Fluoroscopy Flashcards
T/F?
The image intensifiers are surrounded by oil.
False
It is an evacuated glass or ceramic envelope that is itself surrounded by a metal housing.
What is the function of metal housing in II?
To prevent light from getting into the tube and of shielding the device from the effects of magnetic fields.
What are the components of an II? (3)
- input screen
- electron focusing electrodes
- output screen
what is the shape of the input screen?
The input screen is curved with the radius of curvature being approx equal to the distance from the screen to the focal point of the electron beam.
What are the two components of the input screens?
on the outer ( x ray beam side) is the input phosphor layer laid down on a thin metal layer.
On the inner side of the screen there is a thin coating of a material that acts as a photocathode
What is a photocathode?
This is a material that emits electrons when irradiated by light.
Name a common photocathode?
antimony caesium- SbCs3
T/F?
The intensity of light produced in the phosphor and the number of electrons produced in the photocathode are indirectly proportional to intensity of x ray photons.
False
The intensity of light produced in the phosphor and the number of electrons produced in the photocathode are DIRECTLY proportional to intensity of x ray photons.
In II what is the phosphor layer? and why?
Caesium iodide because of its absorption efficiency.
what is the k edge of Caesium and iodide
Caesium= 36 KeV
Iodine= 33 KeV
They fall just below the peak of bremsstrahlung used for clinical situation.
How thick is the phosphor layer?
0.1-0.4 mm thick.
It detects about 60% of incoming X ray photons.
What is the structure of caesium iodide?
It has a crystalline structure, the input ph can be manufactured so that the narrow needle like crystals are laid down perpendicular to the screen.
T/F?
A thicker ph layer causes unsharpness
True
Light produced in the crystals is internally reflected.
this minimises by spread of light in the ph layer.
T/F?
The input screen is maintained as a low negative voltage with respect to the anode.
False
The input screen is maintained as a HIGH negative voltage with respect to the anode.
what is the potential difference ?
about 25 kV
What is the purpose of the potential dfference?
This is so that the electrons generated in the photocathode are accelerated within the vacuum towards the anode and the output screen.
what is the size of the input screen?
when would you use the small vs large size?
150-400 mm in diameter.
smallest size may be used for applications such as fracture fixation in orthopaedic
the largest size are suitable for angio and interventional Radiology.
What input screen size would you use for barium contrast studies for GI?
350mm
T/F?
The input screen is usually bigger than the output screen
True
what is the size of output screen?
25-35 mm
what is the ph layer made of in output screen?
zinc cadmium sulphide- ZnCdS:Ag, to convert the pattern of electron intensities into light.
why is the inner part of the screen covered?
because it is so thin, there is very little light spread in the screen. because the screen gives out light in all direction including backwards the inner part of the screen has to be covered.
what would happen if the inner part of the screen is not covered?
light from the screen would be detected by the photocathode causing a cascade of electron emission that would completely white out the image.
what is the screen covered with?
its covered with an extremely thin coating of Al- 0.5 microm- to prevent whitening out.
This Al coat also acts as the anode.
T/F?
There is a direct relationship between the brightness displayed on the output screen and intensity of x ray photons falling on the input phosphor.
True
ie doubling X-ray exposure doubles the light output.
what is the focusing electrodes?
these are metal rings within the tube that are held at +ve voltages with respect to the photocathode.
They act as an electron lens.
Define gain;
this is the ratio of the brightness of the output ph to that of the input ph.
what two factors are responsible for gain?
- Flux gain
2. Minification gain
what is flux gain?
a single light photon produced in the input ph causes a single e to be emitted from the photocathode.
following acceleration to about 25 keV by the focusing field in the intensifier, each e causes many light photons to be emitted from the output phosphor.
This is described as flux gain and is typically about 50.
what is minification gain?
this is the term used to describe the intensification caused by reducing the image size from the input to output screen.
It is equal to the ratio of the two screens.
T/F?
Gain is a measurable quantity.
False
Gain is NOT a measurable quantity.
What is the conversion factor-Gx?
this is defined as the ratio of the brightness (luminance) of the output phosphor (candela/m2) and the dose rate of at the input surface of the II.
what are the typical values for conversion factor?
what is the unit of the conversion factor?
