Gelatin Phantom Research Articles

Pulse inversion therapy for improved monitoring of blood-brain barrier opening

Microbubble-based ultrasound therapy has enabled non-invasive and reversible opening of the blood-brain barrier (BBB). However, the skull limits our ability to monitor microbubble activity due to high attenuation and beam aberrations. In ultrasound imaging, pulse inversion is used to cancel echoes from linear scatterers by summing the signal obtained from consecutive positive (phase: 0o degrees) and negative (phase: 180o degrees) pulses, thus facilitating imaging of non-linear scatterers such as microbubbles. Here, we adapt the pulse inversion technique to improve monitoring of BBB opening, by transmitting consecutive therapeutic pulses of inverse polarity. Pulse inversion therapy (PIT) was achieved by synchronizing the emission of inverted short pulses (pulse length: 2–3 cycles, PRF: 2 kHz, and pressure: 400 kPa) through a focused 0.5 MHz therapeutic transducer driven by two function generators. A concentric P12-5 linear array was used to passively capture the microbubble emissions. Absolute time-of-flight information was introduced in the beamforming since emission and reception were synchronous. PIT suppressed the signals from linear scatterers within the focal region by up to 6 dB in a gelatine phantom containing microbubbles and in mice in vivo, compared to positive-only pulses. Ongoing in vivo work aims at correlating the BBB opening with the microbubble signal identified by PIT.

P12. - Validation of a contrast orientated algorithm to delineate the target volume from FDG-PET/CT images using a novel gelatin phantom

P12. - Validation of a contrast orientated algorithm to delineate the target volume from FDG-PET/CT images using a novel gelatin phantom

Ultrasonic method of microvibration detection: part II - additional processing method and applications

Introduction In the last 28 years, the scientific community has been using elastography to evaluate the mechanical properties of biological tissue. The aim of this work was the optimization of the UDmV method, presented in Part I of the series, by means of modifying the technique employed to generate the reference sine and cosine functions, used for phase-quadrature demodulation, and determining how this modification improved the performance of the method. Additionally, the UDmV was employed to characterize the acoustic and mechanical properties of a 7% gelatin phantom. Methods A focused transducer, TF, with a nominal frequency of 2.25 MHz, was used to induce the shear waves, with frequency of 97.644 Hz. Then, the modified UDmV method was used to extract the phase and quadrature components from ultrasonic RF echo-signals collected from four positions along the propagation path of the shear wave, which allowed the investigation of the medium vibration caused by wave propagation. The phase velocity, cs, and attenuation, αs, of the phantom were measured and employed in the calculation of shear modulus, μ, and viscosity, η. Results The computational simulation demonstrated that the modification in UDmV method resulted in more accurate and precise estimates of the initial phases of the reference sinusoidal functions used for phase-quadrature demodulation. The values for cs and μ of 1.31 ± 0.01 m·s-1 and 1.66 ± 0.01 kPa, respectively, are very close to the values found in the literature (1.32 m·s-1 and 1.61 kPa) for the same material. Conclusion The UDmV method allowed estimating of the acoustic and viscoelastic parameters of phantom.

Improved tactile resonance sensor for robotic assisted surgery

This paper presents an improved tactile sensor using a piezoelectric bimorph able to differentiate soft materials with similar mechanical characteristics. The final aim is to develop intelligent surgical tools for brain tumour resection using integrated sensors in order to improve tissue tumour delineation and tissue differentiation. The bimorph sensor is driven using a random phase multisine and the properties of contact between the sensor's tip and a certain load are evaluated by means of the evaluation of the nonparametric FRF. An analysis of the nonlinear contributions is presented to show that the use of a linear model is feasible for the measurement conditions. A series of gelatine phantoms were tested. The tactile sensor is able to identify minimal differences in the consistency of the measured samples considering viscoelastic behaviour. A variance analysis was performed to evaluate the reliability of the sensors and to identify possible error sources due to inconsistencies in the preparation method of the phantoms. The results of the variance analysis are discussed showing that ability of the proposed tactile sensor to perform high quality measurements.

