Health System
RADIOLOGY RESEARCH
Henry Ford
NERS/BIOE 481
Lecture 09Nuclear Medicine Detectors
Michael Flynn, Adjunct Prof
Nuclear Engr & Rad. Science
2NERS/BIOE481 - 2014
- General Models
Radiographic Imaging: Subject contrast (A) recorded by thedetector (B) is transformed (C) to display values presented (D)for the human visual system (E) and interpretation.
A
B
Radioisotope Imaging: The detector records the radioactivitydistribution by using a multi-hole collimator.
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V.B.1 – The Anger Camera (10 charts)
B. Nuclear Medicine Detectors
1. Physical Design of the Anger camera
a. Basic components and circuits.
b. The scintillator crystal.
c. Optical PMT coupling
d. PM tubes
e. Detector gantry.
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V.B.1.a – Basic principles and components
• The Anger camera computes anx,y position for each detectedx-ray and increments the countin a corresponding image pixel
• Only events with a full energysum (Z) in the photo-peak areprocessed.
CorrectionTables
computer
Position & Summing Circuits
PulseHeight
Analyzer
Gate
X
Y
Z=energy
colm
row
display
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V.B.1.b – The scintillation crystal.
A. A NaI scintillation crystal with thallium doping istypically ¼ to ½ inches thick.
B. The peripheral boundaries are coated with a granularwhite material to promote reflection of light.
C. The front surface has an optical quality glass sheetfrom which emitted light is detected.
D. The entire assembly is sealed to prevent moisturefrom degrading the crystal material
B
C
D
A
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V.B.1.c – Optical Photomultiplier tube (PMT) coupling.
A. Photomultiplier tubes are coupled to the glasswindow using a coupling grease that has an index ofrefraction designed to maximize the transfer oflight from the crystal thru the glass to the PMT
B. In early designs, filters were place on the glasssurface to shape the response of the PMTs.
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V.B.1.c – Individual PMT response vs scintilation position.
The response of an individual PMT varies withthe relative position of the scintillation event
A
X, mm
PM
TA
response
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V.B.1.c – PMT placement
• PMTs are placed in anhexagonal array oncircular or rectangularcrystal assemblies
• Preamplifier andanalogue to digitalconverter circuits aremounted on the back ofeach PMT and signalrouted to the positionand summing analyzer.
(Ge Millenium, MUSC)
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V.B.1.d – PMT basic principle
• A photomultiplier tube is a vacuum tube consisting of an input window, aphotocathor and an electron multiplier sealed into an evacuated glass tube.
• Light passes through the input window and excites the electrons in thephotocathode so that photoelectrons are emitted into the vacuum.Photoelectrons are accelerated and focused by the focusing electrode ontothe first dynode where they are multiplied by means of secondary electronemission. This secondary emission is repeated at each of the successivedynodes.The multiplied secondary electrons emitted from the last dynodeare finally collected by the anode.
Ham
mam
ats
uPM
TH
andb
ook
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V.B.1.e – Gamma camera detector assembly
• The crystal and PMTassembly is surroundedby ‘mu’ metal to minimizethe influence ofmagnetic fields.
• The assembly is thenmounted in a leadshielded cabinet.
(photos from MUSC)
Detail view showing lead shielding
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V.B.1.e – Gamma camera detector assembly
• The detector assemblyis often mounted in agantry providingcircular rotation forSPECT examinations.
• Reduced examinationtime is achieved byusing two detectors.
Siemens Symbia E Dual
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V.B.1.e – Gamma camera detector assembly
GE Discovery 670 SPECT/CT
Philips SPECT/CT
Dual Head SPECTimaging systems
integrated with anx-ray CT system
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V.B.1.e – Gamma camera detector assembly
Breast - GE Discovery NM750b
Heart - GE Discovery NM530c
NM systemsdesigned for
imaging specificbody parts
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V.B.2 – Position estimation (19 charts)
B. Nuclear Medicine Detectors
2. Position estimation for Anger cameras
a. Response from one PMT
b. Weighted sum estimation
c. Resolution (error in position estimate)
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V.B.2 – Position estimation
The precise estimation of the position of a detectedevent by using the relative responses of a small set ofdetectors with poor resolution is fundamental to theoperation of nuclear medicine and PET imaging devices.
