Nuclear Medicine Nuclear Medicine

InstrumentationInstrumentation

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

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

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 ?

Crystal vs. Light yield

Light

yield

Wavelength (nm)

NaI (Tl)

CsI (Na) CsI (Tl)

410

565420

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

Light guide

Scintillator Light Guide

Light photon PMT photocathod

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