Array Of Pistons Research Articles

Radiation modes and acoustic field confined near acoustic sources.

A method to confine the acoustic pressure distribution near a loudspeaker array is investigated for potential application to personal audio devices. The surface velocity distribution of the loudspeaker array is determined so that more reactive than active acoustic power can be generated. The design is based on the theory of radiation modes, i.e., surface velocity distributions that independently contribute to the complex acoustic power. A computer simulation is conducted for a rectangular array of pistons in an infinite baffle, and it is shown that a locally confined acoustic field can be achieved as expected.

Mathematical Modeling and Analysis of MEMS Deformable Mirror Actuated by Electrostatic Piston Array

SUMMARYIn this report, we constructed a simple mathematical analysis model for a new MEMS deformable mirror using an electrostatic piston array. The deformable mirror has been developed as a wavefront compensation device in adaptive optics for retinal observation. The device realizes a large convex deformation with a low operation voltage because of moving bottom electrodes on the pistons. The constructed model is based on the plate theory and simple superposition of the actuation components. The calculated deformation analyzed using this model agrees well with that analyzed using the finite element method not only in deformation shape but the peak‐valley deformations. In addition, the calculation time is much shorter, so the model can be used for design optimization of the device.

静電ピストンアレイ駆動型MEMS可変形状ミラーの数理解析モデル構築

静電ピストンアレイ駆動型MEMS可変形状ミラーの数理解析モデル構築

Computational analysis of fluid flow within a device for applying biaxial strain to cultured cells.

In vitro systems for applying mechanical strain to cultured cells are commonly used to investigate cellular mechanotransduction pathways in a variety of cell types. These systems often apply mechanical forces to a flexible membrane on which cells are cultured. A consequence of the motion of the membrane in these systems is the generation of flow and the unintended application of shear stress to the cells. We recently described a flexible system for applying mechanical strain to cultured cells, which uses a linear motor to drive a piston array to create biaxial strain within multiwell culture plates. To better understand the fluidic stresses generated by this system and other systems of this type, we created a computational fluid dynamics model to simulate the flow during the mechanical loading cycle. Alterations in the frequency or maximal strain magnitude led to a linear increase in the average fluid velocity within the well and a nonlinear increase in the shear stress at the culture surface over the ranges tested (0.5-2.0 Hz and 1-10% maximal strain). For all cases, the applied shear stresses were relatively low and on the order of millipascal with a dynamic waveform having a primary and secondary peak in the shear stress over a single mechanical strain cycle. These findings should be considered when interpreting experimental results using these devices, particularly in the case when the cell type used is sensitive to low magnitude, oscillatory shear stresses.

Fast ultrasound beam prediction for linear and regular two-dimensional arrays

Real-time beam predictions are highly desirable for the patient-specific computations required in ultrasound therapy guidance and treatment planning. To address the longstanding issue of the computational burden associated with calculating the acoustic field in large volumes, we use graphics processing unit (GPU) computing to accelerate the computation of monochromatic pressure fields for therapeutic ultrasound arrays. In our strategy, we start with acceleration of field computations for single rectangular pistons, and then we explore fast calculations for arrays of rectangular pistons. For single-piston calculations, we employ the fast near-field method (FNM) to accurately and efficiently estimate the complex near-field wave patterns for rectangular pistons in homogeneous media. The FNM is compared with the Rayleigh-Sommerfeld method (RSM) for the number of abscissas required in the respective numerical integrations to achieve 1%, 0.1%, and 0.01% accuracy in the field calculations. Next, algorithms are described for accelerated computation of beam patterns for two different ultrasound transducer arrays: regular 1-D linear arrays and regular 2-D linear arrays. For the array types considered, the algorithm is split into two parts: 1) the computation of the field from one piston, and 2) the computation of a piston-array beam pattern based on a pre-computed field from one piston. It is shown that the process of calculating an array beam pattern is equivalent to the convolution of the single-piston field with the complex weights associated with an array of pistons. Our results show that the algorithms for computing monochromatic fields from linear and regularly spaced arrays can benefit greatly from GPU computing hardware, exceeding the performance of an expensive CPU by more than 100 times using an inexpensive GPU board. For a single rectangular piston, the FNM method facilitates volumetric computations with 0.01% accuracy at rates better than 30 ns per field point. Furthermore, we demonstrate array calculation speeds of up to 11.5 X 10(9) field-points per piston per second (0.087 ns per field point per piston) for a 512-piston linear array. Beam volumes containing 256(3) field points are calculated within 1 s for 1-D and 2-D arrays containing 512 and 20(2) pistons, respectively, thus facilitating future real-time thermal dose predictions.

