IONIZING RADIATION MCQs FOR RADIOGRAPHER EXAM PREPARATION
1. Which statement about the differences between medical imaging using X-rays and a nuclear medicine scan using gamma rays is correct?
A.
An X-ray procedure leaves the patient with residual radioactivity while nuclear
medicine does not.
B.
A gamma ray source can be switched off after which no gamma radiation is produced
while an X-ray source will continue to produce radiation until the source
decays.
C.
X-rays produce an image of internal anatomy while a nuclear medicine scan provides
information about the functioning of an organ or tissue.
D.
A beam of gamma rays is fired at the patient and detected on the other side, while
X-rays are produced by the nucleus of a radionuclide incorporated in the
patient’s body.
Answer
is C: X-rays that pass through the body without being
absorbed by the body are used to produce an image of internal structure. In
nuclear medicine, a radioactive material is incorporated into the body, travels
to certain organs from where a gamma ray is emitted. If physiology is altered
sufficiently to affect the way the radioactive material moves about the body,
the resulting image provides information about how much alteration there has
been.
2.
Conventional radiography—such as a chest X-ray (CXR)—differs from computed
tomography (CT) in what respect?
A.
CT produces an image of all internal anatomy while in CXR, overlying anatomical
structures obscure the view of underlying structures.
B.
In CT the patient is left with some residual radioactivity, but not with CXR.
C.
CT produces a lower absorbed dose of radiation to the patient than does a CXR.
D.
CT involves the use of ultrasound while a CXR results from X-rays.
Answer
is A: CT mages are unobstructed by the “shadow” of
overlying structures.
3. Which of the following imaging modalities does NOT involve the use of “ionizing radiation”?
A.
Mammography
B.
Ultrasound
C.
A scintigram using technetium
D.
A chest X-ray
Answer
is B: Ultrasound produces an oscillation in the
particles of the body, but does not use ionising electromagnetic radiation.
4. When inspecting an X-ray image, the order of densities from blackest to whitest is:
A.
Bone, water, fat, air
B.
Air, fat, water, bone
C.
Air, water, fat, bone
D.
Bone, air, water, fat
Answer
is B: The blackest part on an X-ray image is air, while
the whitest is bone.
5. Which of the following imaging modalities uses X-rays?
A.
Computed tomography (CT)
B.
Single-photon emission computed tomography (SPECT)
C.
Positron emission tomography (PET)
D.
Nuclear medicine scan (scintigram)
Answer
is A: CT machines generate X-rays. The others are all
nuclear medicine procedures.
6. Radiation which is “ionising” includes which of the following?
A.
X-rays and gamma rays
B.
Infrared radiation
C.
Radiation emitted by mobile phones
D.
Microwaves
Answer
is A: The other three choices, while also being forms
of electromagnetic radiation, do not utilise energies that are sufficient to
remove electrons from their atoms.
7. The lead aprons that are used for the protection of staff in diagnostic radiography procedures do not provide protection against the ionising radiation used in nuclear medicine or radiotherapy. Why is this? Because:
A.
Charged particles are much easier to stop (are less penetrating) than photons are.
B.
Gamma rays are more penetrating than x-rays even if both have the same energy.
C.
Such aprons do not cover the arms, feet, head and neck.
D.
The shielding provided by aprons is not sufficient to stop photons with
energies above 100 keV.
Answer
is D: Diagnostic radiography uses X-rays with an average
energy of 70 keV or less, which can be stopped by relatively thin amounts of
lead (or leadlike) material. In order to stop the gamma rays used in nuclear
medicine imaging, the thickness of the “aprons” would make them prohibitively
heavy to wear.
8. When compared to visible light, which is not very penetrating, why can radiation such as X-rays and gamma rays pass right through the human body? Because:
A.
The density of the human body is relatively low.
B.
They have no mass and no charge.
C.
Atoms in the body are mostly empty space.
D.
They have very high energy.
Answer
is D: X- and gamma rays are high energy radiation,
which means high frequency, which means short wavelength, which means that many
of the photons will pass through the body without interacting with any atoms.
