Semmelweis University Medical Biophysics II. Final - Theory topics
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Cours
Medical Biophysics
Établissement
Semmelweis University, Faculty Of Medicine
X-rays: Typical diagnostic wavelength and photon energy range. - 100ev-200 keV min. wavelength
6pm-12nm
Structure and function of the x-ray tube. - A vacuum tube with an anode and a cathode. The cathode is heated and electrons are emitted, they are accelerated by a voltage difference in the tub...
SEMMELWEIS UNIVERSITY Medical
Biophysics II. Final - Theory topics
X-rays: Typical diagnostic wavelength and photon energy range. - 100ev-200 keV
min. wavelength
6pm-12nm
Structure and function of the x-ray tube. - A vacuum tube with an anode and a
cathode. The cathode is heated and electrons are emitted, they are accelerated by a
voltage difference in the tube. When the "brake" against the material of the anode,
they release their high kinetic energy in the form of an x-ray. *x-ray tubes are of
very low efficiency, most of the radiation is dissipated as heat.
The Duane-Hunt-law. - Gives the minimum wavelength of x-radiation produced by
braking radiation (Bremsstrahlung) as a function of voltage by this formula:
minimum wavelength = k/U, k is a 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-rays - 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 energy
is released in the form of x-ray radiation.
,Due to the fact that it's caused by quantum transition (b/w defines energy states) its
spectrum is linear and characterizes the matter of the anode.
Power and efficiency of the x-ray tube. - Efficiency is very low, 1%. Most of the
energy from the accelerating electrons will be dissipated as heat. The anode must
be cooled down and a 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 (tungsten).
Px=cxU^2IZ
Mechanisms and energy dependence of x-ray absorption. - two types of
mechanism: photo-effect and compton-scattering. The photo-effect is the dominant
process and it depends on the atomic number on the thirds power. formula : rho m
= C*wavelength^3*Z^3
thus, it is possible to differentiate btw. different materials in the body and produce
structural image (by the
different absorption level)*for soft tissue imaging (have relatively low atomic
number) we use soft radiation with bigger wavelength to compensate for their low
atomic number.
X-ray contrast media. - To produce an image, the intensity of the radiation leaving
the body is measured. Therefore, contrast in imaging depends on the difference in
absorbance of the x-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 photo effect and Compton scattering. Photo Effect
has a high dependency on atomic number.
CAT-scan: Principles, generations. - A method based on xray which scans part of
the body from different angles and provides a cross-sectional image of the body. x-
,ray alone can't 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 which we are investigating into voxels which are similar to pixels
in an image.
The x-ray summation image. - when the elementary densities along the direction of
the x-ray beam are summed. Produces the projection image.
X-ray image amplifier. - can be used in real time surgery bc the image is being
screened to a monitor.
advantage: reduces the dosage needed for an x-ray image and produce digitizable
optical image without using an x-ray absorption film
disadvantage: the amplified image is miniaturized with respect to the original x-ray
image (spatial resolution decreases).
composed of a device containing 2 luminescence screens, electrode and photo
cathode.
steps of the process:
a. body x-ray image appear 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 photo-electrons 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
DSA. (digital subtraction angiography) - Aim: creating an image which focus on
the blood vessels steps of procedure:
a. x-ray image is take w/o contrast image (native)
b. contrast agent is injected→ additional image is taken (contrast)
, c. by subtracting the native image from the contrast image we produce new images
that concentrate on the vessels
Hounsfield unit, windowing in CAT scan. - Absorption coefficient and density
values for CT imaging are measured in Hounsfield units.
N CT = 1000* (mu-mu W)/(mu W)
Windowing in CTis contrast manipulating technique in which a range of CT
number values (which are density values) can be selected for the grayscale display
of the image while others can be ignored.
For example; soft tissue window, lung window or brain window.
Production of high-energy x-rays. - Medical linear accelerators accelerate electrons
using a tuned-cavity waveguide, in which the RF power creates a standing wave.
Some linacs have short, vertically mounted waveguides, while higher energy
machines tend to have a horizontal, longer waveguide and a bending magnet to
turn the beam vertically towards the patient. Medical linacs use monoenergetic
electron beams between 4 and 25 MeV, giving an X-ray output with a spectrum of
energies up to and including the electron energy when the electrons are directed at
a high-density (such as tungsten) target. The electrons or X-rays can be used to
treat both benign and malignant disease. The LINAC produces a reliable, flexible
and accurate radiation beam.
Volumetric flow rate, stationary flow. - Iv= ΔV/Δt
the change in volume per second (unit: m3 /s )
Stationary flow: a flow that is steady in time.
Bernoulli's law, plasma skimming. - Bernoulli's law: in a steady flow, the sum of
all the energy in a fluid along a streamline is the same everywhere on the
streamline. plasma skimming: The natural separation of red blood cells from
plasma at bifurcations in the vascular tree, dividing the blood into relatively
concentrated and relatively dilute streams.
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