Fluoroscopy Flashcards
What are the three stages of an intensifier tube?
Input stage
Electron optics
Output stage
What are the three components of the input stage of the Image intensifier?
The image intensifier input window comprises a convex metal shield which covers the input face of the image intensifier. provides protection for the sensitive input components of the tube and maintains the vacuum. The input window is usually fabricated from a low Z metal, for example, aluminium [Z = 13] or titanium [Z = 22] foil. Therefore the x-ray beam can enter the image intensifier with minimum attenuation. It also provides mechanical rigidity and maintains the vacuum.
The input phosphor comprises a layer of sodium activated caesium iodide (CsI:Na) deposited on a thin aluminium substrate. as good x-ray absorption efficiency, typically 70 to 90%, plus good x-ray to light (energy) conversion efficiency. CsI:Na is grown as a channelled phosphor comprising tiny needle-like crystals with fibre-optic like characteristics.This makes it possible to use comparatively thick layers of CsI:Na (400 to 500 μm thick) to maximise x-ray photon absorption without causing excessive unsharpness due to scatter and diffusion of light photons.Each absorbed x-ray photon gives rise to ~3000 light photons in the blue part of the spectrum.
The fluorescent emission is then absorbed in a light-sensitive photocathode, normally comprising a very thin layer of caesium/antimony alloy. The photon sensitivity of the photocathode typically lies in the range of 0.1 to 0.15.Absorption of the fluorescent light photons releases a pattern of electrons into the body of the image intensifier tube. Approximately 200 electrons are released per absorbed x-ray photon.
What happens to the photoelectrons released by the photocathode?
are accelerated across the image intensifier tube by applying +25 to +35 kV to the anode, significantly increasing their kinetic energy in the process. The resulting high speed electrons then impact upon the output screen.
What is the typical minification of the image??
The diameter of the output screen is typically a tenth that of the input screen and therefore the image intensifier produces a minified image.
How are the electrons focussed onto the output screen?
As the electrons travel across the tube, they are carefully focused on the output screen via an electric field distribution defined by applying suitable bias voltages to a set of (typically 3) cylindrical metal electrodes.
Why is a fluoroscopy image inverted?
The geometry of image formation in an image intensifier tube requires that the stream of electrons cross at a defined point (or cross-over point) situated in front of the anode. As a result, the image on the output screen is inverted.
How can an II image be magnified?
magnified electronically by altering the distribution of electrode voltages, shifting the cross-over point to a position further away from the anode. Electronic zoom (as this technique is often called) not only magnifies the image but improves the spatial resolution, but at a cost of reducing the input field coverage.
How does the output screen of an II work?
The output screen comprises a thin layer of silver-activated zinc cadmium sulphide (ZnCdS:Ag) crystals deposited on the inner surface of the output window. Bombardment of the output screen with the stream of high speed electrons produces fluorescence in the green part of the optical spectrum.
The output window is the optically transparent port through which the intensified light image exits the image intensifier tube
Why is the output window of the II coated in aluminium?
It forms part of the anode structure
High speed electrons travel through this aluminium layer
The layer is opaque preventing the light emitted by the output phosphor from back-illuminating the photocathode and therefore degrading image intensifier performance. The light is reflected back towards the output increasing the gain of the image intensifier tube
What is halation?
Scatter of light, or halation
How can halation be reduced?
halation, in the output window can seriously degrade the contrast of the image intensifier output image.
Anti-halation techniques typically include use of:
Smoked glass
Special optical coatings
Very thick glass
Fibre-optic bundle
What is brightness gain?
The degree to which an image intensifier tube ntensifies (or amplifies) an x-ray image is described by the brightness gain
how is brightness gain calculated?
by the product of the so-called minification gain (Gminification) and the flux gain (Gflux)
What is minification gain?
describes the increase in brightness due to geometrical demagnification of the image in the image intensifier tube and is given by:
Ainput/Aoutput = (Dinput/Doutput)^2
Ainput and Aoutput are the areas and Dinput and Doutput are the diameters of the image intensifier input and output screens respectively.
