A tissue-mimicking ultrasound test object using droplet vaporization to create point targets.

Ultrasound test objects containing reference point targets could be useful for evaluating ultrasound systems and phase aberration correction methods. Polyacrylamide gels containing albumin-stabilized droplets (3.6 µm mean diameter) of dodecafluoropentane (DDFP) are being developed for this purpose. Perturbation by ultrasound causes spontaneous vaporization of the superheated droplets to form gas bubbles, a process termed acoustic droplet vaporization (ADV). The resulting bubbles (20 to 160 µmm diameter) are small compared with acoustic wavelengths in diagnostic ultrasound and are theoretically suitable for use as point targets (phase errors < 20° for typical f-numbers). Bubbles distributed throughout the material are convenient for determining the point spread function in an imaging plane or volume. Cooling the gel causes condensation of the DDFP droplets, which may be useful for storage. Studying ADV in such viscoelastic media could provide insight into potential bioeffects from rapid bubble formation.

Real-time FDG PET guidance during biopsies and radiofrequency ablation using multimodality fusion with electromagnetic navigation.

PURPOSE: To assess the feasibility of combined electromagnetic device tracking and computed tomography (CT)/ultrasonography (US)/fluorine 18 fluorodeoxyglucose (FDG) positron emission tomography (PET) fusion for real-time feedback during percutaneous and intraoperative biopsies and hepatic radiofrequency (RF) ablation. MATERIALS AND METHODS: In this HIPAA-compliant, institutional review board-approved prospective study with written informed consent, 25 patients (17 men, eight women) underwent 33 percutaneous and three intraoperative biopsies of 36 FDG-avid targets between November 2007 and August 2010. One patient underwent biopsy and RF ablation of an FDG-avid hepatic focus. Targets demonstrated heterogeneous FDG uptake or were not well seen or were totally inapparent at conventional imaging. Preprocedural FDG PET scans were rigidly registered through a semiautomatic method to intraprocedural CT scans. Coaxial biopsy needle introducer tips and RF ablation electrode guider needle tips containing electromagnetic sensor coils were spatially tracked through an electromagnetic field generator. Real-time US scans were registered through a fiducial-based method, allowing US scans to be fused with intraprocedural CT and preacquired FDG PET scans. A visual display of US/CT image fusion with overlaid coregistered FDG PET targets was used for guidance; navigation software enabled real-time biopsy needle and needle electrode navigation and feedback. RESULTS: Successful fusion of real-time US to coregistered CT and FDG PET scans was achieved in all patients. Thirty-one of 36 biopsies were diagnostic (malignancy in 18 cases, benign processes in 13 cases). RF ablation resulted in resolution of targeted FDG avidity, with no local treatment failure during short follow-up (56 days). CONCLUSION: Combined electromagnetic device tracking and image fusion with real-time feedback may facilitate biopsies and ablations of focal FDG PET abnormalities that would be challenging with conventional image guidance.

Clinical utility of real-time fusion guidance for biopsy and ablation.

PURPOSE: To show utility, accuracy, and clinical outcomes of electromagnetic tracking and multimodality image fusion for guidance of biopsy and radiofrequency (RF) ablation procedures. MATERIALS AND METHODS: A combination of conventional image guidance (ultrasound[US]/computed tomography [CT]) and a research navigation system was used in 40 patients undergoing biopsy or RF ablation to assist in target localization and needle and electrode placement. The navigation system displays electromagnetically tracked needles and US images relative to a preprocedural CT scan. Additional images (prior positron emission tomography [PET] or magnetic resonance [MR] imaging) can be fused with CT as needed. Needle aiming with and without tracking were compared, the utility of navigation for each procedure was assessed, the system's off-target tracking error for two different registration methods was evaluated, and setup time was recorded. RESULTS: The tracking error could be evaluated in 35 of 40 patients. A basic tracking error of 3.8 mm ± 2.3 was shown using skin fiducial markers for registration. The error improved to 2.7 mm ± 1.6 when using prior internal needle positions as additional fiducial markers. Real-time fusion of US with CT and registration with prior PET and MR imaging were successful and provided clinically relevant guidance information, enabling 19 of the 40 procedures. CONCLUSIONS: The spatial accuracy of the navigation system is sufficient to display clinically relevant image guidance information during biopsy and RF ablation. Breath holding and respiratory gating are effective in minimizing the error associated with tissue motion. In 48% of cases, the navigation system provided information crucial for successful execution of the procedure. Fusion of real-time US with CT or prior diagnostic images may enable procedures that are not feasible with standard, single-modality image guidance.

