Digital Imaging and Radiography Concepts

attenuation coefficients

Gray tones are based upon attenuation coefficients.

OS

The smallest absolute object sizes that can be resolved by units with spatial frequency of 3.0 is 0.17 mm.

image intensifier and xray tube

The two components that make up the basic fluoroscope that we care about are the image intensifier and xray tube.

lighter

More radiation will absorb more light therefore it will be lighter.

photocathode and image intensifier

The function of the photocathode and image intensifier is that it works like film screens, amplifies light photons creation per xray incident.

image receptor

With digital imaging, one major change is the image receptor.

contrast and exposure

A special factor that can be done in digital radiology is the ability to change contrast and exposure after the xray has been taken.

Scanning

Scanning - field of the image is divided up into an array of small cells.

Sampling

Sampling - intensity of light or radiation measured at each pixel.

Quantization

Quantization - number assigned to each pixel representing discrete, pre-designated gray value.

Matrix

Matrix is a pattern of cells laid out in rows and columns.

pixel

Each cell is called a picture element, or aka pixel.

numeric value and brightness

For radiographic images, numeric value stored in each cell represents brightness.

Sharper the image

Larger the matrix, greater the number of pixels = Sharper the image.

increased resolution

Increasing the number of pixels decreases the pixel size leading to increased resolution.

decreases

Increasing the displayed field of view generally decreases resolution.

Displayed Field of View

Displayed Field of View - The portion of imaging plate that contains relevant anatomic information and is displayed at the monitor screen.

higher resolution

Smaller pixel size = higher resolution.

Spatial Resolution

The ability of the system to resolve small high contrast objects describe Spatial Resolution.

line pairs per millimeter

What is the units for spatial frequency? line pairs per millimeter.

Spatial Frequency

The measurement of image resolution given in LPs/mm of length is Spatial Frequency.

minimum object size

Spatial frequency can also be derived from minimum object size that can be imaged.

inversely proportional to 1/2 the spatial frequency

The smallest absolute object size that can be reproduced is inversely proportional to 1/2 the spatial frequency.

Bit depth

Bit depth - The maximum range of pixel values the computer can store.

gray level

Quantizing values assigned to each pixel is pre-designated for a gray level.

levels of brightness

Bit depths correspond to different levels of brightness.

2^n

Pixel value is calculated by 2^n, where n represents the bit depth.

dynamic range

The range of gray levels made available to construct images = dynamic range.

8 - 12 lp/mm

Standard Radiography approximate spatial resolution is 8 - 12 lp/mm.

6 - 8 lp/mm

Digital Radiography approximate spatial resolution is 6 - 8 lp/mm.

10%

Because of its relatively poor contrast resolution, film based radiography requires a subject contrast difference of at least 10% to enable perception of structures.

as low as 1%

Because of the contrast enhancing capability of digital imaging software, perception of adjacent structures with a subject contrast as low as 1% is made possible.

computed radiography

Digital image receptors are either computed radiography or direct digital capture.

direct digital capture

Digital image receptors are either computed radiography or direct digital capture.

release of light

Whereas the response to x-rays in screen film systems is the release of light by the intensifying screens.

Response to x-ray interaction in computed radiology

Immediate release of light and trapping of electrons.

Response to x-rays in screen film systems

Release of light by the intensifying screens.

Film and computed radiography

Both are latent image.

Types of Direct Digital Radiography

Indirect and Direct Conversion Detectors.

Scintillator

Found in Indirect Conversion Detectors and is responsible for converting x-ray photons into light.

Photodetector

Converts light into electrical charge.

Direct conversion detectors

X-ray directly converted to electrical charges.

DR Direct FP capture element

Amorphous Selenium.

Most common photoconductor in digital radiation

Selenium.