12. SPECT/CT Technology
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Transcript of 12. SPECT/CT Technology
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International Atomic Energy Agency
L 12
SPECT/CT TECHNOLOGY & FACILITY
DESIGN
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Radiation Protection in PET/CT 2
Answer True or False
• The most common isotope used in SPECT/CT scans is 18F
• SPECT scanners work by detecting coincidences of two 511 keV gamma rays
• The facility design concepts are almost identical to those used in designing PET/CT facilities
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Radiation Protection in PET/CT 3
Objective
To become familiar with basic SPECT/CT technology, and review considerations in establishing a new SPECT/CT facility
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Radiation Protection in PET/CT 4
• SPECT cameras
• Image Quality & CImage Quality & Camera QA
• SPECT/CT scanners
• Design of SPECT/CT facilities
Content
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International Atomic Energy Agency
12.1 12.1 SPECT cameras
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Radiation Protection in PET/CT 6
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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Radiation Protection in PET/CT 7
Detector
PhotocathodecathoddDynodes
Anode
Amplifier
PHA
Scaler
Scintillation detector
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Radiation Protection in PET/CT 8
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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Radiation Protection in PET/CT 9
Pulse-height distributionNaI(Tl)
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Radiation Protection in PET/CT 10
Semi-conductor detector as spectrometer
• Solid Germanium or Ge(Li) detectors
• Principle: electron - hole pairs (analogous to ion-pairs in gas-filled detectors)
• Excellent energy resolution
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Radiation Protection in PET/CT 11
Knoll
Comparison of spectrum from a Na(I) scintillation
detector and a Ge(Li) semi-conductor detector
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Radiation Protection in PET/CT 12
Gamma cameraGamma camera
Used to measure the spatial and temporal distribution of a radiopharmaceutical
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Radiation Protection in PET/CT 13
Gamma camera(principle of operation)
PM-tubesDetectorCollimator
Position XPosition YEnergy Z
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Radiation Protection in PET/CT 14
C ounter
C lock
PulsesE nergy windowr
T ime
PHA
ADC
C omputer
Patient
z x y
GAMMA CAMERA
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Radiation Protection in PET/CT 15
PM-tubes
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Radiation Protection in PET/CT 16
Gamma camera collimators
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Radiation Protection in PET/CT 17
Static Dynamic ECG-gated Wholebody scanning Tomography ECG-gated tomography Wholebody tomography
Gamma cameraData acquisition
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Radiation Protection in PET/CT 18
R
Intervaln
Image n
ECG-gated acquisition
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Radiation Protection in PET/CT 19
Scintigraphy seeks to determine the distribution of
a radiopharmaceutical
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Radiation Protection in PET/CT 20
SPECT cameras are used to determine the three-dimensional distribution of the
radiotracer
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Radiation Protection in PET/CT 21
Tomographic acquisition
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Radiation Protection in PET/CT 22
y
z
x
x-position
C ount rate
z
y
Tomographic reconstruction
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Radiation Protection in PET/CT 23
Tomographic planes
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Radiation Protection in PET/CT 24
Myocardial scintigraphy
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Radiation Protection in PET/CT 25
ECG GATED TOMOGRAPHY
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International Atomic Energy Agency
12.2 Image Quality & C12.2 Image Quality & Camera QA
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Radiation Protection in PET/CT 27
•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
•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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Radiation Protection in PET/CT 28
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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Radiation Protection in PET/CT 29
Object Image
Intensity
SPATIAL RESOLUTION
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Radiation Protection in PET/CT 30
Resolution - distance
0
5
10
15
20
25
30
0 2 4 6 8 10 12 14 16
Distance (cm)
FWH
M (m
m) High sensitivity
High resolution
FWHM
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Radiation Protection in PET/CT 31
Optimal Large distance
SPATIAL RESOLUTION - DISTANCE
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Radiation Protection in PET/CT 32
Linearity
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Radiation Protection in PET/CT 33
NON UNIFORMITY
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Radiation Protection in PET/CT 34
NON UNIFORMITY
Cracked crystal
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Radiation Protection in PET/CT 35
NON-UNIFORMITY
