Collimators Flashcards
FOV
total area accepting gamma from area
umbra
area of FOV which the entire crystal has an unobstructed view
penumbra
area of FOV which is visible to only part of the crystal
radius of resolution
radius between the penumbral limits
FOV = ____ + _____
FOV = umbra + penumbra
single aperture collimator
collimator with one hole
typical range for the number of holes a collimator has
4,000 to 46,000
(L)
length of collimator/lead hole
(r)
radius of the hole at the crystal surface
N
number of holes in the collimator
transmission/sensitivity
- area of the crystal not obstructed by lead septa of collimator
- fraction of incident radioactivity transmitted through the collimator
septum
thickness of lead between the hole
angle of acceptance
even wider than penumbral limits due to septal penetration
what affects resolution and sensitivity?
radius and number of of the holes, length of holes
septal thickness, and distance between source and collimator
what improves resolution?
- increasing length of the hole
- decreasing the radius or width of hole (which in turn increases number of holes)
- decreasing distance between source and collimator
hole length for a high resolution collimator
increased hole length
hole length for LEGP or LEAP collimator
medium length hole
hole length for high sensitivity collimator
decreased hole length
what photon energy is used for a high energy collimator?
300 keV
as energy increases, the ____ decreases because _____ increases.
number of holes decrease due to the fact that septum needs to increase
where is spatial resolution best?
at the surface of the collimator
(resolution/sensitivity) is independent of the distance in multi-hole parallel hole collimators.
sensitivity
explain image size in terms of distance.
image size is independent of distance
but scatter makes it appear slightly larger
slant hole collimators
parallel, cylindrical holes at a 15-30 degree slant
what are slant hole collimators good for?
it’s better for oblique views to allow for imaging of organs that are typically observed by an overlying structure while allowing the face of the collimator to be close to the body surface
converging collimator
holes converge to a point at some distance from the face of the collimator
describe the converging collimator
- has magnifying properties (thus used when magnifying needs to be done and it increases resolution slightly)
converging collimators have a compromise between?
sensitivity of LEAP and resolution of pinhole
convergence point
- greatest magnification at the cp
- FOV decreases at the cp
diverging collimator
holes that diverge away from the central axis
when do we typically use a diverging collimator?
when we’re imaging large organs on small crystals
when you increase distance… what occurs?
- increased FOV
- decreased resolution
- decreased sensitivity (assess to less holes with an increased in distance)
flat field collimators
one large hole on uptake probes meant for counting and not imaging
an increase in source to detector distance results in what when using a flat field collimator?
- decrease in sensitivity and resolution
- increase in the FOV
why is lead shielding found extending to the back of the crystal in a flat field collimator as well?
it decreases background counts
in a flat field collimator, where is the thickness of the lead greatest?
closest to the crystal while it decreases with increasing distance
what are the requirements of the flat field collimator?
- need uniform detection efficiency across the FOV and throughout the thickness of the organ
- limit the area seen by crystal so it excludes most radioactivity outside the ROI
what is the isoresponse curve?
needs to make sure that the change in counts detected isn’t due to our equipment or choice of distance
when do you use pinhole collimators?
when you’re imaging small organs
what is the greatest advantage to using a pinhole collimator?
magnification capability
with pinhole collimators, resolution is dependent on what?
- aperture to crystal distance (L)
- source to aperture distance (D)
- aperture diameter (d)
with pinhole collimators, resolution can be improved by what?
- decrease in collimator to source distance and aperture diameter
- increase collimator length
what is the magnification factor that improves resolution?
I/O = L/D
size of the imaged area is affected with the distance from the pinhole collimator. t/f
true
what results in a small imaged area when using a pinhole collimator?
large magnification factor obtained at a close source-to-collimator distance
why are images distorted when using a pinhole collimator?