The other three choices do not distinguish between different types of
radiation.
9. Which of the following is true?
A.
A patient exposed to diagnostic X-rays will emit X-rays for a short time after
the procedure.
B.
A cancer patient treated with a megavoltage beam of X-rays will emit X-rays for
a short time after the treatment.
C.
For a short time after having a bone scan using the radionuclide
technetium-99m, the patient will emit gamma rays.
D.
The human body does not contain any radioactive material unless it has been
exposed to man-made radioactive material.
Answer
is C: A bone scan (any nuclear medicine scan) involves
taking some radioactive materials into the body which then emits gamma rays to
be detected outside the body. The body takes some time to excrete the material
(and much of it decays) so until that happens, the body is more radioactive
than is usually the case.
10. What does the term ionising radiation refer to?
A.
The radiation that is emitted by ionised atoms
B.
That part of the electromagnetic spectrum with wavelengths less than 300 nm
which has enough energy to produce ions
C.
Alpha, beta and gamma rays spontaneously emitted from radionuclides
D.
Radiation with enough energy to produce ionisation in the material which absorbs
it
Answer
is D: If the radiation produces ions when it interacts
with any substance, then it is ionising radiation. Choice B is true but does
not include particulate radiation. Choice C is true but does not include X-rays
or cosmic rays.
11. Which of the following is a correct use of the unit known as the “electron volt” (eV)?
A.
One electron volt is the amount of radioactivity that results in one
disintegration per second.
B.
Radiopharmaceuticals contain gamma photon emitting radionuclides whose energy
is usually in the range 100–250 keV.
C.
One electron volt is equal to 1.9 × 1016 J of energy.
D.
A photon of visible light has energy of about 1.5 MeV.
Answer
is B: eV is a unit of energy (not radioactivity). It
is equal to 1.9 × 10−16 J of energy.
Visible photons have energy of 1.5 eV or less.
12. Which one of the statements about the penetrating ability of radiation is true?
A.
750 keV gamma rays are more penetrating than 750 keV X-rays.
B.
140 keV gamma rays are more penetrating than 60 keV X-rays.
C.
2 MeV beta rays (electrons) are more penetrating than 1 MeV gamma rays.
D.
1 MeV gamma rays are more penetrating than 2 MeV X-rays.
Answer
is B: Gamma rays and X-rays are indistinguishable, once
they have travelled away from their site of production. So a 140 keV gamma or
X-ray is more penetrating than a lower energy one.
13.
What may the term “ionising radiation” be applied to?
A. All electromagnetic radiation
B.
Radiation that produces ions when it interacts with matter
C.
Infrared radiation
D.
Radiation that is emitted by ions
Answer
is B: The production of ions (i.e. removal of
electrons from an atom) is the sign of ionising radiation. Such a change in the
medium through which radiation passes is significant.
14.
What does it mean when an X-ray tube is operated at an accelerating voltage of 120
kV?
B.
The characteristic X-rays will have energy 120 keV.
C.
All of the X-ray photons will have an energy of 120 keV.
D.
The X-ray beam will contain photons with every energy from 0 keV up to 120 keV.
Answer
is A: X-rays are emitted from the tube with a range of
energies (a spectrum) which will range from mid-teens to low 20s of keV
(depending on the amount of “filtration” that the photons pass through) up to
the maximum value which will equal the accelerating voltage.
15. Why do some X-ray photons pass through the human body without deflection? Because:
A.
Carbon, hydrogen and oxygen atoms are transparent to X-rays.
B.
The energy of diagnostic X-rays is too low to produce interactions
C.
The wavelength of X-rays is too long to interact with an object with the dimensions
of the human body.
D.
The interior of atoms is mostly empty space.
Answer
is D: Apart from the positions where electrons and the
nuclei are located, the rest of the body is empty space through which photons
can travel unimpeded. They will interact only if they “hit” these subatomic
particles. The other answers contain errors.
16. How will increasing the filtration of an X-ray beam reduce the intensity of the X-ray spectrum?
A.
