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)

What is directly proportional to the number of x-rays reaching the IR? 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 of the high voltage transformer.

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?