CH. 8 - XRAY TUBE

Type of current used by the x-ray tube?

Direct current

Type of current used by transformers?

Alternating current

Another name for a positive alternation?

Forward bias

Another name for a negative alternation?

Reverse bias; inverse voltage

Another name for incoming voltage?

Line voltage

What does a line voltage compensator do?

Ensures consistent voltage to the x-ray tube and circuit; corrects fluctuations in voltage

What is the only part of an x-ray circuit that is connected to both the primary and secondary circuits?

The x-ray tube

Another name for the primary circuit?

Low voltage circuit

Another name for the secondary circuit?

High voltage circuit

What is the purpose of a transformer?

To increase or decrease voltage

What type of electromagnetic induction does a typical transformer operate on?

Mutual Induction

What are the basic components of a transformer?

- Input
- Output
- Primary coil
- Secondary coil
- Ferromagnetic core

In order for the voltage to change, what else must change?

The number of coils

What are the 2 basic types of transformers?

1. Step-up
2. Step-down

What type of transformer has more coils in the secondary than the primary?

Step-up

What type of transformer has more coils in the primary than the secondary?

Step-down

What are the 4 types of transformer cores?

1. Air Core
2. Open Core
3. Closed Core
4. Shell Core

Describe an air core transformer

2 coils of wire side by side

Describe an open core transformer

2 coils of wire, each having their own iron core

What is the purpose of the iron core found in an open core transformer?

It increases the size of the magnetic field

Describe a closed core transformer

2 coils of wire wrapped around an "iron donut"

Describe a shell core transformer

Primary/secondary coils are wrapped on top of each other in the middle of the transformer

Which type of transformer core is most commonly found in x-ray circuits?

Closed core transformer

Define: Leakage Flux

The loss of magnetic flux at the end of a coil

What is the formula of the turns ratio?

Ns/Np

What type of transformer has a turn ratio > 1?

Step up

What type of transformer has a turn ratio < 1?

Step down

Formula used to calculate voltage change (transformer law)?

Vs/Vp = Ns/Np

What type of relationship is shown with the transformer law dealing with voltage?

Direct

Formula used to calculate current change (transformer law)?

Is/Ip = Np/Ns

What type of relationship is shown with the transformer law dealing with current?

Inverse

Formula for efficiency of a transformer?

Efficiency = (power output/power input)*100

What are the 3 principle causes of transformer loss?

1. Copper Loss
2. Hysteresis
3. Eddy current

The formula (I^2)(R) is also called _______ ________?

Copper Losses

Describe Copper losses.

due to natural resistances

How do you reduce copper losses?

decrease temperature, decrease length, increase diameter, change type of material

Describe hysteresis losses.

molecular friction

How do you reduce hysteresis losses?

use core with higher permeability

Describe Eddy current losses.

small currents within the core itself

How do you reduce Eddy current losses?

laminate the core

What is the main purpose of a rectifier?

Convert AC to DC

Where does rectification occur?

After the step-up transformer and before the x-ray tube in an x-ray circuit

What is a valve tube?

Similar to an x-ray tube except it didn't produce x-rays

N-Type vs. P-Type Silicone

• N-Type: doped with arsenic; has an extra electron (valence of 5)

• P-Type: silicone doped with gallium (valence of 3)

Do electrons flow from N to P or from P to N?

N to P

Modern rectifiers are?

solid-state

Describe half wave rectification.

- X-rays are only produced during 1 alternation
- This can be produced with 0, 1, or 2 rectifiers (if only 1 is present, it must be placed on the anode side)

Describe self-rectification.

- The x-ray tube rectifies the flow itself (same as having 2 rectifiers)
- Lower energy rating
- no rectifiers

Describe full wave rectification.

- Takes the negative alternations and makes them positive
- Produces x-rays during both alternations
- Must have 4 rectifiers to achieve this

True/False: With a 3 phase 12 pulse rectifier, the voltage will never drop to zero.

True

2 ways to achieve rectification

1. suppress negative alternation at rectifier or x-ray tube

2. make negative alternation positive through x-ray tube

Modern day equipment uses what type of rectifier?

