Fluoroscopic Image Display

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Fluoroscopic Image Display

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Television Cameras

3 Methods: Thermionic television camera tube Solid state charge-coupled device (CCD) Active pixel sensors (APSs) or (CMOS)

Coupling I.I. to TV tube, CCD or APS Fiber optics

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Video Viewing System

Closed circuit television Video camera coupled to output screen and

monitor Video cameras

Vidicon or Plumbicon tube CCD APS

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Viewing

The output phosphor of the II is connected by fiber optic cables directly to a TV camera tube when the viewing is done through a television monitor.

The most commonly used camera tube - vidiconInside the glass envelope that surrounds the TV

camera tube is a cathode, an electron gun, grids and a target.

Past the target is a signal plate that sends the signal from the camera tube to the external video device

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Vidicon (tube) TV Camera

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Type of TV camera VIDICON TV camera

improvement of contrast improvement of signal to noise ratio high image lag

PLUMBICON TV camera (suitable for cardiology) lower image lag (follow up of organ motions) higher quantum noise level

CCD TV camera (digital fluoroscopy) digital fluoroscopy spot films are limited in resolution,

since they depend on the TV camera (no better than about 2 lp/mm) for a 1000 line TV system

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Camera tube have a diameter of approximately 1 inch and a length of 6 inches.

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Parts of the camera tube

Glass envelope Electron gun (Cathode) Control grid Electrostatic grids Target

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Camera Tube steps

Light is received by the camera tube. The light from the II is received at the face plate of

the target assembly. Electrons are formed into an electron beam (by the

control grid) at the electron gun. Electrons are burned off by thermionic emission

then focused and accelerated to the target. (made of antimony trisulfide)

12Vidicon Target Assembly

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The electrons scan the signal plate similar to reading a page.

Starting in the upper left across to the right, then back to the left to right.

This is called an active trace. The movement of the electron beam

produces a RASTER pattern. The same pattern occurs in the TV

monitor.

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The signal plate sends the electrical video signal to the control unit which amplifies the signal and synchronizes the pulses between the camera tube and the TV monitor.

TV tube and monitor must be synchronized and duplicated to build a coherent image.

15Synchronization (Sync Signals)

Charge-Coupled Devices (CCDs)

1980s CCD were developed and replaced TV tubes and miniaturized imaging devices.

Light photons enter the silicon layer, ionization of the light separates the e-. A layer of microscopic electrodes beneath the silicon acts as a ground for the freed electrons. Movement of charges can be measured by a circuit.

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Each electrode is connected to a storage capacitor (TFT)

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Video Camera Charged Coupled Devices (CCD)

Operate at lower voltages than video tubes More durable than video tubes

Semiconducting device Emits electrons in proportion to amount of

light striking photoelectric cathode Fast discharge eliminates lag

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CCD’s

Advantages of CCDs

High spatial resolution High SNR High DQE No spatial distortion Unlimited life

Linear response to radiation

Lower patient dose Wider dynamic range

and better contrast resolution than conventional fluoroscopy

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TV camera and video signal (V)

On most fluoroscopy units, the resolution of the system is governed by the number of lines of the television system. MTF

Thus, it is possible to improve the high contrast resolution by increasing the number of television lines.

Some systems have 1,000 lines and prototype systems with 2,000 lines are being developed.

Color TV requires 3 vidicon tubes for each of the wavelengths (red, green, and blue)

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TV Monitors

Monitors Cathode ray tube (CRT)

Liquid crystal display (LCD) Active matrix liquid crystal display

Emissive displays = produce their own light (diodes)

Not found application for medical imaging so far. Plasma screen Light-emitting diode (LED)

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MONITOR

CRT – Cathode Ray Tube Much larger than camera tube – but similar

function The electrons are synchronized by the control

unit – so they are of the same intensity and location as the electrons generated by the pick up (camera) tube.

Soft copy viewingdigital cathode ray tube (CRT)

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TV Monitor

The TV monitor contains the picture tube called cathode ray tube (CRT).

It works like the camera tube. With an electron gun and control grids the

electron beam is fired toward the anode. The TV screen contains small fluorescent

crystals

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Video Field Interlacing

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Different types of scanningINTERLACED

SCANNING

PROGRESSIVESCANNING

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625 lines in 40 msi.e. : 25 frames/s

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Two fields = a frame (525 lines) It take 1/30 of a second. To prevent flicker, two fields are interlaced

to form on television frame. There are 60 fields and 30 frames per

second. Each frame takes 33 ms to form The eye cannot detect flickering above 20

frames/sec.

