imaging 302 test 3

what are the components of an x-ray imaging system?

• patient support couch (table)

• x-ray tube

• operating console

• high voltage generator

whats the operating console

allows technologists to control beam parameters (quality and quantity)

what does kVp adjust

beam quality (penetrability in the beam)

• expressed in terms of half-value layer

what does mAs adjust

mAs adjusts beam quantity (number of photons in the beam)

how many volts can power companies provide to hospital buildings

~220V

x-ray imaging systems require what 3 things in order to operate with the amount of voltage coming into the building

1. a line compensator that monitors and adjusts the voltage feeding the x-ray machine so that it is always exactly 220V regardless of what’s coming into the building

2. an autotransformer that is used when adjusting kV on the console

3. a step-up (high tension) transformer that converts 220V to the thousands of volts (kV’s) required for x-ray production

what are the 3 primary circuits that comprise the circuitry of the x-ray system

• high voltage circuit

• filament circuit

• time circuit

a major kVp adjustment is done in increments of

10 kVp

a minor kVp adjustment is done in increments of

1 kVp

the exact voltage going through the x-ray tube at the time of the exposure is known prior to exposure because the kV meter is placed in ________ after the autotransformer

parallel

the major and minor adjustments of kVp are achieved by the

low-voltage autotransformer

what is the maximum voltage at wall outlet

220V

what is the maximum kilovoltage at x-ray tube

150V

what is the turns ratio of step up transformer

682:1 (fixed)

what is the current crossing from cathode to anode measured in

miliamperes (mA)

the number of electrons emitted by the cathode filament is determined by the

temperature of the filament

• tube current is dependent on temperature

what are the two types of timing circuits (exposure timers)

1. electronic timers (manual timers)

2. automatic exposure control (AEC)

electronic timers (manual timers)

works on the principle of charging a capacitor through a variable receptor

• allow a wide range of exposure times and are accurate

• exposure time must be set manually in addition to kV and mA values

mAs timers

variation of an electronic timer

• monitor the product of the mA and time and terminate the exposure when the desired exposure to the IR is attained

• CR based mobiles utilize mAs timers, while DR based mobiles may use AEC (automatic exposure control)

automatic exposure control (AEC)

automatically terminates exposure when sufficient radiation to provide required image density has reached the image receptor (film)

what are the 2 primary types of AEC systems

1. phototimers (no longer being used)

2. ionization chambers (currently being used)

phototimers

• AEC detector is placed behind cassette

  • piece of lucite (lucite transmits light)

  • coated in 3 areas with a phosphor (these 3 areas correspond to the 3 detector fields)

• area to be utilized is chosen by selecting the detector field(s)

how does AEC using a phototimer work

radiation passes through cassette, strikes phosphor areas and light emitted is transmitted via the lucite to a photomultiplier tube, which excepts the light and changes it into an electrical signal, then terminates the exposure once sufficient light is detected.

how do ionization chambers work

• detector is placed in front of cassette behind patient

  • works on principle of ionization

  • 3 chambers corresponding to the 3 detector fields

• chamber to be utilized is chosen by selecting the detector fields

how does AEC using an ionization chambers work

• at the time of the exposure a positive and negative charge is applied to each plate in the chamber

• as radiation passes through the chamber the air in the chamber is ionized

• electrons flow to the positive plate, resulting in the conduction of a current, which is transmitted through a variable resistance (density control for AEC) to charge a capacitor, which in turn eventually discharges to an electronic switch which will open and terminate the exposure

because AEC exposure time is controlled by aspects of the part being examined, it is important that, what 4 things happen

• centering is accurate

• proper detector fields are chosen

• part is completely covering detector field

• collimation happens

what is the only way to change the dose to the IR and image density when using AEC

adjusting density value

changing what does not affect image density when using AEC

kV

what does a backup timer do

• prevents exposures that are too long

• exposure automatically terminates when back-up time has been reached

• protects tube from overloading

• protects patients from excessive exposure

  • but could result in an image that is too light

what is the minimum response time

the time it takes for the system to respond to the exposure, and produce a diagnostic image (some thin body parts cannot be examined using AEC)

why is the minimum response time on digital fluoro units longer than conventional fluoro units

• must allow for reaction to exposure

• digital reconstruction of image data

• and presentation/viewing on monitor

how will images look if AEC is off more than +- 30% with CR

either exhibit mottle (underexposed) or degraded contrast (overexposure)

what is the average voltage generated in the DR photodiodes used to calculate

the amount of exposure incident on the DR TFT plate

what is voltage rectification

the process of converting alternating current (AC) to direct current (DC) in imaging systems, ensuring a stable and reliable voltage supply for optimal image capture.

