Nuclear Gamma Camera

8/2/2019 Nuclear Gamma Camera

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Scintillation (Anger) camera

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Gamma cameraA gamma camera consists of:

A large scintillating crystal (usually made ofthallium-doped sodium iodide) with about50 photomultiplier tubes on the back.

In front of the detector is a lead collimator.The collimator ensures that only thosephotons with paths parallel to the collimatorholes strike the detector. The direction of

the photons can, thus, be determined.Positional information is obtained byexamining the flash of scintillation lightgenerated by the incoming photon when itinteracts with the crystal. This lightpropagates through the crystal and ispicked up by the PMTs. The most intensesignal is obtained from the PMT nearest theevent, and progressively weaker signalsare found as the distance increases. Byexamining the relative strength of thesignals from all of the PMTs, the location ofthe interaction point can be determined towithin a few millimeters.


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Gamma cameraA position sensitive photo multiplier array.

Patient with

radioactive

tracer

Scintillation.

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Detector CrystalUsed to detect emission photon.

Scintillates (gives off light) when high energy photon interacts.

Amount of light produced is proportional to the number of interactions.

Most commonly made of NaI(Tl) - SPECT

Other materials: CsF, BGO (bismuth germinate), BaF2 - PET.

Emitted photon from radioactive decay interacts in crystal detector via either photoelectric effect orCompton scatter.

Interaction produces secondary electrons in crystal.

If Compton-scattered, photon continues on to next interaction point, where process repeats itself.

Secondary electrons cause ionizations within crystal.

Ionizations produce secondary photons as a result of electron falling from outer electron shells to fill inneratom shells at luminescent centers (due to the inclusion of Tl). Pure NaI doesnt scintillate.


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Crystal thicknessEfficiency- at 250 keV, 50% ofphoton interactions in NaI(Tl)are Compton scatters, thusthicker crystal means morechance of interaction in thecrystal by secondary,Compton-scattered photons.

Thus, for maximum detectionefficiency, we want the crystalto be as thick as possible.

Intrinsic efficiency (Ei) = attenuated fraction (AF)

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ResolutionWe want the location of thescintillation to be as confined aspossible in order to have highlocation precision.

Thus, the Compton-scatteredphoton must escape the crystal so

that it does not interact at adifferent position, thus, producingmultiple scintillation events atdifferent positions.

Spread of light within crystalfollows inverse square law, sincelight spreads spherically.

Intrinsic resolution, Ri, is theminimum size a point source willresolve to on the camera.Generally several mm.

Distance (inches) from primary -ray to thecenter of intensity emitted light


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Crystal

Light Guide

is used to allow light to spread out fromsource position in crystal. Originally, light guide was several cm

thick to improve the uniformity.

Increases uniformity of the camera as it

spreads the light over a larger region,thus allowing those events not directlyunder a photomultiplier to have a verysimilar detection efficiency to those undera PMT. The resolution is decreased.

New cameras have a light guide onlyabout 1 cm or less in order to improveresolution. Uniformity corrections arenow handled by a microprocessorinstead.

Crystal Size

Not all of the crystal detector is useful for imaging, e.g. in a 50 cm crystal, 37 cm is used.Hexagonal size useful for imaging.

Outer portion masked by lead. To prevent distortion in image caused by inefficient collection oflight at periphery of crystal. (i.e. not as many photomultipliers).

Crystal Casing

- NaI(Tl) is a hygroscopic material (i.e. absorbs water).

- Thus, crystal is hermetically sealed against moisture to prevent yellowing, as a yellow crystalabsorbs light.

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Photomultiplier tubes


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Photomultiplier Tubes (PMT)Converts the secondary photon produced in the scintillating detector into an electrical signal.Consists of a photocathode (converts light photon into electrons).

Accelerating dynodes (used to amplify electrical signal from photocathode).

Positioned in a hexagonal (why?) array on the back of the light guide and coupled with lighttransmitting grease.