Equally at all frequencies
B.
More at lower frequencies than at higher frequencies
C.
More at higher frequencies than at lower frequencies
D.
Only at lower frequencies
Answer
is B: Lower frequency radiation is less penetrating
than higher frequency radiation, so the low frequency are preferentially absorbed
compared to high frequency radiation.
17. What is the difference between X-rays and gamma rays?
A.
X-rays are ionising radiation and gamma rays are not.
B.
Gamma rays have higher energies than X-rays.
C.
Gamma rays can be turned off by switching the power supply off.
D.
X-rays are produced in an electrical machine whereas gamma rays emerge from an
atomic nucleus.
Answer
is D: Apart from the difference in how they are
produced, X-rays and gamma rays are the same phenomenon.
18. To what energy can an X-ray tube that is operated at 110 kV accelerate electrons?
A.
110 eV
B.
110 keV
C.
110 J
D.
110,000 keV
Answer
is B: An electron volt is the amount of energy gained
by an electron when it is accelerated by a potential difference of 1 V. 110 keV
is the amount of energy gained by an electron when it is accelerated by a
potential difference of 110 kV.
19. What is the purpose of adding filtration to an X-ray beam? To:
A. Prevent
high-energy photons entering the patient
B. Increase the
mean energy of the beam
C. Decrease the
scattered radiation
D.
Increase the ratio of low energy photons to high energy photons
Answer
is B: X-rays are produced in an X-ray tube with a
range of energies. Filtration will absorb the lowest energy photons more than
higher energy ones. This will result in the average energy of the remaining
spectrum increasing.
20.
Consider the “inverse square law”. Compared to being 1 m from a point X-ray
source, what will be the photon flux at a distance of 4 m from the source?
A.
Sixteen times as great
B.
Eight times as great
C.
One eighth as great
D.
One sixteenth as great
Answer
is D: “Inverse square” means at double the distance,
the flux will be one half squared of the original flux (½)2 = ¼. At 4× the
distance, the flux will be (¼)2 = 1/16.
21. What is the purpose of an intensifying screen?
A.
It converts a small number of X-ray photons into a large number of visible light
photons.
B.
It converts low energy X-ray photons into high energy visible light photons.
C.
It improves the absorption efficiency of X-rays.
D.
It protects the radiologist’s eyes from the damage that would be caused by X-rays.
Answer
is A: It converts X-rays (which have high energies but
are invisible) into visible light photons. Because visible light photons have
low energy, an X-ray photon can be made to produce lots of them.
22. One difference between the X-radiation in the primary beam and the scattered radiation is that:
A.
Photons in the primary beam degrade contrast in radiographic images.
B.
Scattered radiation is more penetrating than the primary beam.
C.
Scattered radiation may be absorbed in the imager.
D.
Scattered radiation is travelling at an angle to the main beam.
Answer
is D: “Scattered” radiation has interacted with an atom
in the medium so is diverted from the direction of the primary beam.
23. The advantage of computed tomography (CT) over conventional radiography is:
A.
CT delivers lower doses than conventional radiography.
B. CT images are
faster to acquire than conventional radiographs
C.
CT produces a cross-sectional image that is not obscured by overlying
anatomical structures.
D.
CT projects a 3D structure onto a 2D image.
Answer
is C: It is the ability to image internal structures
without having them partially obscured by the tissue on either side as is the
case for “conventional” radiography that makes CT such a useful diagnostic
tool. (Also there is no need to cut the person open to see what is inside!)
24. The contrast of a CT image displayed on a monitor may be increased by:
A.
Increasing the window width
B.
Increasing the window level
C.
Decreasing the window width
D.
Decreasing the window level
Answer
is C: Decreasing the width of the “window” means
restricting the range of grey scale displayed on the monitor screen (0–255), to
a more limited range of the available Hounsfield numbers (−1000
for air to 3000 for dense bone). A lung window may be −1250
to +250; a soft tissue window may be −160
to +240; a bone window may be −650
to +1350.
