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asmirt ct exam > Contrast & Dosimetry > Flashcards

Contrast & Dosimetry Flashcards

1
Q

How is CTDI calculated?

A

CTDIvol = CTDIw / Pitch

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2
Q

How is DLP calculated?

A

DLP=CTDIvol x Scan Length

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3
Q

Define absorbed dose

A

The amount of energy deposited by radiation in a mass

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4
Q

What unit is absorbed dose measured in?

A

mGy

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5
Q

Define equivalent dose

A

Dose calculated for individual organs based on absorbed dose to a single organ

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6
Q

What unit is equivalent dose measured in?

A

mSv

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7
Q

Define effective dose

A

Dose calculated for body adjusted to account for the sensitivity of organ to tissue weighting factors

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8
Q

What unit is effective dose measured in?

A

mSv

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9
Q

What is the Australian DRL for CT Brain?

A

880 DLP mGy

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10
Q

What is the Australian DRL for CT Neck?

A

450 DLP mGy

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11
Q

What is the Australian DRL for CT Chest?

A

390 DLP mGy

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12
Q

What is the Australian DRL for CT Abdomen?

A

600 DLP mGy

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13
Q

What is the effective dose for CT Brain?

A

2 mSv

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14
Q

What is the effective dose for CT Chest?

A

1-8 mSv

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15
Q

What is the effective dose for CT Abdomen?

A

10-14 mSv

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16
Q

Name 7 scanner design factors that influence patient dose

A

Beam filtration, beam shaper, beam collimation, detector array, data acquisition system, spiral interpolation, adaptive filtration

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17
Q

Name 8 clinical application factors that influence patient dose

A

kVp, mAs, tube current modulation, pitch, scan range, patient factors, image reconstruction, pathology

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18
Q

What is the threshold of an embryo or fetus to expect detrimental radiation effects

A

100 mSv

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19
Q

What scanning selection factor has the biggest influence on patient dose?

A

kVp

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20
Q

Does kVp affect contrast or brightness?

A

Contrast

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21
Q

Is the photoelectric effect likely to occur at higher or lower energies?

A

Higher

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22
Q

How does mA effect dose?

A

mA has a linear affect on radiation dose

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23
Q

How is effective mAs calculated?

A

Eff mAs = ratio of tube current to pitch

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24
Q

What is k-edge?

A

The energy required to eject a k shell electron

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25
Q

What is the k-edge of iodine?

A

32kEv

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26
Q

What is the approximate average energy of the beam?

A

1/3 kVp

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27
Q

What is a linear attenuation co-efficient?

A

A constant that describes the fraction of attenuated incident photons in a monoenergetic beam per unit thickness of a material which is used to calculate hounsfield units

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28
Q

Describe ionic contrast

A

Ionic contrast dissociates in water and has high osmolality. It dissociates into 2 molecular particles resulting in higher risk of reaction

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29
Q

What is the molecular structure of ionic contrast?

A

Monomer

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30
Q

Describe non-ionic contrast

A

Non-ionic contrast does not dissolve into charged particles when entering the bloodstream with less chance of reaction

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31
Q

Describe low osmolar contrast and its molecular structure

A

Low osmolar contrast is a monomer with increased solubility in water

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32
Q

Describe iso osmolar contrast and its molecular structure

A

Iso osmolar contrast is a dimer that does not dissociate in water

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33
Q

What is a monomer?

A

A molecule that can be bonded to other identical molecules to form a polymer (single benzene ring)

34
Q

What is a dimer?

A

A molecular complex consisting of two identical molecules linked together (double benzene ring)

35
Q

What is the viscosity of Ultravist 370 @ 20 degrees?

A

22mPa.s

36
Q

What is the viscosity of Ultravist 370 @ 37 degrees ?

A

10mPa.s

37
Q

What is concentration of iodinated contrast expressed? Give an example

A

Concentration is expressed as milligrams of iodine per millilitre e.g Omnipaque 300 has 300mgI/mL

38
Q

Define osmolality

A

The concentration of a solution expressed as the total number of solute particles per kg

39
Q

What is the osmolality of blood?

A

275-295 mosm/kg

40
Q

What is the osmolality of Ultravist 370?

A

770 mosm/kg

41
Q

What is the osmolality of Visipaque 320?

A

290 mosm/kg

42
Q

What is the osmolality of ionic contrast?

A

1220-1690 mosm/kg

43
Q

What are the contraindications of oral contrast?

A

Perforation, GI bleed, allergy

44
Q

What are the contraindications for IV contrast?

A

Previous allergy and assessment of severity of reaction, egfr below 30

45
Q

What is contrast induced nephropathy?

A

Impairment of renal function with elevation of serum creatinine >25% from the baseline within 48 hours

46
Q

What are the risk factors for CIN?

A

Known renal insufficiency, diabetes, malignancy, >65 years old, use of NSAIDs

47
Q

What is eGFR?

