Hydrogel based tissue mimicking phantom for in-vitro ultrasound contrast agents studies

Ultrasound medical imaging (UMI) is the most widely used image analysis technique, and often requires advanced in-vitro set up to perform morphological and functional investigations. These studies are based on contrast properties both related to tissue structure and injectable contrast agents (CA). In this work, we present a three-dimensional structure composed of two different hydrogels reassembly the microvascular network of a human tissue. This phantom was particularly suitable for the echocontrastographic measurements in human microvascular system. This phantom has been characterized to present the acoustic properties of an animal liver, that is, acoustic impedance (Z) and attenuation coefficient (AC), in UMI signal analysis in particular; the two different hydrogels have been selected to simulate the target organ and the acoustic properties of the vascular system. The two hydrogels were prepared starting from cellulose derivatives to simulating the target organ parenchyma and using a PEG-diacrylate to reproduce the vascular system. Moreover, harmonic analysis was performed on the hydrogel mimicking the liver parenchyma hydrogel to evaluate the ultrasound (US) distortion during echographic measurement. The phantom was employed in the characterization of an experimental US CA. Perfect agreement was found when comparing the hydrogel acoustical properties materials with the corresponding living reference tissues (i.e., vascular and parenchimal tissue).     

A quality assurance phantom for IMRT dose verification

This paper investigates a quality assurance (QA) phantom specially designed to verify the accuracy of dose distributions and monitor units (MU) calculated by clinical treatment planning optimization systems and by the Monte Carlo method for intensity-modulated radiotherapy (IMRT). The QA phantom is a PMMA cylinder of 30 cm diameter and 40 cm length with various bone and lung inserts. A procedure (and formalism) has been developed to measure the absolute dose to water in the PMMA phantom. Another cylindrical phantom of the same dimensions, but made of water, was used to confirm the results obtained with the PMMA phantom. The PMMA phantom was irradiated by 4, 6 and 15 MV photon beams and the dose was measured using an ionization chamber and compared to the results calculated by a commercial inverse planning system (CORVUS, NOMOS, Sewickley, PA) and by the Monte Carlo method. The results show that the dose distributions calculated by both CORVUS and Monte Carlo agreed to within 2% of dose maximum with measured results in the uniform PMMA phantom for both open and intensity-modulated fields. Similar agreement was obtained between Monte Carlo calculations and measured results with the bone and lung heterogeneity inside the PMMA phantom while the CORVUS results were 4% different. The QA phantom has been integrated as a routine QA procedure for the patient's IMRT dose verification at Stanford since 1999.     

Determination of heterogeneous thermal parameters using ultrasound induced heating and MR thermal mapping

In this paper, a method for the determination of spatially varying thermal conductivity and perfusion coefficients of tissue is proposed. The temperature evolution in tissue is modelled with the Pennes bioheat equation. The main motivation here is a model-based optimal control for ultrasound surgery, in which the tissue properties are needed when the treatment is planned. The overview of the method is as follows. The same ultrasound transducers, which are eventually used for the treatment, are used to inflict small temperature changes in tissue. This temperature evolution is monitored using MR thermal imaging, and the tissue properties are then estimated on the basis of these measurements. Furthermore, an approach to choose transducer excitations for the determination procedure is also considered. The purpose of this paper is to introduce a method and therefore simulations are used to verify the method. Furthermore, computations are accomplished in a 2D spatial domain.     

