Fluoroscopy Flashcards
Who invented fluoroscopy? When?
Thomas Edison
1896
what is fluoroscopy?
produces real time radiographic images using x-rays - dynamic studies
- show structure AND function
what are spot films
digital images taken without interrupting the dynamic exam
- usually better quality than the dynamic images
what is interventional radiology?
angiography
- 2 main areas - neuroradiology and vascular body radiology
- dynamic series and digital (spot film) images
what is angiography?
application of fluoroscopy for the visualization of vessels
fluoroscopy technical factors?
less than 5mA
procedure may require patient to be exposed continuously and/or multiple times, resulting in long exposure times
kVp dependant on anatomy being imaged
does dose tend to be higher or lower than general radiography?
even though mA is lower, patient dose tends to be higher due to prolonged exposure time
Is a high kVp and low mA or low kVp and high mA preferred?
High kVp and low mA is preferred to minimize patient radiation dose
what is automatic brightness control?
operator selects an image brightness level (by selecting anatomy of interest on machine) that is maintained throughout
- kVp, mA or both vary as the tube moves over the body parts dependant on differing thickness and attenuation
Illumination and visual physiology
- principle advantage of image intensified fluoroscopy is increased image brightness
- prior to use of image intensifiers (IIs) operator would have to adapt to objects that were dim
What does IIs do?
raises illumination into the cones vision range, where visual acuity is the greatest
What is an image intensifier (II)?
converts remnant x-ray beams into a high intensity visible light image
what are image intensifiers with input phosphor?
- tube contained within a vacuum surrounded by a glass or metal envelope to provide support
- remnant x-rays interact with the input phosphor (CsI); energy is converted to light
Why use (CsI) as IP?
the mass attenuation peaks in CsI falls within the transmitted x-ray spectrum, increasing absorption of the transmitted x-ray photons
- increased absorption efficiency of the IP - decreased patient dose
CsI IP screens absorb approximately 2/3 of incident beam as opposed to less than 1/3 by ZnCdS (old screens), while being 1/3 as thick
CsI IP crystals
- crystals can be shaped like thin needles and are tightly packed together
- vertical orientation helps direct light with little lateral dispersion and less blurring
- improved spatial resolution compared to turbid form
Advantages in increases IP layer thickness
Higher x-ray absorption efficiency
- More x-rays photons can be absorbed and converted to light
Less patient dose
- Requires fewer x-ray photons to generate the same amount of light photons at the II output window
Disadvantages of increasing IP layer thickness
Decreased spatial resolution
- Light photons are scattered laterally within the phosphor layer, reducing spatial resolution
Typical IP layer thickness
Thickness is a compromise between spatial resolution and x-ray absorption efficiency
- typically measures approx. 300 um
What is a photocathode?
converts light to electrons
- # of e- emitted is directly proportional to intensity of light reaching it
- # of e- emitted is directly proportional to the intensity of the x-ray beam
made up of caesium and antimony - bound directly to the IP
IP shape
to ensure undistorted focusing, all photoelectrons must travel the same distance to a focal point
- IP is curved to allow for this
How do you maximize conversion efficiency in a photocathode?
the light spectrum of the IP should match the sensitivity profile of the photocathode
Accelerating anode
- anode is a plate with a hole in the middle which allows electrons to pass
- II is approx. 50cm long
- 23-35kV across II
electron optics
- electrons emitted from the large cathode end of the II tube must travel the length of the tube and be reduced to the small output phosphor
- the engineering aspects required to maintain this is called electron optics
electrostatic focusing lenses
- as photoelectrons travel from the photocathode to anode, electrostatic forces cause them to diverge
- to reduce this, negatively charges electrostatic (focusing) lenses/electrodes focus electrons on the output phosphor
- electron focusing inverts and reverses the image at the focal point
what is the point of inversion?
the focal point - where the focusing elements reverse the image
What is output phosphor (OP)?
- smaller than the IP
- converts electrons to visible light
how does OP work?
- electrons arrive at OP with high kinetic energy and contain the image of the IP in minified form
- crystal size and layer thickness are reduced to maintain resolution of minified image
coating on OP?
thin Al film coating on vacuum side of OP
- electrons pass through, opaque to light
- prevents light hitting photocathode
- reflector to increase output luminance
side walls absorb light to reduce distortion
What is flux gain?
the ratio of light photons at OP to x-ray photons at IP
photoelectron arriving at OP produces 50-75 times as many light photons as created it - caused due to the acceleration of the photoelectrons
how do you calculate flux gain?
flux gain = # of output light photons/# of input x-ray photons
what is minification gain?
concentration from a large input screen onto a small output screen results in an increase in image brightness
equal number of photoelectrons leaving the photocathode and striking the output phosphor - photoelectrons per area at the output phosphor increases - brighter image
* increases brightness but DOES NOT change contrast*
how do you calculate minification gain?
minification gain = di^2/ do^2
di = diameter of IP
do = diameter of OP
common diameter ranges for minification gain?
