Nuclear Medicine Physics - PowerPoint Presentation

Slide1

Nuclear Medicine Physics

Jerry Allison, Ph.D.Department of RadiologyMedical College of Georgia

Gamma Camera, Scintillation CameraSlide2

A note of thanks to Z. J. Cao, Ph.D.Medical College of GeorgiaAnd

Sameer Tipnis, Ph.D.G. Donald Frey, Ph.D.Medical University of South Carolina

forSharing nuclear medicine

presentation contentSlide3

How to obtain a NM image?Administer radiopharmaceutical (a radionuclide labeled to a pharmaceutical)The radiopharmaceutical

is concentrated in the desired locations.Nucleus of the radionuclide decays to emit g photons

Detect the g photons using a “gamma camera” (scintillation camera, Anger camera)Slide4

Basic principle- rays directed towards a scintillation crystal - NaI(Tl)Multiple PMTs detect light flashes

Signal ( E) is converted to electrical pulsesPulses fed to energy discrimination and positioning circuitsImage of radionuclide distribution formed and displayed2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide5

Nuclear medicine is emission imaging.g photons are emitted from inside of patient.

g energy: 70 to 511 keVRelatively poor image quality due to limited photon number (severe image noise) and poor spatial resolution

Image noise caused by low count density (105 – 106 lower than x-ray imaging)CT is transmission imagingSlide6

BUT: Nuclear medicine is molecular imagingInteraction of the radiopharmaceutical with cells or molecules

 molecular imagingBound directly to a target molecule (111In-monoclonal antibody)Accumulated by molecular or cellular activities (

18F-FDG, 99mTc-sestamibi, 131I

)Molecular or cellular activities (e.g. perfusion for heart, brain, kidney, lungs and metabolism of cancers)



earlier diagnosisSlide7

Major components of gamma camera

p

a

t

i

e

n

t

c

o

l

l

i

m

a

t

o

r

NaI

(Tl) crystal

P

M

T

p

r

e

-

a

m

p

a

mplify

& sum

p

osition

analysis

Pulse Height Analysis

c

o

m

p

u

t

e

r

d

i

s

p

l

a

y

X

Y

ZSlide8

Gamma Camera Components8Slide9

Major components of gamma cameraCollimator

to establish position relationship between g photon source and detector (projection imaging)Scintillation detector (

NaI(Tl))to convert g photons to

blue light photons Photomultiplier tube (PMT)to convert blue photons to electrons and to increase the number of electrons

Electronics

Pulse Height Analysis: estimates energy deposited in each detection (enables scatter rejection)

Position Analysis: center of luminescent intensity

Display

display distribution of radioactivity in patientSlide10

Why collimator? – image formation

Image of a point source is the whole detector.

detector

sources

images

I

mage

of a point

source is a point.

w/o collimator with collimator

image

collimatorSlide11

Why collimator? – image formationto establish geometric

relationship between the source and imageThe collimator has a major affect on gamma camera count rate and spatial resolution

p

arallel-hole collimatorSlide12

Parallel-hole collimatorA collimator with small diameter holes ‘d’ or

long holes provides good resolution but few counts and hence noisy imageDesign principle: to optimize the trade-off between

counts and resolutionThickness of lead between collimator holes (septal thickness) ‘t’ must make septal penetration less than 5%Slide13

Different parallel-hole collimators

low-energy all purpose (LEAP) collimator (Eg < 150 keV)  better efficiency but worse resolution

low-energy high resolution (LEHR) collimator (Eg < 150 keV)

 better resolution but worse efficiency medium-energy all purpose (MEAP) collimator (150 keV <

E

g

< 300

keV

)

high-energy all purpose (HEAP)

collimator for I-131 (

E

g

= 361

keV

) Slide14

Collimators

2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

Most often used: parallel-hole collimatorFor thyroid: pin-hole collimatorFor

brain and heart: converging collimatorSlide15

Pinhole collimatorsingle hole admitting photons low efficiency and small FOV but potentially excellent resolution

Decreasing source-to- detector distance leads tolarger image,

better resolutionhigher count rate.Slide16

Detection of g photons in detectorAn incident g photon may be stopped (absorbed) by or penetrate the detectormore penetration with higher photon energy

g photons recorded as counts (electrical pulses)C

ounts represent concentration and distribution of radioactivity

in the patient16

B

A

B:

penetration

p.e

c.s

c.s

c.s

A: absorption

A: absorption

p.eSlide17

Detection of g photons in detectorThe

pulse height is determined by the energy deposited by a g

photon in the detector.

A penetrating g photon deposits less energy so

the

electrical pulse

is

smaller

.

A photon

scattered

in

the patient

loses energy

so the pulse is

smaller when it

is detected.

S

catter in detector make it impossible to know the entry point of the

g

photon.

17Slide18

Scintillation process in detector Most detectors are

~3/8” of NaI (Tl). Tl (activator) facilitates scintillation at room temperature

As a g photon creates ionizations in the detector

Ionizations free e-

from the atom to create ion-e

-

pairs

The ion-e

-

pairs excite Tl atoms.

Tl atoms return to ground s

tate

by emitting

blue light (~ 3

ev

)

p.e

c.sSlide19

Scintillation process in detector

The detector converts g photons to a number of blue photons.

The number of blue photons is proportional to the energy deposited by g

photone.g. 140 keV  5000 and 70

keV

 2500 blue photons

The number of blue photons determines the number of electrons liberated in the photocathodes of PMTs and in turn, the

electrical

pulse height.

