An MRI tissue equivalent lesion phantom using a novel polysaccharide material

A new polysaccharide material, TX-150, and method is described which will potentially allow formation of stable, multi-compartment MRI phantoms constructed without intervening septa. TX-150 can be made into water based gels which are nominally tissue equivalent. Although contiguous regions of different water content are not possible, as water diffusion will occur until equilibrium is reached, TX-150 gel T1 and T2 values can be adjusted independently, while maintaining a constant water composition, by appropriate additives. Unlike paramagnetic ions and chelates, metal phthalocyanines have been found to bind tightly to TX-150, thus, permitting formation of stable contiguous regions of differing T1 relaxation properties. Phantom T2 values can be effectively modified with 2-2-diphenyl-1 picrylhydrazyl, which has little affect on gel T1 values, to form septumless lesion phantoms of varying T1 and T2.     

A high-resolution phantom for MRI

Assessment of spatial resolution is an important step to test the performance of new sequence techniques-especially ultrafast techniques with dedicated k-space trajectories or interpolation algorithms. Measurement of the modulation transfer function (MTF) is a rather difficult procedure, but using suitable resolution phantoms allows a simple visual evaluation of spatial resolution.In contrast to commonly used test objects with a very restricted number of resolution patterns we developed a phantom containing resolution patterns from 0.1 to 1.5 mm in steps of 0.1 mm. One resolution pattern consists of five parallel Plexiglas strips with the distance of the strips being equal to their thickness. Together with a Plexiglas cuboid the resolution patterns are mounted on a Plexiglas plate on the bottom of the cylindrical phantom. An aqueous solution of manganese chloride is used to fill the phantom. High resolution cross sections (pixel size: 50 microm) through the resolution patterns were measured to confirm the correct dimensions of the phantom. To verify the appropriateness of the 0.1 and 0.2 mm stacks micro-CT images with a pixel size of 25 microm were acquired additionally for both patterns. Besides visual inspection evaluation of the profile function of signal intensity across the stacks demonstrates that the resolution patterns are sufficiently correct. T(1)-weighted SE sequences with slightly different pixel sizes as well as T(1)- and T(2*)- weighted gradient echo sequences were applied to demonstrate some possible applications of this phantom. In conclusion, the proposed phantom is well suited to assess the spatial resolution qualitatively (i.e., visually) and quantitatively over a wide range in steps of 0.1 mm.     

A realistic coronary artery phantom for particle image velocimetry

Myocardial infarction results from the rupture of an atherosclerotic plaque, which occurs in response to both mechanical stress and inflammatory processes. In order experimentally observe flow into atherosclerotic coronary artery morphologies, a novel technique for molding realistic compliant phantom featuring injection-molded inclusions and multiple layers has been developed. This transparent phantom allows for particle image velocimetry (PIV) flow analysis and can supply experimental data to validate computational fluid dynamics algorithms and hypothesis.     

A volume resolution phantom for MRI

Multisite quantitative magnetic resonance imaging (qMRI) of volume requires a small isotropic point spread-function (PSF) that is spatially, temporarily, and platform invariant. A phantom which will allow rapid assessment of this metric throughout the imaged volume without repositioning will assist certification of imaging sites for use in qMRI studies based on volume. This paper presents a phantom design for this purpose with a three-dimensional repeating pattern throughout its 800-cm³ volume. The image of the pattern from the phantom contains a series of positive signal points and lines which can be used to measure the PSF, gradient linearity, gradient orthogonality, and B₀ homogeneity at multiple locations throughout its volume. The phantom is readily constructed, can be filled with any nuclear magnetic resonance signal-bearing liquid, and the design is scalable to cover larger volumes.     

A two-compartment phosphate-doped gel phantom for localized spectroscopy.

Acrylamide gel loaded with phosphate has been used in two compartment phantoms designed to assess localized spectroscopy techniques. A chemical shift difference of 2.3 ppm was produced by changing the pH from 6 to 8.9. The 31P relaxation times were modified by doping the gels with paramagnetic ions. For a T1 of approximately 1 sec nickel doping gave a T2 of approximately 110 msec and manganese doping gave a T2 of approximately 8 msec. Gel phantoms are more robust than their liquid equivalent and are not prone to leakage. The construction of small compartments with very thin walls is possible, making this type of phantom suitable for small bore imaging/spectroscopy systems.     