OP diameter is usually 2.5 - 5cm
IP diameter is usually 10 - 40cm - used to identify II tubes
What is brightness gain
brightness gain = flux gain x minification gain
usually 5,000 - 30,000
What is the brightness gain for a 17cm II tube with a flux gain of 120 and a 2.5cm output phosphor?
brightness gain = (17/2.5)^2 x 120
brightness gain = 5549
What happens to brightness gain over time?
Brightness Gain decreases with tube age and use
- As the II ages, patient dose increases to maintain brightness
what is fluoroscopic magnification?
- Fluoroscopic magnification can be achieved geometrically or electronically
- When source-to-image distance is fixed, both magnification and skin dose increases as the patient moves closer to the x-ray source
Multified image intensification
- present day image intensifiers are capable of multi-field image intensification
- part of or the whole image can be viewed due to the ability to change the diameter of the IP
- the most popular is a tri-field tube
how is MII achieved?
achieved by manipulating the voltage of electrostatic lenses, causing: increased acceleration of electrons and shifting of the focal spot away from the anode
- can be used to penetrate through larger parts and enlarge smaller structures
what is electronic magnification?
Use of the smaller dimension of the multi-field II always results in a magnified image
The magnification factor (MF) is the ratio of the 2 diameters in a MII
How do you calculate MF?
MF = IP (old size)/IP (new size)
How magnified is the image of a 25/17/12 II in the 17cm mode compared with that produced in the 25cm mode?
MF = 25/17
MF = 1.47
what is the cost of magnification?
- In mag mode, minification gain is reduced, and fewer electrons are incident on the OP (because of the reduced field of view at the IP), resulting in a dimmer image
- To maintain the same level of brightness, mA, kV or both is increased by the ABC, which increases patient dose
Magnifications effect on patient dose
increase in dose is equal to the square of the ratio of the 2 IP diameters
A 23/15/10 II tube is used in the 10cm mode. How much higher is the patient dose in this mode compared with the 23cm mode?
dose = (23/10)^2
dose = 5.29 x higher
magnifications effect on image quality
increase in dose = more x-rays per unit area; decreased noise, increased contrast resolution, better image quality
- only the central region of the IP is used in mag mode - increased spatial resolution
advantages of television monitoring?
- Brightness level and contrast can be controlled electronically
- Several observers can view the fluoroscopic image at the same time
- Monitors can also be placed remotely for observers
- Allows for storage of images in electronic form for later playback and image manipulation
what is fibre optic coupling?
fibre optic bundles are only a few millimeters thick and contain thousands of glass fibres per square millimeter
what are advantages of fibre optic coupling?
- Compact assembly – makes it easy to move the II tower
- Rugged – can withstand rough handling
limitations of II systems?
- size
- internal structures must be under high vacuum
- defocusing effect
- geometric limitations
- spatial/contrast resolution vs radiation dose
- veiling glare
why are there size limitations of II systems?
can make it difficult to position
why must internal structures be under high vacuum in II systems?
Air leakage into the II would interfere with the transit of electrons between the PC layer and the OP and degrade image quality
what is the defocusing effect?
- If the voltage of the electrostatic lenses are not adjusted correctly, the electrons will not pass through the appropriate focal point, causing the image to be blurry with a loss of spatial resolution
- The focusing voltages may drift with time and use, requiring readjustments to achieve the desired spatial resolution
What is S distortion?
- Other defocusing effects may be attributed to magnetic field variations such as proximity to an MRI or stray electromagnetic fields from other electronic devices such as power supplies
- Magnetic fields can result in S distortions
what are the geometric limitations of II systems?
Path lengths from curved IP to focus remains constant but, due to the flat OP, distances from focal point to OP are not same
- can result in pin-cushion distortion and vignetting
what is pin-cushion distortion?
Caused due to the curved IP and the flat OP
- Decreases when using magnification mode as electrons from the center of the FOV are more in focus
what is vignetting?
unequal illumination
- longer, diverging paths reduce the concentration of electrons that impact the periphery of the OP, causing the center of the OP to be brighter than the periphery
what are the limitations of spatial/contrast resolution and radiation dose?
