Topic 12

Question 1

Gamma Camera Planar Performance Testing

Answer 1

Three levels of Performance Measurements • Manufacturer Characterisation • Acceptance Testing – Performed within a department before clinical use • Quality Assurance / Quality Control – Routine periodic testing to ensure good system performance

Question 2

Testing Standards

Answer 2

National Electrical Manufacturers Association (NEMA) – American body although accepted worldwide – Lab environment tests to specify equipment performance and allow comparability between systems • Medicines and Healthcare products Regulatory Agency (MHRA) – UK Regulatory Authority – EU (EMEA, CE Mark), US (FDA) • Institute of Physics and Engineering in Medicine – UK Medical Physics Body – Suggests to equipment users tests and their frequency

Question 3

Commonly Performed Planar Measurements

Answer 3

Intrinsic (Uncollimated) Uniformity • Intrinsic Spatial Resolution • Intrinsic Spatial Linearity • Intrinsic Energy Resolution • Count Rate Performance • System (Collimated) Uniformity • System Spatial Resolution (no scatter) • System Sensitivity

Question 4

Making Sense of a Measurement

Answer 4

Radionuclide • Energy Window • Scatter Included ? • Collimator (System Measurements) • Areas of Camera Used – Useful Field of View (UFOV) – Central Field of View (CFOV) – 75% of UFOV • Count Rate / Count / Count Density • Pixel/Matrix Size

Question 5

Uncertainty on Number of counts

Answer 5

The process of Radioactive decay can be described using Poisson statistics • In Poisson statistics – The standard deviation = square root of mean • Example of count uncertainty: – A pixel in a 64 x 64 array has 100 counts – Standard deviation of this measurement is 10 counts (10%) – Moving to 128 x 128 matrix and equivalent count density would give 25 counts in a pixel with s.d. = 5 counts (20%)) • Reduction of pixel size has an adverse effect on uncertainty / noise.

Question 6

The definition of uniformity

Answer 6

The variability of the observed count distribution in the gamma camera from a uniform source – Intrinsic: Without a collimator – Extrinsic or System: with a collimator attached.

Question 7

Causes of non-uniformity

Answer 7

Non-uniform detection efficiency across crystal -

different PMT (energy) gains -

positioning non-linearities -

Optical coupling between PMT and crystal Collimator imperfection

Damaged septa

Uneven hole sizes and angulation errors

Question 8

2 types of Intrinsic uniformity

Answer 8

Integral uniformity and differential uniformity

Question 9

What is integral uniformity?

Answer 9

Integral uniformity assesses GLOBAL non-uniformities

Question 10

Integral uniformity, how do you calculate it?

Answer 10

IU = (max-min/max+min) x100

Question 11

How do you calculate differential uniformity?

Answer 11

DU = (Max-Min/Max+Min) x100

Question 12

What is differential uniformity used to assess?

Answer 12

REGIONAL non-uniformities

Question 13

Why can we never 0% uniformity?

Answer 13

because of poisson statistics

Question 14

What is intrinsic spatial resolution limited by?

Answer 14

Limited by: -statistical fluctuation of light photon distribution PMT, and PMT amplification. - Multiple scattering by gamma photon within the detector

Question 15

What is intrinsic spatial resolution improved by?

Answer 15

Improved by; - Thinner crystal (less light spread) - More, smaller PMTs - Higher gamma ray energy (more light photons produced) - Signal processing and position calculation algorithms

Question 16

How can we determine the intrinsic spatial resolution

Answer 16

get a mask with a slit and get FWHM OR FWTM (TENTH MAX)

Question 17

How can we determine Intrinsic spatial linearity?

Answer 17

Place the mask with slits, and in each row (column) measure deviation of peak count from best line fit. we can do this differentially or integrally.

Question 18

How can you determine the intrinstic spatial linearity differentially and integrally?

Answer 18

Differential - standard deviation of difference between peak locations and fit (typically < 0.2mm) Integral- maximum difference between peak location and fit (typically < 0.4mm)

Question 19

Energy resolution calculation again?

Answer 19

( FWHMx100 / peak energy ) %

Question 20

Better energy resolution allows better discrimination of ______

Answer 20

scattered counts

Question 21

What is count rate performance

Answer 21

Count rate performance characterizes a scintillation camera’s ability to accurately function at count rates which are near the maximum rate of camera operation. (basically if a gamma ray is being processed a second gamma ray cannot be processed after that in what is known as the “deadtime”)

Question 22

What types of deadtime are there?

Answer 22

Paralysable ( the gamma ray is ignored and the deadtime is not extended) and non paralysable (deadtime is extended from subsequent gamma ray).

Question 23

Gamma cameras are paralysable or nonparalysable?

Answer 23

Paralysable

Question 24

What does this graph show?

Answer 24

The count rate performance

The black line is the ideal line. The grey line is how the gamma camera system responds due to the deadtime effects at the end.

Question 25

why do we often use collimator in place when we measure the system?

Answer 25

Closer to “Real” imaging

BUT: (have to say what collimator we are using)

Scatter often included

Higher activity sources often needed - sensitivity greatly reduced with collimator

Question 26

problem with point source to measure System uniformity?

Answer 26

Point source would be seen as a (blurred point) with collimators

Question 27

Spatial resolution limited by?

Answer 27

• Statistical fluctuation of light photon distribution between PMT, and PMT amplification
• Multiple scattering by gamma photon within detector

Question 28

What is spatial resolution improved by?

Answer 28

1. Thinner crystal (less light spread)
2. More, smaller PMTs
3. Higher gamma ray energy (more light photons produced)

• Energy Resolution ∝ 1/(E)1/2

4. – Signal processing and Position Calculation Algorithms