ECSE-4963 Introduction to Subsurface Sensing and Imaging Systems

BMED-4800/ECSE-4800Introduction to Subsurface

Imaging SystemsLecture 6: Nuclear Medicine

Kai E. Thomenius1 & Badri Roysam2

1Chief Technologist, Imaging Technologies, General Electric Global Research Center

2Professor, Rensselaer Polytechnic Institute

Center for Sub-Surface Imaging & Sensing

Homework #3:Using the “Faridana” (Adel Faridani) filtered back projection code example, change filter

parameters for a lower bandpass. Demonstrate loss of spatial resolution.• http://people.oregonstate.edu/~faridana/preprints/

preprints.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.

• http://www.onid.orst.edu/~faridana/preprints/fbp.txt - MATLAB code for filtered back projection– Designer Shepp-Logan phantom– Filter design possibilities– Make sure to use the modified code (fbp2ket.m) available from

http://www.ecse.rpi.edu/censsis/SSI-Course

Recap: CT & Filtered BackprojectionBackprojection reconstructionw. no filtering.

Impact of filter onSinogram.

Backprojection reconstructionw. filter, compare images.

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.

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

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.

Example: FDG• Fluorodeoxyglucose is a

radiopharmaceutical is a glucose analog with the radioactive isotope Fluorine-18 in place of OH

• 18F 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 18F atom becomes a harmless non-radioactive heavy oxygen 18O– that joins up with a hydrogen atom, and forms glucose phosphate that is eliminated via carbon dioxide and water

2-Deoxy-D-Glucose (2DG)

What Happens Upon Radioactive DecayBasic 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)

electronsIsotope Max. Positron Range (mm)

18F 2.6

11C 3.8

68Ga 9.0

82Rb 16.5

Gamma Photon #1

Gamma Photon #2

BANG

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

Energy of a Gamma Ray• A radionuclide has a

typical energy: e.g. 140 keV for 99mTc

• Detection of lower energy scattered gamma- or x-rays degrades contrast and image quality.

• A radioisotope emits one (or more) very sharp energy lines

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

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.

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

Cross-section of an Anger Camera

Collimator Design & Function

Resolution v. Efficiency Trade-off

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

Modern SPECT Scanners

• GE Hawkeye DigiRad Mobile SPECT System

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

Whole Body Imaging

• Bright spots indicate regions where the radioisotope is bound

Cardiac Study

Cardiac Study• Evaluation of the

coronary artery circulation– Myocardial

perfusion• 3D Studies of the

radionuclide activity

Nuclear Medicine Performance Metrics

• Typical performance:– Energy resolution: 9.5 – 10%

• FWHM response

– Spatial resolution: 3.2 – 3.8 mm– Uniformity: 2 – 4%

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

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!

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

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.

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

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!

PET Images

• Single-channel images• Noisy, and blurry

– Not ideal for segmentation– Segment MRI/CT for defining

anatomy– Register the images– Measure activity

PET Radiotracers

• 18FDG 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.

Application in Lung CancerCase 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 nodesTherapy will have to be continued.

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

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: PatientDeceased.

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

1999 image of the year, U. of Pittsburgh

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.

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.

Source Material• http://apps.gemedicalsystems.com/

geCommunity/nmpet/nmpet_neighborhood.jsp

• Siemens & Philips web sites for nuclear medicine & PET

• http://www.crump.ucla.edu/software/lpp/lpphome.html

• http://thayer.dartmouth.edu/~bpogue/ENGG167/13%20Nuclear%20Medicine.pdf

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.

Instructor Contact Information

Badri RoysamProfessor of Electrical, Computer, & Systems EngineeringOffice: JEC 7010Rensselaer Polytechnic Institute110, 8th Street, Troy, New York 12180Phone: (518) 276-8067Fax: (518) 276-6261/2433Email: roysam@ecse.rpi.eduWebsite: http://www.ecse.rpi.edu/~roysabm Secretary: Laraine Michaelides, JEC 7012, (518) 276 –8525,

michal@rpi.edu

Instructor Contact Information

Kai E ThomeniusChief Technologist, Ultrasound & BiomedicalOffice: KW-C300AGE Global ResearchImaging TechnologiesNiskayuna, New York 12309Phone: (518) 387-7233Fax: (518) 387-6170Email: thomeniu@crd.ge.com, thomenius@ecse.rpi.edu Secretary: Laraine Michaelides, JEC 7012, (518) 276 –8525,

michal@rpi.edu