3-phase or high frequency

What are the 2 types of configurations of rectifiers?

1. Delta
2. Wye

What are the different types of circuits? (phase/pulse)

- Single phase (either half wave or full wave)
- 3 phase 6 pulse
- 3 phase 12 pulse
- High-frequency generator

What is the percent ripple for a half wave/ full wave circuit?

100%

What is the percent ripple for a 3 phase 6 pulse circuit?

13-14%

What is the percent ripple for a 3 phase 12 pulse circuit?

3-4%

What is the percent ripple for a high-frequency generator?

<1%

How many impulses in 1 second for half wave?

60

How many impulses in 1 second for full wave?

120

How many impulses in 1 second for 3 phase, 6 pulse?

360

How many impulses in 1 second for 3 phase, 12 pulse?

720

Define: Ripple

The fluctuations in voltage

What is the power rating measured in?

Kilowatts (kW)

What is the formula for power rating (of either a 3 phase or high frequency circuit)?

(mA)(kVp)/1000

What is the power rating formula for a single phase circuit?

(mA)(kVp)(.7)/1000

What are the main components of the control panel?

1. mA selector
2. kVp selector
3. Timer

Which device opens a switch to terminate the exposure?

Timer

What is the purpose of the autotransformer?

It allows you to change your kVp

What is the purpose of the filament circuit?

It allows you to change your mA

What is the purpose of the timing circuit?

It allows you to change your exposure time

What device can you replace a rheostat (of the mA selector) with?

A choke coil

What are the 2 ways to control the number of x-rays produced?

1. Change the exposure time
2. Change the mA

What are the 5 types of timers?

1. Mechanical timer
2. Synchronous timer
3. Electronic timer
4. mAs timer
5. AEC

Describe a mechanical timer.

- Basic type of timer
- Not very accurate; very inexpensive
- Like an old wind up alarm clock

Minimum exposure time for mechanical timer

250 ms or 2.5 sec

Describe a synchronous timer.

- Operates with an AC motor
- Synchronized with a sine curve

Minimum exposure time for synchronous timer

1/60th of a second

Describe an electronic timer.

- Most x-ray tubes use this type
- Shortest exposure time possible

Minimum exposure time for electronic timer

1 ms

Describe an mAs timer.

- Only timer that is located in the secondary circuit
- Located right before the x-ray tube
- No minimum exposure time

Describe an AEC timer.

- Main purpose: to control time
- The technologist chooses kVp and mA, while AEC controls the exposure time

What does AEC stand for?

Automatic Exposure Control

Which timer uses a falling load generator?

mAs timer

What is a falling load generator?

Starts at the highest possible mA and it decreases during the exposure until the wanted mAs is reached

Which 2 timers were the first to be used?

Mechanical and synchronous

What is the main purpose of the AEC?

Provide correct exposure regardless of patient size

What are the components of the AEC?

3 ion chambers placed between the patients and the image receptor

What are the 2 types of AECs?

1. Phototimer
2. Ionization Chamber

What is a PMT?

- Photomultiplier Tube
- Absorbs light, then gives off electrons --> charges a capacitor; once charged, move to thyristor and terminates the exposure

What is a backup timer?

Automatically terminates exposure to avoid tube overload

What is required when new images receptors (or grids) are purchased?

Re-calibration

What is required when new images receptors (or grids) are purchased?

Re-calibration

What does an Ossiliscope give you?

The actual sine wave

The shortest amount of time required for AEC circuitry to work is known as what?

Minimum Response Time

What does a backup timer protect?

It protects the patient from overexposure and the tube from damage

What are the typical values of an x-ray tube?

- 100-1200 mA
- 50-150 kV

What are the typical values of the filament circuit?

- 3-5 amps
- 8-12 volts

The device that allows current to flow in only 1 direction is called a/an:
A. insulator
B. conductor
C. rectifier
D. transformer

C. rectifier

The ratio of a transformer is determined by the:
A. relation to the size of the wire in the primary windings to the secondary windings
B. # of windings in the primary and secondary windings
C. amount of current in the primary circuit in relation to the secondary current
D. resistance in the primary windings in relation to the secondary windings

B. # of windings in the primary and secondary windings