ch24 Flashcards
piezo-electric transducer
A device that converts electrical energy into ultrasound energy by means of a piezo-electric crystal such as quartz.
pd and crystal
Piezoelectric transducers utilize piezoelectric crystals, which generate an electromotive force (e.m.f.) when deformed, either by compression or stretching. Applying a potential difference (p.d.) causes deformation, with reversal leading to expansion. Alternating p.d. induces vibration at the same frequency. Crystals are cut to specific sizes for resonance. Quartz, a common piezoelectric crystal, comprises a silicon dioxide lattice, which, when distorted, creates an electric field and current. Application of electric current causes lattice shape alternation, producing sound waves. Current flows conventionally from positive to negative regions of the crystal
specific acoustic impedance
Z
= ρc //// density ρ of the medium and the speed c of the wave in the medium
intensity reflection coefficient
IR/I0= (Z2 –Z1)^2/ (Z2 + Z1)^2
e reduction in intensity is approximately exponential/// attenuation of ultrasound and x ray in matter
I = I0 e^–μx where μ is a constant for the medium known as the linear absorption (or attenuation) coefficient.
production of x ray
When charged particles undergo acceleration or deceleration, they emit electromagnetic radiation known as Bremmstrahlung or “braking” radiation. The frequency of this radiation depends on the magnitude of the acceleration, with higher acceleration resulting in greater frequency of emitted photons. In the context of generating X-rays, high-speed electrons are accelerated through a potential difference of thousands of volts before striking a metal target. The sudden change in direction and rapid loss of kinetic energy of these electrons upon impact with the target cause large decelerations, leading to the emission of X-ray photons. However, not all of the electrons’ energy is emitted as X-ray photons; a majority is converted to thermal energy in the target metal.
the cut-off (minimum) wavelength λ0
λ0 = hc/e //// where e is the charge on the electron and V is the accelerating potential difference
computed tomography (CT)
produces a 3D image of an internal structure by first combining multiple X-ray images taken in the same section from different angles to obtain a 2D image of the section, then repeating this process along an axis and combining 2D images
of multiple sections
tracers.
A tracer is a chemical compound in which one or more of its atoms have been
replaced by radioactive nuclei of the same element that can then be used to locate
or follow the progress of the compound in living tissues.
annihilate meaning
When particles and antiparticles meet, they annihilate each other, releasing their combined mass as energy in the form of photons.
During the process of annihilation, conservation laws are followed. Namely
» charge: the positive charge and the negative charge on the particle and antiparticle
are equal in magnitude. The overall charge remains as zero.
» mass-energy: when the particle and the antiparticle collide, since mass and energy are equivalent, the total mass-energy before annihilation must equal the total mass-energy after the annihilation.
» momentum: momentum is conserved so the total momentum before annihilation
must equal the total momentum after annihilation
Explain the principles of the generation of ultrasound waves for use in medical diagnosis
quartz crystal B1
alternating p.d. across crystal causes it to vibrate B1
resonance occurs when frequency of p.d. matches natural frequency of crystal B1
natural frequency of crystal is in ultrasound range