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RASTER Pattern

The electron beam moves in the same raster pattern as in the camera tube.

The signal consists of many individual pulses corresponding to the individual location on the camera tube target.

The varying voltage pulses are later reassembled into a visible in by the TV monitor.

Refresh rate

The refresh rate is the measure of how fast the monitor rewrites the screen or the number of times that the image is redrawn on the display each second. The refresh rate helps to control the flicker seen by the user; the higher the refresh rate, the less flicker.

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TV RESOLUTION-Vertical Conventional TV: 525 TV lines to represent

entire image. Example: 9” intensifier (9” FOV)1) 9” = 229 mm

2) 525 TV lines/229 mm = 2.3 lines/mm

3) Need 2 TV lines per test pattern line-pair

4) (2.3 lines/mm) /2 lines/line-pair = 1.15 lp/mm

Actual resolution less because test pattern bars don’t line up with TV lines. Effective resolution obtained by applying a Kell Factor of 0.7.

Example: 1.15 x 0.7 Kell Factor = 0.8 lp/mm

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Kell Factor

The ability to resolve objects spaced apart in a vertical direction.

More dots = more scan lines = more/better resolution

Kell factor for 525 line system is 0.7

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KELL FACTOR

VERTICAL RESOLUTIONABILITY TO RESOLVE OBJECTS SPACED APART IN A VERTICAL DIRECTION

MORE DOTS(GLOBULES) = MORE SCAN LINES = MORE/BETTER RESOLUTION

RATIO OF VERTICAL RESOLUITON # OF SCAN LINES

KELL FACTOR FOR 525 LINE SYSTEM

IS 0.7

Dot pitch

Dot pitch is the measurement of how close the dots are located to one another within a pixel

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TV RESOLUTION-Horizontal Along a TV line, resolution is limited by how fast

the camera electronic signal and monitor’s electron beam intensity can change from minimum to maximum.

This is bandwidth. For similar horizontal and vertical resolution, need 525 changes (262 full cycles) per line. Example (at 30 frames/second):

262 cycles/line x 525 lines/frame x 30 frames/second

= 4.2 million cycles/second or 4.2 Megahertz (MHz)

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Bandpass/Horizantal Resolution

Horizontal resolution is determined by the bandpass.

Bandpass is expressed in frequency (Hz) and describes the number of times per second the electron beam can be modulated.

The higher the bandpass, the better the resolution

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Different types of scanningINTERLACED

SCANNING

PROGRESSIVESCANNING

USED IN DIGITAL

12 2

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4 16

18 6

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820

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1816141210 8 6 4 2

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625 lines in 40 msi.e. : 25 frames/s

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Digital Uses Progressive Scan

1024 x 1024 Higher spatial resolution As compared to 525

8 images/sec (compared to 30 in 525 system)

1080p 1080i

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Digital Uses Progressive Scan

SNR for TV camera tubes

200:1

SNR necessary for DF

Is 1000:1

Each image takes 33 ms

To display

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TV SYSTEMS Images are displayed on the monitor as

individual frames – which tricks the eye into thinking the image is in motion (motion integration)

15 f/sec – eye can still see previous image

Weakest Link - 2 lp/mm resolution

Real Time

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Final Image

The result of hundreds of thousands of tiny dots of varying degrees of brightness.

These dots are arranged in a specific patterns along horizontal scan lines.

Usually 525 scan lines. The electron gun within the picture tube scans from

top to bottom in 1/60 of a second, (262 1/2 lines) called a field.

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Bandpass/Horizontal Resolution

Horizontal resolution is determined by the bandpass.

Bandpass is expressed in frequency (Hz) and describes the number of times per second the electron beam can be modulated.

The higher the bandpass, the better the resolution

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TV RESOLUTION-Horizontal Along a TV line, resolution is limited by how fast

the camera electronic signal and monitor’s electron beam intensity can change from minimum to maximum.

This is bandwidth. For similar horiz and vertical resolution, need 525 changes (262 full cycles) per line. Example (at 30 frames/second):

262 cycles/line x 525 lines/frame x 30 frames/second

= 4.2 million cycles/second or 4.2 Megahertz (MHz)

active matrix liquid crystal display (AMLCD)

Must have sharper resolutionand high speed, each pixel hasits own TFT

Crystals can be aligned by an external electric field

Nematic liquid crystals

Light is twisted along with the crystals.

Varying the amount of twisting vs alignment allows

for more or lesslight and greys in between.

Active matrix liquid crystal displays are superior to cathode ray tube displays.