(Contamination of collimator)
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Radiation Protection in PET/CT 36
NON UNIFORMITYRING ARTIFACTS
Good uniformity Bad uniformity
Difference
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Radiation Protection in PET/CT 37
NON-UNIFORMITY
Defect collimator
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Radiation Protection in PET/CT 38
COUNT RATE PERFORMANCE
(IAEA QC Atlas)
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Radiation Protection in PET/CT 39
Spatial positioning at different energies
Intrinsic spatial resolution with Ga-67 Point source (count rate < 20k cps); quadrant bar pattern; 3M counts; presetenergy window widths; summed image from energy windows set over the 93 keV,183 keV and 296 keV photopeaks.(IAEA QC Atlas)
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Radiation Protection in PET/CT 40
Spatial positioning at different energies
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Radiation Protection in PET/CT 41
Center of Rotation
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Radiation Protection in PET/CT 42
Tilted detector
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Radiation Protection in PET/CT 43
Scattered radiation
photon
electron
Scatteredphoton
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Radiation Protection in PET/CT 44
The amount of scattered photons registered
Patient sizeEnergy resolution of the gammacamera
Window setting
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Radiation Protection in PET/CT 45
PATIENT SIZE
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Radiation Protection in PET/CT 46
Pulse height distribution
Energy
Counts
0
20
40
60
80
100
120
140
20 60 100
120
140
160
Tc99m
Full energy peak
Scattered
photonsThe width of the full energypeak (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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Radiation Protection in PET/CT 47
Window width
20%
10%40%
Increased window width will result in an increased number ofregistered scattered photons and hence a decrease in contrast
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Radiation Protection in PET/CT 48
SCATTER CORRECTION
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Radiation Protection in PET/CT 49
-20
-15
-10
-5
0
0 20 40 60 80 100 120 140
Register 1000 counts Origin of counts
ATTENUATION
I=I0 exp(-µx)
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Radiation Protection in PET/CT 50
Contrast (2cm object)
23% 7% 2%
ATTENUATION
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Radiation Protection in PET/CT 51
ATTENUATION CORRECTION
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Radiation Protection in PET/CT 52
ATTENUATION CORRECTION
Transmission measurements• Sealed source• CT
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Radiation Protection in PET/CT 53
ATTENUATION CORRECTION
Ficaro et al Circulation 93:463-473, 1996
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Radiation Protection in PET/CT 54
Count density
NOISE
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Radiation Protection in PET/CT 55
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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Radiation Protection in PET/CT 56
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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Radiation Protection in PET/CT 57
IAEA-TECDOC-602
Quality control ofNuclear medicine instruments 1991
INTERNATIONAL ATOMIC ENERGY AGENCY IAEAMay 1991
IAEA-TRS-454 Quality Assurance for Radioactivity Measurement in Nuclear Medicine 2006
IAEA QA for SPECT systems (in press)
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Radiation Protection in PET/CT 58
QC Gamma camera
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Radiation Protection in PET/CT 59
Energy resolution
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Radiation Protection in PET/CT 60
Linearity
Flood source or point source (Tc-99m)Bar phantom or orthogonal-hole phantom
1. Subjective evaluation of the image.2. Calculate absolute (AL) and differential (DL)linearity.AL: Maximum displacement from ideal grid (mm)DL: Standard deviation of displacements (mm)
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Radiation Protection in PET/CT 61
Flood source (Tc-99m, Co-57)Point source (Tc-99m)
Intrinsic uniformity: Point source at a large distancefrom the detector. Acquire an image of 10.000.000 counts
With collimator: Flood source on the collimator. Acquirean image of 10.000.000 counts
UNIFORMITY
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Radiation Protection in PET/CT 62
Uniformity
1. Subjective evaluation of the image2. Calculate
Integral uniformity (IU)Differential uniformity (DU)
IU=(Max-Min)/Max+Min)*100, where Max is thethe maximum and Min is the minimum counts in a pixel
DU=(Hi-Low)/(Hi+Low)*100, where Hi is the highestand Low is the lowest pixel value in a row of 5 pixelsmoving over the field of viewMatrix size 64x64 or 128x128
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Radiation Protection in PET/CT 63
UNIFORMITY/DIFFERENT RADIONUCLIDES
D BOULFELFEL
Dubai Hospital
All 4 images acquired with:Matrix: 256 x 256, # counts: 30 Mcounts
Tl 201
Ga 67
Tc 99m
I 131
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Radiation Protection in PET/CT 64
LINEARITY AND UNIFORMITY CORRECTIONS
Dogan Bor, Ankara
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Radiation Protection in PET/CT 65
OFF PEAK MEASUREMENTS
Dogan Bor, Ankara
