Phantom Material Research Articles

Evaluation of water-mimicking solid phantom materials for use in HDR and LDR brachytherapy dosimetry

In modern HDR or LDR brachytherapy with photon emitters, fast checks of the dose profiles generated in water or a water-equivalent phantom have to be available in the interest of patient safety. However, the commercially available brachytherapy photon sources cover a wide range of photon emission spectra, and the range of the in-phantom photon spectrum is further widened by Compton scattering, so that the achievement of water-mimicking properties of such phantoms involves high requirements on their atomic composition. In order to classify the degree of water equivalence of the numerous commercially available solid water-mimicking phantom materials and the energy ranges of their applicability, the radial profiles of the absorbed dose to water, Dw, have been calculated using Monte Carlo simulations in these materials and in water phantoms of the same dimensions. This study includes the HDR therapy sources Nucletron Flexisource Co-60 HDR (60Co), Eckert und Ziegler BEBIG GmbH CSM-11 (137Cs), Implant Sciences Corporation HDR Yb-169 Source 4140 (169Yb) as well as the LDR therapy sources IsoRay Inc. Proxcelan CS-1 (131Cs), IsoAid Advantage I-125 IAI-125A (125I), and IsoAid Advantage Pd-103 IAPd-103A (103Pd). Thereby our previous comparison between phantom materials and water surrounding a Varian GammaMed Plus HDR therapy 192Ir source (Schoenfeld et al 2015) has been complemented. Simulations were performed in cylindrical phantoms consisting of either water or the materials RW1, RW3, Solid Water, HE Solid Water, Virtual Water, Plastic Water DT, Plastic Water LR, Original Plastic Water (2015), Plastic Water (1995), Blue Water, polyethylene, polystyrene and PMMA. While for 192Ir, 137Cs and 60Co most phantom materials can be regarded as water equivalent, for 169Yb the materials Plastic Water LR, Plastic Water DT and RW1 appear as water equivalent. For the low-energy sources 106Pd, 131Cs and 125I, only Plastic Water LR can be classified as water equivalent.

Water equivalence of some 3D dosimeters: a theoretical study based on the effective atomic number and effective fast neutron removal cross section

Effective atomic numbers for photon energy absorption (\( Z_{{{\text{PEA}}_{\text{eff}} }} \)) and their corresponding electron numbers \( (N_{{{\text{PEA}}_{\text{eff}} }} ) \), and effective macroscopic removal cross sections of fast neutrons (ΣR) were calculated for 27 different types of three-dimensional dosimeters, four types of phantom materials, and water. The values of \( Z_{{{\text{PEA}}_{\text{eff}} }} \) and \( N_{{{\text{PEA}}_{\text{eff}} }} \) were obtained using the direct method for energies ranging from 10 keV to 20 MeV. Results are presented relative to water, for direct comparison over the range of examined energies. The effect of monomers that are used in polymer gel dosimeters on the water equivalence is discussed. The relation between ΣR and hydrogen content was studied. Micelle gel dosimeters are highly promising because our results demonstrate perfect matching between the effective atomic number, electron density number, and fast neutron attenuation coefficient of water.

Examination of a spectral-based ultrasonic analysis method for materials characterization and evaluation

Abstract Ultrasonic analysis has long been used as a diagnostic tool for materials and medicine, and measurements that are able to detail internal structure are often desirable. Peak density has previously been used as an assessment for tissue pathology. This quantity has potential for use in the study of tissue and soft materials. However, the use of this parameter is still in its early stages, and the signal processing techniques used are limited. In this work, other signal processing methods commonly used in ultrasonic measurement are applied to the resulting ultrasound pulses transmitted through a soft phantom material to study their effects on peak density calculations. This study examines the outcomes due to the selection of pulse width, the use of standard window functions, and spectral normalization using a reference spectrum. The consequences are assessed through the inspection of individual frequency spectra as well as two dimensional images based upon the peak density parameter. It was found that peak density was highly dependent on the window function employed and was also subject to the pulse width. Spectral normalization was found to have little effect on peak density calculations.

