Types Of Phantoms Research Articles

Measurement of ultrasonic properties of muscle and blood biological phantoms

Oncology hyperthermia refers to an artificial elevation of temperature in biological tissue to remove tumor cells. This temperature increase is reached by applying ultrasound or electromagnetic waves. Working with biological tissues implies a high effort; furthermore, biological material changes its properties with time. Also, it is necessary a knowledge of the handling of biological material and a specialized infrastructure. For these reasons, for some years our research laboratory has dedicated part of its researches to develop mimicking materials to emulate muscle and other tissue ultrasonic properties. A blood phantom was developed in our laboratory for its use in the transit time flow measurement (blood flow). The properties of interest for the muscle and for the blood phantoms are ultrasonic attenuation and ultrasound velocity. This work refers to the phantom preparation and their ultrasonic properties measurement. These phantoms emulate the mentioned ultrasonic characteristics. In the case of muscle, there are two types of phantoms: solid phantom with graphite and phantom with scatterers. The procedure of preparation of the phantoms is described. They have a different composition to that published for Doppler blood phantoms. Some ultrasonic properties of muscle and of blood have been already published and they are referred to one temperature. An originality of the results reported in this paper is that the behavior of the ultrasonic properties is presented at different temperatures: ranging from 22 ∘C to 470 ∘C. This range includes the temperatures used in our experimental work in hyperthermia treatment.

Effect of Fat-saturation Pulse on EPI Time Series in the Presence of B0 Drift

We examined the hypothesis that the fat-saturation pulse in a global off-resonance state caused by magnetic field (B(0)) drift produced signal fluctuation in echo planar imaging (EPI) time series. We performed 3 experiments using 2 types of phantoms, one of which was homemade and contained water and a well emulsified fat source. We found that B(0) drift was approximately +30 Hz in the first 30-min EPI time series and +15 Hz in the second series. We experimentally reproduced the signal fluctuations observed during actual measurement in an artificial global off-resonance state using a fat-saturation pulse, the frequency profile of which also affected the pattern of fluctuation. These results are direct evidence that the fat-saturation pulse is a source of signal fluctuation in the presence of B(0) drift.

Some Physical and Clinical Factors Influencing the Measurement of Precision Error, Least Significant Change, and Bone Mineral Density in Dual-Energy X-Ray Absorptiometry

Dual-energy X-ray absorptiometry (DXA) is the standard method of measuring bone mineral density (BMD) at highly trabecular bone, which can be statistically linked to the risk of fracture. For DXA, precision error (PE) and phantom-based accuracy studies are among the most important routine quality control procedures. A precision study was performed at our institution using International Society for Clinical Densitometry guidelines. Comparing our results with those reported by other investigators, we draw the following general conclusions: the PE was higher for the spine than the hip, which we attribute to the better geometric reproducibility at the hip. The hypothesis that the DXA calculates BMD relative to water was validated. Whether follow-up measurements are performed by the same technologist on the same day—or different technologists on subsequent days—does not appear to have a clinically significant impact on PE or least significant change (LSC). Mixing beam types (i.e., fan and pencil) may affect lumbar PE and LSC measurements more significantly than those of the hip. The use of a single technologist may reduce the PE for the lumbar spine but appears to increase it for the hip. Restricting the patient population to the female gender has the apparent effect of narrowing the gap between lumbar and hip PEs. Finally, the degree of BMD measurement accuracy can be affected by the type of phantom being used (e.g., European Spine Phantom vs Lunar phantom) and the faults in specific DXA edge detection algorithms.

