Ch 13/14 Instrumentation Flashcards

What is the most important application of radioactivity in nuclear medicine?

Radionuclide imaging

What is the purpose of radionuclide imaging?

Obtain a picture of the distribution of a radioactively labeled substance within the body after administration

What is the preferred range of emissions from the radioactivity?

50-500 keV

Why are the gamma rays ideal in the 50-500 keV range?

Penetrates through the body, not powerful enough to pass through the scintillator material.

Why does an imaging system need good energy discrimination compatibility?

Gamma rays that have lost positional information by Compton scattering within the body can be rejected based on reduced energy.

What is an important feature for imaging system detectors?

Good detection efficiency for gamma rays

What type of detector provides both of the valuable features of an imaging system?

A NaI(Tl) detector

What is the energy spectrum of use of a NaI(Tl) detector?

80-300 keV

When did the first attempt of radionuclide imaging occur?

1940s

Who came up with the rectilinear scanner?

Benedict Cassen

When did Benedict Cassen come up with the rectilinear scanner?

1950s

When was the first gamma ray camera capable of recording at all points on the image created?

1953

Who created the collimator (pinhole aperture)?

Hal Anger

What is the correct pathway of radiation from a patient to monitor?

Patient > Collimator > NaI(Tl) crystal > Light guide > PM tube array > Amplifier/Analog Digital Converter > Computer > Monitor

What is the reflective material that surrounds the NaI(Tl) crystal to maximize light output?

TiO2

What shape are the PM tubes?

The PM tubes are round.

What pattern are the PM tubes placed in?

Hexagonal pattern

Why is the PM tube pattern hexagonal?

Maximize coverage of the NaI(Tl) crystal.

How is the count data from the PM tubes assigned?

With X+/X- and Y+/Y- coordinates in a graph

How many photopeak selection methods are there?

2

How does simple energy discrimination work?

All pulse heights that fall within an energy window are accepted

How does the photopeak selection method only useable on digital cameras work?

The photopeak positions and discriminator level settings are computed and stored in the camera. It must be an exact match or else it is rejected.

What does absorptive collimation do?

It projects an image of the source distributions to travel through the detector only from the desired plane to the projector.

What are the 4 types of collimators?

Pinhole, parallel hole, diverging, converging

What are the 4 different events that can be deteeted?

valid events, detection scatter event, object scatter event, septal penetration

Which event has a gamma ray emitted parallel to the collimator, depositing all of its radiation at a single location?

valid events

Which event has a gamma ray emitted parallel to the collimator holes, passing through and causing Compton scattering in the NaI(Tl) crystals?

Detector Scatter Event

Which event has a gamma ray that is not emitted toward the collimator, instead scattering through the body until it is emitted toward the detector?

Object Scatter event

Which event is where the gamma ray is emitted toward the detector, but not straight on and has enough energy to move through the lead collimator?

Septal penetration

What defines the performance of a gamma system?

Sharpness and detail of the images it produces, efficiency of its incident radiation detection, ability to measure the energy of incident gamma rays, and the counting rate it can handle without significant dead time losses

Where can imperfections arise from?

Performance characteristics of the detector, associated electronic circuitry, and the collimator

What does a malfunction of various camera components cause?

image artifacts

What is the measure of the sharpness and detail of a gamma camera image?

spatial resolution

What is the limit of the spatial resolution achievable by the detector and the electronics, ignoring the blurring caused by the collimator?

Intrinsic spatial resolution

What does multiple scattering of gamma ray photons within the detector do to the intrinsic resolution?

It limits the intrinsic resolution, one event can be recorded as two which leads to blurring

What does the statistical fluctuation in the distribution of light photons among PM tubes from one scintillation to the next do to the intrinsic resolution?

It limits the intrinsic resolution

Which is the primary intrinsic resolution limiting factor?

Statistical fluctuation in the distribution of light photons among Pm tubes from one scintillation event to the next

How can intrinsic spatial resolution be measured and characterized?

With a lead mask that contains a number of narrow (1mm) slits placed in front of the gamma camera (no collimator) and a 140 keV Tc99m point source

How does gamma ray energy affect intrinsic resolution?

As the gamma ray energy decreases, less gamma rays are converted into light photons.

What is the intrinsic spatial resolution formula?

1/√(gamma ray energy level)

How does detector crystal thickness influence intrinsic resolution?

Thicker detector results in a greater spreading of scintillation light before it reaches PM tubes.

What type of crystals are ideal for improving intrinsic resolution?

smaller crystals

How does the increased efficiency of collection of scintillation photons affect intrinsic resolution?

It improves intrinsic resolution

What is the average PM tube size?

5 cm diameter

What improves with thicker crystals?

Detection efficiency

What improves with thinner crystals?

Intrinsic spatial resolution

What energy range is the gamma camera designed to provide acceptable detection efficiency while maintaining high intrinsic efficiency while maintaining high intrinsic spatial resolution?

100-200 keV

What is the detection efficiency of a gamma camera for I-131

Poor detection efficiency for high energy gamma ray like I-131

What determines the efficiency for discriminating against low energy scatter photons interacting with the detector via the Pulse Height Analyzer?

Energy resolution of the detector

What factor is the energy resolution dependent on?

Statistical fluctuations in the number of light photons collected from a scintillation event

What is required for good energy resolution?

Light collection efficiency

What is the energy resolution formula?

1/√(gamma ray energy level)

How does the number of light photons released in a scintillation increase with a gamma ray?

Almost linearly

What is the energy resolution for gamma cameras with Tc99m?

9-11% range

What happens as the photopeak becomes narrower?

More efficiency detection of unscattered photons within the chosen energy window

How is the statistical quality of the image improved?

Increased number of valid events recorded

How does energy resolution affect image quality?

Improved energy resolution results in better image quality

What is the phenomenon in which there is a reording of two events at the same time with a high counting rate?

Pulse pile up