BASIC PRINCIPLE OF SPECT AND PE.....T.pptx

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1. BASIC PRINCIPLE OF SPECT AND PET

2. SPECT Tomographic imaging technique using gamma rays. Able to provide true 3D information. Image is presented as cross-sectional slices through the patient, but can be reformatted or manipulated as required. requires injection of a gamma-emitting radioisotope (called radionuclide). (The radioactive isotope decays, resulting in the emission of gamma rays). Used for visualization of functional information

3. Standard planar projection images are acquired from an arc of 180 degrees (most cardiac SPECT) or 360 degrees (most noncardiac SPECT) about the patient. Most SPECT systems use one or more scintillation camera heads that revolve about the patient Transverse images are reconstructed using either filtered back projection (as in CT) or iterative reconstruction methods. Multi-headed gamma cameras can provide accelerated acquisition (dual-headed camera can be used with heads spaced 180 degrees apart, allowing 2 projections to be acquired simultaneously Triple-head cameras with 120-degree spacing )

4. Principles Multiple 2-D images are taken from multiple angle. (Projections) A computer is then used to apply a tomographic reconstruction algorithm to the multiple projections,(Projections are acquired at defined points during the rotation, typically every 3–6 degrees) This dataset may then be manipulated to show thin slices along any chosen axis of the body. Tracer used in SPECT emits gamma radiation that is measured directly.

5. Image acquisition Camera head or heads revolve about the patient, acquiring projection images from evenly spaced angles May acquire images while moving (continuous acquisition) or may stop at predefined angles to acquire images (“step and shoot” acquisition) Each projection image is acquired in frame mode

6. If camera heads produced ideal projection images (i.e., no attenuation by patient and no degradation of spatial resolution with distance from camera), projection images from opposite sides of patient would be mirror images then 180 degree only sufficient . Attenuation greatly reduces number of photons from activity in the half of patient opposite camera head; this information is blurred by distance

7. Schematic diagram of SPECT data acquisition For each projection view, the computer sends a message to the gamma camera tostep to the next viewing angle and, after thecamera sends a message back to the computer that it is ready to acquire, the computer acquires projection image acquisition time. The total SPECT study acquisition time is T=mt, where m is the number of views acquired

8. Transverse image reconstruction After projection images are acquired, they are usually corrected for axis-of-rotation misalignments and for non uniformities Following these corrections, transverse image reconstruction is performed using either filtered back projection or iterative methods

9. Filter kernels Projection images of better spatial resolution and less quantum mottle require a filter with higher spatial frequency cutoff to avoid loss of spatial resolution in the reconstruction transverse images Projection images of poorer spatial resolution and greater quantum mottle require a filter with lower spatial frequency cutoff to avoid excessive quantum mottle in the reconstructed transverse images

10. Iterative reconstruction An initial activity distribution in the patient is assumed Projection images are calculated from the assumed activity distribution, using the known characteristics of the scintillation camera Calculated projection images are compared with actual projection images and, based on this comparison, the assumed activity distribution is adjusted Process repeated several times until calculated projection images approximate the actual ones

11. Calculation of projection images takes into account the decreasing spatial resolution with distance from the camera face If a map of the attenuation characteristics of the patient is available, the calculation of the projection images can include the effects of attenuation Iterative methods can partially compensate for effects of decreasing spatial resolution with distance, photon scattering in the patient, and attenuation in the patient

12. Attenuation and correction X- or gamma rays that must traverse long paths through the patient produce fewer counts, due to attenuation, than those from activity closer to the near surface of the patient Transverse image slices of a phantom with a uniform activity distribution will show a gradual decrease in activity toward the center Attenuation effects are more severe in body SPECT than in brain SPECT

13. A common correction method assumes a constant attenuation coefficient throughout the patient Some SPECT cameras have radioactive sources to measure the attenuation through the patient. After acquisition, the transmission projection data are reconstructed to provide maps of tissue attenuation characteristics across transverse sections of the patient, similar to x-ray CT image These attenuation maps are used during SPECT image reconstruction to provide attenuation-corrected SPECT images.

