Fisica Medica

4 – TC

Corso di laurea in FisicaA.A. 2007-2008

Computed Tomography Principles

• 1. Projection measurement

• 2. Scanner systems

• 3. Scanning modes

Basic Tomographic Principle

The internal structure of anobject can be reconstructedfrom 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) f(x, y)

Computed Tomography Geometry

(First CT was developed in 1972)

reconstructionmatrix

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 attenuationcoefficient, 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 informationwith a larger grey scale

Variation of Linear AttenuationCoefficients with Energy

0.140.160.180.2

0.22

60 70 80

Energy (keV)

µ

WaterFat

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

waterHUµµµµ −

=1000)(

-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 GenerationMultiple 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 3rd Generation

Fourth Generation

Detector ring

Source-rotation

Large fan-beam

Fourth Generation

Detector fan eliminatesring artifacts.

3rd and 4th

Generation Scanners

From Webb, Physics of Medical Imaging

Krestel-Imaging Systems for Medical Diagnosis

Third & Fourth Generations

Fifth Generations

Electron-beamCT 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 PatientTranslation

Important years in helical CT history

Single-slice1989

Dual-slice1992 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 eachrotation 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

Pitch

Pitch = (1)table travel (mm) per gantry rotationbeam collimation (mm)

Information about table travel relative to beam collimation

> 1= 1

Pitch

Volume

Volumetric scanningViewing

Reformatting

x

y

z

Reformating multiple slices into a volume produces a volume “image” with unequal

spatial resolution in x, y, and z.

Reformatting with Interpolation

Stacked Slices(original images)

Interpolated Slices(smoother display)

Scan a volume – view a volume

The image data from the volume can be mergedinto single images.

Coronal Reformat

25, 50, 100, 400, 1000 images can bereconstructedfrom volume.