Analysis of hip geometry by clinical CT for the assessment of hip fracture risk in elderly Japanese women

Two case–control studies were designed to investigate the contribution of the geometry and bone mineral density (BMD) of the proximal femur to bone strength in Japanese elderly women. We also investigated whether clinical CT is useful to assess the risk of hip fracture. Subjects in the neck fracture study included 20 Japanese women with neck fracture (age: mean±SD; 80.1±4.5 years old) and 20 age-matched control women (79.2±2.6 years old). Subjects in the trochanteric fracture study included 16 Japanese women with trochanteric fracture (82.6±5.0 years old) and 16 age-matched control women (80.8±3.8 years old). CT examination of the proximal femur was performed between the date of admission and the date of surgery. The CT scanners used were an Aquillion 16 (Toshiba) and Somatom 64 (Siemens); the scanning conditions including spatial resolution and scanning energy were adjusted, and the same type of reference phantom containing hydroxyapatite was used. QCT PRO software (Mindways) was used to analyze data for BMD, geometry, and biomechanical parameters. Both the neck and trochanteric fracture cases had significantly lower total and cortical BMD, a significantly smaller cortical cross-sectional area (CSA), and a larger trabecular CSA. Both had significantly thinner cortex and smaller distance to center of bone mass, and women with trochanteric fracture had a significantly smaller cortical perimeter in the cross-sectional femoral neck. Women with neck fracture had a longer hip axis length (HAL) and women with trochanteric fracture had a significantly larger neck-shaft angle (NSA). Both groups had significantly lower cross-sectional moment of inertia (CSMI), and only women with neck fracture had a significantly higher buckling ratio (BR) compared to their respective controls. According to the multiple logistic regression analysis, women with neck fracture had a significantly longer HAL, lower CSMI, and higher BR, and women with trochanteric fracture had a significantly smaller cortical CSA of the femoral neck. We conclude that clinical CT may be useful for the assessment of the risk of neck and trochanteric fracture.

WE‐E‐BRD‐02: Development of a Software for Integrating the Medical Accelerator Model with Patient Phantoms Into Monte Carlo Based Dosimetry Platform

Purpose: To develop a new software package that integrates the detailed medical accelerator model with anatomically‐realistic phantoms in the Monte Carlo engine, MCNPX, for the determination of secondary organ doses from secondary photons and neutrons during the treatment. Method and Materials: The accelerator models were developed include 80 MLC leaves, two pairs of tungsten jaws operating at 6 and 18 MV, respectively. The Pregnant Women, Adult Male and Adult Female phantoms were utilized. The software was designed in the Visual C♯.NET and the preliminary effort was on the graphic user interface (GUI) to automatically generate the MCNPX input deck consisting of accelerator and patient according to user‐defined treatment plans. The MLC configuration files were parsed to collect positions of the jaws and MLCs before they were exported into MCNPX by using the “TRCL” with the “LIKE BUT” card. Results: A number of user‐friendly GUI features have been developed for a user to select treatment parameters: selection of an accelerator type, a type of phantom, specify the phantom origin position and the field numbers, parse in the MLC configuration files and save the MLC leaf positions. According to the positions of the jaws and MLCs, the software can automatically output the MCNPX deck files, classified per field, per segment and per MLC leaf position with the accelerator model and computational phantoms. Conclusion: A new software has been demonstrated. The preliminary results indicate that the GUI design and integration features are feasible and versatile. This software is designed to help a user to carry out organ dose studies using the MCNPX input deck without having to handle complex accelerator and phantom information. Such a tool can facilitate the study for assessing and for comparing various new modalities in terms the likelihood for secondary cancer induction.

SU-FF-T-248: Quality Assurance for Total Marrow Irradiation (TMI) Using Helical Tomotherapy

Purpose: Helical Tomotherapy (HT) offers advantages for total marrow irradiation (TMI) patients.The purpose of this study was to perform a more rigorous validation of TMI treatment plans using two types of radiation QA phantoms. Methods and Materials: Cylindrical solid water phantom: Several different delivery quality assurance (DQAs) were generated at different anatomical sites of the PTV of an actual TMI patient. The DQAs were performed using an ion chamber for point absolute dose measurement and EDR film for relative dose profiles. The ion chamberdose and film dose profiles were compared with the expected dose and dose profiles calculated from the treatment planning station (TPS). Rando Phantom: A HT TMI treatment plan was generated using a Rando phantom as the patient. The prescription was 80% of the PTV receives12 Gy. A DQA was also generated based on this plan. The same CTimage set acquired for planning the patient was also used as the CT scan for the DQA phantom. Film profile dose measurements were performed in transverse planes in between slabs making up the phantom. Results: For Rando TMI treatment plan, the mean dose of the PTV was 12.34±0.65Gy and the other organdoses were also well below currently accepted normal tissue tolerances. The absolute dose measurement and isodose distribution comparisons between planned and delivered doses on both coronal and transverse direction were within the clinical tolerance limit. Conclusion: Dose verification to the large TMI PTV delivered by the HT system was performed using an ion chamber and film in a cylindrical solid water phantom at multiple treatment sites along the patient's axis. Both the absolute point doses and relative dose profiles were within the clinical tolerance of +/−3%. A Rando phantom can be used for more rigorous QA by eliminating differences between an actual patient and a phantom.