γTools: A modular multifunction phantom for quality assurance in GammaKnife treatments

PurposeWe present the γTools, a new phantom designed to assess geometric and dosimetric accuracy in Gamma Knife treatments, together with first tests and results of applications. MethodsThe phantom is composed of two modules: the imaging module, a regular grid of 1660 control points to evaluate image distortions and image registration result and the dosimetry module for delivered dose distribution measurements. The phantom is accompanied by a MatLab routine for image distortions quantification. Dose measurement are performed with Gafchromic films fixed between two inserts and placed in various positions and orientations inside the dosimetry module thus covering a volume comparable to the full volume of a head. ResultsTests performed to assess the accuracy and precision of the imaging module demonstrated sub-millimetric values. As an example of possible applications, the phantom was employed to measure image distortions of two MRI scanners and to perform dosimetric studies of single shots delivered to homogeneous and heterogeneous materials. Due to the phantom material, the measured absolute dose do not correspond to the planned dose; doses comparisons are thus carried out between normalized dose distributions. Finally, an end-to-end test was carried out in the treatment of a neuroma-like target which resulted in a 100% gamma passing rate (2% local, 2 mm) and a distance between the real target perimeter and the prescription isodose centroids of about 1 mm. ConclusionsThe tests demonstrate that the proposed phantom is suitable to assess both the geometrical and relative dosimetric accuracy of Gamma Knife radiosurgery treatments.

Spatial Angular Compounding Technique for H-Scan Ultrasound Imaging

H-Scan is a new ultrasound imaging technique that relies on matching a model of pulse-echo formation to the mathematics of a class of Gaussian-weighted Hermite polynomials. This technique may be beneficial in the measurement of relative scatterer sizes and in cancer therapy, particularly for early response to drug treatment. Because current H-scan techniques use focused ultrasound data acquisitions, spatial resolution degrades away from the focal region and inherently affects relative scatterer size estimation. Although the resolution of ultrasound plane wave imaging can be inferior to that of traditional focused ultrasound approaches, the former exhibits a homogeneous spatial resolution throughout the image plane. The purpose of this study was to implement H-scan using plane wave imaging and investigate the impact of spatial angular compounding on H-scan image quality. Parallel convolution filters using two different Gaussian-weighted Hermite polynomials that describe ultrasound scattering events are applied to the radiofrequency data. The H-scan processing is done on each radiofrequency image plane before averaging to get the angular compounded image. The relative strength from each convolution is color-coded to represent relative scatterer size. Given results from a series of phantom materials, H-scan imaging with spatial angular compounding more accurately reflects the true scatterer size caused by reductions in the system point spread function and improved signal-to-noise ratio. Preliminary in vivo H-scan imaging of tumor-bearing animals suggests this modality may be useful for monitoring early response to chemotherapeutic treatment. Overall, H-scan imaging using ultrasound plane waves and spatial angular compounding is a promising approach for visualizing the relative size and distribution of acoustic scattering sources.

Abstract ID: 94 Development of a methodology for formulating radiologically equivalent materials to human tissues

