gamma camera Flashcards
gamma camera and key components
obtains a 2D image from a 3D distribution of radioactivity
multiple detectors on one gantry
- collimator
provides positional info, filters through parallel ionising radiation - scintillator crystal
ionising radiation creates light photons - PM tubes
light photons are converted into electrons then amplified - processing electronics
positional and energy information gathered
image is digitised for display
lead shielding
collimator
need to determine origin of photons
lead on outside stops oblique radiation
lead absorbs rays
increasing thickness of collimator walls, more lead, reduced sensitivity
-parallel hole
-pin-hole
-converging
-fan beam
collimator reduces sensitivity of detector system
energy requirements for collimator
lower energy collimator than required:
inadequate lead to stop obl. radiation
septal penetration -> blurring and streak artefacts
higher energy:
reduces sensitivity
hole width increased to compensate which leads to decreased spatial resolution
spatial resolution
improves with smaller distance to patient
narrow and long holes
thinner crystal
more, smaller pmts
parallel hole collimator sensitivity
improved with shorter and wider holes
importance:
less noisy
better visual contrast
low energy collimators TC-99m
low energy general purpose (LEGP)
better sensitivity
acquire image more quickly
low energy high resolution (LEHR)
better spatial resolution
image detail more important
choosing parallel hole collimators
septal thickness
hole length and width
activity in patient
source position
duration of acquisition
type of scan
required resolution
non parallel hole collimators
pinhole collimators
v good spatial resolution
poor sensitivity
image inverted
for small objects
converging/fan beam collimators
good spatial reso and sensitivity
for brain imaging
converges
gamma camera detector
single thick Nal(Tl) crystal
surrounded by reflective material to maximise light output
shielded
gamma camera crystals
poor sensitivity for higher energies (compton)
thicker crystals:
more light dispersion
poorer spatial resolution
scatter reduction
use energy windowing to reduce the amount of scattered radiation in our final image
& to segregate scattered radiation
- limited energy resolution
dual energy window scatter correction
approximate scattered counts and subtract
triple energy window scatter correction
use 3 windows
multiple energy windows
image formation
(energy window)
take x,y,z info from scintillation
if energy is inside energy window
digitise accepted photons onto an image matrix
sum
apply corrections
corrections
periodic tuning of PMT outputs
different response across pmt tube (energy correction)
difference in sensitivity of pmts (linearity correction)
residual non-uniformities (uniformity correction)
temporal drift of pmts (age, temp..)
difference in individual pmt response
PMT signals and info given
PMT signals give positional info
sum of PMT signals give energy
stabilising pmt output
tuning
temporal drifts in pmt output
irradiate detector with uniform flux of photons
alter pmt gain until consistent ratio of counts in 2 energy windows
energy correction
response greatest at centre of tubes
light lost between
so energy response is spatially dependent
non-uniform pmt response
reduction in response
need constant energy response across field of view
make point by point correction (pixel by pixel)
irradiate detector without collimator w uniform flux
use matrix
set 2 energy windows
one above true photopeak energy
one below
counts in each window should be same
alter if not
linearity correction
detector response varies spatially
different sensitivity of pmts
so spatial non-linearity of system
2 correction matrices
pixel by pixel corrections
no collimator
apply lead mask with slits
look at deviation from lines and correct to align
relatively independent of photon energy
uniformity correction
for remaining non-uniformities
uniform flux of photons
collect counts per pixel
invert image to create correction
levels of quality assurance
acceptance testing: ensuring equipment meets purchase spec
commissioning: ensure system is ready for clinical use
quality control: periodic testing to ensure ongoing performance
pixel/matrix size
reduction of pixel size has adverse effect on uncertainty
more pixels:
better spatial sampling
less counts per pixel, increased noise
standard deviation is square root of mean
uniformity
the variability of the observed count distribution from a uniform source
-non uniform detection efficiency across detector
-collimator imperfections
system uniformity: w collimator
assess integral and differential uniformity
intrinsic / extrinsic
intrinsic: without collimator
extrinsic: with
system spatial linearity
Measures
place lead mask
measure deviation of peak count from best line fit
differential spatial linearity
sd of diff between peak locations and fit
integral
maximum diff between peak location and fit
system spatial resolution measurement
cobalt-57 flood emitting through bar phantom
bar phantom placed on crystal for intrinsic measurements
system planar sensitivity
sensitivity depends on
efficiency of crystal
counts per second/activity in source
count linearity/ count rate performance
measure observed count rate against input count rate
dead time
processing of an event takes a finite time
aka deadtime
if 2 gamma rays enter detector in interval less than deadtime, 1 or 2 is lost
paralysable system
when gamma ray enters the detector within the deadtime of the previous event, the deadtime clock is restarted
gamma cameras are paralysable systems
non-paralysable system
when gamma ray enters the detector within the deadtime from the previous event, the gamma ray is ignored
deadtime clock is not extended
acquisition type
frame mode
image accumulated using pre-defined parameters
energy
matrix size = detector size/pixel size
pixel depth = byte/word
static imaging
after initial uptake period, the distribution is stable over time
acquire data in static mode where we produce one image over a given time period
dynamic imaging
capture change in activity distribution over time to examine a physiological process
produce many images
diff durations
fast frames
noisy
collimation required
ecg gated imaging
imaging of the heart
over many heart beats, counts are accumulated in multiple bins until enough counts have been acquired to show different heart phases
temporal resolution requirements
more frames gives better quantifications
noisy
collimation
whole body imaging
multiple static images
or wholebody mode
patient slowly moved across gamma camera
list mode
stores x y position and energy
stores raw data
used in research
flexible choice of imaging parameters
more data
frame mode
most common in nuc med work
specify ahead of time what parameters we want to use for imaging
eg acquisition time, energy window…
acquisition is accumulated according to the given protocol
less data to store
image visible during data acquisition
choosing matrix/pixel size
how much noise
how much detail
sampling theory
more pixels more noise
sampling theory
pixel size should be at most 1/2 the FWHM spatial resolution
pixel noise
poisson statistics
st dev of pixel value = square root of counts in pixel
reducing noise
collimator - loss of spatial res
bigger pixel size - loss of spatial sampling and res
more activity - more radiation
filtering - loss of res, edge enhancement might create false or over enhanced features
digital image filtering
9 point average filter
simplest filter
averaging filter
9-point average filter
place kernel over image
smoothing
reduces noise
degrades spatial res
9 point smooth kernel
greater emphasis to central pixels which are closer to the replacement value
