Nuclear Imaging Medical Imaging Medical Imaging Nuclear Imaging - - PowerPoint PPT Presentation

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Nuclear Imaging Medical Imaging Medical Imaging Nuclear Imaging - - PowerPoint PPT Presentation

Jul 08, 2023 353 likes •549 views

Nuclear Imaging Medical Imaging Medical Imaging Nuclear Imaging Nuclear Imaging Nuclear imaging is the use of radioactive materials for imaging structure and function inside body. ED X Wu Overview Overview Radioactive Decay and

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SLIDE 1

Nuclear Imaging Nuclear Imaging ED X Wu

Medical Imaging Medical Imaging Nuclear Imaging

Nuclear imaging is the use

  • f radioactive materials for

imaging structure and function inside body.

Overview

  • Radioactive Decay and Radiopharmaceuticals
  • Single Photon Emission Computed Tomography (SPECT)
  • Positron Emission Tomography (PET)
  • Image Reconstruction Methods for SPECT and PET

Overview

  • Radioactive Decay and Radiopharmaceuticals
  • Single Photon Emission Computed Tomography (SPECT)
  • Positron Emission Tomography (PET)
  • Image Reconstruction Methods for SPECT and PET
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SLIDE 2

The Atomic Nucleus

+ + + + + + + + + +

Nucleus consist of protons and neutrons: proton neutron Nomenclature:

X

A Z

X

A

  • r

A := mass number (number of protons + neutrons) Z := atomic number (number of protons) Species with same Z but different A are called “isotopes.” E.g.: 64Zn, 66Zn, 67Zn, 68Zn, 70Zn (49%, 28%, 4%, 19%, 0.6%)

Nuclear Shell Model

Analog to atomic shell model the nucleus can be described with a nuclear shell model, in which protons and neutrons occupy various possible energy states.

Stable Nuclei

Nucleus is most stable when a shell is completely filled with protons and neutrons. (Magic numbers Z or N = 2 ,8,20,28,50,82,126) (Super magic numbers Z and N = 2,8,20,28,50,82,126)

line of stability OR electron capture (EC)

More details in:

Introduction to radiological physics and radiation dosimetry By Frank Herbert Attix

Decay Equation and Half-Life

Unit of Radioactivity or Activity (to define # of decays/disintegrations per unit of time): Curie (Ci) & Becquerel (Bq=1/s)

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SLIDE 3

Decay Equation and Half-Life

β+ , β− decay,

  • r electron capture

possible

N = N0e−λt

N0 := initial number of parent atoms N := parent atoms remaining after time t λ:= decay constant

Half-life (t1/2)? 1 2 = N N0 = e−λT

1/2

=> ln 1 2 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = −λT1/2 => T

1/2 = ln2

λ (Half − Life)

Assignment (due in 2 weeks)

1. What is the activity (in both Ci & Bq) contained in ? 2. Radiocarbon dating or carbon dating is routinely used to determine the age of some excavated species that lived in thousands years of ago. Please describe its principle in ~150 words.

Unstable Nuclei (Alpha decay)

Mass Rich Nuclei:

Nucleus emits alpha particle

a

+ +

What is alpha particle?

Unstable Nuclei (Alpha decay)

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SLIDE 4

Unstable Nuclei (Alpha decay) Unstable Nuclei (Beta- decays)

Neutron Rich Nuclei: Neutron becomes proton under emission

  • f electron

Z increase by 1 (Znew = Zold + 1). A stays the same.

Emax is the max kinetic energy carried by beta-

  • β−-decay

Cs

137 55

Ba +

137 56

β

  • 1

−>

β−-decay

?

Unstable Nuclei (Beta- & Beta+ decays)

In fact, complex reality to conserve energy, momentum and charges!

Unstable Nuclei (Beta+ decays)

Proton Rich Nuclei: Beta+ decay

Z decrease by 1 (Znew = Zold - 1). A stays the same.

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SLIDE 5

Unstable Nuclei (Beta+ & EC decay)

Proton Rich Nuclei:

Sometimes there are two competing decay processes:

  • Beta+ decay (Z decreases by 1)
  • Capture its own electron

(EC decay) Eb + Neutrino: 1.57 MeV

Unstable Nuclei (Isometric transition)

Nuclei with stable number of protons Z and neutrons N in exited state emit γ rays:

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SLIDE 6

Tc −>

99m 43

Tc + γ

99 43

Unstable Nuclei (Isometric transition) Overview

  • Radioactive Decay and Radiopharmaceuticals
  • Single Photon Emission Computed Tomography (SPECT)
  • Positron Emission Tomography (PET)
  • Image Reconstruction Methods for SPECT and PET

