Topic 6. Quality Assurance & Dose Reduction for CT Flashcards

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1
Q

list two reasons why determination of doses is of importance in CT

A
  1. CT is the largest medical contributor of collective dose to the patient population
  2. Irradiation of the image receptor must be sufficient to obtain a usable image
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2
Q

CT invariably results in very ____-________ irradiation of a variety of organs with different radio sensitivities

A

non-uniform

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3
Q

overall dose in CT known as the

A

effective dose

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4
Q

Overall risk from CT exposure may be estimated from

A
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5
Q

CT machines display a crude estimate of the effective dose, this is found using a 3-part computation of?

A
  1. Calculate the CT Dose Index (CTDI) for the given technique factors chosen for the current patient
    CTDI: is a patient independent measure of machine output for the given set of technique factors
  2. Multiply the CTDI vol. by the length of the region irradiated and obtain a dose-length product (DLP)
  3. Another conversion factor (obtained from Monte Carlo) transforms DLP into Effective Dose
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6
Q

CTDI: is a patient independent measure of machine output for the given set of technique factors. what is it?

A

An index of the x-ray output of a CT machine, Invented to give an average dose for a single axial slice.
CTDI phantom is an acrylic cylinder 15 cm long.

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7
Q

True or false? Clinically adequate images can be obtained with lower exposure levels.

A

TRUE. This is driving the development of improved technologies and the re-examination of long accepted protocols.

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8
Q

Distribution of dose being deposited in the phantom at any instant depends on the

A

beam angle

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9
Q

CT Dose index is a measure of dose in a ____ full rotation of gantry

A

single

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10
Q

as many as 30% of CT’s done on children are

A
  • medically unnecessary
  • could be carried out with safer modalities (US, MRI)
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11
Q

TRUE OR FALSE? Dose reductions of up to 50%, still with adequate image quality, have been reported.

A

TRUE, the dose does not need to be high for acceptable image quality. This can be achieved from a simple change of kVp

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12
Q

CT profiles have a bell-shaped graph due to

A
  1. The finite size of the focal spot causes a small amount of shadow blurring at the edges of the primary beam
  2. Compton scatter within the phantom or patient
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13
Q

CTDI 100?

A

defined as the integral dose recorded by the chamber over its entire 100 mm length

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14
Q

measured with the ion chamber in the central hole and in one or more of the peripheral one, what is being measured?

A

CTDI 100

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15
Q

CTDIw?

A

the weighted average of CTDI100 readings

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16
Q

CTDIvol.?

A

CTDIvol. value REMAINS CONSTANT whether 1cm or 100cm of patient anatomy is exposed and therefore doesn’t reflect the total ionizing energy deposited into the scan volume and it is therefore a poor measure of patient dose

17
Q

To better represent the overall risk, CTDIvol. can be integrated along scan length
(DLP)

A
18
Q

The method of conversion from DLP to Effective Dose, is clearly adopted for

A

convenience and extreme simplicity, not for accuracy.

19
Q

For ACCURATE, patient specific mesurments of DOSE you need to employ something like the

A

Patient Air Kerma in Tomography (PAKT) methodology

20
Q

Typical values for the calculated EFFECTIVE DOSES of many CT protocols are

A
  • 1–2 mSv for a head examination,
  • 5–7 mSv for the chest, and
  • 8–11 mSv for the abdomen or pelvis.
21
Q

Clinically adequate images can be obtained w/ ____ → driving the development of improved technologies & the re-examination of long-accepted protocols

A

lower exposure levels

22
Q

Dose reduction of up to 50% (still w/ adequate image quality) have been reported. How is this achieved

A
  • Achieved by changing kVp
  • Reducing mA-s/rotation by 10%
  • Another possibility is to sculpt the edge of the beam at the end of the region of clinical interest. This cuts down the irradiation of tissues outside the region of interest
23
Q

To create a dose reduction program you need to

A
  • Select a different diagnostic modality
  • Consider the overall clinical impact of a study
  • Fit the CT scanning parameters to the patient’s body shape, size, and age
  • Encourage organization to re-appraise the benefit-dise tradeoff of the current scanning
    protocols
24
Q

3 general categories of actual tests (quality assurance tests) dealing w/ :

A
  • Sample clinical images
  • Measurements on a highly specialized ACR CT phantom
  • Dose assessments w/ the CTDI phantom
25
Q

The third category of tests calls for CTDI measurements for three distinct protocols.

The ADULT HEAD, with the SMALL CTDI phantom, should yield a reading below the pass/fail criterion of

A

80 mGy.

26
Q

For the ADULT ABDOMEN protocol with the LARGE phantom, CTDIvol has to be

A

below 30 mGy.

27
Q

The third category of tests calls for CTDI (CT DOSE INDEX) measurements for three distinct protocols.

The pediatric HEAD phantom, with the site’s STANDARD pediatric setting, must be

A

below 40 mGy.

28
Q

The third category of tests calls for CTDI (CT DOSE INDEX) measurements for three distinct protocols.

The pediatric ABDOMEN, with the site’s STANDARD pediatric setting, must be

A

below 20 mGy