4.1 X-ray detectors Flashcards
to determine the image quality of the X-ray image, it is importance to explore how radiation is:
- detected
- measured
- quantified
what is the fundamental principle of optimization?
to obtain an image with good quality, while keeping the patient radiation dose as low as possible, in order to obtain an accurate diagnosis
what are the various types of radiation detectors?
- film-based detectors
- computed tomography detectors
- detectors used in fluoroscopic imaging
- automatic exposure devices
- detectors used in general radiography
the image receptor used in forming a radiograph plays an essential role in determining:
- contrast
- spatial resolution
- noise level of the image
- patient’s radiation dose
what does the type of detector define?
- resolution
- contrast
- can affect the patient’s radiation dose
if we had the choice, we would choose the image receptor with what characteristics?
a better image quality with low patient’s radiation dose
what was the standard choice for recording X-ray images for decades after the early days of radiography?
film (photographic film)
film-based detectors remain in use for many applications such as?
- chest
- lungs
- skeleton
- gastrointestinal tract
what are the numerous advantages of film?
- extremely good spatial resolution
- good sensitivity
- low price
- wide availability
- permanency for record keeping
what does the film used in radiography consist of?
- a film base (made of transparent polyester or acetate sheet): supports one or two emulsion coatings
- emulsion coatings: captures image
what is the emulsion a suspension of?
it is a suspension of tiny silver bromide grains in gelatin
what is the significance of the silver bromide grains when film is exposed to radiation?
when film is exposed to radiation, the photons interact with these grains
when the silver bromide grains are sensitized, what do they form?
a latent image
what is a latent image?
- an invisible image produced when X-rays interact with the grains
- to convert it to a visible image, we have to develop the X-ray film
what happens then to the sensitized and unsensitized grains? what do they end up producing?
- the sensitized grains are retained and blackened during the process of development, rendering those regions opaque
- unsensitized grains are removed, leaving only the transparent base
- the resulting pattern of transparent and opaque regions of film then form the radiographic image
the photo-reactivity of silver halides is what gives film its photo-taking ability, but it also means that film is sensitive to?
- sensitive to damage from other types of radiation
- it is not especially sensitive to X-rays, it is much more sensitive to light photons (visible light)
how can the sensitivity of photographic film be increased? what does this form?
- by placing the film close to one or two thin intensifying screens of fluorescent material (called phosphors)
- this forms a film-screen combination
why do we use intensifying screens made of phosphors?
- phosphors have the ability to absorb the X-rays & emit their energy as light (visible) photon
- then visible photons interact and expose the film emulsion
how do we increase the (1) detection efficiency and the (2) sensitivity of an X-ray film?
(1) to increase the detection efficiency further, the X-ray film can be made with an emulsion on both sides
(2) to increase film sensitivity, we place 2 intensifying screens at the top and bottom
the entire system of intensifying screens, film, etc is encased in?
a casette for handling
although film-based detectors have very good spatial resolution compared to other detectors, what are some of its disadvantages?
(1) while X-ray film can be magnified, very few other manipulations can be made to enhance the quality of images
(2) film has a limited ability to distinguish between subtle variations in absorption of X-rays (which restricts its ability to show image contrast)
how are films viewed?
they are viewed by a radiologist in transmission, illuminated from behind with a light box (in order to enhance contrast)
what is optical density?
- an important parameter when using film
- it measures the amount of X-ray exposure (the perceived extent of film transparency and opaqueness, or film blackness)
what does high optical density and low optical density mean respectively?
- high optical density = more blackness of the film = film has higher exposure to X-rays
- low optical density = less blackening of the film = less exposure to X-rays)
what is a photomultiplier tube?
- a device used in digital radiography
- it is an extremely sensitive detector, and is useful for detection of light of very weak signals
- it uses crystals that absorb X-rays and emits their energy as visible photons
what occurs inside the photomultiplier tube?
- scintillator detectors use a crystal that absorbs x-rays and re-emits their energy as visible photons
- these visible photons next enter the photomultiplier tube, where they hit a photocathode
- a photocathode is a device that converts light to photoelectrons via the PHOTOELECTRIC EFFECT
- however, these photoelectrons are too few to produce an appreciable electrical signal
- to resolve this, the photoelectrons are accelerated by a large voltage toward a positive electrode called a dynode
what are dynodes? what are they used for?
- a positively charged electrode, so it attracts photoelectrons
- they serve as an electron multiplier
- as photoelectrons collide in the dynodes, which results in freeing more electrons, multiplying the signal (hence, “photomultiplier”)
which photon-matter interaction is used in a photomultiplier tube?
the photoelectric effect
how is a large electrical signal produced from a photomultiplier tube?
- the electrons freed from the dynode are in turn accelerated to the next dynode, further multiplying the signal
- many such multiplications produce a very large electrical signal (for the formation of a better image)
in practice, a photomultiplier tube converts _____ to _____?
light photons to electrical signals
indirect imaging systems can either be:
- computed radiography systems (CR)
- indirect digital radiography systems (IDR)
what occurs in indirect imaging?
