G1.

D

By definition 1 curie = 3.7 x 10¹⁰ decays/sec. Therefore the current is 10,135 curie x (3.7 x 10¹⁰ decays/sec) x (1 electron/decay) x (1.6 x 10^-19 Coulombs/electron) =60 x 10^-6 Amp.

G2.

B 

The number of electrons per gram = (Z/A) N_A, where Z is the atomic number, A is the atomic weight and N_A is Avogadro's number.

G3.

A

1 mCi = 3.7x l0⁷decays/sec or 3.7x lO⁷Bq.   So an activity of 3.7x lO⁹Bq= 100mCi.

G4.

D

Absorbed dose per roentgen in tissue requires knowledge of the f-factor.

G5.

C

G6.

C

G7.

E

G8.

C

All x-rays, gamma and betas used for diagnostic and therapy purposes have Q = 1. For neutrons the RBE varies with energy, but for protection a "worst case" value of 20 is used.  See NCRP Report #91.

G9.

B

Exposure is defined only for photon beams, and its SI unit is C/kg. At diagnostic energies, the absorbed dose to bone is greater than that to muscle for the same exposure, because of the predominance of the photoelectric effect.

G10.

B

The Bohr radius of an atom is about 10^-11m while the nuclear radius has a value of 10^-15m. Thus the ratio of the two is (10^-11) I (10^-15) = 10⁴

G11.

C

The mass of the electron in MeV is 0.511. Therefore the mass of the muon in MeV is 207 x 0.511 = 106.

G12.

C

Neutrons are not charged particles, and generally interact with matter by transferring their energy to protons or other light nuclei, which then produce dense ionization tracks.

G13.

A

If the kinetic energy of a particle is » than the rest mass, then the velocity is very close to c.  Electrons have a rest mass of 0.511 MeV. Photons always travel at velocity c.

G14.

B

G15.

D

Binding energy increases as Z increases, and with decreasing distance from the nucleus.

G16.

C

All four elements have complete K- and L-shells of 10 electrons total. The M-shell contains a maximum of 18 electrons (2n2), and would thus contain 6,7, 8, and 9 electrons respectively for the four elements listed. The outer shell, however, contains a maximum of 8 electrons, meaning that potassium and argon both have 8 electrons in the M-shell, but the nineteenth electron in potassium goes into the N-shell. Thus argon is inert, while potassium is highly interactive.

G17.

C

¹⁹⁸Au has a half-life of 2.7 days. For ²²⁶Ra it's 1626 years; for ¹⁰³Pd it's 17 days; for ²²²Rn it's 3.83 days and for ⁶⁰Co it's 5.26 years.

G18.

C

A half-value layer of 1.1 cm of lead, means that this amount of lead will attenuate monoenergetic ⁶⁰Co beam by 1/2. Two half-value layers or 2.2 cm will attenuate the beam by 1/2² = 1/4. Therefore 10 half- values layers or 11 cm will attenuate the beam by a factor of about 1000 since the value of 1/2¹⁰ is 1/1024.

G19.

C

After 10 half-lives the source activity will be reduced to 1/2¹⁰ = 1/1024 = 0.1% .of its original activity. In this case 10 half-lives is l0 x 60 days = 600 days.

G20.

A

G21.

B

Because the half-life of radionuclide X is so much longer than radionuclide Y, the activity of Y builds up to that of its parent X. Thereafter, Y decays at the same same rate it is produced (Ax = Ay) and secular equilibrium is said to exist.

G22.

B

G23.

C

To find the activity required first calculate the number of half lives that have passed in 24 hours. This is simply 24 hr divided by (2.7 days x 24 hr/day) = 0.3703. Therefore if activity to be ordered is N then it satisfies the equation N x (1/2^0.3704) = 10 mCi. Solving for N gives 12.93 mCi.

G24.

B

G25.

C

G26.

C

The other processes don't create empty atomic shells.

G27.

A

G28.

B

Transformers are used in x-ray circuits to step-up voltage from volts to kilovolts required to generate x-rays, and also to step-down voltage (and therefore step-up current) in the filament circuit.

G29.

E

The milliammeter measures the current in mA. It is connected in series with the x-ray tube itself.

G30.

A

Rectifiers allow current to pass in only one direction; an x-ray tube acts as a rectifier in a "self-rectified" circuit.

G31.

D

Thermionic emission is the emission of electrons from the heated filament.  A dual focus enables the focal size to be kept small except when a high power technique is used, when a larger spot allows the heat to be spread over a larger target area.  A small target angle increases the ratio of actual to effective focal spot size. The rotating anode greatly increases the actual focal area, while maintaining a small effective focal spot.

G32.

E

The effective energy of an x-ray beam is approximately 1/3 to 1/2 of the kVp. This may be increased by additional filtration. X-ray energy produced by bremsstrahlung, is independent of the atomic number (Z) of the target material and of the mAs. Subject contrast is a function of x-ray energy.