25. Which one of the following beams is the most penetrating?
A.
2 MeV gamma rays
B.
2 MeV X-rays
C.
4 MeV X-rays
D.
8 MeV X-rays
Answer
is D: Penetrating ability of electromagnetic radiation
increases as energy does.
26. Exposing a foetus or young baby to X-rays should be avoided. What is the cause of the danger most likely to be due to?
A.
Denaturing of cells due to the increase in temperature in cells absorbing radiation.
B.
Damage to a cell’s DNA.
C.
The baby becoming radioactive.
D.
The formation of a blood clot.
Answer
is B: X-rays are ionising radiation. This means that ions
may be formed from the atoms that make up DNA. In this case, the chemical bonds
between atoms will be broken and the molecule will be changed.
27.
Which of the following is NOT a feature of mammography?
B.
Non-ionising radiation is used.
C.
The radiation dose is small (<1 mSv).
D. The X-ray
tube utilises a molybdenum target.
Answer
is B: Mammography uses X-rays which are ionising
radiation, albeit giving the patient a small dose of radiation.
28. Which anatomical plane is usually displayed in a CT scan?
A.
Transverse
B.
Sagittal
C.
Coronal
D.
Longitudinal section
Answer
is A: A cross-section or transverse slice is the
usual image viewed. CT images are displayed as if viewed from the direction of
the feet.
29. Why does an interventional cardiac angiography procedure have the potential to deliver a high dose of radiation to the patient?
A.
A radioactive tracer is injected into the patient.
B.
An extensive region of the torso is irradiated.
C.
Cardiac angiography employs ionising radiation.
D.
The X-ray generator is switched on for 10s to 100s of seconds.
Answer
is D: The long exposure time is the potential problem.
All X-ray imaging uses ionising radiation. A radioactive material is not used
in cardiac angiography.
30. What does the term “ionising radiation” refer to?
A.
Any long wavelength electromagnetic radiation
B.
Radiation that is emitted by ions
C.
Ultrasound radiation
D.
Radiation that produces ions when it interacts with matter
Answer
is D: If ions are produced when radiation passes
through a material, it is termed ionising radiation. It also means that energy
is deposited within that material.
31. When a mammography X-ray tube with an Mo target (characteristic X-ray at 19.6 keV) with an Mo filter is operated at an accelerating voltage of 28 kV, what may be said of the resulting spectrum?
A.
The maximum energy that an X-ray photon can have will be 28 keV.
B.
Most of the X-rays will have an energy of 19.6 keV.
C.
All of the X-ray photons will have an energy of 28 keV.
D.
The X-ray beam will contain photons with every energy from 0 keV up to 28 keV.
Answer
is B: The Mo filter has the effect of stopping photons
with energy between 20 and 28 keV. Characteristic X-rays are present with much
greater intensity than photons in the rest of the spectrum. So the majority of
photons will have energy of about 19 keV. This is the best energy for imaging
the compressed breast.
32. Why will many X-ray photons pass through the human body without deflection?
A.
The carbon, hydrogen and oxygen atoms are transparent to X-rays.
B.
The Compton interaction does not occur at diagnostic X-ray energies.
C.
The wavelength of X-rays is too long to interact with an object with the dimensions
of the human body.
D.
The wavelength of X-rays is too short to interact with many atoms.
Answer
is D: X-ray photons are very small and atoms are
mostly empty space. Furthermore they are uncharged so are not attracted to or
repelled from electrons and protons. To interact, an X-ray photon must pass extremely
close to the nucleus an electron. Hence most pass straight through.
33. If the filtration of an X-ray beam is increased from 2 mm aluminium to 5 mm aluminium, how will the intensity of the X-ray spectrum change?
A.
It will decrease equally at all frequencies.
B.
It will decrease more at lower frequencies than at higher frequencies.
C.
It will decrease more at higher frequencies than at lower frequencies.
D.
It will decrease only at lower frequencies.
Answer
is B: Intensity will certainly decrease. The lower frequencies
are “less penetrating” than higher frequencies and so are more easily stopped
than the higher frequencies.