Suspicious breast lesions: assessment of 3D Doppler US indexes for classification in a test population and fourfold cross-validation scheme.

PURPOSE: To assess the diagnostic performance of various Doppler ultrasonographic (US) vascularity measures in conjunction with grayscale (GS) criteria in differentiating benign from malignant breast masses, by using histologic findings as the reference standard. MATERIALS AND METHODS: Institutional Review Board and HIPAA standards were followed. Seventy-eight women (average age, 49 years; range, 26-70 years) scheduled for breast biopsy were included. Thirty-eight patient scans were partially analyzed and published previously, and 40 additional scans were used as a test set to evaluate previously determined classification indexes. In each patient, a series of color Doppler images was acquired and reconstructed into a volume encompassing a suspicious mass, identified by a radiologist-defined ellipsoid, in which six Doppler vascularity measures were calculated. Radiologist GS ratings and patient age were also recorded. Multivariable discrimination indexes derived from the learning set were applied blindly to the test set. Overall performance was also confirmed by using a fourfold cross-validation scheme on the entire population. RESULTS: By using all cases (46 benign, 32 malignant), the area under the receiver operating characteristic curve (A(z)) values confirmed results of previous analyses: Speed-weighted pixel density (SWPD) performed the best as a diagnostic index, although statistical significance (P = .01) was demonstrated only with respect to the normalized power-weighted pixel density. In both learning and test sets, the three-variable index (SWPD-age-GS) displayed significantly better diagnostic performance (A(z) = 0.97) than did any single index or the one two-variable index (age-GS) that could be obtained without the data from the Doppler scan. Results of the cross validation confirmed the trends in the two data sets. CONCLUSION: Quantitative Doppler US vascularity measurements considerably contribute to malignant breast tissue identification beyond subjective GS evaluation alone. The SWPD-age-GS index has high performance (A(z) = 0.97), regardless of incidental performance variations in its single variable components.

Electromagnetic tracking for thermal ablation and biopsy guidance: clinical evaluation of spatial accuracy.

PURPOSE: To evaluate the spatial accuracy of electromagnetic needle tracking and demonstrate the feasibility of ultrasonography (US)-computed tomography (CT) fusion during CT- and US-guided biopsy and radiofrequency ablation procedures. MATERIALS AND METHODS: The authors performed a 20-patient clinical trial to investigate electromagnetic needle tracking during interventional procedures. The study was approved by the institutional investigational review board, and written informed consent was obtained from all patients. Needles were positioned by using CT and US guidance. A commercial electromagnetic tracking device was used in combination with prototype internally tracked needles and custom software to record needle positions relative to previously obtained CT scans. Position tracking data were acquired to evaluate the tracking error, defined as the difference between tracked needle position and reference standard needle position on verification CT scans. Registration between tracking space and image space was obtained by using reference markers attached to the skin ("fiducials"), and different registration methods were compared. The US transducer was tracked to demonstrate the potential use of real-time US-CT fusion for imaging guidance. RESULTS: One patient was excluded from analysis because he was unable to follow breathing instructions during the acquisition of CT scans. Nineteen of the 20 patients were evaluable, demonstrating a basic tracking error of 5.8 mm +/- 2.6, which improved to 3.5 mm +/- 1.9 with use of nonrigid registrations that used previous internal needle positions as additional fiducials. Fusion of tracked US with CT was successful. Patient motion and distortion of the tracking system by the CT table and gantry were identified as sources of error. CONCLUSIONS: The demonstrated spatial tracking accuracy is sufficient to display clinically relevant preprocedural imaging information during needle-based procedures. Virtual needles displayed within preprocedural images may be helpful for clandestine targets such as arterial phase enhancing liver lesions or during thermal ablations when obscuring gas is released. Electromagnetic tracking may help improve imaging guidance for interventional procedures and warrants further investigation, especially for procedures in which the outcomes are dependent on accuracy.