G33. 

C

Beams with the same peak voltage will have the same maximum energy or equivalently the same minimum wavelength. (E = hc/λ)   (E = hv)

G34.

B

Characteristic x-rays of tungsten are emitted from electrons accelerated to greater than 69 keV. Spectrum I is produced by 50 kVp electrons, spectrum II by 100 kVp electrons.

G35.

C

Photons below 10 keV have been removed by filtration in both spectra.

G36.

B

Spectrum II includes photons up to 100 keV

.

G37.

D

The minimum photon energy from both spectra is about 10 keV.

G38.

D

Spectrum I does not have K characteristic x-rays. The K peeks appear on spectrum II.

G39.

A

The maximum photon energy from spectrum I is 50 keV which is produced by a potential of 50 kVp.

G40.

B

The exposure rate is a function of the intensity and the potential (kVp). The area under II is much greater than under I.

G41.

B

The HVL increases as the potential (kVp) increases.

G42.

E

Either spectra can be produced by single or three phase generators.

G43.

A 

Scatter increases with kVp in the diagnostic range.

G44.

C

Electromagnetic waves are transverse waves. Their component electric and magnetic fields lie in perpendicular planes with respect to each other and to the direction of propagation of the wave.

G45.

D

Gamma rays are electromagnetic radiation with the shortest wavelength. They are the most penetrating of electromagnetic radiation.

G46.

C

Luminescent substances can be placed into two categories depending on the duration of light emission after the source of excitation is removed. When the emission of light ceases immediately (within 10^-8sec) after the excitation is removed, these substances are called fluorescent.  Substances which continue to glow longer than I0^-8 sec after the source of excitation is removed are called phosphorescent.

G47.

C

Reflection, refraction, interference, and diffraction can occur for both transverse and longitudinal waves. Polarization allows only electromagnetic waves with electric fields vibrating in a particular direction to pass through a material while blocking all other waves with electric fields vibrating in different directions. Polarization effects are not observed for longitudinal waves.

G48.

A

Beta-rays (positive or negative) are ionizing particles.

G49.

D

Heat radiation consists of non-ionizing photons with energies of about 10^-2 to 10^-3 eV.

G50.

D 

Visible light consists of non-ionizing photons with energies of about 1.8 to 3.0 eV.

G51.

C

X-rays and gamma-rays are ionizing photons of energies above about 10 keV.

G52.

E

Ultrasound is a mechanical motion that is propagated as a wave through a medium.

G53.

D

X-rays originate from electron shells of an atom, while gamma-rays originate from the nucleus.

G54.

B

Energy is inversely proportional to wavelength.

Thus E_visible = I.2 x 10^-12/6x10^-7MeV = 2eV.

G55.

C

The Compton cross-section increases with electron density (number of electrons / g). The electron density is given by the expression (Z/A) N_A, where Z is the atomic number, A is the atomic weight and N_A is Avogadro's number. For all substances except hydrogen the number of electrons per gram is about 3.0 x 10²³ Because hydrogen has no neutrons its electron density has a value of 6.0 x 10²³

G56.

E

Photons, but not electrons, can be backscattered in Compton interactions. Backscattered photons have the lowest energy while forward scattered photons have the highest energy. Similarly, forward scattered electrons have the highest energy and side-scattered (or 90 degrees) electrons have the lowest energy.

G57.

A

The initial energy of the photon equals the energy of the photon scattered to 0 degrees.

G58.

A

Coherent scattering is also known as Rayleigh scattering. It only occurs for low energy photons.

G59.

D

A decreased wavelength means increased energy after scattering which is not possible.

G60.

C

G61.

B

G62.

B

The photoelectric effect occurs with the tightly bound, inner shell electrons.

G63.

D

Pair production occurs within the electric field of a nucleus.

G64.

C

The Compton effect usually occurs with loosely bound, outer shell electrons.

G65.

E

Triplet production is similar to pair production, only a third electron is required.

G66.

A

No energy is transferred during Rayleigh scattering.

G67.

C

G68.

B

G69.

B

G70.

D

G71.

A

G72.

D

High energy electrons have speeds close to that of the speed of light.  If they pass through the eyeball, their speed exceeds the speed of light in the eye tissue and this causes Cerenkov radiation.

G73.

B

G74.

C

Optical density is the logarithm of the incident intensity divided by the transmitted intensity. Optical density is therefore additive. Thus, 1.5 + 1.5 = 3.0 is the optical density of the sandwich. The antilog of 3 is 1000, meaning that 1/1000 or 0.001 of the incident light is transmitted.

G75.

B

Compton scattered photons travel in random directions and thus contain no useful diagnostic information. The grid absorbs most of these photons, thus improving image contrast.

G76.

C

The probability of a photoelectric interaction is proportional to the cube of Z (atomic number), whereas it is independent of Z for Compton interactions. Thus, a small difference in Z between different materials causes a large difference in the number of interactions which occur at low energies where photoelectric interactions are most common.

