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What are the principles of digital exposure?
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RAD122 - Unit 3 PP
PowerPoint Q and A
| Question | Answer |
|---|---|
| What is the goal of imaging? | to produce a quality image while keeping patient exposure to minimum |
| What are the principles of digital exposure? | -Optimal exposure to plate -mAs, kVp, SID -Scatter control (kVp, grids, collimation) -Smallest FSS -Largest practical SID -Smallest practical OID -Accurate alignment beam, part, IR -Pt modifications |
| Why does digital imaging give a successful image even when under/over exposed? | -Wide dynamic range -PreProcessing (rescaling, Look Up Table) |
| What is over exposure during digital imaging called? | DOSE CREEP |
| What is the risk of failure during digital imaging? | overdosing patient during exposure due to wide exposure range |
| Where does risk of failure occur during imaging? | During automatic rescaling |
| What is the equivalent S#, EI# and LgM # for optimal exposure of 1 mG | S# - 200 (inverse) EI# - 2000 (DIRECT) LgM - 2.2 (DIRECT) |
| What is the difference between pre and post processing? | PRE - The ability to manipulate image before display on monitor (automatic) POST - The ability to manipulate the image after display on monitor (tech can adjust) |
| What are common defects in FPD? | Deflective pixel; Image lag; Line noise; |
| What can be done to correct common defects in FPD? | Flatfielding; signal interpolation; offset voltage |
| Describe potential DR image problems. | DROPPED PIXEL - areas of no data caused by damage; ELEC NOISE - dark noise - always present; INCOMPLETE CHG TXFR - collection element doesn't completely discharge; ELEC MEM ARTIFACT - too rapid sequence of exp; UNEVEN AMPLIFICATION; BACKSCATTER |
| How are problems with DR image addressed? | preprocessing alleviates most FPD errors |
| What is Nyquist frequency? | Signal averaging over the detector aperture width; 1/2 number of pixels/mm; sampling data points 2x per cycle; DETERMINED by pixel pitch; |
| How is pixel pitch determined? | sampling frequency for cassette based PSP systems and by DEL (detector element) size in flat panel systems |
| How to ensure optimal image with lowest patient exposure before exposure? | -dynamic range (flex technique); -auto rescaling (maps image to optimal); -centering/positioning; -collimation; -beam/receptor alignment; -scatter control (grids, kVp) |
| How to ensure optimal image with lowest patient dose after exposure? | -Automatic exposure field recognition (collimation); -Histogram analysis (describes exposure indicator value); -Automatic rescaling (normalization, mapping grayscale VOI to achieve specific display levels) ; |
| What is another way to describe auto rescaling? | EXPOSURE RANGE; if you see exposure range in question, describing AUTORESCALE |
| What is automatic exposure field edge detection? | system chooses which pixel values are valid for image; eliminates signals outside collimated margins (4 edges preferred) |
| What is histogram analysis? | Uses shape of image signal distribution to determine relevant info within exposure field AND exposure indicator values (S#, EI# and LgM #) |
| What is automatic rescaling? | mapping grayscale VOI to achieve specific display levels; specifically, provides images that have uniform display brightness over wide exposure range |
| Define data manipulation. | changing exposure indicators, window width, or window level; Considered changing the LEGAL PATIENT RECORD! BE VERY CAUTIOUS with changing this info b/c alters patient data, alters image, limits image data to radiologist, you have LEGAL LIABILITY |
| CR systems - how can you segment the imaging plate successfully? | up to four images per plate, must be symmetrical; 1, 2 or 4 images per plate, |
| CR - Describe collimated edges for successful and unsuccessful exposure field recognition. | * 4 collimated borders visible, centered on receptor * 2 or 3 collimated borders visible, centered and symmetrical; UNACCEPTABLE 2 or 1 collimated borders visible, not centered on plate |
| CR - what is important in collimating? | Ensure images are evenly distributed, symmetrical in shape and relation to IR or will have errors in histogram analysis (incorrect identification of valid data for image) |
| Describe possible histogram analysis errors. | failure to accurately identify collimated borders; UNEXPECTED ARTIFACTS (PROSTHESIS, SHIELD...); WRONG HISTOGRAM ASSIGNED |
| Describe collimation histogram analysis error. | system needs a defined collimated border with minimum 2 symmetrical/centered borders, 3 or 4 preferred |
| What is image plate saturation? | a digital image, produced with greater amounts of exposure, can reach a point of saturation where image is very dark (BURNED OUT). Saturation requires 8-10x normal exposure (>3000 Kodak, <25 Fuji) |
| What does image plate saturation cause? | LOSS of contrast (this is not the same as LOW CONTRAST) |
| What types of image adjustments can be made at QC station? | BRIGHTNESS and CONTRAST -Window Level - adjust brightness; -Window Width - INVERSELY related to contrast; CHANGING PROCESSING CODE -alters histogram -limits data sent to radiologists -potential legal ramifications |