Number of PMTs dependent upon size of detector crystal.

Circular PMTs often used, but small gaps present in detector where PMTs dont touch.

Energy resolution is dependent upon the amount of light reaching the photocathode.

Each PMT is connected to a circuit containing a preamplifier.

Threshold preamplifier: discriminates large signals from small signals typical of PMT noise.

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Gamma Camera, collimatorCollimator - device used to form a relationshipbetween the originating photon position (i.e.,decay centre), and the position of thesubsequent detection in the gamma camera.

Acts like a lens of a camera

Made of lead sized for radionuclide being used.

Parallel multi-hole - Consists of holes parallelin both X and Z directions separated by leadfoil (septa).

Diverging - Septa angled outward from cameraface in X direction, parallel in Z direction.

Converging / fan-beam Septa angledinwards in X direction, parallel ion Z direction.

Slant hole - Parallel holes but septa are notperpendicular to the camera surface.

Pinhole - Single hole in large box


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Position Encoding Matrix and

Summing AmplifiersFor a scintillation event in the crystal, the amount of light given off is proportionalto the amount of energy imparted in the crystal. This light will be detected bymore than one PMT tube. Thus, depending on how much light the PMT sees,the output signal will be different.

The amount of light seen by a photomultiplier is a function of the distance

between the scintillation event and the PMT. (Except at the edges).The total of all output signals is proportional to the total energy deposited in thecrystal by the incoming -ray. So, if we combine the relative output signals fromeach PMT according to its X-Y location on the detector, we can determine theX-Y location of the scintillation event.


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Pulse Height AnalyzerDetermines the energy of the incident -ray that interacts within the NaI crystal.

Total light produced in the scintillator is proportional to the energy of the -photon.

Output voltage from PMTs gets summed into Z pulse. Size of the Z pulserepresents the energy of the detected gamma photon.

Windowing is also done to bin energies within a certain range into an energychannel. It result is a histogram of measured counts/channel.

Use PHA to select the photopeak of the gamma emitter we are using.

Source of gammas at a depth of 3 in the body we will have:

Transmitted gammas at 140 keV. (30%) Compton scattered gammas at energies < 140 keV. (68%)

Absorbed gammas that do not make it out of the body (2%).

the sum of all three = 100 %.

We want to measure (which of these three?), as the other leads to the imagedegradation.

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NaI can not distinguish a 140keV photon from a 130 keVphoton (energy resolution ofNaI is about 10% at 140 keV).

140 keV photon may look likea 126 keV photon, or a 154keV photon.

Thus, the photo-peak at 140keV is not a spike but rather a

Gaussian shape. TypicalFWHM is 10-20%.


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Quantum mottleIn SPECT, we detect only a few -photons

It introduces noise, quantum mottle.

It obeys Gaussian distribution:

( )

=

2

2

1exp

2

1

NNNP

P probability of observing Ncounts, N mean

value of counts, and standard deviation(SD). determines the spread of a Gaussian

distribution.

Its SD and mean number of counts are relatedasN=

A more useful index of statistical error or precision of measurement that SD is

percent standard deviation (%SD).

NN

N

NSD

100100100% ===

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Contrast versus number of countsOriginal image %SD = 1%

%SD = 3% %SD = 10%

What number of photons

result to such image?


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How many x-ray photons to create animage?

Tube power = 70 kV* 100 mA= 7 kW

X-ray production 1% = 70, Bremsstrahlung

Exposure time 0.1 s = 7 J of X-ray energy

This energy is evenly distributed

We can expect about 0.7 J/m2An x-ray field of 1 dm2 will get 7 mJ.

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How many x-ray photons to create

an image?

