Upload
laurel-fox
View
214
Download
1
Embed Size (px)
Citation preview
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
Scattered (Campton)
Absorbed (Photoelectric)
Non-attenuated
Gamma rays
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
H
S
Rc
L
Rc/(H+L) = S/H Rc L
Septa thickness = T
Resolution v.s. Distance
resolution
Source to collimator distance
High sensitivity
General purpose
High resolution
Collimators
Rc
Septal thickness T is determined by photon energy
– low-energy collimator < 150 keV
– medium-energy collimator < 400 keV
Typical Performance Characteristics
Collimator Types
Suggested Max. Energy
Efficiency Resolution at 10cm
Low energy, High resolution
150 keV 1.84×10–4 7.4 mm
Low energy, General purpose 150 keV 2.68×10–4 9.1 mm
Low energy, High sensitivity
150 keV 5.74×10–4 13.2 mm
Medium energy, High sensitivity
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
– 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.
Conduction band
Valence band
Ioni
zati
on
ener
gy Activator excited statesActivator ground state
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: 230s after glow– Dead time– Position mis-positioning– Wavelength at max. emission: 415 nm
Reflective index: 1.85– Hygroscopic, relatively fragile
Inorganic Scintillators (Crystals)Scintillato
rWave length
Decay constant (ns)
Refraction index
Density (g/cm3)
Light yield
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 ?
Detector response vs. Energy resolution
Output signal amplitude proportional to energy deposited in the scintillator
Energy resolution = 100% Complete electron transfer (ideal condition)
Cou
nt
Photon energyEo/(1+2Eo) Eo
Scatter photon
Non-scatter photon
Photofraction (real condition)
Cou
nt
Photon energyEo / (1 + 2Eo) Eo
Scatter photon
Non-scatter photon
Spreading due to Poisson effect
FWHM
Factors affecting Energy resolution:
Counting statistics + Electronic noise– Causes uncertainty in measured deposited energy
Poisson Statisticsg(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
(x) = ∫{a (x, y, z) * h (x, y, z) } e dy + s (x , z) = Ε { (x , z) }
–∫ (x, y’)dy’
y
radioactivitydistribution
attenuationdistribution
attenuationfactor
scatterDDSR期望值
Gamma Camera
x
y
PMTs
Convert a light photon to electrical charges
scintillator light guide
light photon photocathode
dynodes
10 ~ 12 dynodes
106 e–’s
anode
Outputsignal
e–
一般約 30% photons 可經 light guide 到 PMTs
Pulse Processing: Pre-Amplifying
Preamplifier (preamp):– 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
PMT
50μs
500μs250 ns
C R
Preamplifier
Pulse Processing: 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
PreAmp Amplifier
Positioning logic (Anger)
X- X+
Y-
Y+
PMT arrayX = X+ + X-
Y = Y+ + Y-
Z = X+ + X- + Y+ + Y-
X+
X-
Y+
Y-
……
Position determination
Anger Positioning logic
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:
NaI (Tl)
Detectors
Positioning logic circuit
PHA
A/D
ZX Y
Gatingsignal
SCA
ULD: upper level discriminator
LLD: lower level discriminator
Analog System
Collim
ator C
rystal
PMT array
PreAMPs
SUM
SUM
SUM
AngerRegistormatrix
X/Z
Y/Z
X
Y
Z energy
Summed analog outputs
Digital System
Collim
ator C
rystal
PMT array
PreAMPs
ADC
ADC
ADC
ADC
ADC
ADC
ADC
ADC
ADC
ADC
Analog to DigitalConverters
ADCPMTPMT BUS
Programmable Digital Event
Processor
Individual PMT data to digital event
processor
SUM
SUM
SUM
SUM
SUM
SUM
SUM
SUM
SUM
SUM
PSPMT
X
X
Y
Y
Z
position sensitive PMT– essentially light guide is not necessary– perform multi-positioning within one PMT
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 distanc
e 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 came
ra (-ray)– at low energy, deteriorating spatial resolution
– at high energy, deteriorating detection efficiency
Performance Characteristics:
Count rate:– Mis-positioning
• baseline shift• pile-up• simultaneous detection of multiple events at differen
t locations
– dead time• 0.5~5s• behaves as nonparazable model: 2nd event ignored if
it occurs during the deadtime of the preceding events
ideal
real
baseline shift
pile-up
True count rate, Rt
Obs
erve
d co
unt r
ate,
Ro
nonparalyzable
paralyzable
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– Converts ray directly into electrical signals
– Promising candidate: CdZnTe detector
Novel designs– Scintimammography
• Placed closer to the source by odd geometry• Optimizing resolution & sensitivity
M TPexit window
entrance window
detectorIndium
hump bondsreadout IC
NaI (Tl)
reflectors
fiber
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