Gamma camera

Question 1

Why is patient positioning important ?

Answer 1

Patient detector distance affects image resolution

Drooping couch can cause artefacts

Increased motion artefacts if patient is uncomfortable

Question 2

How does a collimator work ?

Answer 2

Sets acceptance angle of a GC

Question 3

What are the measures of collimator performance ?

Answer 3

Sensitivity,
Resolution,
Magnification
Septal penetration

Question 4

What is the equation for geometric resolution ?

Answer 4

Rg = D + (D/L)(Z+G)

D = width gap betw collimator bars
L = length of collimator holes
Z = Collimator to source distance
G = Collimator to mid crystal distance

Question 5

What is collimator sensitivity dependent on?

Answer 5

Trade off between resolution and sensitivity

Geometric efficiency depends upon:
Fraction of gamma rays reaching each hole
Number of holes allowing gamma rays through

Question 6

What is the equation for minimum septal thickness?

Answer 6

T = 2DW/L-W W > 3/u

D = gap width
W=path length through septa
L = Length of holes

Question 7

Pinhole collimator resolution

Answer 7

RGeff = D + (D / L+G)Z

Question 8

What are the requirements of a scintillation crystal ?

Answer 8

High efficiency for stopping gamma rays

High probability of photoelectric absorption rather than Compton scattering

High conversion efficiency of absorbed energy to light photons

Transparent to its own emissions

Good matching between wavelength of emitted light and photomultiplier response

Short scintillation time
Mechanically robust

Question 9

How does doping effect scintillation crystals?

Answer 9

Promotes fluorescence over phosphorescence

Creates electron traps, alters band gap structure

y = 350-500nm, peak 410nm eff 10% pulse dur 0.25us

Question 10

What is a light guide?

Answer 10

Optical interface between crystal and the PMTs

Usually glass or quartz with a refractive index matched to the scintillator

Question 11

What do the PMTs, pre-amps and ADC do ?

Answer 11

Convert scintillated light to electrons and amplify

pre-amps: amps before signal can degrade

ADC: coverts analogue signal to digital

Question 12

What is the principle of pulse arithmetic?

Answer 12

Signals from each PMT fed through resistors with values which increase in proportion to PMT position.

Using matched resistors for energy output gives the signal for energy discrimination.

No normalisation for energy deposited so signals are larger for higher energy events

Question 13

How is energy windowing used by a gamma camera?

Answer 13

• Used to discriminate between scattered and primary photons
• Scatter reduces energy
• Apply a window (typically ± 10%)

Question 14

Which factors affect an energy peak?

Answer 14

Non uniformity in components responses

Impact of reflections and gap in system

PMT efficiency - variations in pmt output over time

Question 15

Why is gain stabilisation required?

Answer 15

• changes in high voltage
• temperature changes
• changes in magnetic field
• ageing PM tubes

Question 16

What is gain stabilisation ?

Answer 16

Real-time correction:
Reference LED pulses light on the photocathode
PMT output is redirected and compared to a ref voltage.
Now a microprocessor task

Question 17

What are some energy correction issues ?

Answer 17

• varying PMT response across faces
• gaps between tubes
• poorer light collection at crystal edges

fixed window could lead to under acceptance

Question 18

Example energy corrections

Answer 18

Hexagonal PMTs inc fill factor

Pixel-wise corrected applied to energy signals based on an energy correction map

Question 19

What is linearity distortion?

Answer 19

Misplacement of counts due to their position across a PMT

Sensitivity peaks in the middle and is lowest between detectors

Question 20

How does linearity distortion effect images?

Answer 20

Distorts bar phantom image

Causes deviations of a few mm, can misplace small features

Can cause variable magnification

Question 21

How is linearity corrected?

Answer 21

Use a correction matrices, applied similarly to an energy correction.

Obtain a base image by screening detector with a lead mask on top and illum with a flood source

Improves uniformity but not underlying variations

Question 22

How does interaction depth effect an image?

Answer 22

• Gamma rays interaction depth varies randomly
• Inc spread if further from PMT
• Distortion inc with inc thickness

Causes blurring and loss of resolution

Question 23

How to correct for interaction depth?

Answer 23

Standard corrections assume a single depth of interaction

have to review the level of signal spread to estimate a correction

Question 24

What is a sensitivity (uniformity) Map?

Answer 24

Used to correct for:

Small residual effects due to minor imperfections in tuning, gain stabilisation, energy and linearity corrections

Sensitivity variations due to variable crystal stopping power (due to variable thickness or density) or collimator imperfections

Applies a scaling factor to each pixel

Created from a high count flood image (120Mcts)

Question 25

What are the fundamental uncertainties of energy and spatial resolution ?

Answer 25

Fluctuations in signal caused by:

Number of scintillation photons emitted by crystal
Number of photons collected by each PMT
Electron emission by the photocathode
Secondary emission from dynodes 
High voltage supply
Pre-amplifier noise

Poisson stats apply - random