Definitions and Formulas

Question 1

Table Movement / Beam Width = ?

Answer 1

Formula for Pitch

Commonly used to describe table movement.

Question 2

Slice Number x Thickness = ?

Answer 2

Formula for Beam Width

16 slice scanner x 2.5mm thickness =

40 beam width

Question 3

CDTIw(eighted) / Pitch = ?

Answer 3

CDTI vol(ume metric)

Radiation absorbed by patient’s body.
Expressed in Gy.

Question 4

CT Dose Index

(CTDI)

Answer 4

An approximate measure of the radiation dose received in a single CT section or slice.

Question 5

With a 16 slice MDCT at 0.625mm thickness and a table feed of 15mm per rotation, what is the pitch?

Answer 5

15mm / (16 x 0.625mm)

1.5 pitch

Question 6

A 1% increase in attenuation = ———— ————- in HU.

Answer 6

10 increase

Question 7

The pixel value, given in HU, at the center of the WW.

Answer 7

Window Level (WL)

Controls the brightness (density) of the CT image.

Question 8

The range of pixel values assigned a shade of gray in the displayed CT image.

Answer 8

Window Width (WW)

Controls the contrast of the CT image.

Question 9

What is the distance covered for a MDCT helical scan sequence with a 30 second total acquisition time, a 1 second rotation time, 5mm slice thickness, 16 slices per rotation, and a pitch of 1?

Answer 9

Pitch * Acquisition Time * 1/Rotation Time * Beam Width = Scan Coverage

1 * 30 * 1/1 * (5 * 16) = 2400mm

Question 10

Formula for calculation of pixel size

Answer 10

FOV / Matrix size = pixel size

matrix is usually 512