The X-Ray Imaging System (11)

"Below" Saturation Voltage Point

"Below" Saturation Voltage Point ±Not all electrons in space charge cloud will be moved to the anode. ±Those that remains will form a residual space charge, which stays and forms a barrier-layer around the filament. ±This negatively charged barrier would limit / reduce the flow of electrons from cathode to anode. Hint: 'peak' kV used will ensure ALL the electrons will be moved Plus, good for penetration of the body part Plus, keeps the level of mAs (radiation) low

X-Ray Tube Principle: 3 basic requirements for X-Rays to be produced:

- A source of electrons (filament current at the cathode side). - A force to accelerate electrons from the source to the target (kV or kilovoltage). - A target (anode) to slow down or stop the electrons.

Incoming Power (Line Voltage Input)

- A/C incoming power supply - Power comes in at 220 volts / 0.5 to 1 ampere - Electric Power Companies can't always deliver exactly 220 volts of electricity. It can be a bit more or less - Voltage Variation: Sometimes, the electricity they provide might be about 5% higher or lower than 220 volts which can cause inconsistencies in production of high-quality images

Filament Transformer (Step down transformer)

- After mA is selected, current is then sent to the Filament (Step Down) Transformer - Its purpose is to increase the current by reducing the voltage. - In a step-down transformer, as voltage decreases, current increases. - Many x-ray tubes feature two filaments, which are typically represented as large focal spot and small focal spot selections on the control console. - Choice of mA will send information of selection to the Filament Switch (FS Size)

mA Meter

- An mAs timer is an electronic timer that monitors the product of mA and exposure time and stops exposure when the desired mAs value is achieved. - The mAs timer is designed to provide the highest safe tube current for the shortest exposure time for any selected mAs. - To monitor the actual tube current, the mAs timer is typically located on the secondary side of the high-voltage transformer.

Filament Switch (Focal Spot Selector)

- Choice of mA will send information of selection to the Filament Switch (FS Size) - Switch will move to either LARGE or small FS according to mA selected - After mA is selected, current is then sent to the Filament (Step Down) Transformer

Autotransformer (kVp selector)

- It has a single coil, which operates using self-induction - primary side of the autotransformer receives the input voltage (A and A') - The precise kilovoltage (kV) is selected (not produced) on the autotransformer. - After selecting this kv, sends to secondary side of autotransformer (C, D, and E) - typically has major and minor kVp adjustment controls, allowing radiologic technologists to precisely set the required kVp

mA Selector in the Filament Circuit

- The mA selector is represented by mA stations on the control panel (console). These stations typically offer fixed choices for the tube current. - Is a rheostat (variable resistor): adjusts resistance - The mA selector allows the operator to select from the available mA stations to set the desired tube current (mA). It provides a convenient way to control and adjust the current according to the specific radiographic needs.

mA selector

- The mA selector on the control panel offers fixed choices for tube current and is controlled by a device called a rheostat, which is a variable resistor for adjusting resistance. - This selector allows operators to set the desired tube current (measured in milliamperes, mA) conveniently, providing control over the technique's current. - Increasing the current via the mA selector makes the x-ray tube filament hotter, resulting in the emission of more electrons and a higher mA value. - In summary, increasing the mA on the control panel leads to an increase in filament current, filament temperature, and the number of electrons released, making it a critical parameter for controlling the x-ray output.

filament circuit

- This is a separate circuit from the tube circuit. - controls the emission of electrons from the x-ray tube's filament

Secondary side of autotransformer: kVp major and minor selections

- When adjusting kVp on the console, you are controlling the secondary side of the autotransformer. - Used when only a very small step-up or step-down in voltage is required - Selection of kVp sets the "TAPS" on the 2'dary side, which will move to the proper selection point - Once the voltage is adjusted using the autotransformer, the electrical current-voltage is sent to two key components: (i) Step-down Transformer for filament to produce necessary electrons ..... AND ...... (ii) Step-up Transformer to obtain high voltage needed to produce x-rays.

