Ch 14 - Instrumentation

The Gamma Camera: Performance Characteristics

sharpness and detail of the images it produces, the efficiency which it detects radiation, ability to measure energy, and the counting rate it can handle without dead time losses

the performance of a gamma system is defined by the:

no

is a gamma camera capable of producing the “perfect” image of radionuclide distribution

electronic circuitry and the collimator

what are a couple inherent imperfections characteristics that arise from the performance of the detector?

image artifacts

what can also be caused by malfunctions of various camera components?

spatial resolution

this is a measure of the sharpness and detail of a gamma camera image

spatial resolution

sharp edges or small pointed objects produce blurred rather than sharply defined images

intrinsic spatial resolution

the limit of spatial resolution which is achievable by the detector and the electronics (ignoring the blurring caused by the collimator)

multiple scattering of gamma ray photons within the detector and statistical fluctuation in the distribution of light photons among PM tubes from one scintillation event to the next

two factors that affect intrinsic spatial resolution

one event can be recorded as two which leads to blurring

what can happen when multiple scattering of gamma ray photons within the detector via compton scattering occur?

6.4mm

Anger calculated that a detector thickness of ____ has less than 10% of photons misplaced by more than 2.5mm as a result of multiple scattering events

10%

Anger calculated that a detector thickness of 6.4mm has less than ___ of photons misplaced by more than 2.5mm as a result of multiple scattering events

2.5mm

Anger calculated that a detector thickness of 6.4mm has less than 10% of photons misplaced by more than ____ as a result of multiple scattering events

statistical fluctuation in the distribution of light photons among PM tubes from one scintillation event to the next

the primary cause of limited intrinsic resolution is:

a signal to be read from a very thin line of activity that may not necessarily correspond to the actual activity in the patient

with the scintillation material reacting to the radiation that is being emitted events can overlap causing:

lead mask containing 1mm slits placed in from of the camera (no collimator) and a 140 keV Tc99m point source

how can we measure and characterize intrinsic spatial resolution?

no

when measuring intrinsic spatial resolution, is the collimator on?

gamma ray energy and detector crystal thickness

intrinsic resolution also depends on:

less gamma rays are converted into light photons

what happens gamma ray energy decreases?

1 divided by the square root of the energy level

as a general rule, intrinsic resolution is proportional to

there is a greater spreading of scintillation light before it reaches the PM tubes

what happens as the detector crystal thickness increases?

efficiency of collection of scintillation photons

intrinsic resolution improves with increased:

smaller crystals

for nuc med purposes, what general size of crystal is ideal for improving intrinsic resolution?

5 cm diameter

what is the average PM tube size

2-5 cm

how thick are NaI crystals in probes?

6.4-12.7 mm

how thick are NaI crystals in gamma cameras?

thicker crystals

detection efficiency increases in what size of crystal?

thinner crystals

intrinsic spatial resolution improves in what size crystal?

higher energy

as the crystal gets thicker, it allows the _____ emission to be absorbed better

drops

as the crystal gets smaller, the detection efficiency _____ over the energy spectrum

energy resolution

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

Pulse Height Analyzer

the energy resolution determines the efficiency for discrimination against low energy scatter photons interacting with the detector via the ________

light collection efficiency

what is required for good energy resolution?

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

energy resolution, like intrinsic resolution, depends largely on:

linearly

the number of light photons released in a scintillation event increase _____ with gamma ray

1 divided by the square root of the energy

energy resolution improves approximately in proportion to:

9-11%

energy resolution for gamma cameras is typically in the ____ range for Tc99m

130-150 keV

Tc99m is typically imaged with a window set for:

15%

the 130-150 keV window setting for Tc99m corresponds to approx. a ___ range

more efficiency detection of unscattered photons

what is the result in the photopeak becoming narrower

photopeak becoming more narrow

what can increase the number of valid events recorded and improve the statistical quality of the image?

their energy spread within the pulse height spectrum is also smaller

why are gamma rays that are scattered through large angles rejected more efficiently

improves image contrast

what happens to the image with scattered gamma rays are rejected

image quality

improved energy resolution results in better _____

high counting rates

what increases the likelihood of recording two events at the same time?

