Biophyics seminar
Apr. 09-11, 2013
Diagnostic Imaging Methods (2)
X-ray, CT
András M. Kengyel, MD
Institute of Biophysics
X-ray imaging
CT
Problem: X-ray image is a summation of densities
Computed Tomography: cross-sectional image
Godfrey Hounsfield
Tissue attenuation: mass-attenuation coeffitient, density
X-ray film: becomes black when radiated
X-ray image: summation of densities
Allan Cormack
Nobel lauerate in Medicine, 1979
CT
CT instrument
III-IV. generation
Single moving source
Wide fan-beam
Multiple detectors or
detector ring
Spiral CT
D: detector, T: radiation source, X: X-ray
beams, R: rotation
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CT imaging
CT imaging
CT imaging: Different absorption properties of the tissues
Hounsfield Unit (HU)
Total intensity attenuation measured in each direction
„Attenuation matrix”
HU =
µ x − µw
× 1000
µw
Tissue
LEFT
CT image is always looking upwards!
HU
Bone
250-1000
Liver
65
Blood
55
Water
0
Fat
-65
Lung
-500
Spatial resolution: 0,5 – 3 mm
Axial resolution: 5 – 10 mm
CT imaging
A CT image contrast manipulation: „windowing”
MRI
MRI
MRI: outline
1. What is the signal?
- Change in nuclear magnetic properties
(1H nucleus)
p+
NMR
2. How to detect?
(N)MRI
- Excitation with resonance frequency
(Larmor frequency)
- Disturb the spin, wait for recovery, t1, t2 time)
Nuclear Magnetic
Resonance
(Nuclear) Magnetic
Resonance Imaging
3. How to get 3D image?
- Gradeint magnetic field
- Frequency coding
- Phase coding
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Nuclear magnetism
Nuclear magnetism
Moving electrical charge produces magnetic field
In absence of magnetic field: In homogen magnetic field:
Nucleus with unpaired spin: elementary magnet
elementary magnets orient
energy levels split
Random orientation of elementary
magnets
Magnetic moment makes a precessive movement
The precession frequency is quantated
ω = 2πν = γ B0
γ
ν=
B0 = γ B0
2π
p+
B0
Larmor-frequency
γ = magnetogyric factor
Earth: 0,1 mT (Tesla)
B0 = magnetic field
MRI: 1,5 -3 T
Nuclear magnetism
MRI signal
Net magnetization (M)
B0
due to spin excess in different
energy states
Longitudinal signal:
Mz
spin – lattice relaxation
t1 time
antiparallel
B0
Time (s)
Magnetic spin vector
parallel
Transversal signal:
spin – spin relaxation
M
Percession
Magnetic field
Mx
∆E
Low energy state
parallel in case of proton
B0
B0
Radiofrequency
burst (RF)
B
t1 – t2 times
Magnetic field
RF excitation
time
Relaxation
spin-spin (fast)
spin-lattice (slow)
(decoherency)
z
y
●
Transversal signal:
spin – lattice relaxation
spin – spin relaxation
Cause: Interactions between 1H (water)
and the enwironment (e.g. free or
bound water), energytransfer
de-phasing
●
●
●
●
Mz
t1
●
●
t2 time
t1 time
Longitudinal signal:
t2
t2 time
Tissue specific
t1 time
TE: detection time
Settable parameter:
TR: repetition time
Cause: Interactions between
spinning 1H protons
E.g. protein hydration layer t1< free water t1
●
●
●
●
„Top view”
●
Time (ms)
t1 – t2 times
„Side view”
x
t2 time
Detector
High energy state
antiparallel in case of proton
E
Mx
tissue specific,
intrinsic feature
t2 time
t1 time
Idı (s)
TR: repetition time
TE
TR
could be set by the
operator
Idı (ms)
TE: Echo delay time
Detection is always in the transversal plane!
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Contrast enhancement
3D imiging – slice selection
Discriminating tissues: according to different t1, t2 times
ν=γB
proton density [long TR, short TE]
Bz gradient magnetic field
Larmor-frequency
t1 weighting (brain > liquor) [short TR, TE]
t2 weighting (edeme > brain) [long TR, TE]
Radiofrequency
burst
(Larmor
frequency of the
choosen slice)
t1 weighting
t1 weighting: short TR, short TE
t2 weighting
brain tissue more intense
3D imaging
1. (z) Gradient magnetic field – slice
CT vs MRI
phase
2. (x) Frequence coding – column
3. (y) Phase coding - voxel
gradient
magnetic
field
frequence
coding
detection
Detected signal analysis:
Furier transformation
(Complex function could be derived
into basic sine functions)
CT
MR
better spatial resolution
no ionizing radiation
high radiation dose
soft tissue diagnostic (bone less water: translucent)
expensiv (cost/benefit)
(loud)
CT vs. MRI
CT
References
MAIN PURPOSE
Differentiate between tissues
MRI
Suzanne A. Kane: Introduction to physics in modern medicine, T&F 2003
Bradley et al. Magnetic Resonance Imaging, Aspen 1985
Absorption ability of the
tissues
BIOLOGICAL
PROPERTY
Water content, free or
bound water
Reimer et al. Clinical MR Imaging, Springer 1999
X-ray attenuation /
absorption coeffitient
PHYSICAL
PROPERTY
(Pixel intensity)
Change of the
magnetisation signal (t1, t2)
http://www.cis.rit.edu/htbooks/mri/inside.htm
X-ray radiation
METHOD
Hornak, The Basics of MRI
http://www.radiologia.hu/szakma/mro/cikk/3.html
Resonance - Relaxation
Scintillation counter /
photodiode
DETECTION
Spin echo
Attenuation matrix
3D IMAGING
Magnetic gradient
Frequency coding
Phase coding
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