Photothermal strain imaging.

Vulnerable plaques are the major cause of cardiovascular disease, but they are difficult to detect with conventional intravascular imaging techniques. Techniques are needed to identify plaque vulnerability based on the presence of lipids in plaque. Thermal strain imaging (TSI) is an imaging technique based on ultrasound (US) wave propagation speed, which varies with the medium temperature. In TSI, the strain that occurs during tissue temperature change can be used for lipid detection because it has a different tendency depending on the type of tissue. Here, we demonstrate photothermal strain imaging (pTSI) using an intravascular ultrasound catheter. pTSI is performed by slightly and selectively heating lipid using a relatively inexpensive continuous laser source. We applied a speckle-tracking algorithm to US B-mode images for strain calculations. As a result, the strain produced in porcine fat was different from the strain produced in water-bearing gelatin phantom, which made it possible to distinguish the two. This suggests that pTSI could potentially be a way of differentiating lipids in coronary artery.

Piecewise parabolic method for simulating one-dimensional shear shock wave propagation in tissue-mimicking phantoms

The recent discovery of shear shock wave generation and propagation in the porcine brain suggests that this new shock phenomenology may be responsible for a broad range of traumatic injuries. Blast-induced head movement can indirectly lead to shear wave generation in the brain, which could be a primary mechanism for injury. Shear shock waves amplify the local acceleration deep in the brain by up to a factor of 8.5, which may tear and damage neurons. Currently, there are numerical methods that can model compressional shock waves, such as comparatively well-studied blast waves, but there are no numerical full-wave solvers that can simulate nonlinear shear shock waves in soft solids. Unlike simplified representations, e.g., retarded time, full-wave representations describe fundamental physical behavior such as reflection and heterogeneities. Here we present a piecewise parabolic method-based solver for one-dimensional linearly polarized nonlinear shear wave in a homogeneous medium and with empirical frequency-dependent attenuation. This method has the advantage of being higher order and more directly extendable to multiple dimensions and heterogeneous media. The proposed numerical scheme is validated analytically and experimentally and compared to other shock capturing methods. A Riemann step-shock problem is used to characterize the numerical dissipation. This dissipation is then tuned to be negligible with respect to the physical attenuation by choosing an appropriate grid spacing. The numerical results are compared to ultrasound-based experiments that measure planar polarized shear shock wave propagation in a tissue-mimicking gelatin phantom. Good agreement is found between numerical results and experiment across a 40 mm propagation distance. We anticipate that the proposed method will be a starting point for the development of a two- and three-dimensional full-wave code for the propagation of nonlinear shear waves in heterogeneous media.

Noninvasive measurement of wave speed of porcine cornea in ex vivo porcine eyes for various intraocular pressures

The objective of this study was to extend an ultrasound surface wave elastography (USWE) technique for noninvasive measurement of ocular tissue elastic properties. In particular, we aim to establish the relationship between the wave speed of cornea and the intraocular pressure (IOP). Normal ranges of IOP are between 12 and 22mmHg. Ex vivo porcine eye balls were used in this research. The porcine eye ball was supported by the gelatin phantom in a testing container. Some water was pour into the container for the ultrasound measurement. A local harmonic vibration was generated on the side of the eye ball. An ultrasound probe was used to measure the wave propagation in the cornea noninvasively. A 25 gauge butterfly needle was inserted into the vitreous humor of the eye ball under the ultrasound imaging guidance. The needle was connected to a syringe. The IOP was obtained by the water height difference between the water level in the syringe and the water level in the testing container. The IOP was adjusted between 5mmHg and 30mmHg with a 5mmHg interval. The wave speed was measured at each IOP for three frequencies of 100, 150 and 200Hz. Finite element method (FEM) was used to simulate the wave propagation in the corneal according to our experimental setup. A linear viscoelastic FEM model was used to compare the experimental data. Both the experiments and the FEM analyses showed that the wave speed of cornea increased with IOP.