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Radiation Protection in PET/CT 66
TOMOGRAPHIC UNIFORMITY
Tomographic uniformity is the uniformity of the reconstruction of a slicethrough a uniform distribution of activity
SPECT phantom with 200-400 MBq Tc99m aligned with the axis ofrotation. Acquire 250k counts per angle. Reconstruct the data with a ramp filter
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Radiation Protection in PET/CT 67
INCORRECT MEASUREMENT
Two images of a flood source filled with a solution of Tc-99m, which had not been mixed properly
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Radiation Protection in PET/CT 68
Spatial resolution
Measured with: Flood source or point source plus a Bar phantom
Subjective evaluation of the image
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Radiation Protection in PET/CT 69
SPATIAL RESOLUTION
Lead200 mm
50 mm
Screw clipPolyethylene tubingabout 0.5 mm in internaldiameter
Plastic shims
500 mm
Rigid plastic
30 mm
60 mm
5 mm
Intrinsic resolution System resolution
IAEA TECDOC 602
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Radiation Protection in PET/CT 70
Tc-99m or other radionuclide in useIntrinsic: Collimated line source on the detectorSystem: Line source at a certain distance
Calculate FWHM of the line spread function
FWHM: 7.9 mm
SPATIAL RESOLUTION
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Radiation Protection in PET/CT 71
TOMOGRAPHIC RESOLUTION
Method 1: Measurement with theJaszczak phantom, with and without scatter (phantom filled with water and with no liquid)
Method 2: Measurement with aPoint or line source free in air and Point or line source in a SPECT phantom with water
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Radiation Protection in PET/CT 72
SensitivitySensitivity
Expressed as counts/min/MBq and Expressed as counts/min/MBq and should be measured for each collimatorshould be measured for each collimator
Important to observe with multi-head Important to observe with multi-head systems that variations among heads do systems that variations among heads do not exceed 3%not exceed 3%
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Radiation Protection in PET/CT 73
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Radiation Protection in PET/CT 74
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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Radiation Protection in PET/CT 75
Multiple Window Spatial Registration
• Collimated Ga-67 sources are used at central point, four points on the X-axis and four points on the Y axis
• Perform acquisitions for the 93, 184 and 300 keV energy windows
• Displacement of count centroids from each peak is computed and maximum is retained as MWSR in mm
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Radiation Protection in PET/CT 76
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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Radiation Protection in PET/CT 77
Count Rate Performance
• Use of decaying source or calibrated copper sheets to compute the observed count rate for a 20% count loss and the maximum count rate without scatter
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Radiation Protection in PET/CT 78
Pixel size
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Radiation Protection in PET/CT 79
Center of rotationPoint source of Tc-99m or Co-57Make a tomographic acquisition
In x-direction the position will describe a sinus-function. In y-direction a straight line.
Calculate the offset from a fitted cosine and linearfunction at each angle.
Cosine function
Linear function
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Radiation Protection in PET/CT 80
Total performance phantom. Emission or transmission.Compare result with reference image.
Total performance
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Radiation Protection in PET/CT 81
SOURCES FORQC OF GAMMA CAMERAS
•Point source•Collimated line source•Line source•Flood source
Tc99m, Co57, Ga67
<1 mm
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Radiation Protection in PET/CT 82
Phantoms for QC ofgamma cameras
•Bar phantom•Slit phantom•Orthogonal hole phantom•Total performance phantom
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Radiation Protection in PET/CT 83
Phantoms for QC ofgamma cameras
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Radiation Protection in PET/CT 84
QUALITY CONTROLANALOGUE IMAGES
Quality control of film processing: base & fog, sensitivity,contrast
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Radiation Protection in PET/CT 85
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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Radiation Protection in PET/CT 86
•Identification of nuclides
•Control of radionuclide purity
Semi-conductor detectorApplications in nuclear medicine
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International Atomic Energy Agency
12.3. SPECT/CT12.3. SPECT/CT
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Radiation Protection in PET/CT 88
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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Radiation Protection in PET/CT 89
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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Radiation Protection in PET/CT 90
The CT Scanner
• Computed Tomography (CT) was introduced into clinical practice in 1972 and revolutionized X Ray imaging by providing high quality images which reproduced transverse cross sections of the body.
