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CT Theory 1 > Quiz 2 Review > Flashcards

Quiz 2 Review Flashcards

1
Q

Q. Absorbed Dose: What is it and what units is it measured in?

A

Amount of radiation absorbed by the patient. Used to determine threshold for when a deterministic effect will occur. Measured in Grays (G-y).

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2
Q

Q. Effective Dose: What is it and what units is it measured in?

A

Used to measure the risk of partial body exposure from equivalent whole body dose. Accounts for type of radiation and type of tissue. Measured in Sieverts (S-v).

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3
Q

Q. Deterministic Effects: Definition and examples. What Dose is it associated with?

A

Has dose threshold, severity is related to dose received, examples are erythema, necrosis, epilation, cataracts. Associated with Absorbed dose.

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4
Q

Q. Stochastic Effects: Definition and examples. What Dose is it associated with?

A

We deal more with stochastic in CT. Occur by chance, probability increases with dose (response linearity). No threshold. Severity not dependent on dose (only probability). Typically years after exposure. Examples include cancer and genetic effects. Associated with effective dose.

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5
Q

Q. What is response linearity?

A

Probability of effects increase linearly with dose.

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6
Q

Q. What is the law of Bergonie and Tribondeau?

A

Radiosensitivity of a tissue is determined by its metabolic state. (infants and Kids have high metabolic activity, less differentiated cells in fetuses).

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7
Q

Q. What is in plane shielding? Out of plane?

A

In plane is bismuth, used for anterior dose reduction, must not be in scout as it will cause A-C-T-M issues. Out of plane is lead, used to make patient feel safe, cannot be in S-F-O-V and has no impact on image quality.

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8
Q

Q. Where in the CT suite is the greatest exposure?

A

Near gantry aperture.

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9
Q

Q. What replaced maximum dose limits and why?

A

Diagnostic Reference levels (D-R-L) replaced maximum dose limits, because values are too high in max dose limits.

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10
Q

Q. What 3 things can dose measurements be used for?

A

Risk assessment, radiation protection guidelines, dose optimization.

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11
Q

Q. What are 3 measurements we care about in CT?

A

Exposure (dosimeters), absorbed dose, and effective dose.

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12
Q

Q. Which measurements are dose reference limits based on?

A

Based on C-T-D-I measurements.

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13
Q

Q. 2 main uses of dosimeters:

A

Human radiation protection (personal dosimeters) and measurements of dose in medical processes (radiation emitted) from scanner.)

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14
Q

Q. What kind of personal dosimeter is mentioned in the PowerPoint?

A

Thermoluminescent (T-L-D) dosimeter, emits light when heated which shows how much radiation it was exposed to, can be worn for 3 months.

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15
Q

Q. Does the thermoluminescent dosimeter retain a record of previous exposure after it has been measured?

A

No.

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16
Q

Q. What special kind of ionization chamber does CT use, what measurement does it provide?

A

Pencil ionization chambers used to measure C-T-D-I

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17
Q

Q. What is the quality factor of ionizing radiation used in CT?

A

Q=1.

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18
Q

Q. Dosimetry phantoms come in two (blank)s with the same (blank):

A

Two diameters with the same length.

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19
Q

Q. Who takes annual measurements?

A

Medical Physicist.

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20
Q

Q. What are the three primary dose descriptors for CT?

A

CTDI, DLP, EfD.

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21
Q

Q. What is MSAD?

A

CTDI when contiguous slices are used.

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22
Q

Q. What units are used for CTDI measurements?

A

Grays.

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23
Q

Q. What is CTDI_FDA?

A

The mean absorbed dose in the scanned object volume, for use with fixed, large slice widths.

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24
Q

What is CTDI 100?

A

Modifier measurement to accommodate varied slice widths.

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25
Q

Q. What is CTDI_w?

A

Calculates average dose in x and y widths to account for dose distribution in an oblong patient.

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26
Q

Q. What is CTDI_Volume?

A

Calculates helical dose per “slice”; average dose in the z axis.

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27
Q

Q. How is CTDI_Volume calculated?

A

CTDI_vol = CTDI_w / pitch.

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28
Q

Q. What is DLP, how is it calculated?

A

Dose length product, measurement of total exposure from a series. Total z length direction dose. Calculated by multiplying CTDI volume by longitudinal scan length.

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29
Q

Q. What units are used for DLP?

A

Milligreys per centimeter.

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30
Q

Q. What is EfD, and what is it used for?

A

Effective dose is a measurement that attempts to account for the effects particular to the patient’s tissue that has received the dose. Used for risk assessment and comparison of doses to BERT.

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31
Q

Q. How is EfD calculated?

A

Effective dose = DLP x k.

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32
Q

Q. What units does E-f-D use?

A

Sieverts or millisieverts.

33
Q

Q. Differentiate DLP from CTDI Volume:

A

CTDI volume is per slice and unaffected by scan length, DLP is for all slices in series.

34
Q

Q. Who is ultimately responsible for QA?

A

The owner of the equipment.

35
Q

Q. What is the difference between QA and QC?

A

QA , quality assurance, is a systematic collection and evaluation of data Focus is on prevention of defects or deficiencies. QC, Quality control, is a part of QA that monitors and maintains the instrumentation and equipment. Focus is on detecting deficiencies and defects.

36
Q

Q. 3 mandates of QC to run smoothly:

A

1, Q-C tests need to be performed on a regular basis. 2, results need to be recorded and maintained, and 3, results must be interpreted properly.

37
Q

Q. Which authorities determine whether QC procedures are performed by technologist or medical physicists?

A

American college of radiology, international atomic energy agency, health Canada. Manufacturer may also recommend evaluative procedures.