X-rays are absorbed, converted into light and then converted once more into an electric signal
what is the difference between CR and IDR in indirect systems?
- computed radiography: uses a cassette that is put into the reader, then we obtain the digital image on the monitor
- indirect digital radiography: does NOT use a cassette
what is the difference between INDIRECT and DIRECT radiography?
direct digital radiography does not use an intermediate stage, the emergent X-rays cause the system to produce an electrical signal with no conversion of X-rays to light
what produces images with better resolution, direct or indirect radiography? why?
direct digital radiography result in images with better resolution (because there is no intermediate stage)
describe computed radiography
- a system that produces digital radiographic images using imaging plates
- mechanism similar to film screen technology (we use phosphors to absorb x-rays and re-emit their energies as visible photons)
- introduced because it does not require modifications to the x-ray equipment
what occurs in computed radiography?
- the phosphor plate absorbs x-ray photons, which excites the electrons trapped into the storage phosphor
- in order to free the trapped electrons and produce a visible image (latent image), we use a laser beam
- the freed electrons enter the photomultiplier tube and are converted into electric signals
what is the similarity/difference between film-based and computed radiography?
similarity:
- using phosphors
- retaining a latent image following an exposure
difference:
- how we proceed with the latent image
- the latent CR image is scanned using a laser beam and digitized in a CR reader
- film-based latent image is chemically developed
what is the most common imaging system currently?
computed radiography using imaging plates (photostimulable phosphors)
typical CR resolutions range from ___ to ___; what has higher spatial resolution, CR or film screen technology
- 100 - 200 μm
- film screen tech has higher spatial resolution
what are the two main systems that fall under indirect digital radiography?
- systems based on thin film transistor technology (TFT)
- systems based on charged coupled device technology (CCD)
what are the similarities/differences in TFT and CCD systems?
similarity:
- both designs use phosphors/scintillators that produce light when exposed to X-ray radiation
difference:
- differences revolve around how this light is detected and converted into a useful electrical signal
the resolution of a CCD system is between ___ to ___?
100 to 200 μm
in addition to light, what are CCDs also sensitive to? what are the consequences of interacting with it?
- X-rays
- any x-rays interacting with the CCD could create a false signal (and we could obtain false information on the final image)
- therefore, the array cannot be positioned in one with the scintillator crystal where x-rays may interact with it
what are the 2 ways to arrange CCD devices to prevent it from x-ray interactions?
(1) use fiber optical tapers
- positioned at an angle in a different location
- in practice, light photons are focused within the tapering optical fibers (holes)
(2) use a mirror and optical lens arrangement:
- using the mirror, the light photons are directed towards the optical lens and then directed in the holes of the CCD
which type of detectors work in a similar way to ionizing chambers?
direct digital radiography
describe direct digital radiography
- when incident radiation passes into the sensitive volume, it causes electrons to be liberated from their orbits
- this creates (+) and (-) ions to carry the charge from one electrode to the other
- this creates a current
- the image acquisition system converts X-rays to an electrical signal without the need for first converting it to light
compare direct digital radiography and ionization chambers
ionization chambers: chamber filled with air, radiation enters the chamber, ionizes the air, we apply a voltage, a flow of ions occur, result in a current, which is converted into a signal
direct digital radiography:
- instead of air in ionization chambers, we have amorphous selenium, we apply high voltage (we have the negative and positive plate), the (-) plate attracts (+) & vice versa, this creates a current which is converted into the signal, which is converted into the image
in fluoroscopic procedures, what are the 2 types of detectors do we use?
- image intensifier detector
- flat panel detectors (FP)
describe image intensifier detectors
- an analog device, converts x-rays to light to signal to image
- x-rays strike input phosphor, producing light that is amplified and focused onto output phosphor, resulting in an intensified image
- provide good sensitivity and real-time imaging capabilities, but lower in resolution than flat-panel detectors
describe flat panel detectors
- DIRECTLY convert x-ray photons into electrical signals
- provide high spatial resolution and contrast, allowing for high-quality images
- suitable for static and dynamic imaging applications
- commonly used in both diagnostic and interventional procedures
how can you distinguish image intensifiers and flat panel detectors?
- image intensifiers have circular shape
- flat panel detectors have rectangular shape
which has a higher image resolution, image intensifier or flat panel detectors?
flat panel detector has higher resolution due to its rectangular image
used during procedures such as angiography, interventional radiology, surgery
image intensifier detectors
used in digital radiography, fluoroscopy, cone-beam CT, and mammography
flat-panel detectors
what is a better system, CR or DR?
CR:
- greatest versatility
- can be used with unmodified equipment
- available in different cassette sizes
DR:
- much faster (because we don’t have cassettes, so we don’t lose time putting them in the reader)
- have better resolution