The study and development of phantom materials is central to the calculation of patient doses in medical imaging applications. To ensure tissue-equivalence, these materials should have radiation attenuation and scattering properties similar to water or human tissues [ 1 Hermanm K.P. et al. Phys Med Biol. 1985; 30: 1195-1200 Crossref PubMed Scopus (43) Google Scholar , 2 Homolka et al. Phys Med Biol. 2002; 47: 2907-2916 Crossref PubMed Scopus (15) Google Scholar , 3 White D. R Phys Med Biol. 1977; 22: 219-228 Crossref PubMed Scopus (81) Google Scholar ]. Additionally, Monte Carlo techniques are useful to evaluate the radiation transmission and scattering properties of different formulations in order to assure their tissue equivalence when compared with real tissues. In this work the authors evaluate the use of the least squares method as a tool to optimize the formulation of phantom materials. There were developed water-equivalent materials for diagnostic imaging energy range (10–150 keV) as well as mammography phantom formulations for different glandular/adipose breast tissue compositions [ [4] Hammerstein et al. Radiol V. 1979; 130: 485-491 Crossref PubMed Scopus (489) Google Scholar ] for the energy range 8–28 keV. The developed materials are constituted by a thermoplastic base and some additives. The methodology consisted in fitting by the Least Square Method the volume fractions of the components of the phantom material in order to make their linear attenuation coefficient [ [5] Hubbel J.H. Seltzer S.M. 2004, Available:http://physics.nist.gov/xaamdi. Google Scholar ] as close as possible to the linear attenuation coefficient of the reference material such as breast tissue or water. A weight function which reflects the contribution of the X-ray spectra in the suitable energy range was also introduced. The weight function used represents the standard radiation spectra qualities RQR10 and RQR-M2 according to TRS 457 (IAEA) [ [6] IAEA TRS 457 Vienna. 2011. Google Scholar ]. The transmitted and scattered X-ray spectra of the developed compositions were simulated as well as their dose profiles using the PENELOPE Monte Carlo code. The linear attenuation coefficient obtained using the least squares method is in good agreement with the reference material for both water and breast tissue with relative differences lower than 2%. The Monte Carlo method is very useful in determining the X-ray transmission and scattering properties of the materials, before the manufacture of the physical phantom. The study and development of phantom materials is central to the calculation of patient doses in medical imaging applications. To ensure tissue-equivalence, these materials should have radiation attenuation and scattering properties similar to water or human tissues [ 1 Hermanm K.P. et al. Phys Med Biol. 1985; 30: 1195-1200 Crossref PubMed Scopus (43) Google Scholar , 2 Homolka et al. Phys Med Biol. 2002; 47: 2907-2916 Crossref PubMed Scopus (15) Google Scholar , 3 White D. R Phys Med Biol. 1977; 22: 219-228 Crossref PubMed Scopus (81) Google Scholar ]. Additionally, Monte Carlo techniques are useful to evaluate the radiation transmission and scattering properties of different formulations in order to assure their tissue equivalence when compared with real tissues. In this work the authors evaluate the use of the least squares method as a tool to optimize the formulation of phantom materials. There were developed water-equivalent materials for diagnostic imaging energy range (10–150 keV) as well as mammography phantom formulations for different glandular/adipose breast tissue compositions [ [4] Hammerstein et al. Radiol V. 1979; 130: 485-491 Crossref PubMed Scopus (489) Google Scholar ] for the energy range 8–28 keV. The developed materials are constituted by a thermoplastic base and some additives. The methodology consisted in fitting by the Least Square Method the volume fractions of the components of the phantom material in order to make their linear attenuation coefficient [ [5] Hubbel J.H. Seltzer S.M. 2004, Available:http://physics.nist.gov/xaamdi. Google Scholar ] as close as possible to the linear attenuation coefficient of the reference material such as breast tissue or water. A weight function which reflects the contribution of the X-ray spectra in the suitable energy range was also introduced. The weight function used represents the standard radiation spectra qualities RQR10 and RQR-M2 according to TRS 457 (IAEA) [ [6] IAEA TRS 457 Vienna. 2011. Google Scholar ]. The transmitted and scattered X-ray spectra of the developed compositions were simulated as well as their dose profiles using the PENELOPE Monte Carlo code. The linear attenuation coefficient obtained using the least squares method is in good agreement with the reference material for both water and breast tissue with relative differences lower than 2%. The Monte Carlo method is very useful in determining the X-ray transmission and scattering properties of the materials, before the manufacture of the physical phantom.

Evaluation of Stopping-Power Prediction by Dual- and Single-Energy Computed Tomography in an Anthropomorphic Ground-Truth Phantom

