Transcript Nuclear Medicine Instrument

Nuclear Medicine
Instrumentation
Radionuclides
Isotopes Half-life Energy (keV) main decay
 99mTc
6.03 hrs
140
I.T.
 131I
8.05 days
364

 125I
60.2 days
35
E.C.
 123I
13.0 hrs
160
E.C.
 201Tl
73.0 hrs
135, 167
E.C.
 111In
67.2 hrs
247, 173
E.C.
 67Ga
78.1 hrs
300, 185, 93
E.C.
 127Xe
36.0 days
172, 203, 375 E.C.
 133Xe
5.31 days
81

Photon-Matter Interaction
 Photoelectric effect
– entire energy converted into kinetic energy
– high Z material,   Z4E-3
 Compton scattering
– part of its energy converted into kinetic energy
– proportional to electron density,   ZE-1
– predominant interaction in tissue, ( Z   )
Attenuation Effect
 Ina = I0 exp { -∫d}
–  : both photoelectric effect & Campton scatter
Gamma rays
Scattered (Campton)
Absorbed (Photoelectric)
Non-attenuated
Gamera camera
Collimators
Collimator
 Select the direction of photons incident on camera
– defining the integration paths
– Types:
• parallel
• slanted parallel
• fan-beam
• cone-beam
• varifocal cone-beam
• pinhole
• convergent
• divergent
Parallel Collimator
 Resolution : Rc = S (1+L/H)  L,  = S/H
– Distance dependent (DDSR)
 Sensitivity : g  Rc2/L2 = 2 (S(S+T))2
 Septa penetration not considered
L
Rc/(H+L) = S/H  
 Rc  L
S
Septa thickness = T
H
Rc
Resolution v.s. Distance
Collimators
High sensitivity
resolution
General purpose
High resolution
Rc
Source to collimator distance
 Septal thickness T is determined by photon energy
– low-energy collimator < 150 keV
– medium-energy collimator < 400 keV
Typical Performance Characteristics
Collimator
Types
Low energy,
High resolution
Low energy,
General purpose
Low energy,
High sensitivity
Medium energy,
High sensitivity
Suggested
Max. Energy
Efficiency
Resolution at
10cm
150 keV
1.84×10–4
7.4 mm
150 keV
2.68×10–4
9.1 mm
150 keV
5.74×10–4
13.2 mm
400 keV
1.72×10–4
13.4 mm
Scintillator (inorganic)
 Convert a gamma-ray photon to light photons for
subsequent processing by the PMTs
Conduction band
Activator excited states
Activator ground state
Valence band
– A large flat NaI (Tl) crystal (eg., 20”x15”)
– Issue: sensitivity vs. resolution
– Thickness: 1/4” ~ 3/8”
 The thicker the crystal, the better the sensitivity
but the worse (larger) the resolution.
NaI properties
 Stopping power:
– Effective atomic number (Iodine:53, relatively high)
– Density: 3.76 g/cm3
 Light yield: 38 photons/keV (4 eV/per photon)
–
–
–
–
Good light yield, used as reference = 100
Energy resolution (Poisson statics)
no. generated proportional to deposited energy
15% scintillation Efficiency
 Light decay constant: 230s after glow
– Dead time
– Position mis-positioning
– Wavelength at max. emission: 415 nm
 Reflective index: 1.85
– Hygroscopic, relatively fragile
Inorganic Scintillators (Crystals)
Scintillator
Wave
length
Decay
Refraction Density Light yield
constant (ns)
index
(g/cm3)
NaI (Tl)
410
230
1.85
3.67
100
CsI (Na)
420
630
1.84
4.51
85
CsI (Tl)
565
1000
1.80
4.51
45
LiI (Eu)
470-485
1400
1.96
4.08
35
CaF4
435
900
1.44
3.19
50
BGO
480
300
2.15
7.13
15
GSO
410
60
1.9
6.71
16
BaF2
225/310
0.6/620
1.49
4.89
4/20
CdWO4
540
5
2.2
7.9
40
LSO
480
40
1.82
7.4
75
YSO
420
70
1.80
4.54
118 ?
