test 1 Flashcards by Payton Gould

Anode Heel Effect

xrays are more intense on the cathode side of the tube. Intensity of xrays decreases toward the anode side. as xrays are emitted from anode some are absorbed by anode due to heel of target. More xrays are emitted toward the cathode. The cathode side gets put over thicker body part resulting in more even density.

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line focus principle

the relationship between the actual focal spot and the effective focal spot.
shows that by angling the face of the anode, the actual focal spot can remain large while effective focal spot is reduced in size.
greater heat capacity with good image quality

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what causes the anode to crack?

high techniques applied to cold anode.
temp increases too rapidly and the anode cracks.
be sure to warm up the tube

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what causes pitting?

excessive heat placed onto the anode.

causes surface melting and pitting of the target

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Why is tungsten used for filament and anode?

used because it has a high melting point, high atomic # (makes it more efficient in producing electrons), thermal conductivity for efficient xray production, and can withstand high tube currents without cracking or pitting.

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extra focal radiation

xrays produced from outside the focal spot. is undesirable and increases the patient dose and creates unwanted density

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off focus radiation

most off focus radiation is attenuated/weakened by tube housing and collimator located near window of glass housing

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what is oil used for?

oil acts as an insulator to help dissipate the heat

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brehms xrays

produced when a projectile electron passes by nucleus, slows down, and changes its course. it leaves with reduced kinetic energy. the loss of energy reappears as an xray. the more angle on the target, the more energy the electron has/gives off.

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characteristic radiation

created when a projectile electron knocks out an inner k-shell electron so the next shell electron drops in.
have very low energy

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characteristic cascade

outer shell electron leaves a hole, and another electron fills in and so on.

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kinetic energy becomes what when it hits the target?

thermal energy and electromagnetic energy

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4 factors of xray production

separation of electrons
production of high speed electrons
focusing of electrons
stopping high speed electrons in target

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separation of electrons

ex: thermionic emission

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production of high speed electrons

apply the kV and the kV causes electrons to travel at high speed from cathode to anode.
# of electrons is created by how much mA

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focusing of electrons

focal spot (anode)
focusing cup (cathode)

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stopping high speed electrons in target

electrons hit target, transfer kinetic energy to target, electrons slow and almost stop, electrons interfere with orbital electrons which results in conversion of kinetic energy to thermal and electromagnetic energy. 98% heat 2% electromagnetic energy

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leakage radiation must be reduced to?

less than 100mR/hr at 1m

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space charge

electrons come off from the filament during thermionic emission that form a cloud around the filament

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space charge effect

the cloud of electrons not allowing more electrons to be boiled off the filament

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thermionic emission

when outer shell electrons are boiled off and ejected from filament

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focusing cup

filament embedded in cup
allows electrons to flow freely across the tube from cathode to anode
focuses the electrons to a small area on the anode.
made of nickel/molybdenum

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anode

positively charged electrode 
has 2 types 
1. stationary 
2. rotating 
consists of target, stator and rotor.

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target

embedded in molybdenum and graphite.

angled for xrays to exit the tube.

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heat is dissipated in what 3 ways

1. convection 2. conduction 3. radiation

1. convection

fan

2. conduction

stem

3. anode

emits infrared energy

protective housing

metal tube housing | protects against accidental shock

glass envelope

provides insulation from electrical shock | dissipates heat in tube by using oil

housing window

section where xrays are emitted/ exit the tube

cathode

negative side of the tube consists of 2 parts 1. filament 2. focusing cup

filament

very small. emits electrons when heated located in the focusing cup smaller filament can't withstand too much heat used for smaller body part to get better detail use less mAs larger filament used for larger body parts. more mAs

Single phase heat unit

HU= kVp x mA x s

High frequency 3 phase 12 pulse

HU = 1.4 x kVp x mA x S

3 phase 6 pulse

HU = 1.35 x kVp x mA x S

Tube rating chart

Exposure factors lying under the curve Are Safe

1 heat unit

HU= kVp x mA x time

the heel effect: a. exists because some xrays are absorbed by the surface of the anode b. is caused by the charge difference between the anode and the cathode c. depends on the mA and kVp d. is reduced by dual focal spots

the purpose of the cathode focusing cup is to a. alter the filament size b. group the electrons for their passage to the anode c. regulate anode rotation speed d. increase the heat capacity of the tube

a molybdenum shaft is used to connect the anode to the rotor because : a. it is a less dense metal with a high melting point b. it is easily compressed c. it has a high inertia d. it produces 17.5 keV xrays

many xray tubes have 2 filaments: a. because the second filament can be used as a spare when the first burns out b. to provide 2 focal spots c. to allow cooling of the filament by alternating exposures d. to improve tube cooling by sharing the heat between 2 filaments

a technologist can control the quantity of the xrays striking the patient by adjusting the: a. mA b. kvp c. rectification d. anode material

