Tomo & AEC Flashcards
what move during tomo
tube and image receptor
Where is blur created in tomography
planes above and below area of interest
intentional motion unsharpness in tomo
blur
as the tomo angle increases, blur ___
increases
where is blur the greatest
above the focal point because OID
other names for tomo
body section radiography, planography, zonography, laminography, autotomography
most common type of tomo rotation / movement
linear
which part connects the tube to the bucky
armature
device that selects the pivotal point which the armature and image receptor rotate
fulcrum
selects the distance from the image receptor where the sharp focal plane will be imaged
fulcrum
the distance the tube travels, expresed in degrees
tomographic angle / arc / amplitude
point where the x-ray beam intersects
focal point
area of sharply defined structures
focal plane
angle created in the movement of the tube
tomographic angle
angle that beam is on during exposure
exposure angle
another name for tomographic angle
stroke distance
distance in inches the tube will travel in a certain time frame
stroke velocity
distortion of objects outside of focal plane as a result of motion of the tube / IR
blur
if the tomo angle is small, will the section be thick or thin?
thick
what determines the thickness of the sharp image section
tomographic angle / arc / amplitude, otherwise known as stroke distance
where is blur the greatest?
above the focal point
selection of the thickness of the section depends on:
the size of the part, the number of sections to be imaged through part
what factors determine the amount of blur?
distance from focal plane, distance from IR, tube travel orientation, tomographic angle
true or false: SID changes do NOT change tomographic angle
true
large or small FSS for tomo?
small
why do we use long time for tomo
allow the tube to move during the exposure
what steps need to be taken to prepare the tomo unit
inform the patient the tube will move, move tube into starting position, unlock the bucky
tomo sweeps across tissue creating streaks is known as
phantom images
takes multiple exposures onto digital plate to make slices and recreate 3D structures
digital radiography tomosynthesis
mode of tomosynthesis that takes exposures repeatedly, IR then sends each image then reconstructs
stop and shoot
mode of tomosynthesis that uses a continuous beam
sweep
another name for constant beam in tomo
analog
what reconstructs images as a 3D model or radiograph in tomosynthesis?
a computer algorithm
when few projections are acquired in tomosynthesis
blurred ripple (phantom shadows)
peripheral info is cut off (vignetting)
truncation
what causes truncation
sweeping effect of the tube
Part moves during series of exposures creates artifact called
motion
3 types of DRT artifacts
blurred ripple, truncation, motion
AEC intensifying screen converts
photons to visible light
main purpose of automatic exposure control
terminate exposure time when predetermined charge is reached
what do photocells measure
ionization and electric charge, then terminate exposure when predetermined charge is reached
ADC type used by Hodges for photo-fluorography
ionization chamber type, used to react to different areas of anatomy
meaning of ADC
automatic density controls
meaning and purpose of AED
automatic exposure device, created to terminate exposure time automatically
process of determining correct amount of intensity is reached by image receptor for a specific degree of blackening
AEC - automatic exposure control
what do you lose control over when using AEC systems
mAs, time, density
when do ionization chamber type systems terminate exposure?
when the ionization chambers reach a predetermined charge
how many ionization chambers are typically found on an image receptor
3 - 5
single purpose of ionization chamber type systems
terminate exposure time
thickness of the thin parallel plate ionization chamber
6 mm
where electrons are put into motion to trigger the termination of exposure time in ionization chamber types
electrode (wire lead)
what ionizes during exposure in ionization chamber system
air
transmits electrons in current, is charged during ionization
electrode
amount of charge in ionization chamber is equivalent to
amount of radiation hitting the chamber
where does the current travel to in order to terminate exposure time
timer circuit
which sensor will have the greatest influence on density
the sensor receiving the greatest amount of exposure
when using multiple sensors, when is exposure terminated?
when average predetermined voltage is reached
when does a photomultiplier system terminate exposure time
when a predetermined amount of charge reaches the CAPACITOR
component in photomultiplier system that will trigger the exposure timer switches
capacitor
what determines when exposure switch is triggered in photomultiplier system
amount of fluorescence at phototube
what does the phototube convert
visible light to electrons
electrons in motion are called
current
maximum capacity of charge a capacitor can hold
capacitance
purpose of a capacitor
to hold charge
what does the fluorescent screen convert
photons to visible light
brightness in fluorescent screen is directly related to
intensity of the x-ray beam
photomultiplier tube )phototube) is coated with
alkali metal such as potassium and cesium
in a phototube, the amount of current is directly proportional to
the amount of light energy
another name for thyraton or thyristor
“trigger tube”
three electrodes of the thyraton or thyristor (triode)
anode, cathode, grid
part of photomultiplier system that conducts voltage and triggers timer relay switch
thyraton or thyristor
faster trigger tube that allows 1 msec delays
thyristor
part that is triggered by the thyraton (mouse trap)
relay
purpose of the relay
sends current to the exposure switch
when is exposure switch opened
when excessive current increases potential difference on its contacts
system that combines AED with computerized exposure chart
anatomically programmed system
what does the anatomically programmed system pre-select?
kV, mA, FSS, ionization chambers used
what does AEC not control?
image quality
if sensors are collimated off, image will be over or underexposed?
overexposed
purpose of back up time
to protect the unit and patient
at above 50 kV, what mAs are generators required to terminate exposure at?
600 mAs
what percentage of anticipated manual exposure is the back up time set at
150% of anticipated exposure
types of anticipated errors that back up time protects from
inaccurate sensor selection, shielding, tube not centered
minimum response time of AEC
0.05 or 0.001 sec depending on unit and IR
Type of tomography where the patient moves but the tube does not
Autotomography
