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12. SPECT/CT Technology






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  • BGO - Bismuth Gremanate LSO - Lutetium Oxyorthosilicate GSO - Gadolinium Oxyorthosilicate
  • It is important to explain why there is a proportionality between the photon energy absorbed in the detector and the pulse height
  • Explain the origin of the scattered photons (detector and sample) and the completely absorbed photons.
  • The explanation to the better energy resolution in a semiconductor detector is that the energy needed to create a ion-hole pair is much less than the energy to produce a light photon in the scintillation detector. The statistical fluctuation and hence the width of the full energy peak will be much less.
  • The ECG gated acquisition is a type of dynamic acquisition. The RR-interval is divided into a certain number of frames (16 to 32) where each frame then represents a certain phase (t) of the heart cycle. Each time the R-peak in the ECG is detected the collection of data starts in frame 1 during time t, then in frame 2 during time t and so on. The total acquisition time is generally about 10 min so the whole study represents several hundred heart cycles.
  • There are methods to change the radionuclide distribution. In this case (examination of the myocaedium using Tc99 sestamibi) the uptake in the liver and emptying of the gallbladder can be stimulated by giving the patient a fatty meal.
  • This image should be used to explain the principle of filtered back projection. Note that the image contains an animation.
  • The image can be used to illustrate that gammacamera tomography is imaging of a volume where slices can be displayed in any plane.
  • This is an example of a surface rendered image combined with the information in the coronal slices. The patient has an iscemic heart disease.
  • This is an animated image and shows the information in three tomographic planes as well as a surface rendered image to illustrate wall motion.
  • The image to the right is aquired after cleaning of the collimator
  • The lower image is the absolute difference between the upper two images. It clearly shows the ring artifacts.
  • The images in the lower row are acquired using a collimator with 50% lower sensitivity in an 1cm3 area in the center of the field of view. The images in the upper row are from the same patient acquired with a good collimator. It is important to point out the risk of false positive results if the camera is not working perfectly,
  • This image tries to illustrate the problem that could come up if the positioning of an absorption event is dependent on the energy of the photon. If the examination requires a subtraction of two images the result will depend on the positioning rather than the patient, The images are from a parathyroid scan. The left images show the result of subtraction of the upper two images and how the result will be affected by the offset in positioning. The image to the right shows the ”truth”.
  • These are transversal slices of a myocardial tomography. They show the possible effects of an offset in center of rotation. The matrix si z e is 64x64 and the pixel size is 5 mm. Equivalent to an offset in center of rotation is patient movement during acquisition.
  • Remember that compton scattering is the dominating process in the attenuation of photons in soft tissue.
  • In the case of a big patient some of the full energy photons that should have reached the gamma camera will be scattered in the patient. The relation scattered/full energy photons will increase with the volume of the patient.
  • The image can be used to discuss the optimum relation between sensitivity and image quality.
  • This is an illustration to the triple energy window scatter correction. Two narrow windows are used, one on each side of the full energy peak. This information is used to determine the amount of scatter in the full energy window, simply by interpolation. The left image is a planar thallium scan and the middle image is the calculated scatter image which is subtracted from the left image, The result is shown in the right image. I mprovement?
  • This figure tries to explain the origin of the detected photons. It is purely theoretical. Assume an object 20x20x20 cm3 filled with Tc99m. The diagram shows that 12% of the registered photons come from the first cm of the object Only 4% of the registered photons comes from 10cm and 1% from 20 cm
  • This is again the same theoretical phantom and the expected contrast in an image of a 2cm thick object containing no activity.
  • This is an example of how to measure the spatial resolution from the line spread function. It also gives an example of the MTF.
  • The pixel size is used in several application programmes for calculation of distance, surface and volume.
  • These are some examples of the sealed sources used in a nuclear medicine department
  • Note that there different types of generators. This illustrates a dry type with a separate container of saline solution that is changed every time a new elution will be made. In the wet type of generator there is a built in container with enough volume of saline solution for all elutions
  • Example of a transport container for a Tc generator
  • This image shows that extra shielding of the generator should be used.as well as shielding of the elution vial
  • This is a closer look at the top of the generator with the needles where the elution vial and the saline solution vial are placed
  • The shielded elution vial
  • The image can be used for a short explanation of a radiopharmaceutical. The same radioactive substance can be used in labeling of different compunds resulting in radiopharmaceuticals with different properties
  • This is an introduction to the ICRP concept of categorization of hazard which should be used to define some basic building requirements.
  • This is an example of a calculation of the weighted activity. In this case the room for administration of an iodine therapy is a high hazard room
  • These are examples of categorization of hazard for different rooms in a typical nuclear medicine department handling quite large amounts of Tc99m
  • Rooms where work with unsealed sources are taken place should be under negative pressure to minimize the risk of airborne radionuclides to be spread, The sterile environment that might be necessary in preparation of radiopharmaceuticals is achieved in a laminar air flow bench.
