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.

3Normal barium swallow animationhttp://en.wikipedia.org/wiki/Fluoroscopy
http://upload.wikimedia.org/wikipedia/commons/8/80/Normal_barium_swallow_animation.gif


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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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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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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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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 ?

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