Positron Emission Tomography

Universität Bonn

Presenter: Difei Wang

June,2018

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Contents

1

2

3

4

Positron emission

Detected events

Detectors and configuration

Data acquisition

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Positron emission

• Positron emission = 𝛽+decay

𝑝 → 𝑛 + 𝑒+ + 𝜈

• On atomic level

𝑋 → 𝑌 + 𝑒+ + 𝜈𝐴𝑍−1

𝐴𝑍

2 × 511 keV

Positron range

Source: D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey. Positron Emission Tomography: Basic Sciences.

Springer London, 2004.

Collinearity → no physical collimation needed

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Positron emitting nuclide

Nuclide Maximum positron

energy (MeV)

Range in water (mm) Half lifetime

Deduced RMS

11 C 0.96 3.9 0.4 20.4 min

13 N 1.2 5.1 0.6 9.96 min

15 O 1.7 8.0 0.9 2.05 min

18 F 0.64 2.3 0.2 108 min

82 Rb 3.4 18 2.6 1.3 min

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Positron emission

D : distance between two

coincidence detectors

Source: Pat Zanzonico. Positron emission tomography: a review of basic principles, scanner design and performance,

and current systems. Sem Nucl Med 34:87-111, 2004.

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Spatial resolution

Spatial resolution = physical + instrumentation factors

• Physical factors

1. Positron range degrades spatial resolution. The range-related blurring is

reduced by the tortuous path and the spectral distribution of positron

energies.

2. Non-collinearity related blurring depends on the

distance between two coincidence detectors.

Δ𝜃 × 𝐷

whole body scan: ~2 mm

small animal scan: ~ 0.3 mm

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Coincidence event

True coincidence event

energy range : 250 ~ 650 keV Source: Pat Zanzonico. Positron emission tomography: a review of basic principles, scanner design and performance,

and current systems. Sem Nucl Med 34:87-111, 2004.

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Timing window

1. The position of the annihilation

2. Signal processing

3. Scintillation decay time

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Detected Events in PET

1. True

True count rate is linearly proportional to

the activity.

2. Random

Random count rate increases more rapidly

than the true count rate.

Source: D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey. Positron Emission

Tomography: Basic Sciences. Springer London, 2004.

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Detected Events in PET

3. Scatter

Scatter count rate is linearly proportional

to the activity. The scatter-to-true ratio is

independent of timing window.

Source: D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey. Positron Emission Tomography: Basic Sciences.

Springer London, 2004.

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Detector materials

Source: D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey. Positron Emission Tomography: Basic Sciences.

Springer London, 2004.

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Detector materials

Material NaI (Tl) BGO LSO GSO

Density (g/cm3) 3.7 7.1 7.4 6.7

Effective atomic number

Zeff

51 75 66 59

Attenuation coefficient 𝜇

(/cm) 0.34 0.95 0.88 0.70

Light output

(photons/ keV) 41,000 9,000 30,000 8,000

Scintillation decay time (ns) 230 300 40 60

Energy resolution

(% FWHM) 8 12 10 9

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Spatial resolution

• instrumentation factors

Depth of interaction effect (DOI effect):

The relatively thick detector elements lead

to a loss of resolution.

Center :

resolution: 𝑅𝑑𝑒𝑡 = 𝑑/2

Away from the center :

𝑑′ = 𝑑 cos 𝜃 + 𝑥 sin 𝜃

resolution: 𝑅𝑑𝑒𝑡′ ≈ 𝑅𝑑𝑒𝑡 × [cos 𝜃 + 𝑥 𝑑 sin 𝜃]

Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012

x : 2 – 3 cm

d : 0.3 – 0.6 cm

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Block detectors

𝑋 = (𝑃𝑀𝑇𝐴+ 𝑃𝑀𝑇𝐵) − (𝑃𝑀𝑇𝐶+ 𝑃𝑀𝑇𝐷)

𝑃𝑀𝑇𝐴 + 𝑃𝑀𝑇𝐵 + 𝑃𝑀𝑇𝐶 + 𝑃𝑀𝑇𝐷

𝑌 = (𝑃𝑀𝑇𝐴+ 𝑃𝑀𝑇𝐶) − (𝑃𝑀𝑇𝐵+ 𝑃𝑀𝑇𝐷)

𝑃𝑀𝑇𝐴 + 𝑃𝑀𝑇𝐵 + 𝑃𝑀𝑇𝐶 + 𝑃𝑀𝑇𝐷

Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012

(left) D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey. Positron Emission Tomography: Basic Sciences. Springer

London, 2004 (right)

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Modified block detectors

1.Quadrant sharing block design

Each PMT monitors corners of

four different blocks.

Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012

+ : reduce the number of PMTs

− : need more time to process the signal

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Modified block detectors

2. Phoswich

This approach makes use of the

difference in decay times of two

scintillators. The event can be

localized into upper or lower layer.

LSO

GSO

Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012

+ : reduce the DOI effect

− : worse performance

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Detector configurations

Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012

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Detector configurations

2D: Fan beam

3D: Cone beam

Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012

Useful field-of-view

Multi-coincidence fan beam detection: Each detector element is operated in

coincidence with multiple opposed detector elements.

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Data acquisition

𝜃: polar angle

𝜙: azimuthal angle

Source: D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey. Positron

Emission Tomography: Basic Sciences. Springer London, 2004.

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Data acquisition

𝑝 ( 𝑠, 𝜙)

Source: D.L. Bailey, D.W. Townsend, P.E. Valk, and M.N. Maisey.

Positron Emission Tomography: Basic Sciences. Springer London,

2004 (left)

S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear

Medicine. (4th ed). Philadelphia, PA, Saunders, 2012 (right)

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Data acquisition

Direct plane: Crystal ring

collects data from a single

slice.

Cross plane: Crystal ring

collects data from adjacent

rings

Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012

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Data acquisition

Source: S.R. Cherry, J.A. Sorenson, and M.E. Phelps. Physics in Nuclear Medicine. (4th ed). Philadelphia, PA, Saunders, 2012

The axial sensitivity profile for 3-D acquisition reaches its

maximum at the center of the field-of-view.

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Attenuation correction

PET – only system: simultaneous emission/transmission scan

• Blank scan: without the patient, once a day

• Transmission scan: with the patient; all events

• Emission scan: with patient, coincidence events

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Conclusion

Positron emission

Possible events

Detectors

Data acquisition