Topic 6. Quality Assurance & Dose Reduction for CT

Question 1

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

Answer 1

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

Question 2

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

Answer 2

non-uniform

Question 3

overall dose in CT known as the

Answer 3

effective dose

Question 4

Overall risk from CT exposure may be estimated from

Answer 4

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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.

Question 7

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

Answer 7

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

Question 8

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

Answer 8

beam angle

Question 9

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

Answer 9

single

Question 10

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

Answer 10

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

Question 11

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

Answer 11

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

Question 12

CT profiles have a bell-shaped graph due to

Answer 12

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

Question 13

CTDI 100?

Answer 13

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

Question 14

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

Answer 14

CTDI 100

Question 15

CTDIw?

Answer 15

the weighted average of CTDI100 readings

Question 16

CTDIvol.?

Answer 16

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

Question 17

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

Answer 17

Question 18

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

Answer 18

convenience and extreme simplicity, not for accuracy.

Question 19

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

Answer 19

Patient Air Kerma in Tomography (PAKT) methodology

Question 20

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

Answer 20

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

Question 21

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

Answer 21

lower exposure levels

Question 22

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

Answer 22

• 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

Question 23

To create a dose reduction program you need to

Answer 23

• 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

Question 24

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

Answer 24

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

Question 25

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

Answer 25

80 mGy.

Question 26

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

Answer 26

below 30 mGy.

Question 27

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

Answer 27

below 40 mGy.

Question 28

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

Answer 28

below 20 mGy