Imaging test 2 (The x-ray tube)

what are the different types of x-ray tube support systems?

• ceiling support system (most frequently employed)

• floor support system (like the one in lab)

• Floor to ceiling support system

• C-arm support system (most versatile)

Biplanar C-arm angiography system

2 C-arms working together, 90 images simultaneously

what are some parts of an X-ray tube and housing

• expansion diaphragm

• lead cup

• cathode insulator

• xray tube insert

• stator

• rotor

• port

• lead lined shield

• oil

• stator terminal

• anode insulator

• anode

• cathode

what is an x-ray tube lined with and why

Lead lining, for extra focal radiation that’s being produced in an x-ray tube (it absorbs)

what does the expansion diaphragm do

oil pushed through the holes in the lead cup which goes to the expansion diaphragm to keep it inside

• its made of rubber so it allows the oil to flex and expand

what does the oil do inside the x-ray tube housing

The oil inside the x-ray tube housing serves as insulation and cooling. It absorbs heat generated during x-ray production and prevents electrical discharge.

what is the purpose of the tube housing

• holds tube insert

• provides radiation protection

• provides electrical insulation

• contains cooling oil

what is the tube housing made of

made of aluminum and lined with lead

what does electrical insulation protect

it protects you from getting electrocuted because electricity takes the shortest route to ground

what does the tube housing contain

• contains oil and bellows diaphragm

• cooling oil

• opening called a “port” (corresponds to location of the window of the insert)

• collimator attaches to it (at the bottom of housing)

where is the only place that x-rays are allowed to escape the tube from

port

what does the vacum pump do in the x-ray tube

pumps out all the air in the x-ray tube and then seals it off with heat

what is the purpose of a cathode?

create electron cloud to be pulled over to anode to get an x-ray

the insert or envelope (glass or metal) can also be called

it is a vaccumm/ diode tube but it can also be refered to as a thermionic diode (it heats up to boil electrons; two odes, cathode and anode)

why is the vacuum tube made of glass (pyrex)

it has a high melting point, meaning it can withstand high temperatures

why would we use metal for the vacuum tube

• its a little more expensive

• ceramic insulation

• smaller overall size

• larger anode disc

• more surface area for heat to spread out while its spinning meaning its easier to cool

• can expand at different rates without breaking

whats the window

thinner portion under the anode

to produce an x-ray photon you need

1. source of electrons

2. a way to accelerate those electrons to ½ the speed of light

3. abruptly stopping those electrons

what is the cathode

the negative side of the x-ray rube and is composed of a filament and a focusing cup

what is a filament

a small coil of wire (~2mm in diameter and 1 or 2cm long) while hot

• part of the cathode

what are filaments usually made of

thoriated tungsten

what does the filament do?

provides a source of projectile electrons for the creation of x-ray at the anode

• (part of cathode)

what is thermionic emission?

an electrical current is sent through the filament, heating it and causing outer shell electrons to boil off in the vacuum

why do we use a filament made of thoriate tungsten

because Thorium is stronger and helps to prolong its life

• high melting point

• does not vaporize easily

• easily drawn into a thin wire

What is a dual filament in a cathode tube?

In a cathode tube, a dual filament refers to the presence of two filaments that can be independently energized, allowing for different levels of brightness and enhancing the functionality of the display.

• allowing for a large and small focal point

purpose of a small focal point

A small focal spot provides better image detail (higher spatial resolution).  

purpose of a large focal point

A large focal spot allows for higher heat dissipation, enabling the use of higher tube currents (mA) and thus higher X-ray output.

what is happening to the cathode and its filament when you rotor?

current to the filament is being boosted when going to the first position (rotoring) on the exposure switch

• the cathode is getting hot and electrons are being boiled off

filament current (amperage) controls what

the tube current

• higher the mA, hotter the filament gets creating a bigger electron cloud

where are filaments embedded

in a small metal cup called the focusing cup

• Its purpose is to focus the stream of electrons emitted from the heated filament into a narrow beam directed towards the anode

what is the charge of the focusing cup and why

negatively charged

• because of electrostatic repulsion. Since electrons are negatively charged, the focusing cup is given a negative charge to repel them.  

• This repulsive force causes the electrons to converge into a tighter beam, ensuring they strike a small, defined area on the anode (the focal spot).  

what would happen if the focusing cup was positively charged

it would attract the electrons, and they would not form a focused beam

focus cup is modified and actually becomes the

exposure switch

what is the grid (focusing cup)

This grid is used to rapidly switch the X-ray tube on and off, allowing for precise control of the X-ray beam

• the focusing cup is modified and actually becomes the exposure switch

• the charge on cup (now referred to as the grid) controls flow of electrons across tube therefore controlling exposure time

what does the cathode structure provide?

source of electrons

whats the primary way heat dissipates through anode on the surface?

thermal radiation

what is the anode

the positive side of of the x-ray tube

what does the anode do?

it conducts current, radiates heat and supports the target material

anodes can either be stationary or

rotating

what is a stationary anode good for

• its cheaper

• extends the life of the x-ray tube

• it is good for dental x-rays and mammography

why can’t we use a stationary anode for general x-ray

it gets too hot

what are stationary anodes designed for

low dose/ short exposure applications

• cheaper and can compliment a compact, small tube design

whats a perk of a rotating anode

it dissipates heat better

• more expensive than stationary anodes

• bearing mechanisms can wear and become problematic

atomic number of tungsten

74

what is a rotating type anode made of

graphite coated with a tungsten/rhenium alloy, and backed with molybdenum

what is the part that is actually struck by electrons

the focal spot or the focus of the area

on a rotating anode disc what is known as the area struck by electrons

focal track

why is tungsten chosen as the target material for a rotating anode tube

• atomic number (Z #)

• ability to readily dissipate heat

• high melting point

what do anodes typically rotate for most radiographic techniques?

