RAD 354 Chapt. 28 The Digital Image Spatial resolution Contrast resolution Contrast-detail curve Pt. dose considerations

Spatial Resolution The ability of an imaging system top RESOLVE and RENDOR on the image a HIGH CONTRAST object of items in close proximity and similar atomic mass density (shades of gray differenced in close objects to one another)

Spatial FREQUENCY Refers to LINE PAIR! – Line pair per millimeter (lp/mm) The HIGHER the spatial frequency, the HIGHER the SPATIAL RESOLUTION! – In digital imaging, spatial resolution is limited by PIXEL size.

Line Pair Device

Modulation Transfer Function (MTF) The ability of an imaging system to render objects of DIFFERENT SIZES ONTO AN IMAGE (RATIO OF IMAGE TO OBJECT DUE TO SPATIAL RESOLUTION)

Contrast Resolution Shades of gray making various atomic mass densities visible – The HIGHEST possible contrast is TWO shades of gray = BLACK AND WHITE!

Dynamic Range (OD) Dynamic range is really the OD as shown on the H & D curve = ranges from base + fog (.2 or lower for film; and “base OD for digital” = 0) to the solarization point The number of shades of GRAY that an imaging system can produce (remember, the human eye can only see about 30 different shades of gray)

NOISE Image noise limits contrast resolution (SNR) Post processing of the image allows for manipulation and visualization of shades of gray (window and leveling – also called “smoothing”)

Digital Imaging Pt. Dose Digital imaging should reduce pt. dose (in reality, it has done the reverse!) – the “if in doubt, burn it out” syndrome and post image manipulation plus “dose creep” have significantly INCREASED patient dose in digital imaging

Misc. Information Focal spot size determines spatial resolution in film/screen systems Post image manipulation = contrast resolution in digital REGARDLESS os pt. dose Pt. dose in digital SHOULD be LOW because of DQE (probability of a photon interacting with an image receptor due to atomic number, etc.) DQU = measure of x-ray absorption efficency

RAD 354 Chapt. 29 Viewing the Digital Image Photometric Quantities Hard/Soft Copy Active Matrix Liquid Crystal Display Pre-Postprocessing digital image PACS

Photometric Quantities Response of the human eye – Photopic – Scotopic

Luminous Flux Total intensity of light from a source Measured in “luminous flux” in LUMENS

Illuminance Intensity of light incident on a surface – Goes back to the measure of “candle light” on a single square foot of surface

Luminance Intensity Amount of light emitted by a light source – A viewbox is a good example

Luminance Measure of the BRIGHTNESS of a source (like in a digital display) Measured in CANDELA per meter squared = NIT

Cosine Law Relate to inverse square law and the amount of radiation at varied distance – Same for loight at GREATER distances – like the CR vs PERIPHERY of the beam (Pathagora’s Theory – A 2 + B 2 = C 2 )

Hard – Soft Copy Hard = radiograph on film Soft = Viewing image on a cathode ray tube (computer/TV monitor, etc)

Active Matrix Liquid Crystal Display Instead of using a “cathode ray tube” (TV type tube) viewing system, the active matrix, PIXEL by PIXEL, viewing monitor is used Spatial resolution IMPROVES with larger matrix (PIXEL) displays

Preprocessing the Image YOU can program the computer with what YOU want the image to look like (convert an analogue image {radiograph} into a digital image) i.e. Do we want a hand with a DYNAMIC range of 2? – Perhaps a CXR with a dynamic range of 60? – Shows what SCALE of contrast and shaded of gray to display – At the workstation, you tell the computer what anatomic area you radiographed and the computer shows it on the screen as it should look like (hand vs CXR)

Postprocessing Annotation (NOT l or R!!!) upright, decube, etc Window and leveling (expand or shrink the shades of grays)

PACS Ann – Tell your thing!!!