Study of Skin Phantoms by Photothermal Radiometry in Frequency Domain and Multivariate Methods

In this paper the use of the photothermal radiometry technique in the frequency domain (PRTF) and the use of multivariate methods in the study of two types of skin phantoms: (a) one in which skin pigmentation was simulated dyeing the gel phantom and (b) the other consists of exposure of animal skin samples to different degrees of thermal damage. In experiment (a), gel phantoms were prepared with different concentrations of methylene blue (MB). The mean values of the radiometry signal (RS) show significant differences in only those cases in which changes in the concentration of MB were higher than 0.38 mM. This result was confirmed with a t test for independent samples of the data (p < 0.05). The mean values of the amplitude and phase signal do not permit discrimination between phantoms with changes in pigmentation equal to or lower than this value. However, principal component analysis (PCA) demonstrated that it is possible to discriminate between phantoms with changes in molar concentration equal to 0.38 mM (for the phase signal). In the case of experiment (b), the following four groups of pork skin samples were analyzed: one consists of samples of fresh skin, while the other three consist of samples exposed to thermal damage at 45 °C (the exposure time was 4 s) and 80 °C (exposure times were 4 s and 8 s, respectively). The mean values of the RS for each group of samples did not show a clear visual discrimination. However, the t test for independent samples applied to the data demonstrated significant differences only between fresh skin and skin exposure to thermal damage at 80 °C (with exposure times of 4 s and 8 s). PCA was used to discriminate between the four different skin samples.

SU‐GG‐T‐508: Evaluation of the Monte Carlo Dose Calculation Engine of Eclipse Treatment Planning System for Electron Beams

Method and Materials: Eclipse©'s electron Monte Carlo (eMC) (Varian, Palo Alto, CA) engine is a fast implementation of Monte Carlo (MC) method for radiotherapy treatment planning with electron beams. Several types of phantoms, combining low and high density heterogeneities ranging from air through lung to cortical bone, are used to evaluate eMC by comparing its results with measurements and with full MC simulations using DOSXYZnrc (NRC, Canada). Measurements with physical phantoms are done using EBT Gafchromic© films, leading to absolute dose. The phantoms are reproduced virtually in order to exclude the effect of the CTscan (HU — density conversion curve, noise, artifacts, …) in DOSXYZnrc and in Eclipse© for full MC and eMC calculations, respectively. Results: Absolute dose profiles obtained via the three methods (measurements, eMC and MC) are compared. As expected, MC simulations agree better with measurements. The three methods agree quite well in general, within 5% even in regions of high dose gradients, which is a great improvement compared over the previous algorithms. However, larger discrepancies (as high as 10%) between eMC and measurements are found under the small heterogeneities, as was already reported by Ding et al (Phys. Med. Biol. 51, 2781 (2006)). Our differences are smaller though probably due to the fact that that they use smaller heterogeneities which is harder to model due to scatter. This is still under investigation. The effect of the use of CT‐defined phantoms instead of the virtual phantoms is also part of this study. Conclusion: Our preliminary results using virtually designed phantoms show that eMC simulations give a good agreement with measurements within uncertainties. This study is very encouraging and shows that it should now be possible to treat the electron beams with the same kind of precision as is already done for photon beams.

