chapter 1 hardware anatomy and physiology of a …radpacs.weber.edu/images/r_christensen/provo/radt...

10/26/2015

1

Section III: Chapter 6 1

SECTION III - CHAPTER 6

DIGITAL FLUOROSCOPY

RADT 3463 COMPUTERIZED IMAGING

1 RADT 3463 Computerized Imaging

ACKNOWLEDGEMENTS This presentation is a professional collaboration of

development time prepared by:

Rex Christensen

Terri Jurkiewicz

and

Diane Kawamura

References and images are gathered from many sources including

those copyrighted by Elsevier / Mosby Publishing as they appear in:

Bushong, S. C. (2008). Radiologic science for

technologists: Physics, biology, and protection, 9th ed.

Chapter 27, St. Louis, MO: Elsevier Mosby.

Seeram, E. Digital Radiography: An Introduction, Delmar

Cengage Learning.

Section III: Chapter 6 RADT 3463 Computerized Imaging 2

DEFINITION - FLUOROSCOPY

• An imaging modality that produces dynamic or moving images, displayed in real time.

• Study of anatomical structures and the motion of organs and contrast media in organs and blood vessels.

• Identifies the function of organs and blood vessels.


CONVENTIONAL FLUOROSCOPY PRINCIPLES

• X-ray source -> Image intensifier -> Video camera -> Television monitor (analog signal)

• 30 frames per second (fps)


10/26/2015

2


X-RAY TUBE AND GENERATOR

• Continuous – 30 fps @ 33 ms

• Pulsed – lower patient dose (3-10 ms/image), less blurring

• High frequency generators

• Low mA (1-3 mA) and high kV (65-120 kV)

• Switch from fluoroscopic mode to radiographic mode (spot films, radiographic cassettes)


IMAGE INTENSIFICATION

• The brightening of the fluoroscopic image using an image intensifier.


IMAGE INTENSIFIER TUBE


10/26/2015

3


• Parts of the image intensifier

(enclosed in a vacuum tube)

include:

– Input screen (x-ray to light) • Phospher – Cesium Iodide CsI

– Photocathode (photoelectrons) • Phospher - Antimony Cesium SbCs

– Electrostatic lens (focus) • 20 – 30 kV

– Output screen (light - Increase) • Phospher – Zinc Cadmium Sulfide ZnCdS




BRIGHTNESS GAIN (BG)

• Increase in brightness from the input phospher to the output phospher (5,000 – 30,000)

• BG = Minification Gain (MG) x Flux Gain (FG)

MG = Diameter of the input screen2

Diameter of the output screen2

FG= Number of light photons at the output screen

Number of light photons at the input screen

BUT………….


CONVERSION FACTOR (CF)

• The Brightness Gain (BG) method has been replaced by the Conversion Factor (CF)

• This is the light gain at the output phospher

• CF = Luminance of the output screen

Exposure rate at the input screen

Luminance is measured in candela/square meter (Cd/m2)

Exposure rate is measured in milliroentgens/second (mR/sec)

Conversion Factor (CF) ranges between 50-300. The higher being more efficient.


10/26/2015

4

FLUX GAIN • 1000 light photons at the

photocathode • from 1 x-ray photon • photocathode decreased

the # of ë’s so that they could fit through the anode

• Output phosphor = • 3000 light photons (3 X

more than at the input phosphor!)

• This increase is called the flux gain


Magnification

• Magnification enhances the image to help improve diagnostic interpretation.

• Improves spatial resolution

• Controlled by the input

screen diameter


Multi-field Units

• Allows selection of different input phosphor sizes

• Types of multi-field units:

– Dual focus - 9/6 inches

– Tri focus - 12/9/6 inches

• Smaller input magnifies output by moving focal point away from output

• Greater voltage to electrostatic lenses

– Increases acceleration of electrons

– Shifts focal point away from anode

• Requires more x-rays to maintain brightness

Section III: Chapter 6 RADT 3463 Computerized Imaging 15 Section III: Chapter 6 RADT 3463 Computerized Imaging 16

Intensifier Format and Modes

Note focal point

moves farther from

output in mag

mode

10/26/2015

5

Magnification and Patient Dose

• Magnification is used to enlarge small structures or to penetrate through larger parts

