Physics A Answers
A1.
B.
Roentgen is the non-SI unit; in SI units, exposures are expressed in C/kg, where 1R=2.58x10-4 C/kg.
A2.
A-iv;
B-ii;
C-iii;
D-i;
E-v.
A3.
A-iii;
B-iv;
C-i;
D-ii.
A4.
D.
Binding energy. Inner-shell electrons are tightly bound to the nucleus with binding energies of the order of keV; outer-shell electrons are loosely bound with binding energies of a few eV.
A5.
A-iii;
B-i;
C-iv;
D-ii.
A6.
A-ii diagnostic x-rays and 10-10 m;
B-iv; MRI and 7 m radio waves;
C-iii; ultrasound and 1 mm;
D-i; visible light and 5x10-7 m.
A7.
E.
Ultrasound is not ionizing radiation. Note that photons and neutrons are indirectly ionizing radiations, whereas charged particles like electrons and alpha particles are directly ionizing.
A8.
A-True; emission of an electron results in an increase in the nuclear positive charge of one unit;
B-False; the mass number A does not change;
C-False; the emission of a positron reduces the nuclear charge by one;
D-True; isomeric transition involves the emission of gamma rays;
E-True; in alpha decay, a helium nucleus is emitted which contains two protons and two neutrons.
A9.
A-i;
B-ii; neutrinos are emitted in electron capture processes;
C-iii; in pair production, a photon is transformed into an electron / positron pair;
D-v; an electric current is a flow of electrons;
E-iv; 222Rn is an alpha emitter.
A10.
D.
1 Ci is 3.7 x 1010 Bq or 37 GBq, and
10 mCi is 1/100 of that, or 3.7 x 108Bq (370MBq).
A11.
C
The only difference is that half-wave rectification has half the pulses "missing," therefore the output of the full-wave rectified system is twice as large (ripples are the same at 100%).
A12.
A,
The K-shell binding energy for tungsten is 69 keV, so K-shell vacancies cannot be created using 50 keV electrons.
A13.
A
A space charge.
A14.
B
The intensity of x-ray production is approximately proportional to the atomic number Z.
A15.
A-iv; most of the energy is converted into heat;
B-i; only about 1% of the incident energy is converted into x-rays;
C-iii; the incident energy is either converted into x-rays or heat and thus the total must sum to 100%;
D-ii; only 10% of the photons in this beam will be characteristic x-rays with energies of about 65 keV.
A16.
B.
Focal-spot ratings are for exposure times of 0.1s.
A17.
A
The total x-ray tube output will primarily determine the film density.
A18.
B
The highest x-ray beam intensity is at the cathode edge. At the anode edge, there is significant absorption within the anode itself.
A19.
B.
Pair production mainly occurs in the strong electric field near the nucleus. The threshold is 1.022 MeV and, therefore, this interaction is not encountered in diagnostic radiology.
A20.
B.
Scattered photons are generally produced in Compton processes. The incident photon is completely absorbed during the photoelectric effect.
A21.
A-False; the photon disappears in photoelectric absorption and produces a photo-electron;
B-False; Compton scatter has no threshold; 1.022 MeV is the threshold for pair production;
C-False; the photoelectric effect falls off rapidly as 1/E3 above the K-edge
D-True.
A22.
E.
Scattering angle. When the scattering angle is 180 degrees, the backscattered photon has the lowest energy, and the Compton electron the highest energy.
A23.
A-photoelectric; Compton is greater than photoelectric at photon energies > 25 keV; the mean energy for 28 kVp beams is < 25 keV;
B-photoelectric; for calcium with a K-shell binding energy of 4 keV, the Compton effect is greater than the photoelectric effect for energies > 90 keV;
C-Compton effect;
D-photoelectric effect;
E-photoelectric effect.
Coherent processes are never dominant in diagnostic radiology.
A24.
B.
The mass attenuation coefficient is equal to the linear attenuation coefficient (
μ) divided by the density (ρ) (0.1/2 or 0.05 cm2/g).
A25.
C.
Tube loadings must increase since the x-ray tube output is reduced by the added filter. Increased filtration will increase the mean energy of the x-ray beam and therefore decrease subject contrast and skin exposure for a constant film density.
A26.
C.