SU‐GG‐T‐529: Pre‐Clinical Evaluation of a 4D Tracking DmMLC‐Based Delivery to Lung Tumor

Purpose: The aim of this study is to provide a pre‐clinical evaluation of four‐dimensional tracking radiation therapy to lung tumor using a prototype tracking system. The evaluation was based on films dosimetric analysis, time delay measurements and treatment planning DVH analysis by using two types of dynamic phantoms. Method and Materials: The key component of the 4DTRT system was a prototype of TrackBeam. It consists of an image processing tools and a first‐of‐its‐kind dual‐layer micro MLC. DmMLC has two layers of orthogonal leaves which provide advantages in speed and conformality when forming beam aperture for tracking. The TrackBeam was mounted to a Varian Linac and connected to a workstation which process the online MV fluence and controls each leaf's motion. A Quasar dynamic phantom was used for radiographic film irradiation with 4DTRT and also 3DCRT. The phantom has a Gafchromic film insert and a gold marker in the insert. It can move in Sinusoid mode as well as real patient respiratory cycle. Another tissue‐equivalent thorax dynamic phantom was used for DVH analysis after a phantom‐based 3DCRT planning and a 4DTRT planning developed respectively. Results: The synchronization of marker motion and the DmMLC leaf motion was achieved within less than 0.05 seconds. The films analysis indicated that total 29.91% over the tolerance of 5% and 5.09% of over the tolerance of 5% when the 3DCRT and 4DTRT films compared to a static film. The DVH comparisons indicate 4DTRT reduces significant dose to the Ring from 75% (80% volume) to 60% (50% volume). 4DTRT also reduces considerable amount dose to the total lung from 33% (30% volume) to 22% (22%). Conclusion: The 4D tracking using MLC provides a feasible solution delivering conformal dose to lung tumor and spare the surrounding tissue. Conflict of Interest: The work was partially supported by Initia‐RT medical device.

MO-D-AUD B-04: Parameter Optimization for Brachytherapy Robotic Needle Insertion and Seed Deposition

Purpose: To investigate influence of different needle insertion and seed deposition techniques for robotic brachytherapy. To find optimal sets of low, normal and high translational and rotational velocities of the needle for decreasing insertion force, needle deflection and OR time, and increasing seed placement accuracy. Method and Materials: We have developed EUCLIDIAN — a fully automatic robotic prostate brachytherapy system. Robotic system parameters were optimized via preclinical experiments using two types of polyvinylchloride and tissue phantoms, cannula and stylet single-axis force sensors, and six-axis force-torque sensor. Cannula sensor measures the force on the cannula during insertion, withdraw, and axial force exerted by tissue at rest. Stylet sensor measures the force while seed is expelled from the cartridge, during seed travel through the cannula, and at the moment when seed is deposited into tissue. Position of the needle tip and consequently deposition depth into the phantom was measured using optical encoders on the cannula and stylet motors. Cannula and stylet translational velocity range was 5–120 mm/s, and cannula rotation range was 0–30 rev/s. Force patterns were analyzed based on the experimental data. Results: According to the criteria for minimizing insertion force and OR time while maximizing seed deposition precision, it was found that best performances were achieved when cannula and stylet normal speed was 70 ± 10 mm/s and optimal high speed was 100 ± 10 mm/s. Optimal cannula rotation speed range was 15–25 rev/s. In order to avoid seed jam in the cartridge, optimal speed for pushing seed out of the cartridge was 2–5 mm/s. Conclusion: Optimal parameters were programmed in the EUCLIDIAN configuration files. Seed deposition techniques have significant influence on reduction of insertion force, needle deflection and seed deposition accuracy. Future investigation will be on adaptive parameter tuning for specific clinical encounters. Acknowledgement: Supported by NCI-R01-CA091763.

Can the HML's sliced BOMAB phantom be used in any whole body counter with a reduced number of sources?