Use of capillaries in the construction of an MRI phantom for the assessment of diffusion tensor imaging: demonstration of performance

Although diffusion tensor imaging (DTI) shows great potential for the diagnosis of a variety of pathologies, no consensus for an appropriate assessment standard of DTI exists. This study examined the feasibility of using water-filled arrays of glass capillaries to construct a DTI phantom suitable for making repeated and reproducible measurements required in a quality assessment program. Three phantoms were constructed using arrays of capillaries with three inner diameters (23, 48, and 82 μm). Data were acquired using DTI protocols; the fractional anisotropy (FA), mean apparent diffusion coefficient (ADC) and principal eigenvectors of the diffusion tensors were calculated. This study demonstrated four results: (1) echo-planar images show that susceptibility within the capillary arrays does not lead to substantial differences in precessional frequency in regions containing the arrays and neither do the regions show noticeable image distortion; (2) principal eigenvectors of the diffusion tensors agree to within <10.3° of the array orientations; (3) mean FA values (0.18–0.50) and ADC values (1.40–1.93×10−³ mm²/s) within specified regions of interest are in general agreement with simulations after a simple noise correction; and (4) these array performance characteristics are observable using a typical clinical DTI protocol.     

A modified heuristics-based model for simulating realistic pedestrian movement behavior

Pedestrian movement simulation models are used in various areas, such as building evacuation, transportation engineering, and safety management of large events. It also provides effective means to uncover underlying mechanisms of collective behaviors. In this work, a modified heuristics-based model is presented. In this model, the potential collisions in the moving process are explicitly considered. Meanwhile, a series of simulations is conducted in two typical scenarios to demonstrate the influence of critical parameters on model performance. It is found that when facing a wide obstacle in a corridor, the larger the visual radius, the earlier the pedestrian starts to make a detour. In addition, when a pedestrian observes a large crowd walking toward him, he chooses to make a detour and moves in the flow in a uniform direction. Furthermore, the model can reproduce the lane formation pedestrian flow phenomena in relatively high-density situations. With the increase of pedestrian visual radius and the weight of potential collision resistance, more stable pedestrian lanes and fewer moving-through-the-counterflow pedestrians can be observed. In terms of model validation, the density-speed relationship of simulation results accords well with that of the published empirical data. Our results demonstrate that the modified heuristics-based model can overcome the deficiency of the original model, and reproduce more realistic pedestrian movement behavior.     

A two-compartment gel phantom for optimization and quality assurance in clinical BOLD fMRI

Clinical applications of blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) depend heavily on robust paradigms, imaging methods and analysis procedures. In this work, as a means to optimize and perform quality assurance of the entire imaging and analysis chain, a phantom that provides a well known and reproducible signal change similar to a block type fMRI experiment is presented. It consists of two gel compartments with slightly different T2 that dynamically enter and leave the imaged volume. The homogeneous gel in combination with a cylindrical geometry results in a well-defined T*2 difference causing a signal difference between the two compartments in T*2-weighted MR images. From time series data obtained with the phantom, maps of percent signal change (PSC) and t-values are calculated. As an example of image parameter optimisation, the phantom is demonstrated to be useful for accurate determination of the influence of echo time (TE) on BOLD fMRI results, taking the t-value as a measure of sensitivity. In addition, the phantom is proposed as a tool for quality assurance (QA) since reproducible time series and t-maps are obtained in a series of independent repeat experiments. The phantom is relatively simple to build and can therefore be used by any clinical fMRI center.     

Diagnostic value of breast MRI for predicting metastatic axillary lymph nodes in breast cancer patients: diffusion-weighted MRI and conventional MRI

Purposes

To evaluate the diagnostic value of diffusion-weighted MRI (DWI) and combination of conventional MRI and DWI to predict metastatic axillary lymph nodes in breast cancer.

Materials and methods

Two hundred fifty-two breast cancer patients with 253 axillae were included. The morphological parameters on axial T2-weighted images without fat saturation and apparent diffusion coefficient (ADC) values were retrospectively analyzed. An independent t-test/chi-square test and receiver operating characteristics (ROC) curve analysis were used.

Results

On conventional MRI, short and long axis length, maximal cortical thickness, relative T2 value, loss of fatty hilum (p < 0.001 for each), and eccentric cortical thickening (p < 0.003) were statistically significantly different between the metastatic and nonmetastatic groups. The short axis to long axis ratio was not a statistically significant parameter. The ADC value was significantly different between the 2 groups, with an AUC that was higher than that of conventional MR parameters (AUC, 0.815; threshold, ≤ 0.986 × 10–3 mm²/sec; sensitivity, 75.8%; specificity, 83.9%). Using the adopted thresholds for each parameter, a total number of findings suggesting malignancy of 4 or higher was determined as the threshold, with high specificity (90.1%).

Conclusion

Using conventional MRI and DWI, we can evaluate the axilla in breast cancer with high specificity.     

