International Atomic Energy Agency

RAD 466-L 8by

Dr. Halima Hawesa

SPECT/CT TECHNOLOGY & FACILITY

DESIGN

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Objective

To become familiar with basic SPECT/CT technology, and review considerations in establishing a new SPECT/CT facility

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• SPECT cameras• Image Quality & Camera QA • SPECT/CT scanners• Design of SPECT/CT facilities

Content

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What is SPECT Camera

gamma cameras. • The most widely used gamma cameras are the so-

called Anger cameras, in which a series of phototubes detects the light emissions of a large single crystal, covering the field of view of the camera.

• SPECT imaging systems consist of single- or multiple-head gamma cameras which rotate around the patient, thereby acquiring the projections necessary for reconstruction of axial slices.

• SPECT stand for Single Positron Emitting Computing Tomography.

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SPECT Camera Components

• Collimator• NaI(Tl) crystal• Light Guide (optical coupling)• PM-Tube array• Pre-amplifier• Position logic circuits (differential

& addition etc.)• Amplifier (gain control etc)• Pulse height analyser• Display (Cathode Ray Tube etc).

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Scintillators

• Na(Tl) I works well at 140 keV, and is the most common scintillator used in SPECT cameras

Density (g/cc)

Z Decay time (ns)

Light yield (% NaI)

Atten. length (mm)

Na(Tl)I 3.67 51 230 100 30

BGO 7.13 75 300 15 11

LSO 7.4 66 47 75 12

GSO 6.7 59 43 22 15

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Detector

PhotocathodecathoddDynodes

Anode

Amplifier

PHA

Scaler

Scintillation detector

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Pulse height analyzer

UL

LL

Time

Pulse height (V)

The pulse height analyzer allows only pulses of a certain height(energy) to be counted.

counted not counted

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Gamma camera

Used to measure the spatial and temporal distribution of a radiopharmaceutical

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GAMMA Camera

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Gamma camera(principle of operation)

PM-tubesDetectorCollimator

Position XPosition YEnergy Z

Types of collimator1.Pinhole2. Parallel hole3.Diverging 4. Converging

collimators.

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C ounter

C lock

PulsesE nergy windowr

T ime

PHA

ADC

C omputer

Patient

z x y

GAMMA CAMERA

Photons are selected by a collimator, hits the detector crystal, which produce light flashes that are detected and amplified by the photomultipliers, then send to digitizer, and then to computer processor for image reconstruction, then to display on monitor.

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PM-tubes

Detect and amplify the light flash produced by the scintillation crystal.

GAMMA-ray Scintillation Detector

• gamma-ray energy converted to light• Light converted to electrical signal

gamma-Rays

Photomultiplier Tube

Light ElectricalSignal

ScintillationCrystal

Photomultiplier Tubes

• Light incident on Photocathode of PM tube

• Photocathode releases electrons

gamma-Rays Light

ScintillationCrystal

PMTube

Photocathode

-

+

Dynodes

Photomultiplier Tubes

• Electrons attracted to series of dynodes• each dynode slightly more positive than

last one

gamma-Rays Light

ScintillationCrystal

PMTube

Photocathode

-

+

+

+

+

+

Dynodes

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Static Dynamic ECG-gated Wholebody scanning Tomography ECG-gated tomography Wholebody tomography

Gamma cameraData acquisition

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Scintigraphy seeks to determine the distribution of

a radiopharmaceutical

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SPECT cameras are used to determine the three-dimensional distribution of the

radiotracer

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Tomographic acquisition

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Tomographic planes

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Myocardial scintigraphy

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ECG GATED TOMOGRAPHY

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12.2 Image Quality & Camera QA

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• Distribution of radiopharmaceutical• Collimator selection and sensitivity• Spatial resolution• Energy resolution• Uniformity• Count rate performance• Spatial positioning at different energies• Center of rotation• Scattered radiation• Attenuation• Noise

Factors affecting image formation

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Sum of intrinsic resolution and the collimator resolution

Intrinsic resolution depends on the positioning of the scintillation events (detector thickness, number of PM-tubes, photon energy)

Collimator resolution depends on the collimator geometry (size, shape and length of the holes)

SPATIAL RESOLUTION

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Object Image

Intensity

SPATIAL RESOLUTION

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NON-UNIFORMITY

(Contamination of collimator)

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NON UNIFORMITYRING ARTIFACTS

Good uniformity Bad uniformity

Difference

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NON-UNIFORMITY

Defect collimator

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Scattered radiation

photon

electron

Scatteredphoton

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The amount of scattered photons registeredDepends on

1- Patient size2- Energy resolution of the gammacamera3- Window setting

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PATIENT SIZE

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Pulse height distribution

Energy

Counts

020406080

100120140

20 60 100

120

140

160

Tc99m

Full energy peak

ScatteredphotonsThe width of the full energy

peak (FWHM) is determined by the energy resolution of thegamma camera. There willbe an overlap between thescattered photon distributionand the full energy peak,meaning that some scatteredphotons will be registered.

