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 the vidicon 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 phosphor 2. Light to free electrons at the photocathode 3. Electrons with added kenetic energy from their attraction to the anode 4. Intensified light at the output phosphor from flux and minification gain 5. 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 CU Primary Memory Secondary Memory (RAM) ADC DAC Light Camera lens A digital to analog converter is needed if the monitor is not digital

Line 1 Line 511 Interlaced Scanning 262 1/2 Odd Lines scanned = Field 1 Line /2 Even Lines scanned = Field 2 Line 2 2 Fields = 1 Frame. The screen is blanked between fields, so with interlaced scanning there is never a full picture (frame) on the screen. There are fields per second, and 30 frames. Interlaced scanning provides low resolution fluoroscopic monitoring and is used for conventional (not high definition) TV From the next slide on we will look at high definition systems in conjunction with the discussion in chapter 3

Control room High resolution flat screen monitors CCD image intensifiers are easily identified by their 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 good detail viewed further away. Nevertheless, matrix size alone determines spatial resolution.

Field of View (FOV) Image intensifier example } } } 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 mm and 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 River The Canyon averages 19km from rim to rim. Excellent spatial resolution may be measured in meters or 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 SUV can 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