Nuclear Medicine Physics and Equipment 243 RAD 1 Dr. Abdo Mansour Assistant Professor of radiology

2 Lectures No. 11, & 12 Measurement Equipments

1. Single-photon emission computed tomography (SPECT). 2. Positron Emission Tomography (PET). 3 The common nuclear medicine techniques are

4 Positron Emission Tomography (PET)

Definition Positron Emission Tomography (PET) is a nuclear medical imaging technique which produces a three dimensional image of functional processes in the body.

Uses of PET Diagnosis of cancer (especially lung). Staging of cancer. Brain. Cardiac. Oncology.

Benefits of PET scan The information provided by PET examinations is unique and high accuracy. PET gives useful information needed to make a diagnosis or to determine appropriate treatment. PET imaging leads to early detect of disease. PET is less expensive.

8 Positron Radiopharmaceutical Positron nuclides 11 C, 13 N, 15 O, 18 F, 62 Cu, 68 Cu, 75 Br, 38 K and 99m Tc

9 γ - Camera γ camera collimator crystal photomultiplier pulse height analyzer electric element

PET Mechanism Positron Emission Tomography (PET) Positron Emission Tomography (PET) cameras are designed to detect the paired 511 -keV photons generated from the annihilation event of a positron and electron. Following emission, any positron travels only a short distance before colliding with electrons in surrounding matter (into patient). The paired 511 -keV annihilation photons travel in opposite directions ( 180◦ apart) along a line. 10

Annihilation reaction. 11

18 F e+e+ Emission sequences Radionuclide Step – 1

18 F Step – 2

18 F e+e+ e-e- Step – 3

18 F ? Step – 4

18 F “Annihilation” Step – 5

18 F Detector KeV Step – 6

Annihilation coincidence detection Annihilation coincidence detection occurring at high energy ( 511 KeV). * Therefore, a PET camera, needing no collimator, but camera is inherently more sensitive compared to a SPECT. 18

Physics Explanations 1- A short lived radioactive isotope ( 18 F), is injected in to the living subject (usually in to blood circulation ) 2- Host cells uptake radioactivity based on metabolic rate (there is a waiting period while the active molecule becomes concentrated in tissues of interest). 3- Radioactivity generates a positron..

4- Annihilates with electron. 5- Annihilation produces a pair of gamma photons. 6- Gamma ray measured by machine and record data. 7- Millions of these data points are collected and analyzed. 8- PET can then create an image of the required part (such as, the brain ).

21 PET Structure

22 PET Camera Components A common PET detector consists of rings of crystals. The rings may or may not be separated by septa. The individual detector contains a collection of small crystals. A standard detector unit or block consists of a small crystal watched by four photomultiplier tubes.

PET camera 23

24 1. Crystals The basic crystal function of converting gamma photon energy into light photon energy. Thallium-doped sodium iodide (NaI(Tl)) crystals were originally used for PET systems. NaI only has a relatively low density and is less effective at stopping the high energy 511 -keV photons, so sodium iodide doped with Thallium to obtain high density crystal.

25 Photomultiplier Tubes 2- Photomultiplier Tubes Each PMT tubes are attached to many single crystal. The slits between crystal subdivisions channel the light photons toward the PMTs. Localization of the site of impact is achieved by measuring the light detected in each PMT.

Slits between crystals direct light photons toward PMTs. 26

PET Imaging Positron emission tomography (PET) has the advantages of : High energy photon imaging. High Sensitivity. The ability to correct for attenuation. No need for collimation

29 Requirements for PET detector material 1- Good stopping power for 511 keV gamma rays, i.e. high density ( Bismuth germinate crystal). 2- Good energy resolution, i.e. high number of light photons per MeV energy stopped ( Sodium iodide doping with thallium). 3- Low Compton scatter inside detector crystal. 4- Matching of wavelength of fluorescence to response of light detector.

Advantages of PET Imaging high Sensitivity As pointed out earlier, a collimator reduces a camera’s sensitivity because the collimator’s septa cover part of the camera crystal’s face. A collimator is not required for PET, because PET cameras use the detection of directed 511 -keV photons of positron annihilation (high energy).

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