Week 3 Lecture 2 Flashcards
scan time
length of time needed to irradiate the area of interest
what do we need to take into consideration in regards to scan time?
- patient’s condition
- equipment limitations (such as reconstruction capabilities and tube heat limits)
why is the scan speed for MDCT axial acquisition faster than SDCT?
multiple detectors = multiple slices for every full rotation
pitch
table movement during a helical scan
speed of table movement is dependent on?
detector configuration
how do you calculate pitch?
distance travelled by the table per 360 rotate divided by the x-ray beam collimation width
p = d/w
pitch calculations accounts for?
helical scans - thus constant radiation output
what does increasing pitch do?
- 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
table speed and slice thickness have a direct relationship
p = 1 (what is the relationship between table speed and beam collimation?)
when p=1, table speed and beam collimation are the same
what does the pitch ratio 2:1 describe?
table will be moving twice the distance of a slice thickness for each rotation
pitch value ranges from?
0.5-2
pitch value greater than 1.5 =?
- need of more extrapolation
- faster acquisition speeds
- decreases image quality
***effects on image quality are only minimal if the pitch …
pitch does NOT exceed 1.5
pitch with the values less than 1 =?
(p > 1)
- scan overlap
- slow acquisition speeds
- increased patient dose
- improved image quality
what is the formula to calculate SDCT pitch?
p = (table movement in one gantry rotation (d))/(slice thickness or beam collimation (w))
for mdct, each detector row =?
can be used to create a slice
what is the formula to calculate MDCT pitch?
pitch (p) = (table movement in one gantry rotation (d))/(number of slices * slice thickness (w))
what is a limiting factor of scan coverage?
heat - equipment limitations
pitch can be altered to ensure the appropriate amount of anatomy will be irradiated during scan acquisition. t/f
true
pitch can influence _______ and ______ of an acquisition.
pitch can influence scan time and anatomy coverage of an acquisition.
how do you describe pitch in terms of scan coverage?
it influences how much anatomy can be irradiated before an acquisition is automatically stopped by the ct tube heat limits
scan time does not equal pitch. what does that mean?
pitch can be adjusted to make sure the ROI can be scanned within the pre-selected timeframe
what is the formula to calculate scan coverage in a SDCT?
amount of anatomy covered = pitch * total acquisition time * (1/rotation time) * slice thickness
p < 1
overlapping at each view angle from rotation to rotation
↑ p = (↑/↓) scan time
↑ p = ↓ scan time
