digital radiography

lecture given 3/19/2026

what are the advantages of digital imaging?

elimination of darkroom and processing equipment, elimination of hazardous waste (processing chemicals and lead foil), elimination of x-ray film, instant viewing of the image, less radiation dose per exposure

lower cost per image, ability to perform image manipulation (brightness or contrast), remote electronic consultation (teleradiology), remote access of images from outside the office

what are the disadvantages of digital imaging?

high initial cost and replacement costs, image quality is not quite as good as film, sensor size can seem bulky, necessitates a computer and network, lack of hard copy without additional equipment (printer), integration of radiographic software into practice software

digital images are descrete and numeric, meaning…

spatial distribution of picture elements (pixels), gray values

*what are the 2 methods of analog to digital conversion?

sampling and quantization

*sampling

a small range of voltage values are grouped together as a single value

narrow sampling gives a better representation of the data but occupies more memory

*quantized

every single is assigned a value

what are the types of solid state sensors?

charge-coupled devices (CCD), complementary metal oxide semiconductors (CMOS), flat pannel display (FPD)

what are the types of digital sensors?

solid state detectors, photostimulable phosphors (PSP)

explain how a charge coupled device works?

P-N junction forms an electric potential

x-ray radiation releases electrons in silicon (valence band → conduction band)

these electrons flow to the P-N junction

electrodes also create a potential orthogonal to the P-N junction

electrons collect in a well (charge packet)

read out- charges are transferred from one pixel to anther in a bucket brigade fashion

read out amplifier

analog to digital converter

each charge is assigned a number corresponding to a gray level

N type silicon

has free floating electrons

P type silicon

has electron holes

*charge coupled device has ____ conversion

3 step

complementary metal oxide semiconductors (CMOS)

very similar to CCD sensors, other than the readout

a transistor is built into each pixel

charge is directly read out- faster than CCD

this technology is widely used (CPUs, digital cameras) and is cheaper than CCD

has higher noise than CCD, but catching up

what is the only difference between CCD and CMOS?

the readout!

flat panel detector

similar to CMOS but they use amorphous rather than crystalline silicone

selenium is used as direct detector

thin film transistor is underneath each pixels

made larger than CMOS or CCD

more expensive

used in CBCT

*what is the difference between flat panel detectors and CMOS?

flat panel detectors have amorphous rather than crystalline silicon

*photostimulable phosphor

polyester base and europium-doped barium fluorohalide

looks and handles like film

absorb and store energy from x-rays

release this energy as light (phosphorescence) when stimulated by another light of an appropriate wavelength (in a scanner)

erased by whitelight

*when scanning a photostimulable phosphor, what color is the laser and what color does the electron release?

red, green

*contrast

the ability to distinguish different densities in the image

created by attenuation characteristics of the tissue imaged

*contrast resolution

the number of grey values available to use from the system

created by bit depth (computers are 10-12 but the system is 16)

depends on the capacity of the image system

spatial resolution

capacity to distinguish fine detail

smallest pixel sizes ~20 microns resolution of 25 lp/mm

in practice ~16lp/mm digital, 20 lp/mm film

PSP ~7 lp/mm

*what spatial resolution does the human eye have?

6-8 lp/mm (without magnification)

*what are the units of spatial resolution?

line pairs per mm

detector latitude

ability of receptor to capture a range of exposure

full range of human tissues from gingiva to enamel

also be able to detect subtle differences

*explain the detector latitudes graphs

PSP is the most linear in response to exposure

F speed film is lower (?)

high resolution CCD is higher (?)

detector sensitivity

ability to respond to small amounts of radiation

no classification standards for dental digital detectors yet

currect PSP for intraoral imaging allow about 50% dose reduction compared to f speed film with similar diagnostic appearance

image processing

to restore, enhance, or analyze

image restoration- preprocessing

image enhancement- brightness and contrast, sharpening and smoothing, colors, digital subtraction (rarely used)

image analyses

what is an image histogram and what is the purpose?

a plotting of the grey values (x-axis) and the number of pixels that are that grey value (y-axis)

makes images easier to manipulate

how can you make an image brighter? (aka more white)

move the histogram to the right

how can you make an image darker? (aka more black)

move the histogram to the left

how can you increase contrast in an image?

increase the distance in the histogram

how can you decrease contrast in an image?

smush the histogram

do radiologists like more or less contrast?

less, gives more values aka more information

why should you invert images?

can make it easier to see caries/incipient lesions

is edge enhancement aka sharpening always a good thing?

no- if you sharpen too much you will lose too much information

what are some analysis tools for radiographs?

zoom, measurement, digital subtractions

are radiograph measurements always accurate?

no, if the image is foreshortened or elongated

image storage

image archiving and management systems- 200kb to 6MB file size (2D images)

software should prevent permanent deletion or modifying image data

backup considerations

digital imaging and communications in medicine (DICOM)

many digital systems were stand-alne systems and did not necessarily interface with each other

the american college of radiology and national electrical manufacturers association developed a standard called digital imaging and communications in medicine

assures interoperability of imaging and information systems

what are some clinical considerations for solid state intraoral detectors (CCD and CMOS)?

be careful with them as they are expensive, they are rigid, they can’t be sterilized

what are some clinical considerations for PSPs?

it is difficult to distinguish a PSP plate that has been exposed backwards

must be erased

left in a room will appear light

2-5 min scan time