G77.

C

The probability of "outcome A" occurring at least once is equal to 1 minus the probability of "not outcome A" occurring 1000 times in a row:1 - (1-0.001)¹⁰⁰⁰ =0.632

G78.

A

G79.

C

G80.

C

G81.

A

Floppy disks are slow and have a low storage capacity.

G82.

C

Hard discs are the fastest devices other than RAM and have storage capacities exceeded only by optical drives and magnetic tapes.

G83.

D

Optical disks have large storage capacity and intermediate speeds.

G84.

B

RAM is the fastest device because it is non-mechanical. However storage capacity is low.

G85.

D

Tera is the prefix for 10¹².

A Terabyte is 2⁴⁰ bytes or 1,099,511,617,776 bytes.

G86.

A

G87.

C

ASCII is the acronym for American Standard Code for Information Interchange. ASCII defines the codes the computer uses internally to store letters, numbers, and punctuation. ASCII files can be read as text.

G88.

B

The number of bytes needed is 512x 512 x 2 = 524,288 bytes or about 0.5 MB. Remember 16 bits equals 2 bytes.

G89.

B

G90.

D

The binary number has a decimal value of 129. The hexadecimal number 81 also has a decimal value of 129 (= 8 x 16 + 1 x 1)

G91.

C

See ICRU Report #60.

G92.

D

When an unshielded container does not give any reading above background as measured with a sensitive Geiger survey meter, the contents may be disposed of as non-radioactive material.

G93.

B

The half-value layer for an I-125 source is 0.02 mm of lead. Therefore 0.04 mm or two half-value layers will reduce the exposure rate 1/2² = 1/4.

G94.

E

All power lines use alternating current of 60 Hz. The energy of the quanta for such a low frequency is much less than the ionization potential of an atom or molecule. Therefore this is an example of non-ionizing radiation, regardless of the voltage.

G95.

E

Too high a voltage will cause conduction in gasses or recycling in a Geiger tube.

G96.

C

Proportional counters operate in this region.

G97.

B

In region A, some ions recombine before being collected. With sufficient voltage, all are collected. In region C, additional voltage causes multiplication.

G98.

D

Any event will trigger an avalanche, giving pulses of uniform size.

G99.

B

DIAGNOSTIC RADIOLOGY

Answers

D1.

A

The tube current, measured in milliamperes, is the flow of electrons from the filament to the anode and is controlled by the filament circuit which is used to heat the filament.  The space charge is a collection of negatively charged electrons in a small cloud surrounding the tungsten filament. This cloud of negative charges tends to prevent other electrons from being emitted from the filament. Therefore, this limiting of the emission of more electrons is called the space charge effect.  When a metal is heated it is able to absorb thermal energy and some of the electrons in the metal acquire enough energy to allow them to move a small distance from the surface of the metal. This is called thermionic emission and is the source of the x-ray tube current.

D2.

A

Single-phase x-ray units have the most amount of ripple in the kVp waveform. The ripple is nearly 100%. Some of the high frequency and three-phase, 12-pulse x-ray generators have very small amounts of ripple of about 3% to 5%. The maximum voltage regardless of the amount of ripple is the one specified for the x-ray exposures. Hence, the maximum photon energy is the same on these units. Nevertheless, the average photon energy and the amount of x-ray intensity is much less on units that have a high amount of ripple. Units with high ripple have a higher peak mA value and these units require lower heat ratings for the x-ray tubes. Low average photon energy means that the x-rays do not penetrate tissue well.

D3.

D

The single exposure heat ratings are charts that indicate what combination of kV, mA, and time can be utilized without damaging the track of the anode. The anode heat capacity is the maximum cumulative heat that can be placed in the anode without melting it.  The housing heat capacity is the total amount of heat that can be placed into the housing of the x-ray tube without damaging it; this amount of heat is huge, usually in the millions of heat units. Cathode KHU is a fictitious term.

D4.

B

Small anode angles result in a smaller cassette size that can be used without having heel effect cut-off at the edge of the cassette. X-ray tubes are designed by specifying the focal spot size and adjusting the filaments accordingly. The tube angle then determines the heat loading for that particular anode. Smaller angles permit the use of larger areas of the target for electron bombardment (and heat dissipation), yet achieve a small apparent focal spot size. The heel effect increases when smaller anode angles are used. Decreasing the mass of the anode is not directly related to the anode angle. Anodes which have small masses result in a lower anode heat capacity.

D5.

B

Step wedges are used to cause a gray scale which can be used to evaluate the contrast of the image receptor system. The bar patterns are a series of lead lines of varying thicknesses with spaces between them. The wire mesh are metal screens with different spacings and sizes of the screen holes. Hole patterns are different sizes of small holes that can be used by CT and other imaging systems to assess the smallest size that can be seen. A wire impulse response function is used in CT and it can be used elsewhere to get a line spread function (LSF) from which an MTF is then calculated.

