Week 3 Lecture 2

Question 1

scan time

Answer 1

length of time needed to irradiate the area of interest

Question 2

what do we need to take into consideration in regards to scan time?

Answer 2

• patient’s condition
• equipment limitations (such as reconstruction capabilities and tube heat limits)

Question 3

why is the scan speed for MDCT axial acquisition faster than SDCT?

Answer 3

multiple detectors = multiple slices for every full rotation

Question 4

pitch

Answer 4

table movement during a helical scan

Question 5

speed of table movement is dependent on?

Answer 5

detector configuration

Question 6

how do you calculate pitch?

Answer 6

distance travelled by the table per 360 rotate divided by the x-ray beam collimation width

p = d/w

Question 7

pitch calculations accounts for?

Answer 7

helical scans - thus constant radiation output

Question 8

what does increasing pitch do?

Answer 8

• irradiate more anatomy (lengthwise) during a shorter specified acquisition time
• shorter scan time
• decreases patient motion + patient dose
  -while improving imaging of contrast filled vasculature

Question 9

table speed and slice thickness have a direct relationship

Question 10

p = 1 (what is the relationship between table speed and beam collimation?)

Answer 10

when p=1, table speed and beam collimation are the same

Question 11

what does the pitch ratio 2:1 describe?

Answer 11

table will be moving twice the distance of a slice thickness for each rotation

Question 12

pitch value ranges from?

Answer 12

0.5-2

Question 13

pitch value greater than 1.5 =?

Answer 13

• need of more extrapolation
• faster acquisition speeds
• decreases image quality

Question 14

***effects on image quality are only minimal if the pitch …

Answer 14

pitch does NOT exceed 1.5

Question 15

pitch with the values less than 1 =?
(p > 1)

Answer 15

• scan overlap
• slow acquisition speeds
• increased patient dose
• improved image quality

Question 16

what is the formula to calculate SDCT pitch?

Answer 16

p = (table movement in one gantry rotation (d))/(slice thickness or beam collimation (w))

Question 17

for mdct, each detector row =?

Answer 17

can be used to create a slice

Question 18

what is the formula to calculate MDCT pitch?

Answer 18

pitch (p) = (table movement in one gantry rotation (d))/(number of slices * slice thickness (w))

Question 19

what is a limiting factor of scan coverage?

Answer 19

heat - equipment limitations

Question 20

pitch can be altered to ensure the appropriate amount of anatomy will be irradiated during scan acquisition. t/f

Answer 20

true

Question 21

pitch can influence _______ and ______ of an acquisition.

Answer 21

pitch can influence scan time and anatomy coverage of an acquisition.

Question 22

how do you describe pitch in terms of scan coverage?

Answer 22

it influences how much anatomy can be irradiated before an acquisition is automatically stopped by the ct tube heat limits

Question 23

scan time does not equal pitch. what does that mean?

Answer 23

pitch can be adjusted to make sure the ROI can be scanned within the pre-selected timeframe

Question 24

what is the formula to calculate scan coverage in a SDCT?

Answer 24

amount of anatomy covered = pitch * total acquisition time * (1/rotation time) * slice thickness

Question 25

p < 1

Answer 25

overlapping at each view angle from rotation to rotation

Question 26

↑ p = (↑/↓) scan time

Answer 26

↑ p = ↓ scan time