pulse pile up

recording two events at the same time is known as

counting losses and image distortion

two undesireable effects from pulse pile up are

counting losses

cause inaccurate counting rates to be recorded at high counting rates

analog buffers

derandomizers aka

analog buffers

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

paralyzable systems

gamma cameras behave as

window fraction

what does the apparent dead time for a selected energy window depends on?

window fraction

the ratio of the time the system is actively detecting events to the total time, which includes both the detection time and the dead time.

window fraction

fraction of total spectrum counting rate occuring within that window

larger the apparent dead time

the smaller the window fraction the:

image distortion

causes the image to record activity in areas where there isnt any due to pulse pile up distortion of the counts

in the circuitry via the incorporation of a pulse tail extrapolation

how can image distortion be fixed?

both counts to be counted and contribute to the exam

an estimator and subtractor off of known decay schemes for the photon pulses allows for what?

image nonlinearity

straight lines appear with inward or outward bowing

inward

pincushion distortion direction

outward

barrel distortion direction

nonlinearities

result when the X and Y position signals do not change linearly with displacement distance of a radiation source across the face of the detector

image nonuniformity

exposing the detector crystal to a uniform flux of radiation produces a flood field image with a small but noticeable nonuniformities in intensity, even with a properly functioning camera

intrinsic flood images

what kind 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 (Tc99m-pertecnatate)

extrinsic flood field images

what kind of images are acquired with the collimator in place using a disk or thin flood phantom that covers the area of the detector, typically pertechnetate Tc99m-O₄² or a Co57 sheet source are used

nonuniform detection efficiency and image nonlinearities

the two primary causes of gamma camera nonuniformities

nonuniform detection efficiency

what arises from small differences in the pulse-height spectrum for different PM tubes

monthly tuning for Tc99m on the camera

how can the position dependent collection efficiency of scintillation light, particularly for events located over the gaps and dead areas between the PM tubes compared with events located directly over the center of the PM tube be fixed?

edge packing

this can occur when there is a bright ring around the edge of the image

edge packing

this occurs from internal reflection in the scintillator material to be picked up at the outer edge of the camera

higher photon energies

this result in a worsening detector nonuniformity

data

___ is stored in a matrix of offset counts for each PM tube and pixel location that can correct for image nonuniformity

tuning map

every single isotope must complete its own ____ for the camera to be properly calibrated for the scanner

tuning map

what are these?

intrinsic resolution

improvements in camera uniformity also contribute to improvements in what?

counting statistics

older cameras needed to have thicker light guides to ensure that all of the photons were absorbed for good _______

thinner or no light guides at all

how thick are the light guides on newer scanners

image acquisistion

because of constant use and wear on PM tubes, it is important to manually tune the PM tubes for proper ______

due to Earth’s rotating magnetic field influencing changes in the PM tube gain

why is continuous tuning important in SPECT?

the collimator

what is the weak link for the performance of a gamma camera system

absorptive collimation

general collimators emply the principles of:

collimator efficiency

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

small

the collimator efficiency is usually a very ____ percentage

collimator resolution

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

collimator design

A primary consideration in ________ is to ensure that septal penetration by gamma rays crossing from one collimator hole into another is negligibly small.

septal penetration

A primary consideration in collimator design is to ensure that _______ by gamma rays crossing from one collimator hole into another is negligibly small.

negligibly small

A primary consideration in collimator design is to ensure that septal penetration by gamma rays crossing from one collimator hole into another is ______.

high atomic number and density

what characteristic of collimator material should minimize gamma ray interference.

lead

what is the preferred material for collimators

150 keV

low energy collimator have an upper limit of approximately

400 keV

medium-energy collimators have an upper limit of approximately

511 keV

high energy collimators have an upper limit of approximately

collimator efficiency

tc99m can be scanned with a medium energy collimator, but what is sacrificed in the mean time?

septa

if collimator ____ are too thin, they may be virtually transparent to high energy photons

geometry of the collimator holes

collimator performance is affected by the ____

shape, length, and diameter

what features of collimaotr holes affect performance

hexagonal, or circular made up of tightly packed square array on the camera

the ideal shape of collimator holes

collimator resolution and efficiency

what does collimator hole length affect?

collimator resolution

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

point spread function or line spread function

the radiation profile from a point or line source of radiation projected by the collimator onto the detector is also called the:

intrinsic resolution, collimator resolution, scattered radiation, and septal penetration

the sharpness of images recorded with a gamma camera is limited by: