ners-bioe_481-14-09

Health System

RADIOLOGY RESEARCH

Henry Ford

NERS/BIOE 481

Lecture 09Nuclear Medicine Detectors

Michael Flynn, Adjunct Prof

Nuclear Engr & Rad. Science

[email protected]

[email protected]

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.


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.


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


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


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.


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


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)


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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• 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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GE Discovery 670 SPECT/CT

Philips SPECT/CT

Dual Head SPECTimaging systems

integrated with anx-ray CT system

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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)


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.


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).


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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• 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.


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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)


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.


www.dilon.com

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A second tumor, not detected in the mammogram,is identified in a breast radioisotope image.


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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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• 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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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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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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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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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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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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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)


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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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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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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The PET medical cyclotronat Stanford University

Interior view ofthe GE PETtracemedical cyclotron

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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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- Radioisotopes.


- Radiochemistry.




- Time of Flight.





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V.B.4.b – PET Detectors Ring of Photon Detectors


• 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)


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Scintillator TungstenSeptum Lead Shield


Multi-Layer PET Cameras

• Can image several slices simultaneously• Can image cross-plane slices• Can remove septa to increase efficiency (“3-D PET”)


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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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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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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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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• 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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Time of Flight – effect of timing resolution

More precise localization of annihilation eventimproves the noise in the reconstructed image


Standard

TOF

Bettinardi 2011, Discovery 690

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Example from University of Pennsylvania


Philips Gemini TF

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14 mCi FDG, Philips Gemini TF

From: Surti, JNM, 3/2007

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- Radioisotopes.


- Radiochemistry.




- Time of Flight.





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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


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


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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)


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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

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2x2x10mm 1x1x10mmLSO array


Chatziioannou, Tai, Cherry et al, Phys Med Biol, 2002

HamamatsuH7546

64 channel PMT

V.B.4.c – microPET II Detector Development

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1.25 mm cold rods resolved

F– Bone Scan - Mouse

FDG - Mouse Heart

V.B.4.c – microPET II - Images

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