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V.B.2.a – Response from one PMT
The numberof electronsrecorded bya PMT as afunction ofthe positionof incidentgamma rays,x, is peakedat thecenter ofthe PMTand extendsbeyond it’sedges.
PMT 4 response versus input positon
0
100
200
300
400
500
600
-250 -200 -150 -100 -50 0 50 100 150 200 250
x - mm
N4(x
)-
ele
ctr
on
s
Response of a single 50 mm PMT based on it’s solid anglerelative to a detector plane 50 mm away (Barrett eq. 5.188)
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V.B.2.a – Responses from a set of PMTs
The x and ypositionestimatesare usuallydeterminedfrom 1Danalysisbased onthepositions ofthe rowsand columnsof PMTs
The response of 8 columns of PMTs with 50 mm spacingis illustrated using the shape from the prior slide.
PMT responses versus input positon
0
100
200
300
400
500
600
-200 -150 -100 -50 0 50 100 150 200
x - mm
Nn
(x)
-ele
ctr
on
s
1 2 3 4 5 6 7 8
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Position Estimate, Adjusted outside PMT weight
-200
-150
-100
-50
0
50
100
150
200
-200 -150 -100 -50 0 50 100 150 200
x - mm
U(x
),m
m
V.B.2.b – Centroid position estimate
A position estimatebased on the linearlyweighted sum of theeight PMT responsesdeviates slightly froma linear relation.
An improved estimateis obtained byincreasing theweights of theoutside tubes by 1.34
175,125...75,125,175
8,1
)()(
n
n
xnnx
w
NwUNote: Nn(x) from the priorslides has beennormalized by 0.56/500
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V.B.2.b – Centroid position estimate
The sum of the signalfrom all PMTs is usedto estimate the totalenergy depositionand identify thephotopeak for gammarays of a specificenergy..
An improvedestimate with 9%variation is obtainedby increasing theweights of theoutside tubes by 1.34
8,1
)()(
n
xnx NENote: E(x) is computed here in terms of the
number of electrons collected. The total ofabout 960 electron would have a standarddeviation of 3.2% and FWHM of 2.35*3.2 =6.5% which is someone less that usual.
PMT sum signal
0
200
400
600
800
1000
1200
-200 -150 -100 -50 0 50 100 150 200
x - mm
E(x
)-
ele
ctr
on
s
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V.B.2.b Original Anger Camera
• The original Angercamera designs usedpassive electroniccircuits to computeposition from weightedPMT signals.
• A 7 PMT design isshown (Anger 1958) inwhich the charge fromeach PMT is collectedon a capacitor. Thecapacitance value isproportional to thePMT weight.
From Hine, Vol 1, 1967
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V.B.2.b – Transformed PMT response
• A common priordesign used 37 PMTsof 2” or 3” diameterarranged over acircular NaI crystal.
• This photo illustratesa masking plate usedto position each PMT.
• The shape of the PMT response curve can be altered by amask pattern placed between the PMT and the NaI crystal.This can improve linear of the position estimates.
• A disadvantage is that the number of collected electrons isreduced and the precision of the position estimate is worse.
• Non-linear weighting circuits using delay-line elements wereintroduced to shape the response without loss of resolution.
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V.B.2.b – Digital camera logic
All recent Anger cameradesigns have used digital logiccircuits to compute the X andY position of each event andthe Energy in relation to adefined energy window.
CorrectionTables
computer
colm
row
display
ADC ADC ADC ADC ADC ADC ADC
Programmable Digital
Event Processor
XY Position and Energy
Figure adapted from slide #3
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V.B.2.b – Uniformity correction
United States Patent 4,212,061
Knoll et. al. July 8, 1980
RADIATION SIGNAL PROCESSING SYSTEM
Inventors: Glenn F. Knoll, Ann Arbor;
Donald R. Strange, Howell;
Matthew C. Bennett, Jr, Ann Arbor, MI.
“Coordinate signals X, Y arecorrected to their truecoordinate U, V valuesrespectively by accessingtranslation tablerectangular matrix arrayscontaining U,V valuesaddressed by theirrespective correspondingX,Y coordinates, ..”
ADC X
ADC Y
Dx
U = X + Dx
V = Y + Dy
Increment at U,VDy
Vx
Vy
U
V
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PMT 4 response, +/- 2 std dev
0
100
200
300
400
500
600
-150 -100 -50 0 50 100
x - mm
N4
(x)
-e
lec
tro
ns
V.B.2.c – Resolution
Statistical variationin the number ofelectrons detectedcause variation in theestimated position X.