The sliced Bottle Manikin Absorber (BOMAB) phantom was originally proposed as an alternative to a commercially available phantom, but it suffers from the disadvantage of containing over 160 sources that need to be manufactured; however, it was found that the number of slices could be reduced substantially and that two slices in the sliced phantom gave the same performance characteristics over a wide energy range as a conventional BOMAB phantom for a particular counting system. This work explores the adaptability of this phantom to another counting geometry. The response of the Human Monitoring Laboratory's whole-body counter measuring this phantom with a decreasing number of planar sources has been modelled using MCNP5 over a wide energy range (122-2754 keV). It was found that the best agreement was obtained when the phantom contained 10 sources, 1 in the mid point of each section. As this is a different result from a previous finding, any other counting geometry will have to be assessed to determine the optimum loading if the sliced phantom is to be used. Also, it is clear that this type of phantom cannot be used for an intercomparison that will encounter different counting geometries, unless it contains a full loading of sources.

Influence of extremely low energy radiation on artificial tissue: Effects on image quality and superficial dose

The design and slicing technique of artificial soft tissue are presented. Artificial soft tissue has optical penetration properties similar to biological tissues. The soft tissues are made of agar dissolved in water as a transparent tissue (control) incorporated with scatter materials such as polystyrene microspheres and absorbers such as artificial dairy substitute, coffee mate (Carnation Co.). The radiation's interaction with 20 and 40 keV X-ray, and visible light (400–800 nm) with different types of tissue phantoms has been investigated. The half value layer (HVL), attenuation coefficient, energy density and penetration depth through the artificial tissues has been calculated. X-ray radiation depth show significant reduction in soft tissue incorporated with polystyrene microspheres. At extremely low energy (E), the half value layer decreases with increasing the energy, while the attenuation coefficient increase. The calculated values of the half value layers are in very good agreement with experimental results. The calculated values of effective linear attenuation coefficient, are found to be µeff(0.22–0.42). Significant reduction in superficial dose with clear image is found with 10 mm soft tissue filter used. These results suggests: possible enhancement in diagnostic imaging and reduction in excess dose to patients; artificial soft tissue can be used as filter substitute.

Influence of extremely low energy radiation on artificial tissue: Effects on image quality and superficial dose

The design and slicing technique of artificial soft tissue are presented. Artificial soft tissue has optical penetration properties similar to biological tissues. The soft tissues are made of agar dissolved in water as a transparent tissue (control) in- corporated with scatter materials such as polystyrene microspheres and absorbers such as artificial dairy substitute, coffee mate (Carnation Co.). The radiation's interaction with 20 and 40 keV X-ray, and visible light (400-800 nm) with different types of tissue phantoms has been investigated. The half value layer (HVL), attenuation coefficient, energy density and penetration depth through the artificial tissues has been calculated. X-ray radiation depth show significant reduction in soft tissue incorporated with polystyrene microspheres. At extremely low energy (E), the half value layer decreases with increasing the energy, while the attenuation coefficient increase. The calculated values of the half value layers are in very good agreement with experimental results. The calculated values of effective linear attenuation coefficient, are found to be μeff (0.22-0.42). Significant reduction in superficial dose with clear image is found with 10 mm soft tissue filter used. These results suggests: possible enhancement in diagnostic imaging and reduction in excess dose to patients; artificial soft tissue can be used as filter substitute.

Design of mini phantom and measurement of cobalt-60 beam data parameters

Low cost mini phantoms were fabricated indigenously with different water equivalent material such as polymethyl methacrylate and Bee's wax of different shapes (with dome top surface and flat top surface). The beam parameters of the Co-60 machine, such as head scatter correction factor (Sh), phantom scatter correction factor (SP), total scatter correction factor (SC,P), collimator exchange effect were measured. Output ratio measurements were taken for both mini phantom and water phantom for different square and rectangular field sizes. Normalized output ratios were compared with ESTRO published values and (Storchi and Van Gasteren) S and G data. The percentage of variation between the measured and the literature values is about 0.7%. Collimator exchange effect were measured for water and mini phantom for different field size, were compared with ESTRO value. This was found to be 0.5% and 1.0% respectively. Phantom scatter correction factors were calculated for square and rectangular filed sizes; this was compared with ESTRO values, found to be 0.7% for square and 1.0% for rectangular filed size. It was also noted that there were no appreciable variation observed in ion chamber readings of different materials of mini phantoms for dome and flat surfaces. Mini phantom measurements were done for all types of phantoms and the measured values were compared with the existing data and they were in good agreement with the published values.