Area beam equalization: Optimization and performance of an automated prototype system for chest radiography 1

Rationale and objectives To evaluate the feasibility and performance of an x-ray beam equalization system for chest radiography using anthropomorphic phantoms. Materials and methods Area beam equalization involves the process of the initial unequalized image acquisition, attenuator thickness calculation, mask generation using a 16 × 16 piston array, and final equalized image acquisition. Chest radiographs of three different anthropomorphic phantoms were acquired with no beam equalization and equalization levels of 4.8, 11.3, and 21. Six radiologists evaluated the images by scoring them from 1–5 using 13 different criteria. The dose was calculated using the known attenuator material thickness and the mAs of the x-ray tube. Results The visibility of anatomic structures in the under-penetrated regions of the chest radiographs was shown to be significantly ( P <.01) improved after beam equalization. An equalization level of 4.8 provided most of the improvements with moderate increases in patient dose and tube loading. Higher levels of beam equalization did not show much improvement in the visibility of anatomic structures in the under-penetrated regions. Conclusion A moderate level of x-ray beam equalization in chest radiography is superior to both conventional radiographs and radiographs with high levels of beam equalization. X-ray beam equalization can significantly improve the visibility of anatomic structures in the under-penetrated regions while maintaining good image quality in the lung region.

Reshapable physical modulator for intensity modulated radiation therapy.

A new method of generating beam intensity modulation filters for intensity modulated radiation therapy (IMRT) is presented. The modulator was based on a reshapable material, which is not compressible but can be deformed under pressure. A two-dimensional (2D) piston array was used to repeatedly shape the attenuating material. The material is a mixture of tungsten powder and a silicon-based binder. The linear attenuation coefficient of the material was measured to be 0.409 cm(-1) for a 6 MV x-ray beam. The maximum thickness of the physical modulator is 10.2 cm, allowing a transmission of 1.5%. A 16 x 16 square piston array was used to generate a depth pattern in the deformable attenuating material. Each piston has a cross section of 6.37 x 6.37 mm2. The modulator was placed 65 cm from the radiation source of the linear accelerator in the position of the shielding tray. At this position, each piston projects to a 1.0 x 1.0 cm2 area at the isocenter, giving a treatment field of 16 x 16 cm2. The percent depth dose curve and output factor measurement show a slight beam hardening and a 1%-4% increase in scatter fraction when 2.2-4.4 cm uniform thickness filters are in the beam. The surface dose was decreased with the filter in the beam. Ion chamber and verification films were used to verify the entrance dose. The measured absolute and relative doses were compared with the calculated dose. The agreement of measurements and calculations is within 3%. In order to verify the spatial modulation of dose, 1-D dose profiles were obtained using dose calculations. Calculated and measured profiles were compared. The 20%-80% penumbra of the modulator was measured to be 5.5-10 mm. The results show that a physical modulator formed using a 16 x 16 piston array and a deformable attenuation material can provide intensity modulation for IMRT comparable with those provided by currently available commercial MLC techniques.

Area x-ray beam equalization for digital angiography.