A deformable phantom for 4D radiotherapy verification: design and image registration evaluation

Motion of thoracic tumors with respiration presents a challenge for three-dimensional (3D) conformal radiation therapy treatment. Validation of techniques aimed at measuring and minimizing the effects of respiratory motion requires a realistic deformable phantom for use as a gold standard. The purpose of this study was to develop and study the characteristics of a reproducible, tissue equivalent, deformable lung phantom. The phantom consists of a Lucite cylinder filled with water containing a latex balloon stuffed with dampened natural sponges. The balloon is attached to a piston that mimics the human diaphragm. Nylon wires and Lucite beads, emulating vascular and bronchial bifurcations, were uniformly glued at various locations throughout the sponges. The phantom is capable of simulating programmed irregular breathing patterns with varying periods and amplitudes. A tissue equivalent tumor, suitable for holding radiochromic film for dose measurements was embedded in the sponge. To assess phantom motion, eight 3D computed tomography data sets of the static phantom were acquired for eight equally spaced positions of the piston. The 3D trajectories of 12 manually chosen point landmarks and the tumor center-of-mass were studied. Motion reproducibility tests of the deformed phantom were established on seven repeat scans of three different states of compression. Deformable image registration (DIR) of the extreme breathing phases was performed. The accuracy of the DIR was evaluated by visual inspection of image overlays and quantified by the distance-to-agreement (DTA) of manually chosen point landmarks and triangulated surfaces obtained from 3D contoured structures. In initial tests of the phantom, a 20-mm excursion of the piston resulted in deformations of the balloon of 20 mm superior-inferior, 4 mm anterior-posterior, and 5 mm left-right. The change in the phantom mean lung density ranged from 0.24 (0.12 SD) g/cm3 at peak exhale to 0.19 (0.12 SD) g/cm3 at peak inhale. The SI displacement of the landmarks varied between 94% and 3% of the piston excursion for positions closer and farther away from the piston, respectively. The reproducibility of the phantom deformation was within the image resolution (0.7 x 0.7 x 1.25 mm3). Vector average registration accuracy based on point landmarks was found to be 0.5 (0.4 SD) mm. The tumor and lung mean 3D DTA obtained from triangulated surfaces were 0.4 (0.1 SD) mm and 1.0 (0.8 SD) mm, respectively. This phantom is capable of reproducibly emulating the physically realistic lung features and deformations and has a wide range of potential applications, including four-dimensional (4D) imaging, evaluation of deformable registration accuracy, 4D planning and dose delivery.     

Helical antenna arrays for interstitial microwave thermal therapy for prostate cancer: tissue phantom testing and simulations for treatment

Interstitial microwave therapy is an experimental treatment for prostate cancer. The objective of this work was to measure the power deposition (specific absorption rate, SAR) patterns of helical microwave antennae both individually and in array patterns that would be useful for clinical treatment protocols. Commercial helical antenna 3D SAR patterns were measured in muscle equivalent phantoms using a thermographic technique. Two array patterns were tested: a 'square' and a 'crescent' array, both surrounding the urethra. To assess the feasibility of pre-treatment planning, the measured SAR patterns were input to a treatment planning computer simulation program based on a series of trans-rectal ultrasound images from a prostate cancer patient. The simulation solved the Pennes linear bioheat heat transfer equation in prostate tissue, with the aim of achieving a target of 55 degrees C at the prostate periphery while not allowing normal surrounding tissues (bladder, urethra, rectum) to rise above 42 C. These criteria could not be met with the square array but they could be met with the crescent array, provided that the prostate was first dissected away from the rectum. This can be done with a procedure such as 'hydrodissection', where sterile saline is injected to separate the prostate and rectum. The results of these SAR measurements and heat transfer simulations indicate that arrays of helical antennae could be used for safe and effective thermal therapy for prostate cancer.     

A cadaveric breast cancer tissue phantom for phase-contrast X-ray imaging applications

Background : As mammography X-ray imaging technologies advance and provide elevated contrast in soft tissues, a need has developed for reliable imaging phantoms for use in system design and component calibration. In advanced imaging modalities such as refraction-based methods, it is critical that developed phantoms capture the biological details seen in clinical precancerous and cancerous cases while minimizing artifacts that may be caused due to phantom production. This work presents the fabric...     