D6.

C

The grid controls the tube current. When it is sufficiently negative, it cuts off electrons from flowing from the cathode to the anode. The grid does not collimate the x-ray beam. A focusing grid may help focus the electron stream going from the cathode to the anode, but this is one step before actual x-rays are produced. The grid allows fast exposures. The voltage on a grid can be rapidly turned up and down, thus blocking and unblocking the electron stream in very short periods of time, and hence turning the x-ray beam on and off very quickly. The grid can be synchronized with the cine machinery so that the film is exposed at the right time. The grid does not increase heating in the x-ray tube. When the grid is working, it is actually switching the electron beam off intermittently, reducing heating slightly in the x-ray tube.

D7.

D

Increasing the exposure time increases motion unsharpness. At the same mAs and no change in kVp, longer exposures will be associated with an equal decrease in mA current. Consequently, film and patient exposures will not change. No change in kVp means no change in film latitude. The screen/film speed is not affected by exposure time variations.

D8.

E

Although all of the factors listed have some influence on image quality, low contrast visualization is primarily limited by the noise or radiographic mottle in the system.

D9.

D

The heel effect is the decrease in the number of photons on the anode side of the image as opposed to the cathode side of the image.

The space charge effect is a decrease in the tube current as the tube potential is lowered below the saturation voltage.

Reciprocity failure is obtaining less film density for the same amount of mAs as the exposure time either gets very long or very short.

Recombination losses have to do with the measurement of ions in an ionization chamber.

Latent image fading describes a situation where the film density decreases if a long time elapses between the exposure of the film and the processing of the film.

D1O.

C

See answer to D9 above.

D11.

C

D12.

B

D13.

D

D14.

 A

D15.

E

D16.

D

The film/screen combination will decrease exposure to the patient by a factor of 30 to 50. Film alone versus the film/screen combination has higher spatial resolution due to light diffusion effect in the screen. The screen will absorb a relatively smaller number of higher energy, short wavelength x-ray photons, and emit a larger number of lower energy, longer wavelength, light photons. More x-ray absorption occurs at energies slightly higher than the K-edge of the screen phosphor.

D17.

C

Increasing the screen's thickness will stop more photons. However, in order to keep the same film exposure, same film density, one will have to decrease the patient exposure. Since the same total number of x-ray photons will be absorbed by the screen and their energy converted to visible or ultraviolet photons, the noise will remain constant. The screen noise is determined by the total number of detected photons.

D18.

E

The fixer solution is responsible for removing the undeveloped silver grains from the film base. Failure to remove all of the silver grains will over a period of time result in a darkening of the film. Films which are not properly fixed have a brown appearance after a number of years. Thus, the fixer solution is directly related to the archival storage. The fixer also influences the silver reclaiming; however, improper fixing will result in less silver being removed from the film. Therefore, the silver reclamation will decrease. The fixer has no influence whatsoever on film contrast, film speed, or the quantum mottle.

D19.

C

Vignetting is the brightness change from the center to the edge of the image intensifier. The center is brighter than the edge. Flare and veiling glare are related parameters which indicate a degradation in contrast due to light scattering in the image intensifier. Blooming is the excessive brightness that occurs at the outside edge of a highly attenuating object.

D20. 

D

It is important to realize that fluoroscopy results in a high typical patient entrance radiation dose. The typical patient entrance exposure rate is around 1 to 3 R/min. The maximum patient entrance exposure is determined by federal regulations. For a fluoroscopy system with ABC control that does not have a high level mode, the maximum level must be limited to 10 R/min For systems with high-level control, the maximum patient entrance exposure rate is limited to 20 R/min. For systems without ABC, the maximum patient entrance exposure rate is limited to 5 R/min.

D21.

C

The larger caliber of the bowel filled with barium increases subject contrast The mass and linear attenuation coefficients for iodine and barium are not significantly different The atomic number of barium (56) is slightly greater than that of iodine (53).

D22.

B

Bremsstrahlung radiation intensity increases with the tube potential and it is proportional to kVp². Characteristic radiation energy is only dependent on target material atomic number. The escape angle of scattered radiation becomes smaller at higher beam energies; hence, more radiation will scatter in the forward direction (towards the film in radiography). To maintain the same film density, an increase in kVp will be accompanied by a decrease in mA. There would be a small decrease in the focal spot size at higher kVps.

D23.

E

Added filtration reduces soft radiation intensity at the lower end of the energy (E) spectrum which is also the spectrum's longest radiation wavelength (λ) since λ is inversely proportional to E. Added filtration increases the mean energy of the beam, therefore increasing beam effective energy, causing more beam penetration, lower skin absorption, reduced linear attenuation coefficient, and reduced photoelectric absorption, hence reduced subject contrast.

D24-28.

