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Medical Physics 2: Radiology > Fluoro & DSA > Flashcards

Flashcards in Fluoro & DSA Deck (28)
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1

Define fluroscopy

Moving real-time x-ray images 3-30fps

Can also produce high quality stills (Acquisitions)

2

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

3

Benefits of CsI input phosphor

Needle structure acts as its own light guides - leading to improved resolution

4

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

5

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

6

How are electrons focused in an II?

Einzel lens: simple e- lens -> focus only

Acceleration: cathode - anode potential difference of around 25-35 kV

7

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

8

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

9

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

10

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.

11

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

12

Describe flat panel detectors

Indirect A-Si receptor

Similar to digital radiology but up to 30fps

Solves II issues

13

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.

14

FP ABC systems

- Select a pixel area to monitor
- Pixel values are prop to dose
- Can change AEC area

15

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

16

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

17

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

18

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

19

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.

20

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

21

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

22

Deterministic effects

- Skin dose reactions at about 2Gy
- 'Sun burn response' and potential necrosis

23

How do contrast agents work ?

They artificially modify the attenuation coefficients

ex. barium / iodine

24

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

25

What is DSA misregistration?

Movement artefacting caused by motion between the capture of the mask and contrast images.

Can apply a pixel-shift correction

26

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

27

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

28

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