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

What is another name for analog buffers?

derandomizers

What are the electronic circuits that can hold onto the pulse and release it instead of having a backup occur during uptake?

Analog buffer

What causes the image to record activity in areas where there isn’t any activity due to pulse pile up distortion of the counts?

image distortion

What is pincushion distortion?

inward bowing

What is barrel distortion?

outward bowing

What type of images are acquired with the collimator removed using a point source placed far enough from the surface of the gamma camera to give uniform irradiation of the surface?

Intrinsic flood images

What radioisotope is used for intrinsic flood images?

Tc-99m O4-2

What image is acquired with the collimator in place?

Extrinsic flood images

What radioisotopes are used for extrinsic flood images?

Pertechnetate or a Co57 sheet source

What are the two primary causes of gamma camera nonuniformities?

Nonuniform detection efficiency and image nonlinearities

What occurs when there is a bright ring around the edge of the image?

Edge packing

What will result in a worsening detector nonuniformity?

higher photon energies

Improvements in camera uniformity contribute to what?

Improvements in intrinsic resolution

What were used in older cameras to ensure that all of the photons were absorbed for good counting statistics?

Thicker light guides

Why is continuous tuning important in SPECT?

Earth’s rotating magnetic field influences changes in the PM tube gain

What level of activity is the reference source for gamma camera tuning?

30-60 uCi

What is the weak link for the performance of a gamma camera system?

the collimator

What refers to the sharpness or detail of the gamma ray image projected onto the detector?

Collimator resolution

What principles are employed by general collimators?

Absorptive collimation

What is the definition of collimator efficiency?

Fraction of gamma rays striking the collimator that actually pass through it to project the gamma ray image onto the detector

What is the ideal material to use for the collimator?

Lead

What is the upper energy limit of a low energy collimator?

150 keV

What is the upper limit of a medium energy collimator?

400 keV

What is the upper limit of high energy collimators?

511 keV

What occurs if the collimator septa is too thin?

They may be virtually transparent to high energy photons

What does the geometry of the collimator holes affect?

Collimator performance

What is the name of the FWHM of the radiation profile from a point or line source of radiation projected by the collimator onto the detector?

point spread function

What are the two factors that contribute the most to system resolution?

Intrinsic resolution of the detector and electronics and the collimator resolution

What distance is the Tc99m or Co57 point source placed at for intrinsic qc?

5x the UFOV

What is the count rate for the acquisition of data for intrinsic resolution?

Below 30,000 cps

What rotation must be used to ensure that the X/Y coordinates are measured for intrinsic resolution?

0 degree and 90 degree rotation

What are the typical values acceptable for the intrinsic resolution?

2.5-3mm

What are the results for system resolution for Tc99m?

8-14mm

What is involved with system resolution qc?

Two 1mm diameter line sources

How far apart are the line sources placed?

5 cm apart

How far away are the line sources placed from the front face of the collimator?

10 cm

What pattern and conditions does the spatial linearity match for QC?

intrinsic resolution measurement

How is intrinsic uniformity determined?

From flood field images acquired without a collimator

What is the minimum count of each pixel of the image for intrinsic uniformity?

4000 counts

What is the integral uniformity equation?

100 x (max pixel count - min pixel count)/(max pixel count + min pixel count)

What is the differential uniformity equation?

100 x (high - low)/(high+low)

What does the gamma camera behave as?

Paralyzable counting system