Energy deposited in the anode in joules (anode heat loading) is the product of the voltage V, current A, exposure time S, and number of frames if applicable. The tube rating in watts (power deposited) is the product of the kV and the mA with exposure time of 0.1 seconds.

D24.

E

100kV x 2.5 mA x 4 x 60s = 60kJ

D25.

B

100kV x 100mA x 0.01s/f x 5f/s x 5s = 2.5kJ

D26.

D

100kV x 500mA x 0.1s = 5kJ

D27.

C

100kV x 500mA = 50kW

D28.

A

100kV x 25OmA x 0.25s x 4 = 25kJ

D29.

A

The three major sources of blur in radiographs are screen blur, focal spot size, and subject motion. Both screen blur and focal spot blur are functions of magnification. For small magnification in the absence of motion, screen blur is the dominant term.

D30.

B

The 10% point on the MTF curve represents the limit of resolution that the eye observes with a lead line - test pattern, the typical method of determining resolving power.

D31.

D

Subject contrast changes are functions of tissue and beam quality variations. Lower subject contrast is expected at increased beam energy (Higher quality, less photoelectric effect), less tissue thickness, lower tissue density, and lower atomic number. Other film/screen and processor variations will affect radiographic contrast (overall image contrast) but not subject contrast.

D32.

C

When focus-film distance is increased, magnification decreases (with no increase in object-film distance). Geometric unsharpness, also called focal spot blur, decreases with lower magnification. Screen unsharpness increases with screen thickness as a result of increased diffusion of light in the screen before reaching the film. Doubling the exposure time doubles the skin exposure, and doubling the FFD reduces skin exposure to 1/4 of its original value. The combined effect, however, is a decrease in skin exposure. From the answer in B, the combined effect of the three parameters will be 2 x 1/4 x 2 = 1 i.e., there is no change in film exposure or density. The heel effect causes greater attenuation in the anode direction. It can be reduced by using larger anode angle, longer focus-film distance, and smaller field size.

D33. 

C   1,4, and 5 are correct.

Transmittance T is the fraction of incident light passing through the film (I_t/I_o), while optical density OD is log₁₀ (I_o/I_t).  OD = log₁₀(1/T). A film with 10% transmittance has an optical density of 1. Two superimposed films will have a total OD of 2. Film speed is inversely proportional to exposure, hence fast films require less radiation to produce a given density. A film with a gradient of 3.0 is likely to produce high contrast image particularly used in mammography. The 40:1 exposure ratio is defined as the dynamic range. Exposures outside this range are in the toe or shoulder region of the film characteristic curve, and would result in very low image contrast.

D34.

B

The imaging system's target material and geometry, the HVL of the beam, the degree of compression which affects thickness and adiposity, all contribute to the AGD. The focal spot size affects resolution, especially in magnification views.

D35.

E

The MTF of a composite imaging system is equal to the product of the individual components of the system. If MTF1, MTF2, ..., MTFn are the MTFs of the n components of the system, the MTF of the composite imaging system will be given by the following expression:

        MTF = MTF1 x MTF2 x....x MTFn

By definition the MTF of an imaging system always has a value in the range 0 < MTF < 1. Hence, the MTF of a composite imaging system is always less than any MTFi, where i is any of the n components of the imaging system.

D36.

A

Logarithmic transformation as performed in digital fluoroscopy ensures that a structure (such as a vessel) will appear with the same contrast to its background whether it appears in a thick or thin part.

D37.

C

Frame integration in digital fluoroscopy involves averaging or adding together several frames. It reduces the effect of all types of random noise (both electronic and quantum) as opposed to just increasing the dose per frame (which would only decrease x-ray quantum noise).

D38.

D

A narrow window (log relative exposure) is roughly analogous to using high contrast film. After a single exposure, the brightness of each point on the image can be scaled in an analogous way to choosing a film for an appropriate density scale on the film characteristic curve.

D39.

B

Hybrid subtraction in digital fluoroscopy involves combining a simple mask subtraction with dual energy subtraction. It is designed for situations where anatomic motion is a problem.

D40..

B

Electron focusing is not uniform across the entire width of an image intensifier. Peripheral electrons tend to flare out from an ideal course1 resulting in unequal magnification, which produces peripheral distortion and fall off in brightness at the periphery of an image.

D41.

B

Interpolated conversion factor = (175+140 + 184 + 147)/4 = 161.5.

AGD = (161.5 mrads/R)(0.8 R) = 129.2 mrads.

D42.

D

The field size extension should not exceed 2% of SID, and the chest wall edge of the compression paddle should not appear in the mammogram. Average glandular dose in the question should not exceed 3 mGy (0.3 rad). Measured darkroom fog should not exceed 0.05.

D43.