7 mJ correspond to 4.375 * 1013 keV

The mean spectral energy = 43.75 keV

The number of x-ray photons hitting the patient =

1012 About 1 % or 1010 photons will reach the film

This number will be distributed in 1000X1000pixels

Number of x-ray hitting the detector pixel is 10000

The efficiency of the detector = 10 %

1000 photons/pixel will carry the information


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Statistical errorsWhen adding two independent counts, %SD = 21100 NN +When subtracting two independent counts, %SD = ( )100 1 2 1 2 N N N N +

When multiplying or dividing two independent

counts, %SD =( ) ( )22 2%1% NofSDNofSD +

.

Even when there is no radioactive sample near a radiation detector, it will still record

some number of disintegrations known as room background (radon, cosmic rays,etc.).

Example: It is found that in the thyroid uptake measurement of a patient, the neck activity gives 900 countsper minute, whereas the standard is 2500 counts per minute. Calculate the % uptake by thyroid and its

precision.

% uptake = (counts in neck)/(counts in standard)100 = 900/2500 100 = 36%

%SD in neck counts = 100/sqrt(900) = 3.3%, and

%SD in standard counts = 100/sqrt(2500) = 2%

%SD of thyroid uptake = sqrt(3.32 + 22) = 4%

Therefore, thyroid uptake = (36 +- 1.4)%

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Radionuclide Generators: Principles

Short-lived radionuclide are desirable

However, their fast decay entailsproblems

A radionuclide generators, (cow) is thesolution:

A long-lived parent radionuclide is allowedto decay to its short-lived daughter

radionuclide and the latter is chemicallyseparated in a physiological solution.

The most used radionuclide decays asshown

Another useful decay scheme is

131Sn 113mIn 113In

One the equilibrium is established, it can be disturbed by chemical separation (milking) ofthe two radionuclides. After chemical separation, the daughter radioactivity again growsand re-establishes an equilibrium


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Principle of a generatorSuccessive decay and parent/daughterequilibrium. ConsiderA------> B -------> C

If t1/2 of B < t1/2 of A (10 to 50 times).Transient equilibrium: Activity ofdaughter becomes higher than that of theparent and decay with the same rate.(The ratio of amounts of parent/daughterradionuclides is constant)

If t1/2 of B


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Ideal Generators: Characteristics

Easily transportable

Easy separation of daughter from parent in sterile,

pyrogen-free form

High yield of separation

No radionuclidic impurities

Parent with reasonable half life

Daughter with ideal half life and gamma energy Chemistry of the daughter allows hospital preparation

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99Mo/99mTc Generator

Parent: 99Mo as molybdate(99MO4

-2)

Daughter: 99mTc aspertechnetate (99mTcO4

-1)

(what has happened tochange the radical charge?)

Adsorbent material: alumina(aluminum oxide, Al2O3)

Eluent: saline (0.9% NaCl)

Eluate: 99mTcO4


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Eluting (milking)99Mo

Half-life: 66 hr.

Decays by - emission, gamma: 740, 780keV.

High affinity to alumina compared to 99mTc.

99mTc Desirable Characteristics:

Available in a generator form

Emits monoenergetic gamma rays of 140keV

Ideal physical half life: 6 hours

Lack of beta emissions Its daughter (99Tc) has a half-life of

2.12105 Years ---> no extra radiation doseto the patient

Suitable for in-house preparation of manyradiopharmaceuticals.

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Generator quality control

Types of impurities

Radionuclide impurity: 99Mo, 99Mo breakthrough Limit: 0.15 Ci 99Mo/mCi 99mTc at the time of administration. As

a rule of thumb:0.038 Ci 99Mo/mCi of99mTc at elution time isgood for 12 hr(why less?).

Detection Method: the eluate vial is shielded in a lead pot (6 mm)to stop all 140-keV photons of99mTc and count 740-780 keV of99Mo. The shielded vial is assayed in a dose calibrator using99Mo setting.