X-ray properties of an anthropomorphic breast phantom for MRI and x-ray imaging

The purpose of this study is to characterize the x-ray properties of a dual-modality, anthropomorphic breast phantom whose MRI properties have been previously evaluated. The goal of this phantom is to provide a platform for optimization and standardization of two- and three-dimensional x-ray and MRI breast imaging modalities for the purpose of lesion detection and discrimination. The phantom is constructed using a mixture of lard and egg whites, resulting in a variable, tissue-mimicking structure with separate adipose- and glandular-mimicking components. The phantom can be produced with either a compressed or uncompressed shape. Mass attenuation coefficients of the phantom materials were estimated using elemental compositions from the USDA National Nutrient Database for Standard Reference and the atomic interaction models from the Monte Carlo code PENELOPE and compared with human values from the literature. The image structure was examined quantitatively by calculating and comparing spatial covariance matrices of the phantom and patient mammography images. Finally, a computerized version of the phantom was created by segmenting a computed tomography scan and used to simulate x-ray scatter of the phantom in a mammography geometry. Mass attenuation coefficients of the phantom materials were within 20% and 15% of the values for adipose and glandular tissues, respectively, which is within the estimation error of these values. Matching was improved at higher energies (>20 keV). Tissue structures in the phantom have a size similar to those in the patient data, but are slightly larger on average. Correlations in the patient data appear to be longer than those in the phantom data in the anterior-posterior direction; however, they are within the error bars of the measurement. Simulated scatter-to-primary ratio values of the phantom images were as high as 85% in some areas and were strongly affected by the heterogeneous nature of the phantom. Key physical x-ray properties of the phantom have been quantitatively evaluated and shown to be comparable to those of breast tissue. Since the MRI properties of the phantom have been previously evaluated, we believe it is a useful tool for quantitative evaluation of two- and three-dimensional x-ray and MRI breast imaging modalities for the purpose of lesion detection and characterization.     

Dynamic phantom with two stage-driven absorbers for mimicking hemoglobin changes in superficial and deep tissues

In near-infrared spectroscopy (NIRS) for monitoring brain activity and cerebral functional connectivity, the effect of superficial tissue on NIRS signals needs to be considered. Although some methods for determining the effect of scalp and brain have been proposed, direct validation of the methods has been difficult because the actual absorption changes cannot be known. In response to this problem, we developed a dynamic phantom that mimics hemoglobin changes in superficial and deep tissues, thus allowing us to experimentally validate the methods. Two absorber layers are independently driven with two one-axis automatic stages. We can use the phantom to design any type of waveform (e.g., brain activity or systemic fluctuation) of absorption change, which can then be reproducibly measured. To determine the effectiveness of the phantom, we used it for a multiple source-detector distance measurement. We also investigated the performance of a subtraction method with a short-distance regressor. The most accurate lower-layer change was obtained when a shortest-distance channel was used. Furthermore, when an independent component analysis was applied to the same data, the extracted components were in good agreement with the actual signals. These results demonstrate that the proposed phantom can be used for evaluating methods of discriminating the effects of superficial tissue.     

A test material for tissue characterisation and system calibration in MRI

A tissue-equivalent test material for MRI has been produced from a polysaccharide gel, agarose, containing gadolinium chloride chelated to EDTA. By varying the amounts of each constituent, the T1 and T2 of the material can be varied independently. As a result, the entire range of in vivo tissue relaxation times can be covered. Through the mathematical modelling of the 1H relaxation theories for both the gel and chelated paramagnetic ion, it has been possible to create a material with relaxation properties and behaviour predictable as functions of both the Larmor frequency and temperature. The similarity of the material to in vivo tissues, in terms of its biological and physical NMR characteristics, makes it an excellent tissue-equivalent substance, in addition to being an accurate calibration standard for routine MRI.     