C

In mammography, the x-ray beam filtration is usually the same material as the anode material. This material acts as a filter since the highest attenuation occurs slightly above the K- characteristic x-ray energies. Thus, these filters selectively remove x-rays both above and below the characteristic x-rays. In this manner, these filters preferentially transmit the characteristic x rays and filter bremsstrahlung x-rays. Most of the low energy x-rays never penetrate through the tissue in order to interact with the image receptor, hence these x-rays contribute to radiation dose and only contribute in a minor way to image quality. The higher energy x-rays penetrate the tissue well; however, because the photoelectric effect decreases rapidly with photon energy, these photons have less contrast than the characteristic x-ray photons. Hence, it is better to filter the high energy bremsstrahlung photons that have lower contrast and to use the characteristic x-rays which have more contrast

D44.

D

The replenishment rates, developer concentration, and developer temperature directly influence the density and speed of the various film/screen systems. Similarly, the developer immersion time is directly related to film density and the relative speed of the system. Bromine concentration in the developer does affect both the speed and contrast of the mammograms. However, this question talks about nitrate depletion which has no effect on the development of mammography films.

D45.

A

The NCRP recommended maximum whole body and eye weekly doses are 100 and 300 mrems, respectively. Since the lead apron will stop 95% to 98% of the incident radiation to the body, the eye exposure is the critical factor. (300 mrem/wk)I(20 mrem/procedure) =15 Procedures/week.

D46.

B

The image intensifier MTF is less than that for a radiographic film which is 1.

D47.

C

Minification gain is the ratio between the areas of the input and the output phosphors of the image intensifier. Doubling the input phosphor size will increase the ratio between the areas (minification gain) 4 times. Consequently, skin exposure will he reduced by a factor of 4. Due to the increased input field of view, the image spatial resolution will decrease.

D48.

D

For a smaller number of detected photons/mm2 the contrast-diameter curve for detectable objects moves away from the origin. The most significant effect is the inability to detect small objects with low contrast.

D49.

C

D50. A The maximum pulse repetition frequency is determined by the time it takes for a pulse to traverse the maximum useful range and return to the transducer. PRF (max) = (speed of ultrasound)/(2x range) = (1540 m/s)/(2 x 0.1 m) =7.7 kHz

D51.

D

Because fat and other lipids have a molecular chain length that allows fast transfer of magnetic spin properties, these structures have relatively short T2 times. Hence, T2 weighted images would result in the fat appearing darker than the surrounding tissue. T2 images can be produced with long TE and long TR times.

D52.

D

The spatial resolution with MRI imaging is usually limited to the pixel size. In general, the spatial resolution is in the range of 0.6 mm to 1.0 mm minimal detectable size. Hence, the spatial resolution is approximately the same as that of CT scanners. SPECT scanners have much worse spatial resolution of around 6 mm to 10 mm. Mammographic units are capable of visualizing 0.05 mm calcium specks and chest radiographs can visualize objects of around 0.1 mm.

D53.

C

Thinner slices and smaller pixel sizes enclose fewer protons in the voxel space. Therefore, they produce smaller signal levels. More NEX results in averaging the signal which reduces the noise and, therefore this change improves the signal-to-noise ratio. Short TR times result in a smaller signal being used to form the image. The signal/noise ratio improves as the field strength of the magnets increases.

D54. B Fringe magnetic fields often interfere with electronic devices such as biostimulators and pacemakrs.

RF heating effects can occur if metal or wire structures are placed near the ski Magnetic surgical

clips in patients have been known to move during MRI scans and case bleeding in the patients.

Large ferro-magnetic objects brought near a large field strength MRI have been known to go flying

through a room and sometimes injure staaf in the room. For magnetic field strengths below 2 Tesla,

the effect of magnetic fields inducing electrical potential in neurons has not been observed.

However, at much higher field strengths, it has been speculated that such effects may occur~

D55. A The percentage error PE is given by: PE = (aIN) 100 = 100I(N)1TL where a is the standard

deviation; N is the the total number of counts (count rate x time) =3330 x t ~O rate is neglected,

1 = 100I(t x 3330)~, t = 3 minutes.

D56. C RF transmitter problems usually result in the loss of signal, noisy image and possible strealrs or

missing lines in the image. Problems with lekks in the RF shielding also produce noise in the

image. If the gradient fields are not adjusted properly and are not linear; curvatures would be

noticed in structures in MRI images. The eddy currents cause local inhomogeneity problems and

loss of signal. Incorrect shim coil adjustments will also result in inhomogeneities of the naain

magnetic field, but these adjustments do not cause spatial distortions.

D57. A Ti weighted images rely on short repetition time CIR) and short echo time (IE). A short TE' for a

spin echo pulse sequence would be 10 to 20 ms. TE for gradient echo sequences are even shorter.

Range ofTR used in MRJ would be 100 to 3000ms.

D58. B The frequency for a proton sample along the x-axis is f = y (Bo + G~x). Therefore,

Af = ~x Ax = (42.6 MHzfI)(0. 1 mTkm)(4 cm) =17 IcHz.