AMLCD design – gives out more light,

reduces ambient light interference

Better contrast resolution

Less noise Less maintenance

Luminance

Rate of light emitted from a source. Measured in lumen (Lm)

Luminous flux – light as perceived by the human eye.

CRT vs. AMLCD

Lightweight Portable Less expensive More sizes Smaller profile Less heat Longer life Do not produce veiling glare (light

leaking)

LCDs VS CRTs Perfect geometry Uniform sharpness

& brightness Low surface

reflectance (glare) No image flicker No veiling glare

Geometric corrections needed

Uneven sharpness & brightness

Image flicker Veiling glare

LCDs 3 substantial disadvantages

Off-angle viewing degrades rapidly, brightness is less.

Have 1/5 less light intensity of a viewbox. Reducing the contrast and apparent spatial resolution.

Can not transmit true black density. Requires 30 minutes to warm up and

sensitive to extreme tempertures.

When a digital display device is viewed from the side, illumination and image contrast are reduced.

Spatial Resolution improves with the use of higher-

megapixel digital display devices

A 1-megapixel display will have a 1000×1000-pixel arrangement. A high-resolution monitor will have a 5-megapixel display, or a 2000×2500-pixel arrangement

Image Display Resolution

Minimum display resolution of 2.5 LP/mm LCDs resolution is consistent and always

uniform. CRTs must be frequently checked for

deterioration.

Contrast Resolution Dynamic range – the number of different gray

levels or brightness levels that can be represented in the displayed digital image. (gray scale)

Display monitor is the weakest link in the imaging chain, dynamic range compression.

Bit Depth?

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Pixels

Digital Images – Bit Depth

Pixel values can be any bit depth (values from 0 to 1023)

Bit depth = # or gray shades available for image display

Image contrast can be manipulated to stretched or contracted to alter the displayed contrast.

Typically use “window width” and “window level” to alter displayed contrast and brightness

Digital - GrayscaleDigital - Grayscale

Bit depth.Bit depth. Number of gray shadesNumber of gray shades

available for displayavailable for display 8 bit 2568 bit 256 10 bit 102410 bit 1024 12 bit 409612 bit 4096 4 bit 163844 bit 16384

Display Bit Depth1 bit 6 bit 8 bit

2 shades 64 shades 256 shades

DICOM Gray Scale Standard Display Function

EHR

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DICOM Function

Image exchange for both senders and receivers, support for connecting to a database and retrieving image information. Enabling another device to which images have been locally for retrieval Other dimensions deal with image management, patient scheduling information, image quality, media storage, security,

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Image Quality

Problem how to achieve consistency in the image presentation on different monitors, as well as on film, independent of the make or type of characteristics of the media?

Solution: DICOM Grayscale Standard Display Function.

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DICOM Gray Scale Standard Display Function

It specifies exactly what luminance or density level should be produced for a certain input value, based on the Barten curve, which maps the values into a range that is perceptually linear. This means that input values are mapped into a space that is perceived as linear by a human observer.

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Inconsistent image display. The lump visible on the left is almost

invisible on the right

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Viewable area

The viewable area is measured diagonally from one corner of the display to the opposite corner.

Window Level & Width Function

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RECORDING THE IMAGE

STATIC IMAGES

DYNAMIC IMAGES

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Basic Componets of “old” Fluoroscopy “Imaging Chain”

Fluoro TUBE

Primary

Radiation PATIENT

EXIT Radiation

Image Intensifier

ABC Image Recording Devices

Fiber Optics OR

105 Photospot

CINE

VIDICON

Camera Tube

CONTROL

UNITTV

LENS

SPLIT

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Recording the Fluoroscopic Image

Conventional Spot film

Changes the operation of the tube from low fluoroscopic mA to high radiographic mA. 05 – 5 ma (usually ave 1 – 2 ma)

Exposure delayed Masking capabilities

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Fluoroscopy mA vs spot exposure

Radiographic Exposure for cassette spot filmsmA increased to 100 – 200 mAESE 200 mR = 1 image

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Photons used: Fluoro vs Photons used: Fluoro vs RadiographyRadiography

Spotfilm Fluoroscopy

kVp: 85 85mA: 200 3Time (sec): 0.3 0.2*mAs: 60 0.6Ratio: 100 1

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Image recording

Cassette loaded spot film Where is the tube? How should you put the IR into the II slot?You can format the image, 2 on 1, 4 on 1 or 1 on 1Cassette loaded spot film increases patient dose

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CASSETTE SPOT FILMINGvs PHOTOSPOT FILMING

First type of recording used 9x9 cassettes then later up to 14x 14 9 on 1, 4 on 1, 2 on 1 Delay while exposing (anatomy still moving) Radiographic mA - must boost up to 100 – 200 mA for filming And moving cassettes around inside tower Higher patient dose Replaced by Photospot (f/sec) filming

70 & 105 PHOTOSPOT (CAMERA)

Similar to a movie camera, only one frame exposed when activated.

