Mammography and fluoroscopy Flashcards
Mammography: how is the anode heel effect used?
-Use of anode heel effect with cathode over chest wall to capture thicker end of breast and anode over nipple (lower energy beam can penetrate thinner area).
What does PACS stand for? What is the name of files stored in PACS?
-Stands for Picture archiving and communication system
-Images are stored as Digital Imaging and Communications in Medicine (DICOM) files. A file also contains the patient, display preferences and imaging modality.
Mammography: what is the focus detector distance?
-Fixed focus-detector distance (65-66cm)
Mammography: how does the AEC work?
-Automatic exposure control: the detector acts as AEC
Mammography: what is the target material? What x-ray energies are required?
-Needs to produce characteristic x-rays at energies of 17-20 kEv (20-30 for large breasts)
-Usually use Molybdenum
Mammography: what filter material is used for larger and smaller breasts? What is the aim of the filter?
-Usually Molybdenum filter (and target) but not always enough energy for larger breasts.
-Larger breasts: molybdenum target and Rhodium filter
-Aim of filter: removes high energy x-ray photons and to try and make the beam as monoenergetic as possible.
Mammography: Why can you not use a rhodium target and Molybdenum filter?
-The filter would attenuate the rhodium characteristic x-rays.
What are the atomic numbers and k-edges of Molybdenum a Rhodium?
-Molybdenum: K edge 20keV, atomic number 42
-Rhodium: K edge 23keV, atomic number 45
How is magnification in mammography achieved?
-Increasing object-film distance this occurs through greater divergence of the transmitted beam.
Mammography: How does the k-edge compare to the characteristic radiation?
-The k-edge of an element is just higher than its characteristic radiation t/f any element is relatively transparent to its own characteristic radiation.
-Mammography exploits this by using molybdenum as both anode target and filter.
-Digital spatial resolution is better than film mammography (partial volume effect of DR allows higher contrast of smaller details eg micro-calcifications).
What is the typical exposure for glandular breast tissue? What is the dose/risk of inducing a fatal cancer in women of screening age?
-1.5-3mGy
-Dose of 2mGy is the dose that can induce a fatal cancer in 1:50 000 50–65-year-old women
Mammography: how do focal spot size, compression and anti-scatter grids affect spatial resolution?
Spatial resolution is improved by:
-Focal spot size: can be small (0.1-0.15mm) for spot views or can be broad (0.3mm) for full views
-Compression: good b/c leads to less attenuation, reduces scatter, reduces geometric and movement unsharpness, spread tissues out so less overlaying of features.
-Anti-scatter grids
How do thicker breasts affect mammography? How does this affect the kV used?
-Thickness of breast: more radiation absorbed, more scatter, increased beam hardening (less contrast), longer exposure time required.
-Thin breasts: MoMo at kV25
-Thicker breasts: MoRh at kV 32
What is breast tomosynthesis (tomography)?
Breast tomography uses digital radiography to reconstruct planar images of sections of the breast. There are two main methods of acquiring breast tomosynthesis:
1. The x-ray tube traverses along an arc acquiring images as it travels and the detector remains stationary
2. The x-ray tube traverses along an arc and the detector also rotates
The images are then reconstructed using filtered back projection or iterative reconstruction
What are pros of breast tomosynthesis? 3
- Provides enhanced lesion detection
- Reduces false positive recalls
- Allows more precise lesion localisation
What are cons of breast tomosynthesis? 4
- Higher radiation dose (approximately double)
- High contrast objects (e.g. surgical clips) can cause significant artefacts
- Longer interpretation time
- Requires substantially more data storage
What is fluoroscopy?
Fluoroscopy is the use of real time x-ray imaging. It used to utilise image intensifiers, which have been in use since the late 1950’s, but now uses flat panel detectors, which are similar to the digital radiography used in projection radiology.
What is fluorography? What is the matrix size for fluorography?
- Images usually formed and viewed after the x-ray exposure is complete
- Better quality images acquired than in fluoroscopy but at higher doses
- Matrix is typically 1024 x 1024 pixels with each pixel representing 10 bits of grey scale information
*Acquisition of single diagnostic quality images.
fluo: how do you measure brightness gain?
Brightness gain = minification gain x flux gain
fluo: how do you measure minification gain?
o Minification gain = (Dinput / Doutput)2 (where D is diameter of input and output screen respectively)
fluo: how do you measure the image intensifier conversion factor?
o Image intensifier conversion factor (Gx) = L / X’ (where L = luminance of II output, X’ = II entrance dose rate)
fluo: what factors affect brightness gain?
o More minimisation = higher gain
o Gx falls in proportion to the reduction in the area of the input field in zoom setting
o Higher voltage applied across II tube = more flux gain
Xray tube in fluo: input screen - describe the MOA
o X-ray photons enter tube through aluminium or titanium window
o II input window: aluminium or titanium foil to allow x-rays to enter tube and (maintains vacuum).
What is the material of the input phosphor of x-ray tube?
o Layer of Caesium iodide CsI:Na for good x-ray absorption efficiency. Each x-ray photon produces ~3000 light photons in blue spectrum
Xray tube in fluo - What is the role of the input phosphor?
Each x-ray photon produces ~3000 light photons in blue spectrum
o X-ray photons converted into light photons in the input phosphor layer.
What is the role of the photocathode in fluo?
o Fluorescent emission from phosphor absorbed by light-activated photocathode made of SbCs3 which then releases electrons into body of II tube.
o photons are converted into electrons in the photocathode layer.
