BME 501 -
Introduction to BME
Bioelectrical Engineering Part:
Medical Imaging
Reference Textbook: Principles of Medical Imaging,
by Shung, Smith and Tsui
Lecturer: Murat EYÜBO
Ğ
LU, Ph.D.
BME 8720501- Introduction to
Biomedical Engineering
•
Bioelectrical Engineering Part:
•
Medical Imaging ... 3h
•
(X-ray imaging, Computerized Tomography, Medical Ultrasound
Imaging, Nuclear Medicine Imaging, Magnetic Resonance
Imaging)
•
(Dr. B. Murat Eyübo
ğ
lu)
•
Bioelectric phenomena ... 3h
•
(Dr. Ye
ş
im Serina
ğ
ao
ğ
lu)
•
Medical Instrumentation, mathematical modeling of
physiological control
systems... 3h
•
(Dr. Nevzat G. Gençer)
Outline
•
What is medical imaging
•
History
•
Projection Imaging
•
Computerized Tomography (CT)
•
Nuclear Source Imaging (PET, SPECT)
•
Ultrasonic Imaging
•
Magnetic Resonance Imaging
Medical imaging
is a collection
of techniques,
that are developed to measure and display
distribution of
a physical property
in living
subjects, specifically in humans.
Why is it useful?
Medical imaging, not only provides useful
information for diagnosis
but also serves to
assist in planning and monitoring
the
treatment of malignant disease.
Simplified block diagram of a
Medical Imaging System
Which energy types are
used for imaging?
•
X-ray
•
Nuclear (radio-isotope) sources,
•
Ultrasonic waves,
•
Magnetic fields,
•
Electrical currents,
•
Mechanical,
What are the physical
properties of interest?
•
X-ray absorption coefficient,
•
Radionuclide concentration,
•
Ultrasonic properties,
•
Spin density and spin relaxation,
•
Electromagnetic properties,
•
Mechanical properties,
Why are we interested in these physical
properties?
Certain physical property may vary
between different healthy tissue types,
with the physiological state of a tissue type,
Why are there so many imaging
modalities?
•
All imaging modalities are based on the
physics of the interaction
of energy and
matter.
•
Different imaging modalities are based on
physical interaction of different energy types
with biological tissues and thus provide
images of different physical properties
of the
tissues.
History
•
Discovery of X-rays, 1895,
•
Radon transform, 1917,
•
NMR principles, 1946,
•
Nuclear medicine scan, 1948,
•
Ultrasound imaging, 1952,
•
Positron tomography, 1953,
•
Single Photon Emission CT, 1971
•
Development of X-ray CT, 1972,
X-ray Projection Radiography
∫ ∫
∫
+
∞
∞
−
−
+
=
=
(
x
,
y
)
ds
(
x
,
y
)
(
x
cos
y
sin
t
)
dxdy
)
t
(
p
θ
β
β
δ
θ
θ
Radon Transform
Film
X-ray tube
Patient
t
)
t
(
p
θ
)
y
,
x
(
β
Attenuation Coefficients for Biological
Tissues at 60 keV
Tissue
Attenuation
coefficient (cm
-1
)
Blood
0.215
Brain matter
0.210
Water
0.203
Fat
0.185
Bone
0.400
Air
0.0002
X-ray tube design
•
Cathode with focusing cup, 2
filaments (different spot sizes)
•
Anode
–
Tungsten, Z
w= 74, T
melt=
2250 ºC
–
Embedded in copper for heat
dissipation
–
Angled (see next slide)
X-ray tube
•
Working Principle: Accelerated charge causes EM radiation:
–
Cathode filament C is electrically heated (
V
C= ~10V /
I
f= ~5 A) to
boil off electrons
–
Electrons are accelerated toward the anode target (A) by applied
high-voltage (V
tube= 40 –
150 kV);
–
kinetic electron energy:
K
eusually rated in “peak-kilo voltage”
kVp
–
Typical: V
tube= 40 –
150 kVp, I
tube= 1-1000mA
–
Deceleration of electrons on target creates "Bremsstrahlung"
+
-kVp
,
I
tubeC
A
V
C,
I
f+
-•
Tungsten Anode is desirable as:
•
It has high melting point,
•
Little tendency to vaporize,
•
It is strong.
