17 - Computed Tomography III (1)

7/24/2019 17 - Computed Tomography III (1)

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Computed Tomography III

Reconstruction

Image qualityArtifacts


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Simple backprojection

Starts with an empty image matri! and the

"alue from each ray in all "iews is added

to each piel in a line through the imagecorresponding to the ray#s path

A characteristic $%r blurring is a byproduct

A filtering step is therefore added to correct

this blurring


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&iltered backprojection

The raw "iew data are mathematicallyfiltered before being backprojected onto the

image matri In"ol"es con"ol"ing the projection data

with a con"olution kernel

'ifferent kernels are used for "aryingclinical applications such as soft tissueimaging or bone imaging


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Con"olution filters

(ak filter increases amplitude linearly as a

function of frequency) works well when there is no

noise in the data Shepp*(ogan filter incorporates some roll-offat

higher frequencies! reducing high*frequency noise

in the final CT image

+amming filter has e"en more pronounced high*

frequency roll*off! with better high*frequency

noise suppression


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,one kernels and soft tissue

kernels ,one kernels ha"e less high*frequency roll*off and

hence accentuate higher frequencies in the image atthe epense of increased noise

&or clinical applications in which high spatialresolution is less important than high contrastresolution - for eample! in scanning for metastaticdisease in the li"er - soft tissue kernels are used- .ore roll*off at higher frequencies and therefore produce

images with reduced noise but lower spatial resolution


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CT numbers or +ounsfield units

The number CT(x,y)in each piel! (x,y)! of the

image is/

CT numbers range from about -$!000 to 12!000

where -$!000 corresponds to air! soft tissues rangefrom -200 to -$00! water is 0! and dense bone and

areas filled with contrast agent range up to 12!000

water

wateryxyxCT

= 3!4000!$3!4


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CT numbers 4cont53

CT numbers are quantitative

CT scanners measure bone density with

good accuracy-Can be used to assess fracture risk

CT is also quantitati"e in terms of linear

dimensions-Can be used to accurately assess tumor "olume

or lesion diameter


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'igital image display

6indow and le"el adjustments can be madeas with other forms of digital images

Reformatting of eisting image data mayallow display of sagittal or coronal slices!albeit with reduced spatial resolutioncompared with the aial "iews

7olume contouring and surface renderingallow sophisticated 2' "olume "iewing


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Image quality

Compared with *ray radiography! CT has

significantly worse spatial resolution and

significantly better contrast resolution (imiting spatial resolution for screen*film

radiography is about 8 lp%mm) for CT it is about $

lp%mm

Contrast resolution of screen*film radiography is

about 9:) for CT it is about 059:


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Image quality 4cont53

Contrast resolution is tied to the S;R! which is related to

the number of *ray quanta used per piel in the image

There is a compromise between spatial resolution and

contrast resolution

6ell*established relationship among S;R! piel

dimensions 43! slice thickness 4T3! and radiation dose 4'3/

T

SNRD

2

17 - Computed Tomography III (1)

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