Computed tomography (CT)-compatible remote center of motion needle steering robot: Fusing CT images and electromagnetic sensor data

Lung cancer is the most common cause of cancer-related death, and early detection can reduce the mortality rate. Patients with lung nodules greater than 10 mm usually undergo a computed tomography (CT)-guided biopsy. However, aligning the needle with the target is difficult and the needle tends to deflect from a straight path. In this work, we present a CT-compatible robotic system, which can both position the needle at the puncture point and also insert and rotate the needle. The robot has a remote-center-of-motion arm which is achieved through a parallel mechanism. A new needle steering scheme is also developed where CT images are fused with electromagnetic (EM) sensor data using an unscented Kalman filter. The data fusion allows us to steer the needle using the real-time EM tracker data. The robot design and the steering scheme are validated using three experimental cases. Experimental Case I and II evaluate the accuracy and CT-compatibility of the robot arm, respectively. In experimental Case III, the needle is steered towards 5 real targets embedded in an anthropomorphic gelatin phantom of the thorax. The mean targeting error for the 5 experiments is 1.78 ± 0.70 mm. The proposed robotic system is shown to be CT-compatible with low targeting error. Small nodule size and large needle diameter are two risk factors that can lead to complications in lung biopsy. Our results suggest that nodules larger than 5 mm in diameter can be targeted using our method which may result in lower complication rate.

A machine learning alternative to model-based elastography

Model-based elastography methods suffer severe limitations in imaging the complex mechanical behavior of real biological tissues. We adapted the Autoprogressive Method (AutoP) to address these limitations by approaching the inverse problem with machine learning tools. AutoP combines finite element analysis (FEA) and artificial neural networks (ANNs) with force and displacement measurements to develop soft-computational models of mechanical behavior. Unlike model-based elastography methods, only measurement data inform the material properties learned by the ANNs. Because this machine learning approach foregoes the initial model assumption, AutoP can be applied to anisotropic, time-varying, and nonlinear media common in biomedical imaging applications. We first implemented AutoP to characterize linear-elastic gelatin phantoms and ex vivo rabbit kidneys to demonstrate the potential for medical imaging. Those models required an estimate of the interior geometry via segmentation of the B-mode images. In our current work, the capabilities of AutoP are extended by developing a novel ANN architecture. Incorporating spatial information as part of the input to a pair of ANNs working in tandem allows the models to learn the spatially varying mechanical behavior, thus precluding the segmentation requirement. We will demonstrate this new approach to elasticity imaging by presenting elastograms generated by trained ANN material models.

Flash focus ultrasonic images sequences for shear shock wave observation in the brain

Nonlinear shear waves have a cubic nonlinearity which results in the generation of a unique characteristic odd harmonic signature. This behavior was first observed in a homogeneous gelatin phantom using ultrafast plane wave ultrasound imaging and a correlation-based tracking algorithm to determine particle motion. However, in heterogeneous tissue, like brain, the heterogeneities generate clutter that degrades motion tracking to the point where the shock waves and their characteristic odd harmonics are no longer observable. We present a high frame-rate ultrasound imaging sequence consisting of multiple focused emissions that improves the image quality and reduces clutter to generate high quality motion estimates of shear shock waves propagating in the brain. A point spread function analysis is used to characterize the improvements of the proposed imaging sequence. It is shown that the flash focus sequence reduces the side lobes by 20 dB while retaining the same spatial resolution translating to a sensitivity up to the 11th harmonic. The flash focus sequence are then used to acquire high frame-rate (6500 fps) ultrasound movies of an ex-vivo porcine brain in which a shear wave propagates. Using an adaptive tracking algorithm, we compute the particle velocity in a field of view as deep as the brain. It is therefore demonstrated that the proposed method can detect the nonlinear elastic motion and the odd harmonics with sufficient sensitivity to observe the development of a shear wave into a shock wave as it propagates in the brain.

