l11_fluoroscopy
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BME 340: Bioimaging
Lecture 11: Fluoroscopy
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Fluoroscopy
Fluoroscopyis an imaging technique commonly used byphysicians to obtain real-time images of the internal structures
of a patient through the use of a fluoroscope.
In its simplest form, a fluoroscope consists of an x-ray source
andfluorescent screenbetween which a patient is placed.
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Fluoroscopy Modern fluoroscopes couple the screen to anx-ray image
intensifierand CCD video camera allowing the images to beplayed and recorded on a monitor.
The use of x rays, a form of ionizing radiation, requires that the
potential risks from a procedure be carefully balanced with the
benefits of the procedure to the patient.
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Fluoroscopy
Same geometric considerations as screen/film radiography.
General purpose fluoroscopy systems allow frame rates of 30
fps (NTSC).
For film (35mm) recording systems, frame rates as high as
120 fps are possible (cardiac applications).
While physicians always try to use low dose rates during
fluoroscopy procedures, the length of a typical procedure
often results in a relatively high absorbed dose to the patient.
Recent advances include the digitization of the images
captured and flat-panel detector systems which reduce theradiation dose to the patient still further.
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System Components
The X-ray tube, filters, and collimators are similar to those already
discussed for screen/filmradiography. The distinct element in this method is the image intensifier, and we
will be looking at these components in greater detail.
For studies that are to be recorded (e.g. dye angiographic studies) a
video or film mechanism is also required.
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Image Intensifiers
An X-ray image intensifier(XRII) refers to a special image intensifierdevice used in medical imaging involving x rays. It allows for lower x-ray
doses to be used on patients by magnifying the intensity produced in the
output image, enabling the viewer to easily see the structure of the object
being imaged.
An image intensifier is a vacuum tube containing: Input layer to convert X-rays to
electrons
Focusing electrodes (G1, G2, and G3)
Anode (acceleration to 25 35keV)
(part of the output window)
Output phosphor to convert electron
beam to visible light.
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Image Intensifiers The Aluminum input window is curved to allow it to withstand the pressure
differential and is about 1mm thick. A similarly curved Al substrate supports the input phosphor and
photocathode. The curvature begins the focusing process.
X-ray energy incident on the input phosphor is converted to visible light.
Since the phosphor (CsI) is in the form of parallel needles, the phosphor can
be thick without degrading spatial resolution.
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Pincushion distortion effect
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Since the input screen is curved, some pincushion distortion is
present in the output image.
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An acceleratinganodeis used to increase the electron energy
to 25 35keV for impact on the output phosphor, and to carryaway electrons after they impact.
Theoutput phosphor (ZnCdS:Ag) has a green (~530nm)
emission which is well-matched to the spectral sensitivity of
video camera targets. This layer is thin (4-
8m) with small
particles (1.5m) to
preserve spatialresolution.
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Image Intensifiers
The high resolution output phosphor is needed since the 23-35cm input image is focused onto a 2.5cm output target.
To maintain a resolution of 5 line pairs per (lp)/mm at the
input plane, the output must be capable of >70 lp/mm.
A thick output glass window with black coating on the sidereduces veiling (scatter) glare to improve contrast.
The Minification gainis the ratio of the area of the input
phosphor to that of the output phosphor. Therefore, a 9 inch
II with a 1 inch o/p phosphor has a minification gain of 81.
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Image Intensifier Performance
Conversion factor represents the effective gain of the imageintensifier system.
This is the ratio of the output luminance (cd/m2) to the input
exposure rate (efficiency) thus giving units of Cd s m-2mR-1.
Typical range is 100-200 Cd s m-2
mR-1
for new devices. This degrades over time due to phosphor degradation
eventually requiring replacement of the image intensifier.
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Image Intensifier Performance
Brightness gain results from the minification effect as well aselectronic gain and will typically be in the range of 2500 7000 with
larger diameter devices producing larger gains.
Field of view: 35-40 cm devices used for thoracic and abdominal
imaging. 23 cm used for cardiac applications.
As effective diameter of the input phosphor decreases, magnificationincreases, and brightness gain decreases.
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Image Intensifier Performance
Magnification modes are also available in which focusingpotentials are changed to use a smaller region of the input
phosphor.
This results in increased magnification of the image on the
output phosphor. As magnification increases, a smaller area of the II is
visualized.
Use of magnification reduces brightness, which requires an
increase in exposure for compensation. Therefore theminimum magnification necessary will be used out of dose
considerations.
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Image Intensifier Performance
Contrast ratio measures the effect of scatter in the II (veilingglare).
The measurement involves the use of a 2.5cm (1) lead disk
placed at the center of the input phosphor.
Ideally, should be no intensity in the center of the image. Typically an intensity ratio of 15-30:1 is seen between the
occluded and control region of the image. This constitutes
the maximum possible contrast.
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Quantum detection efficiency and measures the X-rayconversion
efficiency taking into account the aluminum window attenuation.
