CT Formulas

Question 1

How do you calculate pitch?

Answer 1

Pitch = travel distance of table per 360 degree rotation divided by the total xray beam collimation width. pitch = TD/TCW

Question 2

what is the formula to calculate HU (CT number) of a tissue given the linear attenuation coefficient of water and the tissue for a particular scanner

Answer 2

(LAC of tissue - LAC of water) / LAC of water * 1000 {in other words: LAC of tissue minus LAC of water divided by LAC of water * 1000 }

Question 3

How do you calculate the minimum object size that a CT scanner can resolve given the limiting resolution in lp/cm? ie. 15 lp/cm

Answer 3

take the reciprocal value of the scanners limiting resolution in lp/cm. Convert it to mm. Then devide by 2. Ie. for a lp/cm of 15. The reciprical of 15 is 1/15 lp/cm. This is equivalent to 10/15 lp/mm. You divide that out and get .6 mm/lp. Then divide by 2 and get .3 mm/lp

Question 4

What is the formula for a peripheral CTA injection duration.

Answer 4

bolus duration = scan time - 5 seconds

Question 5

What is the primary way that temportal resolution may be improved?

Answer 5

decreasing scan time (while covering the same anatomy)