D59. B The chemical shift of the two peeks is 8 ppm (parts per million) which has been calculated relative

to the center frequency of 128 MHz. Therefore, the conversion back to frequency is obtained by:

Af= (128 MHz)(8 x 1~6) =1024 Hz.

D60. C Thyroid burden levels as low as 40 nCi (40 x io-~ Ci) should be measured. Highly sensitive

radiation measuring instruments with adequate shielding will be required. Nal CI· l) probe is an

ideal instrument for this purpose.

~1. D Short scan times reduce patient motion blur. Obviously, some reconstruction methods increase the

spatial resolution or decrease the noise as required. Smaller pixel sizes increase the spatial

resolution for large fields of view such as body scans. low kYps do not have much of an effect on

image contrast since the x-ray beam is heavily filtered. The lower kVps merely reduce the total

number of photons which basically produces a noisier image unless higher mAs are used.

Obviously, the adrninistration of contrast media does enhance certain tissue.

RAPHEX1996 0 Diagnostic Answei: 0 Page 12

1)62. D For small fields of view which are not limited by pixel size, the focal spot size, geometry and

detector size have a significant influence upon the spatial resolution. For large fields~of-view such

as body scans, the pixel size is the predominant factor which determines spatial resolution. Slice

thickness does have an impact upon CT noise and artifacts, but it does not have a signiricant

influence upon the limiting spatial resolution.

1)63. E The two materials which are used to calibrate the CT number scale are air, which is assigned a CT

number of -1000 and water which is assigned a value of 0. Once these two points are determined,

the other CT numbers are deternened by this contrast scale.

D64. A Filmiscreen combinations are limited by the system noise to a contrast discriination level of

around 5% to 8%. CT scanners can disiminate contrast differences as low as 0.3% to 0.5%.

Hence, CT scanners have 20 times better low contrast discrimination than f~screen systems. It

is for this reason that the CT scanners have a main advantage over filmiscreen systems which have

a much better spatial resolution.

1)65. A Helical CT scanners have essentially the same radiation dose and same scatter radiation as axial

CT scans. The scan times are usually of the order of 1 second per slice for helical scans and may

be slightly longer for axial CT scans. Both helical and axial CT scans essentially use the same

kVps. The slice thiclcnesses are a little wider for helical scans with the same slice selection.

1)66. D Th,ical helical CTscan techniques are 120 kYp, 1 to 200 mA and approximately 1 to 2 seconds

per centimeter. Hence, 40 helical slices covering a range of 40 centimeters would require

approximately 40 seconds of scan time. For three-phase units one must multiply the 1.35 by the

kYp by the mAs by the number of scans and divide by 1000. The number of heat units is

approximately 1.3 million heat units for this series of scans. Helical CT scanners usually have x

ray tubes with large anode heat capacities of approximately two to four million heat units. Hence,

this scan series would represent approximately 20% to 50% of the total anode heat capacity.

1)67. E WIPS is a fictitlous term. NEMA stands for the National Electronics Manufacturing Assocation

which sets standards for electronic and electrical devices. RAM stands for random acceas memory

which is the hard memory which disappears when the computer is turned off. The CPU is the

central processing unit which is where the calculations occur in the computer. WORM is the write

once read many times memory so it is a device in which one can store the information and then

retrieve it as many times as one wants.

1)68. A In order to handle multiple complex processing sections, a large central processing unit is required.

This is usually specified in the amount of RAM that is require~ The hard disk is a storage device.

MIPS is the millions of instructions per second, which is the speed of the cornputer. DOS is the disk

operating system, which is the traffic controller and organizer of all software being used. MODEM is

a unit that converts the data into a format that can be trmsmitted over telephone lins.

1)69. D The icon is a system of symbols used to identi~ programs. IlIrallel precesing is performing two

or more sets of computations or analyses simultaneously on the computer. The ethernet is a

coaxial cable that linns the various computers in a network together. CD ROM is a CD disk on

which data can he stored.

D70. E Each number in a binary system represents 2 to some power. The first number to the right

represents 2 to the 0 power which is equal to 1. The second number from the right hand side is 2

to the first power which represents 2. The third number from the right hand side is 2 to the second

power which equals 4. The ones and zeros indicate whether the number is leesent or noL Twelve

bits are required to cover a number range from -1024 to +3096 which is the standard range of

most CT scanners.

RAPHEYI996 0 DiagnosticAnswer: 0 Page 13

D71. C An annular array is a circular arrangement of piezoelectric crystals which can be focused to

provide optimal lateral resolution at different depths in the body. A phased array activates the

elements in a manner so that the ultrasound beam can be steered in different directions and

produces a truncated triangular image. A linear array fires the ultrasound elements in a sequence

and produces a rectangular picture. A sector scanner has a single transoucer which is on a moving

head that produces an image that looks like a triangular section. A variable focus scanner would

be similar to an annular array.

D72. C The speed of an ultrasound wave depends upon the tissue density and compressibility.

D73. B The spatial resolution of ultrasound units is simlar to that of ~I' scanners. The spatial resolution

is around 1 mm.