What happens to the value of minification gain when a zoom mode is selected?
The minification gain reduces in proportion to area when a zoom mode is selected.
What is a typical value of brightness gain?
Gminification ~100 for a modern image intensifier in normal (full-field) mode operation.
For a modern image intensifier tube Gflux is also ~100.
Therefore, for a modern image intensifier in normal (full-field) mode Gbrightness is typically ~10^4.
What measurement has now superseded brightness gain?
The brightness of the input screen is not accessible to direct measurement, and this limits the utility of Gbrightness. For this and other reasons, image intensifier conversion factor (Gx) is now the preferred measure of x-ray image intensifier tube sensitivity and has superseded brightness gain.
How is II conversion factor calculated?
luminance of the image intensifier output [expressed in candelas m^-2] divided by X’ is the image intensifier entrance dose rate [expressed in μGy s^-1].
The Gx of a modern x-ray image intensifier tube typically lies in the range 10 to 30 cd s μGy-1m-2 for the normal (full-field) mode.
What happens to Gx when zoomed and what dose this mean for dose?
When a zoom mode is selected the Gx falls in proportion to the reduction in the area of the input field.
This reduction in Gx normally means that the entrance dose rate is increased (to some degree) when a zoom field is selected to help maintain system sensitivity. Reportedly the value of Gx deteriorates as the image intensifier tube ages.
Regarding an image intensifier:
A. Absorption of x-ray photons in the input window should be kept to maximum
B. Caesium antimonide is a channelled x-ray scintillator
C. Demagnification of the image by the electron optics contributes to image intensifier conversion factor
D. X-ray scatter in the output window is a serious cause of contrast loss
A. False. X-ray photons absorbed in the input window cannot contribute to the image so absorption should be kept to minimum. The input window is usually fabricated from a low Z metal so that the x-ray beam can enter the image intensifier with minimum attenuation.
B. False. Caesium iodide is a channelled x-ray scintillator used as the input phosphor in an image intensifier.
C. True. Demagnification contributes to the brightness of the image on the output screen and thus contributes to the image intensifier conversion factor.
D. False. Scatter of light in the output window is a serious cause of contrast loss.
Which of the following factors affect the brightness gain of an image intensifier tube?
A. The x-ray absorption efficiency of the image intensifier input phosphor screen
B. The electron-optical demagnification factor
C. The voltage applied across the image intensifier tube
D. Selection of a zoom field
A. Incorrect. Brightness gain is the ratio of the brightness of the output screen to the brightness of the input screen. X-ray absorption efficiency of the input screen does not affect this ratio. However, absorption efficiency affects the input dose rate required and therefore affects the conversion factor of the image intensifier.
B. Correct. The demagnification factor determines the minification gain and thus affects the brightness gain.
C. Correct. The applied voltage determines the flux gain and thus affects the brightness gain.
D. Correct. The selection of a zoom field changes the demagnification factor so it affects the minification gain and thus the brightness gain.
How is the image from the output screen translated to a screen to view?
An optical distributor is a light-tight housing which contains the optical components required to transfer the image on the II output screen to the TV sensor (or ancillary recording device such as a fluorographic film camera). Image transfer takes place via a so-called tandem lens pair arrangement, which uses a separate focusing lens for each imaging component.
What structures may be added between the tandem lens pair?
Adjustable iris aperture - A circular iris aperture of adjustable diameter is normally used to calibrate the light intensity illuminating the image recording device. The iris aperture is also used to compensate (at least in part) for the fall in II gain when a zoom field is selected. Opening the iris aperture reduces the need to increase the dose rate to compensate for such a fall in gain.
Electronic light sensor - An electronic light sensor can be mounted between the two lenses to measure the brightness of the II image. This can be used as a real-time feedback signal for an automatic dose (rate) control system.
Beam-splitting mirror - A semi-silvered (or beam splitting) mirror is used to record images via the two imaging channels simultaneously.