Comparison analysis of four processing methods employed in dynamic elastography to estimate viscoelastic parameters of a medium: tests using computational simulation and experiment

Dynamic ultrasound elastography is a method commonly used to quantify the mechanical parameters, such as shear elasticity modulus, and shear viscosity modulus, of biological tissues. Basically, the method employs four processing techniques, in order to estimate the mentioned parameters, denominated: phase velocity dispersion, PVD, attenuation-velocity, AV, complex shear elasticity modulus, CSM, and shear wave velocity, SV. The accuracy and precision of estimated values for and depend on the processing technique and the purpose of this paper is to compare the results obtained with PVD, AV, CSM and SV processing techniques by means of two cases: computer simulation and experiments. In the first case, the processing techniques were evaluated, via simulation, at different conditions for the signal-to-noise-ratio of the ultrasound RF echo signal used to probe the medium with vibrations due to the propagation of shear wave, the vibration amplitude and the viscoelastic properties of the medium itself. The corresponding results are expressed as the magnitudes of the mean and standard deviation of the relative error. In the second case, experiments were conducted based on a dynamic elastographic system with a cylindrical shear wave induced by a repetitive acoustic radiation force, with pulse repetition frequency of 50 Hz. A 4.98 MHz pulse-echo ultrasound system was used to probe the medium, a 3% gelatin phantom, traversed by the shear wave and the received RF echo signals were processed using the four techniques to yield the phantom values of and The results from computer simulation indicate that, in general, the CSM and AV techniques presented highest accuracy and precision. The experimental results obtained in this study are consistent with the values reported in the literature and the values of the shear elasticity modulus estimated by the four processing techniques did not differ significantly.

PROSPECTS OF APPLICATION OF ORIGINAL MULTIROBOTIC SYSTEM FOR BRACHYTHERAPY OF PROSTATE CANCER

The authors analyzed the positioning error for branchytherapy needle inserted with original robotic system «Oncorobot» (Central Research Institute of Robotic And Technical Cybernetics, St. Petersburg) compared with traditional methods of insertion and other technical systems. The gelatin phantom of prostate gland was used for experiments. Needle insertion robotic program was written according to coordinates of dosimetric plan of microsourse localization. A coordinate of the needle tip location was measured after each injection of the needle into phantom and the mean-square error was calculated. It was shown that needle tip positioning error wasn’t more than 0,5 mm at insertion speeds from 3,0 to 30,00 mm/sec. Positioning error enlarged with increase of insertion speed. Needle positioning accuracy of insertion was significantly higher in case of developed robotic system, than in manual brachytherapy. The accuracy indices of the system were correlated with other described robotic systems.

Elastography for Intravascular Ultrasound

The ability to measure the elastic properties of plaques and vessels would be useful in clinical diagnoses, particularly for detecting a vulnerable plaque. This study demonstrates the feasibility of intravascular ultrasound (IVUS) elasticity imaging—combining acoustic radiation force impulse (ARFI) imaging and shear-wave elasticity imaging (SWEI)—for detecting the distribution of stiffness within atherosclerotic arteries in vitro. A dual-frequency IVUS transducer with two elements was used to induce the propagation of a shear wave (by the 8.5-MHz pushing element), which could be simultaneously monitored by the 31-MHz imaging element. The ARFI and SWEI images were reconstructed by measuring the quantified values of peak displacements and the wave velocity of shear wave propagation, respectively. System performance was verified using gelatin phantoms. The phantom results demonstrate that the small stiffness changes with difference of shear modulus of 1.6 kPa can be detected in both ARFI and SWEI images. Moreover, stiffness distributions of atherosclerotic aortas from rabbits were obtained, for which the peak displacement and the shear-wave velocity were 2.6∼4.3 μm and 0.26∼0.32 m/s, respectively, in lipid-rich plaques, and 0.8∼1.2 μm and 3.28∼3.45 m/s in the arterial walls. These results indicate that IVUS elasticity imaging can be used to distinguish the elastic properties of plaques and vessels.