how is voltage rectification achieved

using diodes

how is half-wave rectification achieved

by using a single diode to allow only one half of the AC waveform to pass through, effectively blocking the other half

true or false x-ray ONLY tubes work when current flows from cathode to anode

true (one direction only)

the voltage coming in through the wall outlet is alternating (AC) and changes direction through wiring every ______

1/120th of a second

when drawing a full wave rectification, how are the 4 diodes arranged

in a square-diamond shape

• all 4 of the diodes point in the same direction (from one corner of the diamond to the opposite corner)

how is the AC source and the load connected between the diodes

the AC source is connected between the diodes that are pointing in the same direction, while the load is connected between the diodes pointing in opposite directions

single phase power

Single phase power refers to a type of electrical power distribution where the voltage delivery is provided through one alternating current waveform, commonly used in residential or light commercial applications

three phase power

Three phase power is a method of electrical power distribution that uses three alternating currents, each phase 120 degrees apart, providing a more efficient and stable power supply, typically used in industrial applications

three phase/ 6 pulse power

A system that uses six diodes to convert three-phase AC power into DC power, improving efficiency and reducing ripple

high frequency power

High frequency power refers to electrical power that operates at frequencies above the standard 60 Hz AC and sending it through an inverter circuit

capacitor discharge generators

• capacitor is kinda like a “charge storage device” within a circuit

• often used in conjunction with battery powered mobile x-ray units

when is the capacitor charged and discharged

• the capacitor is charged when technique is set

• capacitor is discharged when exposure button is depressed

when are falling load generators used

in application where the shortest possible exposure time must be achieved to minimize patient motion and optimize image quality.

falling load generator

• the unit defaults to the highest possible mA station as a starting point for the exposure

• as the exposure time goes on, the mA “falls” gradually

• the mA “falls” as a means of improving heat capacity for a single large exposure

  • interventional radiology suites commonly employ falling load generators

what is voltage ripple factor

Voltage ripple factor is a measure of the fluctuations in voltage output in an electrical system, indicating the effectiveness of filtering

• voltage “dips” during an exposure, expressed as a percentage

what is the ripple factor for half wave

100%

what is the ripple factor for full wave

100%

what is the ripple factor for three phase, six pulse

14%

what is the ripple factor for three phase, twelve pulse

4%

what is the ripple factor for high frequency

<1%

• best for x-ray

what is power rating

a means of identifying high-voltage generators in meaningful terms (expressed in watts)

• KW (kilowatts)= max; x-ray tube current (mA) at 100 kVp and 100 mS

how do you calculate power rating

power rating= mA x kVp

• it will be at the maximum mA and kV that are achievable when using a 100mS exposure time (0.1 seconds)

what is the purpose of a beam restricting device

to reduce exposure of the patient to harmful ionizing x-radiation, while maintaining sufficient exposure to the ROI and to improve image quality by limiting the size of the x-ray beam

beam restrictors are not “beam focusing” devices because

while visible light can be focused using a lens, x-ray beams cannot be focused by any means

what are the 3 basic types of beam restricting devices

1. aperture diaphragms

2. cones and cylinders

3. variable aperture collimators (the most common and versatile beam restrictor)

what are aperture diaphragms

• simplest of all collimation devices

• made of a sheet of lead or lead lined metal with an aperture in it, attached to the x-ray tube head

• the aperture is usually designed to cover just less than the size of the IR used at a specific intended SID

• for the most part not being used in medical imaging anymore, but may be seen in applications where a “fixed-aperture” setup is convenient

true or false: aperature diaphragms are non-light localizing

true because they do not provide visual alignment aid.

cones and cylinders

• considered “extension modifications” to the aperture diaphragm

• the shape produced is circular

what is the primary function of cones and cylinders

to reduce the appearance of penumbra at the edges of the exposure field, and may also slightly improve image contrast

variable aperture collimators

aka “collimators”

• devices that have “shutters” that can adjust the size of the radiation beam to limit exposure and improve image quality by reducing unnecessary radiation

• allows for infinite range of field sizes

what is off focus radiation?

Off-focus radiation refers to secondary radiation that originates outside the intended radiation beam. It can contribute to image blur and increase radiation dose to areas outside the target zone.

• “stray photons” are known as off focus radiation and can contribute to image blurring