What is now used as the preferred image recording device in modern IITV fluoroscopy systems?
Solid-state charge coupled device (CCD)
How is a frame transfer CCD made?
Frame transfer CCD chips comprise two conjoined sub-arrays of elements of equal area as shown in the diagram:
One sub-array contains the active light-sensitive elements and is used to capture the initial image which is subsequently transferred to ->
The second sub-array, which is shielded from light and provides temporary charge storage of the preceding image frame during readout
Frame transfer CCDs have large charge capacity, low noise, fast temporal response and wide dynamic range and therefore are ideal for dynamic x-ray imaging.
How is an image made in the CCD?
Each pixel in the CCD array is addressed by an associated electrode. Application of a positive bias voltage to an electrode forms a ‘potential well’ in the region of the silicon substrate below.
Each light photon (from the II output) absorbed in the silicon substrate of the CCD gives rise to an electron-hole pair (where the electron is negatively charged and the hole is positively charged). The holes drain away while signal electrons accumulate in the potential well. The quantity of electronic charge which accumulates at each pixel is directly proportional to the intensity of the incident light and the frame integration time.
Charge packages build up in the light-sensitive array before being transferred to the charge storage section.
The data output register then reads out the image signal from the storage section line-by-line. Meanwhile the next image frame is being acquired in the active area of the CCD. The sequence of charge packets are converted to an analogue video or (using an ADC) a digital signal as appropriate.
What are the technological benefits of CCD sensors?
Small, inexpensive and compact with low-power consumption
Self-scanning image readout (no large electromagnetic deflection coils required)
Negligible lag (temporal unsharpness)
Resilience against burned-in signals at high-lights
Geometrical precision and spatial uniformity
Excellent thermal, electrical and magnetic stability
Excellent serviceability and long life-time
Compatibility with digital x-ray imaging modalities
Regarding an electronic TV camera tube:
A. The video image is produced by absorbing light in the signal plate
B. The lead oxide layer used in a Plumbicon works as an x-ray photoconductor
C. The video output signal is used to reconstitute a visible image on a display monitor
D. The electron gun scans the electron beam across the target layer generating the video signal
A. False. Light is absorbed in the photoconductive target, not the signal plate.
B. False. The photoconductor is exposed to light, not x-rays.
C. True.
D. True.
Regarding a CCD sensor:
A. They are more compact than camera tubes and incorporate their own electromagnetic deflection coils
B. CCD sensors can be used for both real-time imaging and serial exposure applications
C. Spatial resolution can be improved by increasing the number of pixels in the array
D. They generate a digitised output which must be converted to a video signal
A. Incorrect. CCD sensors are more compact than camera tubes but they do not have electromagnetic deflection coils.
B. Correct.
C. Correct.
D. Incorrect. The output from a CCD sensor is an analogue signal.
An IITV fluoroscopy system typically includes certain components.
A. A radiographic intensifying screen
B. A tandem lens pair
C. A photoconductive TV camera tube
D. An array of detector elements
A. Incorrect. An intensifying screen is used in a screen film radiography system, not an IITV fluoroscopy system.
B. Correct. A tandem lens pair is used to couple the TV camera to the output screen of the II.
C. Correct. A photoconductive TV camera tube may be used to capture the image from the output screen of the II (in more modern equipment, a CCD sensor may be used instead).
D. Incorrect. An array of detector elements would be found in a modern fluoroscopy system using a flat panel detector, not in an IITV fluoroscopy system.
What is the base physical parameter used to evaluate x-ray image intensifier television (IITV) fluoroscopy performance?
II entrance dose rate ( The dose delivered to the image intensifier entrance plane per unit time)
What Factors contribute to selection of II entrance dose rate?
The design of the II (e.g. II diameter and conversion factor)
Calibration of the optical iris diaphragm
Design of television systems (e.g. TV sensor type)
The specific needs of the clinical examination (reflecting the trade-off between dose rate and image quality)
How does zoom of an II effect dose?