| Describe function of WINDOW WIDTH | number of values that are included in contrast; INVERSE - wider width, less contrast |
| Describe function of WINDOW LEVEL | increase/decrease brightness of pixels; DIRECT - higher window level, more brightness |
| What is the IR response to low intensity radiation? | image noise |
| Why do CR imaging plates need to be erased every 24 hrs with non-use? | because CR plates are EXTREMELY sensitive to background radiation which will compromise image quality |
| Why are CR plates so sensitive to background radiation? | contain storage phosphor. Stores the charges produced by background radiation 1.5 - 3.5 mSv/yr |
| Which is more intense - scatter or background radiation? | SCATTER; tech should control as much as possible |
| What is the best method to control scatter? | COLLIMATION |
| What is Moire Effect? | ZEBRA PATTERN artifact - double set of grid lines appear on image (2 grids used); CR reader and grid have similar frequencies or are aligned parallel to CR reader scan lines |
| What happens if sampling occurs less than 2x per cycle? | VIOLATION of NYQUIST FREQUENCY; loss of info and fluctuating signal is produced (aliasing); creates wraparound image that appears as 2 superimposed, slightly out of alignment images (Moire effect) |
| Describe Nyquist Frequency relationship to sampling. | 1/2 the sampling frequency |
| What are the causes of aliasing/Moire effect? | Double grid; scan frequency matches grid frequency; Sampling error in CR reader |
| How do you identify the causes of zebra lines? | Must verify grid setup (moving, not doubled) and that grid is not parallel to scan lines of reader; if grid is correct, then problem is in CR reader |
| Describe loss of info in CR systems. | 1. multiple conversions result in loss during transitions; 2. light spread - loss of data; 3. distance (between capture/collection) light spread - loss of data; 4. time - must be read within 1hr (charges escaping) |
| Describe loss of info in DR systems. | fewer conversions result in less opportunity for loss; Nyquist Theory still applies to ensure sufficient signal is sampled; immediate processing of signal minimizes data loss |
| what is AEC? What preceded it? | AEC - automatic exposure control; preceded by phototimer AEC is designed to terminate exposure when appropriate amount of radiation is received at receptor |
| What are the benefits of AEC? | eliminates need for radiographer to set mAs; lower patient dose |
| What are tech responsibilities using AEC? | correct technique (kVp); select correct chamber and density; center anatomy precisely over AEC |
| Describe AEC. | 3 ionization chambers in wall bucky, 5 chambers in table; various combinations of chambers can be selected based on exam; requires anatomy to be centered over chamber; inversely related to kVp (can change receptor exposure by density selection) |
| What is important when using AEC? | CENTERING anatomy over chamber |
| What happens if anatomy is not centered accurately? | image will be underexposed, exposure indicators will be affected, possibly requiring a repeat |
| What affects receptor exposure when using AEC? | -Centering anatomy; -Detector (chamber) selection; -Density control selection (increase or decrease sensitivity of AEC) |
| Describe the causes of overexposure AEC. | incorrect bucky selected; exp time < min response time; incorrect density selected; AEC malfunction; Incorrect chamber selection; prosthesis; artifact (sandbag/lead strip over AEC) |
| Describe the causes of underexposure with AEC. | backup timer too short; incorrect density selected; lack of collimation; incorrect chamber selection; incorrect positioning of anatomy (peripheral to detector, too small (hand), not centered) |
| What is backup timer? | safety mechanism designed to shut off xray if AEC fails |
| Where is the phototimer located? | behind IR |
| Where is AEC located? | in front of IR |
| Describe PHOTODIODE | Phototimer; photomultiplier tube (PMT), photocathode; x-ray exits patient and interacts with fluoro screen, converts x-rays to light, light photons enter photocathode where they are converted to electrons with same intensity as photons that created them |
| Describe AEC | Ionization chamber; between patient and IR, ionization within the chambers occur to predetermined charge, then terminates exposure |
| what are the features of AEC? | AEC is a part of x-ray circuitry minimum response time; back up timer; density controls; detector configuration |
| What is the maximum time the backup timer can continue? | 6 seconds or 600 mAs, whichever comes first |
| What is RADIANT FLUX? | radiant energy that strikes or crosses surface per unit of time or radiant energy emitted by a source per unit time |
| What is PHOTOMETRY? | The study and measurement of light |
| What is LUMINOUS FLUX? | total intensity from source (Lumens) |
| What is ILLUMINANCE? | intensity of light incident on a surface; BRIGHTNESS OF PAGE 1 foot candle (FC) metric = 1 lumen per sq/m 1 fc = 10.8 lux |
| What is LUMINANCE INTENSITY? | property of source of light; the amount of luminous flux emitted per viewing area (ex monitor) lumens; candela |
| What is LUMINANCE? | Similar to luminance intensity / another measure of brightness; the amount of luminous intensity emitted by a source of light; -candela per sq/m -nit |