Maximum value about 70% at 60kVp. S distortion results from static magnetic fields such as the earths, but
can also result from MRI units if fringe fields are not taken into
account when locating equipment.
Images are spatially warped into an S shape through the image.
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Optical Coupling of II
As seen in the fluoroscopy systemdiagram, the small size and
position of the output image from
the image intensifier would
require a ladder to view the
image.
An optical distribution system is
used to split the output to the
video camera for display and a
film camera for recording.
This is basically the same
principle and geometry used for
collimator lights on radiography
systems.
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Video Cameras and Resolution
Closed-circuit (raster scan) videos are typically used totransfer the II signal to video-monitors.
Some systems use the NTSC video standard of 525
horizontal lines, of which about 490 are usable.
Large image intensifier (9 inch, 229 mm) systems use onthe order of 1023 lines (4x video bandwidth).
For the 229-mm (9-inch) systems, effective spatial
resolution is on the order of 4.4 lp/mm.
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Flat Panel Digital Fluoroscopy Flat panel arrays are available for
fluoroscopy as well as for conventional
radiographic applications.
Thin Film Transistor (TFT) arrays use an
image intensifier screen (CsI) to convert
X-rays to visible light photons for
detection.
Some arrays used for single image
applications can also be used for
fluoroscopy by combining pixels to
improve SNR.
Because a vacuum environment is not
required, the cover to flat panel can be ~
1 mm of carbon fiber, which improves
QDE compared to traditional IIs.
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Automatic Exposure Rate Control
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Fluoroscopy Modes
Continuous Fluoroscopy
Field of View and Magnification Modes
Variable Frame Rate (Pulsed) Frame Averaging
Last Frame Hold
Road Mapping
C-Arm CT
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Fluoroscopy Modes
Continuous modefluoroscopy is the basic operating mode.A low rate continuous beam (0.5 4mA, depending on
patient thickness) is used.
Recording and display is at a frame rate of 30fps (33ms /
frame). In most cases patient motion within the 33ms time
frame is not a problem.
FDA maximum exposure = 10 R/min.
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Variable frame rate pulsedfluoroscopy techniques are helpful inreducing motion artifact and patient exposure:
Maintain frame rate but reduce exposure time to about 10ms
while increasing tube current to 6.6mA to reduce motion
artifact. Reduce frame rate to 7.5 or 15fps e.g. during catheter
guidance prior to dye injection to reduce patient exposure.
Digital refresh memory maintains 30fps on the display to
reduce flicker.
Fluoroscopy Modes
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Fluoroscopy Modes
Field of View and Magnification Modes
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Fluoroscopy Modes
Frame averagingis used to improve SNR in images bytrading off temporal resolution.
Fluoroscopy is inherently noisier than conventional
radiography.
System performs real-timeaveraging of a selected
number of frames.
The more frames averaged,
the greater the display lag,
and the better the SNR.
Can be used to reduce
patient exposure.
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Fluoroscopy Modes
Last-frame-holdfreezes the last acquired image frame inmemory and on the display when the foot switch is opened
(rather than leaving a blank screen).
This is a feature for reducing patient exposure since the
system does not have to be on while the next step of an
intervention is planned. The last image is available for
reference.
This technique is useful as a training adjunct and reduced
dose to the patient.
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Fluoroscopy Modes
Road mappingis a dual display technique which permitsviewing of a single image (e.g. following injection of a
small dose of contrast agent to reveal the path of a vessel)
while the real-time display is used (for example) for
advancing a catheter based on the information in the frozen
image.
It is a software-enhanced variant of the last-frame-hold
technique.
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Image Quality in Fluoroscopy There are several components which contribute to the MTF of the
overall system. A 230-mm intensifier can produce 4-5 lp/mm in normal mode and
up to 7 lp/mm at 140 mm.
Direct coupling to film will
retain the spatial resolution ofthe image intensifier.
Video recording/display will
degrade MTF.
Digital capture will degradeMTF according to pixel size
(e.g. 10242at 27cm FOV
1.9 lp/mm.
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Fluoroscopy Systems
C-Arm CT is capable of motorized rotation of about 220
degrees around the patient, which allows two-dimensionalprojection images to be acquired at many different angles
around the patient.
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Spatial Resolution
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Contrast Resolution
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Review
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What is the purpose of image intensifier (II)?
Why can the input phosphor (in II) be thick without degrading spatial
resolution?
Assume that 35cm input image is focused onto a 2.5cm output target. What is
the minimal resolution at the output plane to maintain a resolution of 5 line
pairs per (lp)/mm at the input plane?
As effective diameter of the input phosphor decreases, magnificationincreases, and brightness gain decreases.
Are variable frame rate pulsed fluoroscopy techniques helpful in reducing
motion artifact and patient exposure?
What is the purpose of frame averaging?
Why is the lastframe-hold mode is required? Does flat panel system provide a better resolution than image intensifier ?