Digital Radiography Basic Concepts n Image Quality Concepts

Digital Radiography

Basic Concepts n Image Quality Concepts – Spatial Resolution (limiting resolution) – Noise: Quantum Mottle n Nature of the Digital Image – Spatial Digitization – Analog-to-Digital Conversion n Digital Radiography Factors – Spatial Digitization and Resolution – ADC and Noise – ADC and Dynamic Range

Basic Concepts: Limiting Resolution n Limiting Resolution (simplest form): n Measurement and Units: Bar pattern refers to the smallest, closely spaced objects for which separate images can be seen

Basic Concepts: Limiting Resolution n Limiting Resolution (simplest form): n Measurement and Units: Bar pattern Measured using bar pattern (lead strips separated spaces) and expressed as smallest visible bar size or highest spatial frequency (line-pairs/mm) n Sources Blurring in radiography: – Focal spot (all types of radiography) – Motion (all types of radiography) – Receptor blur - depends on receptor by

Radiography Image Receptor Blur

Dual Receptors (Screens)

Noise and Image Quality

Image Noise: Quantum mottle n Quantum mottle (QM) refers to the “graininess” of x-ray images n QM is caused by using a limited number of xray photons to make an image n QM interferes with ability to details n Using more photons (more m. As) reduces noise but increases radiation exposure

The Nature of the Digital Image Basic Concepts: Resolution and Noise n The Digitization Process n – Spatial Digitization – Analog-to-Digital Conversion (ADC) n Radiation Dose, Noise and Resolution – Resolution versus Dose: receptor thickness – Dose versus Image Noise (Quantum mottle) n Dynamic Range

The Digitization Process n Every “image” starts out in analog form: – “light” image emitted by screen – “light” image from intensifier output phosphor – TV camera voltages – Stimulated light from computed radiography n Analog “image” must be converted (digitized) to matrix of pixels stored as binary numbers – Spatial digitization: generation of pixels – Analog-to-Digital Conversion (ADC)

Spatial Digitization (pixels): Sampling Must “measure” image along many rows (512, 1024, etc) and at many point along each row n Sampling done by: n – detector with discrete “elements” (eg, CCD camera, flat panel detector) or – Raster scan process

Matrix Size, Resolution and Bytes Regular Film/Screen: 5 line-pairs/mm n To “Equal” with Digital Image: n – – n 5 lp/mm = 10 pixels/mm (to see 5 bars+5 spaces) 35 x 43 cm (14 x 17”) image = 350 x 430 mm at 10 pixels/mm = 3500 x 4300 pixels 3500 x 4300 x 2 bytes/pixel (16 bits/pixel) = 30 MB Digital Radiography – Typically 2000 x 2500 pixels maximum (~3 lp/mm)

Digital Spatial Resolution

Spatial Digizitation: ADC

ADC and Noise: How many bits?

Contrast vs Latitude (Dyamic Range)

ADC and Dynamic Range n Suppose we have: – 10 bit ADC: (1024 graylevels) – 1000: 1 dynamic range (e. g. we can measure and record exposures from 1 m. R to 1000 m. R (1 R): n Need 1 m. R difference for different graylevel – Differences between structures to see in image may be < 1 m. R in x-ray intensity reaching the receptor n Alternatives: – “throw out” some dynamic range (limit range) – Increase number of bits (still uncommon)

Digital Detectors Cassette-based: Image Storage Phosphor (CR) n Image Intensifier n Scanned Projection n Direct Digitizing (Full Field) n – – – n CCD Camera Selenium Flat Panel (“Direct” Digital Radiography) Phosphor Flat Panel (“Indirect” Digital Radiography) Future Technology

Digital Detectors Cassette based Image Storage Phosphor (CR) n Image Intensifier n Scanned Projection n Direct Digitizing (Full Field) n – CCD Camera – Selenium Flat Panel (“Direct” Digital Radiography) – Phosphor Flat Panel (“Indirect” Digital Radiography) n Future Technology

CR Clinical Use

Conventional CR Scanning

Flying Spot CR Scan In a conventional flying spot CR reader, stimulated output exposure (scan level) from the IP is proportional to the laser intensity I and dwell time Td

Absorption Efficiency

CR Blur

Dynamic Range (Latitude) n Dynamic Range, or latitude refers the range of exposures which provide useful diagnostic information. For film, is the range of exposures that provide acceptable optical densities (ie, not too dark and not too light)

Dynamic Range

Dynamic Range (Latitude) CR vs Film n Dynamic Range (latitude): range of exposures providing useful diagnostic information Regular F/S: 16: 1 (between 0. 5 and 2. 5 OD) (exposure yielding 2. 5 OD is 16 x exposure yielding 0. 5 OD) n CR: >10, 000: 1 (between minimum and maximum measurable scan levels) n

CR and “Film Density” 1/8 x E 4 x. E

Radiation Dose with DR n How much is enough ? – Image Noise (Quantum mottle) – required image quality n How much is too much? – Patient radiation exposure concerns – possible saturation of parts of image (all black)