Fluoro & DSA Flashcards
Define fluroscopy
Moving real-time x-ray images 3-30fps
Can also produce high quality stills (Acquisitions)
Modern fluroscopy
Output phosphor is couples to a tv camera or a photo-diode array
tube aligned to detector in a C-arm arrangement
some units are capable of cone beam CT too
Benefits of CsI input phosphor
Needle structure acts as its own light guides - leading to improved resolution
How does an II work?
1) X-rays hit an input phosphor (CsI) converting them into visible light
2) The visible light photons are converted into electrons via a photocathode
3) Electrodes focus and accelerate the electrons inc flux (e- /unit area / time)
4) e- strike an output phosphor (ZnCdS) converting them into visible light
5) optical coupling transmits the light to a camera where it is then read out
How does the photo-cathode work ?
Uses the photo-e- effect
E = hv - o
Photons with enough E free electrons
Optimised for CsI spectrum: 420nm
5-10% effic 20 photons per 1 e-
photo-elec energy 2-3 eV
How are electrons focused in an II?
Einzel lens: simple e- lens -> focus only
Acceleration: cathode - anode potential difference of around 25-35 kV
II Output Gain
Flux gain: 1000 photons at out put for each e- (gain of x 50)
Minification gain:
input area / output area
Brightness gain = flux gain x minif gain
What is the conversion factor?
Overall efficiency for input to light output accounting for photo loss.
Defined by ICRU as:
Luminance out / doserate in
units: Cd/m^2 / uGy/s
II Impact of field size on resolution and dose
As field size is reduced resolution increases
Smaller area of input window is mapped to the same output area
reduces minification gain, brightness reduced requires inc dose
Collimation is used to help reduce patient dose
Describe Automatic Brightness Control
A feedback system which aims to optimise image quality by maintaining brightness.
Input exposure parameters are adjusted in response to measured light levels at the output.
4 II limitations
Resolution: limited by cam / out phosphor due to use the minified image
Output limits the system - can reduce noise by inc input CsI thickness
Veiling glare: loss of contrast due to back shine from the output and x-rays penetrating to output
Space-charge effects: trade-off between res and min gain
Image distortion due to e- optics - lensing effects cause periphery issues. Can be affected by external mag fields less than that of a nearby mri room.
Changes as unit is rotated
Describe flat panel detectors
Indirect A-Si receptor
Similar to digital radiology but up to 30fps
Solves II issues
Describe FP field size selection
Old method: Digital magnification - res reduced
Dose inc to maintain brightness
New method: pixel binning for larger field sizes:
Group neighbouring pixels to improve no. photons per pixel
At small field sizes, binning is reduced. But must inc dose to inc snr.
Beyond this use digital mag.
FP ABC systems
- Select a pixel area to monitor
- Pixel values are prop to dose
- Can change AEC area
Define fluorography
Referred to as acquisition mode - used to be a sequence of live images recorded on film to produce a movie.
Now acq is a series or single digital image
fluoro is the live feed (can use last im hold)
can post-process both
Differences between fluoro and acq modes
Fluoro:
lower doserates
live images
fixed aec area (generally)
lower im quality (generally)
Acq
higher dose live images auto saved selectable aec area higher im qual (generally) -Used when a permanent record or high qual im are required
What are power curves? what parameters can they be used to adjust?
The adjustment of the input doserate by an AEC to achieve const. brightness
Parameters altered:
kVp, mAs, pulse width (ms), pulses per second (pps), amount of filtration (mmCu)
Sys use all or a combination of the above
Power curves choice with and without contrast
Without: Inc kVp then mA/ms to minimise dose
with: adjust mA and/or ms as you want to keep kVp near the k-edge
Other fluro optimisation methods (non kVp / mAs)
Filtration
Framerate / pulse per second
Frame averaging = reduces noise (causes lags)
Im processing: edge enhancement, dynamic noise reduction, motion detection, specialised sys.
Ways of reducing fluoro dose
Optimise power curves Reduce pps Change viewing angle on long exposures Use the largest field size possible Collimate to the ROI
May use a grid to improve contrast and reduce noise
How are patient doses monitored?
- Integrated DAP meters
- Estimate skin doserate by Calculating dap to an interventional ref point 15cm below the iso centre
- fluoro on time is the least accurate method
Deterministic effects
- Skin dose reactions at about 2Gy
- ‘Sun burn response’ and potential necrosis
How do contrast agents work ?
They artificially modify the attenuation coefficients
ex. barium / iodine
How does DSA (digital subtraction angiography) work?
1) Record a mask image in ROI before contrast reaches the region
2) Record constrast images when the vessels have filled with contrast
3) Subtract the mask image from the contrast images to create subtraction images highlighting the vessels
What is DSA misregistration?
Movement artefacting caused by motion between the capture of the mask and contrast images.
Can apply a pixel-shift correction
What are the two subtraction methods?
1) Standard subtraction: I2-I1
Does not remove all b/g as it is still dep on tissue thickness
2) Logarithmic subtraction:
ln(i1) - ln(i2) = c(uc - ut)
no dep on T, removes all bg
Impact of DSA on noise
Sub image is effected by the noise of the live and mask images
Sub noise = sqrt 2 . mask noise
A 40% inc is noise
can reduce noise by inc dose or summing a sequence of sub images
Technical requirements of DSA
- A Large detector
- A stable Im sys (minimal motion)
- A small focal spot
- X-rate tube able to pulse quickly
- Im processing power