X-rays characteristics
•
EM radiation at wavelengths 0.1 –
100 keV (10 –
0.01 nm).
•
Diagnostic Range X-rays typically have a wavelength from
100nm –
0.01nm ~1-100 keV.
•
X-ray radiation is thought to be particles traveling at the speed
of light and carrying an energy given by
E=hf .
(Plank constant h=4.13x10E-18 keV/Hz,
1eV=1.6x10E-19Joules)
•
These particles are called QUANTA or PHOTONS.
•
A photon having an energy level greater than a few electron
volts is capable of ionizing atoms an molecules.
Ionization energy for valence electrons < ~10 eV
⇒
X-rays is
ionizing radiation (harmful)
Example: UV light bulb
•
Photon energy > a few
eVolts
may result in ionizing
radiation.
For a UV light bulb:
l=100nm.
results in
f = c/l = 3x10E8 / 1x10E-7 = 3x10E15Hz.
E=h f
= 12eV is ionizing radiation.
Tomo
graphic Imaging
cut
Tomo
graph
ic Imaging
image
3-dimensional subject
Tomographic Imaging
X-ray CT
Detector array
Source
First scan
Second scan
Third scan
Second scan
First scan
First scan
Second scan
Third scan
Fourth scan
θ
θ
θ
δ
β
b
(
x
,
y
)
π
∫ ∫
p
θ
(
t
)
(
x
cos
y
sin
t
)
dt
d
∞
+
−
+
=
Example 1: Backprojection
5
11
7
7
5
5
7
7
5
11
Backprojection
5/5
5/5
5/5
5/5
5/5
11/5
11/5
11/5
11/5
11/5
7/5
7/5
7/5
7/5
7/5
7/5
7/5
7/5
7/5
7/5
5/5
5/5
5/5
5/5
5/5
5
11
7
7
5
Backprojection
5/5
+5/5
5/5
+7/5
5/5
+11/
5
5/5
+7/5
5/5
+5/5
7/5
+5/5
7/5
+7/5
7/5
+11/
5
7/5
+7/5
7/5
+5/5
7/5
+5/5
7/5
+7/5
7/5
+11/
5
7/5+
+7/5
7/5
+5/5
11/5
+5/5
11/5
+7/5
11/5
+11/
5
11/5
+7/5
11/5
+5/5
5/5
+5/5
5/5
+7/5
5/5
+11/
5
5/5
+7/5
5/5
+5/5
5
11
7
7
5
5
7
7
5
11
Backprojection
10/5
12/5
16/5
12/5
10/5
16/5
18/5
22/5
18/5
16/5
12/5
14/5
18/5
14/5
12/5
12/5
14/5
18/5
14/5
12/5
10/5
12/5
16/5
12/5
10/5
5
11
7
7
5
Backprojection
10/5
12/5
16/5
12/5
10/5
16/5
18/5
22/5
18/5
16/5
12/5
14/5
18/5
14/5
12/5
12/5
14/5
18/5
14/5
12/5
10/5
12/5
16/5
12/5
10/5
9
6
5
6
3
9
6
5
6
3
Backprojection
10/5 +9/5 12/5+6/4 +5/316/5 12/5 10/5 16/5 +6/4 18/5+9/5 +6/422/5 18/5+5/3 16/5 12/5 +3/3 14/5+6/4 +9/518/5 14/5+6/4 +5/312/5 12/5 14/5 +3/3 +6/418/5 14/5+9/5 +6/412/5 10/5 12/5 16/5 +3/3 12/5+6/4 +9/510/56
5
Backprojection
10/5 +9/5 12/5+6/4 +5/316/5 +5/3 12/5 +6/4 10/5 +9/5 16/5 +6/4 18/5+9/5 +5/3 22/5 +6/4 +6/4 18/5 +5/3 +9/5 16/5 +6/4 12/5 +3/3 +5/3 14/5 +6/4 +6/4 18/5 +9/5 +9/5 14/5 +6/4 +6/4 12/5 +5/3 +3/3 12/5 +6/4 14/5 +3/3 +9/5 18/5 +6/4 +6/4 14/5 +9/5 +3/3 12/5 +6/4 10/5 +9/5 12/5 +6/4 16/5 +3/3 +3/3 12/5 +6/4 +9/510/59
6
5
6
3
Backprojection
3.8
3.9
6.5
3.9
3.8
4.7
7.1
7.4
7.1
4.7
5.1
5.8
7.2
5.8
5.1
3.9
5.6
6.6
5.6
3.9
3.8
3.9
5.2
3.9
3.8
•
Two
