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Nuclear Medicine Physics
•
Gamma Camera, Scintillation Camera
Jerry Allison, Ph.D.
Department of Radiology
Medical College of Georgia
A note of thanks to
Z. J. Cao, Ph.D.
Medical College of Georgia
And
Sameer Tipnis, Ph.D.
G. Donald Frey, Ph.D.
Medical University of South Carolina
for
Sharing nuclear medicine presentation content
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)
Basic principle
 g- rays directed towards a scintillation crystal -
NaI(Tl)
 Multiple PMTs detect light flashes
 Signal ( E) is converted to electrical pulses
 Pulses fed to energy discrimination and
positioning circuits
 Image of radionuclide distribution formed and
displayed
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Nuclear medicine is emission imaging.
 g photons are emitted from inside of patient.
 g energy: 70 to 511 keV
 Relatively 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 imaging
BUT: Nuclear medicine is molecular
imaging
 Interaction of the radiopharmaceutical with
cells or molecules  molecular imaging


Bound directly to a target molecule (111Inmonoclonal 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 diagnosis
Major components of gamma camera
Pulse Height
Analysis
position
analysis
X
Y
Z
co m p ut e r
amplify & sum
p re - a m p
P M T
NaI(Tl) crystal
d is p la y
c o llim a t o r
p a t ie n t
Gamma Camera Components
8
Major components of gamma camera
 Collimator

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 patient
Why collimator? – image formation
w/o collimator
with collimator
images
image
detector
collimator
sources
Image of a point
source is the
whole detector.
Image of a point
source is a point.
Why collimator? – image formation
 to establish geometric
relationship between the
source and image
 The collimator has a
major affect on gamma
camera count rate and
spatial resolution
parallel-hole collimator
Parallel-hole collimator
 A collimator with small diameter
holes ‘d’ or long holes provides
good resolution but few counts
and hence noisy image
 Design principle: to optimize the
trade-off between counts and
resolution
 Thickness of lead between
collimator holes (septal thickness)
‘t’ must make septal penetration
less than 5%
t
d
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 < Eg < 300 keV)
 high-energy all purpose (HEAP) collimator
for I-131 (Eg = 361 keV)
Collimators
• Most often used: parallel-hole collimator
• For thyroid: pin-hole collimator
• For brain and heart: converging collimator
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Pinhole collimator
 single hole admitting photons
 low efficiency and small FOV but potentially
excellent resolution
 Decreasing source-to-
detector distance leads to
 larger image,
 better resolution
 higher count rate.
Detection of g photons in detector
 An incident g photon may be stopped
(absorbed) by or penetrate the detector

more penetration with higher photon energy
 g photons recorded as counts (electrical
pulses)
 Counts represent concentration and
distribution of radioactivity in the patient
A: absorption
p.e
A: absorption
p.e
B: penetration
c.s
c.s
c.s
B
A
16
Detection of g photons in detector
 The 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.
 Scatter in detector make it impossible to know
the entry point of the g photon.
17
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 state by emitting blue
light (~ 3 ev)
p.e
c.s
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 photon

e.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 pulse height is proportional to g
photon energy deposited in the crystal
Desirable Scintillator Properties
 High , Z  high absorption efficiency
 Improves detector sensitivity
 High light output (conversion efficiency)
 Improves energy discrimination, spatial resolution
 Light output proportional to energy deposited
 Improves linearity
 Transparent to light emissions
 Improves sensitivity
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
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)
21
How good (bad) is NaI (Tl) detector?
 relatively dense, high Z (~ 55)
 good conversion efficiency: ~ 26 eV/blue
photon
 good transparency for blue photons
 blue photons matched with PMTs
photocathode sensitivity
 Compton scatter dominates at Eg > 250 keV
 poor spatial resolution
 fragile and hygroscopic (can absorb water,
turn yellow)
 Hermetically sealed
Photomultiplier tube
Create and amplify electric pulses
 photocathode (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:
610  6 × 107
 very efficient
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 guides
anode connected
to electronics in
the tube base
ultrasensitive to
magnetic field
Weighting factors for
19 tube camera
Y+
16
15
17
6
18
7
19
X-
X+
14
5
13
1
4
12
2
3
11
Y-
8
9
10
Tube
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
X+
20
30
25
15
10
15
25
40
35
30
20
10
05
00
05
10
20
30
35
X20
10
15
25
30
25
15
00
05
10
20
30
35
40
35
30
20
10
05
Y+
20
20
10
10
20
30
30
20
10
40
40
40
10
20
30
40
40
40
30
Y20
20
30
30
20
10
10
20
30
00
00
00
30
20
10
00
00
00
10
• Each PMT provides a weighted X+, X-, Y+ and Y- signal
Energy Signal
Z=
+
x
+
x +
+
y
-
y
• The outputs from all the PMT’s are
summed to estimate energy deposited
Event Location
+
X
=
+
x
-
x
-
y
Z
+
Y
=
+
y
Z
• The X, Y outputs from all the PMT’s are
summed to estimate the center of scintillation
Pulse height analyzer
selects Z pulses of certain voltage amplitudes
 to discriminate against unwanted
(scattered) g photons
V2 (154 keV)
V1 (126 keV)
1
2
3
Absorbed energy spectrum of detector
Penetration/scatter
: energy deposited
in detector is
between 0 and E0.
energy window
photopeak: all
energy of g
photons (E0)
deposited in
detector
Photopeak
All the energy of a g photon (E0) is
deposited in the detector
e.g. E0 = 140 keV for Tc-99m
p.e
p.e
c.s
or
30
Image Formation (Photopeak)
counted
Stops > 99.95% of g s
stopped
Typical efficiency of a LEHR collimator ~ 0.02 %
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Penetration/scatter spectrum
Some of the energy of a g photon (E0)
is deposited in the detector
c.s
c.s
30 keV x-ray
p.e
p.e
x-ray
p.e
32
Scatter
 Major source of image degradation in NM
 Increases image noise and reduces lesion
contrast
 Windowing the photopeak allows suppression
of scatter events (but not complete
elimination)
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Scatter in patient
Photopeak
scatter
Image degradation
Septal penetration
Counted
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Image degradation
Simultaneous detections
detected
2015
detected
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Image degradation
Scatter
detected
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
System spatial resolution
Rsys =
2
R 2int + R col
system resolution Rsys
intrinsic (detector) resolution Rint
collimator resolution Rcol
Rint typically 2.9mm to 4.5mm
Rcol typically 7.4mm to 13.2mm
Rsys typically 1cm
Collimator Resolution
Spatial resolution degrades with increasing pt – collimator distance.
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
Effect of  coll-to-pt distance
Type
Parallel hole
Converging
Diverging
Pinhole
Spat. Res.




Efficiency




FOV




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, DABR
Gamma camera energy resolution
 energy 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 counts
Data acquisition
 collimator: match the radioisotope
 energy window: match the radioisotope
 pixel size: 1/3 ~ 1/2 of spatial resolution

detector size
matrix size 
pixel size
 usually, 64×64, 128×128 or 256×256
 2 bytes in pixel depth
 count rate < 20000/sec
 patient close to the detector
Effect of matrix size
64×64
128×128
Planar NM Imaging
2015
Nuclear Medicine Physics for Radiology Residents
Sameer Tipnis, PhD, DABR
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
counts
Uniformity
a collimator defect
a bad PMT
shift of energy peak
Bar phantom
 made 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 collimator
 intrinsic: take collimator off
and place a Tc-99m point
source 5 × detector size away
pt source
g ray
bar
detector