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V.B.2.c – Resolution
• Variation in the estimate position (x, y) of a detected gammaray results from statistical noise in the number of electronscollected from each PMT.
• For the centroid estimate shown in slide 21, the estimated Xposition comes from the observed set of PMT electron signals(Ni, i=1,8) . In the estimate below this is X rather than U(x) .
• The variance of this is computed using propagation of error.
• The spatial resolution in FWHM is then equal to 2.35sx
8,1
22
8,1
iiix
iii
Nw
NwX
The FWHM based on the weights usedearlier is shown at the left. Poor resolutionat the sides results from the high weightvalues for the outside tubes. This can beavoided by using local estimates with weightsadjusted for the approximate position
4 mm FWHM is typical forearly design Anger cameras.
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V.B.2.c – PMT Position Probability distribution
If the mean number of electrons observed from PMT i for
a gamma ray detected at position x is Mi(x) ,
then the conditional probability of observing ni electrons,
P[ni|x] , is expected to follow a Poisson distribution as wasdiscussed in lecture 05 (or the approximate Gaussian).
X
Mi
22
)(
)(
2
1
!
)(|
n
inxMi
n
i
xMi
ii
e
n
enxMxnP
i
P[ni|x]
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• If we consider the conditional probabilities associated witheach PMT, then the likelihood expression is defined as theproduct of each.
• The maximum likelihood in relation to x can then be taken asas an optimal estimate of the photon interaction position.
• It has been shown that maximizing the log likelihood isequivalent to maximizing the likelihood. This is done byfinding the value of x for which the derivative is zero. Usingthe Poisson distribution, this can be written as.
V.B.2.c – Maximum Likelihood
j
i
ij xnPxnnnP1
1 ||,,2,
0)(
)(
)(|,,2,ln
111
j
ii
j
i i
iij xM
xxM
xxMnxnnnP
x
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• Clinthorne (IEEE, TNS 1987) shows that the prior equation can berearranged as a sum of terms linear in ni ,
• The second term of the weighting factor, e(x) , compensates forchanges in total light collection and is small except at the edges.
• The weighting factor, pi(x) , is a function of x approximately equal tothe derivative of the PMT response function (see Barrett Fig 5.49).
• The value of x for which this weight sum is zero is the maximumlikelihood estimate of the position of the detected gamma ray.
V.B.2.c – Maximum Likelihood
1
1
22
)(
)()(
j
i i
i
xM
xxMx
j
kk
j
jj
i
N
ii
i
iN
ii
xM
xxM
xexpnxexM
xxMn
1
1
11 )(
)(
)(,)()()(
)(0
• The variance, which dictatesthe resolution is otherwiseshown to be equivalent toequation 5.205 in Barrett.
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The MLE weighting factors, Pi(x), are illustrated for tubes withresponse functions following the Barrett model (slide 17).
V.B.2.c – Maximum Likelihood
MLE Weighting factors for 8 PMTs
-0.04
0.00
0.04
-200 -150 -100 -50 0 50 100 150 200
x - mm
ML
Ew
eig
ht,
Pi(
x)
PMT4 at -25 mm
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V.B.2.c – Maximum Likelihood
• Using the mean number of electrons for each tube at x=0 (aboveleft), and the MLE weights as a function of x (above right), theMLE sum is seen to be zero for x=0.
-25
0
25
-150 -75 0 75 150
x - mm
ML
ES
UM
)()0(1
xpxnMLEsum i
N
ii
MLE Weighting factors for 8 PMTs
-0.04
0.00
0.04
-200 -150 -100 -50 0 50 100 150 200
x - mm
MLE
we
igh
t,P
i(x)
PMT4 at -25 mm
PMT responses versus input positon
0
100
200
300
400
500
600
-200 -150 -100 -50 0 50 100 150 200
x - mm
Nn
(x)
-e
lec
tro
ns
ni(x) for 8 PMTs (from slide 18) pi(x) for 8 PMTs (from slide 30)
• For actual valuesof ni observed fora detected gammaray, we would findthe value of x forwhich the MLEsum is zero.
