Chapter 3: Factors of Image Quality

1. Interlaced vs. progressive scanning

2. Matrix size

5. Field of view (FOV)

3. Vertical resolution

4. 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 not covered in the text of chapter 3, but 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 cameral 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.

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 in conjunction with thediscussion in chapter 3

Control room

High resolution flat screen monitors

CCD image intensifiersare easily identified bytheir shape

Spatial Resolution

Measured in line pairs. One line and one space is a line pair

1 mm

= 1 line pair per millimeter of spatial resolution

A Line Pair Test Tool(for Testing Spatial Resolution)

The Test Tool provides line pairs of various sizes to measure spatial resolution

Question: Is the interlaced scheme desirable?

Then why is it used?

What is better than interlaced?

Bandpass or bandwidth = Horizontal resolution

Progressive Scanning

No

It is a remnant of the original technology.

Progressive scanning, which requires a faster of the electron beams

Matrix Size (Vertical resolution = number of lines)

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 large matrix displays better spatial resolution than a small matix.

Matrix Size and 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.

Field of View (FOV)Image intensifier example

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7”

9”

11”11” mode 7” mode

Field of View (FOV)

Computation of Spatial Resolution

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

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

100 mm/400 pixels = .25 mm of anatomy per pixel

How many line pairs?

The Grand Canyon from Landsat

Colorado RiverThe Canyon averages 19km from rim to rim.

Excellent spatial resolution may be measured in metersor kilometers, depending on the field of view.

AP report March 31, 2002

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

Can you name the Factors of Image Quality?

1. Interlaced vs. progressive scanning

2. Matrix size

5. Field of view (FOV)

3. Vertical resolution

4. Horizontal resolution