ct “computer tomography”. contrast mechanisms in x-ray imaging: x-ray absorption x-ray...
TRANSCRIPT
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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