OH 022425 Digital Imaging -How does it work?

Digital imagine is an __________ production of radiographic image

Electronic

Digital imagine uses a _________ instead of silver halide film

Sensor

Pixels

Image composed of thousands of picture elements

Digit

A number that represents each pixel

Each pixel represented by a digit corresponding to ___________

Intensity of radiation absorbed by the corresponding area on the sensor

What is pixel gray scale proportional to?

The digital value for display on the monitor

The darker a digital number is, the closer the number is to _____

Zero

- Lighter it is= closer to 255

Exposure

X-rays directed through patient

Remnant radiation

Pattern of radiation that exits patient

What is remnant radiation recorded on?

Electronic sensor as altered pattern of electrons (voltage)

Voltage

Altered pattern of electrons

What is the relationship between voltage at each pixel and x-ray exposure?

Proportional

- More exposure= greater voltage

- Less exposure= lower voltage

Latent image

Pattern of voltage on the sensor

- Sensor has info that represents pt's tissues, but can't see image yet

Sampling

Analog-digital converter (ADC) samples voltage in small discrete groups

Quantization

ADC then assigns a digit to each voltage level

- Number is assigned to each sampled voltage

Where is a gray scale applied/assigned to?

- Pixel numbers (digits)

Where each pixel is given a degree of darkness from black to clear, proportional to digital value

Gray scale

Assigned to pixel

What are the five steps in creation of digital image?

1. Exposure of pt

2. Voltage on sensor

3. Sampling

4. Quantization

5. Gray scale

What are the two types of digital imaging sensors?

1. Solid state sensor=

- Charge-coupled device (CCD)

- Complementary metal oxide semiconductor (CMOS)

2. Photostimulable phosphor (PSP) sensor

T/F: CMOS sensor is being discontinued

False

- CCD is being discontinued in favor of CMOS sensor

In CCD, where is the silicon layer arranged?

Pixel matrix

How does exposure occur with CCD?

X-rays hit CCD, break covalent bonds freeing electron (photoelectric effect), creating electron-hole pairs

What is voltage like for CCD?

Photoelectrons trapped in "charge packets"

In CCD, each charge packet corresponds to _________

One pixel

What is sampling like for CCD?

"Bucket brigade" into compute through a cable

- Voltage of each pixel sample and quantized by analog/digital converter

In CCD, gray scale assigned to pixels ___________

Based on digital value

In CMOS, what is exposure like?

X-rays hit and ionize silicon atoms

In CMOS voltage, what is each pixel connected to?

Transistor

What is sampling like for CMOS?

Each pixel voltage read directly by the ADC (no bucket brigade)

What is quantization and gray scale like for CMOS?

Signal sent to computer through a cable

What is Photostimulable Phosphor Plates (PSP) also called?

Storage phosphor sensors

What is the PSP like?

- 100% re-usable

- Same size as film

- Flexible

- Thin

- No wires

T/F: PSP sensor is connected to computer by cable

False; it is not

What are the pros and cons of PSP sensor?

Pros= easier placement (like film)

Cons= plate must be removed from mouth and scanned→ delays appearance of image

What is the material used for PSP image plate?

Plate of barium fluorohalide in crystal lattice (the phosphor layer)

Europium ions added as imperfections

What is exposure like for PSP?

- X-rays excite Europium electron to metastable state beyond valence band (the F center)

In PSP, what is the relationship between the number of electrons in the F center and x-ray exposure?

Proportional

- Lower exposure, fewer electrons

- Greater exposure, more electrons

What is the PSP scanned by?

600 nm (red) laser

In PSP, trapped Eu electrons released, _________

Emit light

In PSP, what is light converted to?

Voltage

In PSP, the voltage is ____________ by ADC

Sampled and quantized (digitized)

In PSP, each digital value given a ______________ on computer screen

Gray scale

When should the sensor be processed in PSP? Image will be _________

- As soon as possible

- Too light

With PSP, after the sensor is scanned and the latent image become visible, ____________ floods the sensor to move any remnant Eu electrons out of the F center

White light

With PSP, white light ______ the latent image

Erases

With PSP, what do you do with erased sensors after they fall out of the scanner?

Pick up with bare hands

What are digital image properties?

- Spatial resolution

- Contrast resolution

- Detector latitude

- Dynamic range

What does spatial resolution depict?

Fine detail

What is spatial resolution measured in?

Line pairs/mm

(lp/mm)

Spatial resolution: More lp/mm visible indicates _________ resolution

Better

What does spatial resolution depend partly on?

Size of pixels

- Smaller pixels permit better resolution

Contrast resolution

Ability to distinguish varying degrees of darkness (density)

- Reproduce differences in attenuation

Contrast resolution: ____________ of radiation in tissues

Differential attenuation

Contrast resolution: Does soft or hard tissue absorb more x-rays?

Hard tissue (radiopaque; light)

- Soft tissue absorbs fewer (radiolucent; dark)

Contrast resolution: What does digital radiography (DR) usually capture?

8-bit gray scale (256 shades of gray from black to clear)

Contrast resolution: What pixel value is represented as black?

0

Contrast resolution: What pixel value is represented as clear?

255

Contrast resolution: Digital systems have sufficient contrast resolution at _____________

256 gray shades (8-bit contrast)

What is detector latitude?

Range of x-ray exposures

- Greater latitude= can still see structures even at low or high exposures

Which sensor has greater latitude?

PSP

Detector latitude: What does very low exposure lead to?

Insufficient radiation to produce a signal that differs from noise

Detector latitude: What does very high exposure lead to?

Excessive radiation saturates the image; can't distinguish signal from noise

What does dynamic range indicate?

Range of anatomic structures

What is dynamic range closely related to?

Detector latitude= range of exposure settings

For image display, what must computer monitor show?

- Line pairs/mm

- Dynamic range

Compare the two hard copy image displays: Film Printers and Paper Printers

Film= prints image on transparency

Paper= prints image on paper

What should you be careful for when sharpening an image?

Can make noise look like signal (caries)

What is DICOM standard?

Open data standard whose common principles and guidelines are implemented by manufacturers to communicate digital info in diverse hardware and software environments

What is the current DICOM standard?

DICOM 3.0

T/F: Dentists should ensure that their equipment is DICOM compliant

True

Which sensor is double exposure impossible?

Solid-state digital

Which sensor is it impossible to be backwards?

Solid-state