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ECSE-4963 Introduction to Subsurface Sensing and Imaging Systems
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ECSE-4963 Introduction to Subsurface Sensing and Imaging Systems


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    • 1. BMED-4800/ECSE-4800 Introduction to Subsurface Imaging Systems Lecture 6: Nuclear Medicine Kai E. Thomenius1 & Badri Roysam2 1 Chief Technologist, Imaging Technologies, General Electric Global Research Center 2 Professor, Rensselaer Polytechnic Institute Center for Sub-Surface Imaging & Sensing
    • 2. Homework #3: Using the “Faridana” (Adel Faridani) filtered back projection code example, change filter parameters for a lower bandpass. Demonstrate loss of spatial resolution. • nts.html – A. Faridani: Introduction to the Mathematics of Computed Tomography. Inside Out: Inverse Problems and Applications, G. Uhlmann (editor), MSRI Publications Vol. 47, Cambridge University Press, 2003, pp. 1-46. • - MATLAB code for filtered back projection – Designer Shepp-Logan phantom – Filter design possibilities – Make sure to use the modified code (fbp2ket.m) available from
    • 3. Recap: CT & Filtered Backprojection Backprojection reconstruction w. no filtering. Impact of filter on Sinogram. Backprojection reconstruction w. filter, compare images.
    • 4. Taking Stock of x-ray CT • X-ray images of a live or dead subject look the same! – the core contrast mechanism does not depend on activity, only on structure • Some activities can be sensed (e.g., gross movements can be sensed with cine x-ray) –The kinds of activities with the greatest medical value are of a biochemical nature • They involve the presence/absence, chemical state, spatial distribution, and movements of specific biochemicals in the body • This observation has driven the development of functional & molecular imaging methods.
    • 5. Nuclear Medicine • Basic Idea: – Inject patient with radio-isotope labeled substance (tracer) • Chemically the same as a biochemical in the body, but physically different – Detect the radioactive emissions (gamma rays) • Super-short wavelength • But, can’t achieve the implied high resolution – Detection technology limitations – Not enough photons! – This can be done in 2D: scintigraphy – This can be done in 3D: SPECT/PET • Use filtered back-projection to reconstruct the 3-D image, just like x-ray CT
    • 6. Nuclear Medicine • Imaging is done by tracing the distribution of radiopharmaceuticals within the body. • Radionuclides or radioisotopes are atoms that undergo radioactive decay, and emit radiation. • In nuclear medicine, we are interested in radionuclides that emit x-rays or gamma rays. • A radiopharmaceutical is a radionuclide bound to a biological agent.
    • 7. Example: FDG • Fluorodeoxyglucose is a radiopharmaceutical is a glucose analog with the radioactive isotope Fluorine-18 in place of OH • 18 F has a half life of 110 minutes • FDG is taken up by high glucose using cells such as brain, kidney, and cancer cells. • Once absorbed, it undergoes a biochemical reaction whose products cannot be further metabolized, and are retained in cells. • After decay, the 18 F atom becomes a harmless non-radioactive heavy oxygen 18 O– that joins up with a hydrogen atom, and forms glucose phosphate that is eliminated via carbon dioxide and water 2-Deoxy-D-Glucose (2DG)
    • 8. What Happens Upon Radioactive Decay Basic Idea: – Nucleus emits a positron (an anti-electron) • A short-lived particle • Same mass as electron, but opposite charge – Positron collides with a nearby electron and annihilates • Two 511 keV gamma rays are produced • They fly in opposite directions (to conserve momentum) Nucleus (protons+neutrons) electrons Isotope Max. Positron Range (mm) 18 F 2.6 11 C 3.8 68 Ga 9.0 82 Rb 16.5 Gamma Photon #1 Gamma Photon #2 BANG
    • 9. Gamma Ray – Matter Interactions • 3 basic mechanisms for photon - matter interaction: – Photoelectric Effect (transfer energy to an electron, ejecting it). For < 50KeV – Compton Scatter (lose energy to an electron, and creat e alower-energy photon). For 100KeV – 10MeV – Electron-positron pair Production (For > 1MeV) • Any one of these can happen to the radionuclide gamma-rays. Compton Scatter Pair Production
    • 10. Energy of a Gamma Ray • A radionuclide has a typical energy: e.g. 140 keV for 99m Tc • Detection of lower energy scattered gamma- or x-rays degrades contrast and image quality. • A radioisotope emits one (or more) very sharp energy lines
    • 11. Effects of Gamma Rays on Tissue • Gamma rays cause ionization –Capable of causing damage at the cellular level • Actually used to ultra-sterilize equipment • Used to kill tumors (radiation therapy) – The greatest damage occurs in the 3 – 10MeV range –High energy gamma rays just pass throuigh the body and cause no damage
    • 12. How do we Detect Gamma Rays? • Some crystals (sodium iodide) exhibit the property of scintillation. • Scintillation is a flash of light produced in a transparent material by an ionization event. • When a gamma ray strikes this crystal, it knocks an electron loose from an Iodine atom. • This electron then goes to a lower energy state, and in doing so, emits a faint burst if light • This faint burst of light can be detected using a sensitive device known as a photomultiplier tube (PMT). • Electronic circuits count the number of flashes and these numbers are used to reconstruct images.
