CT

Question 1

In the American College of Radiology (ACR) CT Accreditation program, which examination is allowed to have the lowest contrast-to-noise (CNR) ratio and receive a passing score?

Answer 1

Pediatric Abdomen (CNR > 0.4)

** Note: Adult head and adult abdomen > 1.0 CNR, pediatric head > 0.7 CNR

Question 2

In the American College of Radiology (ACR) CT Accreditation Program, what range of CT numbers in Hounsfield Units (HU) would be allowed (i.e. passing) for water?

Answer 2

-7 to 7

Question 3

What is the correct order for arranging the CT numbers of the following tissues from lowest to highest values?

Kidney
Breast
Lung
Bone

Answer 3

Lung, Breast, Kidney, Bone

Question 4

What is a reasonable estimate for the CTDI_vol of an abdomen scan for an average size adult?

Answer 4

20mGy

Question 5

What is a reasonable approximation to calculate the fetal dose from an abdomen/pelvis CT scan?

Answer 5

CTDI_vol of the scan (better if using the SSDE)

Question 6

How do you calculate pitch?

Answer 6

Table feed per rotation divided by collimation size (number of channels (n) times detector element size (T)).

Question 7

How do you calculate CTDI_w?

Answer 7

= (1/3)CTDI_100_Center + (2/3)CTDI_100_Periphery

Question 8

How do you calculate CTDI_vol?

Answer 8

= CTDI_w / pitch

Question 9

What gating technique is used to minimize patient dose in cardiac CT?

Answer 9

Prospective gating

Question 10

What are the six clinical protocols that need to be reviewed at least annually?

Answer 10

Adult abdomen
Adult head
Pediatric head (~ 1yr old)
Pediatric abdomen (40-50lb)
High resolution chest
Brain perfusion (if applicable)

Question 11

What is the tolerance for the CT scout prescription accuracy test?

Answer 11

Within 2 mm

Question 12

According to the ACR CT QC manual, what are the CNR limits?

Answer 12

Adult head = 1
Adult abdomen = 1
Pediatric head = 0.7
Pediatric abdomen = 0.4

Question 13

According to the ACR CT QC manual, what are the spatial resolution limits?

Answer 13

Adult abdomen = 6 lp/mm
High resolution chest = 8 lp/mm

Question 14

How do you calculate CTDI_100?

Answer 14

= (f * CF * E * L) / (nT)

f = conversion factor for exposure to dose in air (=8.76 mGy/R)
CF = calibration correction factor
E = average measured exposure (in R)
L = active length of pencil ion chamber
n = number of detector elements
T = thickness of detector elements

Question 15

What is the tolerance for radiation beam width measurements?

Answer 15

The larger of 3 mm or 30% of the collimation size

Question 16

Describe the four sections of the ACR CT phantom:

Answer 16

Section 1: Four CT number inserts (mimicing air, bone, polyethelene, acrylic); larger water equivalent insert; 2 sets of slice thickness bars of 0.5mm apart; 4 BBs around edges of phantom for positioning accuracy.

Section 2: Low contrast tests: low contrast insert for CNR calculation, with sets of 6 mm and smaller low contrast targets for visualization

Section 3: Uniformity with geometric accuracy between two BBs 100 mm apart

Section 4: High contrast (spatial) resolution)