Evaluating thin compression paddles for mammographically compatible ultrasound.

We are developing a combined digital mammography/3D ultrasound system to improve detection and/or characterization of breast lesions. Ultrasound scanning through a mammographic paddle could significantly reduce signal level, degrade beam focusing and create reverberations. Thus, appropriate paddle choice is essential for accurate sonographic lesion detection and assessment with this system. In this study, we characterized ultrasound image quality through paddles of varying materials (lexan, polyurethane, TPX, mylar) and thicknesses (0.25 to 2.5 mm). Analytical experiments focused on lexan and TPX, which preliminary results demonstrated were most competitive. Spatial and contrast resolution, side-lobe and range lobe levels, contrast and signal strength were compared with no-paddle images. When the beamforming of the system was corrected to account for imaging through the paddle, the TPX 2.5 mm paddle performed the best. Test objects imaged through this paddle demonstrated < or = 15% reduction in spatial resolution, < or = 7.5 dB signal loss, < or = 3 dB contrast loss and range lobe levels > or = 35 dB below signal maximum over 4 cm. TPX paddles <2.5 mm could also be used with this system, depending on imaging goals. In 10 human subjects with cysts, small CNR losses were observed but were determined to be statistically insignificant. Radiologists concluded that 75% of cysts in through-paddle scans were at least as detectable as in their corresponding direct-contact scans.

Sound speed estimation using automatic ultrasound image registration.

A mismatch between the sound speed assumed for beamforming and scan conversion and the true sound speed in the tissue to be imaged can lead to significant defocusing and some geometric distortions in ultrasound images. A method is presented for estimating the average sound speed based on detection of these distortions using automatic registration of overlapping, electronically steered images. An acrylamide gel phantom containing vaporized dodecafluoropentane droplets as point targets was constructed to evaluate the technique. Good agreement (rms deviation <0.4%) was found between the sound speeds measured in the phantom using a reference pulse-echo technique and the image-based sound speed estimates. A significant improvement in accuracy (rms deviation <0.1%) was achieved by including the simulated sound field of the probe rather than assuming straight acoustic beams and propagation according to ray acoustics.

Rapid elastic image registration for 3-D ultrasound.

A Subvolume-based algorithm for elastic Ultrasound REgistration (SURE) was developed and evaluated. Designed primarily to improve spatial resolution in three-dimensional compound imaging, the algorithm registers individual image volumes nonlinearly before combination into compound volumes. SURE works in one or two stages, optionally using MIAMI Fuse software first to determine a global affine registration before iteratively dividing the volume into subvolumes and computing local rigid registrations in the second stage. Connectivity of the entire volume is ensured by global interpolation using thin-plate splines after each iteration. The performance of SURE was quantified in 20 synthetically deformed in vivo ultrasound volumes, and in two phantom scans, one of which was distorted at acquisition by placing an aberrating layer in the sound path. The aberrating layer was designed to induce beam aberrations reported for the female breast. Synthetic deformations of 1.5-2.5 mm were reduced by over 85% when SURE was applied to register the distorted image volumes with the original ones. Registration times were below 5 min on a 500-MHz CPU for an average data set size of 13 MB. In the aberrated phantom scans, SURE reduced the average deformation between the two volumes from 1.01 to 0.30 mm. This was a statistically significant (P = 0.01) improvement over rigid and affine registration transformations, which produced reductions to 0.59 and 0.50 mm, respectively.