Principles of Radiographic Imaging 6th edition. CH 5 "The X-Ray Tube"

What are the conditions required to make an x- ray

~electrons

~appropriate target material

~high voltage

Vacuum

What does the tube consist of

Cathode

Anode

Enclosed envelope

protective houseing

What is the function of the Cathode ?

1. To produce thermionic cloud.

2. Conduct high voltage to the gap between anode and cathode.

3. To focus electrons toward anode.

what makes up the Cathode Assembly?

1.filament or filaments

2.focusing cup

3. wiring

Is the Cathode + or -?

- negative

What is the filament and where does if reside?

Is a small coil of thin thoriated Tungsten wire.

In the focusing cup

Approximately how thick, wide, and long is the Tungsten filament?

0.1-0.2 mm thick, coil is 1-2 mm wide, and 7-15 mm long.

True or false; Most tubes have dual filaments?

True

What is the melting point of Tungsten?

3,370 degress

True or False; The length and width of the filament has an effect on the x-ray to image fine details?

True

What is the function of the filament?

To provide sufficient resistance to the flow of electrons so that the heat produced will cause thermionic emission to occur.

True or False; Old tubes have a mirrored appearance due to the permanent vaporization of electrons from the filament?

True

What are two problems that can occur due to the vaporization of electrons from the filament?

1. reduce the vacuum

2.cause high voltage aching to the deposits on the glass and can destroy the tube.

True or False; When 10% has been vaporized from the filament it becomes subject to breaking?

True

What percentage can "delayed exposures" shorten tube life by?

50- 60% (5,000-12,000)

In what year did William D Coolidge develop the coolidge tube for the general electric corporation?

A.1920

B.1950

C.1915

D.1925

C. 1915

What else did William Coolidge contribute besides the coolidge tube?

the focusing cup, imbeded Anode target, and anode cooling devices.

What is the focusing cup?

a shallow depression in the cathode assembly designed to house the filament

What is the purpose of the focusing cup?

to narrow the thermionic cloud as it is driven toward the anode

The focusing cup is made of?

Nickel

Does the focusing cup have positive or negative charge?

negative

What is a biased focusing cup?

It maintains the cup at a more negative voltage then the filament.

What is the space charge effect?

As more and more electrons build up in the area of the filament, their negative charges begin to oppose the emission of additional electrons.

What is the maximum range in mA for xray tubes as a result of the space charge effect?

1000-12000

What is the saturation current?

as kVp increases a greater percentage of the thermionically emitted electrons are driven toward the anode

Is the anode positive or negative?

positive

what is the function of the anode?

~Target surface for voltage electrons from the filament

~conducts high voltage from cathode back to the x-ray circuitry.

~Primary thermal conductor.

What does the anode assembly consist of?

anode, stator, and rotor

What are the two types of anode?

stationary and rotating

What is the anode made of? and approximate size?

tungsten-rhenium alloy

molybdenum/ graphite

5-13cm

True or False; it is never important to use a warm up procedure for the Anode?

False

What is the target area?

Precise point at which the photons are created.

focal track

Tungsten focal area around the beveled edge of the rotating anode.

focal spot

Small area on the target on the anode toward which the electrons from the focusing cup of the cathode are directed. X-rays originate at the focal spot.

actual focal spot

the physical area of the focal track that is impacted

effective focal spot size

area projected onto the patient and the image receptor

line focus principle

used to reduce the effective area of the focal spot

True of False; Effective focal spot size is linear with Actual focal spot size?

True; is controlled by length of filament

What is the common angle for target angle?

12 degrees, but can range 7-17 degrees.

As angle is decreased=smaller focal spot size; what is a disadvantage to this?

It can limit size of the primary beam field at short source-to-image reception distance.

True or False; x-ray tube focal spots are square.

False, rectangular

Diagnostic tubes are available with what size of focal spots?

0.1-3mm

Most x-rays have dual-focus spots; why?

One for fine detail and the other for heavy tube loads

Focal spot size increases or blooms as mA is increased?

yes

What is the anode heel effect?

The rotating target is angled thus producing a varied intensity. The intensity of the xray is greater at the cathode side compared to the anode side. Since intensity and density decrease on the anode side of the film place the thinner portion of what is being xrayed towards this end (anode heel effect)

x-rays are produced and emitted at what angles?

45%

The anode side will absorb photons thus creating a weaker beam. A total variation of approximately________parallel between Anode and Cathode axis.

A. 30%

B. 45%

C. 50%

B. 45%

how can the anode heel effect be used to our advantage when x-raying a part with varying thickness

The Cathode end can be placed over the thicker part while the Anode side can be put over the thinner side.

What is the stator?

Induction motor electromagnets comprise the stator and turns the anode.

Where does the Stator reside?

Outside the vacuum of the envelope

True or False; If the stator fails, the rotor will cease to turn the anode?

True

What is the rotor?

Located inside the stator and envelope, the cuff is the true rotor that is affected by the electromagnet field of the stator causing it to turn

What is the rotor composed of?

a hollow copper cylinder, or cuff that is attached to the anode disk molybdenum shaft.

What is the common speed and high speed of the rotor? (rpm)

3,200-3600 rpm for common, 10,000-12,000 rpm

What is inside the rotor?

Steel ball bearings around the shaft anchored to the envelope

High speed anodes can shatter glass around the 5000- 7,000 rpm range. How is this controlled in the rotor so this doesn't shatter the envelope?

Direct current is run directly through the stator to quickly stop the rotor through this dangerous range.

True or False; Can bad bearings be caused by long use at high temps?

True

What is the envelope?

external covering for the anode and cathode assembly.

What is the envelope comprised of?

Glass or Metal, metal becoming more popular as it eliminates the problem of tungsten evaporization.

What is a vaccum?

Primary function is to maintain vacuum between Anode and Cathode.

How is the vacuum created between the Anode and Cathode?

Air is removed until pressure is 10 -5mm mercury (Hg) is achieved. Done through vacuum pupmp through special vent then permanently sealed

How is the vacuum efficient?

By letting electrons flow from Cathode to Anode without encountering gas atoms of air.

What does the protective housing do?

Controls leakage and scatter radiation, isolates High voltage, and provides a means to cool the tube.

What is the protective house comprised of and what does it do?

Cast steele and lined with lead, capable of absorbing most unwanted electrons

What has a window to permit exit for useful photons?

The protective housing

What is leakage radiation?

radiation that emerges from the leaded tube housing in directions other than that of the useful beam. must not exceed 100mR/hr at 1 meter

What is between the envelope and tube housing, and what does it do?

dieletric oil

it insulates high voltage form tube housing and absorbs heat as well

True of False; To permit the oil to expand as it is heated, it is sealed with and expandable gasket?

true

What is off-focus radiation?

Radiation that is not produced at the focal track.

True or False; Off focus radiation photons can cause ghosting of structures adjacent to the edge of the primary beam?

True

off focus radiation can contribute as much as _____% of the total primary beam

25-30%

What is the purpose of Rating Charts and Cooling?

To help Radiographers avoid thermal damage to the x-ray tube.

What do rating charts plot?

mA, Kilovoltage, and time

A heat unit is calculated as...?

kVp x mA x time x rectification constant.

What is the recommendations for extending tube life?

1. Warm up anode using manufactures instructions

2. do not hold rotor switch unnecessarily.

3. use lower mA station when possible.

4. use a lower- speed rotor when possible

5. do not make repeat exposures near tube loading limits

6. do not rotate tube housing rapidly

7. do not use tube when you can hear load rotor bearings