Control room

High resolution flat screen monitors

CCD image intensifiersare easily identified bytheir shape

A modern fluoroscopic suite

Factors of Image Quality:High definition imaging systems

1. Interlaced vs. progressive scanning

3. Matrix size

5. Field of view (FOV)

4. Number of Lines and Vertical resolution

2. Bandwidth and Horizontal resolution

The next slide demonstrates the transfer of energy though the fluoroscopic imaging chain.Even the most primitive system may be digitized by capturing the video signal from thevidicon or plumbicon camera, and sending it through the analog to digital converter (ADC)The next two slides are included as a quick review of the fluoroscopic system before discussing the factors of image quality.

The transfer of energy through the imaging chain follows these basic steps.

1. Remnant x-ray to light at the input phosphor2. Light to free electrons at the photocathode3. Electrons with added kenetic energy from their attraction to the anode4. Intensified light at the output phosphor from flux and minification gain5. Light incident on the target of the camera excites electrons of the target material (globules)6. A separate source of electrons from the cathode of the camera (electron gun) scans the target and discharges the globules one by one (left to right, top to bottom).7. As each globule discharges, an electrical current flows from the camera as the video signal. Each pulse of the signal varies in intensity (modulates) in accord with the excited state of the globule it came from. 8. The video signal energizes the control plate of the cathode (electron gun) of the monitor.9. Electrons are shot toward the phosphor in the same pattern (left to right, top to bottom) as they were scanned by the camera.10. Each pixel on the monitor fluoresces with an intensity proportional to the strength of the pulse of the video signal that struck it. Remember all that?

Fluoroscopic imaging chain converted to digital

ALU

CUPrimaryMemory

SecondaryMemory

(RAM)

ADC

DAC

1011

10

9

8

Light

6

7

1

2

5

3

4

Camera lens

A digital to analogconverter is needed if the monitor is notdigital

Line 1

Line 511

Interlaced Scanning

262 1/2 Odd Lines scanned = Field 1

Line 512

262 1/2 Even Lines scanned = Field 2

Line 2

2 Fields = 1 Frame. The screen is blanked between fields, so withinterlaced scanning there is never a full picture (frame) on the screen. There are 6060 fields per second, and 30 frames. Each frame lasts for 33 milliseconds.

Interlaced scanning provides low resolutionfluoroscopic monitoringand is used for conventional(not high definition) TV

From the next slide onwe will look at high definition systems

Question: Is the interlaced scheme desirable?

Then why is it used?

What is better than interlaced?

1. Progressive Scanning

No

It is a remnant of the original technology.

Progressive scanning: Instead of the every other line,interlaced scheme, every line is scanned at 60 framesa second which requires a faster modulation of the electron beam.

Bandpass or bandwidth = Horizontal resolution

2. Bandwidth and Horizontal ResolutionProgressive scanning requires a faster modulationof the electron beam of the monitor. To write toevery pixel of every line 60 times a second the frequency of modulation (known a the bandwidth or bandpass) must be faster. Bandwidth is measured in Megahertz (MHz). High definition systems start atabout 20 MHz, compared to 3.5 MHz of commercial TV

The resulting improvement is in horizontal resolution.

3. Matrix Size

Matrix size expresses the number of pixels. A standard TV matrix is 525 x 525. A high resolution matrix is 1024 x 1024 or larger. A 1024 matrix gets into mega pixelmatrix size.

A large matrix displays better spatial resolution than a small matix.

Matrix Size vs. pixel size

Smaller pixels make an image look better when viewed close,and larger pixels provide gooddetail viewed further away.Nevertheless, matrix size alone determines spatial resolution.Matrix size and pixel size aretwo different concepts.

Number of lines = Vertical resolution

3. Number of Lines and Vertical Resolution

When the matrix size is increased there are usually morerows and columns of pixels added, though the matrixis not always square. A special purpose monitor for chest x-rays may have 4096 vertical lines of pixels, butonly 1024 across, because that aspect ratio best fits a chest. The number of pixels in the vertical rows is known as the number of lines. High definition systemshave more lines and are often referred to this way.

The resulting improvement is in vertical resolution.

5. Field of View (FOV)

Computation of Spatial Resolution

1. How large is the field of view in mm?2. How many pixels are displayed in it?

To compute you need to know

Field of View (FOV)

A simple example to compute spatial resolution

If the FOV is 100 mmand it is displayed in 400 pixels

How many line pairs are resolved?How much anatomy is displayed in each pixel?

Answer on next screen

Colorado River

The Canyon averages 19km from rim to rim.

The Grand Canyon from the Landsat satellite: Excellent spatial resolution may be measured in meters or kilometers, depending on the field of view.

100 mm/400 pixels = .25 mm of anatomy per pixel400 pixels/100 mm = 4 pixels per mm which is 2 lp/mm

AP report March 31, 2002

Quickbird satellite picture from 280 miles can resolve an object of 2 feet: a person on a golf course appears as a spot on oneor two pixels, an SUV can be distinguished from a pickup.