25-30 Cd/m2
why would gain and conversion factor deteriorates with time?
because of the loss in the detection efficiency of the phosphor.
T/F?
the image in the output ph is a magnified version of the image from the full area of the input screen.
False
the image in the output ph is a MINIFIED version of the image from the full area of the input screen.
The original TV system , used a ……… between the output screen of the image intensifier and the camera tube.
lens
what are the functions of the computer in Digital fluoroscopy?
- image storage
- post processing funtions,including noise reduction, edge enhancement, black and white reversal.
- geometrical inversion
- mapping to display look up table to max contrst
What is the input screen of CCD camera made of?
The input screen of a CCD camera is a thin layer of amorphous silicon that is divided into individual pixel- 1024x 1024
T/F?
CCD are used in domestic market for video recorder and digital camera.
True
T/F?
The CCD cameras are now the standard for image intensifier based fluoroscopy because they have a an increased dynamic range.
true
Fibre optic coupling are now used instead of lens for optical connection.
what is the adv of optic coupling ?
they provide more efficient light collection and improved geometrical integrity.
In ABC system how would the input signal be measured?
It is either the measurement of the light intensity of the output screen or a measurement of the signal from the camera.
T/F?
In general the brightness in the central area of the screen is selected because the user would have centred the image.
true
Fluoroscopy dose rates depend to some extent on the detective…….
Quantum efficiency of the II.
for intensifier with CsI input screen the dose rate is….
65%
what is the input dose rate of image intensifier system?
0.1-0.5 microGy/S, most commonly bwn 0.2- 0.3 microGy/s
T/F?
The lower the input dose rate to the intensifier, the greater is the noise in the displayed image.
True
What happens to gain in magnification;
On magnification the gain of the intensifier reduces that is the brightness in the output image is decreased in proportion to the reduction in area of the field of view.
What would happen if the gain of the TV system is not adjusted?
if the gain of the TV system is not adjusted, then the input dose rate must be increased in inverse proportion to area in order to retain the same level of image brightness on the output screen.
T/F?
The increase in dose is directly proportional to the reduction in area.
False
The increase in dose is INVERSELY proportional to the reduction in area.
In practice increase in camera gain is applied so hen increase in input dose rate for magnified field is less
what happens to dose area product on magnification.
Dose area product is reduced to some extent on some magnification
What is the max ESD rate in standard fluoroscopy?
The standard fluoroscopy settings the ESD rate cannot exceed 100 mGy/min for any field of view.
For the largest field size it cannot exceed 50 mGy/min
What are the typical ESD rates in fluoroscopy?
10-30 mGy/min
Continuous fluoroscopy being pulsed at either —– or —- pulses/s
25 or 30 pulses/s
what is the pulse width ?
in the range of 2-20 ms
where is the additional electrode is built in a grid controlled tube.
In a grid controlled tube an additional electrode is built into the tube between the cathode and anode. This is a circular electrode through which the electrons pass as they are accelerated from the filament( cathode) to the target (anode).
What is a cathode and anode in fluoro?
the electrons pass as they are accelerated from the filament( cathode) to the target (anode).
what voltage is applied to the grid electrode?
a negative voltage- 2kV relative to cathode is applied to the grid electrode, then the e emitted from the target will be repelled and effectively be pushed back into the filament.By switching a -ve volt to the grid the e flow and thus x ray production can be switched off.
Pulsing the beam using this technique is called grid controlled fluoroscopy.
Define fluorography
Recording images produced by an image intensifier can be described by the term fluorography
What is a digital spot images
these are single shot images taken with a single pulse of radiation at a high mA to produce a low noise image.
What dose do the digital spot images require?
Dose in the range of 0.1-5 microGy
what does the total no of images that are stored from a single run depend on?
it depends on RAM.
what causes the blurring in the output phosphor of II?
blurring is caused by the spread of light in the output phosphor.
Resolution is in the range of 4-5 lp/mm
Whats resolution defined as in II?
what is the resolution?
Resolution is in the range of 4-5 lp/mm.
Resolution is defined in terms of the image size at the input face of the image intensifier and not at the position of the patient.
T/F?
The resolution in the monitor display is the same as the II
False
The resolution in the monitor display is more limited
theoretical resolution of 1.7 lp/mm
direct viewing fluoroscopy involved the use of ……….screen.
