Instrumentation chat study guide part 1 gamma cam and detector operation

4 major componenet of gamma camera

1. collimator

2. scintillation crystal (NaI)

3. photomultiplier tubes (PMTs)

4. Position and energy circuits

collimator function

allows photos traveling in the correct direction to reach the crystal and absorbs unwanted off-angle photons

NaI crystl function

cinverts gamma ray energy into visible light

PMTs function

convert light into electrical signals and amplify those signals

positioning and energy cicuit function

determin the X,Y and Z energy of each event, then determine whether the event should be accepted

sequence of gamma photon from patient to camera

1. gamma photon leaves patient

2. passes through the collimator if its direction is acceptable

3. interacts with the NaI crystal

4. produces scintillation light

5. light reaches multiple PMTs

6. PMTs convert light into electrical signals and amplify them

7. positioning circuit calculate X and Y

8. the sun circuit calculates Z, representing energy

9. PHA checks whether the event falls withing the energy window

10. accepted event is assigned to a pixel in the image matrix and stores

how does sodium iodide crystal convert gamma ray energy into light

• gamma ray deposit energy in the crystal

• electrons in crystal become excited

• as electrons return to lower energy states they release energy as visible ligt

• process is called scintillation

why must crystal be protects

• hygroscopic = absorbs moisture

• moistrure can dama crystal and make it cloudy or opaque

• rapid temp changes cause thermal stress and cracking

• 9 degree fereheintet per hour cracking risk

thicker crystal means _ sensitivity

higher

• more photons absorbed

• lower intrinsic spatial resolution

thinner crystal means __ sensitivty

lower

• more photons may pass through

• higher intrinsic spatial resolution

3 functions of PMTs

1. convert scintillation light into electrical signals

2. amplify the electric signals

3. help determine photon energy and position

X ?

horiozontal position

Y ?

vertical positon

Z ?

total energy of the event

Pulse height analyzer function

• determines whether a phtoon has an acceptable energy

• has LLD, UUD, and energy window

energy window

• range between upper and lower level discriminator

• typically 20%

upper level discriminator

rejects events above selected energy range

lower level discriminaotr

rejects events below selected energy range

define peaking

• process of adjusting the gamma cameras energy setting so that the photopeak is cenetered within the selected energy window

why does peaking (energy window alignment) matter

• ensures the correct energy photons are accepted

• prevents excessive rejection of desired photons

• helps reduce scatter contamination

• prevents nonuniformity caused by incorrect energy settings

what can shift a photopeak or change energy response

• PMT gain changes or drift

• changes in PMT high voltage

• temperature changes

• changes in detector electronics

• changes in scintillation light output

• incorrect energy calibration

what is a change in PMT gain

• changes the size of the electrical signal and therefore can shift the measured photopeak

what does energy correct

z pulse/photopeak dfferences

what does linearity correct

X/Y positioning errors

what does unifomity/sensitivity correct

regional count sensitivity differences

energy and linearity corrections are applied…

event by event

PMT gain drift occurs when…

the amplification of a PMT changes over time

autotuning

automatically adjusts PMT voltage and preamplifier gain to maintain stable output

what are the three methods for autotuning

1. photopeak monitoring

2. split photopeak monitoring

3. LED monitoring

what is photopeak monitoring

• method of autotuning

• monitors the photopeak at each PMT and adjusts gain relative to a reference

what is split photopeak monitoring

• method of autotuning

• uses two narrow energy windows on the high side of the photopeak and adjusts high voltage to maintain a predetermined count ratio

what is LED monitoring

• method of autotuning

• uses flashes of light from LEDs and compares each pMTs voltage output to a reference

what is the typical adjustment range for PMT gain drift

plus or minus 3keV

• PMTs outside of range require engineer service

what can a poorly tuned PMT cause

• nonuniformity and positioning errors even when correction maps are being used

Edge packing

• occurs when photos are incorrectly positioned near the outer edge of the detector

  • PMTS at edge have different light detection characteristics

  • positioning calculations become less accurate

  • counts may become artificially concentrated near the detector edges

  • this profuces an abnormal flood pattern

• a positional problem