34. What is a difference between X-rays and gamma rays?
A.
X-rays emerge from a radioactive atomic nucleus whereas gamma rays are produced
in an electrical machine.
B.
Gamma rays can be turned off by switching the power supply off.
C.
X-rays are produced in an electrical machine, whereas gamma rays emerge from an
atomic nucleus.
D.
X-rays can penetrate deeper into solid material than can gamma rays.
Answer
is C: When the electrical supply is turned off, no
more X-rays are produced. Gamma rays continue to emanate naturally from a
radioactive substance until all the
atoms
have decayed.
35. What is the maximum energy that an X-ray tube energised to 140 kV will accelerate electrons to?
A.
140 eV
B.
140 keV
C.
140 J
D.
140,000 keV
Answer
is B: The unit “electron volt” is the amount of energy
gained by an electron when it is accelerated by a potential difference of 1 V.
140 keV is the amount of energy gained by an electron when it is accelerated by
a potential difference of 140 kV.
36. Why is an X-ray beam passed through a metal foil (i.e. filtered) before irradiation a patient? To:
A.
Remove electrons from the X-ray beam
B.
Prevent high-energy photons entering the patient
C.
Increase the mean energy of the beam
D.
Increase the ratio of low energy photons to high energy photons
Answer
is C: The filter material will remove the lowest
energy X-ray photons from the beam. As a result, it will also increase the mean
energy of the beam.
37. What is the purpose of an intensifying screen when used to view an X-ray image?
A.
It converts a small number of X-ray photons into a large number of visible light
photons.
B.
It converts low-energy X-ray photons into high-energy visible light photons.
C.
It improves the absorption efficiency of X-rays.
D.
It improves the conversion efficiency of the detector.
Answer
is A: An X-ray photon has much higher energy than a
visible photon; hence, one X-ray photon has sufficient energy to produce a
great many visible photons. Furthermore, visible light can be seen by human
eyes but X-rays cannot. Such screens mean that a lower radiation dose can be
given to the patient while still producing a useful image.
38. Which element is NOT used in the construction of the anode of an X-ray tube?
A.
Rhenium
B.
Tungsten
C.
Aluminium
D.
Molybdenum
Answer
is C: Aluminum (atomic number 13) is too light to be
used in an X-ray tube anode. It is however used as a filter to remove
low-frequency X-rays from the beam.
39. In which one of the following situations are characteristic X-rays produced?
A.
When an outer shell electron is knocked out of the atom and the vacancy is filled
by another electron.
B.
When electrons bombarding the atom have less than 20 keV of energy.
C.
When the bombarding electrons have LESS energy than the binding energy of the K
shell.
D.
When a K shell electron is knocked out of a target atom and the vacancy is filled
by an outer electron.
Answer
is D: Electron shells are labelled K, L, M, etc. from
innermost to outermost shell. Electrons in the K shell, being closer, are most
tightly bound to the nucleus and require more energy to be knocked out of the
atom. When the hole left behind is filled by another electron, it gives up some
of its energy as a characteristic X-ray photon.
40. The binding energy of an electron in the K shell of a molybdenum atom is 20.0 keV. Given that one of the characteristic X-rays of molybdenum atom is 17.8 keV, what is the binding energy of the other electron energy level?
A.
37.9 keV
B.
20.0 keV
C.
17.8 keV
D.
2.2 keV
Answer
is D: Electrons in the K shell are the most tightly
bound and so have the most binding energy. That is, 20 keV is the most energetic
characteristic X-ray for Mo. If 17.8 keV is a characteristic X-ray, then there must
be an energy difference of 17.8 keV between the 20.0 keV level and the one we
want. 20–17.8 = 2.2 keV.
41. Why is the anode surface of an X-ray tube is inclined at an angle to the bombarding electron beam? So that:
A.
The X-ray beam is directed towards the exit window.
B.
The effective focal spot is smaller than the actual focal spot.
C.
A diverging X-ray beam is produced.
D.