Beam quality is the penetrating power measured as the thickness of Al required to attenuate the beam by 50%.
A27.
A.
kVp affects the subject contrast, which is the difference in x-ray beam intensities emerging from the patient. The other factors determine how subject contrast is transformed into image contrast.
A28.
C.
Rem is the non-SI unit of dose equivalent.
A29.
B
The QF converts dose (rad or Gy) to dose equivalent (rem or Sv). Note that in diagnostic radiology, the QF is generally equal to 1.0.
A30.
A-iii;
B-i;
C-ii.
Photon energy dependant factor used to convert exposures into absorbed dose for a specific absorbing medium.
A31.
A-ii; gray and rad are units of absorbed dose;
B-iii; Becquerel and curie are units of activity;
C-i; sievert and rem are units of dose equivalent;
D-v; C/kg and roentgen are units of exposure;
E-iv; joule and erg are units of energy.
A32.
A
Increasing the developer temperature will generally increase the film density.
A33.
A-iii; mammography requires high contrast; gammas of about 3 are typical;
B-i; chest radiographs need wide latitudes to visualize both the lung and mediastinum;
C-ii; penetration through the abdomen is low, and fast screen/film combinations are used to reduce exposure times and patient dose.
A34.
C.
Wide latitude. Latitude is inversely proportional to contrast, so that a high value of one indicates a low value of the other.
A35.
A-i; GM meters are very sensitive and portable, which makes them ideal for detection of contamination;
B-iii; physicists use ionization chambers to measure the output of x-ray tubes;
C-ii; thermoluminescent dosimeters are replacing film for personnel monitoring due to their superior energy response and ease of processing without the use of chemicals;
D-iv; xeroradiography uses charged selenium plates.
A36.
B.
The intensification factor is typically about 50.
A37.
E.
None are true. Matching the K-edge with the x-ray photon energy improves the x-ray beam absorption efficiency.
A38.
A-True;
B-True;
C-True;
D-True;
E-False. Note that most films are generally sensitive to blue light.
A39.
A
The grid ratio (strip height divided by gap distance) determines how efficiently the scattered radiation is removed.
A40.
C.
The modulation transfer function (MTF) specifies the resolution properties of an imaging system.
A41.
C.
Because the HVL for soft tissue is about 3 cm, the nodule will reduce the x-ray intensity by a factor of 2 and change log relative exposure by log10 2, which is 0.3; changing log exposure by 0.3 changes film density by 0.6 if the gradient is 2.0.
A42.
B.
Focal spot blur increases with focal spot size and magnification. Note that there is no focal spot blur in contact radiography.
A43.
E.
All of the listed factors would generally be expected to increase image contrast.
A44.
C.
Thicker screens will increase the efficiency of x-ray absorption but reduce the spatial resolution performance because of increasing screen blur.
A45.
A-iv; the air gap introduced in the magnification view causes most of the scatter to miss the film;
B-iii; a 12:1 grid is necessary because of the large amount of Compton scatter present in abdominal studies due to the large body thickness;
C-i; because mammography is performed at low (28) kVps, there is less scatter, and a 4:1 grid is usually sufficient;
D-ii; 6:1 grids are less sensitive to lateral decentering, which is an important consideration in portable examinations.
A46.
D.
Increasing the screen thickness does not change the total number of x-rays absorbed to give a specified film density, and therefore image noise will not be affected.
A47.
A-iv; subject contrast is always reduced as kV increases;
B-i; a lower mA will require a longer exposure time, which will increase the motion blur;
C-ii grids improve image contrast by reducing the amount of scatter reaching the screen/film;
D-iii; increasing focal spot size will increase focal spot blur.
A48.
E.
This is a list of the key characteristics of computed radiography.
A49.
A-iii;
B-ii;
C-i.
A50.
E.
Photocathodes convert light to electrons.
A51.
A.
The use of CsI crystals limits the spread of light in the input phosphor and therefore improves spatial resolution.
A52.
E
All of the statements are true.
A53.
D.
The dominant source of image noise is quantum mottle, which is reduced by increasing the number of x-ray photons.
A54.
A
kVp requirements in fluoroscopy and cine are very similar (70 to 90 kVp) and primarily determined by patient penetration.
A55.
B.