Crystal vs. Light yield
NaI (Tl)
Light yield
CsI (Tl)
CsI (Na)
420
410
565
Wavelength (nm)
Detector response vs. Energy resolution
 Output signal amplitude proportional to energy deposited
in the scintillator
 Energy resolution = 100%  
 Complete electron transfer (ideal condition)
Count
Non-scatter photon
Scatter photon
Eo/(1+2Eo)
Eo
Photon energy
Photofraction (real condition)
 Spreading due to Poisson effect
Count
Non-scatter photon
Scatter photon
FWHM
Eo / (1 + 2Eo)
Eo
Photon energy
Factors affecting Energy resolution:
 Counting statistics + Electronic noise
– Causes uncertainty in measured deposited energy
 Poisson Statistics
g(x) = Poisson ((x))
Prob (g(x)) = [(x) g(x)/g(x)!] exp(-(x))
f (n/) = n exp (- )/n!
SNR {n/} = 
E {n/} = 
Var {n/} = 
E {g(x)} = (x)
Var {g(x)} = (x)
Factors affecting Energy resolution:
 1. Incomplete energy transfer
– Detector size
– Attenuation effect: density, effective Z number
 2. Pile-ups & Baseline shifts
Baseline shift
Pile-up
Pile-up and Baseline shift
 Problems occurs at high counting rates
 Both can be reduced by decreasing the pulse width, but
this also increases the electronic noises, thus degrading
energy resolution.
 Baseline shift:
– 2nd pulse occurring during the negative components of the 1st
pulse will have depressed amplitude
– Shift in the energy of the 2nd event
– Corrected by pole zero cancellation or baseline restoration
 Pile-up:
– Two or more pulses fall on top of each other to became one pulse
– Incorrect energy information
– Lost events
What is measured ?
2D vs 3D
attenuation
distribution
radioactivity
distribution
y
–∫ (x, y’)dy’
(x) = ∫{a (x, y, z) * h (x, y, z) } e 
dy + s (x , z)
= Ε { (x , z) }
DDSR
attenuation
scatter
factor
期望值
y
x
Gamma
Camera
Light guide
Scintillator
Light photon
Light Guide
PMT photocathod
PMTs
 Convert a light photon to electrical charges
scintillator
light guide
dynodes
anode
e–
light photon
photocathode
 106 e–’s
10 ~ 12 dynodes
一般約 30% photons 可經 light guide 到 PMTs
Output
signal
Pulse Processing: Pre-Amplifying
PMT
C
R
50μs
250 ns
 Preamplifier (preamp):
Preamplifier
500μs
– To match impedance levels to subsequent
components
– To shape the signal pulse (integration)
• RC = 20~200μs
– To (sometimes) amplify small PMT outputs
– Should be located as close as possible to the PMT
Pulse Processing: Amplifier
PreAmp
 Amplifier
Amplifier
– To amplify the still relatively small signal
– Perform pulse shaping
• Convert the slow decaying pulse to a narrow one
• To avoid pulse pile-ups at high counting rates
Positioning logic (Anger)
Y+
……
X+
X-
Position determination
PMT array
Y-
X = X+ + XY = Y+ + YZ = X+ + X- + Y+ + Y-
X+
XY+
Y-
Anger Positioning logic
NaI (Tl)
Detectors
Positioning
logic circuit
Position determination
X  k (X＋+X－)/Z
Y  k (Y＋+Y－)/Z
A PHA (pulse height analyzer) is to select for counting
only those pulses falling within selected amplitude
intervals or “channels”
A SCA (single channel analyzer) is a PHA having only
one channel:
SCA
Z
X Y
PHA
ULD: upper level discriminator
Gating
signal
LLD: lower level discriminator
A/D
Analog System
Crystal
Collimator
SUM
X/Z
X
SUM
Y/Z
Y
SUM
PMT
array
PreAMPs
Anger
Registor
matrix
Z
energy
Summed analog outputs
Digital System
ADCPMT
Crystal
Collimator
PMT
array
SUM
ADC
SUM
ADC
SUM
ADC
SUM
ADC
SUM
ADC
SUM
ADC
SUM
ADC
SUM
ADC
SUM
ADC
SUM
ADC
PreAMPs Analog to Digital
Converters
PMT BUS
Programmable
Digital Event
Processor
Individual PMT data
to digital event
processor
PSPMT
 position sensitive PMT
– essentially light guide is not necessary
– perform multi-positioning within one PMT
X
X
Y
Y
Z
SPECT scanner
 Multi-head systems:
– 1. Provide higher sensitivity
– 2. Allow simultaneous emission and transmission scans
– 3. More expensive
Performance Characteristics:
 Image Non-linearity
– straight lines are curved
– X and Y signals do not change linearly with the
distance of the detected events
• variations in PMT collection efficiency acrossing its aperture
• variations in PMT sensitivity
• non-uniformities in optical coupling, etc.