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• The experimentalperformance reported byClinthorne (IEEE, TNS1987) indicates the theresolution in FWHM isabout 30% larger for atraditional centroidestimate in relation to aMaximum Likelihoodestimate.
• Position estimates fromMaximum Likelihoodestimates are otherwiseseen to be linear with trueposition.
V.B.2.c – Maximum Likelihood
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VariSEL
PMT Selection
Dynamic IntegratorCountrate AdaptedIntegration Time
TriplePUR
PMT Based, TripePile-Up Recovery
Digital Light Pipe
Energy
Independence
OptiMATCH
One (10 Bit) ADC/PMT
LIPC PositionCalculation
X-Y Position
ZLC256x256 on line energy
& linearity correctionDigitrac
PMT Gain control
PMT Gain Control Signals
Inside Detector
On Top of Detector In Console
TAXI Interface
14 bits each for X,Y and Energy
(Max. 1.5 Million Cts/Sec
V.B.2.c – Modern commercial design
The Siemens H3 design illustrates the digital detectorfunctions provided by most commercial systems.
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V.B.2.c – linearity and resolution
• Modern gammacameras withpropercalibrationprovide goodresolution andlinearity.
• Bar patternphantoms areused routinelyto verifypropercalibration.
3/8
”cry
sta
l(E
.Cam
)5
/8
”cry
sta
l(S
ym
bia
)
From W.D. Erwin
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V.B.3 – Other devices (13 charts)
B. Nuclear Medicine Detectors
3. Other gamma camera devices
a. Segmented crystal designs.
b. Drift photodiodes.
c. CdZnTe (CZT) cameras.
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V.B.3.a – segmented crystal designs
• Early gamma camera designsusing segmented scintillationcrystals were handicappedby the need to use PM tubesto detect light.
• In 1960 Bender and Blauexplored an alternative typeof scintillation camera calledan "auto-fluoroscope." Theirdetector was a mosaic ofcollimated sodium iodidecrystals instead of a largesingle crystal. Unlike theAnger camera, which is nowin common use, the auto-fluoroscope (Baird Atomicmulticrystal camera) hadlimited application.
The Baird multi-crystal camera used293 crystals, 3/8 inch in diameter,arranged in a flat mosaic pattern.Gamma ray event positions were
determined using light pipes to eachcrystal and photomultiplier tubes.
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Pixellated scintillator
Array of position-sensitive PMTs
V.B.3.a – Breast Specific Gamma Imaging (BSGI)
Dilon 6800
• 3 mm squaresegmenteddetectorcrystals (3000+)
• 48 positionsensitive PMTs
• High resolutionin a small field ofview.
www.dilon.com
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Dilon 6800
• The crystals and PMTsare packaged in amaneuverable detectormeasuring 6" x 8" x 4“which can be placed indirect contact with thebreast and chest wall.
• The resolution has thepotential to provideearly detection of smalllesions.
V.B.3.a – Breast Specific Gamma Imaging (BSGI)
www.dilon.com
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A second tumor, not detected in the mammogram,is identified in a breast radioisotope image.
V.B.3.a – Breast Specific Gamma Imaging (BSGI)
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V.B.3.b – CsI & photodiode array
• Cameras using PMTs have generally been designed with NaI.
• NaI scintillators have a spectral emission that is well matchedto the response of PMT detectors (QE about .3 to .35 %).
300 400 500 600 700 800 900 nm
.1.2
.3
NaI(Tl): Bicron(St-Gobain)
PMT: Hammamatsu, hQE
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V.B.3.b – CsI & photodiode array
• Cameras using silicon photodiode detectors rather that PMTs havebeen more recently considered (Engdahl/Knoll USP#5171998).
• CsI scintillators have a spectral emission that is well matched tothe response of silicon diodes
300 400 500 600 700 800 900 nm
.2.4
.6
CsI(Tl): Bicron(St-Gobain)
Typical Photodiode
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V.B.3.b – CsI & photodiode array
Avalanche Photo Diode (APD)
Fiorini, SPIE 4141, 2000
• APDs give the best energy resolution for largeractive areas (1 cm2) and near room temperature.
This structure is calledthe reach through type,having a PN junctionbetween which asubstrate p-layer (lightabsorption region) and a P-layer (avalanche region)are formed. The P layerallows the electric field toeasily concentrate on thePN junction providingadequate gain at lowreverse voltage.