To determine the accuracy of particle range prediction for proton and heavier ion radiation therapy based on dual-energy computed tomography (DECT) in a realistic inhomogeneous geometry and to compare it with the state-of-the-art clinical approach. A 3-dimensional ground-truth map of stopping-power ratios (SPRs) was created for an anthropomorphic head phantom by assigning measured SPR values to segmented structures in a high-resolution CT scan. This reference map was validated independently comparing proton transmission measurements with Monte Carlo transport simulations. Two DECT-based methods for direct SPR prediction via the Bethe formula (DirectSPR) and 2 established approaches based on Hounsfield look-up tables (HLUTs) were chosen for evaluation. The SPR predictions from the 4 investigated methods were compared with the reference, using material-specific voxel statistics and 2-dimensional gamma analysis. Furthermore, range deviations were analyzed in an exemplary proton treatment plan. The established reference SPR map was successfully validated for the discrimination of SPR and range differences well below 0.3% and 1mm, respectively, even in complex inhomogeneous settings. For the phantom materials of larger volume (mainly brain, soft tissue), the investigated methods were overall able to predict SPR within 1% median deviation. The DirectSPR methods generally performed better than the HLUT approaches. For smaller phantom parts (such as cortical bone, air cavities), all methods were affected by image smoothing, leading to considerable SPR under- or overestimation. This effect was superimposed on the general SPR prediction accuracy in the exemplary treatment plan. DirectSPR predictions proved to be more robust, with high accuracy in particular for larger volumes. In contrast, HLUT approaches exhibited a fortuitous component. The evaluation of accuracy in a realistic phantom with validated ground-truth SPR represents a crucial step toward possible clinical application of DECT-based SPR prediction methods.

Experimental analysis of correction factors for reference dosimetry in a magnetic field

Abstract Today, hybrid systems of linear accelerator and MRI scanner are clinically available. Therefore it is important to investigate the feasibility of reference dosimetry with ionization chambers in the presence of a magnetic field and determine correction factors. In this work, correction factors under various conditions that influence the chamber response were experimentally investigated, using a conventional 6 MV linear accelerator together with a stand-alone magnet. We found that the correction factor for a PTW31010 ionization chamber ranges from 0.9873 to 1.009 depending on the magnetic field strength, magnetic field orientation and magnetic field size. The phantom material also does have an influence on the measured signal. Therefore, reference dosimetry with ionization chambers in the presence of a magnetic field is feasible, but requires dedicated correction factors, which depend on the experimental setup.

Optimization of dual‐energy CT acquisitions for proton therapy using projection‐based decomposition

Dual-energy computed tomography (DECT) has been presented as a valid alternative to single-energy CT to reduce the uncertainty of the conversion of patient CT numbers to proton stopping power ratio (SPR) of tissues relative to water. The aim of this work was to optimize DECT acquisition protocols from simulations of X-ray images for the treatment planning of proton therapy using a projection-based dual-energy decomposition algorithm. We have investigated the effect of various voltages and tin filtration combinations on the SPR map accuracy and precision, and the influence of the dose allocation between the low-energy (LE) and the high-energy (HE) acquisitions. For all spectra combinations, virtual CT projections of the Gammex phantom were simulated with a realistic energy-integrating detector response model. Two situations were simulated: an ideal case without noise (infinite dose) and a realistic situation with Poisson noise corresponding to a 20 mGy total central dose. To determine the optimal dose balance, the proportion of LE-dose with respect to the total dose was varied from 10% to 90% while keeping the central dose constant, for four dual-energy spectra. SPR images were derived using a two-step projection-based decomposition approach. The ranges of 70 MeV, 90 MeV, and 100 MeV proton beams onto the adult female (AF) reference computational phantom of the ICRP were analytically determined from the reconstructed SPR maps. The energy separation between the incident spectra had a strong impact on the SPR precision. Maximizing the incident energy gap reduced image noise. However, the energy gap was not a good metric to evaluate the accuracy of the SPR. In terms of SPR accuracy, a large variability of the optimal spectra was observed when studying each phantom material separately. The SPR accuracy was almost flat in the 30-70% LE-dose range, while the precision showed a minimum slightly shifted in favor of lower LE-dose. Photon noise in the SPR images (20 mGy dose) had lower impact on the proton range accuracy as comparable results were obtained for the noiseless situation (infinite dose). Root-mean-square range errors averaged over all irradiation angles associated to dual-energy imaging were comprised between 0.50 mm and 0.72 mm for the noiseless situation and between 0.51 mm and 0.77 mm for the realistic scenario. The impact of the dual-energy spectra and the dose allocation between energy levels on the SPR accuracy and precision determined through a projection-based dual-energy algorithm were evaluated to guide the choice of spectra for dual-energy CT for proton therapy. The dose balance between energy levels was not found to be sensitive for the SPR estimation. The optimal pair of dual-energy spectra was material dependent but on a heterogeneous anthropomorphic phantom, there was no significant difference in range accuracy and the choice of spectra could be driven by the precision, i.e., the energy gap.