DESIGN AND APPLICATION OF A SINGLE-BEAM GAMMA DENSITOMETER FOR VOID FRACTION MEASUREMENT IN A SMALL DIAMETER STAINLESS STEEL PIPE IN A CRITICAL FLOW CONDITION

A single-beam gamma densitometer is utilized to measure the average void fraction in a small diameter stainless steel pipe under critical flow conditions. A typical design of a single-beam gamma densitometer is composed of a sealed gammaray source, a collimator, a scintillation detector, and a data acquisition system that includes an amplifier and a single channel analyzer. It is operated in the count mode and can be calibrated with a test pipe and various types of phantoms made of polyethylene. A good average void fraction is obtained for a small diameter pipe with various flow regimes of the core, annular, stratified, and bubbly flows. Several factors influencing the performance of the gamma densitometer are examined, including the distance between the source and the detector, the measuring time, and the ambient temperature. The void fraction is measured during an adiabatic downward two-phase critical flow in a vertical pipe. The test pipe has an inner diameter of 10.9 mm and a thickness of 3.2 mm. The average void fraction was reasonably measured for a two-phase critical flow in the presence of nitrogen gas.

Empirical dual energy calibration (EDEC) for cone‐beam computed tomography

Material-selective imaging using dual energy CT (DECT) relies heavily on well-calibrated material decomposition functions. These require the precise knowledge of the detected x-ray spectra, and even if they are exactly known the reliability of DECT will suffer from scattered radiation. We propose an empirical method to determine the proper decomposition function. In contrast to other decomposition algorithms our empirical dual energy calibration (EDEC) technique requires neither knowledge of the spectra nor of the attenuation coefficients. The desired material-selective raw data p1 and p2 are obtained as functions of the measured attenuation data q1 and q2 (one DECT scan = two raw data sets) by passing them through a polynomial function. The polynomial's coefficients are determined using a general least squares fit based on thresholded images of a calibration phantom. The calibration phantom's dimension should be of the same order of magnitude as the test object, but other than that no assumptions on its exact size or positioning are made. Once the decomposition coefficients are determined DECT raw data can be decomposed by simply passing them through the polynomial. To demonstrate EDEC simulations of an oval CTDI phantom, a lung phantom, a thorax phantom and a mouse phantom were carried out. The method was further verified by measuring a physical mouse phantom, a half-and-half-cylinder phantom and a Yin-Yang phantom with a dedicated in vivo dual source micro-CT scanner. The raw data were decomposed into their components, reconstructed, and the pixel values obtained were compared to the theoretical values. The determination of the calibration coefficients with EDEC is very robust and depends only slightly on the type of calibration phantom used. The images of the test phantoms (simulations and measurements) show a nearly perfect agreement with the theoretical micro values and density values. Since EDEC is an empirical technique it inherently compensates for scatter components. The empirical dual energy calibration technique is a pragmatic, simple, and reliable calibration approach that produces highly quantitative DECT images.

SU‐FF‐I‐52: Comparison of CR QC Phantoms: The Kodak TQT Program Versus a General Purpose Phantom Program