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V.B.3.b – CsI & photodiode array
Silicon Drift Diode (SDD)
Fiorini, SPIE 4141, 2000
• SDDs shows better performances for moderate cooling (0°C),easily achievable with Peltier stages, or smaller areas (20 mm2)
In an SDD, electrons produced anywhere in the device drift to arelatively small area anode. The small anode area greatly reduces thedevice capacitance providing for low noise detector circuits.
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V.B.3.b – CsI & photodiode array
Fiorini et. al. , Monolithic arrays of silicon drift detectors formedical imaging applications and related CMOS readoutelectronics, NIM-A, 2005
CsI(Tl) and 19 SDDs
.7 mm spacing for irradiation pts.
.5 mm
.5 mm
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V.B.3.b – CsI & photodiode array
The DigiRad 2020tcImager™ detector size is21 cm x 21 cm (8 x 8inches), and the leadingedge dead space is 1.3 cm(0.5 inches). This detectorsize is smaller than mostcommercially-availablegamma cameras today.
Each of the4,096 CsI(Tl)scintillationcrystals isviewed by a
singlephotodiode
Norm
al
Hand
Techneti
um
99
m
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V.B.3.b – CsI & photodiode array Siman W, Kappadath SC; Performancecharacteristics of a new portable gammacamera, Med. Phys. 39 (6), June 2012.
• Digirad Ergo Camera
• CsI pixelated crystal
( 3mm x 3mm x 6mm )
• Resolution measured withTc99m filled capillary tubes
• Offset tubes used to obtainLSF with sub-pixel spacing
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V.B.3.b – CsI & photodiode array Siman W, Kappadath SC; Performancecharacteristics of a new portable gammacamera, Med. Phys. 39 (6), June 2012.
• Digirad Ergo Camera
• CsI pixelated crystal
( 3mm x 3mm x 6mm )
• Resolution measured withTc99m filled capillary tubes
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V.B.3.c – Solid State CZT arrays
Solid state detectors have recentlybeen considered for gamma cameraapplications, particularly detectorsusing Cadmium Zinc Telluride (CZT).
Prototype CZT camera (GE Medical)tested at the Mayo Clinic.
Mueller, J. Of Nucl. Med., 44, 4, 2004
CZT module, Univ. of Arizona
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V.B.3.c – Solid State CZT arrays
D-SPECT cardiac scanner
Detector column:
• CZT sensor (39 · 39 · 5 mm)
• four 16 · 16 pixel detectors
• Square hole tungsten collimator
• pitch, 2.46 mm
• length, 21.7 mm
• septa 0.2 mm).Nine detector columns
D-SPECT cardiac system.
Gambhir, JNM, April 2009
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V.B.3.c – Solid State CZT arrays
The modular CZT technology is nowused in a commercial product, theGE Discovery NM 530c, thatachieves fast cardiac imaging usingan optimized geometry.
GE Alcyone technology.
Herzog, JNM, Jan 2010
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V.B.4 – PET Systems (31 charts)
B. Nuclear Medicine Detectors
4. Designs for PET Systems
a. Pharmaceutical production. (8 charts)
- Radioisotopes.
- Medical Cyclotrons.
- Radiochemistry.
b. PET Cameras. (10 charts)
- Detection Geometry, 2D & 3D.
- Scintillators & resolution.
- Time of Flight.
c. Advanced concepts. (13 charts)
- Radial elongation & interaction depth.
- New PMTs and photodiodes.
- Small animal systems.
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V.B.4.a – Radioisotopes
Common Radioisotopes for PET imaging
Isotope Half life Production Chemistry
18F 2 hours cyclotronVery Good
(replaces H)
15O, 11C, 13N 2 – 20 min cyclotron Excellent
82Rb 2 min generatorOK
(like Na & K)
Adapted from; Moses, 1/26/2007 ppt
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V.B.4.a – Radioisotopes
Examples of [18F] Tracers
Adapted from; GE Healthcare, TRACERlab FXf-n
Tracer Molecular Level Disease Level Example
18F FTHA(fatty acid)
Anaerobicmetabolism
Cardiology Ischemia
Fluoromisonidazole Hypoxia Oncology Poorly perfused tumors
MethylbenperidolDopaminergic D2receptor
PsychiatrySchizophrenia,Addiction
MethylspiperoneDopaminergic D2receptor
PsychiatrySchizophrenia,Addiction
FluorostradiolSteroidmetabolism
OncologyEstrogen DependentBreast Cancer
AltanserineSeratonergic S2receptor
Psychiatry Depression
FLT Fluoro-L-Thymidine
DNA synthesis Oncology Tumor proliferation
FDGGlucosemetabolism
OncologyCardiologyNeurology
Lung Cancer,Myocardial Viability,Alzheimer's
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V.B.4.a – Radioisotopes
Sketch of H2180 target system used at Julich (FRG)
for production of 18F via the 180(p, n)18F process.