Electrical p

ulse

height is proportional to

g

photon energy

deposited in the crystalSlide20

Desirable Scintillator PropertiesHigh , Z  high absorption efficiencyImproves detector sensitivityHigh light output (conversion efficiency)Improves energy discrimination, spatial resolution

Light output proportional to energy depositedImproves linearityTransparent to light emissions Improves sensitivity2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide21

How good (bad) is NaI (Tl) detector?Good

stopping power for low-energy g photons by photoelectric process

at 69 keV, penetration

 0% for a thickness (t) of 3/8”at 140 keV

,

penetration

= 7.7% for t

of 3//8”

at

247

keV

,

penetration

= 48.5% for t

of 3/8”

Slow scintillation decay

(230 ns)

which limits count

rate (avoid pulse pile-up)

21Slide22

How good (bad) is NaI (Tl) detector?relatively dense, high Z (~ 55)

good conversion efficiency: ~ 26 eV/blue photongood

transparency for blue photonsblue photons matched with PMTs photocathode sensitivity

Compton scatter dominates at Eg > 250 keV  poor spatial resolution

f

ragile

and h

ygroscopic (can absorb water, turn yellow)

Hermetically sealedSlide23

Photomultiplier tube Create and

amplify electric pulsesphotocathode (CsSb

): to convert blue light to e-

9 - 12 dynodes: each to increase electrons

3 – 6 times

anode: to collect e

-

gain in

e

-

number:

6

10



6 × 10

7



very efficient Slide24

Photomultiplier tube (PMT)40 to 100 PM tubes (d = 5 cm) in a modern gamma camera

photocathod directly coupled to detector or connected using plastic light guidesanode connected to electronics in the tube base

ultrasensitive to magnetic field Slide25

1

2

85

147

6

15

19

18

17

16

3

4

13

9

10

11

12

Y-

X-

X+

Y+

Weighting factors for

19 tube camera

Each PMT provides a weighted X

+

, X

-

, Y

+

and Y

-

signalSlide26

Energy SignalThe outputs from all the PMT’s are summed to estimate energy deposited

Z = x

+

+ x

-

+ y

+

- y

-Slide27

Event Location

X

+

=

x

+

- x

-

Z

Y

+

=

y

+

- y

-

Z

The X, Y outputs from all the PMT’s are summed to estimate the center of

scintillationSlide28

Pulse height analyzer selects Z pulses of certain voltage amplitudes 

to discriminate against unwanted (scattered)  photons

1

2

V

2

(154 keV)

V

1

(126 keV)

3

Slide29

Absorbed energy spectrum of detector

energy window

photopeak: all energy of g photons (E

0) deposited in detectorPenetration/scatter: energy deposited in detector is between 0 and E

0

. Slide30

Photopeak All the energy of a g photon (

E0) is deposited in the detectore.g. E0 = 140 keV

for Tc-99m30

p.e

p.e

c.s

orSlide31

Image Formation (Photopeak)

counted

stopped

Stops

> 99.95% of



s

Typical efficiency of a LEHR collimator ~ 0.02 %

2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

USEFUL FOR IMAGINGSlide32

Penetration/scatter spectrum32

c.s

c.s

p.e

x-ray

p.e

p.e

30 keV x-ray

Some of

the energy of a

g

photon (E

0

) is deposited in the detector

NOT USEFUL FOR IMAGINGSlide33

ScatterMajor source of image degradation in NMIncreases image noise and reduces lesion contrastWindowing the photopeak allows suppression of scatter events (but not complete elimination)

2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide34

Scatter in patient

scatter

PhotopeakSlide35

Image degradation

Counted

2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSeptal penetrationSlide36

Image degradation

detected

detected2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSimultaneous detectionsSlide37

Image degradation

detected

2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRScatterSlide38

System spatial resolution

system resolution Rsys intrinsic (detector) resolution R

int collimator resolution Rcol

Rint

typically 2.9mm to 4.5mm

R

col

typically 7.4mm to 13.2mm

R

sys

typically 1cmSlide39

Collimator Resolution

Spatial resolution degrades with increasing

pt

– collimator distance.

2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide40

Effect of  coll-to-pt distance

TypeSpat. Res.

Efficiency

FOV

Parallel hole







Converging







Diverging







Pinhole







Increasing collimator to

pt

distance ALWAYS degrades spatial resolution

Parallel hole collimator has very favorable properties

This is the main collimator used in NM

2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide41

Gamma camera energy resolutionenergy spread due to

fluctuation of the blue photon number in the detector, and fluctuation of electric signal in subsequent electronics Energy resolution determines the width of the energy window.

Typical system energy resolution: 9 – 11%

Typical clinical energy window: 20

%,

140±10

%

keV

,

126 – 154

keV

better energy resolution



smaller energy window



acquiring most of the

photopeak

counts but fewer scatter countsSlide42

Data acquisitioncollimator: match the

radioisotope energy window: match the radioisotopepixel size: 1/3 ~ 1/2 of spatial resolution

usually, 64×64, 128×128 or 256×2562 bytes in pixel depth

count rate < 20000/secpatient close to the detectorSlide43

Effect of matrix size

64

×64 128×128Slide44

Planar NM Imaging2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide45

Quality control of gamma camera uniformity: daily, 256

×256, > 4M counts resolution: weekly, 512

×512, > 4M counts

acquisition of new uniformity maps and possible energy map: quarterly, > 30M countsSlide46

Uniformity a collimator defect a bad PMT shift of energy peak Slide47

Bar phantommade of lead stripes with different orientations and spacing in 4 quadrants

to measure extrinsic and intrinsic linearity and spatial resolution extrinsic: place a Co-57 sheet source with the bar phantom on the top of the collimatorintrinsic: take collimator off and place a Tc-99m point source 5 × detector size away

detector

bar

pt

source

g

ray