spect-ct technology and facility design

International Atomic Energy Agency

SPECT/CT TECHNOLOGY & FACILITY

DESIGN

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

Objective

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

• SPECT cameras

• Image Quality & CImage Quality & Camera QA

• SPECT/CT scanners

• Design of SPECT/CT facilities

Content

12.1 12.1 SPECT cameras

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

Detector

PhotocathodecathoddDynodes

Amplifier

Scaler

Scintillation detector

Pulse height analyzer

Pulse height (V)

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

counted not counted

Pulse-height distributionNaI(Tl)

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

Comparison of spectrum from a Na(I) scintillation

detector and a Ge(Li) semi-conductor detector

Gamma cameraGamma camera

Used to measure the spatial and temporal distribution of a radiopharmaceutical

Gamma camera(principle of operation)

PM-tubesDetectorCollimator

Position XPosition YEnergy Z

C ounter

C lock

PulsesE nergy windowr

C omputer

Patient

GAMMA CAMERA

PM-tubes

Gamma camera collimators

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

Gamma cameraData acquisition

Intervaln

Image n

ECG-gated acquisition

Scintigraphy seeks to determine the distribution of

a radiopharmaceutical

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

radiotracer

Tomographic acquisition

x-position

C ount rate

Tomographic reconstruction

Tomographic planes

Myocardial scintigraphy

ECG GATED TOMOGRAPHY

12.2 Image Quality & C12.2 Image Quality & Camera QA

•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

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

Object Image

Intensity

SPATIAL RESOLUTION

Resolution - distance

0 2 4 6 8 10 12 14 16

Distance (cm)

m) High sensitivity

High resolution

Optimal Large distance

SPATIAL RESOLUTION - DISTANCE

Linearity

NON UNIFORMITY

Cracked crystal

NON-UNIFORMITY

(Contamination of collimator)

NON UNIFORMITYRING ARTIFACTS

Good uniformity Bad uniformity

Difference

NON-UNIFORMITY

Defect collimator

COUNT RATE PERFORMANCE

(IAEA QC Atlas)

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)

Spatial positioning at different energies

Center of Rotation

Tilted detector

Scattered radiation

photon

electron

Scatteredphoton

The amount of scattered photons registered

Patient sizeEnergy resolution of the gammacamera

Window setting

PATIENT SIZE

Pulse height distribution

Energy

Counts

20 60 100

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.

Overlappingarea

Window width

10%40%

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

SCATTER CORRECTION

0 20 40 60 80 100 120 140

Register 1000 counts Origin of counts

ATTENUATION

I=I0 exp(-µx)

Contrast (2cm object)

23% 7% 2%

ATTENUATION

ATTENUATION CORRECTION

Transmission measurements• Sealed source• CT

Ficaro et al Circulation 93:463-473, 1996

Count density

Gamma camera

Operational considerations

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

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

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)

QC Gamma camera

Energy resolution

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)

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

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

UNIFORMITY/DIFFERENT RADIONUCLIDES

D BOULFELFEL

Dubai Hospital

All 4 images acquired with:Matrix: 256 x 256, # counts: 30 Mcounts

Tl 201

Tc 99m

LINEARITY AND UNIFORMITY CORRECTIONS

Dogan Bor, Ankara

OFF PEAK MEASUREMENTS

Dogan Bor, Ankara

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

INCORRECT MEASUREMENT

Two images of a flood source filled with a solution of Tc-99m, which had not been mixed properly

Spatial resolution

Measured with: Flood source or point source plus a Bar phantom

Subjective evaluation of the image

SPATIAL RESOLUTION

Lead200 mm

Screw clipPolyethylene tubingabout 0.5 mm in internaldiameter

Plastic shims

500 mm

Rigid plastic

Intrinsic resolution System resolution

IAEA TECDOC 602

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

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

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%

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

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

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

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

Pixel size

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

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

Total performance

SOURCES FORQC OF GAMMA CAMERAS

•Point source•Collimated line source•Line source•Flood source

Tc99m, Co57, Ga67

Phantoms for QC ofgamma cameras

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

Phantoms for QC ofgamma cameras

QUALITY CONTROLANALOGUE IMAGES

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

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

•Identification of nuclides

•Control of radionuclide purity

Semi-conductor detectorApplications in nuclear medicine

12.3. SPECT/CT12.3. SPECT/CT

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

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

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

The CT Scanner

X ray emission inall directions

X ray tube

collimators

X Ray Tube

Detector Arrayand Collimator

A look inside a rotate/rotate CT

A Look Inside a Slip Ring CT

X RayTube

Detector Array

Slip Ring

Note: how most

of theelectronics

isplaced on

the rotatinggantry

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

Conversion of to CT number

• Distribution of values initially measured

values are scaled to that of water to give the CT number

12.5 12.5 Design of SPECT/CT facilities

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

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.

99Mo-99mTc GENERATOR

99Mo87.6% 99mTc

140 keVT½ = 6.02 h

ß- 292 keVT½ = 2*105 y

99Ru stable

ß- 442 keV 739 keVT½ = 2.75 d

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

Mo-99+Tc-99m

Tc-99m

Technetium generator

Radionuclide Pharmaceutical Organ Parameter

+ colloid Liver RES

Tc-99m + MAA Lungs Regional perfusion

+ DTPA Kidneys Kidney function

Radiopharmaceuticals

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

Laboratory work with radionuclides

Administration of radiopharmaceuticals

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

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

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

Administration of 11 GBq I-131

Weighting factor, radionuclide 100Weighting factor, operation 1

Total weighted activity 1100 GBq

Patient examination, 400 MBq Tc-99m

Weighting factor, radionuclide 1Weighting factor, operation 1

Total weighted activity 400 MBq

Patients waiting, 8 patients, 400 MBq Tc-99m per patient

Weighting factor, radionuclide 1Weighting factor, operation 0.1

Total weighted activity 320 MBq

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

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

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

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

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

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

VENTILATION

Sterile roomnegative pressurefiltered air

Dispensationnegative pressure

Corridor

Injectionroom

Fume hood

Laminar airflow cabinets

PassageWork bench

Alarm system

Continous monitoring av air pressure gradients

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

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.

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

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)

Layout of a nuclear medicine department

From high to low activity

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

spect-ct technology and facility design

count rate

nuclear medicine

iaea qc atlas

patient bed

pulse height

full energy

radionuclide

ankara radiation

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