Physics 3 Answers
1
C.
Coherent scatter results in scattered photons but no transfer of energy, and typically accounts for less than 10% of x-ray interactions in diagnostic radiology.
2
E.
In pair production, a positron and electron are created where each has a rest mass energy of 511 keV. They annihilate to produce photons.
3
D.
In pair production, a photon is converted into an electron-positron pair. (The threshold is 1.022 MeV, above which the probability of an interaction increases with Z and with photon energy.)
4
B.
Only the photoelectric effect results in the absorption of the incident x-ray photon.
5
E.
Because the incident photon is absorbed, scattered photons are not produced. (Other products occur directly following PE interactions.)
6
A.
The PE effect is always a maximum for photons with energies just above the K-shell binding energy. (The PE effect decreases with increasing photon energy and decreasing atomic number (Z) and does not depend on outer shell electrons or the molecular structure.)
7
A.
The probability of the photoelectric effect increases rapidly as Z increases (Z3) and decreases rapidly as E increases (1/E3).
8
D.
Lower-energy scattered photons (511- keV photons are annihilation radiation from positrons; characteristic x-rays are produced from inner-shell vacancies; photoelectrons are emitted following photoelectric absorption; internal conversion occurs with nuclear gamma ray emission).
9
C.
Only the PE and Compton scatter are important in radiology. The PE and Compton effects are equal at 25 keV or 75 kVp, with the PE more important at lower energies and vice versa.
10.
A-PE;
B-Compton scatter;
C-Compton scatter;
D-PE;
E-PE;
F-PE.
11.
A-True;
μ falls off very rapidly at low energies [E < 25 keV] and more slowly at higher photon energies;B-True; fractional transmission is e-
μt, where t is the thickness and μ is the attenuation coefficient at the effective [mean] photon energy;C-False; expressed in cm-1;
D-True; mass attenuation coefficient by definition is
μ/ρ;E-False;
μ is independent of tissue thickness (t) and is multiplied with t to compute the transmission e-μt factor.
12
A.
The term measures the fractional transmission, which is independent of incident x-ray intensity (the attenuation coefficient [
μ] is generally a function of photon energy [E], atomic number [Z], and density [ρ]).
13.
A-iii
B-iv
C-i
D-ii
Air is the least attenuating material and lead the most attenuating.
14
D.
By definition,
μ/p, the mass attenuation coefficient, is independent of density (p).
15
C.
The HVL is given by the expression (0.693/0.1 cm, or approximately 7 cm).
16
C.
Each HVL reduces the intensity by 1/2, so the total intensity reduction is (1/2)n for n half-value layers and three half-value layers reduces intensity by a factor of 8.
17
B.
Filters are used to remove the low-energy photons that increase the patient dose but do not contribute to the image.
18
A.
Beam quality is independent of the tube current (mA), which primarily determines the x-ray beam output (intensity).
19.
A-iii;
B-ii; typically 30
μm molybdenum;C-i; chest x-rays performed at high kVps (120 to 140) are usually heavily filtered to reduce contrast and thereby permit visualization of lung/mediastinum.
20
E.
Adequacy of x-ray tube filtration is determined by measuring the HVL, usually at 80 kVp, and ensuring it exceeds 2.5 mmAl.
21.
A-iii;
B-ii;
C-i;
D-iv.
The higher the HVL, the higher the beam quality and mean energy.
22
A.
The HVL expressed in mm Al only depends on the x-ray spectrum; it is thus independent of the intensity of the beam as measured by the exposure.
23
D.
Beam filtration depends on the x-ray tube window and added filters, and is therefore independent of x-ray tube kV/mA.
24
B.
Compton scatter is the primary interaction for soft tissues at high photon energy levels (> 25 keV or > 75 kVp).
25
C.
The high atomic number of barium (Z=56; K-edge 37 keV) results in strong absorption of the incident x-ray beam, and results in very high subject contrast.