3d objects will have different distances depending on its source planes
- magnification of different amounts
L = D
true size
D < L
magnify
D > L
minify
using a pinhole collimator, maximum magnification is best found where?
at collimator surface
counts from a small FOV are projected onto a larger portion of the crystal. what should be done about counting times?
counting times should be increased to maintain enough counts in pixels
star artifact
when high energy RN is imaged with low, medium collimators
- results in septal penetration
the arms of the star in a star artifact is in the direction of?
direction of least amount of lead
fan beam collimator
- cross between converging and parallel-hole collimators
- allows patient data to be spread over the crystal surface
when do we use fan beam collimators?
it is designed for cameras with rectangular heads when imaging smaller organs (ex. brain and heart)
spatial resolution
defined as the ability of an imager to reproduce the details of RN distribution
what are quantitative methods to measure collimator resolution?
- isoresponse line
- line spread function (LSF)
- modulation transfer function (MTF)
isoresponse line
- used to determine the correct distance of collimator to organ
- it is to determine working distance based on uniform sensitivity
how do we determine a Point Spread Function (PSF)?
when we image a POINT SOURCE and plot the intensity profile across its centre, we end up with a bell shaped curve
when using an Point Spread Function (PSF), how do we determine resolution?
by determining the distance for the count rate to fall by 50% (FWHM)
line spread function (LSF)
doing a line profile of count distribution and taking a row of pixels
- plot counts per pixel value
pixel dimension is determined by?
FOV/pixel size
resolution determined by LSF
resolution of collimator = FWHM * pixel dimension
most important factors for resolution =?
intrinsic resolution and collimator resolution
combined effect of these two factors = system resolution
what are the 3 ways to qualitatively analyze resolution?
- PLES
- Hine-Duley
- Four Quadrant
“good resolution”
relates to placement of counts
“good contrast”
relates to variation in count density
what does the modulation transfer function (MTF) do?
assessing the camera’s ability to accurately portray the RP distribution by good reso and good contrast
how do we determine MTF?
- measures the amount of source modulation transferred to the final image
- focusses on converting images from the spatial domain to the frequency domain
contrast or modulation (Ms) is calculated by:
Ms = (Amax - Amin)/(Amax + Amin)
how is modulation (Mi) in the image expressed?
Mi = (Cmax - Cmin)/(Cmax + Cmin)
if MTF measures the amount of source modulation transferred to the final image, then the MTF at a spatial frequency is calculated how?
MTF = Mi/Ms
what value of MTF indicates the best spatial resolution?
1.0
- capture of 100% of the contrast in the image
what value of MTF indicates the worse spatial resolution?
0
low spatial frequency =
homogeneous in counts and has less variation
high spatial frequency =
rapid change in counts
what are some factors that affect MTF?
- source frequency (lesion size)
- distance
- photon energy
- collimator selection
small object = (higher/lower) MTF value
larger object = (higher/lower) MTF value
small = lower MTF
large = higher MTF
↑ distance = (↑/↓) reso = (↑/↓) MTF
↑ distance = ↓ reso = ↓ MTF
↑ energy = (↑/↓) MTF
why?
↑ energy = ↓ MTF
due to more partially absorbed and less efficiency happening in the detector
collimator that has optimal resolution = (↑/↓) MTF
collimator that has optimal resolution = ↑ MTF
what is the formula to determine the resolution of a parallel-hole collimator?
r(c) = d(L+b)/L
d, diameter of hole
L, hole length
b, source to collimator distance
what is the formula to determine collimator efficiency (g)?
g = ((kd^2)/a(d+t))^2
g collimator efficiency
k hole shape constant
d diameter of hole
a length of hole
t septal thickness
efficiency is independent of distance. t/f
true
why does increasing distance result in decrease in resolution?
photons will be interacting with the collimator/crystal at a range of angles making positioning less accurate
why does the length of the hole affect the resolution?
the angle of acceptance is changing which results in the spread of line profile to change as well.
it’ll result in a higher FWHM which means lower reso
when measuring resolution, (larger/smaller) rcoll value is better.
smaller rcoll = better
how do you determine which collimator’s resolution is least affected by changes in source to detector distance?
calculating the Rcoll and the one that is the least difference in Rcoll values when calculated for the different distances