Semianalytical thermal model for subablative laser heating of homogeneous nonperfused biological tissue: application to laser thermokeratoplasty

We present a semianalytical technique to calculate the temperature in homogeneous nonperfused tissue during subablative laser heating. Analytical expressions of the temperature distribution in time and space are provided for collimated beams with Gaussian and top-hat intensity distributions perpendicularly incident on a finite tissue slab. The temperature distribution produced with a collimated Gaussian beam is the triple sum of the product of four functions of separate variables. The semianalytical technique can be used to rapidly calculate the temperature in laser-irradiated tissue at any point in time and space. The model was used to estimate the corneal temperature during pulsed holmium:YAG laser thermokeratoplasty with various boundary conditions at the anterior and posterior corneal surface. The model demonstrates that the corneal temperature during laser thermokeratoplasty (LTK) with a pulsed Ho:YAG laser may be sufficient to induce superficial vaporization of epithelial cells and local thermal damage to the endothelium. The calculations show that convection at the anterior corneal surface does not have a significant effect on the corneal temperature distribution, but that a better knowledge of the cooling effect of the aqueous is required to better estimate the corneal temperature distribution during LTK.     

Feasibility of salvage interstitial microwave thermal therapy for prostate carcinoma following failed brachytherapy: studies in a tissue equivalent phantom

Thermal therapy is an experimental treatment to destroy solid tumours by heating them to temperatures ranging from 55 degrees C to 90 degrees C, inducing thermal coagulation and necrosis of the tumour. We are investigating the feasibility of interstitial microwave thermal therapy as a salvage treatment for prostate cancer patients with local recurrence following failed brachytherapy. Due to the electrical and thermal conductivity of the brachytherapy seeds, we hypothesized that the seeds could scatter the microwave energy and cause unpredictable heating. To investigate this, a 915 MHz helical antenna was inserted into a muscle-equivalent phantom with and without brachytherapy seeds. Following a 10 W, 5 s input to the antenna, the temperature rise was used to calculate absorbed power, also referred to as specific absorption rate (SAR). Plane wave models based on Maxwell's equations were also used to characterize the electromagnetic scattering effect of the seeds. In addition, the phantom was heated with 8 W for 5 min to quantify the effect of the seeds on the temperature distribution during extended heating. SAR measurements indicated that the seeds had no significant effect on the shape and size of the SAR pattern of the antenna. However, the plane wave simulations indicated that the seeds could scatter the microwave energy resulting in hot spots at the seed edges. Lack of experimental evidence of these hot spots was probably due to the complex polarization of the microwaves emitted by the helical antenna. Extended heating experiments also demonstrated that the seeds had no significant effect on the temperature distributions and rates of temperature rise measured in the phantom. The results indicate that brachytherapy seeds are not a technical impediment to interstitial microwave thermal therapy as a salvage treatment following failed brachytherapy.     

T1 and T2 temperature dependence of female human breast adipose tissue at 1.5 T: groundwork for monitoring thermal therapies in the breast

The T₁ and T₂ temperature dependence of female breast adipose tissue was investigated at 1.5 T in order to evaluate the applicability of relaxation‐based MR thermometry in fat for the monitoring of thermal therapies in the breast. Relaxation times T₁, T₂ and T_2TSE (the apparent T₂ measured using a turbo spin echo readout sequence) were measured in seven fresh adipose breast samples for temperatures from 25 to 65 °C. Spectral water suppression was used to reduce the influence of the residual water signal. The temperature dependence of the relaxation times was characterized. The expected maximum temperature measurement errors based on average calibration lines were calculated. In addition, the heating–cooling reversibility was investigated for two samples. The T₁ and T_2TSE temperature (T) dependence could be fitted well with an exponential function of 1/T. A linear relationship between T₂ and temperature was found. The temperature coefficients (mean ± inter‐sample standard deviation) of T₁ and T_2TSE increased from 25 °C (dT₁/dT = 5.35 ± 0.08 ms/°C, dT_2TSE/dT = 3.82 ± 0.06 ms/°C) to 65 °C (dT₁/dT = 9.50 ± 0.16 ms/°C, dT_2TSE/dT = 7.99 ± 0.38 ms/°C). The temperature coefficient of T₂ was 0.90 ± 0.03 ms/°C. The temperature‐induced changes in the relaxation times were found to be reversible after heating to 65 °C. Given the small inter‐sample variation of the temperature coefficients, relaxation‐based MR thermometry appears to be feasible in breast adipose tissue, and may be used as an adjunct to proton resonance frequency shift (PRFS) thermometry in aqueous tissue (glandular + tumor). Copyright © 2015 John Wiley & Sons, Ltd.     