Detection of sonoluminescence signals in a gel phantom in the presence of Protoporphyrin IX conjugated to gold nanoparticles

The particles in a liquid decrease the ultrasonic intensity threshold required for cavitation onset. In this study, a new nanoconjugate composed of Protoporphyrin IX and gold nanoparticles (Au–PpIX) was used as a nucleation site for cavitation. The nonradiative relaxation time of Protoporphyrin IX in the presence of gold nanoparticles is longer than the similar time without gold nanoparticles. The acoustic cavitation activity was investigated via recording of the integrated sonoluminescence signal in the wavelength range of 220–700 nm in a gel phantom by a cooled charge coupled device (CCD) at different intensities of 1 MHz ultrasound. In order to confirm these results, a chemical dosimetric method was utilized, too. The recorded sonoluminescence signal in the gel phantom containing Au–PpIX was higher than the other phantoms. These records have been confirmed by the chemical dosimetric data. Therefore, we anticipate that a new nanoconjugate composed of Protoporphyrin IX and gold nanoparticles can act as an efficient sonoluminescence agent and could be introduced as a novel sonosensitizer for sonodynamic therapy.     

Geometric distortion in clinical MRI systems Part II: correction using a 3D phantom

In this paper, we present the correction of the geometric distortion measured in the clinical magnetic resonance imaging (MRI) systems reported in the preceding paper (Part I) using a 3D method based on the phantom-mapped geometric distortion data. This method allows the correction to be made on phantom images acquired without or with the vendor correction applied. With the vendor's 2D correction applied, the method corrects for both the "residual" geometric distortion still present in the plane in which the correction method was applied (the axial plane) and the uncorrected geometric distortion along the axis normal to the plane. The evaluation of the effectiveness of the correction using this new method was carried out through analyzing the residual geometric distortion in the corrected phantom images. The results show that the new method can restore the distorted images in 3D nearly to perfection. For all the MRI systems investigated, the mean absolute deviations in the positions of the control points (along x-, y- and z-axes) measured on the corrected phantom images were all less than 0.2 mm. The maximum absolute deviations were all below approximately 0.8 mm. As expected, the correction of the phantom images acquired with the vendor's correction applied in the axial plane performed equally well. Both the geometric distortion still present in the axial plane after applying the vendor's correction and the uncorrected distortion along the z-axis have all been "restored."     

Estimation of optical abnormalities in breast phantom by diffuse equation

Optical imaging is an emerging method for bio-imaging. The advantages of this imaging provide non-ionizing and safe radiation, non-invasive and functional medical imaging. Due to diffusion of photons inside biological tissues, its image processing is complicated. So in spite of these advantages, this imaging method has not been progressed like ultrasound imaging and magnetic resonance imaging (MRI). Also, the penetration depth of photons inside biological phantom is low. To overcome this problem, the complicated diffusion of photons through tissue must be modeled. The diffuse equation can be applied to simulate photons through turbid media like biological tissues. In this paper, the diffuse equation is used to study propagation of diffuse photons. The green function method is applied to solve this equation, and then the optical properties of abnormalities in breast phantom are estimated. This fast method can be applied for image processing.     

Geometric distortion in clinical MRI systems Part I: evaluation using a 3D phantom

Recently, a 3D phantom that can provide a comprehensive and accurate measurement of the geometric distortion in MRI has been developed. Using this phantom, a full assessment of the geometric distortion in a number of clinical MRI systems (GE and Siemens) has been carried out and detailed results are presented in this paper. As expected, the main source of geometric distortion in modern superconducting MRI systems arises from the gradient field nonlinearity. Significantly large distortions with maximum absolute geometric errors ranged between 10 and 25 mm within a volume of 240 x 240 x 240 mm(3) were observed when imaging with the new generation of gradient systems that employs shorter coils. By comparison, the geometric distortion was much less in the older-generation gradient systems. With the vendor's correction method, the geometric distortion measured was significantly reduced but only within the plane in which these 2D correction methods were applied. Distortion along the axis normal to the plane was, as expected, virtually unchanged. Two-dimensional correction methods are a convenient approach and in principle they are the only methods that can be applied to correct geometric distortion in a single slice or in multiple noncontiguous slices. However, these methods only provide an incomplete solution to the problem and their value can be significantly reduced if the distortion along the normal of the correction plane is not small.     

Search for a realistic Kaluza-Klein theory

An attempt is made to construct a realistic model of particle physics based on eleven-dimensional supergravity with seven dimensions compactified. It is possible to obtain an SU(3) × SU(2) × U(1) gauge group, but the proper fermion quantum numbers are difficult to achieve.     

