Portable n mobile unit

Portable and
mobile
radiographic unit

Presented by :
Sudil Paudyal
B.Sc .MIT 2nd yr
IOM MMC

Introduction
• Machines used for severely ill and unable patients,
who cannot be brought in the radiology department.

• Can be broadly divided into two types:
 Portable and
 Mobile equipment.

3/26/2012 PORTABLE N MOBILE EQUIP BY SUDIL 2

Portable unit
• Portable - can be carried by one able bodied person
at a time.

• Can be taken around the hospital, some place distant
from hospital or even to a patient’s home.

• Simple

• Can be dismantled for transfer.

Fig: portable radiographic units then (1912) and
now


Components
1. Tube head:
(A) X-ray tube: Self-rectified, Stationary anode,
single focal spot of 1.0mm.

(B) High-tension generator:
 Tube and generator are enclosed in one oil filled
tank, described as tank construction and the whole
enclosure called the tube head.


2. Tube stand:
• support for the tube head,

3. A small control unit:
• Rheostats: to vary the mA and kV.
• Timer (s).
• Exposure switches.


Fig: Schematic diagram of components of portable unit


Advantages
• permit radiographic examination in locations limited by the
availability of a standard electrical supply.

Disadvantages
• Low output a typical maximum is 80 KVp at 15 mA for 1
second.
• Difficult to maintain aspects of radiation protection.


Mobile
• Mobile means capable of being moved.

• Mounted on wheels with the motion brake provided.

• Larger and heavier than portable sets and need to be
motorized or pushed by human power.

• Motorized units require less effort but add to weight
and cost of the unit.

Fig: Mobile units in 1940-1955 and now.

Components
• Tube

• Tube stand

• High tension transformer

• Control unit


Tube:

• Dual focus rotating anode x ray tube .

• Focal spot
 About 1.0mm for fine focus
 2.0 mm for broad focus.


Tube stand:

• A strong vertical column mounted on the base that
supports the cross arm which carries the x ray tube.
• Range of movements:
 Can be split into five major components.
 To prevent damage to cables
movements should have mechanical stops.


 Rotation around the centre of the column, (R)

 Cross arm movements the extension of the tube head away from the centre
of the column ,(A)

 Rotation around the axis of the cross arm, (D)

 Angulations across the long axis of the tube head,(B)

 Vertical movement up and down main column (H)


High tension generator:
• Gives a max output of 300mA and 125KVp.

• Includes full wave rectification provided by means of
selenium rectifiers .

• The H.T. generator, its rectifiers and filament
transformer for tube enclosed in one oil filled
earthed steel tank.

• Connected to the x ray tube by means of high
tension cables.

Control unit:

• Located behind the generator tank on the base of
unit.
• It consists:
Mains on/off switch
mAs selector
kV selector
Focal spot selection
Body part selection
Exposure indicator


Mobile classification
By two ways:
1. By power supply:
• Capacitor discharge unit
• Battery powered unit

2. By output:
 Low Power Mobiles: 10 to 30mA and 40 to 90 kVp.
 Average Power Mobiles:100 to 150 mA and 40-95
kVp.
 High power mobiles : upto 300 mA and 125 kVp.

Capacitor Discharge Mobile Unit
Principle:
• When charge circuit is activated, voltage from HTT charges
a capacitor up to the kilo voltage required for the exposure.

• The capacitor becomes source of power for x ray exposure.
It is disconnected from the charging circuit and connected
to the x ray tube for exposure.

• Its discharge through tube constitutes the mAs of the
radiographic exposure factors.


Fig : showing the working of capacitor discharge unit

CONTD…

• When the charge button is pressed ,the capacitor is
charged through high tension source by connections at
G1andG2.

• when required kV is reached ,charging is automatically
stopped and the lamp indicates ready.

• After charging has stopped when exposure button is
pressed, capacitor is connected to tube at X1 and X2.it
discharges through the tube and constitutes the x ray
exposure.

kV drop:
• In a 1μf design the kV drop is around 1 kV per mAS
used and the effective voltage is 1/3 of the voltage
drop lower than the starting voltage.
• Hence the equivalent kV=Starting kV-1/3 x mAS
• Example for an exposure of 87Kv and 20mAS:
• 87-(1/3 x 20)= 87-6.6 = 80Kv Effective.


