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