Purpose of the Step-Down Transformer:

- adjusts current to cathode filament to produce heat required for the thermionic emission of e- - controls # x-rays: - current = heat - heat = # e- - # e- = # x rays

every x-ray imaging system has three principal parts:

- control console (control panel) - high-voltage generator - x-ray tube

voltage sine wave between primary and secondary side of high-voltage step-up transformer

- no change in waveform in primary (input) side - waveform changes in secondary (output side); amplitue increases

Rectifiers

- process of converting AC to DC - accomplished with diodes - While transformers function with alternating current (AC), x-ray tubes require direct current (DC).While transformers function with alternating current (AC), x-ray tubes require direct current (DC).

When the exposure button is activated..

- the filament heating circuit is immediately activated to heat the filament wire. - The step-down transformer on the secondary side supplies 10-12 volts and 3-6 amps of current to the filament, heating it to at least 2,200°C and above, with hotter filaments emitting more electrons. - Thermionic emission is the process where outer shell electrons of filament atoms are "boiled off" and ejected from the filament, forming a thermionic cloud of freed electrons. - The freed electrons hover around the filament and form a thermionic cloud. - This cloud of electrons, creates a negative space charge effect around the filament. - The space charge in the area becomes so negative that it prevents any more electrons from joining the cloud ......... - This is known as the "saturation point".

Secondary Side (Output) of High-Voltage step-up Transformer

- transforms v to kV; Increases output voltage from autotransformer to kV needed for x-ray production. - For x-rays to be produced: 20,000 volts to 150,000 volts is needed (i.e. 20 kV to 50 kV). - This is where the waveform changes: Amplitude increases!! - Next, the diodes (rectifiers) begin their job.

two separate circuits

- tube circuit (primary and secondary) - filament circuit

Primary side of filament (step-down) transformer

0.5 - 1A, 220 volts

High-voltage transformer (step up transformer)

1. The high-voltage transformer is a step-up transformer, with more secondary windings than primary windings, leading to higher secondary voltage. 2. It takes voltage from the autotransformer, amplifying it from volts to kilovolts, resulting in an increased amplitude. 3. The voltage range typically goes from 100-400 volts on the input side to 25-150 kilovolts on the output side. Next step: - To convert the alternating current (AC) output to direct current (DC) for the x-ray tube, a rectifier is used (next step). - Transformers exclusively function with AC, generating sinusoidal voltage waveforms on both the primary and secondary sides.

filament circuit components

1. input line power 2. auto-transformer 3. mA selector 4. filament switch (FSS selector) 5. filament transformer

tube circuit path

13. Input line power 1. autotransformer 9. timer circuit 4. high voltage transformer 15. rectifer circuit

Line Voltage Compensator

Gets the readout from the line monitor and adjusts voltage to precisely 220 V to maintain constant voltage. - Located on the PRIMARY side of the autotransformer

Saturation Current

The filament current that has risen to its maximum value because all available electrons have been used ± Meaning, the number of electrons produced is controlled by the mA used. ± When more electrons are needed, e.g. a larger patient, what will you do? Filament current (mA) must be increased....NOT kV "Saturation" means ... to FULL capacity (no more room)

Saturation Voltage

Voltage (potentional difference) causing all electrons produced to move from cathode to anode •kV is needed to 'push / move' the electrons. •Increasing kV = would drive a greater % of emitted electrons toward the anode. •If insufficient kV is used, NOT all the electrons will move to the other side. •Saturation point in voltage is needed in order to ensure that most to ALL electrons will be sent to the anode.

Secondary side of filament (step-down) transformer)

Voltage is lowered to 10-12 volts and current is increased to 3 - 6A to heat up the filament.

Line voltage meter

checks on fluctuations (voltage varitations) - measures the incoming line voltage - detects surge or drop - if any fluctuations are detected, line voltage compensator will fix it

Timer Circuit

controls how long the exposure stays on - Located on 2'dary side of Autotransformer and Primary side of high voltage transformer. - Separate from other main circuits of imaging system. - also controls # photons produced. - From here, voltage travels over to the 'primary side' of the high voltage transformer

kVp Meter

displays the voltage output (kvp) used from the autotransformer. - Located on the 2'dary side of the autotransformer - Actually a voltmeter - Reads voltage, but displays as kV

Primary Side (Input) of High-Voltage step-up Transformer

power comes from autotransformer's secondary output = No change in waveform yet.