Fisica Medica 4 – TC
Corso di laurea in Fisica
A.A. 2007-2008
Computed Tomography Principles
• 1. Projection measurement
• 2. Scanner systems
• 3. Scanning modes
Basic Tomographic Principle
The internal structure of an object can be reconstructed
from multiple projections of the
object.
Exponential Attenuation of X-ray
Ray-Sum of X-ray Attenuation
Computed Tomography Principles
• 1. Projection measurement
• 2. Scanning modes
• 3. Scanner systems
Projection & Sinogram
Computed tomography (CT): image reconstruction from projections
P(θ,t) Î f(x, y)
Computed Tomography Geometry
(First CT was developed in 1972)
reconstruction matrix
focal spot detector
Matrix Representation of a Tissue Slice in CT
• Pixel (Picture Element)
• H.U. = [µ - µ(water)/µ(water]*1000
CT Display Scale
linear attenuation coefficient, µ (x,y,z ) reconstructed image
displayed image
Houndsfield units, H.U.
H.U. = [ µ - µ (water)/ µ (water]*1000
Linear Attenuation Coefficients (60 keV)
Tissue
• Brain - Grey
• Brain - White
• Cerebro-Spinal Fluid (CSF)
• Pancreas
• Liver
• Water
• Fat
µ ( χµ −1 )
• .213
• .215
• .208 - .213
• .215
• .221
• .205
• .190
Grey - White Matter Contrast
C = (.215 - .213)cm
-1* 1.0 cm
= .2 %!
CT Number µ (cm
-1) 39
49
.213 .215
CT number allows the computer to present the information
with a larger grey scale
Variation of Linear Attenuation Coefficients with Energy
0.14 0.16 0.18 0.2 0.22
60 70 80
Energy (keV) µ
Water
Fat
Variation of H.U. with Energy
Energy (keV) 60
70 80
µ (water) .205 .193 .184
µ (fat) .190 .179 .171
H.U.(fat)
-73
-73
-73
Image Display
CT Number
- Hounsfield unit
• Air: -1024
• Water: 0
• Bone: +175 to +3071 Viewing Parameters
• Window level (L)
• Window width (W)
• Zoom factor
water water
HU µ
µ µ ) =1000 µ − (
-1024 +3071
0 255
W
L
Computed Tomography Principles
• 1. Projection measurement
• 2. Scanning modes
• 3. Scanner systems
CT Scanner
Data Acquisition System (DAS)
Data Acquisition System (DAS)
First Generation
One detector Translation-rotation
Parallel-beam
160 rays x 180 views
5 minutes/per slice
First Generation CT Scanner
From Webb, Physics of Medical Imaging
Second Generation
Multiple detectors Translation-rotation Small fan-beam
600 rays x 540 views
18 seconds/per slice
Second Generation CT Scanner
From Webb, Physics of Medical Imaging
Third Generation
Multiple detectors Translation-rotation Large fan-beam
800 rays x 1000 views
<1 seconds/per slice
Ring Artifact In 3 rd Generation
Fourth Generation
Detector ring
Source-rotation
Large fan-beam
Fourth Generation
Detector fan eliminates
ring artifacts.
3 rd and 4 th Generation
Scanners
From Webb, Physics of Medical Imaging
Krestel-
Imaging Systems for Medical Diagnosis
Third & Fourth Generations
Fifth Generations
Electron-beam
CT for cardiac
imaging.
Electron Beam CT
Sixth Generation:
Spiral/Helical/Volumetric CT
Continuous &
Simultaneous :
• Source rotation
• Patient translation
• Data acquisition
Volume Scanning
Scan-Translate Patient, Scan-Translate Patient,
Continuous Scan,
Continuous Patient
Translation
Important years in helical CT history
Single-slice 1989
Dual-slice
1992 Quad-slice
1998
Quad-Slice Single-Slice
8 times faster than single-slice
One rotation / sec
Two rotations / sec +
4 slices / rotation
Why is faster better?
•
Improved temporal resolution• Faster scanning causes less motion artifacts
• Breath holding time is reduced
•
Improved spatial resolution• Narrower collimation leads to higher resolution in the z-axis (MPR)
• Narrower collimation reduces partial volume effect
•
Improved contrast media concentration• Higher contrast media concentration due to faster infusion
• Better separation of arterial and venous phases
•
Increased power (mAs)• The widened x-ray beam and sampling of multiple slices for each rotation allows for raised mAs
•
Decreased image noise• A direct effect of raised mAs
•
Efficient x-ray tube utilization• Faster scanning causes markedly less waiting for tube cooling
• More images from x-ray tube during tube life cycle