- Higher spatial/contrast resolutions can be achieved by decreasing the field of view
- Results in an increase in mA and patient radiation dose
- Radiation dose rates increase at a rate of 1/(FOV)2
What is vailing glare?
internal scatter radiation in the form of x-rays, electrons and particularly light can reduce contrast at the OP
Flat panel detector in fluoroscopy?
- Have begun to dominate angiography suites and cardiac catheterization labs
- Smaller FPD allows for more flexible movement
what are the physical characteristics of FPD in fluoro?
- The FPD consists of an array of individual dexels (DELs)
- The size of the entire array ranges from 25 x 25 cm to 40 x 40 cm
- A FPD may contain 1.5 – 5 million individual DELs
- most arrays are indirect solid state detectors
How do flat panel detectors work?
- Phosphor screen scintillates when exposed to x-rays, emitting light that will strike the TFT detectors
- Vertical crystalline channels in the CsI layer form light channels to confine the dispersion of light
- Light is directed towards the detector elements in the AMA
- a-Si acts as a photodiode
what is the process from photodiode to display?
- When light hits the surface of the photodiode (a-Si), it allows the diode to conduct electricity
- In the absence of light on its surface, the photodiode acts like an insulator, preventing the flow of electrons
- Each DEL can quantify the amount of x-ray radiation incident upon its surface
preparation of the photodiode - what happens when the electronic switch is closed?
power supply charges the capacitor
preparation of the photodiode - what happens when the electronic switch is open?
- when no light is incident on the surface of the DEL, the charge remains on the capacitor
- The interaction of x-rays with the scintillator produces light proportionately. The amount of light causes the photodiode to conduct to different degrees (E). As more light is produced, the more charge is drained from the capacitor.
- the capacitor is now left with a remnant charge
how does the photodiode readout occur?
- Another electronic switch is closed and the remnant charge is withdrawn from the storage capacitor and sent to the display system. The loss in charge (E) is related to the amount of x-ray radiation (and light) incident on the DEL.
- By reading each DEL in the FPD array row by row, an electronic image of the distribution of the x-rays that are incident on the FPD can be formed
advantages of FPD system over II systems
- Images obtained with FPD do not exhibit geometric distortions such as “pincushion” effect and S distortions because the individual DELs in the FPD are manufactured in straight rows and columns.
- No vignetting
- No defocusing because there are no electrostatic plates to change focal spot. Each DEL is in a constant position.
what are the 5 limitations of FPDs?
- Difficult to manufacture an FPD array that contains no defective or degraded DELs
- Manufacturers of FPD systems often compensate for defective DELs by using software to interpolate values for those defective elements. This may cause artifacts
- FPD systems are temperature sensitive
- Spatial resolution of FPD systems are limited by the size of the DEL. Smaller DELs improve spatial resolution, but images have more mottle (noise) due to lower SNR. This can be fixed by increasing signal (radiation to the patient).
- Large FPD systems have high data rates.
data rates of large FPD systems?
E.g., a 40 x 40cm FPD system may produce an image composed of 4 million DELs, an image size of 8MB, and a data rate of 240MB/sec
Large data rates such as these are difficult for electronic systems to handle
What is binning?
Size of data rates can be reduced by grouping the data from several DELs together for larger field of views (FOVs)
what is the disadvantage of binning?
less spatial resolution because the effective area of each image pixel is larger
what is the advantage of binning?
lower data rates and less image mottle than ungrouped DELs
Magnification in FPD systems?
- For smaller FOVs, collimation is used to select only the central portion of the FPD for imaging, spreading less anatomy across the display monitor
- When smaller FOVs are used, data rates are lower, and binning is no longer required
Magnification effect on dose with FPD
- With smaller FOVs, magnification makes the image noise more apparent to the observer
- Increased radiation is can reduce the optical perception of noise - However, the increase is substantially less than with II systems
what is ghosting in FPD systems?
- phosphorescent light in the scintillation surfaces undergoes a period of decay - light emissions from previous image may persist
- bright internal light source flashes to reset
Digital fluoroscopy
- images appear with an inverted grayscale (black/white reversed) compared to standard radiographs
- tube can still operate in radiographic mode; constant energization would fail because of thermal overloading and patient dose would be extremely high; therefore images are obtained by pulsing the beam; pulse progressive fluoroscopy
what is pulse-progressive fluoroscopy?