14. SPECT collimators Most commonly used is the high-resolution parallel-hole collimator Fan-beam collimators mainly used for brain SPECT FOV decreases with distance from collimator If used for body SPECT, portions of the body are excluded from the FOV, creating artifacts in the reconstructed images

15. Multihead SPECT cameras Two or three scintillation camera heads reduce limitations imposed by collimation and limited time per view. Permits use of higher resolution collimators for a given level of quantum mottle Requirement for electrical and mechanical stability of the camera heads.

16. SR-Comparison with conventional planar scintillation camera imaging In theory, SPECT should produce spatial resolution similar to that of planar scintillation camera imaging In clinical imaging, its resolution is usually slightly worse Camera head is closer to patient in conventional planar imaging than in SPECT Short time per view of SPECT may mandate use of lower resolution collimator to obtain adequate number of counts

17. In planar imaging, radioactivity in tissues in front of and behind an organ of interest causes a reduction in contrast Main advantage of SPECT is markedly improved contrast and reduced structural noise produced by eliminating the activity in overlapping structures SPECT also offers promise of partial correction for effects of attenuation and scattering of photons in the patient

18. Advantages of SPECT. Improved image contrast. Improved quantification of cardiac function, tumor/organ volume determination, the quantification of radioisotope uptake. Problems of gamma-ray attenuation and scatter may be better handled by SPECT (although, as yet, not completely), over planar projection imaging. SPECT acquires 2-D projection of a 3-D volume.

19. SPECT applications 1.Brain: Perfusion (stroke, epilepsy schizophrenia, dementia[Alzheimer])Tumors 2.Heart: Coronary artery disease,Myocardial infarcts. 3.Bone scan. 4.Tumor and tumor staging.

20. Improvements in SPECT technology The application of multiple gamma camera heads. Noncircular orbits. The application of non-uniform attenuation correction methods. Gated SPECT perfusion scans with 99mTc agents and 201-Tl, also gated SPECT blood pool. SPECT systems lack anatomical resolution-Hybrid technique SPECT/CT .

21. SPECT/CT SPECT/CT system was only introduced in 1999. The advantage of using CT data for attenuation correction of emission data. low-power X ray tube with separate gamma and X ray detectors mounted on the same slip ring gantry. X-ray system operated at 140 kV with a tube current of only 2.5 mA (low dose) It provides a high photon flux that significantly reduces the statistical noise associated with the correction in comparison to other techniques (i.e., radionuclide's used as transmission

22. The total imaging time is significantly reduced because of fast acquisition speed of CT scanners. the anatomic images acquired with CT can be fused with the emission images to provide functional anatomic maps for accurate localization of radiopharmaceutical uptake. CT images are acquired as transmission maps with a high photon flux and are high-quality representations of tissue attenuation and thus can provide the basis for attenuation correction.

23. POSITRON EMISSION TOMOGRAPHY

24. Positron Decay. Also known as Beta Plus decay. A proton changes to a neutron, a positron (positive electron), and a neutrino Mass number A does not change, proton number Z reduces. The positron may later annihilate a free electron, generate two gamma photons in opposite directions. These gamma rays are used for medical imaging (Positron Emission Tomography)

25. INTRODUCTION Is a NM imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted by a positron emitting radionuclide (tracer). Three-dimensional images of tracer concentration within the body are then constructed by computer analysis. If biologically active molecule chosen for PET is FDG an analogue of glucose, the concentrations of tracer imaged then give tissue metabolic activity, in terms of regional glucose uptake.

26. short-lived radioactive tracer isotope is injected into the living subject (usually into blood circulation). The tracer is chemically incorporated into a biologically active molecule. There is a waiting period while the active molecule becomes concentrated in tissues of interest; then the subject is placed in the imaging scanner. The molecule most commonly used for this purpose is fluorodeoxyglucose (FDG).

27. As the radioisotope undergoes positron emission . it emits a positron, an antiparticle of the electron with opposite charge. The emitted positron travels in tissue for a short distance (typically less than 1 mm) The encounter annihilates both electron and positron, producing a pair of annihilation (gamma) photons moving in approximately opposite directions. These are detected when they reach a scintillator in the scanning device, creating a burst of light which is detected by photomultiplier tubes.

