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..
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
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
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
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
Control Console Transformers Tube Functional Position
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
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
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?
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
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
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