Transcript The x-ray imaging system

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 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 (stepup), 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?