chp 6 -143 sem 2

Medical x-ray units can be classified as either

diagnostic or therapeutic

time selection controls length of exposure

timer circuit

rotor switch changes speed of rotating anode

stator

controls kvp level through kvp selection

autotransformer

Diagnostic procud:

General procedures, head procedures, fluoroscopy, cardiac cath

Therapeutic:

Treatment

Range of diagnostic units: tube current:

10 to 1,200 mA

Range of diagnostic units: exposure time

0.001 to 10 seconds

Range of diagnostic units: potential difference

25 to 150 kVp

Range of therapeutic units: tube current

below 20 mA

Range of therapeutic units: exposure time

1 to 60 minutes

Range of therapeutic units:x-ray energy

4 to 40 meV

most common xray tables called

Bakelite or Carbon graphite fiber

usually found on fluoro tables. Also called ‘dished’.

curved tables

advantages of curved tables

more comfortable, allows the body part to be placed closer to the film,

Decrease in OID

disadvantages of curved tables

difficult to keep the patient in an oblique or lateral position, useless for tabletop radiography, unable to perform decubitus or cross-table exams

most in-table grids are_____ which will

“reciprocating” -- this will blur out grid lines

90 – 15 table-tilts

90 degrees in one direction (feet down) and 15 degrees in the other direction (head down)

most tables are at a fixed height (usually

30 to 40 inches or 75 to 100 cm)

Ancillary equipment has -

foot board: used for gastro studies, shoulder supports: used when head

tilted down, hand grips, compression bands: May restrain or be used during exam

five types of tube supports:

overhead, floor-to-ceiling, floor, mobile, C-arm

Overhead suspension system cont. it can rotate 360 degrees around the column it can roll or pitch up to

60 degrees

the most flexible and most expensive system

overhead suspension system

cheaper and less flexible than overhead system

Floor-to-ceiling Suspension Systems

floor-to-ceiling mounted “c-arm” (also ‘U-arm’) used specifically to radiograph the cranium, facial bones, and cervical spine

“Franklin Head Units”

does not include a table patient sits between tube and image

receptor can obtain very precise angles and is easy on the patient

“Franklin Head Units”

usually contains a Bucky unit used for chests, c-spines, sinuses, AC

joints, weight bearing studies, etc.

Upright Units (Chest Stands)

upright Bucky unit with an automatic film feed from a storage box (100 14x17 films) to receive magazine great for doing upright chest films, but lousy for doing anything else

Chest Units or Chest Changers

used for dental or mandibular x-rays described with tomography summer

semester

Panoramic Units or Panoramic Tomography

Incoming line current or Main power Comes in the form of:

Single-phase power or Three-phase power (Multiphase Power)

Nearly all x-ray equipment operates on

210-220V

U.S. supplied electricity rate to all buildings

60 Hz alternating current

incoming line: Alternating Current or AC , negative side

Can also have incoming Line changed to DC or Direct Current also called a full-wave rectified, single phase power

the tube will drop to zero (every time it crosses the line on the sine wave) 120 times a second using an incoming line of 60 Hertz

Single phase

Three phase incoming Line, alternating current, will drop To zero

Three phase incoming line With Direct Current, full Wave rectified

T H R E E - P H A S E I N C O M I N G L I N E W /

D I R E C T C U R R E N T , F U L L W A V E

R E C T I F I E D

Every time the power begins to drop it is boosted back up by the next incoming wave of power. Voltage never reaches zero

Two circuits in an x-ray unit:

Main; tube, or high voltage circuit, Filament; low voltage circuit

The Main or tube circuit supplies

the x-ray tube with modified power and to provide

potential difference (kVp)

The Main or tube circuit purpose

to produce x-rays.

The Filament circuit supplies

the filament with modified power.

The Filament circuit purpose

create enough current to thermionically emit the proper amount of

electrons

Modifies incoming line power to produce thermionic emission from the filament wire.

the filament circuit

The incoming voltage is stepped down to _____ in filament circuit and Amperage (current) is modified to ______

6-12, 3-5

When you set the mA station (100, 200, 400, etc.) you are actually adjusting the number of _____flowing through the filament circuit

amps

The amperage/current in the filament circuit is adjusted with a

variable resistor (rheostat) – mA selector

Once the technique is set and the exposure begins the current is sent to the ______ to be modified before it reaches the filament.

step-down transformer

The tube (main) circuit modifies the incoming power so that the radiographer can control the

voltage, amperage, exposure time

Turn Power on and off

main switch

Prevent overload (fire)

circuit breakers

A remote-controlled device to allow the flow of electrons through the circuit that activates the rotating anode

exposure switch

Which side of circuit will radiographer stand?

low voltage side Exception is mobile radiography where the cord

must be 6 feet minimum.

