Diagnostic Imaging - Interaction of Ionizing Radiation w/ Tissue (Image Production Processing) Flashcards
What are the different digital processing systems for images?
CR - computed radiography processing
DR - digital radiography processing
Compare CR and DR.
CR: cheaper, more time consuming, need to remove/replace IR, less efficient QDE, more artifacts, less portable, cassettes easier to replace, prone to dust, damage of CR reader/PSP
DR: more expensive, faster, no need for repositioning, better QDE, less artifacts, very portable, IR sensitive to cold/damage, better qlty image
Describe the process of CR.
Primary excitation: xray passes through upper layer to phosphor, reacts w/ electrons w/i phosphor, exciting them to higher energy levels, majority of electrons drop back to initial state, remaining get trapped, stores image w/i plate.
Secondary excitation: plate removed from cassette & scanned by laser beam, beam stimulates trapped electrons, causes light photons to be emitted. Light produced channeled to photomultiplier using optics
Photomultiplication
Digitisation
Plate erasure
The number of light photons produced by secondary excitation is directly proportional to
the number of electrons that were trapped in the storage phosphor (in CR)
How should an imaging plate be maintained?
- store away from radiation
- store cassettes in an upright position
- check imaging plates for damage
- dust-free environment
- erase imaging plates before use
- screen cleaning
- dirt on intensifying screens or PSP plates can appear as artefacts & mimic pathology
What are practical disadvantages of CR?
slower than DR
not as portable, less useful for off-site imaging
CR reader artefacts
PSP need to be replaced/maintained
need to erase cassettes prior to using
need to keep PSP clean & dust free
need to be processed relatively quickly
prone to scatter radiation
IDR stands for…
indirect digital radiography
IDR transforms x-ray energy to
light, then transforms light into an electric impulse
What are the two types of IDR?
Charge-coupled device
Flat panel detector
What two chemical compounds are used in a scintillator?
Gadolinium oxysulfide (Gadox)
Cesium iodid (CsI)
Flat Panel Detector uses
photodiode and TFT
photodiode converts light into
an electrical signal
TFT converts electrical charge into
an image displayed on monitors
How should flat panel detectors be maintained?
- wireless system batteries/ tablets must be charged
- may need to warm up detector to required temp
- careful handling systems/ electric board
What is DDR?
Direct digital radiography
What takes place in DDR?
Photodiode converts x-ray energy to electrical signal which is then converted to TFT to produce digital image. Then sent to PACS
What are characteristics of the optimal image?
- demographic info
- correct markers
- good positioning and centering
- geometric integrity/sharpness
- radiation safety/protection
- optimal exposure, brightness, contrast & grey scale, with minimal noise
- no preventable artefacts
How is the spatial resolution in CR?
Size of pixels detected by field of view
how is the spatial resolution in DR?
array of detectors linked together to form matrix, in which individual detector elements form the pixels of the matrix
Contrast is affected by
- exposure factors - kVp/mAs
- subject region of interest
- dynamic range of imaging system
- artefacts
- quantum mottle/noise
In digital systems, contrast is a result of…
selecting appropriate look-up table setting, providing an appropriate scale of displayed contrast for exam selected
Image contrast is affected by…
subject contrast
Contrast digital processing means
the dynamic range is wider, copes better w/ wider range of exposure
Underexposure means the image has a…
grainy appearance/quantum mottle
Overexposure is often tolerated because
software brings image to required greyscale value
What does it mean to window the image?
matching the dynamic range to maximise the contrast
Window level is the
midpoint of the greyscale values
Window level controls
image brightness
Window width is the number of
shades of grey
Window width controls
image contrast
Exposure indicators/index (EI) is the measure of
the amount of exposure received by the image receptor
EI is dependent on
mAs
total detector area irradiated
beam attenuation
EI is an indicator of
image quality
What is wide exposure latitude?
Range of exposures used to obtain very wide diagnostic images
What is the benefit of wide exposure latitude?
fewer repeat films due to incorrect exposure settings
What is the risk of wide exposure latitude?
overexposure of image
What does increased dynamic range do?
Allows assessment of soft tissues and bone in the same exposure by altering the image in the monitor
What is post-processing of images?
the ability to manipulate the image to change the contrast, brightness, edge enhancement, etc.
What are some common problems with technique?
CR reader artifacts
algorithm used
positioning and centering
exposure
What makes a noisy image?
pixelated, mottled
Quantum noise often causes…
underexposure, so the image appears pixelated
How can you fix quantum noise?
increase exposure to improve image qlty
Electronic noise is caused by…
photon noise
Saturation due to electronic noise can lead to overexposure which means…
too many photons reach the receptor
Chessboard patterns can be caused by…
overexposure
Radiographs should not be repeated in digital radiography due to…
mild to moderate overexposure
brightness
contrast
Radiographs may need to be repeated for…
gross overexposure
underexposure
Scatter radiation decreases
tissue contrast
What are ways of reducing scatter?
- collimate the primary beam
- Choose best exposure factors
- use a grid
What are the best exposure factors in DR?
Lower KV means more photoelectric effect, greater contrast, less Compton interactions = less scatter
When should you use a grid in DR?
if tissue depth is >10cm or >15 cm in the thorax
What is a grid?
a number of parallel strips of lead held apart & parallel to each other by x-ray-transparent interspace material
What is the function of a grid?
to reduce scatter affecting IR/radiographs
Where do you place a grid?
above IR, under the patient
What is a linear grid?
- lead strips are parallel
- decrease in number of photons at the periphery because more are absorbed
What is a focused grid?
lead strips are angled progressively from the center
What is an advantage of a grid?
increases contrast by reducing scatter reaching IR
What is a disadvantage of a grid?
increases exposure factors
must be properly aligned
+/- grid lines
when buying a DR system, what are some things to consider?
Cost
reputation of supplier/panel manufacturer
Where to have it serviced
warranty on detector/tablet
storage of images
can you trial the system?