Video-tactile pneumatic sensor for soft tissue elastic modulus estimation

BackgroundA new sensor for estimating elasticity of soft tissues such as a liver was developed for minimally invasive surgery application.MethodsBy measuring deformation and adjusting internal pressure of the pneumatic sensor head, the sensor can be used to do palpation (indentation) of tissues with wide range of stiffness. A video camera installed within the sensor shell is used to register the radius of the contact area. Based on finite element model simulations and the measured data, elastic modulus of the indented soft tissue can be calculated.Results and conclusionsThree phantom materials, namely plastic, silicone and gelatin, with varied stiffness were tested. The experimental results demonstrated that the new sensor can obtain highly reliable data with error less than 5%. The new sensor might be served as an instrument in laparoscopic surgery for diagnosis of pathological tissues or internal organs.

Mass attenuation coefficient of tannin-added Rhizophora spp. particleboards at 16.59–25.56 keV photons, and 137Cs and 60Co gamma energies

The aim of this study was to determine the suitability of tannin-added Rhizophora spp. particleboards as phantom materials in the application of low- and high-energy photons. The tannin-added Rhizophora spp. particleboards and density plug phantoms were created with a target density of 1.0g/cm3. The elemental composition and effective atomic number of the particleboards were measured using energy dispersive X-ray analysis. The mass attenuation coefficient of the particleboards for low-energy photons were measured using the attenuation of X-ray fluorescence. The mass attenuation coefficients of high-energy photons were measured using the attenuation of 137Cs and 60Co gamma energies. The results were compared to the calculated value of water using XCOM calculations. The results showed that the effective atomic number and mass attenuation coefficients of tannin-added Rhizophora spp. particleboards were similar to those of water, indicating the suitability of tannin-added Rhizophora spp. particleboards as phantom materials for low- and high-energy photons.

Mechanical behaviour of tissue mimicking breast phantom materials

The mechanical properties of soft tissue have long been of interest in biomedical research and applications, and increasingly for breast cancer imaging. In this paper, the mechanical properties of three different materials used to emulate the mechanical behaviour of real breast tissue are measured: agar, gelatine, and silicone, to assess their suitability for use in phantoms in systems assessing tissue mechanics for diagnostics. Two widely recognised measurement procedures are used. Quasi-static uniaxial compression was performed under a strain rate of 0.5 mm min−1 up to 15% strain with preloads of 0.05, 0.1, and 0.2 N, was used to measure the elastic moduli. Dynamic testing over a frequency range of 0.1–50 Hz for agar and 0.1–100 Hz for gelatine and silicone with the same preload was used to measure the storage moduli. Elastic and storage moduli were (5–81 kPa, 17–85 kPa, 5–112 kPa) and (3–128 kPa, 10–109 kPa, 5–73 kPa) for agar, gelatine, and silicone, respectively at the three preloads. All materials can be cast into arbitrary shapes and are suitable for tissue-mimicking phantoms. Silicone was the most consistent across the different preloads and frequencies, and can provide a range of stiffness ratios of adipose to tumour tissue that match experimentally reported values. More specifically, silicone samples for skin, adipose and tumour tissues show nonlinear stress–strain characteristics at 3 preloads characterised using hyperelastic parameters by fitting Neo-Hookean, Mooney Rivlin and Ogden models. Silicone also does not contain water, so environmental influences do not affect its mechanical properties as much as the other materials, and is thus more durable for consistent re-use. Finally, breast shaped mimicking silicone phantoms were fabricated for in vitro trials of a digital image elasto tomography breast cancer screening system assessing changes in mechanical properties.