T or F: an increase in focal spot size will increase the heat capacity of the tube

True

T or F: an increase in anode rotation speed will increase te heat capacity of the tube

True

T or F: an increase in target angle will increase the heat capacity of the tube

False

T or F: the effective focal spot will increase with an increase in the anode angle

True

T or F: the effective focal spot will increase with an increase to the anode rotation speed

False

The effective focal spot will increase when changing from 100 to 400 mA

True

the transfer of heat by ___________ is increased by mounting a fan on the tube housing a. convection b. conduction c. radiation d. convention

the heel effect is more pronounced a. with smaller SID b. with a large focal spot c. with a large target angle d. with a higher speed anode rotation

the disadvantage of a small target angle is a. insufficient heat capacity b. greater field coverage c. a greater anode heel effect d. more uniform radiographic density

thermionic emission is the emission of a. thermions b. electrons from a heated cathode c. electron from a heated anode d. xrays from the tube housing

the tube current (mA) is changed by changing the a. filament current b. anode voltage c. focal spot size d. exposure time

to extend xray tube life the technologist should 1. perform warm up exposures at the beginning of each day 2. extend the time that the boost current button is held down 3. avoid repeated exposures at or near the tubes capacity a. 1 and 2 b. 1 and 3 c. 2 and 3 d. 1 2 and 3

which chart should be consulted to ensure that adequate time has passed before making an additional exposure a. a tube rating chart b. an anode cooling curve c. a heat unit index

characteristic radiation is produced when a. electrons are stopped at the cathode b. a vacancy in an electron orbit is filled c. a vacancy in the nucleus is filled d. electrons are stopped at the anode

xray tube filtrations filter out a. low energy electrons b. high energy electrons c. low energy xrays d. high energy xrays

as kVp is increased, the production of xrays a. decreases b. increases c. remains the same

what are the 4 factors affecting xray quantity

1. mAS 2. kVp 3. distance 4. filtration

less mA =

lighter image

more mA =

darker image

longer the exposure time =

darker image

shorter exposure time =

lighter image

Inverse square law

when the distance is doubled the same radiation is spread over an area 4 times as great _i1_ = _ (d2)squared_ i2 (d1) squared

as SID decreases density...

increases

as SID increases density...

decreases

Direct square law density maintenance

when the SID is increased the mAs must be increased by SID squared to maintain constant optical density _mAs1_ = _(d1)squared (SID)_ mAs2 (d2)squared(SID

Xray quality =

kVP = penetrating power

Xray quantity =

mAs

high kVp =

more gray low contrast

low kVp =

black and white high contrast

15% rule

a 15% increase in kVp will have the same effect on the optimal density as doubling the mAs a 15% decrease in kVp will have the same effect on the optimal density as decreasing the mAs by half 10 mAs @ 92kVp will be twice as dark as 10 mAs W@80 kVp because 92kV is 15% more than 80kV

to increase density

multiply the kVp by 1.15 | makes image twice as dark

to decrease density

multiply the kVp by 0.85 | makes image half as dark

Maintaining density

if you're increasing kVp by 15% divide the original mAs by 2 if you're decreasing kVp by 15% multiply the mAs by 2

filter

any material to effectively absorb photons from the xray beam

what is the most common filtering material

Aluminum because its readily available, inexpensive and easily shaped

other filters include

copper, tin, gadolnium

Inherent filtration

filtration in the path of the xray beam | within the tube, the oil, and glass envelope

added filtration

filtration added to port of xray tube. positioned between the tube housing and the xray beam collimator

total filtration

added filtration + inherent filtration

minimum filtration is

2.5 mm of Aluminum or equivalent

half value layer

the amount of filtration needed to reduce the intensity of xray beam to one half its original value HVL evaluates the tube efficiency

beam attenuation

the reduction in xray intensity due to xray absorption and scattering occurs as a result of photon interactions with atomic structures

differential absorption

process whereby some of the xray beam is absorbed in the tissue and some passes through anatomy

backscatter

occurs when a scattered photon travels back in the direction of the incident photon or beam happens in larger patients or with higher kVp

transmission

xray photon passing through anatomic part without any interaction with the atomic structures lower kV =less xray transmission higher kV = more transmission

exit radiation

remaining xray beam leaving the patient | also known as remnant

fog

unwanted density created from scatter | obscures small details