    • 13. Cross-section of an Anger Camera 1. Shield Around Head 2. Mounting Ring 3. Collimator Core 4. Sodium Iodide Crystal 5. Photomultiplier Tubes Named after Hal Anger
    • 14. Cross-section of an Anger Camera
    • 15. Collimator Design & Function Resolution v. Efficiency Trade-off
    • 16. SPECT Instrument • The “gamma camera” is a 2-D array of detectors • One or more gamma cameras are used to capture 2-D projections at multiple angles • Use filtered back-projection to reconstruct 3-D image! – Actual sinograms appear “noisy” due to the fact that we don’t have enough photons – Quantum-limited imaging 3-camera SPECT instrument
    • 17. Modern SPECT Scanners • GE Hawkeye DigiRad Mobile SPECT System
    • 18. Nuclear Medicine Images • Typical image: – 64 by 64 pixels • Intensity gives “counts per pixel” • Pseudocolor often used. • Nuclear med imaging modes: – Static – Dynamic – MUGA – Whole Body – SPECT
    • 19. Whole Body Imaging • Bright spots indicate regions where the radioisotope is bound
    • 20. Cardiac Study
    • 21. Cardiac Study • Evaluation of the coronary artery circulation – Myocardial perfusion • 3D Studies of the radionuclide activity
    • 22. Nuclear Medicine Performance Metrics • Typical performance: – Energy resolution: 9.5 – 10% • FWHM response – Spatial resolution: 3.2 – 3.8 mm – Uniformity: 2 – 4%
    • 23. Strengths & Limitations of SPECT • Main Strengths: – Low cost: cheaper instrumentation & cheaper longer-lived and easily obtained radio-pharmaceuticals – quick acquisition and simple reconstruction – Can be made nearly portable – Can be shaped to suit custom applications – Can be made to acquire time series – Can be gated to sync with other signals (e.g., ECG) – Multiple camera heads (typ. 2 – 3) can speed up acquisition • Main Weakness: – Low resolution: Reconstructed images typically have resolutions of 64×64 or 128×128 pixels, with the pixel sizes ranging from 3–6 mm.) – Attenuation of gamma rays leads to underestimation of activity in deep regions – Intense areas of activity result in a lot of “streaking” artifacts
    • 24. Ways to Improve Upon SPECT • Better reconstruction algorithms – Model the point spread of photons more accurately – Model the non-uniform attenuation of gamma rays in the body (leveraging x-ray CT) • Build combo “x-ray CT & SPECT” systems • Use both the photons: PET – Since a pair of gamma rays at 180o are produced, try to detect pairs of photons instead of single photons • Detect photon timing: TOF-PET – The difference in photon arrivals can tell us where the decay event occurred!