Experimental MRI-SPECT insert system with Hybrid Semiconductor detectors Timepix for MR animal scanner Bruker 47/20

Multimodal medical imaging based on Magnetic Resonance is mainly combinated with one of the scintigraphic method like PET or SPECT. These methods provide functional information whereas magnetic resonance imaging provides high spatial resolution of anatomical information or complementary functional information. Fusion of imaging modalities allows researchers to obtain complimentary information in a single measurement. The combination of MRI with SPECT is still relatively new and challenging in many ways. The main complication of using SPECT in MRI systems is the presence of a high magnetic field therefore (ferro)magnetic materials have to be eliminated. Furthermore the application of radiofrequency fields within the MR gantry does not allow for the use of conductive structures such as the common heavy metal collimators. This work presents design and construction of an experimental MRI-SPECT insert system and its initial tests. This unique insert system consists of an MR-compatible SPECT setup with CdTe pixelated sensors Timepix tungsten collimators and a radiofrequency coil. Measurements were performed on a gelatine and tissue phantom with an embedded radioisotopic source (57Co 122 keV γ ray) inside the RF coil by the Bruker BioSpec 47/20 (4.7 T) MR animal scanner. The project was performed in the framework of the Medipix Collaboration.

Ultrasound-guided central line placement: is a gelatine phantom a good and affordable alternative?

Ultrasound-guided central line placement: is a gelatine phantom a good and affordable alternative?

Impedance and admittance control for respiratory-motion compensation during robotic needle insertion - a preliminary test.

Many robotic needle-biopsy systems have been developed to enhance the accuracy of needle-biopsy intervention. These systems can reduce the intervention time and the radiation exposure of clinicians. However, respiratory-motion compensation is needed to ensure the accuracy and efficiency of needle biopsy intervention. Human respiratory-motion data were acquired using three inertial measurement units (IMUs), and respiratory motion was simulated using the Stewart-Gough platform. Robotic needle intervention was performed using impedance and admittance control algorithms for respiratory-motion compensation using the Stewart-Gough platform and a gelatin phantom. The impedance and admittance control algorithms can be used to compensate for respiratory motion during robotic needle insertion. The admittance control algorithm exhibits better performance than the impedance control algorithm. The impedance and admittance control algorithms can be applied for respiratory-motion compensation during robotic needle insertion. However, further study is needed for them to become clinically feasible.

A Real-Time Estimator for Needle Deflection During Insertion Into Soft Tissue Based on Adaptive Modeling of Needle–Tissue Interactions

This work proposes a real-time estimator for needle tip deflection and needle shape during needle insertion into soft tissue. The estimator is based on an adaptive quasi-static mechanics-based model for needle–tissue interactions. The model uses Euler–Bernoulli beam theory to model the needle as a cantilever beam that experiences loads imposed by the tissue. The modeled needle–tissue interactions consist of a distributed load along the inserted needle portion and tissue cutting-related point load at the needle tip. We propose a closed-form solution to quantify the magnitude of these needle–tissue interaction loads based on force and torque measured at the needle base. The model adaptively adjusts the shape of the distributed load as the needle is inserted. Experiments are carried out into gelatin phantom and porcine tissue to validate the deflection estimate's performance. The newly proposed model's performance is compared against a previously proposed quasi-static model for needle-deflection estimation. It is shown that the novel model outperforms the previously proposed model.

