Physics B Answers

1.

A-u;

B-i;

C-iv;

D-iii.

2.

A-iii; 222Rn and its daughters emit alpha particles;

B-iv; MR uses RF pulses for imaging;

C-ii, radiation therapy uses gamma emitting radioactive sources such as 60Co;

D-i; electrons with a spectrum of energy levels are emitted during beta minus decay.

3.

A

 X-rays, like all photons, are not particles and have no rest mass.

4.

A-iii; I and 33 keV;

B-i; O and 0.5 keV;

C-ii; Ca and 4 keV;

D-iv; W and 70 keV.

Note that the K-shell binding energy always increases with atomic number Z.

5.

A-iii; the resonance frequency for protons at 1 T is 42 MHz, which corresponds to a wavelength of 7 m, which is in the radiofrequency region of the electromagnetic spectrum;

B-ii; diagnostic x-rays are high frequency electromagnetic radiation with energy in the range of 20 to 150 keV;

C-i; light photons have an energy in the range 1 eV (red) to 3 eV (blue);

D-iv; this is in the form of heat; as bodies get hotter, the photon energy increases;

E-v; gamma rays are used in nuclear medicine.

6.

C.

Ionization occurs when electrons are ejected from a neutral atom leaving behind a positively charged atom.

7.

A-iv; isomers refer to excited energy levels of the given nucleus;

B-i; isobars have the same mass number;

C-iii; isotones have the same number of neutrons;

D-ii; isotopes have the same number of protons.

8.

E. All of the statements are true.

9.

A

X-rays are generally produced by electrons, whereas gamma rays are generated in nuclear processes. All of the physical properties of x-rays and gamma rays are identical.

10.

E.

None are correct. For the same kVp, the three-phase generator has the same maximum energy, more photons, and a higher HVL.

11.

E.

Tube kVp determines the maximum energy of electrons striking the anode and therefore the maximum x-ray photon energy that can be produced in a bremsstrahlung process.

12.

A-iii; Mo and 19.6 keV;

B-iv; I and 32.3 keV;

C-ii; W and 67.2 keV;

D-i; Pb and 84.9 keV.

The characteristic x-ray energy increases with Z and is a little less than the corresponding atomic K-shell binding energy.

13.

C.

Multiformat cameras are used to produce film images of video displays. Pinhole and slit cameras measure focal spot dimensions directly. Star and parallel bar phantoms measure the loss of spatial resolution due to the finite size of the focal spot.

14.

D.

All are generally true.

15.

C.

Reduced to 1/3. The mAs should be kept constant to maintain the same film density.

16.

A-True; output increases approximately as kV2

B-False; output depends on anode Z, but not heat capacity per se;

C-True; output is approximately proportional to Z;

D-True; output is directly proportional to tube current:

E-False; filters increase the average photon energy but reduce the tube output;

F-False; x ray production is independent of anode rotation speed whose purpose is to spread anode heat loading.

17.

C.

Leakage radiation.

18.

E.

None of the statements are true.  Coherent scatter is the result of x-rays that have changed direction but do not deposit energy in the patient.

19.

B

Energy will be deposited and produce a large number of ionizations as the photoelectron loses energy.

20.

A-False; scattered photons have less energy and therefore lower frequencies and longer wavelengths;

R-True; photoelectric effect;

C-True;

D-True; coherent scatter.

21.

A.

Compton scatter is the major interaction in fluoroscopy which results in backscattered x-rays.

22.

C.

Pair production has a threshold energy of 1.022 MeV, which is much greater than that used in diagnostic radiology.

23.

E.

All of the given values are reasonable for the transmission of primary x-ray beam for an average patient (there will also be scattered radiation).

24.

E.

All of these statements are generally true.

25.

A-False; patient entrance skin exposure is reduced as the effective energy is increased

B-True;

C-False; higher effective energy levels reduce subject contrast;

D-True; requires a higher mAs because the filter reduces the beam intensity;

E-True; because the tube output must be increased, the anode loading must increase.

26.

D.

