasound", and "elastography ultrasound" were used to identify studies related to this investigation. RESULTS: Based on this literature review, QUS investigations performed on peripheral nerves can be categorized into three main groups: (1) B-mode echogenicity measurements, which are affected by a variety of post-processing algorithms applied during image formation and in subsequent B-mode images; (2) ultrasound (US) elastography, which examines tissue stiffness or elasticity through modalities such as strain ultrasonography or shear wave elastography (SWE). With strain ultrasonography, induced tissue strain, caused by internal or external compression stimuli that distort the tissue, is measured by tracking detectable speckles in the B-mode images. In SWE, the propagation speed of shear waves, generated by externally applied mechanical vibrations or internal US "push pulse" stimuli, is measured to estimate tissue elasticity; (3) the characterization of raw backscattered ultrasound radiofrequency (RF) signals, which provide fundamental ultrasonic tissue parameters, such as the acoustic attenuation and backscattered coefficients, that reflect tissue composition and microstructural properties. CONCLUSIONS: QUS techniques allow the objective evaluation of peripheral nerves and reduce operator- or system-associated biases that can influence qualitative B-mode imaging. The application of QUS techniques to peripheral nerves, including their strengths and limitations, were described and discussed in this review to enhance clinical translation.

Repeatability, Reproducibility and Sources of Variability in the Assessment of Backscatter Coefficient and Texture Parameters from High-Frequency Ultrasound Acquisitions in Human Median Nerve.

Ultrasound (US) is an increasingly prevalent and effective diagnostic modality for neuromuscular imaging. Gray-scale B-mode imaging has been the dominant US approach to evaluating nerves qualitatively or making morphometric measurements of nerves, providing important insights into pathological changes for conditions such as carpal tunnel syndrome. Among more recent ultrasound strategies, high-frequency ultrasound (often defined as >15 MHz for clinical applications), quantitative ultrasound and image textural analysis offer promising enhancements for improved and more objective approaches to nerve imaging. In this study, we evaluated the repeatability and reproducibility of backscatter coefficient (BSC) and imaging texture features extracted by gray-level co-occurrence matrices (GLCMs) in homogeneous tissue-mimicking reference phantoms and in median nerves in the wrists of healthy participants. We also investigated several practical sources of variability in the assessment of quantitative parameters, including influences of operators, and participant-to-participant variability. Overall, BSC- and GLCM-based outcomes are highly repeatable and reproducible after operator training, based on measurement of descriptive statistics, repeatability coefficient (RC) and reproducibility coefficient recommended by Quantitative Imaging Biomarker Alliance (QIBA RDC). GLCM parameters appear more reproducible and repeatable than BSC-based parameters in healthy participants in vivo. However, such variability noted here must be compared with the value ranges and variability of the results in pathological nerves, including median nerves afflicted by trauma, overuse syndromes such as carpal tunnel syndrome and after surgical repair.

Evaluation of Ultrasonic Attenuation in Primary and Secondary Human Liver Tumors and Its Potential Effect on High-Intensity Focused Ultrasound Treatment.

Primary and secondary liver tumors are completely different diseases but are usually treated similarly using high-intensity focused ultrasound (HIFU). However, the acoustic parameters of these tissues are not well documented. In this study, attenuation coefficients were evaluated in fresh primary (N = 8) and secondary (N = 13) human liver tumor samples recovered by hepatectomy. The average attenuation coefficients of the primary and secondary liver tumors were 0.10 ± 0.03 and 0.20 ± 0.04 Np/cm/MHz, respectively. The average attenuation coefficients of the liver tissue surrounding the primary and secondary tumors were 0.16 ± 0.07 and 0.07 ± 0.02 Np/cm/MHz, respectively. Numerical simulations performed using these values revealed that completely different HIFU ablation patterns were created in primary and secondary liver tumors using the same exposure parameters. The dimensions of a typical HIFU lesion were two times larger in secondary liver tumors than in primary tumors. HIFU treatment parameters should be set properly according to the acoustic properties of the diseased liver tissue.

Fast and Selective Ablation of Liver Tumors by High-Intensity Focused Ultrasound Using a Toroidal Transducer Guided by Ultrasound Imaging: The Results of Animal Experiments.

This study demonstrated that high-intensity focused ultrasound (HIFU) produced with an intra-operative toroidal-shaped transducer causes fast, selective liver tumor ablations in an animal model. The HIFU device is composed of 256 emitters working at 3 MHz. A 7.5 MHz ultrasound imaging probe centered on the HIFU transducer guided treatment. VX2 tumor segments (25 mg) were implanted into the right lateral liver lobes of 45 New Zealand rabbits. The animals were evenly divided into groups 1 (toroidal HIFU ablation), 2 (surgical resection) and 3 (untreated control). Therapeutic responses were evaluated with gross pathology and histology 11 d post-treatment. Toroidal transducer-produced HIFU ablation (average ablation rate 10.5 cc/min) allowed fast and homogeneous tumor treatment. Sonograms showed all ablations. VX2 tumors were completely coagulated and surrounded by safety margins without surrounding-organ secondary HIFU lesions. HIFU group tumor volumes at autopsy (39 mm3) were significantly lower than control group volumes (2610 mm3, p < 0.0001). HIFU group tumor metastasis (27%) was lower than resected (33%) and control (67%) group metastasis. Ultrasound imaging, gross pathology and histology results supported these outcomes. HIFU procedures had no complications. Rabbit liver tumor ablation using a toroidal HIFU transducer under ultrasound imaging guidance might therefore be an effective intra-operative treatment for localized liver metastases.

Imaging of Thermal Effects during High-Intensity Ultrasound Treatment in Liver by Passive Elastography: A Preliminary Feasibility in Vitro Study.

High-intensity focused ultrasound is a non-invasive modality for thermal ablation of tissues through locally increased temperature. Thermal lesions can be monitored by elastography, following the changes in the elastic properties of the tissue as reflected by the shear-wave velocity. Most studies on ultrasound elastography use shear waves created by acoustic radiation force. However, in the human body, the natural noise resulting from cardiac activity or arterial pulsatility can be used to characterize elasticity through noise-correlation techniques, in the method known as passive elastography. The objective of this study was to investigate the feasibility of monitoring high-intensity ultrasound treatments of liver tissue using passive elastography. Bovine livers were heated to 80°C using a high-intensity planar transducer and imaged with a high-frame-rate ultrasound imaging device. The dynamics of lesion formation are captured through tissue stiffening and lesion expansion.