Effect of zoom field on dose rate - In principle, if the same TV signal is to remain constant the fall in gain will invoke a proportionate increase in II entrance dose rate. In a modern IITV channel however, the TV signal is stabilised partly by increasing II entrance dose rate and partly by opening the iris diaphragm to increase light transmission. In this way, the dose penalty from selecting an II zoom field is moderated but without affecting the TV signal. Fluoroscopy systems often provide two or three automatic brightness control (ABC) dose rate selections, and correspondingly these settings result in differing levels of II entrance dose rate.
What are the 2 sorts of distortion II tubes are subject to?
pincushion and S-distortion
What is pincushion distortion?
Pincushion distortion
This manifests itself as a non-linear increase in magnification toward the periphery of the image field. This arises from the convex shape of the input phosphor screen and the diverging geometry of the x-ray beam.
What is S-distortion?
This arises if the stream of electrons crossing the II interact with the Earth’s magnetic field and/or a stray magnetic field from an adjacent piece of electrical equipment. Movement of the electrons through a magnetic field results in a lateral displacement of the electron trajectories which increases in magnitude toward the periphery of the field.
As a result linear structures in the image field, such as the x-ray beam collimators, exhibit a distinctive S-shaped curvature (hence the name). The degree of S-distortion will vary with the orientation of the imaging chain with respect to the Earth’s magnetic field. S-distortion is minimised by shielding the II tube housing with anti-magnetic alloy (such as Mu-metal).
What is vignetting?
Associated with the pincushion distortion, is a radial variation (decrease) in image brightness toward the periphery of the image field. The degree of vignetting increases for large field II tubes. Vignetting can reflect a 20 to 30% reduction in signal intensity from the centre to the peripheral rim of the displayed image field.
How does scatter of xray, light and electrons effect the image?
Together these effects cause a veiling glare over the image field which results in a reduction in displayed contrast. The effect of veiling glare adds to the effect of x-ray scatter in the patient but obviously is not moderated by the presence of an anti-scatter grid.
How does zoom on an II effect contrast ratio?
Real II systems typically produce a contrast ratio in the range 10 to 20. Veiling glare reduces when a zoom field is selected and this is reflected in an improvement (increase) in contrast ratio.
What is temporal unsharpness?
Lag is the term used to describe the temporal unsharpness of an x-ray IITV fluoroscopy system.
Presence of lag in fluoroscopy can seriously degrade image quality producing blur, artefacts and smearing of moving structures in the image.
What is the main source of lag in an x-ray IITV fluoroscopy system?
The predominant source of lag is the TV sensor. TV lag arises from the persistence of the photosensitive material and temporal integration in the TV readout mechanism, which lead to the carry-over of the recorded signal into subsequent TV frames.
What does the Spatial resolution of the overall x-ray IITV fluoroscopy system depends upon?
the unsharpness (or blur) of
X-ray image intensifier - defined by the: Unsharpness of the input phosphor screen, Focusing of the electron optics and Unsharpness of the output phosphor screen
TV channel - which depends upon depends on the Properties of the photo-sensor layer,
TV scan format (e.g. standard 625-line vs. high definition 1250-line) which essentially defines the vertical resolution and the
Bandwidth of the camera electronics which essentially defines the horizontal resolution
Display
WHat is the limiting spatial resolution of an x-ray IITV fluoroscopy system?
The limiting spatial resolution is the finest periodic test pattern that can be discriminated visually. It corresponds (approximately) to the high frequency limit of the modulation transfer function (MTF).
The typical limiting spatial resolution of a large field (say 35 cm) II is ~2.5 lp mm-1.
Why does spatial resolution vary across the field?
The spatial resolution of an IITV fluoroscopy system varies across the image field (allied to the vignetting). Spatial resolution is normally maximum at the centre of the image field but falls towards the periphery.
What is quantum noise?
X-ray quantum noise arises from the random fluctuations in the fluence (number per unit area) of x-ray photons which form the image.