The regulatory minimum HVL is 2.5 mm Al at 80 kVp and 3 mm would be a typical value. The HVL should not be confused with the filtration, which is the actual amount of Al added at the x-ray tube to increase the HVL.

27.

A.

The LET determines the radiation quality.

28.

D.

Exposure is ionization by photons per unit mass in air, whereas absorbed dose is the energy absorbed per unit mass.

29.

D.

The f-factor (rad/R) is about 3 for bones and about 1.0 for soft tissues at diagnostic x-ray energies.

30.

A-iii;

B-i;

C-iv;

D-ii;

1 gray = 100 rad;

1 sievert = 100 rem;

1 R = 2.58 x 10-4 C/kg;

Ci = 3.7x 1010 Bq.

31.

B.

Photometers are used to measure the illuminance level (lux) in a room or brightness levels (cd/m2) of a viewbox or display screen. Densitometers are used to measure the optical density of film.

32.

A.

Fog will reduce image contrast, and this will be most marked at low film densities.

33.

A

A thicker crystal will stop more photons and thereby increase the absorption efficiency.

34.

A-iii; CR imaging plates use photostimulable phosphors that store x-ray energy as trapped electrons and release light when stimulated with a laser scanner;

B-iv; photo-multiplier tubes absorb light and emit electrons;

C-i; scintillation crystals such as Nal absorb gamma rays and emit light;

D-ii; screens absorb x-rays and convert between 5% and 20% of this energy into light.

35.

E.

Bremsstrahlung is produced when electrons interact with matter.

36.

E.

Because less radiation is required, all the items listed will be reduced.

37.

D.

Modern rare earth screens are normally much faster (commonly rated as 400 speed) than calcium tungstate (commonly rated as 100 speed).

R38.

A-iii; single screens and emulsions are used to achieve high spatial resolution by minimizing blur;

B-i; chest radiographs require wide latitude;

C-v; in CT, films are printed using lasers that only use single emulsions;

D-iv; the limited penetration requires the use of relatively fast systems with no special requirements for spatial resolution;

E-ii; xeroradiography uses selenium photoconductors as the detection medium, which require a special powder development process to visualize the latent image.

39.

D.

Geometric unsharpness will not be affected by the introduction of a Bucky grid.

40.

A.

Grids remove scatter and therefore improve image contrast.

41.

E.

Reducing screen speed (use of thinner screens) will reduce screen blur. Note that there is no focal spot blur in contact radiography because there is no magnification.

42.

A.

Doubling the mA will double the exposure rate and require only half the exposure time to give the required film density.

43.

A-i; DSA exposure times are very short;

B-iii; a typical abdomen is 23 cm thick and longer exposures are required to adequately expose the film;

C-ii; the exposure time should be short enough to minimize heart motion;

D-iv; mammography exposure times are generally longer because of the low photon energy levels used and the low x-ray tube outputs for Mo targets operated at 28 kVp.

44.

A.

Subject contrast is determined by the x-ray beam (kVp and filtration) and the tissue characteristic (atomic number, density, and thickness), and is independent of the choice of screen/film combination.

45.

A-iv;

B-iii;

C-ii;

D-i.

46.

D.

Focal spot size normally does not affect the entrance skin exposure.

47.

B.

Doubling the kVp would increase the screen/film exposure by much more than a factor of two.

48.

A.

Decreasing kVp always increases subject contrast. Changes in screen/film combination and film processing affect image contrast but do not affect the subject contrast.

49.

B

Input phosphor thickness is the primary determinant of the blur associated with the image intensifier. The TV camera will affect fluoroscopic performance, but not the II itself.

50.

E.

The minification gain is the ratio of the input area to the output area, or (12:1)2.

51.

D.

The II potential is in the range of 25,000 to 30,000 volts and is direct current, not alternating current.

52.

B.

1 mR/s. A typical II conversion factor is 100 cd/m2 per mR/s.

53.

D.

54.

A-True;

R-False;

C-False;

D-True.

In exact framing, the entire circular II image is recorded and only 80% of the film is used. In overframing, the II image is magnified so that the entire film is used, but only 64% of the image is recorded.

55.

A-ii;

B-iii;

C-iv;

D-i.

56.