Purpose: Recently, the American Association of Physicists in Medicine (AAPM) issued two Task Group reports (AAPM Reports 74 and 93) covering acceptance testing and QA for CR systems. An important component of CR QA programs is routine testing of system performance by the technologist. Testing should be simple to perform, be easy to evaluate, and probe most facets of system operation. To this end a quality control phantom for routine testing is recommended. CR QA phantoms can be vendor‐specific, or general purpose for cross‐platform use. We compare our clinical experience with both types of phantom and discuss their relative merits. Method and Materials: The optional Kodak Total Quality Tool (TQT), and associated analysis software, is designed for acceptance testing and routine QA of the Kodak DirectView CR system. Our institution has used the TQT for more than two years. The Fluke EZ CR/DR “DIN” test tool (DTT) is a general purpose phantom incorporating a standard test pattern. The DTT, with an in‐house analysis package, has been used with our Fuji CR systems for over four years. Results: Both phantoms measure parameters recommended by the AAPM reports. A detailed comparison of measurement capabilities including spatial resolution and linearity, exposure linearity and calibration, low contrast resolution, uniformity, and noise and artifact evaluation is presented. The documentation and integrated analysis features of the Kodak QA program led to easy implementation and management. The DTT analysis used standard imaging tools and subjective measures. While the DTT is useful for cross‐platform comparisons, the TQT appears to be more sensitive in detecting potential equipment problems. Conclusion: Advantages of the TQT program include: tests specific to the reading mechanism, an integrated user interface, and acceptance and understanding by service personnel. The non‐proprietary DTT is useful for comparing different systems using a set of standardized measures.

SU‐FF‐I‐29: Optimizing Radiation Dose and Image Quality in a Multi‐Detector Computed Tomography

Purpose: The study was carried out to find optimal scanning parameters for an MDCT at Ramathibodi hospital. Method and materials: Two types of phantom were employed to assess image quality and standard ionization chamber was used to measure radiation dose. Tube current, slice thickness, pixel size and pitch were varied from routine techniques to evaluate factors affecting image quality (spatial resolution, contrast detectability and image noise) and dose (CTDIair) with aim of optimization. Results: From our study showed that, optimization of radiation dose and image quality could be achieved by proper selection of scanning parameters. Reduction of tube current could be made 22 – 28%, pitch could be doubled from 0.75 to 1.5 (50% dose reduction) in slice thickness less than 7.5 mm without any significant effects on noise and contrast. In examinations that do not require high spatial resolution, larger pixel size could be made and could result in 15 – 17% dose reduction. Conclusions: We concluded from our study that, optimal scanning parameters were 115, 105, 150 and 170 mA for head (above/below posterior fossa) and abdomen (pre‐ and post‐ contrast) protocols, respectively. Slice thickness should be made thinnest possible before the penumbra dose penalty becomes significant. Pitch and pixel size should be selected based on clinical requirements.

Phantom‐based evaluation of geometric distortions in functional magnetic resonance and diffusion tensor imaging

Phantom-based evaluation of geometric distortions in functional MRI and diffusion tensor imaging (DTI) was investigated. An acrylic water-filled phantom with a grid structure was designed and manufactured to provide accurate geometric information over the volume measured in human brain imaging. The grid structures were well detected in data acquired using a 3-T MRI scanner with echo-planar imaging (EPI) sequences commonly applied in functional MRI and DTI. A method for quantifying distortions in the phantom data was presented and applied for the images. The validity of the phantom for EPI was evaluated by quantitatively comparing the distortions present in and induced by the phantom and a human brain when imaged under identical conditions. The results suggest that the new phantom can reveal geometric distortions easily undermined by standard MRI phantoms. For example, prominent variability in the distortions was found as a function of the orientation of the diffusion-sensitizing gradient. Possible future applications for this type of phantom include quality assurance and calibration of the hardware and software used in EPI-based functional MRI and DTI.

An ultrasonic theoretical and experimental approach to determine thickness and wave speed in layered media

This work presents an ultrasonic method to characterize the layers of a stratified medium, using independent measurements of wave speed and thickness of each layer. The model, based on geometrical acoustics, includes refraction. Two transducers are used: one active (3.4 MHz) and a hydrophone as a receptor, which is moved laterally through 15 positions. The distance between the transducers and the delay between the echoes, from the interfaces separating the layers, received by them are used to estimate the speed and thickness. Three types of layered phantoms were used: Ph1 made with alcohol/acrylic, Ph2 made with polyvinyl chloride/water/acrylic, and Ph3 made with acrylic/water/polyvinyl chloride. The experimental results for speed of sound and layer thickness presented an experimental mean relative error, for thickness and wave speed, lower than 7.0% and 6.6%, respectively.