• Patient dose is INCREASED in the magnification mode

• Dose is dependent on the size of the Input Phosphor (IP)

• FORMULA:


MAGNIFICATION MODE

FORMULA

IP OLD SIZE

IP NEW SIZE = % mag


PT dose in MAG MODE

IP OLD SIZE 2

IP NEW SIZE 2 = ↑(x) pt dose


Image Quality Characteristics

• Spatial Resolution

• Contrast Ratio

• Noise

• Artifacts


10/26/2015

6

Spatial Resolution • The ability to resolve fine details of the object

being viewed (patient)

• Input screen is convex – better resolution in the center

• Resolution gets better as the Input diameter gets smaller

• Measured in line pairs per mm (lp/mm) - how close lines can be to each other and still be visibly resolved. The more line pairs the better the resolution (spatial frequency).


Spatial Frequency


• One line pair = the line and an interspace the

same width as the line

Spatial Frequency


An imaging system with high spatial

frequency has better spatial resolution

APPROXIMATE SPATIAL RESOLUTION - MEDICAL IMAGING SYSTEMS

Gamma camera 0.1 lp/mm

Magnetic resonance imaging 1.5 lp/mm

Computed tomography 1.5 lp/mm

Diagnostic sonography 2 lp/mm

Fluoroscopy 3 lp/mm

Digital radiography 4 lp/mm

Computed radiography 6 lp/mm

Radiography 8 lp/mm

Mammography 15 lp/mm

Line pair gauges

GOOD RESOLUTION POOR RESOLUTION


10/26/2015

7

Spatial Resolution

• A 1024 x 1024 image matrix

is sometimes described as a

1000-line system

• Spatial resolution (how

much information is stored

within the space given) is

determined by both the

image matrix and by the

size of the image intensifier.


Pixel Size = Image intensifier Size / Matrix Noise

• Low mA produces high amount of noise

• If you increase the mA to minimize the noise you increase patient dose.


How Noise Effects Contrast


Artifacts • Image lag – continuous emission of light from the screen after

the radiation beam has been turned off.

• Vignetting - reduction of an image's brightness or saturation at the periphery compared to the image center.

• Veiling glare – light is scattered in the intensifier tube

• Distortion artifacts:

– Pincushion

– S distortion

– Barrel Distortion


10/26/2015

8

Artifacts – Vignetting


FALL-OFF OF BRIGHTNESS

AT PERIPHERY (EDGES)

OF THE IMAGE

Artifacts – Veiling glare


Scatter in the form of

x-rays, light &

electrons can

reduce contrast of

an image intensifier

tube.

Artifacts – Distortion


Geometric problems in

shape of input screen

• Pincushion –

rectangular grid used with

a round input screen

• S distortion –

electromagnetic field is

close to the intensifier

Fluoroscopic Television Chain


10/26/2015

9

Fluoroscopic Television Chain

Video Camera

• Television pick-up camera

• Charged Couple Device (CCD) – more common

– More compact

– No image lag

– No spatial distortion

– High dynamic range 3000:1

• Connected to the image intensifier by an image distributor

• Converts light to an electrical signal

Display Monitor

• Cathode Ray Tube (CRT)

• Liquid Crystal Display (LCD)

• Scanning: – Interlaced - odd/even

– Progressive – sequentially (important in digital fluoro)


VIDEO CAMERA - CHARGE-COUPLED DEVICE

Charge-coupled

device is mounted

to the output

phosphor of the

image-intensifier

tube and is coupled

through fiber optics

or a lens system


DISPLAY MONITOR

Conventional Fluoroscopy System Digital Fluoroscopy System

Interlaced Mode Progressive Mode

Signal-to-noise ratio 200:1 Signal-to-noise ratio 1000:1

Conventional Fluoroscopy System

• Usually a 525-line system

Limitations restrict application in digital techniques

1. Interlaced mode of reading the target of the TV

camera can significantly degrade a digital

image

2. Conventional TV camera tubes are relatively

noisy (compare signal-to-noise ratios on table)


DISPLAY MONITOR

Interlaced Mode

• 2 fields

• 525-line system/2 = 262½

lines

• 262½ lines are read

individually in 1/60 s (17 ms)

to form a 525-line video

frame in 1/30 s (33 ms)