from; Qaim, NIM, 1989
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While the radiusincreases with energy,the time to completeone orbit is constant.The accelerationfrequency istherefore constant
V.B.4.a – Medical Cyclotrons
The cyclotron, one of the earliest types of particle accelerators,makes use of the magnetic force on a moving charge to bendmoving charges into a semicircular path between accelerations byan applied electric field. The applied electric field accelerateselectrons between the "dees" of the magnetic field region. Thefield is reversed at the cyclotron frequency to accelerate theelectrons back across the gap.
from; hyperphysics
m
qBcyclotron
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V.B.4.a – Medical Cyclotrons
The PET medical cyclotronat Stanford University
Interior view ofthe GE PETtracemedical cyclotron
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V.B.4.a – Medical Cyclotrons
Forged steel rings used inthe CTI medical cyclotron.
(Scot Forge, Illinois, USA)
Siemens Eclipse ST baseon the CTI cyclotron
CTI molecular imaging, Inc (Knoxville, Tenn)was acquired by Siemens in 2005
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V.B.4.a – Radiochemistry
Radiopharmaceuticalproduction is done within a‘hot cell’ using remotemanipulators.
Automated FDGproduction system
(GE Tracerlab)shown within a hot
cell (UC Davis).
hot cells, Zurich,Inst. Radioph. Sc.
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V.B.4.a – RadiochemistrySCIENCE VOL 310 16 DECEMBER 2005
Automated FDGproduction system
(GE Tracerlab)shown within a hot
cell (UC Davis).
SCIENCE VOL 310 16 DECEMBER 2005
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V.B.4 – PET Systems (32 charts)
B. Nuclear Medicine Detectors
4. Designs for PET Systems
a. Pharmaceutical production. (8 charts)
- Radioisotopes.
- Medical Cyclotrons.
- Radiochemistry.
b. PET Cameras. (10 charts)
- Detection Geometry, 2D & 3D.
- Scintillators & resolution.
- Time of Flight.
c. Advanced concepts. (13 charts)
- Radial elongation & interaction depth.
- New PMTs and photodiodes.
- Small animal systems.
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V.B.4.b – PET Detectors Ring of Photon Detectors
Adapted from; Moses, 1/26/2007 ppt
• Radionuclide decays,emitting e+.
• e+ annihilates with e– fromtissue, forming back-to-back 511 keV photon pair.
• 511 keV photon pairsdetected via timecoincidence.
• Positron lies on line definedby detector pair (known asa chord or a line ofresponse or a LOR).
• Detect Pairs of Back-to-Back 511 keV Photons• Detect Pairs of Back-to-Back 511 keV Photons• No Collimator Needed -> High Efficiency
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V.B.4.b – PET Detectors
Early PET Detectors
• Single crystal coupled tosmall PMT.
• Single ring and segmentedmultiple ring designs.
BGO Scintillator Crystal(Converts g into Light)
Photomultiplier Tube(Converts Lightto Electricity)
3 — 10 mm wide(determines in-planespatial resolution)
10 — 30 mm high(determines axialspatial resolution)
30 mm deep(3 attenuation lengths)
Adapted from; Moses, 1/26/2007 ppt
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Scintillator TungstenSeptum Lead Shield
V.B.4.b – PET Detectors
Multi-Layer PET Cameras
• Can image several slices simultaneously• Can image cross-plane slices• Can remove septa to increase efficiency (“3-D PET”)
Adapted from; Moses, 1/26/2007 ppt
Planar Images “Stacked” to Form 3-D ImagePlanar Images “Stacked” to Form 3-D Image
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V.B.4.b – PET Detectors Modern PET Detectors
Hexagonal Detector
• Segmented LYSO crystals.
