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Med Imaging > gamma camera > Flashcards

gamma camera Flashcards

1
Q

gamma camera and key components

A

obtains a 2D image from a 3D distribution of radioactivity
multiple detectors on one gantry

  1. collimator
    provides positional info, filters through parallel ionising radiation
  2. scintillator crystal
    ionising radiation creates light photons
  3. PM tubes
    light photons are converted into electrons then amplified
  4. processing electronics
    positional and energy information gathered
    image is digitised for display

lead shielding

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2
Q

collimator

A

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

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3
Q

energy requirements for collimator

A

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

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4
Q

spatial resolution

A

improves with smaller distance to patient

narrow and long holes

thinner crystal

more, smaller pmts

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5
Q

parallel hole collimator sensitivity

A

improved with shorter and wider holes

importance:
less noisy
better visual contrast

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6
Q

low energy collimators TC-99m

A

low energy general purpose (LEGP)
better sensitivity
acquire image more quickly

low energy high resolution (LEHR)
better spatial resolution
image detail more important

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7
Q

choosing parallel hole collimators

A

septal thickness
hole length and width
activity in patient
source position
duration of acquisition
type of scan
required resolution

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8
Q

non parallel hole collimators

A

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

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9
Q

gamma camera detector

A

single thick Nal(Tl) crystal
surrounded by reflective material to maximise light output

shielded

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10
Q

gamma camera crystals

A

poor sensitivity for higher energies (compton)

thicker crystals:
more light dispersion
poorer spatial resolution

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11
Q

scatter reduction

A

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

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12
Q

image formation

(energy window)

A

take x,y,z info from scintillation
if energy is inside energy window
digitise accepted photons onto an image matrix
sum
apply corrections

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13
Q

corrections

A

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

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14
Q

PMT signals and info given

A

PMT signals give positional info

sum of PMT signals give energy

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15
Q

stabilising pmt output
tuning

A

temporal drifts in pmt output
irradiate detector with uniform flux of photons
alter pmt gain until consistent ratio of counts in 2 energy windows

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16
Q

energy correction

A

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

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17
Q

linearity correction

A

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

18
Q

uniformity correction

A

for remaining non-uniformities

uniform flux of photons
collect counts per pixel
invert image to create correction

19
Q

levels of quality assurance

A

acceptance testing: ensuring equipment meets purchase spec

commissioning: ensure system is ready for clinical use

quality control: periodic testing to ensure ongoing performance

20
Q

pixel/matrix size

A

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

21
Q

uniformity

A

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

22
Q

intrinsic / extrinsic

A

intrinsic: without collimator
extrinsic: with

23
Q

system spatial linearity
Measures

A

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

24
Q

system spatial resolution measurement

A

cobalt-57 flood emitting through bar phantom

bar phantom placed on crystal for intrinsic measurements

25
Q

system planar sensitivity

A

sensitivity depends on
efficiency of crystal

counts per second/activity in source

26
Q

count linearity/ count rate performance

A

measure observed count rate against input count rate

27
Q

dead time

A

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

28
Q

paralysable system

A

when gamma ray enters the detector within the deadtime of the previous event, the deadtime clock is restarted

gamma cameras are paralysable systems

29
Q

non-paralysable system

A

when gamma ray enters the detector within the deadtime from the previous event, the gamma ray is ignored

deadtime clock is not extended

30
Q

acquisition type
frame mode

A

image accumulated using pre-defined parameters

energy
matrix size = detector size/pixel size
pixel depth = byte/word

31
Q

static imaging

A

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

32
Q

dynamic imaging

A

capture change in activity distribution over time to examine a physiological process

produce many images
diff durations

fast frames
noisy
collimation required

33
Q

ecg gated imaging

A

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

34
Q

whole body imaging

A

multiple static images
or wholebody mode
patient slowly moved across gamma camera

35
Q

list mode

A

stores x y position and energy
stores raw data
used in research

flexible choice of imaging parameters
more data

36
Q

frame mode

A

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

37
Q

choosing matrix/pixel size

A

how much noise
how much detail

sampling theory

more pixels more noise

38
Q

sampling theory

A

pixel size should be at most 1/2 the FWHM spatial resolution

39
Q

pixel noise
poisson statistics

A

st dev of pixel value = square root of counts in pixel

40
Q

reducing noise

A

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

41
Q

digital image filtering
9 point average filter

A

simplest filter
averaging filter
9-point average filter
place kernel over image

smoothing
reduces noise
degrades spatial res

42
Q

9 point smooth kernel

A

greater emphasis to central pixels which are closer to the replacement value

Med Imaging (14 decks)

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