Transcript Biomedical Imaging I - METU | Department Of | Electrical

Nuclear Medicine Imaging
Overview
• Nuclear medicine:
Therapeutic and diagnostic use of radioactive substances
• Radioactivity:
• Naturally occurring radioisotopes (radioactive isotopes) discovered 1896
by Becquerel
• First artificial radioisotopes produced by the Curies 1934 (32P)
 “Radioactivity,” “Radioactive”
• 1947 - Kohman: “Radionuclide” = nucleus of measurable half-life
• 1935 - Hevesy uses 32P for metabolic studies with Geiger-Muller counter
• 1949 - First radionuclide imaging by Cassen of 131I uptake in thyroid gland
(scintillator+PMT, scanner, collimator,1/4” spatial resolution)
• 1957 - Anger camera (planar imaging)
• 1960 - Kuhl & Edwards construct Mark IV scanner (~10 years before x-ray
CT)
• 1977 – Kayes & Jaszczak develop SPECT independently
• 1950 – first PET attempts
• 1976 – First commercial PET (Phelps & Hoffman at CTI)
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VII.2
Radionuclide Imaging
• Characteristics:
• The distribution of a radioactive agent inside the body is imaged
• Projection and CT imaging methods
• Imaging of functional or metabolic contrasts (not anatomic)
– Brain perfusion, function
– Myocardial perfusion
– Tumor detection (metastases)
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VII.3
Nuclear Stability
• The neutrons and protons which form the nucleus of an atom are held
together by a combination of forces such as gravitational and electrostatic
forces. The protons tend to repel each other. This means that, as bigger
atoms are but together, it becomes more difficult for the nucleus to be stable
as one collection of particles.
• The only reason that the nucleus is stable is that neutrons bind the other
particles together, which is why the heavier atoms have more and more
neutrons.
• As a general rule, there are about equal number of neutrons and protons in
a nucleus. But, in heavier atoms, a greater proportion of neutrons have to be
added to maintain the stability of the atom.
• The nucleus of many atoms is not stable. Nuclei with infavourable
neutron/proton ratio will disintegrate or decay into stable nuclei by
spontaneous emission of nuclear particles.
electron
neutrion
• Example:
neutron rich nucleus :
n  p   e    e  energy
proton rich nucleus :
p   n  e    e  energy
positron
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VII.4
Nuclear Stability
Neutron rich
unstable element
Proton rich
unstable element
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VII.5
Definitions
• Isotope: Nuclides of same atomic number Z but different N (and A)  same
element
• Nuclide: Species of atom characterized by the constitution of its nucleus (in
particular N, Z)
• Radionuclide: Nuclide of measurable half time
• Radioactive decay : the process by which an unstable nucleus is transformed
into a more stable daughter nucleus by emitting nuclear particles.
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VII.6
Examples of Radioactive Decay
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VII.7
Nuclear Activity
• Radioactive decay is described by
N (t )  N0et
99mTc
• N(t), N0: number of radionuclide at
time t = 0 and t, resp.
• : decay constant [1/t]
T1/ 2 
0.693

• Activity A = average decay rate [decays per second]
A t   
dN  t 
dt
  N t  
A  t   A0e t
• Nuclear activity is measured in curie: 1 [Ci] = 3.7  1010 decays/sec
(orig.: activity of 1 g of 226Ra)
• Practical: 1 mCi, mCi. SI unit is becquerel [Bq] = 1 decay/second
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VII.8
Interaction of Nuclear Particles and Matter
• Alpha particles
• Helium nucleus (4He++), mostly occurring for parent with Z > 82
• ~ 3-9 MeV (accounts for the kinetic energy of the alpha particle
+ kinetic energy of the product nucleus)
• + 2 charge large mass  strong interaction (ionisation: attracts eşectrons
from other atoms which become cations)
• Mean range in air: Rm = 0.325  Ealpha3/2
• Beta particles
• Causes Bremsstrahlung (white, characteristic)
• “Wiggly” motion in matter (low mass)
• Gamma rays
• Electromagnetic waves produces in nuclear processes ( < 0.1 nm, E >
10 keV)
• Identical to x-ray interaction (for E > 1.02 MeV: pair production and
photo disintegration [emission of alpha, n, or p from nucleus])
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VII.9
Radionuclides in Clinical Use
• Most naturally occurring radioactive isotopes not clinically useful (long T1/2,
charged particle emission)
• Artificial radioactive isotopes produced by bombarding stable isotopes with
high-energy photons or charged particles
• Nuclear reactors (n), charged particle accelerators (Linacs, Cyclotrons)
99
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T1/ 2 2.5d
Mo 
 99mTc  e  
VII.10
Radionuclide Generator
• On-site production of
99mTc
T1/ 2 6 h
Tc 

