Physics Chapter 16 Flashcards
Advantages of DR over film screen
No processing Image can be post processed Easier to store No transportation Viewed in multiple places
Types of DR
CR (Computer Radiography) SPR (Scanned Projection Radiography) Indirect DR-CsI/CCD Indirect DR- CsI, GdOS/TFT Direct DR- a-Se/TFT
SPR (Scanned Projection Radiography)
Previously used in dedicated chest units,
now mostly used in CT
Fan collimated xray beam
CsI scintillation phosphor
Linear array of CCD’s
Uses scanning motion
takes several seconds and results in motion blur on chest radiographs
Elements of DR
Capture Element- Where the xray is captured. This is where xray interaction occurs.
Coupling Element- Transfers the xray generated signal
Collection Element- collects either light photons or electrons.
Capture Element
Cesium iodide Gadolinium Oxysulfide Amorphous selenium (Amorhous means it is without shape)
Coupling Element
Lens
Fiber optic assembly
Contact Layer- Amorphous Silicon
Amorphous selenium
Collection Element
Photodiode- light sensitive device that collect light photons
Charge-coupled device (CCD)- light sensitive device that collect light photons
Thin film transistor (TFT)- charge sensitive device that collects electrons.
Charge-coupled device CCD
Solid-state device that converts visible light photons to electrons.
Light Sensitive element
Silicon-based semiconductor
Three principle imaging characteristics of CCD’s
Sensitivity- responds to very low levels of light
Dynamic range- wide range of light intensity, more responsive to low levels. About the same dynamic range as 400 speed film.
Size- very small, 1-2 cm, but can be tiled to 100 x 100 micrometers
Photodiode
type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.
Thin Film Transistor TFT
collects electric charges
sends a signal when prompted
positioned as matrix so it can be read on pixel-by-pixel, column-by-column basis
Display screen made with TFT is liquid crystal display (LCD)
Collects electrons
TFT display screen
liquid crystal display (LCD)
common in notebook and laptop computers
has a transistor for each pixel( for each tiny elements that control the illumination of your display)
Transistor at each pixel means that current that triggers pixel illumination can be smaller and therefore can be switched on and off more quickly
TFT
aka Active Matrix Display Technology(and contrasts with ‘passive matrix’ which does not have a transistor at each pixel)
TFT or active matrix display is more responsive to change.
Indirect DR CsI/CCD
X rays~Light~electric signal
Capture Element- Cesium Iodide *xray to light
Coupling Element- Fiber Optic Bundles *light
Collection Element- CCD *light to electric signal
Cesium Iodide (CsI) emits light when stimulated by xray.
Light is transmited through fiber optic bundles
CCD converts the visible light to an electric signal
Indirect DR CsI, GdOS/TFT
Xrays~light~electric signal
Captture Element- CsI, GdOS *light
Coupling Element- Amorphous silicon *light to electric signal
Collection Element- TFT *electric signal
Phosphor (CsI, GdOS) emits light when stimulated by xray
Amorphous silicon converts light to an electric signal
TFT- collects electric charges
Affected by Fill Factor
Face of each pixel must not only contain the capture element, but also the TFT and a storage capacitor
Approximately 20% of the pixel doesn’t contribute to image
smaller pixels reduce the fill factor
Charge is collected in the storage capacitor until signal is read by switching action of the TFT
Direct DR
Xray~electric signal
No light used
Capture Element-Amorphous Selenium
Collection Element- TFT
Xray beam interacts directly with Amorphous Selenium
Produces charge pair
Charge is collected at the bottom of the selenium layer
Charge is transmitted by the TFT
Line Spread Function
How far the light spreads
High line spread function blurs the image
Line spread is higher with film screen than digital imaging
digital does not have better spatial resolution than film screen because it is limited by pixel size
Even with line spread, filmscreen imaging produces better special resolution than DR due to pixel size.
Digital Mammography
DMIST
DMIST (Digital Mammography Imaging Study Trial)
Investigation to see if Digital Mammography could perform as well as film screen
Suspicion was digital could not perform as well because the spatial resolution was not as good.
However, it was discovered that because of the greater contrast resolution digital may be even better for some types of breast tissue
Digital Mammography Tomostynthesis
Similar to tomography or CT of breast tissue
Lower dose than CT
Produces 3D voxel of breast tissue
CsI
Cesium iodide
GdOS
Gadolinium Oxysulfide
TFT
Thin film transistor
a-Se
Amorphous Selenium
Image-forming xray beam interacts directly with a-Se producing a charged pair of electrons. a-Se is both the capture element and the coupling element.
a-Se is used in Direct DR.
a-Si
Amorphous Silicon
CsI/a-Si
Indirect DR process by which xrays are converted first to light and then to electric signal
CsI/CCD
Indirect DR process by which xrays are converted first to light and then to electric signal
a-Se
Amphorous Selenium
Direct DR process by which xrays are converted to electric signal.
Resolution
Contrast resolution is more important than spatial resolution for soft tissue radiography.
CCD
has a higher sensitivity for radiation and a much wider dynamic range than screen-film image receptors.
CCD
light-sensing element
three principal advantageous imaging characteristics: sensitivity, dynamic range, and
CR
Capture Element-BaF PSP
Coupling Element- Lens/Fiber Optics
Collecting Element- Photodector
SPR
Capture Element- NaI/CsI
Coupling Element- None
Collecting Element- Photodector
Indirect DR
Capture Element- CsI
Coupling Element- Fiber Optics
Collecting Element- CCD/CMOS
Indirect DR
Capture Element- CsI, GdOS
Coupling Element- Contact layer (Amphorous Silicon)
Collecting Element- TFT
Direct DR
Capture Element- a-Se (Amphorous Selenium)
Coupling Element- none
Collecting Element- TFT
DRT (Digital Radiographic Tomosynthesis)
purpose of all forms of tomography is to improve image contrast. SPR is reemerging with modifications as an adjunct to DRT.
Pixel limitations
Spatial resolution in DR is pixel limited
As pixel size is reduced, spatial resolution improves with increase in patient radiation dose.
Fill Factor
With smaller pixels, fill factor is reduced, xray intensity must be increased to maintain adequate signal strength.
Direct DR
No scintillation phosphor is involved
Image-forming xray beam interacts directly with amorphous selenium (a-Se)_ producing a charged pair. a-Se is both the capture element and the coupling element.
Xrays incident on the a-Se create electron hole pairs through direct ionization of selenium.
Created charge is collected by a storage capacitor and remains there until the signal is read by the switching action of the TFT
Line Spread Function of Mammography
largest for screen-film mammography and least for amorphous selenium (a-Se) digital mammography
Line Spread Function of Mammography
Light from a radiographic intensifying screen spreads and exposes a large area of the film. Results in limited spatial resolution.
The signal emitted during CR also spreads, limiting spatial resolution.
Line spread functions indicate the relative degree of spatial resolution.
