X-Rays: Instrumentation Flashcards
What is the purpose of filtration of an X-Ray beam?
Removes lower keV photons that would only contribute to skin dose.
What photon-tissue interaction process is responsible for contrast in an image?
Photoelectric effect.
How does the probability of a photoelectric effect interaction vary with energy?
1/(E^3)
How does the probability of a photoelectric effect interaction vary with atomic number?
Z^3.
What is the equation to define contrast in DR?
C=(I1-I2)/I1 =1-EXP[x(μ1-μ2)]
μ is the attenuation coefficient.
x is the thickness of the medium.
What does radiographic contrast depend on? (2)
The thickness of the object.
The difference in attenuation coefficients.
How does radiation contrast change with energy?
Decreases with increasing energy.
Why aren’t all DR exams done at lower energies to maximise contrast?
It increases dose to the patient.
How does Compton scatter affect an image?
Compton scatter deflects photons, which reduces contrast if they are captured by the detector.
How does the probability of Compton scatter change with increasing atomic number?
It is independent of atomic number.
Where is Compton scatter dominant?
Higher energies and thicker sections.
Apart from a reduction in contrast, what other problems does Compton scatter cause?
Staff and patient dose.
How can scattered photons be removed from an image?
Use of an anti-scatter grid - parallel lead holes that only allow perpendicular photons to the image receptor.
(Image filters to reduce noise.)
On a DR image, what is used as a measure of noise?
What statistical distribution models this?
Standard deviation is used as a measure of noise.
Noise is governed by discreet random processes, so is described by Poisson statistics.
How does the signal to noise ration (SNR) vary with an increase in the number of photons (N)?
Signal Increase with N and noise increases with N^0.5.
SNR = Signal/Noise
= N/(N^0.5)
= N^0.5
What are the three types of noise and where at what range are they significant?
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.
What can affect the spatial resolution of a system? (5)
Pixel size of the detector. Spread of signal in the detector. Focussing of electrons (Image Intensifier). Focal Spot Size. Magnification.
Explain how Digital Radiography produces an image. (5)
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.
How does an indirect receptor form a signal from X-Rays, and what material is commonly used?
X-Rays -> Light -> Charge -> Signal
Amorphous Silicon Flat Panel (also used in flurosocopy).
How does a direct receptor form a signal from X-Rays, and what material is commonly used?
X-Rays -> Charge -> Signal
Amorphous Selenium Flat panel
How does a CR receptor form a signal from X-Rays?
X-Rays -> Delayed Light -> Signal
What are the components of a direct or indirect detector?
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.
What phosphor is commonly used for an indirect (A-Si) detector?
Caesium Iodide
GdOS also sometimes used
Describe in detail how an indirect detector forms an image.
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.
What property of CsI improves the spatial resolution of the receptor?
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.
What is the k-edge of CsI?
33keV.
What advantage does the k-edge of CsI offer?
Increased chance of photoelectric effect.
How can the efficiency of the CsI phosphor be improved?
Increase thickness.
Describe in detail how an image is formed in a direct detector?
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.
How is the spatial resolution of a direct detector increased?
High electric field pulls electrons and holes in opposite directions minimising sideways spread. (increases the size of the exclusion zone)
What is the k-edge of a-Se?
12.7keV - too low for diagnostic x-ray but good for mammography.
How can the efficiency of the a-Se detector be improved?
Increase thickness.
What types of phosphor does a CR cassette use?
Powder phosphor - individual crystals held together with a binder.
How does the powder phosphor capture X-Rays?
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.
Needle plate phosphors are now available for CR cassettes, what material are they made from?
Caesium Bromide (CsBr)
What advantages does a CsBr Phosphor have over a traditional powder phosphor fro a CR cassette?
Better image quality - less light spread
Higher packing density - no binder - greater x-ray absorption.
Where area automatic exposure controls (AECs) situated in DR detectors?
Behind the grid but in front of the image plate.
What is the function of an AEC?
Monitor the air kerma from the tube and terminate when a pre-determined limit is reached.
The must take into account patient thickness.
Explain how an Image Intensifier works.
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.