• Hex PMT detector arrays.
• Philips Pixelar
Block Detectors
• Segmented LSO crystals.
• 4 PMT detectors.
• Position from Anger logic
• Siemens HiRez
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V.B.4.b – PET Detectors
PET scintillators – stopping power and timing
LSO LYSO GSO BGO LuAP LaBr3
AttenuationLength
1.15 1.2 1.4 1.04 1.04 2.1
Energyresolution
11% 10% 10% 13% 7-9% 3%
Light Yield 1.0 1.2 < 0.5 < 0.2 0.5 2.0
Decay Time 40 ns 40 ns 60 ns 300 ns 17 ns 35 ns
TimingResolution
450 ps 450 ps na na 500 ps 400 ps
LSO - Lu2SiO5:Ce
LYSO – Lu[Yt 10%]2SiO5:Ce
GSO - Gd2SiO5:Ce
LuAP – LuAlO3:Ce
Adapted from : Philips, IEEE 2006 MIC
LYSO is currently used commercially because of availability and cost
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V.B.4.b – PET Detectors
The improved light emission of LSO relative to thatfor BGO, that was used in earlier PET systems,produces better position estimates and resolution
A B
Siemens PET
A. BGO scintillator. B. LSO scintillator (HiRez)
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V.B.4.b – PET Detectors
Adapted from; Moses, 1/26/2007 ppt
Time-of-Flight in PET
• Can localize source alongline of flight.
• Time of flight informationreduces noise in images.
• Time of flight camerasbuilt in the 80’s with BaF2and CsF.
• These scintillators forcedcompromises thatprevented TOF fromflourishing.
• TOF now commerciallyavailable using LYSO.
• Variance Reduction Given by 2D/ct• 500 ps Timing Resolution -> 5x Reduction in Variance!
c = 1 foot/ns
500 ps timing8 cm localization
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V.B.4.b – PET Detectors
• The D-690 is a multi-ring system with 13,824 LYSOcrystals with dimensions of 4.2x6.3x25 mm3.
• The detection unit is a block of 54 (9x6) individualLYSO crystals coupled to a single squarephotomultiplier tube with 4 anodes.
• The D-690 has 24 rings of detectors for an axial fieldof view (FOV) of 157 mm. The transaxial FOV is 70 cm.
c = 1 foot/ns
The timing coincidence ofa recent PET system withLYSO detector crystalswas recently reported as544 ps FWHM
--------------------Bettinardi et. al.,Physical Performance of the newhybrid PET/CT Discovery-690,Med. Phys. 38 (10), Oct. 2011.
GE PET/CT Discovery 690
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V.B.4.b – PET Detectors
Time of Flight – effect of timing resolution
More precise localization of annihilation eventimproves the noise in the reconstructed image
Adapted from : Philips, IEEE 2006 MIC
Standard
TOF
Bettinardi 2011, Discovery 690
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V.B.4.b – PET Detectors
Example from University of Pennsylvania
Adapted from : Philips, IEEE 2006 MIC
Philips Gemini TF
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V.B.4.b – PET Detectors
14 mCi FDG, Philips Gemini TF
From: Surti, JNM, 3/2007
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V.B.4 – PET Systems (31 charts)
B. Nuclear Medicine Detectors
4. Designs for PET Systems
a. Pharmaceutical production. (8 charts)
- Radioisotopes.
- Medical Cyclotrons.
- Radiochemistry.
b. PET Cameras. (10 charts)
- Detection Geometry, 2D & 3D.
- Scintillators & resolution.
- Time of Flight.
c. Advanced concepts. (13 charts)
- Radial elongation & interaction depth.
- New PMTs and photodiodes.
- Small animal systems.
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• Penetration of 511 keVphotons into crystal ringblurs measured position.
• Blurring worsens asdetector’s attenuationlength increases.
• Also known asParallax Error orRadial Astigmatism.
• Can be removed (in theory)by measuring depth ofinteraction.
V.B.4.c – Blur from Radial Elongation
From: William W. MosesLawrence Berkeley Nat. Lab.Dept. of Functional Imaging
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1 cm
Resolution Degrades Away From Center...Resolution Degrades Away From Center...