Acoustic and Elastic Properties of Glycerol in Oil-Based Gel Phantoms

Phantoms are important tools for image quality control and medical training. Many phantom materials have been proposed for ultrasound; most of them use water as the solvent, but these materials have disadvantages such as dehydration and low temporal stability if not properly stored. To overcome these difficulties, copolymer-in-oil gel was proposed as an inert and stable material; however, speed of sound for these materials is still lower than what is described for most biological tissues. Here, we propose the glycerol dispersion in oil-based gels to modify the acoustic and elastic properties of copolymer-in-oil phantoms. We manufactured copolymer-in-oil gels using styrene-ethylene/butylene-styrene (SEBS) in concentrations 8%–15%. We used 2 types of mineral oils with different viscosities. Glycerol was added in a volume fraction 0%–30% of the total amount of liquid. The acoustic (i.e., speed of sound, attenuation and backscattering) and the mechanical (i.e., density and Young's modulus) properties of the samples were within the range of values observed for soft tissues. The acoustic parameters of the samples were dependent on oil viscosity and glycerol concentration. The speed of sound ranged 1423 m/s – 1502 m/s, while the acoustic attenuation and the ultrasonic backscattering increased by adding glycerol. The density and the Young's moduli were less affected by the presence of glycerol. We conclude that glycerol can be used to control the acoustic parameters of copolymer-in-oil gels. Additionally, it opens the possibility of incorporating other oil-insoluble substances to control further properties of the phantom.

Effective atomic number estimation using kV-MV dual-energy source in LINAC

Dual-energy computed tomography (DECT) imaging can measure the effective atomic number (EAN) as well as the electron density, and thus its adoption may improve dose calculations in brachytherapy and external photon/particle therapy. An expanded energy gap in dual-energy sources is expected to yield more accurate EAN estimations than conventional DECT systems, which typically span less than 100kV. The aim of this paper is to assess a larger energy gap DECT by using a linear accelerator (LINAC) radiotherapy system with a kV X-ray imaging device, which are combined to provide X-rays in both the kV- and MV-energy ranges. Traditionally, the EAN is determined by parameterising the Hounsfield Unit; however, this is difficult in a kV-MV DECT due to different uncertainties in the reconstructed attenuation coefficient at each end of the energy spectrum. To overcome this problem, we included a new calibration step to produce the most likely linear attenuation coefficients, based upon the X-ray spectrum. To determine the X-ray spectrum, Monte Carlo calculations using GEANT4 were performed. Then the images were calibrated using information from eight inserts of known materials in a CIRS phantom (CIRS Inc., Norfolk, VA). Agreement between the estimated and empirical EANs in these inserts was within 11%. Validation was subsequently performed with the CatPhan500 phantom (The Phantom Laboratory, Salem). The estimated EAN for seven inserts agreed with the empirical values to within 3%. Accordingly, it can be concluded that, given properly reconstructed images based upon a well-determined X-ray spectrum, kV-MV DECT provides an excellent prediction for the EAN.

Study of dosimetric characteristics of a commercial optically stimulated luminescence system

AbstractBackgroundOptically stimulated luminescence dosimeters (OSLDs) have a number of advantages in radiation dosimetry making them an excellent dosimeter for in vivo dosimetry. The study aimed to study the dosimetric characteristics of a commercial optically stimulated luminescence (OSL) system by Landauer Inc., before using it for routine clinical practice for in vivo dosimetry in radiotherapy. Further, this study also aimed to investigate the cause of variability found in the literature in a few dosimetric parameters of carbon-doped aluminium oxide (Al2O3:C).Materials and methodsThe commercial OSLD system uses Al2O3:C nanoDotTM as an active radiation detector and InLightTM microStar® as a readout assembly. Inter-detector response, energy, dose rate, field size and depth dependency of the detector response were evaluated for all available clinical range of photon beam energies in radiotherapy.ResultsInter-detector variation in OSLD response was found within 3·44%. After single light exposure for the OSL readout, detector reading decreased by 0·29% per reading. The dose linearity was investigated between dose range 50–400 cGy. The dose response curve was found to be linear until 250 cGy, after this dose, the dose response curve was found to be supra-linear in nature. OSLD response was found to be energy independent for Co60 to 10 MV photon energies.ConclusionsThe cause of variability found in the literature for some dosimetric characteristics of Al2O3:C is due to the difference in general geometry, construction of dosimeter, geometric condition of irradiation, phantom material and geometry, beam energy. In addition, the irradiation history of detector used and difference in readout methodologies had varying degree of uncertainties in measurements. However, the large surface area of the detector placed in the phantom with sufficient build-up and backscatter irradiated perpendicularly to incident radiation in Co60 beam is a good method of choice for the calibration of a dosimeter. Understanding the OSLD response with all dosimetric parameters may help us in estimation of accurate dose delivered to patient during radiotherapy treatment.

