Lecture 5: Fluoroscope and Digital Imaging Concepts

Digital Units

Gray tones are based upon attenuation coefficients

Smallest absolute object sizes

Resolved by units with spatial frequency of 3.0 is 0.17 mm

Basic components of fluoroscope

The image intensifier and xray tube

More radiation

Will absorb more light therefore it will be lighter

Function of the photocathode and image intensifier

Works like film screens, amplifies light photons creation per xray incident

Major change in digital imaging

The image receptor

Special factor in digital radiology

The ability to change contrast and exposure after the xray has been taken

Scanning

Sampling

Quantization

Matrix

Picture element

Each cell is called a picture element, or aka pixel

Numeric value in radiographic images

Stored in each cell represents brightness

Larger matrix

Greater the number of pixels = Sharper the image

Increasing number of pixels

Decreases the pixel size leading to increased resolution

Increasing displayed field of view

Generally decreases

Displayed Field of View

Smaller pixel size

Higher resolution

Ability to resolve small high contrast objects

Describes Spatial Resolution

Units for spatial frequency

Line pairs per millimeter

Measurement of image resolution

Given in LPs/mm of length is Spatial Frequency

Spatial frequency derived from

Minimum object size

Smallest absolute object size reproducible

Is inversely proportional to 1/2 the spatial frequency

Bit depth

Quantizing values assigned to each pixel

Is pre-designated for a gray level

Bit depths correspond to

Different levels of brightness

Pixel value calculation

Is calculated by 2^n

Range of gray levels for images

Is dynamic range

Standard Radiography spatial resolution

Approximate is 8 - 12 lp/mm

Digital Radiography spatial resolution

Approximate is 6 - 8 lp/mm

Film based radiography subject contrast difference

Requires at least 10% due to poor contrast resolution

Digital imaging software contrast perception

Can perceive adjacent structures with a subject contrast as low as 1%

Digital image receptors

Are either computed radiography or direct digital capture

Original Fuji digital receptor

Was known as computed radiography

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

Is both immediate release of light and trapping of electrons

Film and computed radiography

Are both latent image

Types of Direct Digital Radiography

Are Indirect and Direct Conversion Detectors

Scintillator in Direct Conversion Detectors

Is responsible for converting x-ray photons into light

Photodetector function

Converts light into electrical charge

X-ray directly converted to electrical charges

Describes direct conversion detectors

DR Direct FP capture element

Uses Amorphous Selenium

Most common photoconductor in digital radiation

Is Selenium