Time-resolved photoacoustic measurement for evaluation of viscoelastic properties of biological tissues

In this letter, we proposed a method for viscoelastic characterization of biological tissues based on time-resolved photoacoustic measurement. The theoretical and experimental study was performed on the influence of viscoelasticity effects on photoacoustic generation. Taking the time delay between the photoacoustic signal and the exciting laser, the viscoelasticity distribution of biological tissues can be mapped. To validate our method, gelatin phantoms with different densities were measured. We also applied this method in discrimination between fat and liver to confirm the usefulness of the viscoelastic evaluation. Furthermore, pilot experiments were performed on atherosclerosis artery from an apolipoprotein E-knockout mouse to show the viscoelastic characterization of atherosclerotic plaque. Our results demonstrate that this technique has the potential for visualizing the biomechanical properties and lesions of biological tissues.

K-space data processing for magnetic resonance elastography (MRE).

Magnetic resonance elastography (MRE) requires substantial data processing based on phase image reconstruction, wave enhancement, and inverse problem solving. The objective of this study is to propose a new, fast MRE method based on MR raw data processing, particularly adapted to applications requiring fast MRE measurement or high elastogram update rate. The proposed method allows measuring tissue elasticity directly from raw data without prior phase image reconstruction and without phase unwrapping. Experimental feasibility is assessed both in a gelatin phantom and in the liver of a porcine model in vivo. Elastograms are reconstructed with the raw MRE method and compared to those obtained using conventional MRE. In a third experiment, changes in elasticity are monitored in real-time in a gelatin phantom during its solidification by using both conventional MRE and raw MRE. The raw MRE method shows promising results by providing similar elasticity values to the ones obtained with conventional MRE methods while decreasing the number of processing steps and circumventing the delicate step of phase unwrapping. Limitations of the proposed method are the influence of the magnitude on the elastogram and the requirement for a minimum number of phase offsets. This study demonstrates the feasibility of directly reconstructing elastograms from raw data.

Real-time electromagnetic seed drop detection for permanent implants brachytherapy: Technology overview and performance assessment.

To describe the principles and report on the performance of a novel real-time electromagnetic (EM) seed drop detection technology for permanent implants brachytherapy procedures. A novel EM hollow needle prototype was recently developed by Philips. It possesses standard 3D tracking capability as well as a seed drop detection mechanism, both performed from a single custom built EM sensor. The detection mechanism is based on the magnetic permeability changes in the sensor as the seeds pass through. Drop position estimates are generated by the tracking information at the dropping instants. Three validation experiments were carried out in this study. First, the robustness of the detection mechanism was tested in free air with four different seed types. Detection waveforms were measured and commented. The accuracy of the seed drop position estimates was then evaluated using both 2D and 3D experiments. The procedures consisted of dropping seeds in phantoms, recording the drop position estimates, and finally registering the resulting spatial distributions on reference ones obtained by accurate modalities. Seeds were dropped on a specially designed plastic support adapted to brachytherapy template dimensions for 2D experiments, and true seed positions (reference distribution) were obtained by optical detection. In 3D experiments, seeds were dropped in edible gelatin and reference distributions were obtained by localizing the implants from CT scans of the phantoms. All four seed types were correctly detected by the needle prototype. In total, 250 seeds were dropped on the plastic support, and 96 were dropped in gelatin phantoms. The detection rate was 100% in both cases. The minimum, maximum, and average drop position errors were, respectively, 0.1(+1.6/ - 0.1), 2.9(+1.4/ - 1.5), and 0.9(+1.4/ - 0.7) mm for 2D, and 0.1(+1.0/ - 0.1), 2.1(+1.1/ - 0.8), and 0.6(+1.2/ - 0.5) mm for 3D experiments. The hollow needle prototype combines both EM tracking and automatic seed drop detection in a compact and convenient form. The EM detection mechanism is robust, and the seed drop position estimates appear sufficiently accurate for potential integration of the technology to current brachytherapy treatment planning systems. In that context, it would serve as a valuable tool for rapid dosimetry validation in real-time treatment delivery.