38
Q

Q. What situations require immediate testing?

A

Replacement of major components, equipment service or adjustments.

39
Q

Q. What is the estimated average bert in the united states?

A

3 millisieverts per year.

40
Q

Q. what is The unit of x-ray exposure in air?

A

the roentgen (R).

41
Q

q. What is effective dose, and what measurements does it use?

A

The amount of energy absorbed per unit mass, represented by r-a-d (radiation absorbed dose). The S-I unit for rad is grey.

42
Q

q. How many rads are in 1 grey?

A

100, so one centigrey = 1 rad.

43
Q

Q. what is the unit of air kerma?

A

Same as for absorbed dose, gray.

44
Q

Q. when we apply the quality factor (1, for ct,) to the radiation absorbed dose, what do we get and what is the SI unit used for this?

A

Dose equivalent is the result, measured in Sieverts.

45
Q

Q. What is rem?

A

roentgen equivalent man, a non S-I unit used to represent dose equivalent.

46
Q

Q. smaller scan field of view = (blank,) dose.

A

greater.

47
Q

Q. Why is the collimation opened wider than the number of detectors in MDCT? What is the resulting impact on patient dose?

A

To overcome the penumbra effect and produce a uniform detector response. Result is increased dose to patient.

48
Q

Q. What is an alternate name for over-ranging?

A

Z-axis overscanning.

49
Q

Q. What kind of acquisition results in over-ranging, and why does it happen?

A

Helical. At least an extra half of a rotation is required to acquire data for the start and end of the desired image volume.

50
Q

Q. What impact would decreased slice spacing have on patient dose?

A

Increase dose.

51
Q

Q. What impact would decreased slice spacing have on patient dose and why?

A

Increase dose because thinner slices need more photons to maintain signal noise ratio.

52
Q

Q. Where does SFOV base its measurements on?

A

Isocenter.

53
Q

Q. How much can dose be increased by missing isocenter in the x direction and in the y direction?

A

X mis-centering can result in an 18 to 41% dose increase, and y mis-centering can result in a 140% dose increase.

54
Q

Q. Which technical factor does dose have a linear relationship with?

A

mAs

55
Q

Q. Greater thickness = _____ mAs = _____dose.

A

More, higher.

56
Q

. What kind of detector has the greatest DQE, and how does this impact patient dose?

A

Scintillation detector has greater DQE than xenon, higher DQE = lower patient dose.

57
Q

Q. Which factors affecting dose are not under our control?

A

Noise, SNR, inherent filtration.

58
Q

Q. What impact does filtration have on patient dose and on image quality?

A

Lowers dose, increases quality.

59
Q

Q. What impact can beam geometry have on patient dose?

A

Using 360 degrees of radiation will increase patient dose compared to 180 degrees, as will overscanning. However it’s worth it because of the diagnostic value.

60
Q

Q. What is partial shielding?

A

Internal body organs act as dose-reducing partial shields for other organs of the body. This is due to the attenuation properties of x-rays, and exposure. As tissue thickness increases, larger, thicker organs will absorb more radiation, enabling other structures to be exposed to smaller amounts of potentially harmful ionizing radiation.

61
Q

Q. What location and size is the R-O-I in a C-T number accuracy test?

A

Middle of the phantom, 200-300 pixels.

62
Q

Q. what are the pass/fail criteria for the C-T number accuracy test?

A

Pass/fail based on mean value of 0 plus or minus 4 hounsfield units for a water phantom.

63
Q

Q. What location and size is the R-O-I in a C-T noise test?

A

Middle of the phantom, 40% of diameter.

64
Q

Q. what are the pass/fail criteria for the C-T number accuracy test?

A

Standard deviation no greater than 10.

65
Q

Q. How many R-O-I are there in a C-T uniformity test?

A

5, 1 center, 4 peripheral.

66
Q

Q. what are the pass/fail criteria for the C-T uniformity test?

A

Mean at the center R-O-I must be within 2 hounsfield units of each of the peripheral ones with no value over 5.

67
Q

Q. What type of phantom is used in a C-T number linearity test?

A

Non uniform phantom containing known objects (attenuation values are known).

68
Q

Q. what are the pass/fail criteria for the C-T number linearity test?

A

Hounsfield units of regions of interest match expected attenuation values (mean). These values are plotted on a graph which should be linear.

69
Q

Q. How do we detect beam hardening?

A

With C-T uniformity tests.

70
Q

Q. What are the acceptable limits for C-T slice thickness test?

A

For a slice greater than or equal to 2mm, limit is plus or minus 1mm. For a thickness of 1 to 2mm, limit is 50% of thickness. For a thickness of less than 1mm, limit is plus or minus 0.5mm.

71
Q

Q. How many Region(s) of interest are used in a calibration of C-T number text?

A

2, one is in the center of the uniform water filled phantom, one is in the air next to it.

72
Q

Q. what are the criteria for the calibration of C-T number test?

A

Mean value of water is 0 plus or minus 4 Hounsfield units, and mean value of air is minus 1000 plus or minus 10 hounsfield units.

73
Q

Which tests are subjective?

A

Low and high contrast tests.

74
Q

Which algorithms need to be run with a low contrast test?

A

Edge enhancement (bone) and smoothing (soft tissue).

75
Q

How much more likely is a 1yr old to develop cancer than a 50 year old from the same dose?

A

6x

76
Q

2 ways to reduce the time in a scan:

A

Limit the number of scans, adjust pitch

77
Q

Where is secondary shielding required as per SC35?

A

Room walls (protects public) (based on equipment, not workload)

78
Q

What thickness of lead shielding is required for any person who remains in the room during a scan?

A

0.35mm of lead for kVp’s of 100 to 150.

CT Theory 1 (4 decks)

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