dawha-components of x ray machine

8/6/2019 Dawha-components of x Ray Machine

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(A)SKETCH THE DIAGRAM OF A ROTATING ANODE X-RAY(A)SKETCH THE DIAGRAM OF A ROTATING ANODE X-RAY

TUBE SHOWING THE INSERT AND THETUBE SHOWING THE INSERT AND THE HOUSINGHOUSING

(B)DESCRIBE THE FUNCTION OF EACH MAJOR(B)DESCRIBE THE FUNCTION OF EACH MAJOR

COMPONENT.COMPONENT.

(C)DISCUSS WHY A CHANGE IN THE TUBE FILAMENT(C)DISCUSS WHY A CHANGE IN THE TUBE FILAMENT

HEATINGHEATING CRRENT PRODUCES A CHANGE IN THECRRENT PRODUCES A CHANGE IN THE

CURRENT.CURRENT.

(D)ROTATING ANODE IS OFTEN PREFERRED TO(D)ROTATING ANODE IS OFTEN PREFERRED TO

STATIONARY ANODE X-RAY. GIVE REASONS.STATIONARY ANODE X-RAY. GIVE REASONS.


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(B) MAJOR COMPONENT INCLUDE THE FOLLOWING:

1. The rotating anode2 The cathode

3. Glass envelope4. Tube Housing.

A brief overview on an x-ray tube. It is a component of the x-ray machine which basically consists of cathode and the

anode enclosed within a glass envelope.

types(i) Single filament/focus tube contains one filament

(ii) Dual filament/focus tube- Has two filaments(iii) Grid controlled Has Voltage controlled connections on the projectile electrons

which turns on and off quickly to produce very short exposure times used in digitalprocedures. e.g. fluoroscopy.


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.

It is described as an energy converter, receiving electrical energy at thecathode and converting it into x radiation (1%) and heat(99%) at theanode end.

. The process involves generation of electrons by thermionic emission fromthe filament of the cathode which are then accelerated to a high speedand driven across to the anode by an applied Voltage power across the

two ends.. The fast moving electrons are then suddenly brought to stop at the

anode, thus losing their kinetic energy in the form of x-radiation andheat energy.

. Heat is an undesirable by product and thus Morden x-ray tubes are

designed to maximize x-ray production and dissipate heat rapidly.


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(1) THE CATHODE . This is the negative electrode of the X-ray tube consisting

of a small coil of wire (filament) recessed in a focusingcup

. Types Single filament Cathode

_ Dual filament cathode.

. Basic Functions (i) Controlled production of electrons at the filament

through conversion of electrical energy into thermalenergy.

(ii) Directing the projectile electrons into a desiredfocusing beam size by means of a focusing cup.


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:The desired current is selected from a selector over the filamenttransformer.

:Application of a high amount of current over the filament can causethermal damage and vaporisation of the filament leading to tube failure.

A diagram.


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THE FOCUSING CUP

:It is the device which contains the filament designed to condense theprojectile electrons into a desired beam size and thus a desired focalpoint on the anode.

:Its shape and size is guided by the size of the filament positioned inside

it.

Types _ Single cup _Dual cup. Electrons are negatively charged, hence, tends to repel each other as

they travel towards the anode. The focusing cup which is also negativelychange provides a greater force of repulsion on the electrons, preventingthem from fanning out, thus bringing them into a focal stream.


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2 THE ANODE. It is the positive electrode of the x-ray tube consisting of a rotating

anode disc as the target ancord into the induction motor component.. The rotating anode is made up of bevel shaped anode disc as the

target or a base containing the surface target. It is joined to the rotor bya connecting shaft and a stator of rods outside the neck of the tube as

shown in the above diagram.. The rotor is made of copper and soft iron fabricated into one and

supported on a bearing system.. The stator consist of series of electromagnets equally spaced

around the neck of the tube. Current flowing in the stator is designed to

sequentially energise the electro magnets thereby inducing magneticfields.


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.The induced magnetic fields then acts on the rotor causing it to rotate in

synchrony with the activated stators..The connecting shaft is made of molybdenum, a strong metal with low

thermal conductively so as to prevent excessive heat from damaging thesupporting bearing which could lead to tube failure.

. The rotor and the connecting shaft are sealed in the evacuceted

glass envelope. Anode targets are usually made of tungsten for the following

reasons._ Higher atomic number (z)__ efficient x-ray production_ Good thermal conductivity

_ High melting point (3410oc)_ Low rate of evaporation

1.


