Chapter 5, 6 physics

If the primary beam is collimated from one of the activated ionization chambers the resulting image will be

If the primary beam is collimated from one of the activated ionization chambers the resulting image will be

over-exposed

The minimum response time is the length of time necessary for the AEC to respond to the ionization and send a signal to

4 requirement of x-ray production

tungsten anode, high voltage, a source of electrons and a vacuum

Anode

the positive side of the x-ray tube; serves as a target surface for high-voltage electrons from the filament, conducts the high voltage from the cathode back into the x-ray generator circuitry, and serves as the primary thermal conductor

Anode assembly

consists of the anode, stator, and rotor.

Anode cooling charts

Permits the calculation of the time necessary for the anode to cool enough for additional exposures to be taken.

Anode heel effect

Due to the geometry of the angled anode target, the radiation intensity is greater on the cathode side.

actual focal spot:

the physical area of the focal track that is impacted.

Cathode

The negative side of the x0rat tube; produces a thermionic cloud, conducts the high voltage to the gap between the cathode and the anode, and focuses the electron stream as it heads for the anode.

Dual-focus

a two-filament arrangement within the x-ray tube.

Effective focal spot

the area of the focal spot that is projected out of the tube toward the object being radiographed.

Envelope

the entire cathode and anode assembly is housed within this glass or metal structure.

Extrafocal radiation

Photons that were not produced at the focal spot; also called off-focus radiation

Filament

a small coil of thin thoriated tungsten wire.

Focal point

the portion of the anode where the high-voltage electron stream will impact.

Focal spot

the portion of the anode where the high-voltage electron stream will impact.

Focal track

The portion of the anode where the high-voltage electron stream will impact. When discussing a rotating anode, this describes the circular path that will be impacted by the electron beam.

Focus

the portion of the anode where the high-voltage electron stream will impact.

Focusing cup

A shallow depression in the cathode assembly designed to house the filament.

Fractional focal spot

A very small focal spot; usually a fraction of a millimeter.

Grid-biased

A type of tube that quickly regulates the flow of electrons, producing x-ray photons.

Grid-controlled

A type of tube that quickly regulates the flow of electrons, producing x-ray photons.

Grid-pulsed

A type of tube that quickly regulates the flow of electrons, producing x-ray photons.

Housing cooling charts

Permits the calculation of the time necessary for the housing to cool enough exposure and the resultant density on the film; also known as characteristic curve, sensitometric curve, and D log E curve.

Leakage radiation

any photons that escape from the housing except at the port.

Line-focus principle

Used to reduce the effective area of the focal spot.

Off-focus radiation

Photons that were not produced at the focal spot; also called exrafocal radiation.

Protective housing

the entire cathode and anode assembly is housed within this metal structure.

Radiographic tube rating charts

a guide regarding the most common technical factor combinations that can be used without overloading the tube.

Rotating anodes

An anode assembly that turns during exposure.

Rotor

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

Saturation current

As kVp increases, a greater percentage of the thermionically emitted electrons are driven toward the anode.

Space charge effect

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

Stationary anodes

an anode assembly that is immobile.

Stator

Induction-motor electromagnets that turn the anode.

Target

the portion of the anode where the high-voltage electron sream will impact.

Tube rating charts

Provides a guide regarding the most common technical factor combinations that can be used without overloading the tube.

Window

A structure where the primary x-ray beams exit the envelope, which allows less absorption or scatter of photons.

Wiring

The parts within the cathode assembly that connect the elements of the assembly together.

rectification

the conversion of alternating current to direct current.

how to calculate Heat Units (HU)

kVp x mA x time x rectification constant

how to extend the x-ray tube’s life

• Follow recommended tube warm-up procedures

• Avoid excessive rotaring \n • Don’t take repeated exposures if the tube is reaching it \n heat limits \n • Avoid rough handling of x-ray tube head \n • Listen for unusual sounds \n • Report to service engineer

1 rectification

1.00

high frequency rectification

1.40

OFF-FOCUS RADIATION

Produces “ghosting” of image and reduces image quality

TUBE FAILURE ON THE ANODE SIDE

Bad bearings \n Overheating of the anode \n • Example: Long exposures with high mAs or repeated exposures with high mAs. \n Both caused by long use at high temperatures. \n • Listen to sounds of x-ray tube as you use it \n • Unusual sounds can be an early warning of tube failure \n Avoid rough, fast movement of tube

WARM-UP PROCEDURE

Gradually warms anode to prevent cracking \n Helps maintain vacuum \n Tube warm up needs to be done if the tube has been idle for more than 2 hours

ANODE LAYERING

• Assists with heat loading \n • Backed with molybdenum and/or graphite \n • Increases x-ray tube thermal capacity \n • Used to double heat loading capabilities

ANODE ASSEMBLY

Three functions: \n 1. Target surface for x-ray production \n 2. Conducts high voltage to maintain a closed- \n circuit pathway \n 3. Serves as primary thermal conductor

GRID-BIASED TUBES

• Precise control of thermionic cloud \n • Changes charge of focusing cup from negative \n to positive \n • Permits very short, rapid sequencing of \n exposures \n • Commonly used in: \n • Angiography \n • Pulsed fluoroscopy

TUBE FAILURE ON THE CATHODE SIDE

Tube arcing \n o Vaporized tungsten collection on envelope \n • Filament breakage \n o Rough handling of the tube

THERMIONIC EMISSION

the ejection of electrons from the surface of the wire due to increased heat, causing an electron cloud; also called the space charge cloud.