X-ray Tube:
• Rotating anode tube with single focal spot size of 1.2mm
and heat storage capacity of 80,000 HU.
+
Grid controlled:
• Uses a third electrode i.e. focusing cup to control flow of
electrons from filament to target.
• Voltage across filament-grid produces electric field along
path of electron beam that pushes e-s closer.
• Exposure start & stop controlled by voltage on grid.
• Large enough negative voltage on grid blocks tube current
from cathode to anode.

Control Unit
• Automatic charging circuit for charging the capacitor.
• The kV range is from 30kv to 125kv. If pre selected Kv is
altered after charging, the tube voltage is automatically
adjusted to the new value.
• A single mAs control, rather than mA and exposure time
being selected separately. The units are usually designed
to operate at a high, fixed mA value, so that exposure
time becomes the main variable by default.
• mAs control has 15 steps range from 2 mAs to 60 mAs.


Advantages:
• less bulky and easily controllable.

• No need for special installations as power is only required
to charge the capacitor.

• The voltage waveform across x ray tube is uniform rather
than pulsating. It has no relationship with ac mains.


• Shorter exposure time by use of grid controller.

Limitations:
• Limited mAs output.
• The maximum output range is 30-50mAs so cannot
be used for heavy body part such as spine and
abdomen.


Battery Powered Mobile Unit
• Also called cord less mobile unit.
• Ni-Cd batteries are used as a source of energy for x ray
exposure .
• The generator is referred to as constant potential
generator (CPG).
• Power supply is used to charge the batteries.
• The batteries produces low voltage DC charge.


Major Components of battery Powered
X Ray machine


Advantages

• Stores considerable energy to generate x rays. can
store 10,000 mAs.

• Make exposures independent of power supply.

• Supplies constant output of KV and mA through out
the exposure.

• Used in case of emergency power failure.

Disadvantages

• Batteries must be charged

• Requires regular batteries maintenance

• Heavy


Care and maintenance:

• The unit should be left connected to the mains
power supply of (200v or 115v):
– Every night. – During weekends.
– At all times when the unit is idle (not being used).

• Naked flames or lighted cigarettes should not be
held near the batteries when they are being charged.

• The acid level in the batteries should be checked
every 2 weeks.

Mobile Image Intensifiers Units

• Used in operating theatre for fluoroscopy examination.

• This reduces the number of radiographs taken and saves
the time during surgery.


Essential features of mobile unit for
fluoroscopy
• The x ray tube

• The image intensifier

• High tension generator

• Control consol


The x ray tube and image intensifier:-

• Mounted opposite to each other at the end of C –arm.

• X ray tube head is at the lower end of C arm and the
image intensifier is at the upper end with input phosphor
facing towards the x ray tube.
• The tube head houses the x ray tube.
• The x ray tube is stationary anode and two focal spots.

• A small focal spot of size 0.6mm is used for fluoroscopy
and a large focal spot of size 1.8mm is used for
radiography.

• The C arm is mounted on a cross arm which extend from
a carriage carried on a vertical support rising from the
control console.

• The x ray tube and the image intensifier are held directly
opposite to each other with x ray beam permanently
centered to the input phosphor.

• At the back of I.I. is the television pick up tube, so that
image can be viewed on monitor.


• The input phosphor of the image intensifier is cesium
iodide which gives good contrast.
• A secondary grid of grid ratio 5:1 is used.


Movements

• The vertical support can be raised and lowered and the
cross arm can be extended through the carriage.

• The x ray tube and image intensifier are attached to the
c arm and move together.

• The c arm can be moved in any direction. so the
equipment enables a good range of positions and
projections to be used without wastage of time.


High tension generator:

• The x ray tube and high tension generator are together
in the tube head.

• High tension generator are single phase with full wave
rectification provided by silicon rectifiers i.e. the
generator is two pulse.


Fig: Block diagram of sequence of transference of signal


Control unit :
• Kilovoltage for fluoroscopy or radiography range
from 50kv to 105kv.

• mA settings or fluoroscopy range from 0.1to
3.0mA.for radiography the Kv and mA are linked .i.e.
 50mA at 55kv
 40mA at 80 Kv
 30mA at 105 Kv

• The timer for radiographic exposure is electronic and
gives range from 0.1 to 3.0 sec.


• For the television chain, circuit includes controls which
allow the image to be transposed right to left and
inverted top to bottom.

• There is control for memory circuit with two magnetic
disc that is image storage is possible.

• There is facility for pulsed fluoroscopy with electronic
selector: the rates provided vary from 1 flash per sec to 1
flash every 5 sec.
• This allows to monitor dynamic events without subjecting
the patient to continuous radiation.

