Pet I

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The functional images differ from standard nuclear medicine is that they show

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metabolism and blood flow on a cellular level.

Front 2

PET produces

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cross sectional functional images of the body.

Front 3

fMRI/functional MRI

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Is the other only modality which also shows function but this is mostly based on regional changes in blood flow.

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FDG

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Fluorodeoxyglucose

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Finger Exposures
Tc-99m
I-131
F-18

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4mrem/sec
3mrem/sec
19mrem/sec

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What is PET?CT used for?

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oncology
cardiology
neurology

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What are 2 ways proton rich isotopes turn form unstable to stable....protons to neutrons???

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1. Positron emission
2. Electron Capture

Isotopes with too many protons are unstable

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Interaction with an electron causes

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annihilation

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PET scanners are only looking for

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511 keV photons.

1.02 MeV or 2 photons at 511 keV.

Front 10

The fundamental basis for a PET scanning is

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annihilation coincidence detection.

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ACD

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annihilation coincidence detection

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Why is collimation in PET not required?

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Due to ACD or annihilation coincidence detection

Front 13

In PET scanners, groups of

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detectors are arranged in a ring around the patient. A group of crystals will be viewed by 1 PMT.

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Septal rings

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are lead shields placed between the detectors to eliminate cross talk between detectors.

Front 15

When the septal rings are removed......

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scatter increases.

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LOR/Line of Responses

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When 2 events are detected in coincidence, a straight line is drawn between the 2 detectors registering the event.

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Why is a high count rate required for PET?

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For every coincidence event detected, at least 100 unmatched events are discarded.

Front 18

In order for the PC to record an event......

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1. Two events simultaneously
2. Energy information is recorded...position in pulse height analyzer

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Detected events =

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T + R + S

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True

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good data

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Random

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occur when two unrelated events are detected within the coincidence window. Randoms are estimated and corrected for

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Scatter

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is usually of lower energy below 511 KeV and can be removed with absorbers placed in front of the crystals.

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As we increase the activity......

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eventually the randoms will exceed the number of true coincidence events.

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Coincidence Detection

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The rate of random event detection is also greatly effected by the width of the time window. A larger time window allows more randoms to be detected.

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2D

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LOR/ lines of response are oimited to the same detector ring or adjacent detectors. Thus, shielding between rings is used.

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2D limits counting and

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lowers sensitivity.

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3D

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any LOR is permitted and NO SHIELDING BETWEEN RINGS IS USED.

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3D allows for 5 times more sensitivity however,

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randoms and scatter increase as well.....

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New detectors had to be made due to

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the 511 keV penetrating photons. Improved stopping power and detector efficiency.

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Heavier elements and are better photon attenuators

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BGO
LSO
GSO

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Energy window and detector size

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effect detector sensitivity.
BGO has the greatest sensitivity.

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Since no collimation is used in PET.......

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sensitivity is 10 to 20 times greater and allows for a lower dose adm. patient.

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Scatter degrades the

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image

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Compton Scatter

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for lower atomic numbers materials

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Photoelectric absorption

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for higher atomic numbers. Thus, BGO crystals with high density and high atomic number reduces the number of scatter events in the crystals

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Inside a person

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Compton scatter

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Inside a BGO crystal

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photoelectric effect

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Light yield

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the amount of light produced per unit energy. NaI(Tl) crystals have high light yield despite their low stopping power

Front 39

The light produced by different detectors crystals varies in

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amount, wavelength and time of decay

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Time of decay is

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the amount of time it takes for light output to reach zero after a scintillation is initiated.

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A _______ decay time desirable so the detector can produce more scintillations in a short time

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fast

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time of decay is related to the ________ ______ of the detector,

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dead time

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BGO has a _______

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long decay time of 300 nsec

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Spatial Resolution on A PET
2 limits exist due to....

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1. range of positrons in tissues
2. The fact that the 180 degrees separation of positron annihilation photons is not exactly true.

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1). Positrons can travel in tissue just as beta particles do....their range if a function to their energy so

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with higher energy positrons travel further

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For Fluorine 18, positrons have a maximum range of 2.6 mm however the actual effect on resolution is less than

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1 mm

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2.) Annihilation photons deviate from 180 degrees path because at the moment of annihilation,

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the positron-electron pair is not completely at rest. The result is an angular spead.

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Therefore, the combined action of these two effects limit PET spatial resolution to

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1.5-3.0 mm

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These effects and detector resolution together determine the system resolution. Modern PET scanners are capable of final image resolution in the range of

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8 mm

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Attenuation Correction

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In PET scanning, a transmission scan is performed to correct for the patient attenuation of different densities in the patient.

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This is an effective method since 5 11 photons travel in a

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straight line.

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By measuring the total attenuation along the line of travel

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attenuation can be corrected from the emission image.

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Dedicated PET scanners use internal sources how ever, in a PET/CT

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the CT scan is used for attenuation correction.

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Daily QA PET
A daily ________ scan is required.

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blank = This is similar to a flood on the camera. A uniform source is used to expose the PET detectors. This may be internal rods sources or a cylinder.

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The purpose of the blank scan is to determine if the scanner is OK for clinical use.

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More specifically, the scan can be used to monitor system stability and to determine which detectors are more or less sensitive then average

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Normal uptake of FDG

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Brain, Breast, Bowel, Heart, Liver, Salivary glands, stomach, thyroid, urinary

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Brain

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high uptake in gray matter due of metabolic substrate.

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Myocardial uptake

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variable, depends on substrate availability

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Kidney

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route of excretion (filtered, poorly absorbed from renal tubules and excreted unchanged.)

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Muscles

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variable depends on muscle tension or activity shortly or during FDG uptake phase

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Bone marrow

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mild upatke

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liver

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high initial uptake with rapid clearance due to relatively high glucose -6-phosphatase activity

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lungs

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low uptake (low initial extraction of FDG)

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GI tract

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variable uptake

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Mouth, oropharynx,pharynx

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mild to moderate uptake

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Breast, thyroid, thymus,

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mild to moderate uptake

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BIOdistribution of Fdg

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brain = 6.9 % OF INJ. DOSE
liver = 4.4
heart = 3.3
red marrow = 1,7
kidneys = 1.3
lungs = 0.9

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Breast Uptake

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- Breast blushing
especially in patient who breast-feed
resume breast feeding after 10 t/12

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Recent Surgery

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will show uptake

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Non-fasting patient

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will have bowel uptake

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hip replacement and patient exercise

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will show uptake in muscle activity

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PET Artifacts

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Brown fat can take up FDG. Shivering can cause uptake in brown fat so it is important to keep the patient warm during the uptake period.

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Muscles will also take up FGD and patient should remain calm and still during the uptake period.

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Strenuous exercise should be avoided 24 hrs prior to exam. Shivering and talking may also cause muscular activity.

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Smokers may exhibit uptake in

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the intercostal muscles

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Patient chewing gum will

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show uptake.

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Artifacts can also be caused by mismatch in the PET vs CT scan due to motion, esp. .....

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respiration scans of the thorax
- This can cause a curvilinear cold band parallel to the dome of the diaphram.

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High density objects in the CT will cause over correction in the PET scan making false hot spots

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Common causes are dental implants, cardiac pacemakers, central intravenous lines, and metallic limb protheses.

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Oral or intravenous contrast can cause

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errors in attenuation correction.