 99Tc   140 keV
99m
Al2O
3
•
99mTc
keV)
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is the single most important radionuclide in clinical use (gamma @ 140
VII.11
Radiopharmaceuticals and their uptake in the body
In nuclear medicine imaging a radioactive isotope is introduced into
the particular part of the body which is to be investigated.
Ex: in order to follow heart, introduce the activity into the blood stream.
Ex: In order to follow tyroid gland, introduce radioactive iodine (as tyroid
absorn iodine)
In some cases, neither of the two methods are possible.
 attach the radioactive subtance to another chemical which is chosen
because ıt is preferentially absorbed by part of the body. The chemicals to
which radipactive labels are attached are called radiopharmaceuticals.
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VII.12
Radiopharmaceuticals (cont.)
If a chemical compound has one or more of its atoms substituted by
a radioactive atom then the results is a radiopharmaceutical.
For more detailed information: see Belcher & Velter “Radionuclides in
medical diagnosis”, 1971
Selection of isotopes:
1) choose an isotope so that the resultant radiopharmaceutical is in the correct
chemical form which will allow it to be absorbed by the particular organ to be
imaged.
2) the energy of radiation must be suitable to the detectors to be used. Optimum
energy range for gamma cameras is 100-300 keV. Efficiency drops beyond this
range
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VII.13
Selection of isotopes (cont.)
3) T1/2 must not be too short, otherwise it will decay before the radiopharmaceutical
can be delivered. It must not be too long, otherwise the patient will be unnecessarily
exposed to ionization.
T1/2 (ideal) is a few hours.
Exception: Se is used for pancreas scanning. T1/2 is 120 days.
4) radiation dose delivered to patient must be as low as possible
5) radiopharmaceutical must be available, it should be cheap.
The radionuclide that fulfills most of the above criteria is Technetium _ 99m (99m Tc),
which is used in more than 90% of all nuclear medicine studies.
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VII.14
Properties of
99mTc:
• T1/2 = 6 h
• radiates 140 keV gamma ray
• the short half time and absence of Beta emission allows low radiation
dose to patient.
• The 140 keV gamma radiation allows for 50% penetration of tissue at a
thickness of 4.6 cm.
Applications:
•
•
99mTc-Sestamibi
(myocardial perfusion, cancer)
99mTc-labeled hexamethyl-propyleneamine (brain perfusion)
Other gamma emitters:
123 I, 111 In, 67
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Ga,
201 Tl, 81 Kr m
VII.15
Positron emitters:
•
11
C
, T1/2 = 20 min
– many organic compounds (binding to nerve receptors, metabolic activity)
•
13
N
, T1/2 = 10 min
– NH3 (blood flow, regional myocardial perf.)
•
15
O
, T1/2 = 2.1 min
– CO2 (cerebral blood flow), O2 (myoc. O2 consumption), H2O (myoc. O2
consumption & blood perfusion)
•
18
F, T1/2 = 110 min
– 2-deoxy-2-[18F]-fluoroglucose (FDG, neurology, cardiology, oncology, metabolic
activity)
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VII.16
Imaging
As long as the photons emanating from the radionuclide have sufficient energy
to escape from the human body in significant numbers, images can be generated
that portray in vivo distribution of the radiopharmaceutical.
Nuclear medical imaging may be divided into three categories:
1) conventional or planar medical imaging,
2) Single photon emission computed tomography (SPECT),
3) Positron emission tomography (PET).
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VII.17
Conventional or planar imaging
The three-dimensionally distributed radiopharmaceutical is imaged onto a planar or
two-dimensional surface producing a projection image.
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VII.18
Detection of Gamma Radiation
• Scintillation detectors most commonly used
• Crystals: NaI(Ti), BGO, CsF, BaF2
• Criteria: Stopping power, response time, efficiency, energy resolution
• Ion collection detectors (ionization chambers) not used because of low
efficiency, slow response
• Semiconductor detectors (diodes): very high energy resolution, fast but small
and high cost
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VII.19
Scintillation Camera (Anger Camera)
• Imaging of radionuclide distribution in 2D
• Replaced “Rectilinear Scanner”, faster, increased efficiency, dynamic imaging
(uptake/washout)
• Application in SPECT and PET
• One large crystal (38-50 cm Dia.) coupled to array of PMT
1.
2.
3.
4.
5.
Enclosure
Shielding
Collimator
NI(Ti) Crystal
PMT
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VII.20
Anger Logic
The Anger camera is a system for achieving a large number of resolvable
elements with a limited number of detectors. It thus overcomes the previous
difficulty of having the resolution limited by the number of discrete detectors.
The principle is based on estimating the position of a single event by
measuring the contribution to a number of detectors.
GAMMA-RAY PHOTON
Cameras of this general
type have a single crystal
viewed by arrays of detectors
SCINTILLATING CRYSTAL
with the detected outputs
followed by a position computer
LIGHT PHOTONS
to estimate the position of
PHOTODETECTOR
x1
x2
I1
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x
each event.
I 1 x1 + I 2 x2
I1+I2
I2
VII.21
Applications
1. Thyroid imaging: The thyroid gland is situated in the lower part of the neck at a
depth of about 1 cm. The purpose of thyroid is to secrete the hormone thyroxin
which is carried in the blood stream and controls a number of body functions:
• stimulate metabolism
• influence growth
• control mental development
• store iodine
underactive thyroid :
• mental dullness,
• low temperature
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• decrease in metabolism
VII.22
Imaging of thyroid can be useful for the following purposes:
1. To determine the amount of thyroid tissue left after surgery or radiotherapy
for thyroid disease,
2. To detect thyroid metastases associated with thyroid cancer,
3. To show the comparative function of different parts of the glands,
4. To measure the size and position of the thyroid prior to surgery or other
treatments of the disease.
To obtain images, the patient is given an oral dose of 30mCi
KI (potassium iodide). The scan is taken 24hrs later.
131 I
131I
in the form of
emits  rays (336 keV).
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VII.23
Collimators I
Collimator restricts
the acceptance angle
Geometry
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VII.24
Single Photon Emission Computed Tomography (SPECT)
• If one or more gamma cameras are
attached to a computer controlled
gantry, which allows the detectors to
be rotated around a patient, multiple
views (or 2D projections) of the 3D
pharmacutical distribution can be
acquired.
• First SPECT 1963 (Mar IV) used array
of detectors
• Rotation, Translation
• High count rates
• Many components
• Mostly single-slice
• Rotating camera:
• Multiple slices
• Multi-camera systems
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VII.25
SPECT Image
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VII.26
SPECT Artifacts
• Reconstruction methods similar to x-ray CT
• X-ray attenuation: X-ray from source is attenuated by tissue 
unknown concentration of tracer and unknown distribution of tissue
absorption.
• Corrective measures:
1) Transmission measurement with external source to determine tissue
absorbtion
2) Assume constant absorption and use geometric mean of two
measurements 180 apart, which is independent on d
3) Iterative reconstruction
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VII.27
Using the Geometric Mean
Let there be an activity A at depth d from detector I. Assume that the object has
a constant attenuation coefficient. Then the fraction of photons reaching that
detector (C1) is proportional to e-mx, that is
m d
C1  Ae
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Survival
probability
VII.28
Geometric mean (cont.)
The fraction of photons reaching the second detector (C2) is:
 m (T  x )
C2  Ae
If the geometric mean is used, then
CGM  (C1C2 )1/ 2

