X-Rays: Instrumentation

Question 1

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

Answer 1

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

Question 2

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

Answer 2

Photoelectric effect.

Question 3

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

Answer 3

1/(E^3)

Question 4

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

Answer 4

Z^3.

Question 5

What is the equation to define contrast in DR?

Answer 5

C=(I1-I2)/I1 =1-EXP[x(μ1-μ2)]

μ is the attenuation coefficient.

x is the thickness of the medium.

Question 6

What does radiographic contrast depend on? (2)

Answer 6

The thickness of the object.

The difference in attenuation coefficients.

Question 7

How does radiation contrast change with energy?

Answer 7

Decreases with increasing energy.

Question 8

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

Answer 8

It increases dose to the patient.

Question 9

How does Compton scatter affect an image?

Answer 9

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

Question 10

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

Answer 10

It is independent of atomic number.

Question 11

Where is Compton scatter dominant?

Answer 11

Higher energies and thicker sections.

Question 12

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

Answer 12

Staff and patient dose.

Question 13

How can scattered photons be removed from an image?

Answer 13

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

(Image filters to reduce noise.)

Question 14

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

What statistical distribution models this?

Answer 14

Standard deviation is used as a measure of noise.

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

Question 15

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

Answer 15

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

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

Question 16

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

Answer 16

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.

Question 17

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

Answer 17

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

Question 18

Explain how Digital Radiography produces an image. (5)

Answer 18

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.

Question 19

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

Answer 19

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

Amorphous Silicon Flat Panel (also used in flurosocopy).

Question 20

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

Answer 20

X-Rays -> Charge -> Signal

Amorphous Selenium Flat panel

Question 21

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

Answer 21

X-Rays -> Delayed Light -> Signal

Question 22

What are the components of a direct or indirect detector?

Answer 22

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.

Question 23

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

Answer 23

Caesium Iodide

GdOS also sometimes used

Question 24

Describe in detail how an indirect detector forms an image.

Answer 24

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.

Question 25

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

Answer 25

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.

Question 26

What is the k-edge of CsI?

Answer 26

33keV.

Question 27

What advantage does the k-edge of CsI offer?

Answer 27

Increased chance of photoelectric effect.

Question 28

How can the efficiency of the CsI phosphor be improved?

Answer 28

Increase thickness.

Question 29

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

Answer 29

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.

Question 30

How is the spatial resolution of a direct detector increased?

Answer 30

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

Question 31

What is the k-edge of a-Se?

Answer 31

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

Question 32

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

Answer 32

Increase thickness.

Question 33

What types of phosphor does a CR cassette use?

Answer 33

Powder phosphor - individual crystals held together with a binder.

Question 34

How does the powder phosphor capture X-Rays?

Answer 34

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.

Question 35

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

Answer 35

Caesium Bromide (CsBr)

Question 36

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

Answer 36

Better image quality - less light spread

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

Question 37

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

Answer 37

Behind the grid but in front of the image plate.

Question 38

What is the function of an AEC?

Answer 38

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

Question 39

Explain how an Image Intensifier works.

Answer 39

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.