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MED-107 Medical Physics II: Medical Imaging and Radiotherapy > 4.1 X-ray detectors > Flashcards

4.1 X-ray detectors Flashcards

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1
Q

to determine the image quality of the X-ray image, it is importance to explore how radiation is:

A
  • detected
  • measured
  • quantified
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2
Q

what is the fundamental principle of optimization?

A

to obtain an image with good quality, while keeping the patient radiation dose as low as possible, in order to obtain an accurate diagnosis

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3
Q

what are the various types of radiation detectors?

A
  • film-based detectors
  • computed tomography detectors
  • detectors used in fluoroscopic imaging
  • automatic exposure devices
  • detectors used in general radiography
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4
Q

the image receptor used in forming a radiograph plays an essential role in determining:

A
  • contrast
  • spatial resolution
  • noise level of the image
  • patient’s radiation dose
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5
Q

what does the type of detector define?

A
  • resolution
  • contrast
  • can affect the patient’s radiation dose
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6
Q

if we had the choice, we would choose the image receptor with what characteristics?

A

a better image quality with low patient’s radiation dose

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7
Q

what was the standard choice for recording X-ray images for decades after the early days of radiography?

A

film (photographic film)

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8
Q

film-based detectors remain in use for many applications such as?

A
  • chest
  • lungs
  • skeleton
  • gastrointestinal tract
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9
Q

what are the numerous advantages of film?

A
  • extremely good spatial resolution
  • good sensitivity
  • low price
  • wide availability
  • permanency for record keeping
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10
Q

what does the film used in radiography consist of?

A
  • a film base (made of transparent polyester or acetate sheet): supports one or two emulsion coatings
  • emulsion coatings: captures image
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11
Q

what is the emulsion a suspension of?

A

it is a suspension of tiny silver bromide grains in gelatin

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12
Q

what is the significance of the silver bromide grains when film is exposed to radiation?

A

when film is exposed to radiation, the photons interact with these grains

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13
Q

when the silver bromide grains are sensitized, what do they form?

A

a latent image

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14
Q

what is a latent image?

A
  • 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
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15
Q

what happens then to the sensitized and unsensitized grains? what do they end up producing?

A
  • 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
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16
Q

the photo-reactivity of silver halides is what gives film its photo-taking ability, but it also means that film is sensitive to?

A
  • 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)
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17
Q

how can the sensitivity of photographic film be increased? what does this form?

A
  • by placing the film close to one or two thin intensifying screens of fluorescent material (called phosphors)
  • this forms a film-screen combination
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18
Q

why do we use intensifying screens made of phosphors?

A
  • 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
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19
Q

how do we increase the (1) detection efficiency and the (2) sensitivity of an X-ray film?

A

(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

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20
Q

the entire system of intensifying screens, film, etc is encased in?

A

a casette for handling

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21
Q

although film-based detectors have very good spatial resolution compared to other detectors, what are some of its disadvantages?

A

(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)

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22
Q

how are films viewed?

A

they are viewed by a radiologist in transmission, illuminated from behind with a light box (in order to enhance contrast)

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23
Q

what is optical density?

A
  • 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)
24
Q

what does high optical density and low optical density mean respectively?

A
  • 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)
25
Q

what is a photomultiplier tube?

A
  • 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
26
Q

what occurs inside the photomultiplier tube?

A
  • 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
27
Q

what are dynodes? what are they used for?

A
  • 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”)
28
Q

which photon-matter interaction is used in a photomultiplier tube?

A

the photoelectric effect

29
Q

how is a large electrical signal produced from a photomultiplier tube?

A
  • 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)
30
Q

in practice, a photomultiplier tube converts _____ to _____?

A

light photons to electrical signals

31
Q

indirect imaging systems can either be:

A
  • computed radiography systems (CR)
  • indirect digital radiography systems (IDR)
32
Q

what occurs in indirect imaging?

A

X-rays are absorbed, converted into light and then converted once more into an electric signal

33
Q

what is the difference between CR and IDR in indirect systems?

A
  • 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
34
Q

what is the difference between INDIRECT and DIRECT radiography?

A

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

35
Q

what produces images with better resolution, direct or indirect radiography? why?

A

direct digital radiography result in images with better resolution (because there is no intermediate stage)

36
Q

describe computed radiography

A
  • 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
37
Q

what occurs in computed radiography?

A
  • 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
38
Q

what is the similarity/difference between film-based and computed radiography?

A

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

39
Q

what is the most common imaging system currently?

A

computed radiography using imaging plates (photostimulable phosphors)

40
Q

typical CR resolutions range from ___ to ___; what has higher spatial resolution, CR or film screen technology

A
  • 100 - 200 μm
  • film screen tech has higher spatial resolution
41
Q

what are the two main systems that fall under indirect digital radiography?

A
  • systems based on thin film transistor technology (TFT)
  • systems based on charged coupled device technology (CCD)
42
Q

what are the similarities/differences in TFT and CCD systems?

A

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

43
Q

the resolution of a CCD system is between ___ to ___?

A

100 to 200 μm

44
Q

in addition to light, what are CCDs also sensitive to? what are the consequences of interacting with it?

A
  • 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
45
Q

what are the 2 ways to arrange CCD devices to prevent it from x-ray interactions?

A

(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

46
Q

which type of detectors work in a similar way to ionizing chambers?

A

direct digital radiography

47
Q

describe direct digital radiography

A
  • 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
48
Q

compare direct digital radiography and ionization chambers

A

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

49
Q

in fluoroscopic procedures, what are the 2 types of detectors do we use?

A
  • image intensifier detector
  • flat panel detectors (FP)
50
Q

describe image intensifier detectors

A
  • 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
51
Q

describe flat panel detectors

A
  • 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
52
Q

how can you distinguish image intensifiers and flat panel detectors?

A
  • image intensifiers have circular shape
  • flat panel detectors have rectangular shape
53
Q

which has a higher image resolution, image intensifier or flat panel detectors?

A

flat panel detector has higher resolution due to its rectangular image

54
Q

used during procedures such as angiography, interventional radiology, surgery

A

image intensifier detectors

55
Q

used in digital radiography, fluoroscopy, cone-beam CT, and mammography

A

flat-panel detectors

56
Q

what is a better system, CR or DR?

A

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

MED-107 Medical Physics II: Medical Imaging and Radiotherapy (16 decks)

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