 md
 ( Ae
 (T  d )
)( Ae
)

1/ 2
 mT / 2
 Ae
which is totally independent of source depth. Provided an outline of the body,
a simple correction can be applied to the combined opposed projections.
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VII.29
Iterative Reconstruction method
p (t )   A( x, y ) ( x cos  y sin   t )e 
 m ( s ) ds
dxdy
  m ( s ) ds 

   A( x, y )e
 ( x cos  y sin   t )dxdy


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VII.30
The image domain can be discretized and acquired ray sums can be expressed by:
p (k ) 
 f
i( k , )
k ,
i
m j l kj ,

Ai e

where
Ai
: activity contained in the ith voxel,
p(k) : projection data at angle , the sum of weighted activity (or ray sum)
along the kth ray at angle of view ,
fi k,  : fractional volume of the ith element that is contained within the kth ray,
mi
: the attenuation coefficient of the ith element (corresponding to the
energy of the photon),
lj k,  : length of the portion of the kth ray that is contained within the ith
element
exp(- mj lj k, ) : attenuation factor for radiation originating from the ith element.
The index j denotes elements lying along the kth ray between the ith element
and the boundary of the object nearest the detector.
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VII.31
Iterative method
• Assume attenuation distribution, find Ai
• Calculate attenuation distribution using Ai
• Find new estimate for Ai using the calculated attenuation coefficients,
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VII.32
Positron Emission Tomography
• Use with positron emitters (beta-plus)
• Positron annihilates with electron of nearby atom
 two gamma quanta each at 511 keV leave under
180
• “Tagging” of radiation:
• Windowing
• Coincidence detection (“electronic collimation”)
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VII.33
PET Detectors
• Individual Coupling:
Expensive, packing problematic,
high count rate
• Block Design:
Digital encoding, longer dead time,
more economic, somewhat reduced
resolution
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VII.34
PET Image Resolution
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VII.35
PET Resolution compared to MRI
• Modern PET ~ 2-3 mm resolution
PET
MRI
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VII.36
Functional Imaging
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VII.37