SEMMELWEIS UNIVERSITY Medical
Biophysics II. Final - Theory Exam 2026
Questions and Answers
X-rays: Typical diagnostic wavelength and photon energy range. - Correct Answer-
100ev-200 keV min. wavelength
6pm-12nm
Structure and function of the x-ray tube. - Correct Answer-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 Answer-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 Answer-A continuous spectrum illustrating
the "breaking" radiation released by accelerating electrons. Increasing the voltage
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,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 Answer-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 Answer-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
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,Mechanisms and energy dependence of x-ray absorption. - Correct Answer-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 Answer-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 Answer-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
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, divide the area which we are investigating into voxels which are similar to pixels
in an image.
The x-ray summation image. - Correct Answer-when the elementary densities
along the direction of the x-ray beam are summed. Produces the projection image.
X-ray image amplifier. - Correct Answer-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
©COPYRIGHT 2025, ALL RIGHTS RESERVED 4
Biophysics II. Final - Theory Exam 2026
Questions and Answers
X-rays: Typical diagnostic wavelength and photon energy range. - Correct Answer-
100ev-200 keV min. wavelength
6pm-12nm
Structure and function of the x-ray tube. - Correct Answer-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 Answer-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 Answer-A continuous spectrum illustrating
the "breaking" radiation released by accelerating electrons. Increasing the voltage
©COPYRIGHT 2025, ALL RIGHTS RESERVED 1
,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 Answer-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 Answer-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
©COPYRIGHT 2025, ALL RIGHTS RESERVED 2
,Mechanisms and energy dependence of x-ray absorption. - Correct Answer-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 Answer-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 Answer-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
©COPYRIGHT 2025, ALL RIGHTS RESERVED 3
, divide the area which we are investigating into voxels which are similar to pixels
in an image.
The x-ray summation image. - Correct Answer-when the elementary densities
along the direction of the x-ray beam are summed. Produces the projection image.
X-ray image amplifier. - Correct Answer-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
©COPYRIGHT 2025, ALL RIGHTS RESERVED 4