  • If there are regulations about air pressure gradients they should be continously monitored and an alarm system introduced
  • This illustartion is from a Nuclear Medicine department in India. Does it follow the general rule to separate high activity areas from low activity areas and to separate working areas from patient areas?

12. SPECT/CT Technology 12. SPECT/CT Technology Presentation Transcript

  • Answer True or False
    • The most common isotope used in SPECT/CT scans is 18 F
    • 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 & C amera QA
    • SPECT/CT scanners
    • Design of SPECT/CT facilities
  • 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
  • Scintillation detector Amplifier PHA Scaler
  • 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
  • Pulse-height distribution NaI(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 Knoll
  • Gamma camera Used to measure the spatial and temporal distribution of a radiopharmaceutical
  • Gamma camera ( principle of operation) PM-tubes Detector Collimator Position X Position Y Energy Z
  • PM-tubes
  • Gamma camera collimators
  • Gamma camera Data acquisition
    • Static
    • Dynamic
    • ECG-gated
    • Wholebody scanning
    • Tomography
    • ECG-gated tomography
    • Wholebody tomography
  • ECG-gated acquisition R Interval n Image n
  • Scintigraphy seeks to determine the distribution of a radiopharmaceutical
  • SPECT cameras are used to determine the three-dimensional distribution of the radiotracer
  • Tomographic acquisition
  • Tomographic reconstruction
  • Tomographic planes
  • Myocardial scintigraphy
  • 12.2 Image Quality & C amera QA
  • Factors affecting image formation
    • 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
  • SPATIAL RESOLUTION 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
  • Resolution - distance High sensitivity High resolution FWHM
  • SPATIAL RESOLUTION - DISTANCE Optimal Large distance
  • Linearity
  • NON UNIFORMITY Cracked crystal
  • NON-UNIFORMITY (Contamination of collimator)
  • NON UNIFORMITY RING ART I FACTS Good uniformity Bad uniformity Difference
  • NON-UNIFORMITY Defect collimator
  • Spatial positioning at different energies Intrinsic spatial resolution with Ga-67 Point source (count rate < 20k cps); quadrant bar pattern; 3M counts; preset energy 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 Scattered photon
  • The amount of scattered photons registered Patient size Energy resolution of the gammacamera Window setting
  • Pulse height distribution Full energy peak Scattered photons The width of the full energy peak (FWHM) is determined by the energy resolution of the gamma camera. There will be an overlap between the scattered photon distribution and the full energy peak, meaning that some scattered photons will be registered. FWHM Overlapping area Energy Counts 0 20 40 60 80 100 120 140 20 60 100 120 140 160 Tc99m
  • Window width 20% 10% 40% Increased window width will result in an increased number of registered scattered photons and hence a decrease in contrast
  • ATTENUATION Register 1000 counts Origin of counts I=I 0 exp(- µx)
  • ATTENUATION Contrast (2cm object) 23% 7% 2%
    • Transmission measurements
    • Sealed source
    • CT
  • ATTENUATION CORRECTION Ficaro et al Circulation 93:463-473, 1996
  • NOISE 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
  • QC GAMMA CAMERA Acceptance Daily Weekly Yearly Uniformity P T T P Uniformity, tomography P P Spectrum display P T T P Energy resolution P P Sensitivity P T P Pixel size P T P Center of rotation P T P Linearity P P Resolution P P Count losses P P Multiple window pos P P Total performance phantom P P P: physicist, T:technician
  • IAEA-TECDOC-602 Quality control of Nuclear medicine instruments 1991 INTERNATIONAL ATOMIC ENERGY AGENCY IAEA May 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)
  • UNIFORMITY Flood source (Tc-99m, Co-57) Point source (Tc-99m) Intrinsic uniformity : Point source at a large distance from the detector. Acquire an image of 10.000.000 counts With collimator: Flood source on the collimator. Acquire an image of 10.000.000 counts
  • Uniformity 1. Subjective evaluation of the image 2. Calculate Integral uniformity (IU) Differential uniformity (DU) IU=(Max-Min)/Max+Min)*100, where Max is the the maximum and Min is the minimum counts in a pixel DU=(Hi-Low)/(Hi+Low)*100, where Hi is the highest and Low is the lowest pixel value in a row of 5 pixels moving over the field of view Matrix 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 Ga 67 Tc 99m I 131
  • TOMOGRAPHIC UNIFORMITY Tomographic uniformity is the uniformity of the reconstruction of a slice through a uniform distribution of activity SPECT phantom with 200-400 MBq Tc99m aligned with the axis of rotation. 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 Lead 200 mm 50 mm Screw clip Polyethylene tubing about 0.5 mm in internal diameter Plastic shims 500 mm Rigid plastic 30 mm 60 mm 5 mm Intrinsic resolution System resolution IAEA TECDOC 602
  • SPATIAL RESOLUTION Tc-99m or other radionuclide in use Intrinsic: Collimated line source on the detector System: Line source at a certain distance Calculate FWHM of the line spread function FWHM: 7.9 mm
  • TOMOGRAPHIC RESOLUTION Method 1: Measurement with the Jaszczak phantom, with and without scatter (phantom filled with water and with no liquid) Method 2: Measurement with a Point or line source free in air and Point or line source in a SPECT phantom with water
  • 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%
  • 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 rotation Point source of Tc-99m or Co-57 Make 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 linear function at each angle. Cosine function Linear function
  • Total performance Total performance phantom. Emission or transmission. Compare result with reference image.