Experimental and computational analyses of the effects of slice distortion from a metallic sphere in an MRI phantom

Susceptibility artifacts due to metallic prostheses are a major problem in clinical magnetic resonance imaging. We theoretically and experimentally analyze slice distortion arising from susceptibility differences in a phantom consisting of a stainless steel ball bearing embedded in agarose gel. To relate the observed image artifacts to slice distortion, we simulate images produced by 2D and 3D spin-echo (SE) and a view angle tilting (VAT) sequence. Two-dimensional SE sequences suffer from extreme slice distortion when a metal prosthesis is present, unlike 3D SE sequences for which--since slices are phase-encoded--distortion of the slice profile is minimized, provided the selected slab is larger than the region of interest. In a VAT sequence, artifacts are reduced by the application of a gradient along the slice direction during readout. However, VAT does not correct for the excitation slice profile, which results in the excitation of spins outside the desired slice location and can lead to incorrect anatomical information in MR images. We propose that the best sequences for imaging in the presence of a metal prosthesis utilize 3D acquisition, with phase encoding replacing slice selection to minimize slice distortion, combined with excitation and readout gradient strengths at their maximum values.     

Stochastic resonance in a realistic model for surface adsorption

We study a model for a monolayer single adsorbate system used to describe pattern formation on adsorbates with lateral interactions, when it is submitted to pressure oscillations. Through numerical and analytical (based on a two-state approximation) methods to analyze the existence of stochastic resonance in such a bistable system. This is a first step toward the study of resonant phenomena in adsorbate systems with moving fronts and/or with presence of micro-reactors or spots.     

Use of supersymmetric isospectral formalism to realistic quantum many-body problems

We propose a novel mathematical approach for the calculation of resonances in weakly bound systems. For any potential, families of strictly isospectral potentials (with very different shape) having desirable and adjustable features can be generated. For systems having no bound ground state, an isospectral potential with a bound state in the continuum is possible. The quasi-bound state in the original shallow potential will be effectively trapped in the deep well of the isospectral family, facilitating more accurate calculation of resonance energy. Application to ⁶He, ⁶Li and ⁶Be yield excellent results. Another application is the calculation of Efimov states in weakly bound threebody system. We present the result of ⁴He trimer, where the first excited state is claimed to be an Efimov state.     

Tridimensional dosimetry for prostate IMRT treatments using MAGIC-f gel by MRI

Intensity-modulated radiation therapy is able to deliver complex dose distributions and a system able to determine tridimensional dose distribution during the quality assurance of the treatments would be of great interest. In this context polymeric gels can be a useful dosimeter. This work aims to apply this tridimensional dosimetry technique for two prostate IMRT treatments, using MAGIC-f as a gel dosimeter combined with the magnetic resonance imaging to determinate the dose distributions. Dose images obtained by the gel were compared with the treatment planning images and the gamma index was calculated as a quantitative comparison. In both plans a high pass rate was achieved in the gamma analyses (95.5% and 94.0% for plans 1 and 2 respectively) showing that they were approved in the tridimensional quality assurance.     

Use of the WWR-SM reactor for developing neutron-capture therapy in Uzbekistan

A reconstruction of the horizontal channel of the WWR-SM research nuclear reactor at the Institute of Nuclear Physics, Academy of Sciences of Republic of Uzbekistan, is necessary to develop the method of neutron-capture therapy of human cancer. To perform such a reconstruction, the neutron flux is measured at the channel input and the neutron transport through various filter materials forming an epithermal neutron beam at the output is calculated.     

Effects of nonlinear propagation, cavitation, and boiling in lesion formation by high intensity focused ultrasound in a gel phantom

The importance of nonlinear acoustic wave propagation and ultrasound-induced cavitation in the acceleration of thermal lesion production by high intensity focused ultrasound was investigated experimentally and theoretically in a transparent protein-containing gel. A numerical model that accounted for nonlinear acoustic propagation was used to simulate experimental conditions. Various exposure regimes with equal total ultrasound energy but variable peak acoustic pressure were studied for single lesions and lesion stripes obtained by moving the transducer. Static overpressure was applied to suppress cavitation. Strong enhancement of lesion production was observed for high amplitude waves and was supported by modeling. Through overpressure experiments it was shown that both nonlinear propagation and cavitation mechanisms participate in accelerating lesion inception and growth. Using B-mode ultrasound, cavitation was observed at normal ambient pressure as weakly enhanced echogenicity in the focal region, but was not detected with overpressure. Formation of tadpole-shaped lesions, shifted toward the transducer, was always observed to be due to boiling. Boiling bubbles were visible in the gel and were evident as strongly echogenic regions in B-mode images. These experiments indicate that nonlinear propagation and cavitation accelerate heating, but no lesion displacement or distortion was observed in the absence of boiling.