 Image Non-uniformity
– flood field-image shows variations in brightness
– non-uniform detection efficiency and nonlinearities
• differences in pulse-height spectrum of the PMTs
Performance Characteristics:
Spatial Resolution
– overall resolution R2 = Ri2 + Rc2
– affecting image contrast and visualization of small structures
– introduce bias
– intrinsic resolution Ri
•
•
•
•
•
•
•
crystal thickness (light distribution)
crystal density, effective Z number (multiple scattering)
light yield (statistical variations in pulse heights)
degraded with decreasing -ray energy (light yield)
improves with increased light collection and detection efficiency
improves with image uniformity and digital positioning
expected resolution limit for NaI (Tl) = 2mm
– collimator resolution Rc
• collimator design
• source to collimator distance
Performance Characteristics: cont’d
 Detection Efficiency:
– Crystal thickness, density, effective Z number
• almost 100% at up to 100 keV, but drops rapidly with
increasing energy to about 10~20% at 500 keV
– Collimator efficiency
– affecting image noise
– introduce variance while quantitative studying
 100 ~ 200 keV is the best optimal energy of Anger
camera (-ray)
– at low energy, deteriorating spatial resolution
– at high energy, deteriorating detection efficiency
Performance Characteristics:
Count rate:
baseline shift
– Mis-positioning
• baseline shift
pile-up
• pile-up
• simultaneous detection of multiple events at different
locations
nonparalyzable
ideal
paralyzable
real
– dead time
• 0.5~5s
True count rate, Rt
• behaves as nonparazable model: 2nd event ignored if
it occurs during the deadtime of the preceding events
SPECT reconstruction:
 Issues: attenuation, scatter, noise, DDSR, sampling geometry
 Filtered Backprojection (FBP)




– ignore attenuation, DDSR
– usually no scatter correction
– ad hoc smoothing for controlling image noise
Iterative Reconstruction
– OSEM
– allow attenuation, and DDSR corrections
– optimal noise control
– usually no scatter correction
– needs attenuation map
Analytical approaches uniform attenuation
Simultaneous Emission, Attenuation map Reconstruction
Dynamic SPECT by interpolation vs. timing
Newer developments:
 Coincidence Imaging (PET like)
– Low cost
– Poor sensitivity and resolution
–  ray septa penetration
 Simultaneous Transmission and Emission Imaging
– Registered attenuation map
– Spill-down scatters from the transmission source
– Truncation error remains unsettled ……………………..
 Dual Isotope Imaging
– Increase diagnosis specificity
– Issues: spill-down scatters from high to low energy window
Newer developments: cont’d
 Small-animal gamma camera
– Small FOV, higher resolution
 Depth-of-interaction (DOI) detectors
– Better spatial resolution
– Allow use of thicker NaI crystal
 Semi-conduction imager
exit window
entrance window
reflectors
P M T
NaI (Tl)
detector
– Converts  ray directly into electrical signals
– Promising candidate: CdZnTe detector
readout IC
 Novel designs
– Scintimammography
• Placed closer to the source by odd geometry
• Optimizing resolution & sensitivity
fiber
Indium
hump bonds
Newer developments: cont’d
 Novel designs
– CERESPECT
• A single fixed annular NaI (Tl) crystal completely surrounding the
patient’s head
• A rotating segmented annular collimator
– Modular systems:
– SPRINT II brain SPECT
• 11 modules in a circular ring around the patient’s head, each module
consists of 44 one-dimensional bar NaI (Tl) scintillation camera
• Rotating split or focused collimators
– FASTSPECT
• A hemispherical array of 24 modules for brain imaging
• Each module views the entire brain through one or more pinholes
• Stationary system, easy dynamic imaging