Calibration of the 125I-induced x-ray fluorescence spectrometry-based system of in vivo bone strontium determinations using hydroxyapatite as a phantom material: a simulation study

Objective. The calibration of in vivo x-ray fluorescence systems of bone strontium quantification, based on 125I excitation, is dependent on a coherent normalization procedure. Application of this procedure with the use of plaster of Paris (poP) as a phantom material requires the application of a coherent conversion factor (CCF) to make the calibration functions transferable between the phantom material and human bone. In this work we evaluate, with the use of Monte Carlo simulation, the potential benefit of employing a newly developed hydroxyapatite phantom material into the calibration protocol. Approach. Simulations being performed on bare bone phantoms, as the emission spectrum in this case is equivalent to an emission spectrum of an adequately corrected measurement for soft tissue attenuation of emitted strontium signal. We report that the application of hydroxyapatite phantoms does in fact remove the need for a coherent correction factor (CCF). Main results. The newly developed phantoms can thus be used for the calibration of in vivo bone strontium systems removing one step of the calibration protocol. Calibration is, however, limited to cases in which the concentration is relative to the amount of calcium in the specimen, which is, the most useful quantity in a clinical sense. Determining concentrations on a per-mass-of-material basis, that is, a concentration not normalized to the calcium content of the phantom/bone, results in large biases in estimated bone strontium content. Significance. The use of an HAp phantom material was found to remove the need for a CCF. It was also found that in the case of an incomplete conversion ratio when preparing the phantom material that there would be little effect on the differential coherent cross-section and thereby the coherent normalization-based calibration protocol.

Evaluation of shear wave generation methods for intraoperative magnetic resonance elastography in a clinical transcranial focused ultrasound system

The current standard for FDA approved real-time monitoring of transcranial Focused Ultrasound (FUS) ablation is MR-thermometry. Ultrasound-elastography and Magnetic Resonance Elastography (MRE) have shown success in monitoring FUS ablation in various parts of the body. Introducing shear wave vibrations into the brain during transcranial-FUS becomes technologically challenging given the stringent constraints of the operating space in water filled transcranial phased array devices and since patients are fixed in stereotactic headgear during treatment. This study investigates the feasibility of using intraoperative-MRE during transcranial-FUS ablation in the ExAblate 4000 (InSightec, Ltd.) platform. Acoustic radiation force induced vibration using the ExAblate 4000 transducer and common MRE surface actuation methods are evaluated for shear wave generation. Elastic phantom experiments were used to gauge the acoustic power levels and time incurred during vibrations from MRE scanning while in the FUS transducer. These values were compared to FUS and MR parameters used in clinical transcranial-FUS treatments. MRE derived 3D displacement fields within the phantom materials are evaluated for all vibration methods. The cooling water in the ultrasound traducer influenced the MR signal using current MRE pulse sequences. Future work will involve integrating MRE into sequences less sensitive to cooling water media and minimizing intraoperative-MRE scan time.

