X-Ray Generators, MMED Physics, Lesson 3

X-Ray Generators

X-Ray Generators

A transformer is a device that either increases or decreases the voltage in a circuit. A rectifier changes alternating current into direct current.

                230V, 60 Hz

                Adjust mA, kVp and Exposure time

                Low voltage transformer for the filament circuit and

High Voltage transformer and Group of Rectifiers for the high-voltage circuit.

Immersed in oil that acts as an insulator and prevents sparking of various components. 

TRANSFORMERS

Filament heating requires potential difference of 10V.

Electron acceleration in the x ray tube requires potential difference of between 40 000 to 150 000 V.

Potential and Potential Difference are measured in Volts. Voltage meter measures the potential difference.

§  Primary circuit and Secondary circuit

The core of a transformer is laminated  - to prevent eddy currents – which waste power and appear as heat in the transformer core.

§  When current flows through a primary coil, it creates a magnetic field within the core, and this magnetic field induces a current in the secondary coil. 

A current only flows in the secondary circuit when the current is increasing or decreasing.  No current flows while the magnetic field is in steady state.

v  Laws of transformers

The voltage in the two circuits is proportional to the number of turns in the two coils

·         Np/Ns = Vp/Vs

Step up transformers and step down transformers.

·         Step up transformers increase the voltage and decrease the current. Used to supply the high voltage to the x-ray tube.

·         Step-down transformers decrease the voltage and increase the current. (remember that no transformer is 100% efficient). Supplies the power that heats up the filament.  (10V and 3-5 Amperes)

The second law of transformers is simply a restatement of the law of conservation of energy. A transformer cannot create energy. An increase in voltage must be accompanied by a corresponding decrease in current. The product of the voltage and current in the two circuits must be equal.

·         Vp Ip = Vs Is

§   The product of voltage and current is power,

·         Watt = Volt x Current

·         W = VI

The autotransformer

v  Provides voltage for the x-ray tube filament circuit.

v  Provides voltage for the primary of the high-voltage transformer

v  Provides a convenient location for the kVp meter that indicates the voltage to be applied across the x-ray meter

v  Single winding – wound around a laminated closed core

v  Filament circuit

Ø  The filament circuit regulates current flow through the filament of the x-ray tube.

The filament is a coiled tungsten wire that emits electrons when it is heated by current flow – thermoionic emission. (3-5 A with 10 V heats the filament).

Ø  The power to heat the x-ray tube filament is provided by a small step-down transformer called the “filament transformer”

Ø  Precise control over of filament heating is critical, because a small variation in filament current results in a large variation in x-ray tube current.

Using resistors to reduce voltage – Ohm’s Law:  V= IR

§  Volts = Current x Resistance

§  Volts = Amperes x Ω

v  High-Voltage Circuits

F3.5 – is important.

Ø  Need a step down filament transformer.

Ø  Have a 150 kVp step up transformer – both are immersed in oil for maximum insulation.

Two meters measure kVp and mA.

“Prereading peak kilovolt meter”: The kVp meter can be placed between the autotransformer and the step up circuit. It will measure the high voltage indirectly using the ratio of the number of turns in the transformer.

mA meter in the secondary coil: to record current flow accurately. (Remember that transformers are not 100% efficient – nothing is). The reader needs to be grounded and placed in the center of the coil  - to minimise the operator risk for electric shock. (the center of a coil has zero potential).

Rectification

v  Rectification is the process of changing alternating current into direct current – rectifier

v  Half-wave rectification

Half wave rectification is not desirable as:

Ø  you need double the exposure time, and

Ø  the anode may become hot enough to emit electrodes to produce a current in the inverse half-cycle that may bombard the filament and destroy it.

v  Rectifiers: a rectifier is a device that allows an electrical current to flow in one direction but does not allow current to flow in other directions

Small solid state, modern rectifiers, with longer lives then vacuum-tube type rectifiers – are made from silicon, with its four valence electrons.  

Semiconductor: The heart of a solid state rectifier is a semiconductor, which usually a piece of crystalline silicon.

A semiconductor describes a material that at a low temperature acts like an insulator, but at normal temperatures acts like a conductor.

v  N-type semiconductors. Donor

Impurity atom with 5 valence electrons added to the four valence silicon lattice. (N for negative – electron donated by the impurity (usually arsenic or antimony.)

v  P-type semiconductors. Acceptors

Impurity atom with a 3 valence electrons added to the 4 valence silicon lattice. Need to find an additional electron to plug the “hole” (the absence of an electron is called a hole). P-for positive particle. The “hole” moves in a direction opposite to the electron. Indium, Gallium and Aluminum.

v  P-N Junctions. Diode. Solid state rectifiers are diodes.