These random fluctuations are known as dynamic noise. The fluctuations in photon fluence are described by Poisson statistics (counting statistics).
What are the 3 sources of noise in an IITV image?
Dynamic x-ray quantum noise (the primary factor)
II structure (or fixed pattern) noise, due to the random micro-structure of the II input and output screens
TV noise arising in the camera electronics which is added to the video signal
How is mean fluence of the x-ray photons contributing to the image calculated?
Nx (the fluence of x-ray photons in the II entrance plane) x η (the detective quantum efficiency (DQE) of the II) x Τ (the temporal integration time of the IITV process (including the lag of the TV camera))
How is SNR related to II entrance fluence?
SNR can be improved by increasing the II entrance fluence (dose); however this will come at a cost of increased patient dose
The improvement in SNR with dose is slow because of the half power law:
A four-fold increase in dose only results in a two-fold (square root of 4) improvement in SNR
While a 100-fold increase in dose only results in a 10-fold (square root of 100) improvement in SNR
SNR is better improved by using an II with a greater DQE as this will not incur a dose penalty to the patient. This can be achieved by using an II with an input window of higher x-ray transmission and/or one with an input screen of greater x-ray absorption efficiency
What is the threshold contrast detail detection (TCDD) test?
The overall image quality of an x-ray IITV fluoroscopy system can be evaluated via the threshold contrast detail detection (TCDD) test. TCDD uses a specialised test object which comprises an array of disc-shaped details of varying (and calibrated) subject contrast and diameter. Trained observers view the fluoroscopic images and determine the minimum (or threshold) contrast which is visible for details of each size.
How should threshold contrast reduce in relation to II entrance dose rate?
reduce in proportion to the square root
Which of the following cause a non-uniform image field in IITV fluoroscopy?
A. Inverse square law
B. Conversion factor
C. Anode heel effect
D. Vignetting
A. Incorrect. All parts of the II input screen are (almost) the same distance from the source of x-rays so the inverse square law has an imperceptible effect on image uniformity.
B. Incorrect. The conversion factor is constant across the II input screen.
C. Correct. The anode heel effect causes the intensity of the x-ray beam to decrease towards the anode side of the x-ray tube and thus contributes to the non-uniformity of the image.
D. Correct. Vignetting causes a decrease in brightness toward the periphery of the image field.
Regarding the spatial resolution of an IITV based imaging system:
A. In the normal field selection, typically lies in the range 5 to 10 lp per mm
B. Spatial resolution of an IITV based imaging system improves when an II zoom field is selected
C. Spatial resolution of an IITV based imaging system is superior to that of a SFR system
D. Spatial resolution of an IITV based imaging system can be measured using a contrast detail test object
A. False. The spatial resolution of an IITV based imaging system is typically around 1 lp/mm.
B. True. The use of a zoom field magnifies the image seen on the TV screen and therefore decreases the degradation of spatial resolution caused by the TV system.
C. False. The spatial resolution of an IITV based imaging system (around 1 lp/mm) is inferior to that of a SFR system (around 5 to 10 lp/mm).
D. False. A contrast detail test object is used to assess overall image quality by determining the threshold contrast for details of different sizes. This will depend on a combination of factors including not only spatial resolution but also contrast and noise.
SNR in IITV fluoroscopy can be increased by:
A. Minimising the II entrance dose rate
B. Using an II with a thicker layer of CsI:Na
C. Selecting an II zoom field
D. Replacing the CCD sensor with a Vidicon camera tube
A. Incorrect. A lower II entrance dose rate is associated with a decrease in SNR.
B. Correct. A thicker layer of CsI will have greater absorption efficiency.
C. Incorrect. Selecting a zoom field will not, of itself, change the photon fluence and therefore will not affect the SNR. However, in reality the situation is more complicated because most systems automatically increase the II entrance dose rate when using a zoom field.
D. Correct. Replacing the CCD sensor (which has negligible lag) with a Vidicon camera tube (which has large lag) will increase the SNR.
