Ī weakness of ARFI imaging is its inability to identify the tissue’s composition. The repetition of this reference push-track procedure over other spatial locations allows the creation of a 2D image by aligning the displacements of each location in time relative to its pushing pulse. To observe the recovery of tissue as it returns to its original configuration, approximately 4–6 ms of tracking are required. Thereafter, with the help of conventional US, a series of tracking lines are acquired for monitoring displacement and recovery of the tissue. Afterward, a radiation force impulse is induced in the same location to create a slight displacement. To obtain displacement information for a spatial location through ARFI imaging, a reference line is first acquired by a conventional US pulse. This technique utilizes a single transducer for both transmitting the radiation force and tracking the resulting displacement of tissue. ĪRFI transmits a brief acoustic radiation force (0.003–1 ms) to generate a localized displacement in tissue. As a result, this technique offers a qualitative color-coded or grayscale elastogram representing relative tissue stiffness. ARFI is based on the principle that as the US pulse passes a tissue, the displacement of soft tissues is larger than the displacement of hard tissues. In this technique, the tissue is excited internally by a focused US pulse. Whatever their origin, these stresses can be advantageous, as in the case of shot peening or the thermal toughening of glass.Īrmin Schneider, Hubertus Feussner, in Biomedical Engineering in Gastrointestinal Surgery, 2017 5.4.5.1 Acoustic Radiation Force Impulse ImagingĪRFI imaging is a form of strain elastography which is mainly used for liver, thyroid, and breast imaging. They might arise because of misfits between different phases, such as the matrix and reinforcement of a composite, or due to anisotropic thermal expansion of the differently oriented grains of a polycrystal, or at an atomic level due to the presence of dislocations and defects. These residual stresses are called microstresses (see Residual Stresses: Macro and Micro Stresses). In other cases the misfit is accommodated over relatively short distances. In many cases these misfits span large distances, for example, the macrostresses between reinforcements and concrete in prestressed concrete. Whatever their cause or classification, they originate from misfits ( Fig. They may be categorized by cause (e.g., thermal mismatch stresses), by the scale over which they self-equilibrate (e.g., macro- or microstresses), or according to the methods by which they can be measured. Residual stresses are difficult to assess, because they are self-equilibrating and so, unlike applied stresses, they cannot be calculated from the forces, impulses, and couples experienced by a material, component, or assembly. * This span has single conductor phases (no bundle spacers to be removed). Exposures are totaled for danger areas under each phase of each modeled span assuming either end fails. Note: Total Station Risk is calculated for the whole station per year. K = a coefficient accounting for 53 m spans not modeled explicitly (= 14/6)į = 19.5 - for all lines connected to Middleport TS (230 kV) P i prProbability of Accident (per span).With bundle spacers removed from 53 m spans With bundle spacers present in 53 m spans The mismatch there is primarily due to the unmodeled high-frequency dynamics. Excellent agreements are found in all but the very high frequency range. The magnitude and phase plots of its transfer functions are compared to the experimental FRF data in Figures 6.3 and 6.4. The final discretetime state-space realization has 10 states. The key identification parameters are given in Table 6.1. The MF parameterization is chosen for the curve-fitting, which is complemented by the ERA method for state-space realization. The experimental FRF is preconditioned to eliminate the second order direct current (dc) zeros from acceleration measurement. The sampling rate is set at 256 Hz, and the frequency resolution is set at 0.125 Hz. For this purpose, a DSPT Siglab spectrum analyzer is used to measure the FRF data. The goal of the identification is to capture accurately the first five pairs of the complex poles of the structure. The accelerations of these floors are selected as the system measurement outputs and are sensed by PCB accelerometers. The system is excited by impulse force produced by a PCB hammer and applied individually at the 16th, 12th, 8th, and 4th floors. The first identification target is a 16-story steel structure model shown in Figure 6.2.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |