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Mosby CT review > Chapter 3 Rad Safety > Flashcards

Chapter 3 Rad Safety Flashcards

1
Q

Describe the process of photoelectric absorption

A

Incident ray comes in and hits inner shell electron and becomes absorbed by the atom

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2
Q

Describe the process of Compton scatter

A

Incident ray comes in and hits outer shell electron. Incident ray loses energy and then changes direction when leaving the atom

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3
Q

Attenuation of an Xray beam depends on 2 things what are they

A
  1. beam quality

2. tissue density

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4
Q

TREND

patient-to- detector distance decreases, then the dose will…..

A

increase

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5
Q

What is the difference between Focus-to- isocenter distance and Focus-to- detector distance

A

Focus-to- isocenter distance describes difference from Ct tube to center of gantry

Focus-to- detector distance describes distance from Ct tube to detector array

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6
Q

What happens when you position the patient off center relative to the gantry

A

unwanted variation of dose and exposure

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7
Q

What material lines the inner CT x-

ray tube and promotes filtration of unwanted beams

A

Aluminum. 6-9 mm

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8
Q

Describe inherent absorption efficiency

A

the detectors ability to capture x-ray quanta and provide a response

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9
Q

Describe geometric efficiency

A

The spatial arrangement of everything in

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10
Q

Describe the process of over beaming? How does it impact the image ?

A

When the detectors are exposed to undesirable “penumbra”.

It does not contribute to the formation of the CT image

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11
Q

What technique do some Ct manufactures employ to control the position of the x ray beam on detectors?

A

focal spot tracking

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12
Q

To expose the widened detector arrays to xray beams equally MDCTs utilize what shape of beam?

What beam shape was employed in the traditional single slice scanners?

A

cone

fan

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13
Q

At what point does overranging occur?

A

When radiation dose is applied before and after the acquisition volume. (half rotaion before and after spiral)

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14
Q

What is used during the reconstruction process to reduce noise in a CT image?

A

Adaptive filters ( noise reduction algorithms)

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15
Q

What are 4 reasons radiation dose is usually higher in MDCT scanners vs SSCT

A
  1. Decreased focus to detector distance
  2. Cone beam utilization
  3. More phases in acquisition
  4. Thinner sections
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16
Q

Why is it necessary to optimize scanning parameters

A

follows ALARA principles to minimize patient radiation dose

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17
Q

The relationships between milliampere (mA) setting, scan time, and radiation dose is inversely or directly proportional?

A

directly

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18
Q

The relationships between kvp setting and radiation dose is inversely or directly proportional?

A

directly and EXPONETIAL

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19
Q

For which scans can reducing kvp not have a negative effect on image quality ?

A

Pediatric CT, and CT angiography

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20
Q

What does the ‘Dose Profile” reference in Ct ?

A

the section of the body exposed to radiation

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21
Q

Thinner slices lead to more or less noise?

A

more

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22
Q

When you increase the collimation on a camera are slices getting thinner or thicker?

A

thinner

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23
Q

With MDCT scanners does more over beaming occur when you have wider or thinner collimation?

Wider collimation increases/ decreases dose efficency?

A

thinner

increases

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24
Q

Decreasing patient does increases/ decreases image noise ?

What are two solutions used in image reconstruction to create a less noisy image

A

increases

  1. using the correct reconstruction kernel
  2. using the correct window setting
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25
Q

Decrease in patient size increases/ decreases image noise?

A

decrease

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26
Q

During SSCT acquisition what range in pitch will allow you to take the picture in shorter time

A

pitch > 1

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27
Q

How does increasing pitch in MDCT scanners affect image noise?

A

increases noise

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28
Q

Identify whether x, y, or z references the angular or longitudinal axis

A

x and y axis = Angular

z= Longitudinal

29
Q

current is measured in what units

A

milliampere (mAs)

30
Q

Describe angular tube current modulation

A

mAs is adjusted based on the difference and thickness of the part as the tube rotates

31
Q

How do CT scanners maximize radiation dose reduction

A

Employ both angular and longitudinal current modulation techniques

32
Q

What complex material can be used as a selective shield to reduce dose to sensitive areas? ( hint: one is impregnated into the other)

A

Rubber with bismuth inside

33
Q

What is the disadvantage of bismuth shielding?

What can you use to reduce this scattered streaking artifact ?

A
  • streaking artifact

- Foam spacer between patient and bismuth shield

34
Q

Room shielding requirements should account for what factors?

A
  1. daily work load
  2. scanner position
  3. window and door construction
35
Q

If a family member must absolutely be in the room with the patient during the scan what must they wear ?

A

Appropriate lead shielding

36
Q

What is CTDI?

Is it calculated for all slices in a CT scan ?

A

A measure of the dose received in a single CT slice.

No. It is calculated for only the central slice

37
Q

What equipment is used to measure CTDI? How do they work together to collect the CTDI index

A

Dosimeter
Head and body phantom

you place the dosimeter which is usually a small ionization chamber inside of the phantoms and then scan them

38
Q

CTDI is calculated for the central slice or all of the slices?

A

central

39
Q

Whats the difference between the three CTDI’s:

CTDI 100

CTDI vol

CTDI w

A

CTDI-100: uses a pencil shape dosimeter 100 mm in length

CTDI-w: Weighted dose index that accounts for variance in dose distribution.

CTDI-vol: accounts for the dose measured in each section during a helical scab

40
Q

What is the equation for CTDI-w? CTDI-vol?

A

CTDI-w:
2/3( periphery exposure)- 1/3(central exposure)

CTDI-vol: CTDI-w/ pitch

41
Q

True/ False. In the acquired field of view of a CT image the radiation dose varies depending on which area you are looking at

A

True

42
Q

Between CTDI-w and CTDI-vol which uses step-and-shoot and which uses helical scanning to obtain radiation measuremnets

A

CTDI- vol: helical scanning

CTDI-w : Step and shoot

43
Q

Between CTDI-w and CTDI-vol which axes are utilized to approximate radiation measurements in an acquired image

A

CTDI-w : uses the x/y axis

CTDI-vol: uses z-axis

44
Q

What does MSAD stand for?

What does it measure?

A

Multi slice average dose

Average collective dose to each slice in the center of a scan

45
Q

What are some equation to calculate MSAD ?

A

MSAD= (T/I) x CTDI
T* is slice thickness
I* is image spacing

MSAD= CTDI/ pitch

46
Q

Multi slice average dose (MSAD) accounts for the effects of what during axial scanning?

A

Image spacing

47
Q

Overlapping scanning increase/decrease the patient radiation dose.

A

increase

48
Q

How does slice thickness and image spacing effect the multi slice average dose ( MSAD)

A

Slice thickness > image spacing = increase in MSAD

Slice thickness < image spacing = decrease in MSAD

Slice thickness= image spacing –> MSAD= CTDI

49
Q

What is another word that refers to ‘Image spacing ‘

A

bed index

50
Q

During helical or spiral scanning what factor controls the MSAD

A

pitch

51
Q

True/ False. MSAD measurement is more accurate at either end of the acquisition rather than in the center of the scan

A

false

52
Q

What does DLP stand for?

A

Dose length product

53
Q

What are some equations DLP?

A

DLP= (MSAD) x (slice width) x (# of slices)

slice*= pre-patient collimator setting ONLY. This applies for CTDI- vol as well.

DLP= (CTDI-vol)x (scan length)

54
Q

What do the following stand for:
DICOM
RDSR

A

Digital imaging and communications in medicine

Radiation dose structured report

55
Q

What is the RDSR

A

it is an archive of all the patinet exposure information .

Includes: voltage, scan time , pitch , width , acquisition length

56
Q

Does the radiation dose report (RDSR) only keep archived records of quality scans or do they save EVERYTHING including those with low quality or other issues?

True/ False . Industrial standards requires this data to be automatically archived to patients chart after scan.

A

saves everything

true

57
Q

A dose check system employs 2 types of alerts what are they?

What’s the purpose of a dose check system?

A

Dose notification
Dose alert

Notify tech if a patient is anticipated to receive over the recommended amount of exposure

58
Q

What’s the difference between dose notification and dose alert

A

” Notification” refers to the technical setting
“ Notification” requires tech to confirm settings before acquisition starts
( ie… date/time, dose value, comments, etc.)

“Alert” refers to the impending acquisition at that time
“Alert” requires tech to adjust settings to confirm dose OR confirm setting and enter initials to proceed.
“Alert” sounds when the CTDI-vol exceeds that which is recommended for s prescribed CT with multiple acquisitions

59
Q

industry standards require CT scanners to employ the following to keep patient dose to a minimum:

A
  1. Automated dose check
  2. Adult and Ped reference protocols
  3. DICOM RDSR’s
  4. Automatic exposure control
60
Q

True/ False. across the field of view of an image there is a dose gradient

A

true

61
Q

Dose patient size affect the dose gradient exhibited on a CT image? How?

A

Yes

Larger patients have more massive gradients where the difference in absorbed dose is greater. Pediatric patients don’t have a gradient since they are so small

62
Q

Why do pediatric patients don’t normally have a dose gradient?

A

Radiation intensity upon patient entry and exit is equal to each other and therefore the distribution along the field of view is uniform

63
Q

True/ False. When all technical parameters remain constant , absorbed dose is typically higher in smaller patients

A

true

64
Q

What are 5 ways recommended to reduce pediatric dosage in CT?

A
  1. get rid of CT orders with improper indications
  2. reduce multiphase scanning
  3. Lower KVP
  4. Lower mAs
  5. Increase pitch
65
Q 
True/ False .
Protocol optimization               ( adjusting mAs, kVP , and pitch) should be done based on age not weight
A

false

66
Q

Who developed the ‘Image Gently’ campaign ? What is its purpose?

A

Alliance for radiation safety in pediatric imaging

develops guidelines to reduce radiation exposure to pediatric patients

67
Q

What are the 3 guidelines listed in the ‘image gently’ campaign?

A
  1. implement “child size” mA’s and kVp doses
  2. need only one acquisition phase
  3. Image only the area that is needed
68
Q

What is the advantage of using iterative reconstruction techniques over filtered back projection

A

reduces noise and therefore lowers patient exposure

Mosby CT review (8 decks)

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