Near Tomograph Center 14 cm from Tomograph Center
Point Source Images in 60 cm Ring Diameter Camera
From: William W. MosesLawrence Berkeley Nat. Lab.Dept. of Functional Imaging
V.B.4.c – Blur from Radial Elongation
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V.B.4.c – Improved reconstruction using the PSF
Siemens HD-PET, introduced in2008, uses the shape of thedetected PSF to improve theestimate of the line of response(LOR) and achieve 2mm F18 FWHM
With HD
Without HD
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• PMT Provides Timing Pulse• PMT Provides Timing Pulse
• PD Array Identifies Crystal of Interaction
• PD+PMT Provides Energy Discrimination
• PD / (PD+PMT) Measures Depth of Interaction
LSO Array(crystals 3x3x30 mm3)
64 ElementPD Array
1” Square PMT
Custom IC
Flex Board
24 mm
30
mm
Image of CollimatedGamma Rays
V.B.4.c – Depth-Encoding PET Detector Module From: William W. MosesLawrence Berkeley Nat. Lab.Dept. of Functional Imaging
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V.B.3.a – position sensing PMT
New position sensing PMT designs have been used forPET detectors but have high cost per unit area.
Hammamatsu256 channelsquare PMT(16 x 16)
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RMD, Inc.
Hamamatsu Photonics
• Advantages:• High Quantum Efficiency -> Energy Resolution• Smaller Pixels -> Spatial Resolution• Individual Coupling -> Spatial Resolution
• Challenges:• Dead Area Around Perimeter• Signal to Noise Ratio• Reliability and Cost• # of Electronics Channels
Steady Progress Being MadeSteady Progress Being MadeV.B.4.c – Avalanche Photodiode Arrays
From: William W. MosesLawrence Berkeley Nat. Lab.Dept. of Functional Imaging
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APD Analog of a Position-Sensitive PMTAPD Analog of a Position-Sensitive PMT
*Data and image courtesy of K. Shah, RMD, Inc.
Flood Map,–20° C28 mm
LSO Array
• 15% fwhm EnergyResolution
• 3 ns fwhm TimingResolution
V.B.4.c – Position-Sensitive APD (PSAPD)
From: William W. MosesLawrence Berkeley Nat. Lab.Dept. of Functional Imaging
79NERS/BIOE481 - 2014
V.B.4.c – Micro-PET for small animal imaging
• Crump Institute forBiological Imaging, UCLA
• Micro-PET commercialized byCondord Microsystems, Inc
Mouse prostate Ca model,
Metatasizes to bone,
bone scan, F18, 1.0 mCi, 8 mins
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V.B.4.c – Micro-PET System
• Number of detector modules:30
• Ring diameter:172 mm
• Animal port:160 mm
• Transaxial field-of-view:112 mm
• Axial field-of-view:18 mm
• Volumetric resolution:6 µL
• Sensitivity:200 cps/µCi
• Coincidence time window:12 ns
• In-plane bed wobble:0.76 mm radius
• Axial bed motion:0.1 mm accuracy
• Number of matrices:64 (3-D only), 1.54 Mbytes
• Sinogram size:100 views x 120 angles
• Sampling distance1.125 mm for non wobbled acquisitions
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V.B.4.c – Micro-PET detector
• Crystal material:LSO
• Crystal size:2x2x10 mm
• Crystal pitch:2.25 mm (both in-plane and axially)
• Crystal array:64 (8x8 crystals/PMT)
• PMT:30 Phillips XP 1722
• Number of crystals:1920 (240 per ring x 8 rings)
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DEPARTMENT OF BIOMEDICAL ENGINEERING
17,640 LSO crystals(0.95x0.95x12.5 mm)
15 cm ring diameter
8 cm transverse FOV
4.9 cm axial FOV
~1.2 mm resolution
~2.5% sensitivity
microPET II
V.B.4.c – Micro-PET II, Small Animal PET
83NERS/BIOE481 - 2014
2x2x10mm 1x1x10mmLSO array
DEPARTMENT OF BIOMEDICAL ENGINEERING
Chatziioannou, Tai, Cherry et al, Phys Med Biol, 2002
HamamatsuH7546
64 channel PMT
V.B.4.c – microPET II Detector Development
84NERS/BIOE481 - 2014
DEPARTMENT OF BIOMEDICAL ENGINEERING
1.25 mm cold rods resolved
F– Bone Scan - Mouse
FDG - Mouse Heart
V.B.4.c – microPET II - Images
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