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SEMMELWEIS UNIVERSITY Medical Biophysics II. Final - Theory topics Exam
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SEMMELWEIS UNIVERSITY MedicalBiophysics II. Final - Theory topics Exam
X-rays: Typical diagnostic wavelength and photon energy range. - Correct Answers -
100ev-200 keV min. wavelength
6pm-12nm
Structure and function of the x-ray tube. - Correct Answers -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. - Correct Answers -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. - Correct Answers -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 - Correct Answers -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. - Correct Answers -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 i
s 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. - Correct Answers -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. - Correct Answers -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. - Correct Answers -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. - Correct Answers -when the elementary densities along
the direction of the x-ray beam are summed. Produces the projection image.
X-ray image amplifier. - Correct Answers -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) - Correct Answers -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. - Correct Answers -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. - Correct Answers -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. - Correct Answers -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. - Correct Answers -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.
Stokes drag law. - Correct Answers -Describes the force drag acting on a particle being
proportional to the radius of the particle.
Laminar and turbulent flow. - Correct Answers -laminar:
fluid layers do not mix, position of the flowing particles relative to each other is constant
turbulent: (above critical flow velocity). fluid layers mix, slower flow (compared to
laminar flow)
Real fluids: Newton's law of friction. - Correct Answers -Real fluids are those in which
friction exists between its layers. Fluids which adhere to this law are "Newtonian fluids".
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