Medical Physics Flashcards
(23 cards)
How are X-rays produced
▪️The metal filament is heated
▪️thermionic emission occurs
▪️electrons are produced
▪️electrons accelerate toward the target anode
▪️when they strike the target anode their kinetic energy is absorbed by the electrons of the target anode
▪️As a result the electrons of the target anode undergo excitation jumping from their lower energy levels to higher energy levels and emitting electromagnetic radiations of wavelength 10^-10m HENCE X-RAYS ARE PRODUCED
What controls the intensity of X-rays
The magnitude of Tube current
What happens when tube current increases
More electrons emitted, more number of X-rays produced hence intensity increases
What happens when pd increases
▪️When pd increases electrons gain more energy
E=hf
so X-ray photons are emitted with a greater frequency
▪️emitted electrons have a range of KE so X-ray photons produced also have a range of frequencies
Attenuation of X-rays
Defined as the loss in power or loss in intensity of a radiation as it passes through a living matter
Exposures of
Bone
Soft tissue
Flesh
In Bone the X-ray film has very little exposure I.e white
In soft tissue the X-ray film has very little exposure I.e grey
In flesh the X-ray film has very little exposure I.e black
What is half value thickness
It is defined as the thickness of the medium which causes the intensity of X-ray to reduce to half of its original value
What is half life
The time taken for an activity to reduce to half of its initial value
What does the quality of an image depend upon
Sharpness
Contrast
What is sharpness
It is defined as the ease with which the EDGES of a structure can be determined. A sharp image is one in which the EDGES ARE WELL DEFINED
What is contrast
It is defined as the ease with which ONE STRUCTURE CAN BE DIFFERENTIATED FROM THE OTHER. A good contrast is one which has a WIDE RANGE OF EXPOSURES i.e it shows areas of little or no blackening as well as areas with heavy blackening
What factors affect the sharpness of X-rays
- Size of the target anode
2. Size of the aperture
How does the size of the target anode affect the sharpness of X-rays
If the beam of the X-ray has a greater width it is less focused or less sharp and if the beam of the X-ray has a lesser width it is more focused or more sharp
I.e
Reducing the size of the anode increases the sharpness of the X-ray beam
How does the size of the aperture affect the sharpness of X-rays
If the aperture is large sized the beam is less focused i.e lacks sharpness and if aperture is small sized the beam is more focused i.e sharpness improves
So
Reducing the size of the aperture improves the sharpness of the X-ray beam
Factors that affect the contrast of X-rays
- Exposure time
- Use of back light
- Stomach patients
How does exposure time affect the contrast of X-rays
If u increase the exposure time within the limit contrast improves
I.e it goes from
Grey ➡️ Black
White ➡️white
How does the use of back light affect the contrast of X-rays
Black ➡️ Black
White ➡️ back light ➡️ more passes through, brighter than before and hence a good contrast is established
How do stomach patients affect the contrast of X-rays
Barium drink is very strong and absorbs X-rays creating an artificial contrast
How is ultrasound produced
Ultrasonic transducer
How does A scan measure the thickness between various boundaries
Using gel as a coupling medium a short burst of ultrasound is made to enter the body
At each boundary between the two medium a certain fraction of ultrasound gets reflected while a certain fraction gets transmitted
The reflected pulse is picked up by the transducer which converts it into a voltage pulse
The voltage pulse is processed, amplified and observed on the CRO screen
The later an echo is received the greater is the scale through which it is amplified before it is displayed on the cro screen (as the echos obtained from the region deep inside the body are of low intensities)
What is a B scan
It is a combination of an A scan which is taken from a variety of different angles
Individual pulses are gathered, analysed and processed by a computer which superimposes these multiple echos on top of each other thereby gathering a 2D image
What is a radio tracer
It is a substance that consists of radioactive material. It is injected into the body and then eventually accumulates in the area of the body under examination. The radiations emitted from the trader is used for image construction
What is the advantage of a glucose based traced
Different tissues or organs take it up at different rates. Since cancer cells are more metabolically active than surrounding healthy cells they absorb glucose at a higher rate and this emit radiations at a higher rate