3400 RPM

what do anodes typically rotate for high speed rotation for high mA techniques?

10,000 RPM

the _____ of the anode connects the anode to the rotor

stem

what is the stem usually made of and why

molybdenum for its poor heat conducting properties

what rotates the anode

an induction motor

what 2 parts do the induction motor consist of?

1. rotor

2. stator

what causes the induction motor to spin

stator magnets are energized sequentially and attract ferromagnetic strips embedded in the rotor causing it to spin

what is the stator?

• The stator is the fixed, non-rotating part of the motor.  

what is the coast time for a new tube?

~60 secs

what happens if there is a grinding noise when rotating

bearings are broken and it needs to be replaced

what is typically the target in a stationary anode

tungsten alloy block embedded in copper

what is the line focus principle

Allows for high heat capacity with a small effective focus

• The line focus principle is a concept in radiography where the x-ray tubes produce a narrow beam of x-rays that focuses on a line, rather than a point, enabling sharper images and improved detail in the radiographic film.

anode cooling depends on

actual focus

is a bigger or smaller effective focus better?

smaller

what kind of focal spot will you get with a big bevel angle?

a big focal spot

• we don’t want this

what 2 things happens when the bevel angle is too small

1. anode heel effect

2. heats up too quickly

what is the anode heel effect

Intensity of the X-ray beam is not uniform across the field

• “The variation in intensity along the long axis of the tube due to absorption of the beam by the heel of the anode

the intensity of the beam is always greatest towards which end?

cathode end

How can the anode heel effect be used to your advantage?

it allows for optimized imaging by positioning the thicker body part towards the cathode side of the x-ray beam, resulting in higher image density and improved contrast due to the variation in x-ray intensity across the beam

(ex: in T spine)

whats an ideal bevel angle

12-17 degrees

do you get a bigger or smaller anode heel effect when you have a bigger bevel angle

smaller; less anode level effect

bevel angle affects

• anode heel effect

• size of the useful field

how to effectively use the Anode heel effect to your advantage:

• position the thickest part of the anatomy under the cathode

• use larger field size (14×17) to accentuate and emphasize the effect

• use short SID (using more divergent rays, thus producing a more noticeable anode heel effect)

what is off focus radiation?

the production of x-rays outside the focal spot

• extends the size of the effective focus

• reduces image contrast somewhat

• can result in exposure to tissues outside the collimated field

what is the fundamental reason that x-ray tubes age and fail

excessive heat

• remember that 99% of electron energy is converted into heat at the anode

what are the 3 way heat dissipation occurs

1. conduction

2. convection

3. radiation

what is the average general duty/ heat capacity the x-ray tube has before it will begin burning up ?

350,000 HU (heat units)

how are heat units calculated for a single phase unit?

HU= kV x mA x time

how are heat units calculated for a 3 phase/ high frequency unit?

HU= kV x mA x time x 1.35

why do we multiple by 1.35 for a 3 phase/ high frequency unit?

because there is 35% more heat

what are some anode problems due to heat?

• pitting on surface of the target

• cracked anode (max exposure on a cold anode)

• bearing ceased (grinding noise and decreased coast time- less than 1 min is really bad)

what are some cathode problems due to heat?

thinning of the filament wire

• may result in breakage and therefore no radiographic exposure

what is the primary cause of general problems in x-ray tubes due to heat?

vaporization of tungsten from the cathode

what is a potential consequence of tungsten deposits forming on the inside of the tube insert

increased inherent filtration

what does gassing in an x-ray tube result in

loss of vacuum and unreliable mA readings

what is the most common reason for replacing x-ray tubes is

arching

• electron beam jumps down to tungsten on inside of tube

what is perforation caused by

repeated arching

what are some protective devices on an x-ray tube

• heat monitors (monitor with thermometer)

• over load protection circuitry (won’t let you expose until it cools down)

• water cooling (expensive and requires plumbing but it is used instead of oil cooling)

what is the wamr up procedure

• 3 consecutive exposures using AP abdomen technique

• OR

• newer units initiate an automatic tube warm up protocol at start up

why do we want to use short boost times (rotoring)

it reduces the heat applied to filament

what are 3 type of rating charts

1. tube rating chart

2. anode cooling chart

3. housing cooling chart

what is a tube rating chart

tells you what your maximum exposure time that you can use will be for a certain kV and mAs value

what is the purpose of a cooling chart

tells you how long it will take for the tube to cool down after so many heat units

failure may occur due to overheating of entire tube assembly in a general fluoroscopic tube due to

long exposure times even though mA values are low (2-4 mA)

• high kV= low mA

how do grid controlled x-ray tubes work

• the focusing cup acts as the grid (chokes off electron flow)

• voltage applied to grid controls flow of electrons through the tube and therefore controls the exposure time

• used when short exposure times are required (digital fluoro)

metal insert tubes

• insert made of metal

• use of ceramic electrical insulation

• high heat capacity

• insert grounded therefore reducing arching and better control of off-focus radiation

• used where high heat capacities are reached (CT scanners and interventional units)

what are straton x-ray tubes

the whole tube rotates and is bathed in oil

• bearings are replaceable since they are located outside of the tube