The emerging beam is parallel.
Answer
is B: A small focal spot will produce a sharper X-ray
image. A small focal spot will also mean a large amount of heat is deposited in
that part of the anode. Inclining the anode angle to about 7° will spread the
bombarding electrons onto a larger area, while making the size of the emerging
X-ray beam narrower.
42. The bremsstrahlung interaction produces all of the following except one. Which one?
A.
Characteristic radiation
B.
Heat in the anode
C.
A bombarding electron with less kinetic energy
D.
Photons with a range of energies
Answer
is A: Bremsstrahlung radiation is emitted when a
fast-moving electron is slowed down by passing close to a nucleus in the target
anode. Characteristic radiation is emitted when an electron bombarding the
anode collides with a K shell electron and knocks it out of the atom.
43. The benefit of having a small focal spot on the anode of an X-ray tube is that:
A.
Scattered radiation is minimised.
B.
Heat production in the anode is minimised.
C.
The resulting image will be sharply defined.
D.
The anodes may be made smaller.
Answer
is C: A small focal spot ensures that the produced
X-rays come from close to a “point” source. Such a source will produce an X-ray
image with very little “penumbra”, that is, the image will be sharper.
44. Given that the absorption of photons by the photoelectric effect increases with the Z of the absorbing atom and decreases with energy of the photon, which of the following is likely to be true?
A.
The photoelectric effect is of greater importance in soft tissue than in bone.
B.
The photoelectric effect is of greater importance in bone than in soft tissue.
C.
There is no marked difference in the occurrence of the photoelectric effect in
soft tissue than in bone.
D.
Photons with energy greater than 60 keV are more likely to undergo the photoelectric
effect than photons of energy less than 60 keV.
Answer
is B: Bone contains Ca and P which have atomic numbers
of 20 and 15, respectively. The elements C, H and O which predominate in soft
tissue have the lower Z values of 6, 1 and
8, respectively. Hence, the photoelectric interaction is more common in bone.
45. What is the “heel effect” that is displayed in an X-ray spectrum is due to?
A.
Poor collimation
B.
Absorption of X-rays as they pass out of the anode
C.
The use of wedge filters in the X-ray beam
D.
Operating the X-ray tube with the beam horizontal
Answer
is B: X-ray photons are produced at different depths
within the anode. Hence, they traverse different distances through the anode
before they emerge. This ensures that those traversing the greatest distance suffer
the greater absorbance. So a noticeable modification to the spectrum (a “heel”)
with angle from the anode is discernible.
46. The photoelectric effect produces all of the following except one. Which one?
A.
Characteristic radiation (a secondary X-ray photon)
B.
A scattered X-ray photon
C.
An energetic photoelectron
D.
A dose of radiation to tissue
Answer
is B: An incoming photon with sufficient energy is
totally absorbed by an electron which is then ejected from the atom as a “photoelectron”.
As the energy of the incoming photon is all absorbed, there is not scattered
photon.
47. Consider 100 X-ray photons (all of the same energy) that are fired at a 4 mm thick sheet of aluminum and that 80 pass through without interacting. What is the linear attenuation coefficient (in cm−1) for Al at this energy?
A.
0.2 cm−1
B.
0.5 cm−1
C.
2.0 cm−1
D. 5.0 cm−
Answer
is B: 80 out of 100 photons (80%) pass through. Thus,
20% are stopped. Hence, 20% per 4 mm, or 5% per 1 mm is = 0.05 mm−1
= 0.5 cm−1 as there are 10
mm per cm.
48. An X-ray beam with a high HVL (half-value layer) is:
A. Said to be “softer” than a beam of low HVL
B.
Less penetrating than a beam of low HVL
C.
Likely to produce a larger absorbed dose to the skin
D.
More penetrating than a beam of low HVL
Answer
is D: Having a HVL of “x” mmAl means that half of the
photons in the beam are able to penetrate “x” mmAl. Hence, the higher is the
HVL, the more penetrating is the beam. Also it will be said to be “harder” and
to produce a lower absorbed dose to tissue.