Based in the polarity of the diode, current will flow or not flow depending on the direction of the current.

v  Forward bias and reverse bias

Forward bias the negative electrode of a battery is connected to the N-type material and the positive terminal to the P-type material. Allowing current to flow in one direction – rectifier.

v  Silicon rectifiers

Silicon rectifiers are arranged in cylindrical stack of a bundle of electrons – can operate at 150 kVp and 1000 mA.

Half wave rectification

Note diode direction to be inserted on diagram.

Full wave rectification

v  The current is twice as large, and needs half the exposure time.

v  F3-17.

v  Remember the current is flowing from zero to its maximum all the time.

v  The x-rays are generated during the central high voltage portion of the cycle. The rest of the time its just produces heat or non-diagnostic low energy x-rays. Figure 3-18

TYPES OF GENERATORS

v  Three phase generators

Produce an almost consistent potential difference across the x-ray tube.

v  Three phase transformers – delta and wye

Three phase transformers have three sets of primary and three sets of secondary windings. There are two configurations – the delta (TRIANGLE) and the wye (LOOKS LIKE LETTER Y).

Ø  Six-pulse, six-rectifier

Delta (TRIANGLE) primary transformer and a wye-wound (Y) secondary transformer. F3-21

Ø  Ripple factor

Value of 13,5% (half percent less than South African VAT). This ripple factor is only 3,5% in a twelve pulse circuit.

v  Load ripple-factor, and is always greater than theoretical ripple

The load accentuates the ripple.

Ø  Six-pulse Twelve rectifier

Delta (TRIANGLE) primary transformer and 2 wye-wound (Y) secondary transformers. F3-23

Ø  Twelve pulse recitifier

Delta (TRIANGLE) primary transformer and one delta and one wye-wound (Y) secondary transformer. F3-24

More efficient consistent production of high energy diagnostic x-rays – and less wastage on lower energy, non diagnostic relevant x-ray production. Higher tube ratings for extremely short exposure times.

Power storage generators

v  Capacity Discharge Generators

v  Battery-Powered Generators

v  Medium-frequency generators. In a transformer, the voltage induced in the secondary coil is proportional to the rate of change in the current of the primary coil.

Nearly a consistent voltage to the x-ray tube that is not dependent on the power supply. Operation at a high frequency (>6500 Hz) results in a more efficient smaller transformer for portable units.

V = output voltage, F= frequency, n=number of windings, A=core cross-sectional area

A=CROSS SECTIONAL AREA – not Amperes

v  TRANSFORMER RATING

Transformer rating is the maximum safe output of the secondary winding.

Remember kV x mA = watt.

v  Kilowatts

Ø  kW = kV x mA / 1000

For single phased generators – use the root mean square voltage:

kW = kV x mA x 0.7 / 1000

RMS = peak /  = .707 peak

For three phase circuits, RMS voltage is 0.95 peak. 

Kilowatt ratings of x-ray generators are determined when the generator is under load, taken to be at 100 kVp

v  EXPOSURE SWITCHING (no radiologist know and few physicist care – so not important)

Ø  Primary switching and

Primary switching occurs most generally.

§  Silicon-controlled rectifiers (SCRs) or thyristers

Ø  secondary switching

generally used in units designed for rapid, repetitive exposures or where extremely short exposures are required.

The secondary switch must be insulated to withstand high voltage currents.

Ø  Primary versus Secondary Switching

Primary switches are - Easier and cheaper. Minimum Exposure time to 1 or 2 ms.

These are used in special-purpose generators like angiography or cineflurography.  Exposure rates of 80/second, exposure time 0.5ms and power output of 150 kW. 400 to 600 mA.

v  FALLING LOAD GENERATORS

Ø  The purpose of a falling load generator is to produce an x-ray exposure in the shortest possible exposure time by operating the x-ray tube at its maximum kilowatt rating for the entire exposure

Short exposure times are achieved because falling load generators operate at a higher mA then a fixed-tube mA technique.

Problems: operating the tube at a high mA – causes:

 maximum focal spot booming;

Heating the anode at its maximum capacity with each exposure shortens the x-ray tube life.

Falling load generators are expensive

v  EXPOSURE TIMERS

Ø  Electronic Timers

The length of exposure is determined by the time required to charge a capacitor through a selected resistance.

Ø  Automatic Exposure Control (Phototimer)

§  Entrance or exit types

Phototimers located in the front of the cassette are entrance types – or behind a cassette are exit types.

§  Photomultiplier Phototimers

Most common type of automatic exposure control.

§  Ionization Chamber Autotimers

§  Solid-State Autotimers

§  Miscellaneous Autotimer Topics

§  Pulse-Counter Timers

Ø  Pulse Counting timers