lectures Flashcards
x-ray: typical diagnostic wavelength and photon energy range
100 eV -200KeV
6pm- 12 nm wavelength
structure and function of the x-ray tube
a vacuum tube with anode and cathode.
The cathode is heated and electrons are emitted, they are accelerated by a voltage difference in the tube.
when they “brake” against the material of the anode, they release their high kinetic energy in the form of an x-ray
** x ray tubes are very low efficiency, most of the radiation is dissipated as heat.
the Duane Hunt law
give the minimum wavelength of x radiation produced by braking radiation ( Bremsstrahlung )
as a function of voltage by this formula :
min wavelength = k/voltage
k is constant
spectrum of Bremsstrahlung
a continuous spectrum illustrating the breaking radiation released by accelerating electrons.
Increasing the voltage will result in electrons with higher kinetic energy, x-ray photons of higher frequencies will be emitted. changing the heating of the cathode will result in more electrons. thus a higher current. The area under the curve represents the total emitted power.
production of characteristic X-ray
occurs when an electron with sufficient energy ejects an inner shell electron from the atoms of the anode.
This state is unstable so an electron from an outer shell fills the vacancy and the excess of energy is released in the form of x-ray radiation.
due to the fact that its cause by quantum transition its spectrum is linear and characterizes the matter of the anode.
power and efficiency of the x-ray tube
efficiency - very low! 1%
most of the energy from the accelerating electrons will be dissipated as heat. The anode must be cooled down and material of high melting point should be used. ( tungsten )
can be calculated as the ratio between the power in (P=U*I) and the power out.
POWER- is proportional to the voltage, the current and the atomic number of the anode.
Px=cxUUIZ
mechanism and energy dependence of x-ray absorption
two types of mechanism: Compton scattering and photo effect.
The photon effect is the dominant process and its dependency on the atomic number is on the power of the third.
formula:
m= C*(wavelength in the 3rd power )( Z in the third power )
thus it is important to differentiate between different materials in the body and produce a structural image by the different absorption level )
**for soft tissues imaging (have relatively low atomic number ) we use soft radiation with a bigger wavelength to compensate for their low atomic number.
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x-ray contrast media
to produce an image, the intensity of the radiation leaving the body is measured. Therefore, the contrast in imaging depends on the difference in absorbance of the x-ray radiation, thus on the attenuation coefficient of the media ( which is a product of the mass attenuation coefficient and the density).
The two main absorption processes are Compton scattering and photo effect.
The photon effect is the dominant process and its dependency on the atomic number is on the power of the third.
formula:
m= C*(wavelength in the 3rd power )( Z in the third power )
- CAT scan - principles and generations.
a method used based on x-ray which scans part of the body from different angles and provides a cross-sectional image of the body.
x-ray alone can not give us information on the depth of the body, by summation of a lot of x-ray images that were taken from different angles we can divide the area we are interested in to voxels that are similar to pixels in an image.
generation : ??
the x-ray summation image
when the elementary densities along the direction of the x-ray beam are summed. producing the projection image.
x-ray image amplifier
can be used in real-time surgery because the image being screened to a monitor.
advantages: the amplified image is miniaturized with respect to the original x-ray image (spatial resolution decreases )
composed of a device containing 2 luminescence screens, electrodes, and photocathode.
steps of the processes :
a. Body x-ray image appears on the first luminescence screen ( the attenuated radiation from the body is proportional to the number of the optical photons.)
b. luminescence light emitted to the photocathode produces photoelectrons which are proportional to the intensity of the radiation. those electron being accelerated and focused by high voltage
c. the electron reaches the second luminescence screen and through the electron lens thus we get a real image. reversed miniaturized and very luminous
- CAT- scene image reconstruction?
?????????
- DSA ( digital subtraction angiography )
aim: creating an image that focuses on the blood vessels. steps:
a . an x-ray image is taken without contrast image (native )
b. contrast agent is injected > additional image is taken (contact )
c. by subtracting the native image from the contrast image we produce a new image that concentrates on the vessels.
sound as a wave
Sound is a mechanical wave - a “pressure wave” which are
sinusoidal oscillations. It is a longitudinal wave in liquid and in
gases, which means that the oscillation is parallel to the direction of
propagation. In solids the waves can be either longitudinal or
transverse.
- Acoustic impedance, the reflection of sound, reflectivity
להתסכל בתשובות
Acoustic impedance: quantity that shows to what extent the medium
resists when particles are brought into motion, practical equation:
( product of the density and the speed of sound in the
specific medium) (Z also can be measured by the ratio of the square
root of density and compressibility)
reflection: At the boundary of 2 media part of the intensity of the sound reflect into the opposite direction and not penetrate the body . THe extent of this phenomena is measured by the reflectivity equation.
**ultrasound diagnostic method are based on that phenomenon .
reflectivity : measured by the ratio of the reflected intensity according to this formula :להתסכל
when r=1 max reflection
when R=0 no reflaction