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70 & 105 PHOTOSPOT (CAMERA)

Photo spot camera will take the image right off the output phosphor

This requires less patient dose 70 & 105 mm roll film General rule, larger film format = better

image quality but at increased patient dose.

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CASSETTE SPOT FILMINGvs PHOTOSPOT FILMING

Photospot (f/sec) filming – Set at control panel from 1 f/sec – 12 f/sec Used for rapid sequence:

Upper Esophogram Voiding Cystourethrograms (Peds)

Lower patient dose

Photospot filming

100 mR Frame rate12/s

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Recording the Fluoroscopic Image

Dynamic systems Cine film systems Videotape recording Static digital spot filming systems

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Cine Film Systems

Movie camera intercepts image 16 mm and 35 mm formats Record series of static exposures at high speed 30 – 60 frames per second

Offer increased resolution At the cost of increased patient dose

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Cinefluorgraphy aka CINE

35 or 16 mm roll film (movie film) 35 mm ↑ patient dose / 16 mm – higher quality images produced 30 f/sec in US – (60 frames / sec)

THIS DYNAMIC IMAGE CAPTURE = HIGHEST PATIENT DOSE (10X greater than fluoro)

(VS SINGLE EX DOSE IS ↓)

87Cine Cinefluorography is used most

often in cardiology and neuroradiology.

The procedure uses a movie camera to record the image from the image intensifier.

These units cause the greatest patient doses of all diagnostic radiographic procedures, although they provide very high image quality.

The high patient dose results from the length of the procedure and relatively high inherent dose rate.

For this reason special care must be taken to ensure that patients are exposed at minimum acceptable levels.

Patient exposure can be minimized in a number of ways. The most obvious means of limiting exposure is to limit the time the beam is on.

CINE - 2mR per frame (60f/sec)

400 mr per “look”

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Synchronization

Camera shutters and x-ray pulsed fluoro happen at the same time

Only exposes pt when shutter is open to record image

Patient radiation dose ↑ as #/f/sec ↑

(filming a TV show – pattern seen)

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Framing frequency

Number of frames per second Cine – division of 60 (7.5, 15,30,90,120) Organ if interest determines f/s rate Patient exposure?

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Video disc

Referred to as electronic radiography. Fluoroscopic radiation continues only long enough

to build up a useful image on the display monitor. The image is stored as a single television frame on

the video disc recorder. There is about a 95% reduction in patient dose.

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Video tape

Utilizes VHS or high-resolution tapes. Patient’s exposure to radiation is not

increased. Used for barium swallows.

Fluoroscopy & Digital Photospot

Real-time fluoroscopy operates at 30 f/sec the human eye can perceive 3-5 frames of image information.

Last image will appear very noisy.

Digital spot images are acquired through the II display system (TV tube or CCD)

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Fluoroscopy & Digital Photospot

Real-time fluoroscopy operates at 30 f/sec the human eye can perceive 3-5 frames of image information during that second.

Last image hold will appear very noisy.

Digital spot images are acquired through the II TV tube to ADC or CCD.

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Digital Spot Imaging

Half the image resolution of photospot images, 2.4 lp/mm

Digital spots are acquired with a much higher mA (radiographic mode). 50 – 100 x one frame of fluoroscopy

Digital images can be windowed/leveled, smoothing algorithms can reduce noise and enhance edges of images.

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• 1 frame of fluoro digital spot

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Digital Subtraction Angiography

Computer controlled image matrix size, dynamic range and image acquisition rate.

Matrix size requirement will determine imaging rate capability. 512 x 512 = 30 images per sec. 1024 x 1024 only 8 image per second can be acquired.

Limitation is enormous quantities of data transfer.

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Postprocessing image subtraction - DSA

Temporal subtraction Dual Energy subtraction Hybrid subtraction

Using both subtraction techniques

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Temporal Subtraction

Time Exposures made over time will be subtracted

from each other. Ex: scout, during, post Automatic injector and x-ray generator work

in tandem.

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Dual energy subtraction

2 images acquired at very different kVp. Each pixel value can be compared to

determine how much change occurred. Tissues are identified. Specific ranges of

value can be subtracted from the image.

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Questions?