6.5 Medical Imaging Flashcards
What is the basic structure of an X-ray tube;
components – heater (cathode) where electrons are emitted by hot filament, cup shaped to focus beam on target metal
Anode (target metal)
High voltage supply
How are x-ray photons produced from an x-ray tube?
X-ray tubes produce X-rays by accelerating electrons in a high-voltage electric field then rapidly decelerating them via collisions with a hard metal anode (positive electrode) e.g. tungsten
What happens when the electrons smash into an anode like tungsten?
They decelerate and some of the KE converts into EM energy, in the form of x-ray photons. The tungsten anode emits a continuos spectrum of x-ray radiation.
Why is the tungsten anode rotated in a x-ray tube?
most of the KE of the electrons are converted to thermal energy, so rotating avoids overheating.
Describe what happens in terms of electrons when an electron hits tungsten atom
- Incoming electron
- inner “tungsten” electron ejected
- outer electron drops down to fill gap
- x-ray photon emmited
What are 2 ways of increasing the intensity of X-ray beams?
- Increase tube voltage - Gives electrons more KE so knocks out deeper shell electrons in tungsten atom
- Increase current supplied to filament - Temp rises, frees more electron per second producing more x-ray photons per second
The intensity of an X-ray beam decreases (attenuates) exponentially with what?
The intensity of an X-ray beam decreases (attenuates) exponentially with distance from the surface(x)
Intensity=Io e ^ -(μx)
x is in cm
μ is in cm^-1
How are X-rays attenuated?
By absorption and scattering
What are the 3 causes of X-ray attenuation?
- The photoelectric effect
- Compton scattering
- Pair production
What is the photoelectric effect?
Photon absorbed by electron, which is ejected from its atom. The gap in the shell is filled by another electron, which emits a photon.
What is the Compton effect?
(X-ray) photon interacts with an (orbital) electron
The (scattered) photon has a longer wavelength
AND
The electron is ejected (from the atom at high speed)
What is pair production?
high energy Incoming photon decays and produces electron-positron pair
How much energy is absorbed by a material is dependent upon what?
Atomic number
What is simple scattering (another X-ray attenuation mechanism)
X-rays of energy 1-20 keV will reflect off layers of atoms or molecules in the material as they do not have enough energy to undergo more complex processes
Describe the use of contrast media when X-rays are used to produce images of internal body structures (4 marks)
- Iodine / barium (used as contrast material)
- *High Z number / large attenuation coefficient / large absorption coefficient (used to improve image contrast)
- Contrast media are ingested / injected into the body
- *Scan shows outline / shape of soft tissue
Describe the use of contrast media when X-rays are used to produce images of internal body structures (4 marks)
- Iodine / barium (used as contrast material)
- High Z number / large attenuation coefficient / large absorption coefficient (used to improve image contrast)
- Contrast media are ingested / injected into the body
- Scan shows outline / shape of soft tissue
What components are needed for CAT scan
Computerised axial tomography (CAT) scanning; components – rotating Xtube producing a thin fan-shaped X-ray beam, ring of detectors, computer
software and display
What are 3 advantage of CAT scan over X-ray image?
- X-ray image is 2D / CT scan produces 3D image
- Greater detail / definition / contrast with CT scan / ‘soft tissues can be seen’
- Image can be rotated
Describe a CAT scan
X-ray tube rotates around (the patient) / X-ray beam passes through the patient at different angles
A thin X-ray beam is used
Image(s) of slice(s) / (cross) section(s) through the patient are taken
X-ray tube moves / spirals along (the patient)
The signals / information(from the detectors) are used by the computer (and its software) to produce a 3D image
Explain how the production of a CAT scan image differs from that of a simple X-ray image
- Simple X-ray is one directional / produces single image
- CT image(s) taken at different angles / X-ray tube is rotated
- Computer processes data / image constructed from
many slices
Name 2 medical tracers and what they do
Technetium-99m emits γ radiation, halflife of 6 hours, decays to stable isotope
flourine-18 used in pet scans, half life 110 mins, undergoes beta plus decay
Describe the use of medical tracers to diagnose the condition of organs
- Tracer is injected into the body / placed inside the
body / circulates the body - Tracer is absorbed by organ / shows blockage
- Beta detector / gamma camera (is used to detect radiation from the body)
What are the components in a gamma camera? (5)
- Lead shield-stops radiation from other sources entering the camera
- Lead collimator - A piece of lead with thousands of vertical holes in it - only γ rays parallel to the holes pass thorugh
- Sodium iodide crystal - emits a flash of light (scintillates) whenever a γ-ray hits it
- Photomultiplier tubes - Detect the flashes of light from the crystal and turn them into pulses of electricity
- Electronic circuit - collects the signals from the photomultiplier tubes and sends them to a computer for processing unto an image to help diagnose patients
Evauluate how useful gamma cameras are
- Diagnose patience without surgery
- Cheaper then pet scans but still expensive
- uses ionising radiation which is bad for you