Fabrication and characterisation of phantom material made of Tannin-added Rhizophora spp. particleboards for photon and electron beams

Particleboards made of Rhizophora spp. with addition of tannin adhesive were fabricated at target density of 1.0 g/cm3. The physical and mechanical properties of the particleboards including internal bond strength (IB) and modulus of rupture (MOR) were measured based on Japanese Industrial Standards (JIS A-5908). The characterisation of the particleboards including the effective atomic number, CT number and relative electron density were determined and compared to water. The mass attenuation coefficient of the particleboards were measured and compared to the calculated value of water using photon cross-section database (XCOM). The results showed that the physical and mechanical properties of the particleboards complied with Type 13 and 18 of JIS A-5908. The values of effective atomic number, CT number and relative electron density were also close to the value of water. The value of mass attenuation coefficients of the particleboards showed good agreement with water (XCOM) at low and high energy photon indicated by the χ2 values.

Technical Note: On the accuracy of parametric two-parameter photon cross-section models in dual-energy CT applications.

To evaluate and compare the theoretically achievable accuracy of two families of two-parameter photon cross-section models: basis vector model (BVM) and modified parametric fit model (mPFM). The modified PFM assumes that photoelectric absorption and scattering cross-sections can be accurately represented by power functions in effective atomic number and/or energy plus the Klein-Nishina cross-section, along with empirical corrections that enforce exact prediction of elemental cross-sections. Two mPFM variants were investigated: the widely used Torikoshi model (tPFM) and a more complex "VCU" variant (vPFM). For 43 standard soft and bony tissues and phantom materials, all consisting of elements with atomic number less than 20 (except iodine), we evaluated the theoretically achievable accuracy of tPFM and vPFM for predicting linear attenuation, photoelectric absorption, and energy-absorption coefficients, and we compared it to a previously investigated separable, linear two-parameter model, BVM. For an idealized dual-energy computed tomography (DECT) imaging scenario, the cross-section mapping process demonstrates that BVM more accurately predicts photon cross-sections of biological mixtures than either tPFM or vPFM. Maximum linear attenuation coefficient prediction errors were 15% and 5% for tPFM and BVM, respectively. The root-mean-square (RMS) prediction errors of total linear attenuation over the 20keV to 1000keV energy range of tPFM and BVM were 0.93% (tPFM) and 0.1% (BVM) for adipose tissue, 0.8% (tPFM) and 0.2% (BVM) for muscle tissue, and 1.6% (tPFM) and 0.2% (BVM) for cortical bone tissue. With exception of the thyroid and Teflon, the RMS error for photoelectric absorption and scattering coefficient was within 4% for the tPFM and 2% for the BVM. Neither model predicts the photon cross-sections of thyroid tissue accurately, exhibiting relative errors as large as 20%. For the energy-absorption coefficients prediction error, RMS errors for the BVM were less than 1.5%, while for the tPFM, the RMS errors were as large as 16%. Compared to modified PFMs, BVM shows superior potential to support dual-energy CT cross-section mapping. In addition, the linear, separable BVM can be more efficiently deployed by iterative model-based DECT image-reconstruction algorithms.

Development of a permeable phantom for dynamic contrast enhanced (DCE) imaging quality assurance: material characterization and testing

The wide-spread use of contrast-enhanced imaging is often combined with pharmaco-kinetic modeling to estimate the level of perfusion and/or permeability of the tissue or tumour under investigation. Despite the availability of advanced experimental methods and phantoms to test the accuracy of blood flow and transfer rates, the reliable estimation of tissue permeability following kinetic modeling against a known truth is still outstanding. This work reports on the first two phases of the development of a tissue mimicking permeable hepatic perfusion phantom being tested under DCE-CT: material selection and fabrication, as well as property testing. The required properties of these synthetic materials include a tailored porous structure, tumor mimicking pore size and tumor mimicking permeability. Three phantom materials were investigated for their capabilities to address these requirements, i.e. polydimethylsiloxane (PDMS), thermoplastic polyurethane (TPU) and agar gel. Porous structures were made using different techniques such as phase separation micro-molding (for PDMS) and a particular leaching process (for PDMS and TPU). Manual pores were made on agar gel and all were characterized under SEM, micro-CT scan and pycnometer. TPU was selected as the phantom material of choice moving forward due to its high homogeneity in pore size ranging from 200 to 400 μm, which mimics tumor vasculature the best. TPU also has the highest permeability of the three materials and its stable structure provides good repeatability.