An area beam equalization technique has been investigated in order to generate patient-specific compensating filters for digital angiography. An initial image was used to generate the compensating filter, which was fabricated using a deformable compensating material, containing CeO2, and an array of square pistons. The CeO2 attenuator thicknesses were calculated using the gray level information from the initial unequalized image. The array of pistons was pressed against a uniform thickness of attenuating material to generate a filter for x-ray beam equalization. The filter was subsequently inserted into the x-ray beam for the final equalized radiograph. It was positioned close to the focal spot (magnification of 8.0) in order to minimize edge artifacts from the filter. The equalization of x-ray transmission across the field exiting from the object significantly improved the image quality by preserving local contrast throughout the image. The contrast-to-noise ratio (CNR) in the equalized images was increased-by up to fivefold. Phantom studies indicate that equalized images using a relatively small array of pistons (e.g., 8 x 8) produce significant improvement in image quality with negligible perceptible artifacts. Animal studies showed that beam equalization significantly improved fluoroscopic and angiographic image quality. X-ray beam equalization produced an image with a relatively uniform scatter-glare intensity and it reduced the scatter-glare fraction in the previously underpenetrated region of the image from 0.65 to 0.50. Also, x-ray tube loading due to the mask assembly itself was negligible. In conclusion, area beam equalization reduces the scatter-glare fraction and significantly improves CNR in the previously underpenetrated region of the image.

Transient Radiation from Pistons in an Infinite Planar Baffle

An approach is presented to compute the near- and farfield transient radiation resulting from a specified velocity motion of a piston or array of pistons in a rigid infinite baffle. The approach, which is based on a Green's function development, utilizes a transformation of coordinates to simplify the evaluation of the resultant surface integrals. A simple expression is developed for an impulse response function, which is the time-dependent velocity potential at a spatial point resulting from an impulse velocity of a piston of any shape. The time-dependent velocity potential and pressure for any piston velocity motion may then be computed by a convolution of the piston velocity with the appropriate impulse response. The response of an array may be computed using superposition. Several examples illustrating the usefulness of the approach are presented. The farfield time-dependent radiation from a rectangular piston is discussed for both continuous and pulsed velocity conditions. For a pulsed velocity of time duration T it is shown that the pressure at several of the field points can consist of two separate pulses of the same duration, when T is less than the travel time across the piston.

Reception by an Infinite Array of Pistons and Reflection by an Infinite Plane with Mixed Impedances

A plane wave is assumed to be incident upon an infinite array of circular pistons or infinite strips in a rigid-plane baffle. The forces on the pistons, the velocities of the pistons, the power absorbed by the array, and the amplitude of the reflected wave are calculated. Some numerical results for circular pistons are presented. Under optimum conditions, small pistons absorb the same power as large pistons.

Piston-Type Spherical Radiation

In a previous paper, it has been shown that it is possible to specify a desired acoustic pressure pattern and to calculate the continuous velocity distribution on the surface of a spherical transducer that will produce it. A computer program has been written to yield the transducer velocity distribution required to produce a prescribed pressure pattern. A program has also been developed that permits the acoustical field pressure to be calculated when a velocity distribution is prescribed over the surface of the transducer. These two programs are the basic tools in this research. A desired far-field pressure pattern is selected and the continuous transducer velocity distribution computed. This distribution is then approximated by an array of piston lying in a spherical surface and the pressure pattern which the array produces is calculated. This is compared with the original desired pattern. The present paper discusses the problem of approximating the continuous velocity distribution by an array of pistons. In particular the effects of piston size, signal frequency, and transducer radius are discussed. [Work supported by U. S. Navy Ordnance Systems Command.]

Radiation Loading on Pistons and Arrays of Pistons

Expressions are presented whereby the radiation load on a piston or an array of pistons may be determined from physical principles and factors illustrated by various types of well-known radiating surfaces, wave guides, and conical horns. On the assumption that the radiation loading on a piston may be defined sufficiently by its low- and high-frequency characteristics, then the total surface area and not the geometrical shape of the piston is the principal determining factor. In particular, the real component of the load on each unit area of a small piston is directly proportional to the vibrating surface area. The reactive component of the load, however, can be determined only within a factor between one and two in terms of the area. The infinite array of pistons on a plane and the finite array of pistons on a spherical surface are two examples for which accurate expressions of the radiation load are possible in terms of the characteristic properties of wave guides and conical horns. The principal term of the expression is proportional to the radiation load on the array if it is assumed that all of the array surface is vibrating actively. The factor of proportionality is the fractional portion of the surface that is actually active. The secondary term depends on the free-space characteristics of one of the units in the array and also on the spacing between the units.