    • Point source
    • Collimated line source
    • Line source
    • Flood source
    Tc99m, Co57, Ga67 <1 mm
  • Phantoms for QC of gamma cameras
    • Bar phantom
    • Slit phantom
    • Orthogonal hole phantom
    • Total performance phantom
  • Phantoms for QC of gamma cameras
  • QUALITY CONTROL ANALOGUE IMAGES Quality control of film processing: base & fog, sensitivity, contrast
  • QUALITY ASSURANCE COMPUTER EVALUATION 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
  • Semi-conductor detector Applications in nuclear medicine
    • Identification of nuclides
    • Control of radionuclide purity
  • 12.3. 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
    • 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 in all directions X ray tube collimators
  • A look inside a rotate/rotate CT X Ray Tube Detector Array and Collimator
  • A Look Inside a Slip Ring CT X Ray Tube Detector Array Slip Ring Note : how most of the electronics is placed on the rotating gantry
  • 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 Design of SPECT/CT facilities
  • Nuclear medicine application according to type of radionuclide 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
  • 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 .
  • 99 Mo- 99m Tc GENERATOR 99 Mo 87.6% 99m Tc  140 keV T½ = 6.02 h 99 Tc ß - 292 keV T½ = 2*10 5 y 99 Ru stable 12.4% ß - 442 keV  739 keV T½ = 2.75 d
  • Technetium generator Mo-99 Tc-99m Tc-99 66 h 6h NaCl AlO 2 Mo-99 +Tc-99m Tc-99m
  • Technetium generator
  • Technetium generator
  • Technetium generator
  • Technetium generator
  • Technetium generator
  • Radiopharmaceuticals Radionuclide Pharmaceutical Organ Parameter + colloid Liver RES Tc-99m + MAA Lungs Regional perfusion + DTPA Kidneys Kidney function
    • Radiopharmaceuticals used in nuclear medicine can be classified as follows:
    • ready-to-use radiopharmaceuticals
    • e.g. 131 I- MIBG, 131 I- iodide , 201 Tl- chloride , 111 In- DTPA
    • instant kits for preparation of products
    • e.g. 99m Tc-MDP, 99m Tc-MAA, 99m Tc-HIDA, 111 In-Octreotide
    • kits requiring heating
    • e.g. 99m Tc-MAG3, 99m Tc-MIBI
    • products requiring significant manipulation
    • e.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 hazard 50-50000 MBq Medium hazard >50000 MBq High hazard
  • Categorization of hazard Weighting factors according to radionuclide Class Radionuclide Weighting factor A 75 Se, 89 Sr, 125 I, 131 I 100 B 11 C, 13 N, 15 O, 18 F, 51 Cr, 67 Ga, 99m Tc, 111 In, 113m In, 123 I, 201 Tl 1.00 C 3 H, 14 C, 81m Kr 127 Xe, 133 Xe 0.01
  • Categorization of hazard Weighting 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
  • Categorization of hazard Administration of 11 GBq I-131 Weighting factor, radionuclide 100 Weighting factor, operation 1 Total weighted activity 1100 GBq Weighted activity Category < 50 MBq Low hazard 50-50000 MBq Medium hazard >50000 MBq High hazard
  • Categorization of hazard Patient examination, 400 MBq Tc-99m Weighting factor, radionuclide 1 Weighting factor, operation 1 Total weighted activity 400 MBq Weighted activity Category < 50 MBq Low hazard 50-50000 MBq Medium hazard >50000 MBq High hazard
  • Categorization of hazard Patients waiting, 8 patients, 400 MBq Tc-99m per patient Weighting factor, radionuclide 1 Weighting factor, operation 0.1 Total weighted activity 320 MBq Weighted activity Category < 50 MBq Low hazard 50-50000 MBq Medium hazard >50000 MBq High hazard
  • Category of hazard (premises not frequented by patients) Typical results of hazard calculations High hazard Room for preparation and dispensing radiopharmaceuticals Temporary storage of waste Medium hazard Room for storage of radionuclides Low hazard Room for measuring samples Radiochemical work (RIA) Offices
  • Category of hazard ( premises frequented by patients) Typical results of hazard calculations High hazard Room for administration of radiopharmaceuticals Examination room Isolation ward Medium hazard Waiting room Patient toilet Low hazard Reception
  • Building requirements Category Structural shielding Floors Worktop surfaces of 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 account e.g. waiting room
  • Building requirements Category Fume hood Ventilation Plumbing First aid of 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
        • 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 room negative pressure filtered air Dispensation negative pressure Corridor Injection room Fume hood Laminar air flow cabinets Passage Work bench
  • Continous monitoring av air pressure gradients Alarm system
  • 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
  • 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.
  • 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
    • 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