In fluoroscopy the visibility of a low contrast detail depends upon:
A. The II entrance dose rate
B. The II conversion factor
C. The contrast ratio of the II
D. The degree of geometrical distortion
A. Correct. The II entrance dose rate determines the level of noise in the image, and this affects the visibility of a low contrast detail.
B. Incorrect. The II conversion factor is the light output per unit entrance exposure rate. It does not have a direct effect on image quality.
C. Correct. The contrast ratio is a measure of the contrast loss in an II, and this affects the visibility of a low contrast detail.
D. Incorrect. Geometrical distortion affects the appearance of the image (for example, straight lines appear curved) but it does not affect the visibility of a low contrast detail.
What are the two modes in which Fluoroscopy systems can operate?
Continuous mode fluoroscopy- In continuous mode fluoroscopy x-rays are delivered continuously to the patient albeit at a comparatively low dose rate. The TV system operates asynchronously with the x-ray source. In the UK, continuous mode normally operates at 25 frames per second and has a signal integration time of 40 milliseconds.
Pulsed mode fluoroscopy - In pulsed mode fluoroscopy x-rays are delivered as a series of pulses of short duration and comparatively high intensity; these pulses are synchronised with the TV readout process. Pulsed fluoroscopy systems typically operate at 30 frames per second and each x-ray pulse is ~ 3 to 10 milliseconds in duration. Lower pulse rates, such as 15 or even 7.5 frames per second, can be selected if clinically appropriate to reduce dose. Pulsed mode fluoroscopy offers much better spatio-temporal resolution than continuous mode fluoroscopy. This is seen when imaging rapidly moving anatomy, such as coronary vessels in cardiac angiography or structures within the young infant.
How is a grid-controlled xray tube used for pulsed images?
Grid-controlled x-ray tubes
Such an x-ray tube incorporates a third (fine-mesh) electrode, mounted between the cathode filament and the anode target.
When the grid is biased close to earth (zero) potential the current will flow across the tube undisturbed producing x-rays.
However, when a large negative bias voltage is applied to the grid this instantaneously arrests tube current flow and therefore the x-ray exposure. This makes it possible to generate a rapid sequence of very short x-ray pulses
How does an ABC function?
The feedback signal required to control the x-ray generator is derived by measuring either the II light output with a photo-sensor or by electronically sampling the corresponding video signal.
In its most basic form an ABC operates by regulating the mA while the kV is set manually. A modern ABC more commonly uses a combination of mA and kV regulation.
Two such algorithms include:
One which favours keeping the kV low in order to maximise subject x-ray contrast, while allowing mA and therefore patient dose to rise
Another algorithm favours a higher kV setting and a lower mA, which leads to lower patient entrance skin dose but at a price of reduced subject x-ray contrast
What is the average skin dose in fluoro for a patient?
Patient entrance skin dose rates in IITV fluoroscopy typically lie in the range:
3 to 10 mGy per minute for the average patient
10 to 30 mGy per minute for the larger patient
In the UK the maximum entrance skin dose rate limit for a standard patient is 100 mGy per minute (for all II fields). In practice in the UK, a lower maximum dose rate limit of 50 mGy per minute is commonly adopted.
What techniques are available to reduce patient skin dose in fluoroscopy?
Using a patient table manufactured from a low x-ray absorption material, such as carbon fibre
minimising the fluoroscopy x-ray beam on time
Tightly collimate the x-ray beam onto the clinical region of interest
Select the ABC mode with the lowest dose rate which achieves the required diagnostic information
Only use an II zoom field when justified by an improvement in diagnostic information
When available, always use pulsed fluoroscopy and select minimum acceptable pulse rate
Use the appropriate x-ray beam spectral filter to minimise patient entrance skin dose rate
Increase the distance between the x-ray source and patient entrance surface to reduce divergence of the x-ray beam entering the patient
The gap between the patient and the II entrance should be minimised
Periodically shift the projection angle to help prevent over-irradiation of the same area of skin
The x-ray anti-scatter grid should be removed if possible
Use the last-image-hold facility to digitally capture and store a positional reference image (e.g. when positioning a catheter during angiography)
Take note of on-board dose measurement system which typically displays values of instantaneous dose area product (DAP) rate, cumulative DAP etc.