10/26/2015

10

DISPLAY MONITOR

Progressive Mode

• The video signal is read and the

electron beam of the TV camera

tube sweeps the target

assembly continuously from top

to bottom in 33 ms

• The video image is formed

similarly on the TV monitor with

no interlace of one field with

another occurs


Interlaced Mode

Progressive Mode

DISPLAY MONITOR


Compared to cathode ray

tubes (CRT), flat panel

monitors are:

1. Easier to view

2. Easier to manipulate

3. Provide better images

4. Light in weight

5. Easy to See

6. Easy to mount or

suspend in an

angiographic room

Digital Fluoroscopy with Image Intensifiers


• Projecting a radiographic image on an image-

intensifying fluorescent screen coupled to a

digital video image processor.

DIGITAL FLUOROSCOPY

Advantages

• Low dose fluoroscopic imaging (digital average,

last frame hold)

• Pulsed fluoroscopy and variable frame rate

• Speed of image acquisition

• Postprocessing to enhance image artifacts

• Uses hundreds of mA settings compared to 5Ma

or less in conventional

• Digital Subtraction Angiography (DSA) and non

subtraction acquisition and display

• Image distribution and archiving, PACS


10/26/2015

11

DIGITAL FLUOROSCOPY IMAGING SYSTEM If x-ray tube were energized continuously

thermal overloading would cause tube failure

patient dose would be high or exceeded quickly

Pulse-Progressive Fluoroscopy

Images obtained by pulsing the x-ray beam


DIGITAL FLUOROSCOPY IMAGING SYSTEM

PULSE-PROGRESSIVE FLUOROSCOPY

INTERROGATION TIME

• Time required for

the x-ray tube to be

switched on and

reach selected

levels of kVp and

mA

EXTINCTION TIME

• Time required for

the x-ray tube to be

switched off

High frequency generators

must be fast enough to

have interrogation and

extinction times of less than 1 ms



PULSE-PROGRESSIVE FLUOROSCOPY

DUTY CYCLE

• The fraction of time the x-ray tube is energized

• The illustration shows the x-ray tube is

energized for 100 ms every second which

equals a duty cycle of 10%

(100/1000 = 0.1 = 10%)

• Can result in significant

radiation dose reduction



Operating Console: The right side module

contains:

• Computer-interactive

video controls

• A pad for cursor and

region-of interest

(ROI) manipulation • May use trackball, joystick

or a mouse instead


10/26/2015

12


Monitors • Two or more monitors are

used

• One is used to edit • Patient data

• Examination data

• Annotate final image

• One is used for subtraction

images


Computers in Digital Fluoroscopy

• Digital fluoroscopy employs the

use of minicomputers and

microprocessors

• Computer capacity is an

important factor in determining:

1. Image quality

2. The manner and speed of

image acquisition

3. The means of image

processing and manipulation


DIGITAL SUBTRACTION ANGIOGRAPHY

• Usually image storage occurs in primary

memory where data acquisition and transfer can

be as rapid as 30 images per second

• A system might be capable of acquiring 30

images per second in the 512 x 512 matrix

mode



Data Transfer Limitation

• If a higher spatial resolution image is required

and the 1024 x 1024 mode is requested, then

only 8 images per second can be acquired

• Limitation on data transfer is imposed by the

time required to transport enormous quantities of

data from one segment on memory to another


10/26/2015

13

Digital Fluoroscopy with Flat-Panel

Detectors (FPD)


• FPDs are used in regular radiographic imaging.

• When used in Fluoroscopy it is referred to as Dynamic FPD

IMAGE CAPTURE - FLAT PANEL IMAGE RECEPTOR

Flat Panel Image Receptors (FPIRs)

• Composed of cesium iodide (CsI) / amorphous

silicon (a-Si) pixel detectors

• Much smaller and lighter and is manipulated

more easily than an image intensifier

• Provides easier patient manipulation and

radiologist / technologist movement

• There are no radiographic cassettes



• In contrast to an image-intensifier tube, a flat

panel image receptor is insensitive to external

magnetic fields

May allows advanced application in:

• Cardiology Radiology

• Neurovascular Radiology

• Interventional - Vascular Radiology

• Image-guided catheter - magnetic tip in vessels is

manipulated remotely by two large steering magnets

located on either side of the patient



DF equipped with

a flat panel image

receptor


10/26/2015

14

POSTPROCESSING – LAST IMAGE HOLD


Displays the last image continuously when the x-ray

beam is turned off

POSTPROCESSING – TEMPORAL FRAME

AVERAGING

• Averages the current frame with previous frames to reduce the noise in the image.