| What is reflective glare? | when ambient or room lighting reflects off the surface of the monitor or display screen minimize this by turning down the room lights (using LCD instead of CRT) |
| What is veiling glare? | Orth p326/Papp p183 -light scatering in display that induces a diffuse luminance veils intended image -internal glare (inside CRT) caused by electron backscatter, lateral leakage of light from phosphor, and other scattered light **degrades CONTRAST** |
| Describe a CRT. | type of monitor used to display images; consists of vacuum tube, electron gun (cathode) w/focusing and accelerating grids and a deflecting coil for steering electron beam. Anode is reflective phosphor coating on inside of front screen; raster pattern scan |
| What is the electron gun? | emits a stream of electrons, which go through three modifications to control the shape, direction and quality of the electron streem |
| What is the electron beam? | created at the electron gun (cathode), it is the beam that delivers data to the phosphor screen anode. It scans across the phosphor screen in a left to right pattern with a slight diagonal angle called a raster pattern. |
| What is the Aluminum reflector? | the fluorescent screen that is activated by the electron beam |
| What is the difference between a spot pixel and a picture element? | spot pixel is round (CRT, electron); picture element is square (LCD) |
| How does SNR affect CRT displays? | must have a high SNR to combat the high noise levels present TV 200:1 Fluoro 500:1 Dr CRT 1000:1 |
| Describe methods of display available for digital imaging. | MONITOR LCD - liquid crystal display CRT - cathode ray tube AMLCD - plasma display MONITOR=weakest link in dig sys (resolution) DIGITAL FILM PRINT DIGITAL COMMUNICATION NETWORKS PACS, RIS, HIS, DICOM, Teleradiology, HL7 |
| Describe LCD | flat panel monitor using liquid crystals that consist of liquid crystal cells, polarizer filters, and a backlight ; Crystals change molecular orientation when electrical field is applied |
| what are the advantages of LCD over CRT? | *compact and lightweight *doesn't defocus (no electron beam) *better grayscale *less electricity/low heat *unaffected by magnets *little/no flicker *greater contrast resolution *higher luminance *greater resolution/contrast |
| Why does LCD give the appearance of greater resolution and contrast? | light is emitted at 90' angle to faceplate and there are lower black levels (min luminance) |
| What are the disadvantages of LCD over CRT? | *limited viewing angle (40' either side) called angular dependence *subject to dead pixels |
| Describe plasma displays. | flat panel plasma displays consist of active matrix of tiny fluorescent bulbs emit light from each pixel. Thin yet have a high luminance output, wide angle of viewing EXPENSIVE and short lifespan |
| What are the advantages of CRT? | *high contrast rate *excellent viewing angle *can be used in hot/cold temps |
| What are the disadvantages of CRT? | *large size/weight *prone to geometric distortions *high power consumption/high heat *susceptible to screen burn *flicker at refresh rates <85Hz *sensitive to mag interference *blurred edges *glass envelopes contain lead/barium (shielding) |
| Describe digital communication networks | PACS - stores imaging RIS - radiology info sys (internal documentation, not for pt) HIS - hospital info sys (pt record EMR) DICOM- header used to transfer info btwn diff imaging modalities HL7 - Comm std to allow exch alphanumeric data, integrates tex |
| Describe PACS | *picture archiving and communication system; *divided into acquisition, display, and storage system; *Communication btwn modalities *Use common language DICOM |
| Describe DICOM | Digital imaging and communications in Medicine universal std for exch medical images; specifies std formats for info objects and for stds for workflow |
| Describe RIS | radiology info system supports functions within the medical imaging dept - ordering/scheduling exams, maintain pt database, transcription, reporting and bill prep decreases possibility of improper ID exams |
| Describe HIS | hospital info system (EPIC in this area) contains the patient's full medical info from billing to inpatient ordering; any possible service to patient |
| Describe EMR | Electronic Medical record houses the patient's private health records including prior imaging or medical procedures, lab results, pathology reports, notes from Drs/Nurses, etc; allows interface with ancillary services to view reports in common format |
| Describe HL7 (Health Level 7) | communication standard configured for info systems PACS, RIS, HIS, and EMR can exch alphanumeric data. integrates textual data with patient demographics, admission, and discharge orders, types of procedures, radiology reports, admin info, clinical data |
| Describe characteristics of LCD displays. | liquid crystals are linearly arranged by electrical current to dipole state 2 thin glass plates with liquid crystals between, one glass has rows, the other columns |
| What is contained in a DICOM header? | *patients name *ID info *date of procedure *ordering MD *location of service |
| What are important concepts in medical imaging? | Patient demographics: tech markers: prep of image for interpretation |
Created by:
Larobbins
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