D74. A Cavitation and heating effects are the primary adverse biological effects from ultrasound

procedures. It has been speculated that ultrasound proceures might also affect the nervous system

in fetuses. It is highly unlikely that either heating or cavitation effects would ever occur at the

power ranges used for normal scans with diagnostic units.

D75. B In isomeric transitions, the photons can either be directly emitted or they can interact with an

orbital electron to cause an internal conversion effect. The internal conversion electrons are

ejected. This creates a vacancy in the orbital shells. Following a rearrangement of the electrons in

the orbital shells, either a fluorescent x-ray or an Auger electron may be emitted.

D76. D In a 33.5 hour period, technetium-99m essentially undergoes more than S half-lives. This reduces

the amount of activity by more than a factor of 32. In the same Ireriod of time the molybdenum-99

contamination in the generated eluent decays by less than one half-life.

D77. B

D78. E

D79. D

D80. C

D81. A

D82. E The limiting spatial resolution of most scintillation cameras is between 6 mm and 10 mm. At 140

keY photon energies, there is little septal penetration of the collimator. The collimator hole siecs

are generally much smaller than the limiting spatial resolution provided by the scintillation

camera. Compton scattered photons do degrade image quality but they are not necessarily the

limiting factor in the spatial resolution. For the matrix size described the pixel size is

approximately 1 mm.

D83. C SPECT studies must correct for attenuation and small ditferences in detector uniformities have an

appreciable effect upon the reconstruction. Similarly, Compton scattered photons and tissue

inhomogeneities are importanL SPECT studies involve single photon emission and not PositrouL

D84. C Most guidelines suggest that the critical organ dose in nuclear medicine studies be limited to less

than 5 CGy (5 rads).

D85. D For therapeutic thyroid ablations using the administration of 131~ the typical amount of activity

administered is usually 100 mCi (4000 MBq). The corresponding dose is listed for this amount of

activity~

RAPHEY1996 0 DiagnosticAnswers 0 Page 14

D86. D 4.5 HVL£ are needed to attenuate the beam to 95% of the initial value. The beam HVL in this

question is 0.5 mm14.5 = 0.111 mm of lead equivalent. The lead equivalent apron thickness of

0.25 mmis about 2.25 HYLs, which will attenuate about 77.6% of the scattered beam.

D87. E There are no regulations limiting the radiation dose to patients undergoing radiological

procedures. The physicians are expected to weigh the risk(benefit analysis to determine whether a

procedure should be performed.

D88. D In general, the rule for pregnant patients is to limit the pre~xposure dose limits to less than 4 mSv

(500 mrem) during the entire gestation period. However, therapeutic abortions are usually not

considered until the radiation dose limit to the fetus excels 50 to 100 mSv.

D89. C The CPCRD recommends regulatory agencies use the following: the effective dose equivalent cmi

be determined by multiplying the reading on the external badge at the collar level by a factor of

0.3. This yields a value of 500 jiSv per month.

D90. C Radiation shielding is usually calculated for three different source terms: the primary beam,

scattered radiation, and leakage radiation. For scatter radiation, the direction is assumed to be

isotropic. Hence, the use factor is always equal to a factor of 1 and no use factor is given in the

formulas. All of the other factors are important in calculation of the required radiation shielding

for the walls.

1)91. D

1)92. B

1)93. E

1)94. A

1)95. C

RAPHEX1996 0 DiagnosticAnswer: 0 Page 15

D86. D 4.5 HVL£ are needed to attenuate the beam to 95% of the initial value. The beam HVL in this

question is 0.5 mm14.5 = 0.111 mm of lead equivalent. The lead equivalent apron thickness of

0.25 mmis about 2.25 HYLs, which will attenuate about 77.6% of the scattered beam.

D87. E There are no regulations limiting the radiation dose to patients undergoing radiological

procedures. The physicians are expected to weigh the riskibenefit analysis to determine whether a

procedure should be performed.

D88. D In general, the rule for pregnant patients is to limit the pre~xposure dose limits to less than 4 mSv

(500 mrem) during the entire gestation period. However, therapeutic abortions are usually not

considered until the radiation dose limit to the fetus excels 50 to 100 mSv.

D89. C The CPCRD recommends regulatory agencies use the following: the effective dose equivalent cm'

be determined by multiplying the reading on the external badge at the collar level by a factor of

0.3. This yields a value of 500 ~Sv per month.

D90. C Radiation shielding is usually calculated for three different source terms: the primary beam,

scattered radiation, and leakage radiation. For scatter radiation, the direction is assumed to be

isotropic. Hence, the use factor is always equal to a factor of 1 and no use factor is given in the

formulas. All of the other factors are important in calculation of the required radiation shielding

for the walls.

~1. D

[)92. B

1)93. E

1)94. A

1)95. C

RAPHEX1996 0 DiagnosticAnswer: 0 Page 15