Topic 11

Question 1

Spatial resolution definition?

Answer 1

Ability to detect contrast in high (small ) spatial frequency details function of line pairs per mm

Question 2

Modulation transfer function?

Answer 2

MTF expresses how spatial frequencies are transferred through system

Question 3

What does the spatial resolution depend on ?

Answer 3

1 ) Source focal spot 2 ) Magnification 3 ) Detector resolution

Question 4

Which MTF would you choose?

Answer 4

Screen a because it extends into higher frequencies.

Question 5

How do you calculate the MTF with the bar pattern test object?

Answer 5

amplitude of modulation at u/amplitude of modulation at 0

Question 6

The point spread function

Answer 6

describes the response of an imaging system to a point source or point object.

Question 7

The line spread function

Answer 7

is the response when subdividing the pixel surface in ideally infinitely small strips and expressing the relative efficiency of each

Question 8

To measure the PSF

Answer 8

In principle one should have a pencil beam of negligible transverse direction and scan the pixel surface with it.

Question 9

To measure the LSF?

Answer 9

A “blade” of radiation scanned in 1D only.

Question 10

Instead of measuring the LSF what do we measure instead?

Answer 10

The edge response function.

Question 11

How do you measure the Edge response function?

Answer 11

A sheet of tungsten, stick it front of the detector. Then shift the tungsten while the xrays are on and then for every shift, you look at each pixel response. If its isotropic and the detector responds in the same way, the more you uncover the more you count. This is a mechanism to sample the edge response function. Then all you do is take the derivative of this and get the line spread function.

Question 12

A shortcut to measuring the edge response function?

Answer 12

Take an image of a slanted edge of a blocker such as tungsten.

Question 13

The ideal detector LSF is what shape? and what is it in reality?

Answer 13

rectangular. rounded edges

Question 14

How can you get the MTF from the LSF?

Answer 14

Fourier transform the LSF and take the modulus

Question 15

What is the assumption made from the slanted edge method of obtaining the ERF?

Answer 15

That all of the pixels respond in the same way.

Question 16

What are the three types of noise in x-rays?

Answer 16

Quantum noise

• X ray photons distributed randomly in space and time. governed by the poisson statistics. so std = sqrt(counts).
• so you try to bring it close to* poisson limit.
• • AlWAYS present in any x-ray image. A statistical process.*

Electronic noise ( due to the detector + readout electronics: read noise, dark current, etc.) No electronic device is perfect. if you have an integrator. the pixel will collect data. the electrical charge is stored. if you switch on the transistor you have a transient. you cannot switch on something and not have parasitic noise all over the place. you have dark noise. when its being switched off.

Film-grain noise (in films) - ​

Structural noise : overlying & underlying anatomic structures

Question 17

How can you characterise the noise?

Answer 17

Standard deviation,

Frequency.

Question 18

What is quantum mottle?

Answer 18

The number of photons contributing to image per cm^2

Varies due to random fluctuations

Proportional to 1/√N

Question 19

The noise power spectrum

Answer 19

This is the noise power spectral density, expressing how power (= amplitude of the corresponding Fourier coefficient) in a random signal distributed between frequencies

Question 20

How do you calculate the Noise Power Spectrum?

Answer 20

Calculated as the FT of the autocorrelation function. (basically a mathematical tool for finding repeating patterns). which expresses how correlated noise is as a function o time/distance.

Question 21

What is the autocorrelation function?

Answer 21

same function convolved with itself. same function shifted in space, integrated over the axis.

if there are hidden frequencies in that function that are not obvious to see, when you shift it, one on top of the other and there is a hidden frequency which is contained in the function with a higher coefficient than other frequencies when you put them in phase with eachother you will have a bigger overlap.

If they overlap it will provide a peak - finding repeating patterns.

Question 22

The NPS is a function of /…..?

Answer 22

Exposure

Question 23

What does the MTF and NPS tell you about the system?

Answer 23

The MTF - tells you how contrast is transferred by your system

The NPS - How noise is transferred by the system as a function of frequency.

Question 24

What is a quantum accounting diagram?

Answer 24

A diagram that shows the number of x-ray photons at each stage of image processing in the xray detector.

(any level smaller than N, would mean a deterioration of the system. This is called the primary quantum sink.)

Question 25

What is the detection efficiency for a monochromatic beam? with a single mu.

Answer 25

Question 26

What is the detection efficiency for a polychromatic beam?

Answer 26

Question 27

What is the definition of detective quantum efficiency?

Answer 27

Question 28

What is the noise equivalent quanta?

Answer 28

If you have a detector thats not as good as a photon counter, you will assume that all detectors are a photon counter but they just recieve fewer x-rays - this is an assumption to make the maths easier. And this value is called the Noise Equivalent Quanta. (NEQ).

The reduced number of counts that a perfect detector should recieve to give the same noise performance.

Question 29

How do you calculculate the Normalised NPS?

Answer 29

NPS(V) / (Large area signal)^2

Question 30

How do you calculate the NEQ(v) Noise equivalent Quanta?

Answer 30

(MTF(v))²

^{= _________________}

NNPS

The MTF provides the response of the system (i.e. the signal) at each frequency, and the (normalised) NPS gives its variance (the noise).

Question 31

The three levels of performance measurements

Answer 31

1) Manufacturer characterisation
2) Acceptance testing
- Performed within a department before clinical use
3) Quality assurance / Quality control
- Routine periodic testing to ensure good system performance

Question 32

Testing standards

Answer 32

• 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 (IPEM)

– UK Medical Physics Body

– Suggests to equipment users tests and their frequency

Question 33

Commonly performed planar measurements

Answer 33

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 34

Making Sense of a Measurement

Answer 34

• 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 36

Uncertainty on number of counts

Answer 36

The process of radioactive decay can be described using poisson statistics