    • 25. Better Algorithms • Filtered back-projection algorithm – produces a background artifact, discussed earlier – Noisy reconstruction • The Maximum Likelihood algorithm produces a better reconstruction for the same data Filtered Back-Projection Maximum Likelihood
    • 26. Positron Emission Tomography: PET • Several gamma-detector rings surround the patient. • When one of these detects a photon, a detector opposite to it, looks for a match. • Time window for the search is few nanosecs. • If such a coincidence is detected, a line is drawn between the detectors. • When done, there will be areas of overlapping lines indicating regions of radioactivity.
    • 27. Emission Detection • If detectors A & B receive gamma rays at the approx. same time, we have a detection • Hard sensor and electronics design problem, expensive A B Ring of detectors
    • 28. Image Reconstruction • We can sort our set of detections by angle, and view the data as a set of angular projections • Use filtered back-projection algorithm!
    • 29. PET Images • Single-channel images • Noisy, and blurry – Not ideal for segmentation – Segment MRI/CT for defining anatomy – Register the images – Measure activity
    • 30. PET Radiotracers • 18 FDG is probably the most widely used PET tracer. • HIGH FDG pick- up by tumors first reported in 1980 at Brookhaven NL. • Can also be used to measure rate of metabolism in the brain.
    • 31. Application in Lung Cancer Case Study: • 55-year old female • Lung Cancer • 2 cycles of chemo & radiotherapy PET results: • Increased uptake of FDG in lung nodules • Increased uptake of FDG in lymph nodes Therapy will have to be continued.
    • 32. SPECT vs PET • Both are Major Functional imaging tools – SPECT: Single-photon Emission Computed Tomography • cheap and low-resolution • Tells us where blood is flowing – PET: Positron Emission Tomography • expensive but higher-resolution PET image Showing a tumor
    • 33. How Does PET Compare With Other Imaging Modalities? • PET provides images of molecular-level physiological function • Extends capabilities of other modalities. – Like MR & CT, it uses tomographic algorithms – Like Nuclear Medicine, the images represent distributions of radiotracers. • But that’s where the similarity ends… CT Scan MRI Scan PET Scan Report: Normal Report: Normal Report: Patient Deceased.
    • 34. Other Imaging Instruments • Structure imaging: – CT & Magnetic Resonance Imaging – Ultrasound Imaging • Functional Imaging: – Nuclear Imaging • Positron Emission Tomography • Single-Photon Emission Computed Tomography • Combined Modalities – Functional & structural imaging 99 image of the year, U. of Pittsburgh
    • 35. PET/CT Scanners • Generation of PET & CT images in a single study • The image data sets are registered and fused. – Anatomic data from CT – Metabolic data from PET • Colorectal Cancer shown in images.
    • 36. Steps in imaging • Imaging done by a gamma camera. • A radionuclide is infused into the patient’s blood. – Usually, the radionuclides have a specific physiological role. – This gives the clinical specificity to the procedure. • Concentrations of the agent emit greater quantity of gamma rays. • These are mapped by the camera head.
    • 37. Source Material • mmunity/nmpet/nmpet_neighborhood.jsp • Siemens & Philips web sites for nuclear medicine & PET • pphome.html • GG167/13%20Nuclear%20Medicine.pdf
    • 38. Summary • Introduction to Nuclear Medicine, SPECT and PET imaging. – Additional examples of agents (probes) introduced to reveal subsurface phenomena. – Today’s focus on radioactive labeling. • Review of instruments – Relatively straightforward devices. – Signal-to-noise ratio challenges, need to limit exposure. • Powerful clinical tools. • Much of today’s research focused on PET and extensions of PET technology.
    • 39. Instructor Contact Information Badri Roysam Professor of Electrical, Computer, & Systems Engineering Office: JEC 7010 Rensselaer Polytechnic Institute 110, 8th Street, Troy, New York 12180 Phone: (518) 276-8067 Fax: (518) 276-6261/2433 Email: Website: Secretary: Laraine Michaelides, JEC 7012, (518) 276 –8525,
    • 40. Instructor Contact Information Kai E Thomenius Chief Technologist, Ultrasound & Biomedical Office: KW-C300A GE Global Research Imaging Technologies Niskayuna, New York 12309 Phone: (518) 387-7233 Fax: (518) 387-6170 Email:, Secretary: Laraine Michaelides, JEC 7012, (518) 276 –8525,