basic strategies for producing an image that
doesn’t have the blurring seen in the preceding
example:
–
Backproject, and then perform a second,
repair operation on the image to correct the
blur (
Backprojection–Filtering
algorithms),
–
Modify the projection data in an appropriate
manner, so they will produce an unblurred
image,
before
backprojecting
(
Filtered
Filtered Backprojection
Backprojected image represents a blurred
version of the original distribution:
{
}
{
}
ρ
β
β
β
β
F
(
x
,
y
)
F
(
x
,
y
)
1
r
1
*
)*
y
,
x
(
)
y
,
x
(
2
b
2
b
=
⇒
=
⋅
This blurring effect can be removed as,
{
}
{
F
(
x
,
y
)
}
F
)
y
,
x
(
2
1
2
b
bf
ρ
β
β
=
−
⋅
Filtering can be applied to projections prior to
backprojection which is computationally more
effective:
{
}
{
θ
ρ
}
θ
1
{ }
ρ
1
1
1
1
F
p
(
t
)
p
(
t
)*
*
F
F
−
⋅
=
−
Filtered Backprojection
Measure projections from
all possible view angles
Backproject the
Convolve all
projections with
the filtering
function
h(t)
Performance of CT
•
Spatial resolution of 1 mm. (minimal distance
between two pixels which can be
discriminated is 1 mm.)
•
Contrast resolution of 1 % (i.e, pixel density
which is 1% different than the background
density can be discriminated.)
•
Soft tissue contrast is low.
•
Invasive : X-rays are harmful for living
Nuclear Source Imaging
•
Planar Scintigraphy :
–
Radioisotopes (radionuclides) are injected
to the body,
–
They emit radiation which can be detected
by photon detectors and the position of the
isotopes can be determined,
–
Two-dimensional representations of the
projections of three-dimensional activity
distributions are reconstructed.
Nuclear Source Imaging
•
Emission Computed Tomography
:
is a
technique to obtain cross sectional images of
activity,
–
SPECT:
Single gamma ray is emitted per
nuclear disintegration.
–
PET:
Two gamma rays are emitted when
a positron from a nuclear disintegration
annihilates in tissue.
SPECT and PET
∫ ∫
∞
+
−
∫−
+
=
A
(
x
,
y
)
(
x
cos
y
sin
t
)
e
dxdy
)
t
(
p
sds
)
s
(
β
θ
δ
θ
θ
Neuroblastoma
SPECT
CT
SPECT
DUAL
PET perfusion
scan of heart
Advantages and Disadvantages
of Nuclear Source Imaging
•
Functional images can be obtained,
•
Spatial resolution is poor,
•
Good tissue specific contrast,
Ultrasonic Imaging
•
Body is probed by Ultrasonic waves,
•
Ultrasound wave propagates through the
body,
•
Fraction of the ultrasound waves are reflected
at various tissue interfaces along the wave
path, producing echoes,
•
The reflected echo signals are measured and
used to reconstruct the reflection coefficient
distribution along the path.
