Digital Fluoroscopy Flashcards
Difference between conventional fluoro and digital is
the way the image is processed
In digital fluoroscopy, A computer is placed
between the tv camera and tv monitor
In digital fluoroscopy, a video signal from camera is routed through ______, manipulated and ___________ to view
computer, transmitted to tv monitor
The advantage of digital fluoro over conventional fluoro is
the speed of retrieving the image and the ability to use postprocessing to enhance the image
In DF the spatial resolution is determined by
the image matrix and the size of image intensifier
Formula for Digital fluoro pixel size:
Pixel size = image intensifier size
matrix
The differences in the equipment between conventional and digital fluoro include the following additions for DF:
a more complex operating console that includes two monitors, more keys, computer-interactive video controls and a pad for cursor and ROI manipulation.
When two monitors are utilized, one is used to _____________ while the other is used to ____________.
edit pt. data, exam data and annotate final images, display subtracted images
In DF, Tube current measured in:
hundreds of mA compared to less than 5mA
Tube is not continuously energized, Images obtained by
pulsing x-ray beam
With Pulse-progressive fluoroscopy,The rate to acquire an image ranges from
1 per second to 10 per second
It requires how much time to produce one video frame?
1/30th of a second.
Because of pulse-progressive fluoroscopy, the x-ray generator must be capable of:
switching on & off very rapidly
What is the definition of Interrogation time?
time required for x-ray tube to be switched on and reach selected levels of kVp & mA
What is the definition of Extinction time?
time required for x-ray tube to be switched off
DF systems must incorporate 3-phase or high frequency generators with interrogation and extinction times of less than ____
This will accommodate switching on and off as rapidly as needed
1ms
Benefits of Pulse Progressive Fluoro
• Utilizes high mA by operating the undertable tube in radiographic mode.
– Reduces quantum mottle and improves signal-to-noise ratio.
• Delivers radiation in rapid, high intensity pulses, about 30 per second.
– Protects tube from thermal overload.
– Aids in controlling patient dose.
• Utilizes a 1023-line display monitor, and scans in progressive mode.
– Improves resolution.
– Reduces flicker on the monitor.
The CCD converts visible light to
electrons
Sensitive component of a CCD is:
layer of crystalline silicon
When silicon is illuminated, __________ is generated
an electrical charge
CCD is placed
on output phosphor of image intensifier tube, Coupled by fiber optics or a lens system
Principal advantages of CCDs:
High spatial resolution High signal-to-noise ratio High detective quantum efficiency (DQE) No warm-up require, No lag No spatial distortion, No maintenance Unlimited life Unaffected by magnetic fields Linear response & lower dose
The spatial resolution of a CCD is determined by
its physical size & pixel count
Because of its flat surface ___________ as with tv cameras (pincushion distortion*)
no distortion with CCD
Advantages of CCD
- Because CCD has a high DQE, the CCD has higher sensitivity to light (detective quantum efficiency)
- Lower level of electronic noise than tv camera
- Higher signal-to-noise (SNR) and better contrast resolution
- All of this results in lower patient dose
- Warm up is not required (because of CCD’s response to light)
What is lag?
the continuation of luminescence after the x-ray source has stopped
What is blooming?
overflow of electrons into a detector element
True or false: CCD has an unlimited lifetime and requires no maintenance.
True
Most important feature of CCD is
its linear response
Whereas other image receptors have S-shaped response, this one is linear
Particularly helpful for subtraction imaging
The linear response of CCD gives:
improved dynamic range & better contrast resolution
CCD’s will eventually be replaced by
flat panel image receptors (FPIRs)
Flat panel image receptors (FPRIs) are composed of:
cesium iodide/amorphous silicon pixel detectors
Advantages of FPIR over CCD
Distortion-free images
Constant image quality over the entire image
Improved contrast resolution over the entire image
High DQE at all dose levels
Rectangular image area coupled to similar image monitor
Unaffected by external magnetic fields
A 525-line system is adequate for video,however, for DF higher spatial resolution is obtained with:
1000-line systems
In DF, the camera tube reads in ______ mode.
progressive
The electron beam of tv camera sweeps target assembly continuously from top to bottom in (how much time?)
33ms (1/30s)
As the video image is formed on the television monitor:
There is no interlacing of one field with another
This produces a sharper image with less flicker
All analog electronic devices are inherently noisy. Why?
Due to heated filament and voltage differences always produce very small electric current flowing in any circuit
This is known as background electronic noise
Noise limits what?
contrast resolution and conveys no useful information/obscures electronic signal
A high signal to noise ratio gives us
a much better image with more clarity and more detail
As mAs is increased the SNR is
increased. This happens at the expense of increased pt dose
Conventional tv camera tubes have a SNR about
200:1
This is not sufficient for DF
Minimum SNR for digital is
1000:1
Important characteristics of a DF system that are computer controlled and affect your image:
Image matrix size – affects image quality
System dynamic range – shades of gray
Image acquisition rate – speed with which image is acquired
Image processing & manipulation
The output signal from tv camera tube is transmitted by cable to
an analog-to-digital converter (ADC)
The ADC accepts the continuously varying tv camera output signal, the analog signal, and
digitizes it
The output of the ADC is transferred to
main memory and manipulated so that a digital image in matrix form is stored
The dynamic range of each pixel, the number of pixels and the method of storage determines
the speed with which the image can be acquired, processed, and transferred to an output device
The dynamic range of each pixel, the number of pixels and the method of storage determines the speed with which the image can be acquired, processed, and transferred to an output device
Why?