28. Simultaneous or coincident detection of the pair of photons moving in approximately opposite direction. (accepted within a timing-window of a few nanoseconds) Most significant fraction of electron-positron decays result in two 511 keV gamma photons being emitted at almost 180 degrees to each other. It is possible to localize their source along a straight line of coincidence.(also called the line of response)

29. Annihilation coincidence detection Positrons emitted in matter lose most of their kinetic energy by causing ionization and excitation When a positron has lost most of its kinetic energy, it interacts with an electron by annihilation The entire mass of the electron-positron pair is converted into two 511-keV photons, which are emitted in nearly opposite directions

30. Annihilation Coincidence Detection Detect two events in opposite directions occurring “simultaneously "Time window is 2-20 ns, typically 12 ns No detector collimation is required(Higher sensitivity) less wasteful of photon.

31. True, random, and scatter coincidences A true coincidence is the simultaneous interaction of emissions resulting from a single nuclear transformation A random coincidence, which mimics a true coincidence, occurs when emissions from different nuclear transformations interact simultaneously with the detectors A scatter coincidence occurs when one or both of the photons from a single annihilation are scattered, but both are detected

32. Design of a PET scanner Scintillation crystals coupled to PMTs are used as detectors in PET. Signals from PMTs are processed in pulse mode to create signals identifying the position, deposited energy, and time of each interaction Energy signal is used for energy discrimination to reduce mispositioned events due to scatter and the time signal is used for coincidence detection

33. Early PET scanners coupled each scintillation crystal to a single PMT. Size of individual crystal largely determined spatial resolution of the system Modern designs couple larger crystals to more than one PMT Relative magnitudes of the signals from the PMTs coupled to a single crystal used to determine the position of the interaction in the crystal

34. Scintillation materials Material must emit light very promptly to permit true coincident interactions to be distinguished from random coincidences and to minimize dead-time count losses at high interaction rates Must have high linear attenuation coefficient for 511-keV photons in order to maximize counting efficiency

35. Most PET systems use crystals of bismuth germanate (Bi4Ge3O12, abbreviated BGO) BGO light output is less and light is emitted slowly.Inorganic crystal like lutetium oxyorthosilicate (Lu2Sio4O)LSO and gadolinium oxyorthosilicate(Gd2SiO4O) GSO both activated with cerium are newer crystal of choice They have Faster light emission than BGO produces better performance at high interaction rates

36. 2D data acquisition In 2D (slice) data acquisition, coincidences are detected and recorded within each detector ring or small groups of adjacent detector rings PET scanners designed for 2D data acquisition have thin annular collimators (typically tungsten) to prevent most radiation emitted by activity outside a transaxial slice from reaching the detector ring for that slice Fraction of scatter coincidences reduced because of the geometry

37. Comparison of PET with SPECT PET scanner more efficient than scintillation camera due to use of annihilation coincidence detection instead of collimation; also yields superior spatial resolution Spatial resolution in SPECT deteriorates from edge toward center; PET is relatively constant across transaxial image, best at center Attenuation less severe in SPECT; accurate attenuation correction possible in PET (with transmission source) Cost: SPECT ~US$500,000; PET ~US$1M - $2M

38. Factors affecting availability Main factors limiting availability of PET are the relatively high cost of a dedicated PET scanner and, in many areas, the lack of local sources of F-18 FDG. Positron emitter - short half life require nearby cyclotron facility. Multi head SPECT cameras with coincidence circuitry and SPECT cameras with high-energy collimators provide less expensive, although less accurate, alternatives for imaging FDG

39. Clinical application of PET. Characterizing brain disorders such as Alzheimer disease and epilepsy and cardiac disorders such as coronary artery disease and myocardial viability Diagnosis and staging of non small cell lung cancer. Diagnosis and staging of colorectal cancer. Malignant melanoma.(to see distant mets) Hodgkin and Non-Hodgkin Lymphoma. Esophageal Carcinoma Head and Neck Cancer. Breast Carcinoma.