Physics of Nuclear Medicine, SPECT and PET Jerry Allison, Ph.D. Department of Radiology Medical College of Georgia Augusta University

Outline Radionuclides in Nuclear Medicine Radiation Dose Gamma Camera Basics SPECT (Single Photon Emission Computed Tomography) PET (Positron Emission Tomography

Radionuclides used in nuclear medicine Less than 20 radionuclides but hundreds of labeled compounds © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps, 4th edition, 2012

Effective dose of NM procedures

Dose Definition Effective dose E (Sv): measure of absorbed dose to whole body, the product of equivalent dose and organ specific weighting factors Whole body dose equivalent to the nonuniform dose delivered Equivalent dose: averahe absorbed dose across an organ or tissue with radiation specific weighting factors (Sv)

How to obtain a NM image? Administer radiopharmaceutical (a radionuclide labeled to a pharmaceutical) The radiopharmaceutical concentrates in the desired locations Nucleus of the radionuclide decays to emit photons (g , x-ray) Detect the photons using a “gamma camera”

Gamma Camera Basics p a t i e n c o l m r d P M T - amplify & sum position analysis Pulse Height Analysis u s y X Y Z

Photomultiplier tube (PMT) 40 to 100 PM tubes (d = 5 cm) in a modern gamma camera photocathode directly coupled to detector or connected using plastic light guides ultrasensitive to magnetic fields

Why collimator? – image formation w/o collimator with collimator detector sources images image collimator Image of a point source is the whole detector. Image of a point source is a point.

Why collimator? – image formation to establish geometric relationship between the source and image The collimator has a major affect on gamma camera sensitivity (count rate) and spatial resolution parallel-hole collimator

Collimators Most often used: parallel-hole collimator For thyroid and heart: pin-hole collimator For brain and heart: converging collimator 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

Collimator Summary Collimator must be matched to energy of radionuclide Efficiency changes little with distance to source (patient) Resolution falls off quickly with distance to source (patient) Low energy: < 160 keV Medium energy: < 250 keV High energy

Energy spectrum of detector Septal penetration & scatter: energy deposited in detector is between 0 and E0. photopeak: all energy of g photons (E0) deposited in detector energy window

Photopeak All the energy of a g photon (E0) is deposited in the detector e.g. E0 = 140 keV for Tc-99m p.e c.s or

Septral penetration & scatter spectrum Some of the energy of a g photon (E0) is deposited in the detector NOT USEFUL FOR IMAGING c.s p.e 30 keV x-ray p.e x-ray

Modern Camera Design Most cameras use rectangular heads Most cameras are designed to do SPECT imaging The dual head is the most common design

Tomographic imaging (SPECT) SPECT (Single Photon Emission Computed Tomography) Tomographic imaging (SPECT) Tomographic images can be produced by acquiring conventional gamma camera projection data at several angles around the patient Similar to CT

© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps Sinogram © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps

© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps

Filtered Back Projection Attenuates streaks by filtering the projections © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps 1D projections convolved with a ramp filter (actually a ramp with some roll-off) © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps

Iterative Reconstruction Quantitatively more accurate Can model various corrections Collimator Scatter System geometry Detector resolution Slow Being used increasingly in SPECT Improve comments on limitations of filtered back projection.

Measured Projection (P) Assume Some Image (I) Calculate Projections (P’) Calculation Includes Attenuation Scatter Blur with depth Compare to Measured Projection (P) Use P’ & P to form corrections Image estimate may be a uniform image Assumed image undergoes forward projection to produce sinogram Form New Image (I’) Is I-I’< * Done

Data Collection Image matrix is collected Each image row makes a slice 64 x 64 or 128 x 128 Each image row makes a slice Multiple slices can be added to reduce noise Anything higher than 128 x 128?

Attenuation correction: Chang Method I(x) = I0e-mx Assume uniform attenuation m = linear attenuation coefficient of soft tissue (0.15 per cm for Tc-99m) X is tissue thickness along projection from emission data Get SPECT images that are attenuated and not attenuated

Attenuation correction: Transmission measurements X-ray source (SPECT/CT) Non-diagnostic CT Diagnostic CT

PET (Positron Emission Tomography) Positron decay characteristics Coincidence and angular correlation Time of flight PET detector/scanner design Data corrections

Positron is an Anti-particle When a particle and and antiparticle interact they annihilate Both particles are destroyed Two photons(Gamma-rays) are created Two photons are emitted in ~opposite directions (± 0.25 degrees for F-18) Gamma 1 Gamma 2 + -

PET Imaging Concepts Where was the event? ? Coincidence

Where was the event?