A new test phantom with different breast tissue compositions for image quality assessment in conventional and digital mammography

Our objective is to describe a new test phantom that permits the objective assessment of image quality in conventional and digital mammography for different types of breast tissue. A test phantom, designed to represent a compressed breast, was made from tissue equivalent materials. Three separate regions, with different breast tissue compositions, are used to evaluate low and high contrast resolution, spatial resolution and image noise. The phantom was imaged over a range of kV using a Contour 2000 (Bennett) mammography unit with a Kodak MinR 2190-MinR L screen-film combination and a Senograph 2000D (General Electric) digital mammography unit. Objective image quality assessments for different breast tissue compositions were performed using the phantom for conventional and digital mammography. For a similar mean glandular dose (MGD), the digital system gives a significantly higher contrast-to-noise ratio (CNR) than the screen-film system for 100% glandular tissue. In conclusion, in mammography, a range of exposure conditions is used for imaging because of the different breast tissue compositions encountered clinically. Ideally, the patient dose-image quality relationship should be optimized over the range of exposure conditions. The test phantom presented in this work permits image quality parameters to be evaluated objectively for three different types of breast tissue. Thus, it is a useful tool for optimizing the patient dose-image quality relationship.     

A tool for validating MRI-guided strategies: a digital breathing CT/MRI phantom of the abdominal site

Dynamic magnetic resonance imaging (MRI) is emerging as the elected image modality for organ motion quantification and management in image-guided radiotherapy. However, the lack of validation tools is an open issue for image guidance in the abdominal and thoracic organs affected by organ motion due to respiration. We therefore present an abdominal four-dimensional (4D) CT/MRI digital phantom, including the estimation of MR tissue parameters, simulation of dedicated abdominal MR sequences, modeling of radiofrequency coil response and noise, followed by k-space sampling and image reconstruction. The phantom allows the realistic simulation of images generated by MR pulse sequences with control of scan and tissue parameters, combined with co-registered CT images. In order to demonstrate the potential of the phantom in a clinical scenario, we describe the validation of a virtual T1-weighted 4D MRI strategy. Specifically, the motion extracted from a T2-weighted 4D MRI is used to warp a T1-weighted breath-hold acquisition, with the aim of overcoming trade-offs that limit T1-weighted acquisitions. Such an application shows the applicability of the digital CT/MRI phantom as a validation tool, which should be especially useful for cases unsuited to obtain real imaging data.     

A complex flow phantom for medical imaging: ring vortex phantom design and technical specification

Abstract

Cardiovascular fluid dynamics exhibit complex flow patterns, such as recirculation and vortices. Quantitative analysis of these complexities supports diagnosis, leading to early prediction of pathologies. Quality assurance of technologies that image such flows is challenging but essential, and to this end, a novel, cost-effective, portable, complex flow phantom is proposed and the design specifications are provided. The vortex ring is the flow of choice because it offers patterns comparable to physiological flows and is stable, predictable, reproducible and controllable. This design employs a piston/cylinder system for vortex ring generation, coupled to an imaging tank full of fluid, for vortex propagation. The phantom is motor-driven and by varying piston speed, piston displacement and orifice size, vortex rings with different characteristics can be produced. Two measurement methods, namely Laser-PIV and an optical/video technique, were used to test the phantom under a combination of configurations. Vortex rings with a range of travelling velocities (approximately 1–80?cm/s) and different output-orifice diameters (10–25?mm) were produced with reproducibility typically better than ±10%. Although ultrasound compatibility has been demonstrated, longer-term ambitions include adapting the design to support comparative studies with different modalities, such as MRA and X-ray-CTA.