- Emitted as a series of short pulses rather than continuously
- At reduced frame rates, pulse fluoroscopy can reduce dose substantially
- Images may be acquired at 15 frames/second as opposed to 30 f/s - Each image is displayed multiple times in sequence to provide a 30 f/s display
- Can be performed at lower frame rates (ex. 7, 5 or 3 f/s) at the expense of a “choppy” display when imaging rapidly moving regions like the heart
- Simply reducing the pulse rate would result in an increase in image noise
- machine must be capable of switching on and off very rapidly
what is the interrogation time?
Time required for the x-ray tube to be switched on and reach the selected kVp and mA
what is the extinction time?
Time required for the x-ray tube to be switched off
for pulse-progressive fluoroscopy what must the interrogation and extinction times be?
less than 1ms
what is the duty cycle?
the fraction of time the x-ray tube is energized
What are some advantages and disadvantages of using a FPD compared to an II?
FPD is smaller, does not have geometric distortions, no defocusing, no vignetting
How do we use mag-mode with FPDs? What advantages and disadvantages are associated with this?
reduce field of view of IP, increases dose but better image quality, and contrast resolution and decreased noise
Why do we use pulsed fluoroscopy?
less patient dose
What is last image hold?
- In conventional fluoroscopy, image is displayed on the video monitor only when the x-rays are turned on to produce an image on the II
- Modern fluoroscopy systems often use last image hold, where the last image is kept on the monitor after x-ray exposure is terminated
- saves patient dose
What is spot collimation with last image hold?
- enables the operator to collimate a rectangular or square region of interest anywhere within the general field of view
- the operator can define the collimation of interest on LIH
- as soon as the collimation of interest is confirmed by a press on joystick, the last image hold is superimposed over the collimated part to preserve the anatomical and/or device-relevant information during the fluoroscopic operations that would normally be hidden by the collimator blades
- allows for asymmetrical collimation
what is digital subtraction angiography (DSA)?
temporal subtraction;
- computer assisted technique where an image obtained at one time is subtracted from an image obtained later
- If, during the intervening period, contrast material was introduced into the vasculature, the subtracted image will contain only the vessels filled with contrast material
2 methods: mask mode and Time-interval difference (TID) mode
how do you preform Mask mode DSA?
- The patient is positioned in the fluoroscopic unit to ensure that the anatomy of interest is positioned within the field of view
- Power injector is armed and readied to deliver contrast
- The injector is fired and after a delay of 4 – 10 s, before the bolus of contrast medium reaches the anatomic site, an initial pulsed exposure is made
- When the exposure is initiated, the mA increases 20 – 100 times and activates a program of pulse image acquisition
- The image obtained is stored in primary memory and is displayed on video monitor A
- From this image, a mask image is produced by the computer by reversing the acquired image
- As subsequent images are acquired, the mask image is subtracted from each and the result stored in primary memory
- At the same time, the subtracted image is displayed on video monitor B
what are misregistration artifacts?
occur when the patient moves slightly between the mask image exposure and subsequent contrast exposures
what is pixel shifting?
The computer can be directed to reregister misregistration artifact images by pixel shifting the mask to the right or left, up or down, so that anatomy is superimposed again, minimizing artifacts
what is remasking?
Remasking is another option in which a different image in the series is selected as the mask image
How do you obtained TID DSA?
- In a cardiac study, image acquisition begins 5 seconds after injection at the rate of 15 images per second for 4 seconds
- A total of 60 images is obtained in such a study
- These images are identified as frame numbers 1 through 60
- If a TID of four images is selected, the first image to appear will be of frame 1 subtracted from frame 5
- The second image will contain the subtraction of frame 2 from frame 6; the third will contain the subtraction of frame 3 from frame 7 and so forth
What is dual energy subtraction?
- Uses 2 different x-ray beams alternately to provide a subtraction image that results from differences in photoelectric interaction
- Occurrence of photoelectric interactions varies much more for iodine because of its k-edge effect compared to bone and soft tissue
- Subtraction of the 2 images removes bone and soft tissue, but iodine remains
Temporal subtraction
- single kVp
- normal beam filtration
- simple arithmetic image subtraction
- motion artifacts are a problem
- total subtraction of common structures achieved
- subtraction possibilities are limited by number of images
What are the 2 methods of dual energy subtraction?
- Alternately pulsing the x-ray beam at different kVp values
- Alternately introducing metal filters into the x-ray beam
energy subtraction
- Rapid kVp switching
- filter switching is used
- complex image subtraction
- motion artifacts are greatly reduced
- residual bone may survive subtraction
- more types of subtraction images possible
what is road mapping?
- DSA application
- useful for placement of catheters and wires in complex and small vasculature