Zinc sulphide fluorescent screen.
what are the two ways of reducing noise in fluoroscopy?
Increasing mA, but this would increase the dose
Frame averaging- the level of smoothing depend on no of frames added.
useful if there is no movement between frames, otherwise the image will be blurred.
how is veiling glare produced?
it is produced by scattering effects in the image intensifier.
It is mainly from light scattering in the output window of the intensifier, but it is also caused by x ray and light scattering in the input phosphor and e scattering in the tube itself.
what is the effect of veiling glare?
It reduces contrast , darker region appear lighter because of the surrounding lighter areas.
It causes the central area of the image being brighter than the priphery- this is known as vignetting.
What is vignetting?
This is when the central area of the image is brighter than the priphery.
T/F?
the larger the image intensifier the greater is the veiling glare.
True
what are the two main types of geometrical distortion?
Pin cushion distortion
S-type distortion.
what is pin cushion distortion?
This is when there is a magnification towards the edges of the image is produced by the curvature of the input screen.
What is S type distortion
It imposes curvature on straight line features and is caused by ambient magnetic field influencing the path of the electrons from input to output screens of the intensifier.
How do you test the SR in fluorsocopy?
using a grid test object, usually without any attenuating material in the beam.
A low kV is used to minimise scatter.
Why use a low kV in grid test object
to minimise scatter.
T/F?
When testing it is usual to make the test grid parallel to the matrix of the TV system.
False
The test grid is usually angled at 45 with respect to matrix of TV system.
How do you test the contrast resolution?
with a low contrast test object-
Leeds test object
what is a Leeds test object?
These are used ti test the contrast resolution.
They are flat disks approximately 6mm thick and 200-300mm in diameter.
They contain high atomic no material that produce varying levels of contrast in the final image.
what are the three steps of obtaining DSA?
- the non contrast image is taken to show anatomy. usually two frames are obtained (a,to stabilise the x ray factors and b, stored in teh computer memory- the mask image.
- contrast image is taken- second frame
- the mask is subtracvted from the 2nd image = third frame
how could you minimise misregistration in DSA?
By pixel shifting, this could be done only over the full area of the image.
it could be done manually or using automated techniques.
What is the effect of DSA on SNR?
subtracting images causes a decrease in SNR, ie the image appears noisier
To counteract this effect DSA requires increased mA in comparison with unsubtracted image acquisition.
what is dual energy subtraction?
At low kV, the image will display high contrast between bone and soft tissue
at high kv the contrast of bone is reduced, image contrast is influenced by tissue density rather than atomic number.
subtracting low from high minimises visualisation of bone and improves soft tissue contrast.
How do you maximise visualisation of bony structures in dual energy subtraction?
by subtracting high kv from low kv.
T/F?
The detectors used for fluoroscopy are the same as those used for DR.
True
what type of detectors are used in fluoroscopy?
amorphous silicon detectors with CsI scintillators.
The detective quantum efficiency for these are comparable with that of imaging intensifiers-about 65%
what are the advantages of the fluoroscopy detectors?
Increased DR and improved SR.
what is the contrast ratio of image intensifier systems?
they have a limited contrast ratio of 30:1
why would the image intensifier have a limited contrast ratio?
because of high background signal and veiling glare.
T/F?
For a flat plate detector the range of signals that can be detected is very much greater and makes full use of the 14 bit depth that is typically available.
True
what is the pixel size for flat plate detectors?
what is the contrast resolution?
150 microm- giving high contrast resolution of about 3 lp/mm
(displayed SR of II which is no better than 1-1.2 lp/mm)- pixel size of 400 microm.
T/F?
The SR improves with with magnification using a flat plate detector.
False
The SR does NOT improve with with magnification using a flat plate detector, other than through the display of a magnified image, because the pixel size remains the same.
why would the SR of flat plate detectors remain the same and does not improve with magnification?
Because the pixel size remains the same.
T/F?
The resolution of the II improves with magnification.
The resolution of image intensifier based system improves with magnification so that the two systems are comparable.
T/F?
With flat plate systems there is no distortion in the displayed image as for II based systems.
True
why would you use flat plate detectors?
to provide superior image quality.
