CT “Computer tomography”. Contrast mechanisms in X-ray imaging: X-ray absorption X-ray...

CT

“Computertomography”

Contrast mechanisms in X-ray imaging:X-ray absorption

X-ray absorption mechanisms:1. Photoelectric effect2. Compton scatter3. Pair formation

Problem:X-ray image is a summation image

CT History1972

Godfrey Hounsfield

„Siretom” head scanner (1974)128x128 image recorded using

the Siretom scanner (1975)

Allan Cormack

1979 Nobel Prize in Medicine

CT Foundations I

sourcedetector

CT Foundations II

µx: linear attenuation coefficient

Scanning I

I. generationSingle moving source

Single moving detector

II. generationSingle moving source

Narrow fan-beamMultiple moving detectors

Scanning II

III-IV. generationSingle moving source

Wide fan-beamMultiple detectors or

detector ring

closed gantry open gantry

CT Image Reconstruction

1. Algebraic reconstruction techniques

2. Direct Fourier reconstruction

3. „Filtered Back Projection”

CT-image:4000 detectors

1000 projections512x512 matrix

16 bit depth

CT Image:Density matrix

€

NCT =1000μ−μwμw

Density(“CT Number”):

Hounsfieldunits

µ: attenuation coefficient of voxelµw: attenuation coefficient of water

Contrast Manipulation of CT Image:„windowing”

Spiral CT

New CT Developments, Trends

Virtualendoscopy

Angiography

3Dreconstruction

MRI

“MagneticResonanceImaging”

Nuclei with nuclear spin:elementary magnets

μ i = γLMagnetic moment:

=magnetogyric ratioL=angular momentum

In absence of magnetic field:Random orientation of elementary magnets

In magnetic field:elementary magnets energy levels

orient splitB0 parallel

antiparallel

E

B0

E

B

Precession

Precession orLarmor frequency:

ω0 = γB0

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B0

M

Low energy stateparallel in case of proton

High energy stateantiparallel in case of proton

Net magnetization (M)due to spin excess in different energy states

Excitationusing radio frequency (RF) radiation

Resonance condition: Larmor frequency

M

Net magnetization

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Spin-lattice relaxationT1 or longitudinal relaxation

t

Mz

T1 relaxation time: depends on interaction betweenelementary magnet (proton) and its environment

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Spin-spin relaxationT2 or transverse relaxation

Mxy

t

“free induction decay” (FID)

T2 relaxation time: depends on interaction betweenelementary magnets (protons)

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1970: detection of lengthened relaxation times in cancerous tissues1972: theoretical development of human in vivo 3D NMR1977: first human MRI image

Inventor of MRI:Raymond V. Damadian

(1936-)

MRI:Net magnetization of the human body takes place

“indomitable”

Paul C. Lauterbur(1929-)

1971: development of spatially resolved NMR

voxel:volume element

pixel:picture element

Image

MRI imaging I:Spatial resolution

Definition and addressing ofelementary 3D image points (voxels):by using gradient magnetic fields

MRI imaging I:Spatial resolution

By

Bx

Bz

MRI imaging II:Color (grayscale) resolution (contrast)

Based on relaxation times

MRI imaging II:Color (grayscale) resolution (contrast)

Based on spin density and relaxation times

T1-weighing T2-weighingProton density-weighing

MRI technology

Magnet: superconducting (liquid He)

Resolution enhancement: with surface RF coils

Excitation with pulse sequences90˚ 90˚ 90˚

Detection and analysis:Fourier transform of temporal signal

t

MRI:Image manipulation I

Reslicing in perpendicular plane

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MRI:Image manipulation II

Spatial projection(„volume rendering”)

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Bloodflow

Image slice

Saturatedspins

Unsaturatedspins

MRI:Non-invasive angiography

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MRI:Non-invasive angiography

arteria carotis Circulus arteriosus Willisii

MRI movieBased on high time resolution images

Opening and closing of aorta valve

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Functional MRIfMRI

High time resolution image sequencesrecorded synchronously with physiological processes

Effect of light pulses on visual cortex

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MRI információ szuperponálásaegyéb információval (PET)

Superposed MRI and PET image sequence

PET activity: during eye movementVolume rendering

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