Chemical impurity: Al+3

Effect: interferes with labelling:

Sulfur colloid: precipitate

labeling of RBCs: agglutination


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Radionuclide productionPrimary sources:reactor, cyclotron;Secondary sources:generators:*A -----> *B ------> C

Nuclear Reactor

The most important reaction (SPECT): 98Mo (n, ) 99Mo------> 99mTc

Starting material and products have the same chemical identity.

Nuclear reactors are primarysource of thermal neutrons,

0.025 eV.

Easily captured by nuclides:

A

ZX (n, ) A+1

ZX

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CyclotronsCyclotrons or accelerators aresources of a large number of high-energy (MeV range) chargedparticles.

The reactions are characterized byan energy threshold (why?).

Example : 68Zn (p,2n) 67GaStarting material and product havedifferent chemical identity.

Radionuclides with high specificactivity are typically produced.

Cyclotrons are expensive.

Radionuclides decay by + orelectron capture.


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Fission-produced radionuclidesThe process of splitting a heaviernucleus into two small nuclei is calledfission. Example: 23592U+

10n

14156Ba +

9136Kr + 4

10n.

Large number of neutrons isproduced.

They are captured, in turn, initiatinganother reactions called chainreaction

Uncontrolled chain reaction atombomb, controlled chain reaction nuclear reactor. Example includesproduction of131I

Starting material and products aredifferent.

As a result, high specific activityradionuclides are produced.

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Planar imaging

The simplest way to use a gamma camera isanalogous to a plain x-ray film. The camera isplaced adjacent to the patient and both patientand camera are kept still while the signalaccumulates. This results in a single planar viewof the patient (see slide below).

The advantage of this method: fast andcomputationally simple

The disadvantage: structures that overlay eachother along the line of sight to the camera aredifficult to distinguish. In addition, gamma raysarising from tracer concentrations on the far sideof the patient tend to be scattered and absorbed(attenuated) by the patients body, renderingthem difficult to see.


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Planar images

Oncology, Whole body

Front and back view

Not Tomographic

Low

spatialresolution

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Single Photon Emission CT,

SPECTThe two gamma camerasrotate around the patient.

A little thought will show thatthe same math used in CTto compute (x,y) can beused to compute theconcentration of theradioisotope.


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Reconstruction in SPECT

Gamma cameras rotate around the patient (no x-ray tubes).

Each planar projection consists of a ray sum of the activity onto the2D image plane.

For each row in a projection image sequence, a sinogram, p(, ) isobtained.

It can be reconstructed to produce a tomographic slice of thepatient, using e.g. FBP.

When each slice is reconstructed, it is possible to stuck them

together into a 3D activity distribution.The 3D image can then be sliced and diced in any directions(transverse, sagittal, oblique) for viewing.

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Sampling Linear sampling

In order to reconstruct tomogram properly, we need tosample with high spatial frequency

Sampling period, < 0.52 FWHM of Gaussian response ofgamma camera to a point-source, 1% aliasing.

However, if is too small, number of counts decrease, (andwhat?)

Angular sampling

Cannot acquire many angles as in CT - too few photons.

If we sample over 180, # of angles = (D/2)/ = (/2)N. Atthe edge of FOV.

D= N

N number of pixels


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Sampling artefacts

180 360

Signal originating from radionuclide from the edge of FOV has wider spread than thesignal from a proximal radionuclide. It causes errors when performing FBP.

To correct this error, use 360 rotation geometry to acquire data.

Photon attenuation also causes errors

As photons emanate from within the patient, they will be attenuated as they traverse to thedetector.

Photons travelling further through the medium will be attenuated more than photons close tothe camera.

when sampling in 180 rotation geometry, effect is pronounced.

For highest resolution, mount the camera the closest to the patient elliptical orbitingof the heads?

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Image reconstruction in SPECTTraditionally used FBP, however, not always optimal

low counts contrary to CT

FBP makes use of a ramp filter, it amplifies noise, i.e. high frequencynoise in FBP data.

Since most useful information is in low-frequency range, LPF is applied.