Reflectivity of normally incident waves
Materials at interface
Reflectivity
Brain-skull bone
0.66
Fat-bone
0.69
Fat-blood
0.08
Muscle-blood
0.03
Muscle-liver
0.01
Soft tissue-water
0.89
Ultrasound Imaging
Burst of US wave is transmitted
x
Reflected wave is measured
x
dx
)
x
(
f
)
c
x
2
t
(
p
)
t
(
p
r
=
∫
t
−
∞
+
∞
−
Ultrasound Imaging
Advantages and Disadvantages
of Ultrasound
•
Functional images can be obtained,
•
Involves no ionizing radiation,
Magnetic Resonance Imaging
Magnetic Resonance Imaging
MAGNET
GRADIENT COILS
Use of gradient fields in MRI
[
]
{
j
(
G
x
)
t
(
G
y
)
t
}
dxdy
exp
)
y
,
x
(
M
K
)
t
(
S
=
∫∫
−
γ
x
+
γ
y
y
The emitted magnetization signal is measured
which is the 2-dimensional Fourier Transform
of the spin density (proton density) distribution.
First in-vivo MRI experiment in 1977,
by Damadian, Minkoff and Goldsmith
MR Images of human head
Advantages and Disadvantages
of MRI
•
Superior spatial resolution,
•
Good soft tissue contrast,
•
Functional imaging is possible,
•
Involves no ionizing radiation,
Electrical Impedance
Tomography
EIT :
cross-sectional
imaging of electrical
impedance
•
injected EIT
•
induced EIT
ACEIT ventilation scan
Right lung
Left lung
ANTERIOR
4th intercostal space level dynamic ventilation scan
Mediastenum
Advantages and Disadvantages
of EIT
•
Functional images can be obtained,
•
Good soft tissue contrast,
•
Involves no ionizing radiation,
•
Poor and position dependent spatial
resolution,
X-Ray Imaging -
1:
History
and Physics
background
Modified from SUNY Downstate Medical Center BMI Lecture Notes
Reference Textbook: Principles of Medical Imaging,
Discovery of x-rays
X-ray history on the web:
http://www.xray.hmc.psu.edu/rci/centennial.html
Physical Institute,
University of Würzburg, Germany.
Wilhelm
Wilhelm
Konrad
Konrad
R
R
ö
ö
ntgen (1845
ntgen (1845
-
-
1923)
1923)
(photographed in 1896)
First x-ray images
Radiograph of the hand of Albert von
Kolliker, made at the conclusion of
Roentgen's lecture and demonstration at
the Würzburg
Physical-Medical Society
The famous radiograph made by
Roentgen on 22 December 1895, and
sent to physicist Franz Exner
in Vienna.
This is traditionally known as "the first
Complex Atoms
•
Number of protons
Z
: Atomic number (determines
element)
•
Number of neutrons
N
: Neutron number
•
Number of protons + neutrons
A
m=
Z
+
N
: Mass number
Na
22
11
--
-+ + + + ++ + + +
K-shell (n=1, strongly bound)
L-shell (n=2)
M-shell (n=3, weakly bound)
...
Atom and electronic transitions
•
Electrons (-) are organized in shells around nucleus (+)
•
Higher shell (greater shell radius) = higher electronic energy
•
Electronic transitions between shells require
or release
energy
+
-n
= 1
n
= 2
E = h
ν
= E
3- E
2E = h
ν
= E
2- E
1Absorption
Emission
Excitation
Relaxation
Energy scheme
•
Binding energy (
BE
): energy binding electron to atom
•
Ionization energy
I
K,L,…= -
BE
: amount of energy needed to remove
electron from atom
•
BE
counted in negative units of electron volts (eV)
•
At infinity,
BE
= 0.
Continuum
Zero
K
L
M
N
E
•
Binding energy for
53I: -33.2 keV
(K), -4.3 keV
(L), -0.6 keV
(M)
Energy units
•
SI unit: 1 Joule [J] = 1 Nm = 1 kg m
2s
-2•
Electron volt [eV]: The potential energy of one elementary
charge gained/lost (e = 1.6×10
-19C) when crossing a potential
difference of 1V:
1 eV
= 1.6×10
-19C ×1 V = 1.6×10
-19[A s V] = 1.6×10
-19J
+