The distance along one dimension is 250 mm, which is used for 256 pixels, so each has a linear dimension of about 1 mm.
A56.
C.
The CPU performs all the arithmetic and logical operations in a computer.
A57.
A-False; random access memory loses its information when the power is switched -off;
B-True;
C-False; floppy disks hold about 1 MByte of information, whereas hard disks can store up to 1000 times more;
D-False; magnetic tapes are relatively slow devices because the information is stored serially;
E-True; optical jukeboxes can store over 1 TByte (10*12 Bytes) of information.
A58.
D.
PACS are currently very expensive; their justification is in their potential to cut operating costs (e.g., film, clerks).
A59.
A-ii;
B-iv;
C-iii;
D-v;
E-i.
A60.
E.
Patient temperature does not significantly affect the attenuation properties of the tissue relative to water.
A61.
D.
Limiting spatial resolution will increase because the pixel size is reduced when the matrix size is increased.
A62.
B.
At a higher kVp, attenuation coefficient differences are reduced and subject contrast is decreased.
A63.
B.
Phase-encoding artifacts only occur on MR images.
A64.
C
The mA determines the intensity of the x-ray beam, hence image noise, but has no direct effect on spatial resolution.
A65.
A,
Displayed contrast is the only factor that will be affected by the display window width.
A66.
A.
Beam-hardening artifacts occur because the average photon energy of an x-ray beam increases as it passes through the patient. The preferential loss of lower energy x-rays depresses the CT numbers because an apparent increase in x-ray beam penetration.
A67.
A-37 Bq (10*-9 Ci);
B-370 kBq (10*-5 Ci)
C-370 MBq (10*-2 Ci);
D-3.7 GBq (1 Ci)
E-3.7 x 10*13 Bq (10*3Ci);
F-3.7 x 10*17 B (10*7 Ci).
A68.
D.
99mTc generators can be eluted with saline on a daily basis. The parent 99Mo has a half-life of 67 hours, whereas the daughter 99mTc has a half-life of 6 hours. The growth of daughter activity reaches its maximum after about 4 daughter half-lives, or 24 hours.
A69.
E.
All of the statements are true.
A70.
A.
Spatial resolution decreases with increasing distance from the collimator face.
A71.
B.
Septal penetration always increases with increasing photon energy.
A72.
C.
Only MUGA studies absolutely require a computer to place the acquired counts into different parts of the cardiac cycle.
A73.
E.
Gamma camera imaging time has no effect on organ doses.
A74.
A-True;
B-False; the r factor would be needed to estimate the external radiation exposure level near the patient, but not the thyroid dose;
C-True;
D-True;
E-True.
A75.
B.
Crystal impurities are found in solid crystals such as film grains or semiconductor devices, not radiopharmaceuticals.
A76.
D.
Multiple views are obtained (typically 64 or 128 as the gamma camera is rotated around the patient).
A77.
C.
The range of the positrons is only 1 mm in soft tissue. PET makes use of the subsequent 511 keV gammas emitted when the positron annihilates with an electron.
A78.
C
Lymphocytes are very radiosensitive.
A79.
E.
All of the statements are true.
A80.
Aiii;
B-i;
C-ii.
A81.
D.
The entrance skin dose for a lateral lumbar spine is about 10 mGy (1 rad) so that the dose to the embryo will likely be 0.5 to 1 mGy (5~100 mrad).
A82.
E.
All of the statements are correct.
A83.
C
Most diagnostic radiology lead aprons have a 0.5 mm lead equivalence.
A84.
D.
None of these are requirements for leaded glasses.
A85.
A-True;
B-True;
C-True; domestic radon may be responsible for 10% of lung cancers in the United States;
D-False; radon is an alpha emitter;
E-False; radon's half-life is just under 4 days.
A86.
B.
Because beam attenuation is proportional to frequency, beam penetration will be markedly reduced.
A87.
C.
dB = 10 x 1og10(I/Io).
A88.
E.
All of the statements are true.
A89
D.
Refraction is governed by Snell's law and results in the ultrasound beam changing direction when passing from one medium to another.
A90.
C.
The Q factor is the bandwidth of the frequencies generated by the ultrasound transducer. A high Q indicates a narrow bandwidth and relatively pure frequency.
A91.
