CT Image Processing and Display

Question 1

Each 1% contrast between adjacent objects amounts to a difference in pixel value of approximately ___ HU.

Answer 1

10 HU

Question 2

The ability of an object to attenuate the x-ray beam is what?

Answer 2

linear attenuation coefficient

Question 3

What is back-projection?

Answer 3

a computer reconstruction process that projects the CT image on to a matrix.

Question 4

A soft tissue algorithm yields what type of image? What about a bone algorithm?

Answer 4

ST = smooth; preferable for eval of ST structures
bone = sharp; sharpens edges, good for bony detail

Question 5

The algorithms combined with back-projection in MSCT is called what?

Answer 5

the filtered back-projection

Question 6

Define interpolation.

Answer 6

mathematical process that uses data from tube positions above and below a given slice to create an image (i.e the slices above and below what is being scanned)

Question 7

MSCT scanners use what type of interpolation? How does it work?

Answer 7

180IL (InterpoLation). data from a scan of a slice and another at 180 degrees are used to create an image

Question 8

What is the benefit of interpolation?

Answer 8

the extra data from the same slice on the z-axis reduces blurring and improves image quality

Question 9

What is z-filtering?

Answer 9

multidetector row CT systems use it to further the image quality by using multiple data from slices higher and lower than just one slice away

Question 10

Define partial volume averaging.

Answer 10

this is an average of all the attenuation values in a single pixel. this happens when more than one type of tissue share the same pixel.

Question 11

Name two ways to reduce the effects of partial volume averaging.

Answer 11

1. reduce section width

2. reduce pixel dimension

Question 12

What is the formula for pixel dimension?

Answer 12

pixel dimension = DFOV / matrix size

Question 13

Where is the z-axis for a scan of an extremity?

Answer 13

the z-axis runs parallel to the long axis of the part, from proximal to distal (or vice versa)

Question 14

What happens to a pixel whose CT number is outside the range of the window width?

Answer 14

if the HU is below the range, it will appear black. If it is above, it will appear white.

Question 15

What is the range of HU if the WL is 200 and the WW is 1000?

Answer 15

-300 to 700; anything above 700 or below -300 will appear as white or black respectively.

Question 16

What is the range of HU if the WL is 35 and the WW is 150?

Answer 16

-40 to 110. anything above 110 or below -40 will appear as white or black respectively.

Question 17

Put simply, the window width controls what? The window level controls?

Answer 17

WW: contrast
WL: brightness

Question 18

What is an isotropic voxel?

Answer 18

a voxel whose x-, y-, and z-axis are equal in length

Question 19

What is an orthogonal plane? Give an example.

Answer 19

planes that are at right angles to each other. coronal and sagittal is an example of one.

Question 20

What is a MIP image? What is the benefit?

Answer 20

a reformatted image in which lower attenuation structures are not displayed. good for viewing bone and/or contrast-enhanced soft tissues/vessels

Question 21

What is a min-IP image? What is the benefit?

Answer 21

the opposite of a MIP. structures with low attenuation are displayed while higher ones are not. good for showing the biliary tree, colon, trachea, and lungs.