imaging system characteristics

what is the focal spot?

the area on the anode target where electrons hit and x-rays are produced

where do the electrons hit to produce x-rays?

the anode target

what is the effective focal spot?

the projected focal spot seen from the IR

what is the difference between the actual and effective focal spot?

actual is where electrons hit the target and effective is how that focal spot appears when projected onto the IR

what is the typical anode angle range?

5-15 degrees

what is focal spot blooming?

expansion in the size of the focal spot as exposure increases

what happens to the focal spot during blooming and how does it affect image quality?

the focal spot expands which can decrease image sharpness

what focal spot sizes are commonly found in dual-focus x-ray tubes?

0.5/1.0 mm and 0.6/1.2 mm

what are the two focal spot sizes in a dual-focus x-ray tube used?

small focal spot and large focal spot

what is the typical focal spot size for a microfocus (fractional) tube?

0.3 mm or less

what imaging procedures commonly use microfocus tubes?

mammography and angiography

why is a small focal spot useful?

provides more detail

why is the focusing cup negatively charged?

because electrons are negatively charged, so the negative cup repels and directs them toward the anode

what focal spot is associated with 300 mA or higher?

large focal spot

what focal spot is associated with 200 mA or lower?

small focal spot

what is a limitation of the small focal spot?

limited heat capacity

what is penumbra?

the region of shadow at the edges of an object’s image caused by the focal spot

what happens to penumbra when the focal spot size increases?

penumbra increases

what happens to penumbra when the focal spot size decreases?

penumbra decreases

what happens to penumbra when OID increases?

penumbra increases

what happens to penumbra when SID increases?

penumbra decreases

what is the primary purpose of filtration?

to reduce patient exposure

what type of x-ray photons does filtration absorb?

lower energy photons coming from the x-ray tube

what happens to the x-ray beam when filtration removes low-energy photons?

hardens the beam

what does it mean when filtration “hardens” the x-ray beam?

it increases the penetrating ability of the x-ray beam

what is inherent filtration?

filtration that is built into the x-ray tube

what is the amount of inherent filtration in an x-ray tube?

0.5 mm aluminum (Al) equivalent

what is added filtration?

filtration that is placed in the x-ray beam to remove low energy photons

what are the two sources and amounts of added filtration?

1.0 mm aluminum and 1.0 mm collimator mirror

what is the amount of total filtration?

2.5 mm Al minimum

why are low-energy x-ray photons filtered out?

they are more likely to be absorbed by the patient rather than contribute to the image

what is the purpose of using a compensating filter?

to provide uniform radiographic density for uneven body parts

what are two types of compensating filters?

wedge and trough

what is voltage ripple?

the variation in the peak voltage of the x-ray tube waveform

what happens to exposure time when voltage ripple is lower?

shorter exposure times

which type of generator has the least voltage ripple?

high frequency

what is the voltage ripple of a high frequency generator?

1%

what is the voltage ripple of a three-phase, six-pulse generator?

14%

what is the voltage ripple of a three-phase, twelve-pulse generator?

4%

what is the voltage ripple of single-phase half-wave and full-wave generators?

100%

as voltage ripple decreases, what happens to generator efficiency?

efficiency increases

what does AEC detect?

radiation that has passed through the patient

what is the purpose of AEC?

automatically terminates exposure after a set amount of radiation reaches the sensing device

what type of device can sense the radiation in AEC?

ionizing chamber

how does an ionization chamber help AEC?

it detects radiation and converts ionization into an electrical signal that tells the system when to stop the exposure

what does AEC automatically control?

time

with AEC, what exposure factors does the technologist still select?

kV and mA

what does the technologist NOT directly control when using AEC?

time

why is proper positioning of the AEC cells important?

the selected cell must be under the anatomy of interest so it receives the appropriate amount of radiation and terminates the exposure correctly

where are the ionization chambers located?

between the grid and IR

What does APR stand for?

automatically programmed radiography

what is automatically programmed radiography (APR)?

a system that provides preselected exposure factors