Implement radiation safety education/training for staff
Ensure compliance with Ionising Radiation (Medical Exposure) Regulations 2000
What are the three sources of stray radiation leading to staff dose?
Leakage of x-rays from the tube housing - typical tube leakage 5 μGy per hour at 1 m distance
X-rays scattered from the patient (the dominant cause of staff irradiation)
Secondary scatter from structures in the room
How do you keep staff dose to a minimum?
Fluoroscopic equipment must be designed, selected (prior to purchase) and operated with staff radiation dose safety in mind
Staff must always wear a lead apron and where relevant other personal radiation shields to protect eyes, thyroids, gonads, hands
Individual staff doses must be routinely monitored
Lead-rubber drapes and movable lead glass shields should be available for added protection
Operators should be aware of x-ray beam on warning lights and audible warnings
Staff must try to maintain a maximum distance from the patient
Generally, minimising patient dose minimises staff dose
Regarding automatic brightness control (ABC) system:
A. Maintains constant fluorographic film density for patients of different sizes
B. Maintains the patient entrance dose rate for patients of different size
C. Maintains the II entrance dose rate for patients of different size
D. Maintains the II entrance dose rate when an II zoom field is selected
A. False. The ABC uses the output of the image intensifier during fluoroscopy. It is not relevant when the equipment is operated in fluorographic mode using film.
B. False. The ABC maintains the II entrance dose rate. The patient entrance dose rate must vary to achieve this, with larger patients requiring a higher entrance dose rate.
C. True. The ABC uses a combination of mA and kV regulation to maintain the II entrance dose rate and thus keep the brightness of the displayed image constant.
D. False. II entrance dose rate (and therefore patient skin dose rate) normally increases when an II zoom field is selected.
Patient entrance surface dose rate is typically reduced by several factors. Which appear/appears in the list below?
A. Rejecting scattered x-rays by introducing an air gap between the patient and the II
B. Increasing the thickness of a copper beam spectral filter
C. Reducing the fluoroscopy pulse rate from 30 to 15 frames per second
D. Selecting an II zoom field
A. Incorrect. The gap between the patient and the II entrance should be minimised. Introducing an air gap between the patient and the II requires an increase in the patient entrance surface dose rate in order to maintain the II entrance dose rate.
B. Correct. An appropriate x-ray beam spectral filter is used to reduce patient entrance skin dose rate.
C. Correct. Reducing the pulse rate from 30 to 15 frames per second will halve the average patient entrance surface dose rate.
D. Incorrect. Patient entrance surface dose rate normally increases when an II zoom field is selected.
Staff dose can be reduced by several factors. Which appear/appears in the list below?
A. Minimising the x-ray beam on time
B. Using an overcouch x-ray source rather than an undercouch x-ray source
C. Ensuring that the patient is wearing a lead apron
D. Advising the operator to increase his/her distance from the patient
A. Correct. Minimising the x-ray beam on time reduces staff dose. It also reduces patient dose.
B. Incorrect. Staff dose is higher using an overcouch x-ray source rather than an undercouch x-ray source.
C. Incorrect. Lead aprons should be worn by staff.
D. Correct. Staff must try to maintain a maximum distance from the patient.
What are the advantages of a digital IITV system over what went before it?
The introduction of digital IITV systems made it possible to acquire high quality dynamic x-ray images at comparatively low patient dose and view them in a flexible and ergonomically efficient way.
Digital IITV images can be computer enhanced to improve the presented information.
Image results can be archived digitally and/or on film via laser hardcopy as preferred.
As a result of these advances, digital IITV systems have broadened the range of clinical imaging applications which can be supported in the fluoroscopy suite
What are the differences between fluorography and fluoroscopy?
Automatic brightness control and automatic exposure control systems are used in fluoroscopy and fluorography respectively.
