X-Rays: Instrumentation Flashcards Preview

Medical Physics: Imaging w/ Ionising Radiation > X-Rays: Instrumentation > Flashcards

Flashcards in X-Rays: Instrumentation Deck (39)
Loading flashcards...
1
Q

What is the purpose of filtration of an X-Ray beam?

A

Removes lower keV photons that would only contribute to skin dose.

2
Q

What photon-tissue interaction process is responsible for contrast in an image?

A

Photoelectric effect.

3
Q

How does the probability of a photoelectric effect interaction vary with energy?

A

1/(E^3)

4
Q

How does the probability of a photoelectric effect interaction vary with atomic number?

A

Z^3.

5
Q

What is the equation to define contrast in DR?

A

C=(I1-I2)/I1 =1-EXP[x(μ1-μ2)]

μ is the attenuation coefficient.

x is the thickness of the medium.

6
Q

What does radiographic contrast depend on? (2)

A

The thickness of the object.

The difference in attenuation coefficients.

7
Q

How does radiation contrast change with energy?

A

Decreases with increasing energy.

8
Q

Why aren’t all DR exams done at lower energies to maximise contrast?

A

It increases dose to the patient.

9
Q

How does Compton scatter affect an image?

A

Compton scatter deflects photons, which reduces contrast if they are captured by the detector.

10
Q

How does the probability of Compton scatter change with increasing atomic number?

A

It is independent of atomic number.

11
Q

Where is Compton scatter dominant?

A

Higher energies and thicker sections.

12
Q

Apart from a reduction in contrast, what other problems does Compton scatter cause?

A

Staff and patient dose.

13
Q

How can scattered photons be removed from an image?

A

Use of an anti-scatter grid - parallel lead holes that only allow perpendicular photons to the image receptor.

(Image filters to reduce noise.)

14
Q

On a DR image, what is used as a measure of noise?

What statistical distribution models this?

A

Standard deviation is used as a measure of noise.

Noise is governed by discreet random processes, so is described by Poisson statistics.

15
Q

How does the signal to noise ration (SNR) vary with an increase in the number of photons (N)?

A

Signal Increase with N and noise increases with N^0.5.

SNR = Signal/Noise
= N/(N^0.5)
= N^0.5

16
Q

What are the three types of noise and where at what range are they significant?

A

Quantum Noise (proportional to N^0.5) - dominates over the clinical range.

Electronic Noise - Fluctuations in electronics of the system - Can be significant at low doses, assumed to be constant.

Fixed pattern noise - Pixel to pixel variation - Should be reduced in DR due to flat field calibrations. Like signal so proportional to N.

17
Q

What can affect the spatial resolution of a system? (5)

A
Pixel size of the detector.
Spread of signal in the detector.
Focussing of electrons (Image Intensifier).
Focal Spot Size.
Magnification.
18
Q

Explain how Digital Radiography produces an image. (5)

A

Photons captured by fluorescent screen, or directly.

Signal digitised and assigned to a pixel.

Image processed and display varied.

Has linear response over large dynamic window,

Flat-field calibrations can be used to increase uniformity.

19
Q

How does an indirect receptor form a signal from X-Rays, and what material is commonly used?

A

X-Rays -> Light -> Charge -> Signal

Amorphous Silicon Flat Panel (also used in flurosocopy).

20
Q

How does a direct receptor form a signal from X-Rays, and what material is commonly used?

A

X-Rays -> Charge -> Signal

Amorphous Selenium Flat panel

21
Q

How does a CR receptor form a signal from X-Rays?

A

X-Rays -> Delayed Light -> Signal

22
Q

What are the components of a direct or indirect detector?

A
Material to absorb X-Rays.
Array of electronics to collect signal
- Array of thin film transistors (TFTs)
- Each Transistor forms a pixel
- TFTs usually deposited in glass.
- Signal from pixels transferred to a computer to create an image.
23
Q

What phosphor is commonly used for an indirect (A-Si) detector?

A

Caesium Iodide

GdOS also sometimes used

24
Q

Describe in detail how an indirect detector forms an image.

A

X-Rays absorbed in phosphor and produce light photons.
Photodiode array converts light to electric charge.
Signal captured by a-Si TFT array.
Each pixel 0.1-0.2mm square.
The image is read out in a progressive scan from top to bottom.
The charge stored is converted to a digital signal and the image is built up.

25
Q

What property of CsI improves the spatial resolution of the receptor?

A

Needle-like structure allows for TIR of photons, reducing sideways spread.
Other phosphors have more advantageous k-edge but do not have needle-like structure, so it is counteracted by the spread of the light.

26
Q

What is the k-edge of CsI?

A

33keV.

27
Q

What advantage does the k-edge of CsI offer?

A

Increased chance of photoelectric effect.

28
Q

How can the efficiency of the CsI phosphor be improved?

A

Increase thickness.

29
Q

Describe in detail how an image is formed in a direct detector?

A

X-Rays absorbed in a-Se - create electron/hole pair.
a-Se is a photoconductor - become more electrically conductive upon irradiation.
Voltage applied across a-Se layer.
Charge in a-Se is swept to the electrons and stored in pixels.
Image is read out.

30
Q

How is the spatial resolution of a direct detector increased?

A

High electric field pulls electrons and holes in opposite directions minimising sideways spread. (increases the size of the exclusion zone)

31
Q

What is the k-edge of a-Se?

A

12.7keV - too low for diagnostic x-ray but good for mammography.

32
Q

How can the efficiency of the a-Se detector be improved?

A

Increase thickness.

33
Q

What types of phosphor does a CR cassette use?

A

Powder phosphor - individual crystals held together with a binder.

34
Q

How does the powder phosphor capture X-Rays?

A

Poor packing density (much less than 100%), phosphor must be thin to avoid light spread (increasing spatial resolution). Phosphor, therefore, relies on K-Edge to capture X-Rays.

35
Q

Needle plate phosphors are now available for CR cassettes, what material are they made from?

A

Caesium Bromide (CsBr)

36
Q

What advantages does a CsBr Phosphor have over a traditional powder phosphor fro a CR cassette?

A

Better image quality - less light spread

Higher packing density - no binder - greater x-ray absorption.

37
Q

Where area automatic exposure controls (AECs) situated in DR detectors?

A

Behind the grid but in front of the image plate.

38
Q

What is the function of an AEC?

A

Monitor the air kerma from the tube and terminate when a pre-determined limit is reached.
The must take into account patient thickness.

39
Q

Explain how an Image Intensifier works.

A

X-Rays incident on input phosphor (CsI) and produce light photons.
Light incident on photocathode and produces electrons.
Electrons accelerated across a vacuum.
High energy electrons incident on output phosphor (ZnCdS) and produce more light (flux gain from acceleration of electrons -increased energy)
Area of input phosphor is larger than output leading to minification gain.
Light detected be camera and image is displayed.