Xray tube in fluo: What is the role of the electron optics?
- Electron focusing: positively charged electrodes along tube direct electron path to create exact but minified and inverted image on input screen
- Magnification: achieved via electronically focusing electron beam. Magnified images use less signal and so need a higher dose but improve the resolution
Xray tube in fluo - What is the charge differential between the input screen and output screen?
- Input screen at negative charge compared to output screen to direct electron towards output screen.
Xray tube in fluo - Output screen: what is the phosphor made of?
- Output screen made of thin layer of silver activated zinc cadmium sulphide crystals (ZnCdS:Ag) that convert electrons into light photos that then leave through the output window
Xray tube in fluo - Output screen - what is halation and how can this be minimised?
- Scatter of light (halation) in the output window can seriously degrade the contrast of the output tube. This can be minimised with: smoked glass, very thick glass, fibre-optic bundle.
Display of image in fluo: explain how a TV camera works
o Electronic TV camera uses electron beam that scans across photoconductive target to create flow of electrons, the rate of which corresponds to the amount of light photons striking that area. (not really used anymore)
Display of image in fluo - how is the resolution determined in a TV camera?
-Vertical resolution is determined by the number of scan lines
-Horizontal resolution is determined by the bandwidth of the system (higher bandwidth = lower resolution)
Display of image in fluo: CCD device sensors - explain MOA
-Each pixel has an associated electrode. Each light photon gives rise to an electron-hole pair. The negative electrons accumulate in a potential well. These packages are then transferred to the light shielded storage array. Data is read from storage section line by line. The accumulation of charge is directly proportional to the intensity of the incident light
-Then sent to flat panel detector
Display of image in fluo - explain how a flat panel detector works
-Utilises same technology as digital radiography -there is a flat panel of detectors that provide a direct electronic readout instead of requiring the conversion of analogue to digital like in IITV.
-Most commonly indirect dynamic FP detector with CsI:Tl x-ray scintillator layer superimposed onto a-Si high resolution active matrix
T or F: CCD + flat panel has an improved DQE compared to IITV camera
True
T or F: CCD + flat panel has video signal that emerges in digital form, reducing electronic noise
True
T or F: II ITV camera has better temporal resolution than CCD, with a matrix size of 2048 x 2048
False - this is the resolution of CCD
IITV has poorer temporal resolution and has a circular field, meaning it doesn’t cover corners.
CCD produces better quality images
What is the approximate range of bits per pixel for CCD?
-Greyscale of 12-14 bits per pixel
T or F - Detective quantum efficiency for CCD is 10-20% better than IITV so can afford to reduce patient dose
True
T or F - Zoom can be used for CCD and it increases spatial resolution as ir does in IITV
False - zoom is available but it doesn’t increase the spatial resolution as it does in IITV
How does AEC work in fluorography?
o Alters kV and mA to ensure stable quality of images. This, in turn, alters the patient dose
o Done by measuring II light output with a photo-sensor or electronically sampling video signal
What is the ABC/AEC for fluroscospy and fluorography?
-ABC: automatic brightness control - used in fluoroscopy
-Automatic exposure control: used in fluorography and radiography
Describe the use and the curve in fluoroscopy: A curve
-Curve A: Anti-isowatt curve: increase kV and mA as radiological thickness increases.
Describe the use and the curve in fluoroscopy: B curve
-Curve B: iodine contrast imaging -holds kV at a 60-65kV (optimum spectrum for imaging iodine). Curve has high imaging quality but high dose
Describe the use and the curve in fluoroscopy: C curve
-Curve C: high kV curve – dose minimisation curve (low dose) – mostly for paeds.
*see curve page 65 Farr’s 3rd ed.
Digital processing: fluo - describe greyscale processing
Grey scale processing with greyscale range compression: suppress or highlight intensities and improve contrast balance. Uses analogue (video) circuit or via a look-up-table (LUT)
Digital processing: fluo - describe spatial filtering
Spatial filtering: similar to edge enhancement in projection radiography. Improves displayed spatial resolution. Best used for high image contrast (barium GI studies).
Digital processing: fluo - describe temporal filtering
Temporal filtering: decreases level of dose by summing current image with previous frames, averaging out signal and resulting in smaller proportion of noise.
Dose to patient is measured in?
Measured in skin dose rates
What is the greatest contribution to staff dose in fluo?
Greatest contribution to staff dose is from scatter, amounts to ~0.1% of patient dose at a distance of 1m
What is a Leed’s test and when is it used?
Used for quality control in fluoroscopy – consists of many circular details of different diameters and thicknesses.
Describe how an image is taken in DSA
- Acquire mask image (IM) to record anatomical background.
- Contrast injected. Series of images acquired which show arrival and run-off of contrast (contrast medium enhanced image, IC)
- Image frames subtracted via digital processor. Any structures that are common to set 1 and set 2 are subtracted (i.e. all background anatomy but not the contrast filled structures)
- Amplify contrast signal to boost displayed contrast of the vessels
Describe misregistration - artefact in DSA
The movement of a structure by even 1 mm can cause misregistration - structure in wrong place
Dual energy radiography - how can you get an image with high soft tissue contrast?
Uses a composite of two images obtained with different tube voltages.
They can then be subtracted from one another to form an image with high soft tissue contrast (subtract low kV from high kV)
Dual energy radiography - how can you get an image with high bone contrast?
Uses a composite of two images obtained with different tube voltages. They can then be subtracted from one another to form an image with high bone contrast (subtract high kV from low kV).