Fluoroscopy
low current (0.5-5 mA), continuous or near-continuous x-ray exposures
relatively low signal to noise ratio (SNR)
prioritises temporal resolution for procedures
512 x 512 pixel matrix with 8-bit greyscale
real-time imaging viewed on a display monitor in the clinical room
Fluorography
relatively intense (50-1000 mA), pulsed x-ray exposure (pulses are of short duration and applied at 1-12 pulses/second)
relatively high SNR
prioritises spatial resolution for diagnostic purposes
1024 x 1024 pixel matrix with 10-bit greyscale
images usually viewed after acquisition
What are the 2 forms of grayscale processing in digital fluorography?
Greyscale range compression, typically using a non-linear response curve, is often used to suppress or highlight intensities and improve the contrast balance of the image - this can be implemented via an analogue (video) circuit or a digital look-up-table (LUT)
Contrast and brightness adjustment is used to improve the presented contrast of the viewed image or to optimise hardcopy image quality
WHat is spatial filtering?
similar to the contour or edge enhancement used to enhance computed radiography (CR) and digital radiography (DR) images) is employed to improve the displayed spatial resolution of the images acquired with a digital IITV system
How does temporal filtering reduce noise?
this technique uses a sliding average of the current frame with a set of [N -1] preceding frames.
The noise content of the final displayed image reduces with the square root of the number of frames averaged
This technique uses a digitally generated lag (temporal unsharpness) to smooth the noise fluctuations, and therefore it works best for quasi-static image content.
What is road mapping?
Road mapping is a subtractive image processing technique which is widely used during cardiovascular diagnosis and intervention. Here, a digital fluorographic image is first acquired depicting the contrast medium enhanced vessel of interest.
This reference image is then digitally subtracted from the fluoroscopy image in real time removing the common background anatomy. This leaves a reference (or road) map, against which the live catheter or guide-wire is more clearly visible.
Road mapping is used to reduce the time required to route the catheter through the vessel and therefore the radiation dose delivered to the patient.
A digital fluorography system:
A. Requires a much larger receptor entrance dose than a screen film combination used for radiography
B. Offers a spatial resolution which exceeds that of a screen film radiography combination
C. Records hardcopy images using fluorographic film
D. Records hardcopy images using a laser printer
A. Incorrect. In fluorography (digital spot imaging) the receptor dose per frame is typically less than 5 μGy, the exact amount depending on the quality required. This is comparable to the dose to a typical screen film combination used for radiography.
B. Incorrect. The spatial resolution of a digital fluorography system is lower than that of a screen film radiography system.
C. Incorrect. Fluorographic film was used in old analogue systems but is not used in a digital fluorography system.
D. Correct. Selected image frames can be hard copied on film for distribution to clinical colleagues using a laser printer.
Regarding digital fluorography:
A. Last-image-hold requires the acquisition of supplementary image frames
B. Road mapping presents a live image of the catheter against a frozen image of the vessels
C. Dynamic flat panel detectors can operate with lower dose rates than IITV systems
D. The spatial resolution of a flat panel detector improves in proportion to the magnification factor when a zoom field is selected
A. False. The last-image-hold facility is used to capture and temporarily store an image during fluoroscopy. This provides the radiologist with an on-going image on the reference display monitor without the need to expose the patient to further radiation unnecessarily.
B. True. Road mapping is a subtractive image processing technique which is widely used during cardiovascular diagnosis and intervention. A digital fluorographic image is first acquired depicting the contrast medium enhanced vessel of interest. This reference image is then digitally subtracted from the fluoroscopy image in real time removing the common background anatomy. This leaves a road map against which the live catheter or guide-wire is more clearly visible.
C. True. The DQE of dynamic flat panel detectors is 10 to 20% higher than that of a modern IITV, so a lower dose per frame can be used.
D. False. As a fixed matrix array is used, the spatial resolution of a flat panel detector does not improve when a zoom field is selected (unlike IITV).
