Lecture 4C Handout

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guide. Typical Gamma Camera. Parameters. NaI(Tl) crystal ~ 50cm X 30cm ... function versus position. (light sharing —-> spatial resolution) x. E x7 x6 x5 x4 x3.
Overview of today’s lecture •

Nature of nuclear radiation - Isotopes used in nucl. med.



Detection methods



Counting statistics



Imaging systems - Planar gamma scintigraphy

The Planar Gamma Camera

Siemens e.cam

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Gamma Camera Instrumentation Typical Gamma Camera Parameters NaI(Tl) crystal ~ 50cm X 30cm PMTs ~ 7.5 cm (3”) 30 – 50 PMTs total Collimators holes (hex) ~ 2–6mm

Electronics

acquisition processing display computer

boards

PMTs light guide

crystal collimator

Crystal and light guide Light Guide

3/8” thick ~ 9.5 mm Crystal

NaI(Tl) Density Attenuation Coefficient µ (@140 keV) PE fraction Light output Decay time Wavelength

3.67 g/cm3 2.64 cm-1 —-> 1-e-(2.64/cm)(0.95cm) = 92% ~80% 40/keV —-> 40*140 = 5,600 scint. photons 230 nsec 410 nm

Light guide distributes scintillation light over PMT array

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Light response function versus position (light sharing —-> spatial resolution) E

xˆ =

" xi i

! Ei

Intrinsic spatial Resolution: < 4 mm FWHM < PMT size!

" Ei i

x1

x2

x3

x4

x5

x6

x

x7

PMT signals; Ei

PMTs LG

Crystal

γ

γ absorption creates 1000s of scintillation photons

Spatial Positioning

From: The Essential Physics of Medical Imaging (Bushberg, et al)

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Gamma Camera Energy Spectra Summed signal from all PMTs

Scattered events have changed direction, hence, they will be mis-positioned by the image generation algorithm ---> this tends to diffuse sources and reduce image contrast

Energy Windows

• Balance between accepting all good events (importance of sensitivity) and rejecting scattered events. • Most gamma cameras can acquire data using multiple energy windows. Allows for simultaneous imaging of different radioisotopes, for example Tc-99m (140 keV) and I-131 (364 keV).

Collimators - Septal Penetration Detector (NaI(Tl)) Collimator septa, µ

l

d Minimum septa thickness, t, for