• Tissues are therefore not superimposed on the image as they are in conventional projections
• The technique offered in particular improved low contrast resolution for better visualization of soft tissue, but with relatively high absorbed radiation dose
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Radiation Protection in PET/CT 91
The CT Scanner
X ray emission inall directions
X ray tube
collimators
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Radiation Protection in PET/CT 92
X Ray Tube
Detector Arrayand Collimator
A look inside a rotate/rotate CT
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Radiation Protection in PET/CT 93
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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Radiation Protection in PET/CT 94
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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Radiation Protection in PET/CT 95
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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International Atomic Energy Agency
12.5 12.5 Design of SPECT/CT facilities
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Radiation Protection in PET/CT 97
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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Radiation Protection in PET/CT 98
Sealed sources in nuclear medicine
Sealed sources used for calibration and quality control of equipment (Na-22, Mn-54, Co57, Co-60, Cs137, Cd-109, I-129, Ba-133, Am-241). Point sources and anatomical markers (Co-57, Au-195). The activities are in the range 1 kBq-1GBq.
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Radiation Protection in PET/CT 99
99Mo-99mTc GENERATOR
99Mo87.6% 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
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Radiation Protection in PET/CT 100
Mo-99 Tc-99m Tc-99 66 h 6h
NaCl
AlO2
Mo-99+Tc-99m
Tc-99m
Technetium generator
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Radiation Protection in PET/CT 101
Technetium generator
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Radiation Protection in PET/CT 102
Technetium generator
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Radiation Protection in PET/CT 103
Technetium generator
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Radiation Protection in PET/CT 104
Technetium generator
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Radiation Protection in PET/CT 105
Technetium generator
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Radiation Protection in PET/CT 106
Radionuclide Pharmaceutical Organ Parameter
+ colloid Liver RES
Tc-99m + MAA Lungs Regional perfusion
+ DTPA Kidneys Kidney function
Radiopharmaceuticals
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Radiation Protection in PET/CT 107
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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Radiation Protection in PET/CT 108
Laboratory work with radionuclides
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Radiation Protection in PET/CT 109
Administration of radiopharmaceuticals
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Radiation Protection in PET/CT 110
Categorization of hazard
Based on calculation of a weighted activity using weighting factors according to radionuclide used and the type of operation performed.
Weighted activity Category< 50 MBq Low hazard50-50000 MBq Medium hazard>50000 MBq High hazard
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Radiation Protection in PET/CT 111
Categorization of hazardWeighting factors according to radionuclide
Class Radionuclide Weighting factorA 75Se, 89Sr, 125I, 131I 100
B 11C, 13N, 15O, 18F,51Cr, 67Ga, 99mTc,111In, 113mIn, 123I, 201Tl 1.00
C 3H, 14C, 81mKr127Xe, 133Xe 0.01
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Radiation Protection in PET/CT 112
Categorization of hazardWeighting factors according to type of operation
Type of operation or area Weighting factor
Storage 0.01
Waste handling, imaging room (no inj),waiting area, patient bed area (diagnostic) 0.10
Local dispensing, radionuclide administration,imaging room (inj.), simple preparation,patient bed area (therapy) 1.00
Complex preparation 10.0
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Radiation Protection in PET/CT 113
Categorization of hazard
Administration of 11 GBq I-131
Weighting factor, radionuclide 100Weighting factor, operation 1
Total weighted activity 1100 GBq
Weighted activity Category< 50 MBq Low hazard50-50000 MBq Medium hazard>50000 MBq High hazard
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Radiation Protection in PET/CT 114
Patient examination, 400 MBq Tc-99m
Weighting factor, radionuclide 1Weighting factor, operation 1
Total weighted activity 400 MBq
Weighted activity Category< 50 MBq Low hazard50-50000 MBq Medium hazard>50000 MBq High hazard
Categorization of hazard
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Radiation Protection in PET/CT 115
Patients waiting, 8 patients, 400 MBq Tc-99m per patient
Weighting factor, radionuclide 1Weighting factor, operation 0.1
Total weighted activity 320 MBq
Weighted activity Category< 50 MBq Low hazard50-50000 MBq Medium hazard>50000 MBq High hazard
Categorization of hazard