Three types of exposure timers:

electronic timer, mAs timer, automatic exposure control (AEC)

most complicated and expensive, but the most accurate timer most common timer used in modern radiographic equipment

electronic timer

operates by charging a silicon-controlled rectifier (SCR) which then terminates the exposure and accurate to 1 millisecond

electronic timer

used in capacitor discharge units

mAs timer

A falling load generator will

utilize the tube’s loading potential to a greater extent

D I S A D V A N T A G E O F U S I N G F A L L I N G L O A D

G E N E R A T O R :

the operator will not know what mA the machine will select and must rely on the mAs timer to terminate the exposure Also, shorten tube life due to higher mA setting being used, shorten tube life due to higher mA

automatic exposure controls (AEC) or automatic exposure devices (AED) are programmed to

automatically terminate the exposure when a predetermined exposure has been reached

they only control the exposure time – the mA and kVp must be set

AEC

- two types of AECs:

Phototimer or photomultiplier and Ionization chamber

older device that used a thyratron to regulate the exposure its placed under the patient and the cassette

phototimer

the problem with phototimer AEC

photomultiplier is behind the cassette, it is often inaccurate

process of phototimer AEC

light energy to electrons by photomultiplier tube and when the number of electrons reaches a preset value, the exposure is terminated

ionization chamber must be____ thin and radiolucent and it is placed _____

5mm, between the patient and the cassette

in the ionization chamber the x-rays

ionize the gas in the chamber and the liberated electrons cause a current to a wire to the timer in circuit

in the ionization chamber when the predetermined amount of current

is produced the exposure is

terminated

All AECs have a Minimum Reaction Time of

0.05 second 1/20th sec

the Minimum Reaction Time of AECs is

the length of time necessary for the AEC to respond to the radiation and for the generator to terminate the exposure

ionization chambers are more efficient at

providing the exact time necessary to produce the density required

will terminate the exposure at a specific time even if the AEC does not read the predetermined exposure and should always be set when using AEC

Backup Time

the backup time cannot exceed the tube limit and it should be set to _____ of the anticipated manual mAs

150% (x mAs by 1.5)

if you think you may need about 50 mAs – set the backup timer for

75 mas

U.S. Public Law 90-602 states that generators must terminate the exposure at ____mAs for exposures ____ 50 kVp and ____mAs for exposures _____ 50 kVp

600 above, 2000 below

All of the technologist-operated controls are located on the ____ voltage side of the circuit to minimize the hazard of electrical shock.

low

The main and filament circuit combine here to complete the basic circuit.

control console

Monitors electricity coming in.

line monitor

Keeps constant source of power.

line compensator

control console consists of

line monitor, line compensator, rotor/exposure switch, mA selector(rheostat), time selector, kvp selector (autotransformer)

Used to increase kVp level for x-ray production

Step up transformer

helps prevent you from being electrocuted

ground

what measures current

mA meter

Changes AC to DC

Rectification bridge

used in filament circuit to increase current flow to the cathode

Step down transformer

Single-phase (1Ø) generator

Full-wave rectified current produces 2 pulses per cycle or 120 pulses per second, the voltage drops to zero twice per cycle, 100% “ripple”

Three-phase, six-pulse (3Ø 6P) generator

Full-wave rectification produces 6 pulses per cycle or 360 pulses per second, Current never drops to zero,13 to 25% ripple

Produced approximately 35% MORE average photon energy that single-phase

Three-phase, six-pulse (3Ø 6P) generator

Three-phase, 12-pulse (3Ø 12P) generator

12 diodes, 1 wye, and two delta connections 4 to 10 percent ripple

Produces about 40% more average photon energy than single-phase

Three-phase, 12-pulse (3Ø 12P) generator

Uses AC to DC Converters to change the frequency of the incoming current from 60 Hz to about 6,000 Hz

high frequency

1

main breaker

2

exposure switch

3

autotransformer

4

timer circuit

5

high voltage transformer

6

4diode rectification circuit

7

filament circuit variable resistance

8

filament step-down transformer

9

xray tube