Single Photon Emission Computed Tomography (SPECT)

99mTC

Anger Camera

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SLIDE 7

Anger (γ) Camera Anger (γ) Camera

Single Photon Emission Computed Tomography (SPECT) Single Photon Emission Computed Tomography (SPECT)

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SLIDE 8

Phantom Test

SPECT Measurements SPECT Measurements

α(x,y) vs. µ(x,y)

unknown activity & absorption cross-section

I( ,θ)

g(s,θ) = α(x, y)exp − µ(x, ydl

L1 Lmax

∫ ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ d

Lmin L max

∫ l

L1 Lmax problem is ill-posed! ⇒different combinations of α and µ can yield same measurement

SPECT Measurement SPECT Measurement

α(x,y) vs. µ(x,y)

unknown activity & absorption cross-section

L1 Lmax scattering can degrade image ⇒use of collimators necessary! However spatial resolution limited to 8-15mm.

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SLIDE 9

Overview

  • Radioactive Decay and Radiopharmaceuticals
  • Single Photon Emission Computed Tomography (SPECT)
  • Positron Emission Tomography (PET)
  • Image Reconstruction Methods for SPECT and PET
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SLIDE 10

Overview

  • Positron Emission Tomography (PET)
  • Basic Principles
  • Radioisotope production
  • PET Tracers
  • Clinical Examples

http://www.crump.ucla.edu/lpp

Principles of PET

In unstable nucleus (more protons than neutrons) proton converts to neutron under emission of positron. (Z decrease by 1 , A stays same) Positron travels limited distance in tissue.

Principles of PET

Positron combines with electron of nearby atom and emits two 511 keV X-ray photons that travel in opposite direction.

+/- 0.25 degree

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SLIDE 11

Principles of PET Principles of PET Principles of PET

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SLIDE 12

Principles of PET Principles of PET Principles of PET

  • 18-FDG is a glucose analog that is widely used to image glucose metabolism
  • O-15 for study of blood flow or perfusion
  • More recently, PET tracers are employed for molecular and cellular imaging
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SLIDE 13

Overview

  • Radioactive Decay and Radiopharmaceuticals
  • Single Photon Emission Computed Tomography (SPECT)
  • Positron Emission Tomography (PET)
  • Image Reconstruction Methods for PET and SPECT

CT Transmission Measurement CT Transmission Measurement µ(x,y)

unknown absorption cross-section

X-ray source

( measurable attenuation, shadowgram )

I = Ioexp − µ(x, y)dl

L

∫ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ g ≡ ln Io I ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ (Signal) g(s,θ) = µ(x, y)dl

L

∫

PET Measurement PET Measurement

α(x,y)

unknown activity cross-section ( measurable coincident)

g(s,θ) = c α (x, y)dl

L

∫

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SLIDE 14

PE Tomography PE Tomography PE Tomography PE Tomography PE Tomography PE Tomography PE Tomography PE Tomography

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SLIDE 15

PE Tomography PE Tomography PE Tomography PE Tomography

g(s,θ) = c α (x, y)dl

L

∫

To obtain image from projection data use filtered backprojection algorithm, etc.

Multi-ring PET Some Problems of PET Some Problems of PET

(1) resolution limited to 2-5 mm because of positron mean-free path before annihilation. (2) False Coincidence Events: (a) unrelated photons arrive at same time (<20ns) (~15% of all signals) (b) one or both photons of an annihilation event are scattered (e.g. Compton scattering) (10-30% of signal) (3) Unknown photon absorption profile (2) relatively high radiation dose to patient

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SLIDE 16

Some Problems of PET Some Problems of PET Some Problems of PET Some Problems of PET Some Problems of PET Some Problems of PET

Anti-scatter collimator Small detectors

2D to 3D PET Configurations 2D to 3D PET Configurations

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SLIDE 17

Transmission Measurement (as in CT) Transmission Measurement (as in CT)

µ(x,y)

unknown absorption cross-section

X-ray source

( measurable attenuation, shadowgram )

I = Ioexp − µ(x, y)dl

L

∫ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ g ≡ ln Io I ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ (Signal) g(s,θ) = µ(x, y)dl

L

∫

PET Measurement PET Measurement

α(x,y)

unknown activity cross-section ( measurable coincident)

g(s,θ) = c α (x, y)dl

L

∫

PET vs. SPECT

New Trend

  • CT/PET

Another approach to PET: Time-of-Flight (TOF) ?

  • Electronics requirement?

Or TOF electronics

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SLIDE 18

Emerging Technology

  • PET/CT scanner
  • PET/MRI scanner

PET/CT scanner PET/CT scanner