Chapter 27 Flashcards
What is attenuation?
Attenuation is the reduction of intensity of the electromagnetic radiation as it travels through a medium/tissue. It can occur through absorption or scatter of photons
What is contrast?
Difference in intensity between the brightest and darkest parts of the image
What does the contrast of an X-ray image dictated by?
The difference in attenuation coefficient of different images:
If they are of similar attenuation coefficients, image intensity will be similar for different tissues so contrast will be poor
The energy of the X ray photon:
If its too high, they won’t be attenuated by hard or soft tissue. Image will be black so contrast is poor
What are soft X rays?
Low energy X ray photons which are easily attenuated
What are hard X rays?
High energy X ray photons which are easily transmitted by soft and hard tissue
How are X rays produced in an X ray tube?
- Small current is run through the filament and electrons are emitted via thermionic emission
- Large pd exists between the cathode (filament) and anode (tungsten metal) and the tube which is evacuated so no gas atoms are there
- Electrons are accelerated and acquire energy due to the pd
- Electrons strike the tungsten metal and interact with the atoms and decelerate
- Energy of electrons is manifested as heat (99%) and x ray photons (1%)
What are the 4 methods of attenuation?
Simple scatter:
X ray photons scatter while conserving energy
Photoelectric effect:
X ray photons absorbed by low energy electrons and cause them to be liberated
Compton scatter:
X ray photon transfers some of its energy to liberate an outer shellelectron. X ray photon scattered with energy lower than before
Pair production:
X ray photon interacts with nucleus of an atom via strong force, causing a electron/positron pair to be produced
Transmitted intensity equation
I=I0e^-mewx
What are contrast mediums?
Substances which increase details of images within soft tissues. They do this by increasing the attenuation coefficient of soft tissue. Iodine and Barium sulfate are used.
What are the components of CAT scanner?
X ray tube
Detector array
Computer
How does a CAT scanner work?
- Patient lies stationary on a bed.
- X ray tube moves around the ring, direction a thin, fan shaped beam of x rays in a short pulse towards the patenting
- Detectors opposite the X ray tube detect the signals which are sent to the computer
- Once a revolution is completed, the signals from all detectors are added up, producing a slice
- Bed is moved through the scanner causing slices to be produced every revolution
- Slices added together to produce a 3D image
Advantages of CAT scanners
Can be used to create 3D images
Can distinguish between soft tissues of similar attenuation coefficients
Disadvantages of CAT scanners
More expensive and take longer to be produced
Radiation dosage is much larger
Harder for patient to remain still
What is a gamma camera?
Detects gamma radiation, emitted from a radioactive tracer, inside the patient.
How do gamma cameras work?
Gamma photons emitted from within the patient and travel towards the collimator.
Gamma photons which aren’t aligned to the axis of the tube are absorbed.
The gamma photons which are aligned, travel along the tubes to reach the scintillator.
The gamma ray photons strikes the scintillator and excites electrons of the atoms in the scintillator, emitting visible light photons.
The visible light photons travel through the light guide, into the photomultiplier tubes and convert the light photons into electrical signals.
The signals are processed by the computer and an image is produced.
What are the properties of gamma tracers?
Gamma emitters
Short half-life
What medical tracers are used?
Technetium 99m
Fluorine 18
Advantages of gamma cameras
Shows function of organ
What is PET scanner?
Detects gamma ray photons created from annihilation events. Can be used to create 3D images and cross-sectional images of body by using a ring of detectors.
Disadvantages of gamma cameras
Images are in poorer resolution compared to X ray images
How are gamma rays produced for PET scans?
- Fluorine 18 compound is consumed
- Positron travels less than a mm and annihilate when it meets an electron
- Ring of detectors registers a positron emission when the opposite detectors detect the 2 gamma photons
- Computer maps out location of fluorine 18 and image is produced
Advantages of PET scanners
Shows function and location of issues
Non invasive, can locate certain types of cancers
Disadvantages of PET scanners
Poor resolution images
Ionising radiation
Quite expensive and is not convenient
What is ultrasound?
Sound waves with a frequency greater than 20,000 hz
What is a transducer?
Device which produces ultrasound waves by causing a crystal to vibrate very rapidly
What is the piezoelectric effect?
When a pd is applied to the opposite faces of a crystal, causing it so expand or contract.
How does the piezoelectric effect work in transducers?
Transducer converts electrical energy into sound energy and converts sound energy to electrical energy using the piezoelectric effect.
When a high frequency alternating pd is applied across the opposite faces of the crystal, it causes the crystal to expand and collapse. The frequency of pd matches the natural frequency to ensure maximum amplitude causing ultrasound pulses to be produced.
When ultrasound is reflected back onto the transducer, this causes the crystal to vibrate, making it compress and expand, producing a pd which is detected as an electric signal
What are A scans?
Single transducer is used to image along a straight line. Records thickness of bones or the distance between the lens and retina of an eye. Ultrasound pulses are sent out in a single direction and the time for the reflected pulses are recorded. Then a calculation can be made to determine how far below the surface is.
What are B scans?
2D images
Transducer is connected to a computer while the transducer is being moved along the body
A computer plots a series of dots for each position of the transducer. This corresponds to the location of the boundaries. The brightness indicates the intensity of the reflected ultrasound wave. Dots form an image when projected together
What is the Acoustic impedance?
Product of density of the medium and the speed of the ultrasound. It relates to the percentage of ultrasound reflected.
Why is coupling gel used?
Without it, most of the ultrasound waves will be reflected due to the very different acoustic impudences of air and water. Coupling gel has a similar acoustic impedance to skin so not much ultrasound waves will be reflected, making the image better
How does Doppler ultrasound imaging work?
Frequency of reflected sound changes if reflected off moving bodies. This can be used to determine the amount/direction of blood flow in arteries/vessels/organs.
If the blood is moving towards the transducer, frequency increases. If its moving away, frequency decreases.
Transducer must be held at an angle for it to have a component in the plane of the blood flows velocity.