A

Estimated glomerular filtration rate. How well the kidneys filter waste from the blood and is indicative of kidney damage

48
Q

eGFR has not been accurately established in which groups?

A

Malnourished, amputees and frail patients

49
Q

Which group is most at risk of CIN?

A

eGFR <30mL/min

50
Q

What is the association of IV contrast and asthma?

A

This group experiences a 6x increased risk of hypersensitivity to reaction and is high in those with unstable disease

51
Q

What is the association of IV contrast and beta blockers?

A

May increase severity of reaction

52
Q

What is the association of IV contrast and hyperparathyroidism?

A

If TSH is abnormal but T3 and T4 are normal contrast can be administered. These patients cannot have radioactive thyroid treatment 8 weeks pre/post IV contrast

53
Q

What is the association of IV contrast and sickle cell disease?

A

May experience temporary worsening pain

54
Q

What is the association of IV contrast and pregnancy?

A

Infants born to patients who have had IV contrast during pregnancy should have heel prick test for neonatal hyperthyroidism

55
Q

What is the association of IC contrast and breastfeeding?

A

No cessation of breast feeding or discarding required

56
Q

What is the association of IV contrast and phaeochromocytoma?

A

None

57
Q

What is the association of IV contrast and Myasthenia Gravis?

A

Possibility of worsening symptoms e.g. breathing difficulties

58
Q

What is the association of IV contrast and Interleukin -2 (IL-2) immunotherapy?

A

Risk of delayed anaphylactic reaction if therapy has been within the past 6 months

59
Q

Define anaphylaxis

A

A generalised allergic reaction which often involves more than one body system (e.g. skin, respiratory, gastrointestinal, cardiovascular) and can rapidly become life threatening

60
Q

What is the chance of a severe reaction to IV contrast?

A

1 in 100,000

61
Q

What is the chance of a fatal reaction to IV contrast?

A

1 in 170,000

62
Q

What are the signs of a minor reaction?

A

Flushing, nausea, pruitus (itch), vomiting, headache, mild uritcaria (rash)

63
Q

What treatments can be given for a minor reaction?

A

Non-sedating antihistamines (urticaria), anti-emetics (vomiting)

64
Q

What are the signs of a moderate reaction?

A

Severe vomiting, marked urticaria, bronchospasm, other respiratory symptoms, facial/laryngeal oedema, vasovagal

65
Q

What treatments can be given for a moderate reaction?

A

Non-sedating antihistamines and adrenaline (urticaria)
Adrenaline, salbutamol, oxygen (6-10L) (other symptoms)
Remain supine if possible and elevate legs, hartmanns IV (hypotension or vasovagal)

66
Q

What are the signs of a severe reaction?

A

Hypovolaemic shock (rapid loss of blood or fluid), respiratory arrest, convulsions, cardiac arrest

67
Q

What treatments can be given for a severe reaction?

A

Adrenaline, IV fluids, other drugs as required

Remain supine, sit up (dyspnea), oxygen 6-10L, maintain airway and suction, ECG, observations

68
Q

What is the time frame for a delayed contrast reaction?

A

1 hour to 1 week

69
Q

What are the signs of a delayed reaction?

A

Maculopapular rash (hives), angioedema, urticaria, erythema, painful salivary gland swelling

70
Q

What is the recommended pre-medication?

A

13 hours before: 50mg prednisolone
1 hour before: 25mg prednisolone
1 hour before: 25mg phenergen
1 hour before: 40mg cimetidine

71
Q

What IV lines cannot be used for contrast administration?

A

PICC lines (not pressure rated), CVC

72
Q

What IV lines can be used for contrast administration?

A

Power PICC, portacath

73
Q

What is thyrotoxicosis?

A

Increased secretion of thyroid hormones due to IV contrast administration with hyperparathyroidism developing over the following 2-12 weeks

74
Q

What groups are at risk of CT thyrotoxicosis?

A

Graves disease, multinodular goitre, elderly and endemic iodine deficiency

75
Q

Although kV and dose is not linear, how are they related?

A

Dose is approximately the square root of the kV

76
Q

What is the photoelectric effect?

A

Absorbed x-ray photons interact with inner shell electrons

77
Q

How is the photoelectric effect proportional to kV?

A

Inversely proportional to third power of the kV (1/E)^3

78
Q

How is the photoelectric effect proportional to the atomic number?

A

Directly proportional to the third power of the atomic number (Z^3)

79
Q

How should mAs be modified from adult to newborn in CT brain?

A

Reduced a factor of 2-2.5

80
Q

How should mAs be modified from adult to newborn in CT body?

A

Reduced by a factor of 4-5

81
Q

When taking into account less intra-abdominal fat in paediatrics when can the mAs be halved?

A

For each 4-6cm reduction in body diameter

asmirt ct exam (5 decks)

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