RATING CHARTS AND COOLING CURVES

Designed to protect x-ray \n tube and lengthen tube life \n o Tube rating charts \n o Anode cooling curves \n o Housing cooling curves

Automatic exposure control

a device programmed to terminate the radiographic exposure time; also called automatic exposure control.

Automatic exposure device

A device programmed to terminate the radiographic exposure time; also called automatic exposure device.

C-arm tube suspension system

A type of radiography equipment that utilizes a c-shaped arm device to support the tube and the image receptor that allows the tube and image receptor to be rotated to new positions.

Compression band:

A piece of equipment used to restrain a patient or compress abdominal tissue.

Exposure switch

a remote-control device that permits current to flow through circuit

Filament circuit

a small coil of thin thoriated tungsten wire.

Fixed

a piece of equipment that remains in one place and is movable.

What are the three types of tube supports

overhead suspension, floor to ceiling, floor suspension.

Floor suspension system

a type of radiography unit that uses a tube support column mounted on the floor.

overhead suspension

allows the tube to move around room and to be locked into place.

Floor to ceiling suspension system

a type of radiography unit that uses a pair of rail for longitudinal positioning.

Footboard

a piece of equipment used with a tilting table that allows the patient to stand when the table is upright.

Handgrips

a piece of equipment that allows the patient to grip when the table is being tilted for added support reassurance.

Head units

specialized radiography equipment used to conduct cranial studies.

Incoming-line current

The alternating current that is supplied to buildings; usually supplied in a three-phase power cycle; also called mains.

Ionization chamber

an automatic-exposure control device used to terminate the exposure after a desired exposure has been reached. is located in front of image receptor and is 5mm parallel plate chamber. it is radiolucent at correct kvps. Typically there are 3 detector positions and are outlines on the wall bucky.

Main circuit

supplies the x-ray tube with properly modified power.

Main switch

the switch that generates the power to the x-ray tube.

Mains

the alternating current that is supplied to buildings; usually supplied in the three-phase power cycle; also called the incoming-line current.

Mammography units

specialized radiography equipment for studies and imaging of the breast tissues.

Milliampere-second timer

a device used in falling-load generators and some capacitator units to monitor the product of mA and time on the secondary side of the high-voltage step-up transformer.

Minimum reaction (or response) time

the length of time necessary for the AEC to respond to the radiation and for the generator to terminate the exposure. which should be .001 seconds

Mobile systems

a type of radiography equipment that can be brought to the patient anywhere the patient may be brought to the patient anywhere the patient may be.

Multiphase power

combination of several waveforms of current slightly out of step with one another.

Overhead suspension system

a type of radiography unit that allows control of longitudinal and transverse positioning as well as vertical distance.

Panoramic dental units

specialized radiography equipment for combined tomography of fascial and dental structures.

photo timer

often used to describe all automatic exposure controls; this type of AED is rarely used on modern radiography.

Shoulder supports

a piece of equipment that keeps the patient from sliding off the table when the table is tilted head down.

Simulator units

specialized radiography equipment that allows verification of radiation therapy prior to actual treatment.

Single-phase power

power supply that allows the potential difference to drop to zero with every change in the direction of current flow.

Therapeutics

methods used to treat and rehabilitate a disease, disorder, or traumatic injury.

Tilting

a piece of equipment that is movable and can be angled according to need.

Timer

device used to end the exposure at an accurately measured preset time.

Tomography units

specialized radiography equipment that allows movement in an arc during exposure.

Urologic units

specialized radiography equipment that allows study of the urological and genital structures.

Voltage ripple

the net voltage produced during full-wave rectification. (refers to the change in voltage with each pulse.)

What are two classifications of xray tubes

Diagnostic and therapeutic

single phase

represents 1 line of current - 2 pulses per 1 hertz (cycle)

multi-phase

represents 3 lines of current - 6 pulses per 1 hertz (cycle)

half-wave rectification

blocks the electrons moving in the opposite direction

full-wave rectification

the polarity of the electrons are switched so they all flow the same way. Allows all electrons to be used so there are no gaps in the current.

phase

number of wave cu

pulse

number of voltages per wave cycle

frequency

number of pulses (when single phase turns to multiphase it increases the frequency.

There are 3 types of generators

Single-phase generator, multiphase generator, high-frequency

single phase generator

is an inadequate supply of power. is full-wave rectified. 2 pulses per hertz and 120 pulses per second. voltage ripple is at 100%.

multi-phase generator

(3 waves of current) three-phase six-pulse = voltage ripple of 14%.

three-phase twelve-pulse = voltage ripple of 4%

still not an adequate source of power and is not used in xray.