2. Digital Radiographic Imaging Flashcards
What is the typical size of a DICOM for a radiograph?
4-12MB
Define spatial resolution. What feature of digital radiographic imaging is most important for this?
- How small an object can be detected
- PIXEL SIZE!
More generally better, although at a point, there will be no improvement in image quality -> other technical and hardware factors
What is BIT DEPTH? What formula can calculate this?
- Refers to number of grey shades present within an image
- Determined by 2n
Where n = number of 0s and 1s permitted per pixel
e.g. BIT depth 4 = 24 = 16 grey shades
DIRECTLY RELATED TO IMAGE FILE SIZE
How does a Computed Radiography (CR) plate work?
- Flexible plate coated with photostimulable phosphor (PSP)
- > Xray attenuation pattern stored as latent image in PSP => CHANGES IN ELECTRON ENERGY BANDS
- Latent image read out by scanning laser in plate reader
=> Visible light emitted, hits PMT -> converted to electronic signal
- Exposed to bright white light to return electrons to ground state
List two advantages of CR systems
- Damage to plates -> Relatively inexpensive to replace
- May help obtaining non-standard views as mobile
What are the 3 types of DDR (Direct Digital Radiography) systems?
1) Indirect flat panel
2) Direct flat panel
3) Charge-coupled device (CCD)
Describe how an indirect flat panel DDR system works
- Xrays converted to VISIBLE LIGHT (Hence indirect) by XRAY INTENSIFYING SCREEN
–> Layered onto panel with array of photodiodes
=> Convert to electronic signal
- Read out by a thin-flim transistor array -> electronic file
- Many detector elements required for good spatial resolution
- Commonly capable of Bit depth 14
- Many photodiodes (can have 6-7million), complex electronics
EXPENSIVE
- One disadvantage: Some light diffusion in intensifying screen can lead to blurring. In reality, technology negates this. Comparable to Direct.
Describe how direct flat-panel detectors work
Xrays strike photoconductor -> TYPICALLY AMORPHOUS SELENIUM (effcient absorber)
=> Electrons liberated and collected to form charge
- Charge read out directly by thin-film transistor array
- Processed -> Electronic file
- Similar matrix size and bit depth (typically 14) to indirect
- NO VISIBLE LIGHT EMITTED. Key difference
How many grey shades can the human eye detect?
50 - 100 simultaneous shades
-> probably little benefit in generating >14-16 Bit depth (16000+ shades!)
Describe how a Charged-Coupled Device (CCD) works
- Small chip, used in cameras etc. Only a few centrimetres cf. 43x43cm DR plates
- Sensitive to visible light:
Xrays -> intensifying screen -> fibreoptic connection / focusing lenses to chip - Pixels in chip accumlate electronic charge, read-out and converted to file
May be millions of pixels on surface of CCD!
- GEOMETRY NOTEWORTHY: Small -> needs focussing cf indirect DDR where intensifying screen directly layered onto detector
IMPORTANT:
image quality depends more on quality of light collection / focusing than on quality of chip
some LOSS OF LIGHT and DISTORTION between screen and CCD -> can lead to image degradation
**spatial resolution more affected by these factors in CCD cf pixel density of DDR**
Use: Dental, (mammography) where small area imaged! Less neeed for focusing
Disadvantages:
- housing quite large to accomdate focusing -> often built within table
- Requires vertical relationship betweeen components in housing, not portable, cant do horizontal readily
Describe the differences between preprocessing, processing and post processing
Pre:
- Corrections to raw data beyond control of user
- E.g. inhomogeneity of light collection, imade distrotion, dead pixels
Processing:
- Some controlb user -> conversion of corrected raw data at xray station
- E.g. optimize contrast, edge enhancement, minimise noise etc.
- Often presets e.g. thorax / abdo / limb
Post-processing:
- In Dicom viewer -> 100% user controlled
- magnifcation, blackness, rotation, measurment etc.
List 6 advantages of digital XR
1) Reduced expendable supply cost / dark room cost
2) Contrast optimization and exposure latitude
3) Postprocessing
4) Image accesibility / storage
5) Portability
6) Oppurtunity to change image paradigm…
How does spatial resolution compare between imaging modalities?
Screen film approximately 2x that of digital (0.08mm vs 0.17mm)
What factors contribute to inherent spatial resolution in Screen film vs digital radiography?
It is the combination of the below factors that ultimately produces overall spatial resolution.
NB: Despite difference between two modalities -> EXCELLENT in both
What digital alternative to scatter grids is available? What are the advantages of this?
Antiscatter algorithm
- Dont need increased mAs
- No grid artefact
=> User preference cf using grid
Define spatial resolution vs contrast resolution
- Spatial: Ability to resolve small objects -> contribution of multiple factors
- Contrast resolution (= contrast optimization): Unique to digital -> relates to BIT DEPTH. Relates to range of gray scales available for assignment to individual pixels. In analog - black or white only. Think about film screen image of multiple areas of different thickness below…
What is exposure latitutude?
- The ability of a digital system to compensate for inappropriate kVp / mAs combinations to produce an image
=> comparatively ‘narrow’ in analog imaging
NB: Confusingly sometimes called dynamic range, but this term is used for other things too, including contrast resolution. AVOID
