Function Of Phantom Thickness Research Articles

Light-cell interactions in depth-resolved optogenetics.

Light as a tool in medical therapy and biological research has been studied extensively and its application is subject to continuous improvement. However, safe and efficient application of light-based methods in photomedicine or optogenetics requires knowledge about the optical properties of the target tissue as well as the response characteristics of the stimulated cells. Here, we used tissue phantoms and a heart-like light-sensitive cell line to investigate optogenetic stimulation through tissue layers. The input power necessary for successful stimulation could be described as a function of phantom thickness. A model of light transmission through the tissue phantoms gives insights into the expected stimulation efficiency. Cell-type specific effects are identified that result in deviations of the stimulation threshold from the modelled predictions. This study provides insights into the complex interplay between light, tissue and cells during deep-tissue optogenetics. It can serve as an orientation for safe implementation of light-based methods in vivo.

SU-E-I-24: Method for CT Automatic Exposure Control Verification

Purpose: Design of a phantom and a simple method for the automatic exposure control (AEC) verification in CT. This verification is included in the computed tomography (CT) Spanish Quality Assurance Protocol. Methods: The phantom design is made from the head and the body phantom used for the CTDI measurement and PMMA plates (35×35 cm2) of 10 cm thickness. Thereby, three different thicknesses along the longitudinal axis are obtained which permit to evaluate the longitudinal AEC performance. Otherwise, the existent asymmetry in the PMMA layers helps to assess angular and 3D AEC operation.Recent acquisition in our hospital (August 2014) of Nomex electrometer (PTW), together with the 10 cm pencil ionization chamber, led to register dose rate as a function of time. Measurements with this chamber fixed at 0° and 90° on the gantry where made on five multidetector-CTs from principal manufacturers. Results: Individual analysis of measurements shows dose rate variation as a function of phantom thickness. The comparative analysis shows that dose rate is kept constant in the head and neck phantom while the PMMA phantom exhibits an abrupt variation between both results, being greater results at 90° as the thickness of the phantom is 3.5 times larger than in themore » perpendicular direction. Conclusion: Proposed method is simple, quick and reproducible. Results obtained let a qualitative evaluation of the AEC and they are consistent with the expected behavior. A line of future development is to quantitatively study the intensity modulation and parameters of image quality, and a possible comparative study between different manufacturers.« less

SU‐FF‐T‐52: A Test for Water Equivalence of Solid Phantoms in the Kilovoltage Photon Energy Range

Purpose: To test water equivalence of solid phantoms in the kilovoltage photon energy range Method and Materials: Three solid water‐type phantoms and perspex were compared with water. Technetium‐99m radionuclide was confined within and collimated by a lead container to produce a narrow beam of photons of 5 mm diameter. The transmitted gamma rays were measured as a function of phantom thickness using a Phillips Skylight gamma camera. An energy window of ±10% was set to exclude low energy scatter. This setup was modeled within the EGSnrc Monte Carlo code V4.2.2 and the FLURZnrc user code. The total fluence was scored after passing through the different phantom thickness. The linear attenuation coefficient was calculated from the resultant transmission curves and compared with published NIST data using the XCOM program. Results: The measured transmission values for the Plastic Water, RMI‐457 Solid Water and RW3 phantoms were in good agreement with those for water, with a maximum difference of 1.5%. However the transmission values through perspex differed by up to 4% as compared to water. The agreement between EGSnrc and measured transmission values was good, with a maximum difference of 1.3% for all phantom materials. The linear attenuation coefficients derived from measurements and EGSnrc calculations agreed to within 2.4%. For all phantom materials, the NIST linear attenuation coefficient was greater than the measured and EGSnrc calculated coefficients. Differences between experimental values and published NIST data are attributed to the energy resolution of the scintillator crystal within the gamma camera and limitations in the source description for the Monte Carlo calculations. Conclusion: This work has demonstrated a simple method for testing water equivalency of solid phantoms at low photon energies using gamma ray transmission values determined from measurements and Monte Carlo calculations.

Technique charts for Kodak EC-L film screen system for portal localization in a 6MV X-ray beam

Port films are used in radiotherapy for visual evaluation of the radiation fields and subsequent quantitative analysis. Common port films suffer from poor image quality compared to the simulator-diagnostic films and is desirable to determine the appropriate exposure required for the best image contrast. The aim of this work is to generate technique charts for the Kodak EC-L film screen system for use in a 6MV x-ray beam. Three homogeneous water phantoms were used to simulate head-neck, thorax and abdomen dimensions of adult human, correspondingly. The film screen system was calibrated in a 6MV x-ray beam and under various irradiation conditions. The film screen system behavior was studied as a function of phantom thickness, field size and air gap between the phantom and the film screen system. In each case the optimum film exposure which produces the maximum image contrast was determined. The generated technique charts for the EC-L film screen system and for a 6 MV x-ray beam are used in our radiotherapy department for daily quality assurance of the radiotherapy procedure.

