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3/30/2015 1 Nuclear Medicine Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body (versus transmission of photons through body like x-ray, CT) • Radiotracers (unstable “radioactive” nuclides) are attached to each molecule of a substance. • Image localization occurs due to natural biodistribution in the body of the body of the substance (inject, inhale, ingest) and measurement (with scintillation camera) of ionizing radiation emitted when radioactive atom undergoes decay => FUNCTIONAL IMAGING MODALITY • Planar, 2D images (scintigraphy) produced or tomographic images produced (PET, SPECT)

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Page 1: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

1

Nuclear MedicineIntro & Physics

from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links

NM - introduction

• Relies on EMISSION of photons from body (versustransmission of photons through body like x-ray, CT)

• Radiotracers (unstable “radioactive” nuclides) areattached to each molecule of a substance.

• Image localization occurs due to naturalbiodistribution in the body of the body of thesubstance (inject, inhale, ingest) and measurement(with scintillation camera) of ionizing radiationemitted when radioactive atom undergoes decay =>

FUNCTIONAL IMAGING MODALITY• Planar, 2D images (scintigraphy) produced or

tomographic images produced (PET, SPECT)

Page 2: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

2

• Functional vs. Anatomic

NM - introduction

Bone scan ( planar scintigraphy)vs.

Projection x-ray

Myocardial perfusion scan (PET)(distribution of blood flow)

vs.Projection x-ray (flouroscopy)

(anatomy of coronary arteries)vs.

Tomographic x-ray (CT)(anatomy of heart muscle)

• On your own: define nucleon, atomic number, massnumber, nuclide, radionuclide, isotope, isobar, isotone,isomer.

• Radioactive isotopes serve as the source of ionizingradiation we image in NM

• Mass defect: sum of masses of parts of atoms is morethan mass of atom => there is “missing” energy acc toE=mc2

• Binding energy: “missing energy”. Applies to protonsand neutrons in nucleus and to orbiting electrons(nuclear binding energy and electron binding energy)

• Radioactive decay: process by which an atomrearranges its protons and neutrons to end up withlower energy. Occurs spontaneously. Releases energy.

NM - physics

Page 3: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

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• In general, ratio of protons to neutrons determines stable/unstable

• For stability at higher atomic numbers, have more neutrons than protons.

• One way of conceptualizing radioactive decay is the attempt of an atomthat is “off” the line of stability to reach the line.. Changes the proton toneutron ratio in the process of decay

NM - physics

Red = “line of stability”

• “Radioactivity”: how many radioactive atomsare undergoing decay per second. Has nothingto do with type/energy of radiation

• Units: curie, [Ci], 1Ci=3.7x1010 disintegrationsper second (dps)

• SI Units, becquerel, [Bq], 1Ci=3.7x1010 Bq

• NM imaging range: mCi, MBq

NM - physics

Page 4: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

4

• Radioactive Decay Law

• N = number of radioactive atoms in source(constant)

• λ = decay constant. Units: inverse time, sec-1

• Assume N0 atoms at t=0, then number ofatoms at time, t , is

NM - physics

dNN

dt

0t

tN N e also the radioactive decay law

• Also, if radioactivity is A=number of atomsdisintegrating per unit time

• Big difference in detection:

– In CT, x-ray, measure total energy of beam, intensity

– In NM, measure dps => you must be able to count andseparate γ-rays, cannot just use measure of intensity

NM - physics

0t

t

dNA N

dt

A A e

(also the radioactive decay law)

Page 5: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

5

• Decay factor, DF, =

• Half life:

• Half life and decay constant have fixedrelationship for all radionuclides. Constantnumbers per radionuclide => differentapplications choose different radionuclides

NM - physics

1/ 2 1/ 2

0

1

2

t tA

eA

te

1/ 2

0.693t

• Modes of radioactive decay governtypes of ionizing radiation produced– alpha decay: results in emission of

alpha particle (2 protons + 2neutrons)

– beta decay: results in emission of abeta particle (like electrons)

– positron decay*: results in emissionof a positron (antimatter electrons),which rapidly annihilate withelectron to produce 2 gamma rays

– isomeric transition*: results inemission of a gamma ray (remember:x-rays and gamma rays areindistinguishable after emission.Difference is in origin – nucleus vs.electron cloud)

NM – physics - ionizing radiation

Particulate ionizing radiation

electromagnetic ionizing radiation

* Used in PET *Used in planar scintigraphy and SPECT

Page 6: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

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• 1500 known radionuclides

• 200 can be purchased

• ~dozen suitable for NM1. “clean” gamma ray emitters (don’t also emit alpha

and beta particles that contribute to dose and notimage formation) or positron emitters.

