Computerized Tomography

1st CT Scanner

two slice NaI detetor withh 80×80 image matrix and 8 shades of gray

1st generation of CT

rotate/translate tube and detectors
two NaI detectors
5 minute acquisition time

2nd generation of CT

rotate/translate
an array of detectors
narrow fan beam
one minute acquisition time

3rd generation of CT

rotate/rotate
wide fan beam
1-2 second image acquisition
bow-tie filter was used to equalize image noise for varying thickness body parts

4th generation of CT

rotate/stationary (only x-ray tube rotates)
1-2 seconds per scan
found to have a wobble artifact due to detectors and x-ray sources on independent axes

modern CT

helical scanning
slip-ring technology - allows continuous rotation
multidetector array (4-320 channels of spatial data)
modern CT most closely resembles the 3rd generation

Ray

a single transmission measurement thru a patient made by a single detectora

projection (or view)

series of rays that pass thru the patient at the same orientation

Sinogram

projections are stored for each angular orientation in data file

intensity formula

assuming homogeneous, monoenergetic, narrow beam geometry

iterative solution

approximate solution to linear attenuation coefficients of each voxel by guessing at them and then adjusting using an algorithm until result matches closely with measured
high computational demands (in modern tech)

Improved image quality

backprojection

process begins by taking our ray projection and backprojecting the result along the ray-line
take our projection data, smear it back along project path evenly - leaving a blurring artifact (fix with filtering)

convolution

process of applying a reconstruction kernel to account for the backprojection blur

filtered backprojection

leads to streaking artifacts around high Z materials when there is complete attenuation of the beam

Analytical algorithms

solve all equation for unknown attenuation coefficients
too much for our current computing power

CT numbers (HUs)

Air CT number

-1000

lung ct number

-300

fat ct number

-90

water ct number

0

white matter ct number

30

gray matter ct number

40

muscle ct number

50

bone ct number

1000 (up to 3000 for dense bone)

titanium ct number

5000

CT number use in radiotherapy

used for heterogeneity corrections in dose calculations
- attenuation coefficients can be related to electron density to correct dose distributions (aka using CT numbers)

electron density to HU calibration curve

created by scanning a phantom of known electron density
plot CT number verses electron density

considerations for scanning phantom

• energy

• location

how much does a change in HU affect the dose calculation?

depends on exactly how much of the material with a different HU values are present
rule of thumb: 10% change in HU value yields a 2% change in calculated dose

window width

determines contrast of image
change the gradient from black to white

level

is the ct number at the center of the window

Pitch

Pitch = table increment per rotation (mm) /Beam collimation

pitch = 1

we gather similar information a contiguous axial scan

pitch < 1

implies overlapping slices (higher dose and increased image quality)

pitch > 1

implies gaps between slices (reduced dose and decreased image quality)

Cone beam CT

slow - about 60 seconds
lower quality than a scan compared to CT due to large field generating a lot more scatter that degrades image quality

CT detectors

most use solid-state scintillators
CdWO_4 (Cadmium Tungstate), Gadolinium Based, Proprietary Ceramic Scintillator