Annihilation Detection In coincidence counting an event is ONLY registered if a signal is received from two detectors within a narrow window of time. A few nanoseconds is usually used. Coincidence

Time-of-Flight PET In “Time-of-Flight” pet, use of a very small time window (<100 picoseconds) can localize an annihilation event to within a few cm along the line of coincidence. Time-of-Flight PET can improve SNR. Coincidence

PET Scanner Ring (multiple rings) with lots of little detectors (up to 23,040) Rings have axial coverage of up to 26cm. Detectors must have good stopping power Detector must be fast for accurate coincidence measurements Lutecium silicate LSO (LYSO) is commonly used (&BGO) Modern detector materials? Philips Vereos Digital PET 23, 040 detectors

PET scanner PET scanners lack conventional collimation so they have a high geometric efficiency Some had septal rings to reduce cross talk from ring to ring When rings in 2D When rings out 3D Septa 2D growing out of favor? Septa

Detector Needs High Stopping Power Light Output Short Decay Time Much higher gamma ray energy (511 keV) Light Output Not as important because each gamma ray leaves a lot of energy in the crystal Short Decay Time Very important because of high count rate Limits activity given to patient

Events in PET Scanners

Trues

Trues Rtrue = AO g2 gACDe-mT gACD~ h/2D for ring(s) Where Rtrue = true coincidence rate Ao = Administered activity g = intrinsic efficiency gACD = geometric efficiency e-mT = object attenuation h = detector thickness D = detector diameter

Scatter A 511 keV photon can give up 340 keV in a single 180 degree scatter Scatter most probable about 45 degrees from incident direction, leaving a scattered photon of 285keV

Scatter-to-True Ratio .2 - .5 brain .4 -2 body Scatter (and Trues) are proportional to administered activity Random-to-True ratio higher for 3D since there are no septa to eliminate scatter

RRnd = CTW Rtrue Rtrue CTW = timing window Random

Random-to-True Ratio .1 – 2 brain .1 -1 body Random-to-True Ratio high near high activity (Bladder) Random-to-True Ratio high near high activity (Bladder)

Corrections PET scanners use energy discrimination (pulse height analysis) system like the gamma camera to help eliminate scatter Randoms are corrected for by measuring coincidence rates with a delay of time between 511 keV photon arrivals (so there are no trues). Actually measures anticoincident 511 keV photons

Attenuation Correction Like all radionuclide imaging there is a problem due to attenuation. It is much less for PET than for Tc-99m imaging Correction is important for quantifying the metabolic activity of lesions (SUVs)

Attenuation Correction CT data reconstructed to make a attenuation map of the body Attenuation map information is used in image reconstruction Replace w/ CT data

PET: CT Based Attenuation Correction Get PET images that are attenuated and not attenuated See Darko’s 2015 head and neck lecture © Nuclear medicine physics : a handbook for students and teachers, International Atomic Energy Agency, 2014

SPECT vs PET PET SPECT (Simultaneous acquisition) (Step-and-shoot acquisition) 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

SPECT & PET SPECT – 2 views from opposite sides Res. ~ collimator res., which degrades rapidly with increasing distance from collimator face PET – Simultaneous acquisition Res. ~ detector width; is max in center of ring SPECT sensitivity ~ 0.02% Huge losses due to absorptive collimators PET sensitivity- 2D ~ 0.2%; 3D ~ 2% or higher High sensitivity due to ACD (electronic collimation) Allows higher frequency filters / higher spatial resolution 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

October 7, 2015 -- Researchers at the University of California, Davis (UC Davis) have received a five-year, $15.5 million grant to develop what they are calling the world's first total-body PET scanner. National Cancer Institute and will fund the Explorer project, led by Simon Cherry, PhD, distinguished professor of biomedical engineering and Ramsey Badawi, PhD, a professor of radiology. The total-body PET scanner would image an entire body all at once, and it would acquire images much faster or at a much lower radiation dose by capturing almost all of the available signal from radiopharmaceuticals. … the design would line the entire inside of the PET camera bore with multiple rings of PET detectors. … such a total-body PET design could reduce radiation dose by a factor of 40 or decrease scanning time from 20 minutes to 30 seconds http://www.auntminnie.com/index.aspx?sec=sup&sub=mol&pag=dis&ItemID=112051

References Physics in Nuclear Medicine: Simon Cherry, James Sorenson and Michael Phelps, 4th Edition, Elsevier, 2012 International Atomic Energy Agency, SPECT/CT TECHNOLOGY & FACILITY DESIGN, https://rpop.iaea.org/ SPECT Single Photon Emission Computed Tomography, David S. Graff PhD, http://www.slideshare.net/david.s.graff/spect-presentation Quantitative capabilities of four state-of-the-art SPECT-CT cameras; Alain Seret, Daniel Nguyen and Claire Bernard, EJNMMI Research 2012, 2:45 Characterization of the count rate performance of modern gamma cameras, M. Silosky, V. Johnson, C. Beasley, and S. Cheenu Kappadath, Medical Physics 40, 032502 (2013) Nuclear medicine physics : a handbook for students and teachers, International Atomic Energy Agency, 2014

References Physics in Nuclear Medicine: Simon Cherry, James Sorenson and Michael Phelps, 4th Edition, Elsevier, 2012 Physics of PET-CT, David S. Graff PhD, http://www.slideshare.net/david.s.graff/pet-ct-presentation The Challenge of Detector Designs for PET, Thomas K. Lewellen, AJR:195, August 2010 Basics of PET Imaging; Physics, Chemistry, and Regulations, Gopal B. Saha, Springer, 2005