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Objective and Camera Lenses
Published in Robert J. Parelli, Principles of Fluoroscopic Image Intensification and Television Systems, 2020
Although relative aperture is the correct technical term, the much more common term used to describe this characteristic of a lens is called speed. Basically this has to do with the light-gathering ability of the lens and is of most importance in photographic systems in determining the amount of exposure necessary to produce a negative of the correct density. As any camera fan knows, this is usually referred to as the “f” number. It is a dimensionless number expressing the ratio of the focal length divided by the diameter of the entrance of the pupil of the lens. Figure 2.3 illustrates this characteristic. The definition of aperture ratio, or the f/number, can be expressed mathematically as
Physics
Published in Peter R Hoskins, Kevin Martin, Abigail Thrush, Diagnostic Ultrasound, 2019
Kevin Martin, Kumar V Ramnarine
The simulations in Figure 2.22 show that when the f-number is low, e.g. 2.7, the focusing action is strong, i.e. there is a large reduction in beam width, but the length of the focal zone, referred to as the depth of focus, is quite short. For a higher f-number, e.g. 4, the focusing action is weaker. That is, there is less reduction in the beam width, but the depth of focus is greater. Hence, weaker focusing is associated with more moderate reductions in beam width and a longer focal zone. The depth of focus dF can be estimated from the following equation:
EM behavior when the wavelength is much smaller than the object
Published in James R. Nagel, Cynthia M. Furse, Douglas A. Christensen, Carl H. Durney, Basic Introduction to Bioelectromagnetics, 2018
James R. Nagel, Cynthia M. Furse, Douglas A. Christensen, Carl H. Durney
A common way to specify the focusing power (or equivalently, the light-gathering capability) of a lens is by its f-number, sometimes given as f# or f/. The f-number of a lens is defined by
Sonication strategies toward volumetric ultrasound hyperthermia treatment using the ExAblate body MRgFUS system
Published in International Journal of Hyperthermia, 2021
Kisoo Kim, Muhammad Zubair, Matthew Adams, Chris J. Diederich, Eugene Ozhinsky
We have found that a less focused transducer allowed a wider range of off-axis steering (Figure 2(a)). Using the partial array activation, two different sizes of sonication cells with four focal spots were implemented to demonstrate the feasibility of various heating volumes. Heated regions were generated between the focal spots, forming a square shape in a transverse plane since the heat distributions of each focal spot were overlaid continuously. There were many control parameters such as the number of focal spots, cell size, heating time, and beam sweeping time. In this study, four focal spots were used to create a cell, based on the duration of each focal point (500 ms). The results for the sonication cell method demonstrated that 60% of array elements allowed off-axis distances between 5 and 10 mm along the focal depths between 100 and 160 mm while avoiding unwanted heat gain induced by the second lobes (Figure 3(b)). However, we did not evaluate the steering performance according to a wide range of f-numbers using the ExAblate body transducer. Further study is needed to investigate an optimized off-axis distance depending on the f-number.
The histotripsy spectrum: differences and similarities in techniques and instrumentation
Published in International Journal of Hyperthermia, 2023
Randall P. Williams, Julianna C. Simon, Vera A. Khokhlova, Oleg A. Sapozhnikov, Tatiana D. Khokhlova
While the focal length of the transducer is dictated largely by the depth of the treatment target, the desired F-number for an application therefore dictates the aperture used for the transducer. In general, transducer arrays used for intrinsic threshold histotripsy have the largest apertures, which can be in the range of approximately 10 cm to 20 cm [44,45,77], although smaller transducers have been used for in vivo experiments in rodents [83]. Shock-scattering histotripsy tends to use transducers with apertures of about 10 cm to 15 cm [36,79,80]. Boiling histotripsy can be realized using the smallest transducer apertures, in the range of approximately 4 cm to 14 cm [11,25,33, 37,76,84,85].
Evaluating treatments with rifabutin and amoxicillin for eradication of Helicobacter pylori infection in adults: a systematic review
Published in Expert Opinion on Pharmacotherapy, 2022
Elias Xirouchakis, Sotirios D. Georgopoulos
An overall estimation of published studies was carried out. Our primary end point was successful eradication of H. pylori based on Intention to Treat (ITT) analysis. Secondary end points were results based on Per-Protocol (PP) analysis and safety of treatment. Extracted variables for presentation were a) authors names, b) country of origin, c) year of publication, d) treatment regimen, d) line of treatment, e) duration of treatment, f) number of patients, and g) eradication rate and absolute numbers of patients. Safety of treatment was evaluated considering h) adherence and i) adverse effects. Separate analyses were made for therapy as first-, second-, and third- line and beyond.