• Reduces noise by 44%

• This is sometimes called “over-sampling” an image


Postprocessing - Edge Enhancement


A.Original image

B.Blurred image

C.A and B digitally

subtracted

D.C is added to the

original (A) image

to produce the

edge-enhanced

image

Postprocessing Images

• Image contrast can be enhanced

electronically using subtraction

techniques

• Subtraction techniques provide

instantaneous viewing of the subtracted

image during passage of a bolus of

contrast medium


Digital fluoroscopy provides better

contrast resolution through

postprocessing of image subtraction.

http://en.wikipedia.org/wiki/File:Cerebral_Angiogram_Lateral.jpg

10/26/2015

15


DSA Techniques

1. Temporal Subtraction (used most often)

2. Energy Subtraction

3. Hybrid Subtraction (combines temporal and

energy subtraction)


Temporal Subtraction

Two methods are commonly used to obtain the

temporal subtracted image are:

1. The mask mode

2. The time interval difference mode (TID)



Mask Mode –Temporal

Subtraction


Mask mode results in successive

subtraction images of contrast vessels.

DIGITAL SUBTRACTION

ANGIOGRAPHY

Mask Mode – Temporal Subtraction

A, The preinjection mask.

B, A postinjection image.

C, Image produced when the preinjection mask is

subtracted from the postinjection image.



10/26/2015

16

Time-Interval

Difference Mode –

Temporal

Subtraction


Time-Interval Difference mode produces

subtracted images form progressive

masks and following frames

DIGITAL

SUBTRACTION

ANGIOGRAPHY

Time-Interval

Difference Mode –

Temporal

Subtraction

Misregistration

Artifacts - due to

patient motion occurring

between the mask

image and a

subsequent image.


DIGITAL SUBTRACTION

ANGIOGRAPHY

Energy Subtraction

• Based on abrupt

change in photoelectric

absorption at the K

edge of contrast media

compared with that for

soft tissue Illustration shows the

probability of x-ray

interaction with iodine,

bone, and muscle as a

function of x-ray energy


DIGITAL SUBTRACTION

ANGIOGRAPHY

Energy Subtraction

• When the incident x-ray energy is sufficient to

overcome the K-shell electrons binding energy

of iodine, an abrupt and large increase in

absorption occurs

• Graphically, this increase is known as the K absorption edge


The probability of photoelectric

absorption in all three decreases

with increasing x-ray energy.

DIGITAL SUBTRACTION

ANGIOGRAPHY

10/26/2015

17

Hybrid Subtraction

• Combines temporal and energy subtraction

techniques.

• Produces highest quality digital fluoroscopy

images if patient motion can be controlled Section III: Chapter 6 RADT 3463 Computerized Imaging 65

DIGITAL SUBTRACTION ANGIOGRAPHY PATIENT DOSE – DIGITAL FLUOROSCOPY

• Should result in reduced patient dose

• Uses pulsed beams to fill one or more 33-ms

video frames

• mA settings are higher with digital fluoroscopy

but the fluoroscopic dose rate is lower than

continuous analog fluoroscopy



PATIENT DOSE Approximate Patient Dose in Representative Fluoroscopic Examinations

Patient Dose

Imaging Mode Conventional Digital

5 minutes

fluoroscopy

20 rad

(200 mGy)

10 rad

(100 mGy)

3 spot films-

normal mode

0.6 rad

(6 mGy)

0.2 rad

(2 mGy)

3 spot films-

mag 1 mode

1.0 rad

(10 mGy)

0.3 rad

(3 mGy)

Total dose 21.6 rad

(216 mGy)

10.5 rad

(105 mGy)


QUESTIONS??

chapter 1 hardware anatomy and physiology of a …radpacs.weber.edu/images/r_christensen/provo/radt...

Documents