Principles of Imaging 2: Mobile Radiography

what are the two types of mobile radiography units

Fluoroscopic C-arm unit and radiography mobile unit

how much voltage ripple is present in each generator:

• single phase: _____

• 3 phase 6 pulse: _______

• 3 phase 12 pulse: _______

• high frequency: ________

• battery operated: _______

• single phase: 100%

• 3 phase 6 pulse: 14%

• 3 phase 12 pulse: 4%

• high frequency: <1%

• battery operated: <1%

pro of capacitor discharge unit?

con of capacitor discharge unit?

lightweight and maneuverable

not battery operated and must be plugged into outlet before using

what is a capacitor

stores a charge for each exposure and releases the charge during exposure

how much does kV drop during an exposure from a capacitor

1kV per mAs, up to 30%

radiation leakage

residual x-rays may be present after an exposure is terminated, shock hazard

what are some strategies to minimize leakage

-grid biased/controlled tube

-metal wire mesh, actually the focusing cup itself, negative charge runs to grid to stop exposure

-collimation

battery operated units:

energized by powerful storage batteries

10-16, 12 volt packs of sealed, lead acid, batteries

battery operated units produce a current similar to what generator

high frequency

battery operated units have a typical focal spot size of ______ and a thermal capacity of ________heat units

0.75mm, 275,000 heat units

what are all battery operated units equipped with

“dead man” switch and special circuitry to monitor battery strength

characteristics of digital mobile units

• battery operated

• patient exams selected from worklists

• fast imaging viewing

• no wait time between exposures

• onboard storage of 3,000 exposures

• no more cassette processing

• exposures immediately transmitted or printed within seconds

• image data can be transferred to the hospital IT system for archival and documentation

pediatric mobile radiography

stickers on machines to make a fearful experience into fun experience, patients may remain more relaxed during an exam and obtain a high quality image

characteristics of a c-arm unit

• fluoro unit

• tube and image intensifier on c-arm

• maneuverable and flexible

• utilized in many departments

• have 2 monitors (live imaging and static/saved images)

technical factor selection:

• mobile units have single mAs control, mA and time not selected seperately

• designed to operate at a high, mixed mA

• provides a wider exposure latitude

what does a high mA allow for

shorter exposure time

AEC:

• mobile units

• paddle ionization chamber

• positioning challenges

where is the paddle placed for AEC

behind the IR

what type of grid ratio do we want to use for mobile radiography? Why?

low grid ratio (6:1, 8:1) for a wider positioning latitude

what type of grid frequency do we want for mobile radiography

higher frequency, short dimension, and wide focal range (SID)

off centering to a focused grid by as little as _____ can cause cut-off

5 degrees

of how much can estimates be for distance to avoid significant exposure differences

15%

Time: high mA enables ______ exposure time

Distance: exposure cord must be min. of ____ ft.

Shielding: min. of _____ mm lead

Time: high mA=short exposure time

Distance: cord at minimum of 6 ft.

Shielding: minimum of 0.5 mm lead

where is the primary source of tech exposure

the patient

when c-arm is in a vertical position the tube is ______ the patient and there is less dose to the _____ and _____

below, neck and face

what does a 30% tilt of c-arm increase

dose by a factor of 4

minimum source to skin distance is ____ per federal regulation of c-arm mobile unit

12”

the exposure rate for a c-arm unit should not exceed ________ at tabletop

10 R/min

when using a high frequency generator the tech has to use ______ exposure factors

higher