X-ray dose to the skin in patients undergoing percutaneous transluminal coronary angioplasty.

Dose measurements were performed with an ionization chamber placed on the surface of a polystyrene phantom to estimate the radiation dose to the skin from fluoroscopy in patients undergoing PTCA and to define parameters predicting for high-risk irradiation conditions. Dose rate changes were analyzed as a function of phantom thickness, X-ray source-to-phantom distance, image intensifier-to-phantom distance, and field size. Skin dose calculations were made in 38 PTCA patients to validate the model. Thickness was the most important factor determining dose rate changes in the phantom's surface. Dose rate increased by a factor of almost 4 with each 10-cm increment in thickness, doubled upon decreasing the field diameter from 17 cm to 14 cm, and increased by a factor of 1.2 to 1.8 upon decreasing the source-to-phantom distance or by increasing the image intensifier-to-patient distance. All these parameters may significantly increase the dose to the skin and augment the risk for skin injuries post-PTCA.

Position-dependent scatter response functions: will they make a difference in SPECT conducted with homogeneous cylindrical phantoms?

The author explains why it is possible to perform accurate quantitative SPECT when scatter correction is based on stationary and non-stationary scatter functions. This is achieved by comparing the variations of scatter parameters as a function of phantom thickness. The results show that the decrease of scatter fraction with phantom thickness and the decrease of values of scatter kernel inside the field of view are about equal. The deviation of the position-dependent slope from the average value is small for central distributions. These observations explain why estimations of scatter projection by non-stationary convolution and by stationary convolution are comparable when SPECT measurements are conducted with uniform cylindrical phantoms. It is concluded that investigations on the perceived superiority of non-stationary over stationary scatter subtraction in SPECT should be conducted with elliptic phantoms that deviate appreciably from cylindrical shape.

Quantification techniques for dual-energy cardiac imaging.

We have previously reported a motion immune dual-energy subtraction technique in which x-ray tube voltage and x-ray beam filtration are switched at 30 Hz between 60 kVp (2.0-mm Al filter) and 120 kVp (2.0-mm Al + 2.5-mm Cu filtration). In this paper we consider the suitability of these dual-energy images for quantitative measurements of iodine thickness and volume. Optimized iodine signal-to-noise ratio (S/N) was measured as a function of phantom thickness. Using a fixed mAs, the S/N of the dual-energy images was found to decrease by sevenfold as lucite thickness increased from 10 to 25 cm. For the same increase in lucite thickness S/N for time subtraction images decreased by fivefold. Image quality in two human volunteers was subjectively judged to be good. In order to quantitate physiological parameters such as ejection fraction and left ventricular volume, energy dependent corrections for scatter and veiling glare, beam hardening, detector nonuniformity, heel effect, and uncanceled bone signals were developed. Since the dual-energy technique does not completely cancel bone, a preinjection dual-energy subtraction image was used to estimate integrated bone contributions to iodine volume measurements. In a phantom measurement simulating exercise ventriculography, the known (Vk) and videodensitometrically measured (Vm) volumes of 19 mg/cm3 solution of iodine were related by Vm = 0.95 Vk + 1.50 cm3 (r greater than 0.99).

A signal processing model of diagnostic x-ray scatter.

A model of scatter is developed from a signal processing approach. The scattering process is viewed as a nonlinear filter (NLF), which transforms a two-dimensional signal representing phantom thickness into a two-dimensional signal of scattered radiation. The NLF point spread function (PSF) is derived from a single scattering model, using the principles of Compton scattering and x-ray attenuation. The PSF is characterized by three approximations: a constant geometric shape, a volume that depends on the phantom thickness, and a width that depends on the phantom-to-detector distance. This leads to a closed form expression for the scatter-to-primary ratio as a function of phantom thickness, field size, photon energy, source-to-phantom distance, and phantom-to-detector distance. The NLF model is compared with previously reported measurements using constant thickness phantoms, and discrepancies are discussed. The good agreement found between the NLF model and measured data shows that the functional dependence of scatter on the above parameters, previously only explained in terms of empirical models or Monte Carlo simulations, can be incorporated into a signal processing model.