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Other metals used as target include_ molybdenum __used in mammography machines._ Tungsten- rhenium alloy_ Rhodium ____ also used in mammography machines

. Copper in addition to those mentioned above is also used as anode

base material due to its excellent thermal conductivity.. Anode discs are made to rotate at 3600 r/min but high speed tubes

run at 10,000 rpm with a greater heat dissipation.accelerated projectile electrons from the cathode are then suddenly

brought to stop at the target surface of the anode losing their kineticenergy and producing x radiation and heat in the process.


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The functions of the anode are thus summarised into_ Conversion of electronic energy (in kev) into x radiation and heat._ Immediate dissipation of the heat so created in the process aided

by the rotating nature of the anode.

- It receives electrons emitted by the cathode and conducts them away

into the circuit cable._ Provides mechanical support for the target_ determining the effective focal sport size by the use of the line focus

principle.


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(3). THE GLASS ENVELOPE. It is an air-tight glass devices which is designed to tightly surround the

two electrodes of an x ray tube.. The cathode and anode assembly sealed in an air tight glass

envelope is known as the insert of the X-ray tube.

. Glass has a low relative heat expansivity, thus special alloys whichexpands at the same amount as glass when heated is used to seal thegaps between the metal and glass component.

.majority of glass tubes are mode of Pyrex glass which enables it

withstand tremendous heat that is generated in the procedure.


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. The glass tube size and shape is designed such that the twoelectrodes are positioned for enough apart to avoid electrical dischargedbetween the two. Size ranges from 20-35cm long and 15cm in diameter.

. The vacuum space provides an ideal place for effective conductionof the tube current and thus efficient x-ray production.

. The glass envelope also provide a mechanical support for theassembly.. It also provides for an exist window for the useful x-ray beam. It is

a thin section of the glass approximately 5cm square in dimension.


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4 TUBE HOUSING.It is the metal enclosure over the x-ray tube which contains a special type

of oil surrounding the tube. It also has electrical connecting wires, a fan,and x- ray beam filters.

The inside of the metal enclosure is lined with lead except over the

windows port.

. The housing provides mechanical support for the x ray tube as wellas protect it from damage.

. The housing and its lead lining prevents leakage radiation to amaximum allowed standard of 100mR/hr at a distance of 1m from thefocal spot with the machine operating at maximum capacity

. Only useful x ray beam is allowed to pass through the house

window.


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. The filament heating current describes the large current suppliedto the filament in order to produce the needed thermal energy togenerate electrons by a process called thermionic emission.

. Typically the filament current ranges from 2-5 ampere which ishundred of times larger than the tube current.

. Large current is applied to the filament to raise the thermal energy sogenerated to a glowing temperature so as to boil off orbital electronsfrom the filament.

. At this threshold thermal energy , a slight increase in the filament

current will lead to a large number of electrons to be ejected out of theirorbit forming the tube current as they are driven to the anode by a high

voltage power.


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. Filament current are controlled and selected from discrete taps on

the filament transformer.. The tube current which describes the projectile electrons is also

called the MA which is the unit of its measurement.1MA =6.24x1015

. Tube currents which ranges from 50-1000MA are produced withvaried intensities selected from fixed stations which are the samediscrete taps of the filament transformer used for the control and

selection of filament current.. The x-ray tube current is thus adjusted by controlling the filament

current.. A change in the filament heating current therefore causes a change

in the thermionic emission of electrons from the filament and thus a

change in the tube current.


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. It is important to note that 99% of energy of the projectileelectrons slamming at the anode is converted to heat and only 1% givesoff the x radiation.

. The amount of heat so produced is capable of damaging the anodeand thus causing a tube failure.

. Rotating anode are disc- shaped structures of 6-15cm diameterproviding a wider target area as it rotates .. Projectile electrons thus slam on a wider area and with the good

thermal conductivity of the anode material, which helps in speedy

dissipation of the heat so generated.. It therefore increases the heat capacity of the tube thus prolonging

tube life .

Stationary anode on the other hand has a smaller target embedded


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. Stationary anode on the other hand has a smaller target embeddedin a wider stationary anode base. This makes it prone to heat damage asmore heat is generated by streams of projectile electrons repeatedlyslamming on the same spot. tube failure is thus commoner withstationary anode.

. Because of the high thermal capacity of rotating anode tube, itaffords the use of high voltage and current to generated more energetic

x rays for desired diagnostic use.. Overall better image quantity is thus achieved with a rotating

anode machine.. Longer exposure times is better tolerated with a rotating anode

machine than a stationary anode type.. Longer operation time can equally be tolerated with a rotating

anode x ray machine.. Longer tube life is achieved.. Use of a rotating anode machines in the long-run is thus more cost

effective.


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dawha-components of x ray machine

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