because the larger the image matrix the longer it will take and
The larger the matrix the fewer (slower) the images per second produced
The time to transfer data is made slow due to
the time required to conduct the enormous quantities of data from one segment of memory to another
Principal advantage of DF exams are
the image subtraction techniques and ability to visualize vasculature with a venous injection of contrast material
Digital fluoroscopy provides better contrast resolution by
postprocessing image subtraction. It provides instant viewing of subtracted images during injection of contrast
The subtraction techniques used to enhance contrast are:
Temporal
Energy
Hybrid – combination of two
Only some digital capable of producing this
Temporal Subtraction – consists of
number of computer-assisted techniques whereby an image obtained at one time is subtracted from an image obtained at a later time
Two methods of temporal subtraction:
Mask mode
Time-interval difference mode (TID)
Mask Mode
patient is positioned under fluoro to ensure that region of anatomy under investigation is within the FOV of the image intensifier
Power injector is armed and readied to deliver contrast material through venous or arterial entry
Mask mode Requires an increase in
in x-ray tube current with activation of pulse image
During mask mode, Injector is fired and
an initial x-ray pulsed exposure is made before contrast reaches anatomic site
the mask image is:
(pre-contrast) is stored in primary memory and displayed on video monitor A
Additional images (to the mask image) are taken with contrast and
stored in another memory location.
The mask image is subtracted from these and stored in primary memory. These subtracted images are displayed on video monitor B.
By subtracting the static object, we can
see arteries much more clearly than just viewing it dynamically while contrast is flowing in vessels
Advantages of digital subtraction
The digital subtraction allows a better analysis of the opacified arteries
These images are stored in memory and can later be recalled for closer examination
image integration
Because the video system is slow to respond and noise is high, several video frames may be necessary to make an image.
Image integration improves ________ but also increases _______ because more frames are required.
contrast resolution, pt dose.
Sometimes the initial mask image is inadequate due to:
pt motion
improper technique
noise
remasking:
- Later images (#5 image instead of #1) may be used as the mask image.
- The examiner can even integrate several images using the composite image as the mask.
- A typical exam may require a total of 30 images in addition to the mask image
time interval difference mode
produces subtracted images from progressive masks and following frames
First image = frame 1 subtracted from frame 5
Second image = frame 2 subtracted from frame 6, etc.
The images observed during time interval difference mode convey the flow of the contrast medium dynamically.
Observing dynamically as opposed to a slower rate in mask mode
the flow of the contrast medium dynamically.
Observing dynamically as opposed to a slower rate in mask mode
TID image shows it to be relatively free of motion artifacts but
with less contrast than mask-mode imaging
TID imaging is principally applied in
cardiac monitoring
Misregistration occurs when:
there is patient motion between the mask image and a subsequent image
The subtraction image will contain misregistration artifacts
Cause of Misregistration
The same anatomy is not registered in the same pixel of the image matrix
Misregistration artifact can be eliminated by
reregistration of the mask.
superimposition of images
shifting the mask by one or more pixels
Energy subtraction
uses two different x-ray beams alternately to provide a subtraction image resulting from differences in photoelectric interaction
What is energy subtraction based on?
the change in photoelectric absorption of contrast media for soft tissue compared to bone
“K-absorption edge”
When the incident x-ray energy is sufficient to overcome the K-shell electron binding energy of iodine, an abrupt and large increase in absorption occurs
Describe the X-rays used for energy subtraction:
A low (70) kVp mask is created for soft tissue
As well as a high (90) kVp mask for bone
The two are subtracted with a resulting image
Disadvantage of using energy subtraction:
Requires a method of providing an alternating x-ray beam of two different emission spectra (70 & 90)
What are the two methods of energy subtraction?
Alternately pulsing the x-ray beam at 70 then 90 kVp
Alternately utilizing different metal filters in beam
Hybrid subtraction
the technique of combining temporal and energy subtraction
How is hybrid subtraction achieved?
The mask and each subsequent image are formed by an energy subtraction technique
If patient motion is controlled, hybrid imaging can produce the highest quality DF images
Roadmapping
Special application of DSA Initial images acquired in same manner Mask acquired Contrast injected Subtraction image acquired
What is the main difference in roadmapping?
Catheter is fluoroscopically advanced & image is formed by subtracting from second mask
Static images with DF are also lower dose per frame than
with a 100-mm spot film camera
Digital fluoro
produces series of dynamic images obtained with x-ray beam and image intensifier
Digital subtraction angiography
a type of fluoroscopy technique used in interventional radiology to clearly visualize blood vessels in a bony or dense soft tissue environment. Images are produced using contrast medium by subtracting a ‘pre-contrast image’ or the mask from later images, once the contrast medium has been introduced into a structure.
registration
lining up of the mask with the correct pixels within the matrix.
