The X-ray Imaging System

Week 4-5

Bucky slot cover

 During fluoroscopy the Bucky tray is moved to the end of the table  This leaves an opening in the side of the table about 5cm  Approximately at what level is the bucky?

Bucky slot cover

• The opening should automatically be covered with at least 0.25 mm Pb equiv..

Fluoroscopy

The Control Console

• The control console is device that allows the technologist to set technical factors (mAs & kVp) and to make an exposure.

• Only a legally licensed individual is authorized to energize the console.

Control Panel • All the electric circuits connecting the meters and controls are at low voltage to minimize the possibility of shock.

Operating Console Controls:

 Line Compensation, kVp, mA and time  Quantity = # of x-rays  Milliroentges (mR) or (mR/mAs)  Quality = the pentrability  Kilovolts peak (kVp)

Operating Console has meters to measure

 kVp, mA, & exposure time  Modern units only display mAs  Units with ACE’s will have a separate meter for mAs

Control Panel

AEC

 Automatic Exposure Control  Uses an ionization chamber  Technologist sets kVp, mA, back-up time & sensors  Exposure terminates the IR has proper OD  Patient positioning must be absolutely accurate

AEC Sensors

APR

 Anatomically Programmed Radiography (Ch 15)  Radiologic Technologist selects on the console a picture or a written description of the anatomic part to be imaged and the patient body habitus  A computer selects the appropriate kVp and mAs.

APR

 The whole process uses an AEC  Precise patient positioning over the phototiming sensor is critical

APR

X-RAY CIRCUITY

Contributions by Mosby, Thompson Publisher, Carlton, Bushberg, and the WWW.

3 Divisions of Circuit Board • PRIMARY (CONTROL PANEL) yellow • SECONDARY (HIGH VOLTAGE) blue • FILAMENT (LOW CURRENT) purple

Functional Position

Control Console Transformers Tube

Line Compensation

 Most imaging systems are designed to operate on 220 V. (some 110 V or 440 V)  However power from the wall is not always accurate continuously

Line Compensation

 Wired to the autotransformer is the line compensator   Designed to maintain the accurate voltage required for consistent production of high-quality images Today’s line compensators are automatic and are not displayed on the control panel

Line Compensator

Autotransformer

   The power for the x-ray imaging system is delivered first to the autotransformer  The autotransformer works on the principle of electromagnetic induction It has one winding and one core There are a number of connections along its length

Autotransformer • A’s = primary connections & power into the transformer • Other connections allow for variations of voltages

Autotransformer

 Is designed to step up voltage to about twice the input voltage value  The increase in voltage is directly related to the number of turns

kVp selection

X-ray tube current or Filament circuit

 A separate circuit crossing from cathode to anode  Measured in milliampers (mA)  What determines how many x-rays are created?

X-ray tube current or Filament circuit

 # of e- is determined by the temperature of the filament. The hotter the filament the more e  Are their any limiting factors to thermionic emission?

mA selection

Filaments

Operate at currents of 3 to 6 amperes (A)

Question?

What is directly proportional to the number of x-rays reaching the IR?

Exposure Timers

 The timer circuit is separate from the other main circuits of the imaging system  It is a mechanical or electronic device whose action is to “make” and “break” the high voltage across the x-ray tube  This is done on the

primary side

voltage transformer. of the high

mAs Timers

 Monitors the product of mA and exposure time  Terminates the exposure when the desired mAs value is reached  Located on the

secondary side

of the high-voltage transformer since actual tube current must be monitored

mAs Timers

 Designed to proved the highest mA for the shortest exposure  Modern X-ray machines have falling-load generator  Automatically adjusts to the highest mA at the shortest exposure time possible

AEC Control

 AEC measure the quantity of radiation reaching the IR  Automatically terminates when the IR has received enough radiation for desired OD  Two types are common

Flat, parallel plate ionization chamber

 Located between the patient and the IR  Made radiolucent  Ionization w/in the chamber creates a charge; calibrated to produce a given OD on the IR

Photomultiplier

(Photodiode)

detector assembly

  Located behind the IR Contains a fluorescent screen and a photomultiplier  The photomultiplier detects the light from the fluorescent screen until the desired OD on the IR is reached terminating the exposure

AEC’s

 Upon instillation must be calibrated by the service engineer  Technologists selects the desired OD which then sets the mA & kVp

AEC’s

 A back up timer usually automatically set to prevent over exposure if the AEC fails  Should be set to 1.5 times the expected exposure time Why?

 When the ionization chamber or photodiode reaches the preset level, a signal is returned to the operating console to terminate the exposure

High-Voltage Generator

 Responsible for increasing the output voltage from the autotransformer to the kVp necessary for x-ray production  3 parts: High-voltage transformer (step up), filament transformer (step-down) and rectifiers

High voltage transformer

 Or step up transformer  Connected to the Major and Minor kVp selector  Increases the volts from the autotransformer to kilovolts

Step Up Transformer

Voltage Rectification

  Converts AC to DC current During the negative cycle current can only flow from anode to cathode  E- must travel cathode to anode – DC current keeps e- traveling in the correct direction, cathode to anode  Attracted to the positive anode

Voltage Rectification

X-Ray Tube Circuit

Filament transformer

 Or step down transformer  Reduces the current to the filament

High-Voltage Generation – converts 220 volts of AC to kilovolts of DC

 The generator is a FIXED component of the imaging system, not under the control of the technologist  Three basic types: single phase, three phase, and high frequency  The generator affects the quality and quantity of photons produced

How does this effect technique selection?

Questions on imaging systems?