49. Which statement about the Compton effect is NOT correct?
A.
A Compton interaction is more likely in bone than in muscle tissue.
B.
The deflected photon has less energy than the original photon.
C.
An electron is ejected from the atom.
D.
Compton events contribute to scattered radiation.
Answer
is A: A Compton interaction occurs when a photon uses
some of its energy to eject an electron, while the remaining energy becomes a
scattered photon. The probability that an X-ray photon will undergo a Compton
interaction depends on the density of the issue and the energy of the photon.
Tissues such as muscle, blood and solid organs create the greatest amount of
scatter radiation in the body. Bone tends to absorb more of X-rays.
50. Use the “inverse square law” to compare to being 1 m from an X-ray source, with the photon flux at a distance of 3 m from the source. The flux will be:
A.
Nine times as great
B.
Six times as great
C.
One sixth as great
D.
One ninth as great
Answer
is D: “Inverse square” means at double the distance,
the flux will be reduced to one half squared of the original flux (½)2 = ¼. At
3× the distance, the flux will be (1/3)2 = 1/9.
51.
Imagine that the X-radiation from a diagnostic machine was measured by a dose-area-product
meter that could count individual photons. For a particular exposure, 8 × 1012
photons were collimated to pass through a 10 cm × 10 cm aperture at 1 m from
the X-ray source. What would be the number of photons passing through a 20 cm ×
20 cm aperture at 2 m from the X-ray source?
B.
2 × 1012 photons
C. 16 × 1012
photons
D.
4 × 1012 photons
Answer
is A: By the inverse square law, a 10 cm × 10 cm beam
at 1 m from the source would diverge to a 20 cm × 20 cm beam at 2 m from the
source (that is to four times the area). However, the same number of photons
would be passing through this wider area.
52. Why are iodine and barium ideal for use as contrast agents in imaging of soft tissue?
A.
They are high Z atoms so the
probability of a Compton scatter is high.
B.
The binding energy of their K shell is in the middle of the diagnostic energy range.
C.
The energy of their characteristic X-rays is in the middle of the diagnostic energy
range.
D.
They are high Z atoms so the
probability of a photoelectric interaction is high.
Answer
is B: As the binding energy of their K shell is in the
middle of the diagnostic energy range, many X-ray photons will be absorbed by I
and Ba. Hence, organs containing these elements will appear white on the image
and so will contrast with the grey shades of the surrounding soft tissue.
53.
One difference between the X-radiation in the primary beam and the scattered radiation
is that:
A.
Scattered radiation contributes dose to the surroundings, primary beam does
not.
B.
Scattered radiation is more penetrating than the primary beam.
C.
Scattered radiation is not absorbed by the patient while photons in the primary
beam are.
D.
Primary X-ray photons that have passed through the patient contribute to patient
dose.
Answer
is A: Scattered radiation has had its direction of
travel altered and so may irradiate people standing nearby. Scattered radiation
is less penetrating than the primary beam and may be absorbed by the patient
and so contribute to their radiation dose. Choice D is wrong as emerging
photons do not contribute to dose.
54. The detectors in a CT scanner measure which one of the following?
A.
The linear attenuation coefficient of each pixel
B.
The grey scale value of the image
C.
The Hounsfield unit of each pixel
D.
The average linear attenuation coefficient
Answer
is D: The average linear attenuation coefficient along
the path from where the beam enters the patient’s body to where it emerges.
This number is then attributed to every pixel along the path. A variety of techniques
is then used to create a cross-sectional image of anatomy and to assign a “Hounsfield
unit” to each pixel of the image.
55.
If a CT (Hounsfield) number associated with a given tissue is close to zero,
then what is the tissue likely to be?
A.
Mostly water
B.
Predominantly lung
C.
Mainly muscle
D.
It probably contains bone
Answer
is A: The numerator of Hounsfield number is calculated
by subtracting the “linear attenuation coefficient” of water from the attenuation
coefficient of the tissue. If the tissue coefficient is close to that of water,
the result will be close to zero.
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