A-True;
B-False; TGC amplifies returning echoes dependent on depth;
C-True; prevents an air layer between the transducer and patient that would reflect virtually all the incident beam;
D-False; stores digital image data which are converted into video signal for display on a TV monitor.
A92. B. TGC increases the echo intensity with increasing echo time to account for increasing signal attenuation with tissue depth.
A93. D. The typical PRF is 1 kHz, not 100 kHz. The penetration depth would be less than 1 cm for a PRF of 100 kllz.
A94. A Axial resolution is equal to half the pulse length; therefore, increasing the pulse length reduces axial resolution.
A95. C Doppler provides information about flow from the change in ultrasound frequency produced by a moving object.
A96. A-False; nuclei with even numbers of protons and neutrons (e.g., 4He) have no net magnetic moment; WTrue; for protons this is 42 MHz/tesla; C-False; when aligned parallel, the energy is lower; D-False; only the excess in the lower energy orientation contribute to the signal (about 3 per million).
A97. D. The Larmor equation (2irf~ = y B) determines the RF frequency, where f is the Larmor frequency, B is the applied fie'id, and y is the gyromagnetic ratio.
A98. B. T2 is the characteristic time that describes loss of phase coherence in the transverse plane in the absence of magnetic field inhomogeneity effects.
A99.
A-True; power dissipation in the resistive coils is several tens of kW;
B-False; maximum fields are about 0.5 T;
C-True; permanent magnets have poles similar to horseshoe magnets which limit the fringe fields;
D-True; the large mass of permanent magnets is one of their disadvantages because of siteing difficulties;
E-True; most magnets are shimmed to achieve a uniformity of about 1 ppm;
F-True; there are a number of whole body 4 T magnets in operation.
A100.
D. RF coils do not require water cooling.
A101.
E. Optical densitometers are used to measure the density of film and are not affected by magnetic fields.
A102.
A-False; RF frequency is determined solely by the type of nucleus and the magnetic field strength;
B-True; several seconds for water and cerebrospinal fluid;
C-False; T2 for all solids including bone is very short and is the main reason why bone gives no detectable MR signal;
D-True;
E-True; T1 and T2 generally increase with malignancy, but there are wide ranges for both normal and malignant tissues.
A103.
A. Two-dimensional Fourier transforms are generally employed for planar reconstruction on current commercial systems.
A104.
C. CSF has a long T2 value and will appear bright on T2 weighted spin echo sequences.
A105.
B. For biological tissues, T2 is generally independent of magnetic field strength.
A106.
A-ii
B-iii;
C-i.
Lipids have a short Ti and fluids have a long Ti.
A107.
A-iii;
B-i;
C-ii.
A108.
D. Refraction gives rise to artifacts in ultrasound, not MRI.
A109.
A-ii;
B-iii;
C-iv;
D-i.
Fat has the lowest density, and calcifications have the highest. Cancer is only slightly more dense than fibroglandular tissue.
A110.
B. The contrast between fibroglandular and malignant tissues increases with reducing kVp because the photoelectric effect predominates at these low kVps.
A111.
A, Dedicated mammography units currently use either three-phase or high-frequency generators. Half-wave rectification would reduce the x-ray tube output and significantly increase exposure times.
A112.
D. Object contrast, which refers to the physical differences between areas of the imaged breast, is not affected by grids.
A113.
D. kV is checked on an annual basis and requires special kVp meters used by physicists.
A114.
A-iii; classic requirements for screen/ film mammography;
B-i; classic requirements for xeroradiography;
C-ii; microfocal spot size is always used in magnification mammography to minimize focal spot blur;
D-iv; two views are required to localize the lesion stereoscopically.
A115.
D. Ultrasound does not visualize microcalcifications.
A116.
D. Electron microscopy is used for in vitro tissue analysis.
A117.
D. The standard deviation omega is SR10,000 or 100; 99% of counts will lie between the mean +/- 3 omega or 10,000 +/- 300.
A118.
C. 10:1; signal is 100, the noise is SR100, or 10, so the SNR is 10:1.
A119.
A-i;
B-iii;
C-ii.
A120.
B. When the threshold criterion becomes more lax so that the TPF increases, the FPF also increases, and specificity, which is (1 FPF), is reduced.