A-iii;

B-i;

C-ii.

57.

A-ii;

B-iii;

C-i.

58.

E.

None of the answers are correct; with 8 bits per pixel, the maximum number of gray levels is 28 or 256.

59.

A

COBOL is a programming language, not an input/output device.

60.

A-v; CR is 2 k2;

B-i; nuclear medicine is usually 642 or 1282;

C-iii; CT is generally 5122;

D-ii; 128 x 256 to 2562 are common MR matrix sizes;

E-iv; most current DSA is 1 k2.

61.

D.

CT can differentiate between two adjacent tissues that differ in attenuation properties by as little as 5 HU (i.e., 0.5% difference), and thus has excellent contrast discrimination.

62.

D.

63.

B.

lodinated contrast has a high atomic number (Z = 53 for iodine), which increases x-ray absorption and the attenuation coefficient and, therefore, the computed HU value.

64.

B.

Increasing the matrix size will reduce the pixel size and thereby improve spatial resolution.

65.

E.

All are possible reasons for missing a small (low contrast) tumor. Thick sections contribute to volume averaging; window settings affect displayed contrast; a large table index may result in the lesion falling between sections; and a bone reconstruction algorithm will increase image noise.

66.

A.

Focal spot size affects spatial resolution, but all the other parameters affect image noise, which impacts the ability to detect low contrast objects.

67.

E.

None of the doses listed are correct. Typical eye lens dose is approximately 40 mGy (4 rad).

68.

A-True;

B-False; 1 curie = 3.7 x 1010 disintegrations per second;

C-True; 1 g of 226Ra produces 3.7 x 1010 disintegrations per second;

D-True;

E-True.

69.

D.

99mTc emits 140 keV photons. The daughter product (99Tc) is radioactive, with a half-life of 210,000 years.

70.

A-iv;

B-iii;

C-i;

D-ii

71.

A-Transient; 99Mo half-life is 67 hours, and 99mTc half-life is 6 hours;

B-Secular; 82Sr has a 25-day half-life, and 82Rb has a half-life of 1.3 minutes;

C-Secular; 113Sn has a half-life of 120 days, and 113mIn has a half-life of 100 minutes.

72.

A-iv; ¹⁵O is a positron emitter;

R-ii;  ^99mTc at 140 keV;

C-iii; ¹³¹I at 365 keV;

D-i;  ²⁰¹Tl at 70 keV.

73.

B.

Pulse pile-up occurs when two absorption events occur simultaneously and the total energy deposited in the crystal is summed as one event.

74.

D.

Larger collimator holes allow more primary photons to pass through to the Nal crystal.

75.

C.

The biological half-life is determined by biological clearance (i.e., no biological clearance corresponds to an infinite biological half-life).

76.

A-ii;

B-iii;

C-iv;

D-i.

77.

E.

All of the statements are true.

78.

B.

The issue of room shielding for dispensing radiopharmaceuticals is irrelevant.

79.

C.

Coincidence detection of two 511 keV annihilation photons is used in PET, which results in high count rates and short imaging times.

80.

C.

Leukemia is a stochastic effect. All the other effects listed are deterministic and have a threshold dose below which the effect does not occur.

81.

A.

Zero. The eye lens dose will be about 160 mGy (16 rad), which is below the threshold dose of 2 Gy (200 rad) for cataract induction.

82.

A-False; dose limits are set to prevent deterministic effects such as skin erythema;

B-True; in the 1930s, genetic effects were considered the main risk, but in the last 30 years, the estimated cancer risks have increased and the genetic risks decreased;

C-True; the latent period is shorter for leukemia (a few years) than for solid cancers (5- to 20-year latent period);

D-True; ICRP, UNSCEAR, and BEIR all publish radiation risk estimates, but uncertainties in these risk estimates at low occupational exposure levels are very large and much debated;

E-True; this is the best estimate of the radiation risk currently available from groups such as the ICRP, UNSCEAR, and BEIR.

83.

C.

Higher grid ratios increase the patient dose. The other factors listed reduce patient doses.

84.

D.

All will reduce the patient skin dose.

85.

E.