FWHM

Overlappingarea

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Window width20%

10%40%

Increased window width will result in an increased number ofregistered scattered photons and hence a decrease in contrast

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ATTENUATION CORRECTION

Transmission measurements• Sealed source• CT

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ATTENUATION CORRECTION

Ficaro et al Circulation 93:463-473, 1996

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Count density

NOISE

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Gamma camera

Operational considerations

• Collimator selection• Collimator mounting• Distance collimator-patient• Uniformity• Energy window setting• Corrections (attenuation, scatter)• Background• Recording system• Type of examination

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Acceptance Daily Weekly YearlyUniformity P T T PUniformity, tomography P PSpectrum display P T T PEnergy resolution P PSensitivity P T PPixel size P T PCenter of rotation P T PLinearity P PResolution P PCount losses P PMultiple window pos P PTotal performance phantom P P

P: physicist, T:technician

QC GAMMA CAMERA

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Sensitivity

Expressed as counts/min/MBq and should be measured for each collimator

Important to observe with multi-head systems that variations among heads do not exceed 3%

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Multiple Window Spatial Registration

• Performed to verify that contrast is satisfactory for imaging radionuclides, which emit photons of more than one energy (e.g. Tl-201, Ga-67, In-111, etc.) as well as in dual radionuclides studies

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Count Rate Performance

• Performed to ensure that the time to process an event is sufficient to maintain spatial resolution and uniformity in clinical images acquired at high-count rates

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Total performance phantom. Emission or transmission.Compare result with reference image.

Total performance

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Phantoms for QC ofgamma cameras

• Bar phantom• Slit phantom• Orthogonal hole phantom• Total performance phantom

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QUALITY CONTROLANALOGUE IMAGES

Quality control of film processing: base & fog, sensitivity,contrast

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Efficient use of computers can increase the sensitivity and specificity of an examination.* software based on published and clinically tested methods* well documented algorithms* user manuals * training* software phantoms

QUALITY ASSURANCECOMPUTER EVALUATION

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SPECT/CT System

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TYPICAL SPECT/CT CONFIGURATION

The most prevalent form of SPECT/CT scanner involves a dual-detector SPECT camera with a 1-slice or 4-slice CT unit mounted to the rotating gantry; 64-slice CT for SPECT/CT also available

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SPECT/CT• Accurate registration• CT data used for attenuation

correction

Localization of abnormalities• Parathyroid lesions (especially for

ectopic lesions)• Bone vs soft tissue infections• CTCA fused with myocardial perfusion

for 64-slice CT scanners

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The CT Scanner

X ray emission inall directions

X ray tube

collimators

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X Ray Tube

Detector Arrayand Collimator

A look inside a rotate/rotate CT

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A Look Inside a Slip Ring CT

X RayTube

Detector Array

Slip Ring

Note: how most

of theelectronics

isplaced on

the rotatinggantry

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What are we measuring in a CT scanner?

• We are measuring the average linear attenuation coefficient µ between tube and detectors

• The attenuation coefficient reflects how the x ray intensity is reduced by a material

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Conversion of to CT number

• Distribution of values initially measured• values are scaled to that of water to

give the CT number

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Radionuclide• Pure emitter ()

e.g. ; Tc99m, In111, Ga67, I123

• Positron emitters (ß+) e.g. : F-18

• , ß- emitters e.g. : I131, Sm153

• Pure ß- emitters e.g. : Sr89, Y90, Er169

• emitters e.g. : At211, Bi213

Diagnostics Therapy

Nuclear medicine applicationaccording to type of radionuclide

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RADIOPHARMACEUTICALS

Radiopharmaceuticals used in nuclear medicine can be classified as follows:

• ready-to-use radiopharmaceuticalse.g. 131I- MIBG, 131I-iodide, 201Tl-chloride, 111In- DTPA• instant kits for preparation of productse.g. 99mTc-MDP, 99mTc-MAA, 99mTc-HIDA, 111In-Octreotide • kits requiring heatinge.g. 99mTc-MAG3, 99mTc-MIBI• products requiring significant manipulatione.g. labelling of blood cells, synthesis and labelling of radiopharmaceuticals produced in house

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Radionuclide used with SPECT

99mTc - Technetium

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99Mo-99mTc GENERATOR

99Mo 87.6% 99mTc

140 keVT½ = 6.02 h

99Tc

ß- 292 keVT½ = 2*105 y

99Ru stable

12.4%

ß- 442 keV 739 keVT½ = 2.75 d

Molybdenum 99

Technetium-99m is a metastable nuclear isomer of technetium-99, symbolized as 99mTc. The "m" indicates that this is a metastable nuclear isomer

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Mo-99 Tc-99m Tc-99 66 h 6h

NaCl

AlO2

Mo-99+Tc-99m

Tc-99m

Technetium generator

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Technetium generator

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Technetium generator

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Radionuclide Pharmaceutical Organ Parameter

+ colloid Liver RES

Tc-99m + MAA Lungs Regional perfusion

+ DTPA Kidneys Kidney function

Radiopharmaceuticals

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Laboratory work with radionuclides

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Administration of radiopharmaceuticals

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SUMMARY OF SPET/CT• SPECT cameras are scintillation cameras, also called

gamma cameras, which image one gamma ray at a time, with optimum detection at 140 KeV, ideal for gamma rays emitted by Tc-99m

• SPECT cameras rotate about the patient in order to determine the three-dimensional distribution of radiotracer in the patient

• SPECT/CT scanners have a CT scanner immediately adjacent to the SPECT camera, enabling accurate registration of the SPECT scan with the CT scan, enabling attenuation correction of the SPECT scan by the CT scan and anatomical localization of areas of unusually high activity revealed by the SPECT scan

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SPECT/CT CLINICAL ALLPLICATIONS

• Refer to the pdf file included with this lecture (spect-appl-L8)