Ramp filter

f

LPF

f

=

BPF

f


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FilteringDetermination of optimal BPF is an important task in SPECTParticular types of projection data (cardiac, bone) requiredifferent filters

Since SPECT is a low-resolution imaging modality than CT, with array size of

6464, 128128 vs. 512512, iterative reconstruction algorithm can be used.

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Positron Emission Tomography

A tracer compound is labelled with a positron emitting radionuclide andinjected into the subject

The subject is placed within the field of view (FOV) of a number of-detectors.

After a short travel (~ 1 mm) the radionuclide decays emitting positrons.

These positrons annihilate on contact with electrons within the body.

Each annihilation produces two 511 keV photons travelling in oppositedirections along the so-called line of response (LOR).

The detector electronics is configured to detect two coincident events withina nano-second time window - from the same annihilation.

These "coincidence events" can be stored in arrays corresponding toprojections through the patient and reconstructed using standardtomographic techniques.

The resulting images show the tracer distribution throughout the body of thesubject.


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PET radionuclidesProton-rich isotopes may decay via positron emission.

A proton in the nucleus decays to a neutron, a positron and a neutrino. The

daughter isotope has an atomic number one less than the parent:

116C =

115B + e

+ + n

Properties of commonly used positron emitting radio-isotopes

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Two photon production

When e+ reach thermal energies, they interact

with tissue electrons by annihilation.

Two 511-keV photons anti-parallel in the

positrons frame are produced

Variations in the momentum in free annihilationresult in an angular uncertainty in the direction of

the 511 keV photons of around 4 mrad.

In a PET camera of diameter 1 m and active

trans-axial FOV of 0.6 m this results in a

positional inaccuracy of 2-3 mm.


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Coincidence detectionIn a PET camera, each detector

generates a timed pulse when it registers

an incident photon.

These pulses are combined in

coincidence circuitry, if the pulses fall

within a short time-window, they are

deemed to be coincident.

A coincidence event is assigned to a line

of response (LOR) joining the two

relevant detectors.

The collimation is realized by the so-

called electronic collimation, not physicalcollimator.

physical collimator. This is known as

electronic collimation.2 major advantages:

improved sensitivity (10 times for 2D-

PET compared with SPECT)

improved uniformity of the point source

response function (PET image resolution

5-10 mm, SPECT 15-20 mm)

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AttenuationThe attenuation factor of the photons

along the LOR from v, is the same forany position along the line of response.

By measuring the coincidence signal

as a positron-emitting source is moved

around the object within the FOV, it is

possible to obtain attenuation factorsfor each LOR.

Isotope distribution can be measured enables quantitatively.

In SPECT techniques, where the attenuation factors increase with increasing

distance from the detectors, there is no simple way to correct for photon

attenuation.

For 511 keV photons in human tissue the half-value layer is 7 cm. Attenuationfactors in human studies can rise to around 50 for LORs crossing large dense

areas, for example those crossing the shoulders perpendicularly to the sagittal

plane.


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Coincidence eventsCoincidence events in PET: true,scattered, random and multiple.

True coincidences occur when

both photons from an annihilation

event are detected by detectors in

coincidence.

In a scattered coincidence, at

least one of the detected photons

has undergone a Compton

scattering event prior to detection

- resulting coincidence event will

be assigned to the wrong LOR .

Random coincidences occur when two photons not arising from the same annihilation event

are incident on the detectors within the coincidence time window of the system.

Define t , the coincidence resolving time of the system, such that any events detected with a

time difference of less than t are considered to be coincident (see section 5.4). Let r1 be thesingle event rate (singles rate) on detector channel 1. Then in one second, the total time-

window during which coincidences will be recorded is 2t r1. If the singles rate on detectorchannel 2 is r2, we can say that the number of random coincidences R12 assigned to the LOR

joining detectors 1 and 2 is given by2112 2 rtrR =

Nuclear Gamma Camera

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