None of the statements are true. ESE values are a few R per minute. The legal maximum is 10 R/min, but may be exceeded if a high dose option is used, or if the image is being recorded. Doubling the distance from the patient will reduce the scatter exposure rate by a factor of four.

86.

A-iii;

B-iv;

C-ii;

D-i.

87.

C.

It is only Compton scattered photons that have a significant probability of reaching the operator.

88.

E.

The average US citizen receives an average annual exposure, including medical exposures, of about 3.6 mSv (360 mrem) per year. The additional dose to a person living in Leadville, as a result of both the elevation (higher cosmic background) and living in the Colorado plateau (higher external background), would result in an average annual dose > 4 mSv (400 mrem) per year.

89.

D.

There is no significant risk of skin irradiation by beta particles.

90.

D.

Acoustic impedance is the product of the velocity of sound and the density. For air, both velocity of sound and density are low; therefore, the acoustic impedance of air is very low.

91.

A-ii;

B-iii;

C-i.

92.

D.

The echo strength is not determined by the frequency per se, and the echo intensity will be lower at higher frequencies due to increased tissue attenuation.

93.

C.

As in the case of light, the angle of reflection is equal to the angle of incidence.

94.

B.

The crystal thickness (t) is generally one half the wavelength (λ) and thus determines the resultant transducer frequency (f = v/λ = v/2 x t), where v is the speed of sound in the crystal.

95.

E.

Low Q transducers have a wide band of ultrasound frequencies, and they also have short pulses, a characteristic that improves axial resolution.

96.

D.

Continuous wave Doppler does not -provide spatial information, only frequency shifts, which are related to movement such as blood flow.

97.

B.

98.

A-ii;

B-iii;

C-iv;

D-i.

99.

E.

Cavitation is the creation and collapse of tiny bubbles, and only occurs at high ultrasound intensities.

100.

C.

For any fixed angle, the ultrasound frequency shift is directly proportional to blood flow.

101.

A-iii;

B-ii;

C-i.

102.

A-False;

B-True;

C-False;

D-False;

E-True;

F-True.

103.

A-iii;

B-iv;

C-i;

D-ii.

104.

E.

Resistive magnets dissipate large amounts of power in the coils and generally require water cooling. The practical upper magnetic field strength is about 0.5 T.

105.

B.

Gradient coils permit the origin of the MR signal to be determined by superimposing small gradients (mT/m) on the large fixed field.

106.

A-False; these are light flashes induced by time-varying magnetic fields;

B-True;

C-True;

D-False; applied RF can result in tissue heating;

E-False.

l07.

A.

Reverberation echoes occur in ultrasound imaging, not in MRI.

108.

E.

None of the answers are correct. A Fourier analysis decomposes the signal into its frequency components.

109.

D.

EPI obtains the image from a single "excitation" by rapidly switching the gradients, and there is no repetition of a basic pulse sequence separated by a repetition time TR, as in SE or IR imaging sequences.

110.

C.

The image reconstruction technique does not affect the signal-to-noise ratio.

111.

A.

The frequency encoding gradient does not affect the image acquisition time, but determines matrix size.

112.

D.

Spectroscopy is commonly performed on the naturally occurring metabolite of ³¹P

113.

A-iv; adipose tissue has a low density

B-iii;

C-ii; carcinoma is slightly more attenuating than fibroglandular tissue;

D-i; the high atomic number of calcium results in high attenuation coefficient.

114.

E.

All of the listed statements are true.

115.

D.

Increasing exposure time may result in film reciprocity law breakdown and hence require an increase in exposure to maintain film density. All other factors reduce the mean glandular breast dose.

116.

B.

Xeroradiography has a wide latitude.

117.

D.

The spatial resolution of MR (< 1 line pair/mm) is markedly inferior to that of screen/film (15 to 20 line pairs/mm).

118.

E.

All of the listed statements are correct.

119.

A-iv;

B-iii;

C-ii;

D-i;

120.

A-True;

B-False; increasing the area under the ROC curve corresponds to improved imaging performance;

C-True

D-False; an ideal performance would correspond to an area of 1.0

E-True.