2. Want “correct” energy gamma rays, 70-511keV Unlike CT, x-ray, attenuation CONFOUNDS our image

formation. Contributes to dose, reduces detected signal =>want higher energy gamma rays

• BUT, higher energy = less likely to interact with detector

3. Half life: need to form images in minutes, not secondsor hours

NM – physics - radiotracers

• Want radionuclides that are useful and safe to“trace”

• Want monoenergetic decay => energy-sensitive detection can filter out Comptonscatter

• Technetium-99m (Tc-99m) is most commonlyused gamma ray producing radionuclide inNM. ½ life of 6 hrs. easily produced on site.Can be tagged to variety of molecules. Keep itin mind as example.

NM – physics - radiotracers

Page 7: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

7

KEY POINTS - physics

• NM produces images that depict the distribution of a radiotracer;the distribution is governed by body function, not structure.

• NM makes use of radionuclides; these are unique nuclear speciesrepresenting radioactive atoms, which emit ionizing radiation uponspontaneous decay.

• A given radionuclide is characterized by its decay mode (indicatestype of ionizing radiation) and half-life (time for half the radioactiveatoms to decay, on average

• Radioactive decay is a random process (governed by Poissondistribution); described by non-exponential relation and is afunction of time and the decay constant (probability of decay),which is a characteristic of a given radionuclide.

• Radiotracers make use of radionuclides that emit radiation ofappropriate type and energy, have half-lives that are appropriate,and are chemically inert.

Overview – NM – Scintigraphy, SPECT, PET

Page 8: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

8

Planar Imaging(Scintigraphy)

SPECT

PET

Emission Detection

Single gamma photon(produced by isomeric transition)

Anger cameraProjection imaging

Single gamma photon(produced by isomeric transition)

Rotating Anger cameraTomographic due todetection/reconstruction

Positron →Annihilated→2 gamma photons (511keV photons)emmitted [cons. of energy] back toback [cons. of momentum]

Tomographic due to emissionNo corresponding projection mode

AnnihilationCoincidenceDetectionbased on Angercamera principle

Overview – NM – Scintigraphy, SPECT, PET

Planar scintigraphy : SPECT :: Projection x-ray : CTnothing: PET :: Projection x-ray : CT

Planar scintigraphy SPECT PET

Overview – NM – Scintigraphy, SPECT, PET

Page 9: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

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Overview –Planar Scintigraphy

Overview – SPECT

Page 10: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

10

Overview – SPECT

Planar scintigraphy imagesat different angles.

Same line of detectors ineach image is pulled toform the sinogram for thatslice.

Overview – PET

https://www.inkling.com/read/nuclear-medicine-ziessman-4th/chapter-5/positron-emission-tomography

Page 11: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

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Overview –PET

Each detector pair, and thereby each line of response (LOR) corresponds to a particularpixel in the sinogram depending on its orientation angle and distance from the center ofthe gantry. Therefore, for each coincidence detection, the LOR for that detection isdetermined, the pixel in the sinogram associated with that LOR is located, and the valuein the pixel is incremented.

Fahey et al., “Data Acquistion in PET Imaging”, Journal of Nuclear Medicine Technology, 2002.

Overview – PET “versus” SPECT

SPECT, April 2012PET, May 2012

Higher resolutionLess sensitive (used on anatomieswith less attenuation)

Lower resolutionMore sensitive

Page 12: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

12

Overview – PET “versus” SPECT

Overview – functional + anatomic

Functional (PET) + anatomic (MRI)

Functional (PET) + anatomic (CT)

Page 13: NM - introduction - Weebly...Intro & Physics from Medical Imaging Signals and Systems , Chapter 7, by Prince and Links NM - introduction • Relies on EMISSION of photons from body

3/30/2015

13


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