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Radiation Protection in PET/CT 116
Category of hazard(premises not frequented by patients)Typical results of hazard calculations
High hazardRoom for preparation and dispensing radiopharmaceuticalsTemporary storage of waste
Medium hazardRoom for storage of radionuclides
Low hazardRoom for measuring samplesRadiochemical work (RIA)Offices
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Radiation Protection in PET/CT 117
High hazardRoom for administration of radiopharmaceuticalsExamination roomIsolation ward
Medium hazardWaiting roomPatient toilet
Low hazardReception
Category of hazard(premises frequented by patients) Typical results of hazard calculations
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Radiation Protection in PET/CT 118
Building requirements
Category Structural shielding Floors Worktop surfacesof hazard walls, ceiling
Low no cleanable cleanable
Medium no continuous cleanable sheet
High possibly continuous cleanable one sheet folded to walls
What the room is used for should be taken into accounte.g. waiting room
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Radiation Protection in PET/CT 119
Building requirements
Category Fume hood Ventilation Plumbing First aidof hazard
Low no normal standard washing
Medium yes good standard washing & decontamination
facilities High yes may need may need washing & special forced special decontamination ventilation plumbing facilities facilities facilities
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Radiation Protection in PET/CT 120
Design Objectives
•Safety of sources•Optimize exposure of staff, patients and general public
•Maintain low background where most needed•Fulfil requirements regarding pharmaceutical work
•Prevent uncontrolled spread of contamination
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Radiation Protection in PET/CT 121
VENTILATION
Laboratories in which unsealed sources, especially radioactive aerosols or gases, may be produced or handled should have an appropriate ventilation system that includes a fume hood, laminar air flow cabinet or glove box The ventilation system should be designed such that the laboratory is at negative pressure relative to surrounding areas. The airflow should be from areas of minimal likelihood of airborne contamination to areas where such contamination is likely
All air from the laboratory should be vented through a fume hood and must not be recirculated either directly, in combination with incoming fresh air in a mixing system, or indirectly, as a result of proximity of the exhaust to a fresh air intake
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Radiation Protection in PET/CT 122
VENTILATION
Sterile roomnegative pressurefiltered air
Dispensationnegative pressure
Corridor
Injectionroom
Fume hood
Laminar airflow cabinets
PassageWork bench
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Radiation Protection in PET/CT 123
Alarm system
Continous monitoring av air pressure gradients
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Radiation Protection in PET/CT 124
Fume hood
The fume hood must be constructed of smooth, impervious, washable and chemical-resistant material. The working surface should have a slightly raised lip to contain any spills and must be strong enough to bear the weight of any lead shielding that may be required
The air-handling capacity of the fume hood should be such that the linear face velocity is between 0.5 and 1.0 metres/second with the sash in the normal working position. This should be checked regularly
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Radiation Protection in PET/CT 125
Sinks
If the Regulatory Authority allows the release of aqueous waste to the sewer a special sink shall be used. Local rules for the discharge shall be available. The sink shall be easy to decontaminate. Special flushing units are available for diluting the waste and minimizing contamination of the sink.
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Radiation Protection in PET/CT 126
Washing facilities
The wash-up sink should be located in a low-traffic area adjacent to the work area Taps should be operable without direct hand contact and disposable towels or hot air dryer should be available An emergency eye-wash should be installed near the hand-washing sink and there should be access to an emergency shower in or near the laboratory
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Radiation Protection in PET/CT 127
Shielding
Much cheaper and more convenient to shield the source, where possible, rather than the room or the person
Structural shielding is generally not necessary in a nuclear medicine department. However, the need for wall shielding should be assessed e.g. in the design of a therapy ward (to protect other patients and staff) and in the design of a laboratory housing sensitive instruments (to keep a low background in a well counter, gamma camera, etc)
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Radiation Protection in PET/CT 128
Layout of a nuclear medicine department
From high to low activity
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Radiation Protection in PET/CT 129
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