Advancements
 High frequency generator mobile units

 Computed radiography mobile x ray unit

 Direct radiography mobile x ray unit


High Frequency Generator
• Recently high frequency generators are more popular
for use in radiology for mammography and mobile
units.

• One of significant advantages is its compact design.

• Converts low frequency (60 HZ) to high frequency
(KHZ) pulsed DC, by altering waveform of incoming
electrical circuit which is subsequently increased to
kilovolts, rectified and sent to the x-ray tube.


• Produces a nearly constant voltage wave form with
extremely low ripple(<1%).

• Provides great x-ray quality(effective energy) and
quantity. More efficient.
• More expensive than previous mobile units.


• Uses inverter circuit.
• The DC power supply produces
a constant voltage from either a
single phase or three phase
input line source.
• inverter circuit creates the high
frequency AC waveform.
• This AC current supplies the
high voltage transformer and
creates a waveform of fixed
high voltage and corresponding
low current.


• After rectification and smoothing two high voltage
capacitor on the secondary circuits accumulates
electron charges. These capacitors produce a voltage
across the x-ray tube, that depends upon the
accumulated charges


Fig: Block diagram of high frequency generator


Mobile CR


Digital portable unit


Construction


• Includes a transport case housing several
components, a digital x-ray image sensor and x-ray
generator.
• Sensor is coupled to a stand that is mounted to
transport case.
• Generator is positioned opposite to centre of sensor.
• A pt. support unit is disposed b/w image sensor and
x-ray generator.
• Image acquisition unit connected to generator and
image sensor and mounted on transport case.
• An image display unit is connected to image
acquisition unit and spaced from generator.


• Tube is positioned above stand to facilitate image
generation
• Image acquisition unit includes a display to allow the
user to view information relating to control and
operation of digital x-ray sensor and tube.
• Display comprises of a touch screen display. An
image display unit may also be provided to permit
the radiologist to view images at a distance from the
tube.


• The image sensor is a thin film transistor (TFT) based
digital x-ray image sensor. Preferably, digital image
sensor is about the same size as standard medical x-
ray films. The sensors can be grouped into two
categories:
• 1)indirect x-ray sensors
• 2) direct x-ray sensors


 indirect x-ray sensors: feature a TFT photodiode
array that is sensitive to visible light. These sensors
are covered by an x-ray to light converter such as
gadolinium oxysulfide or cesium iodide which
converts each incident x-ray photon into light
photons. The light photons are converted into
electronic charge at each pixel photodiode.


 direct x-ray sensors: is coated with amorphous
selenium which converts incident x-rays directly into
electronic charge at each pixel.

• In both cases electronic charge distribution collected
by pixel array is proportional to intensity distribution
of x-ray photons. This charge distribution is digitized
and stored in a computer.


• In order to display images
generated by image sensor
image display unit is provided
with software that at least
supports images in DICOM
and JPEG format.


Table : Tube parameters of digital portable


Digital mobiles
• The digital mobile units are the ultimate solution to
mobile X-ray imaging for digitization in ER,
traumatology, intensive care units (ICU), in patient
wards and pediatrics.

• The system represents an evolutionary move in
mobile diagnostic imaging equipment and includes
unique features in terms of operability, mobility and
image quality.


• The system meets today’s and future clinical
demands providing the premium solution for

immediate image processing

an efficient clinical workflow

a rapid display of high-quality clinical images

fast integration into the hospital network


Digital mobile units


Features

• Generator: High frequency generator

• Kvp Range: 40 - 100 Kvp, adjustable in 1 kV increments

• mA range: Fixed, 15 mA and 30 mA

• mAs Range: 0.15 – 120 mAs

• Exposure Time: 0.01 - 4.00 s


• Indicators: Digital display of kVp, mAs and Time, X-Ray,
Ready, mA Station and Fault Indicators

• Exposure Switch: Detachable two position switch

• X-Ray Tube: Stationary Anode
Filtration-2.7 mm aluminum
Target Material: Tungsten
Anode Capacity: 30,000 HU Storage
Focal Spot: 1.0 mm
Target Angle: 15 degree.

• Detectors: f p ds amorphous selenium or silicon

Questions
• What are the features of portable unit?
• What are the features of mobile unit?
• How does a capacitor discharge mobile unit works?
• What are the major components of battery powered
mobile unit?
• What are the features of mobile image intensifier
unit?
• How does a high freqency generator works?
• Description of digital portable unit?
• What are the features of digital mobile units?

References
• Chesney’s equipment for student radiographers
• Christensen’s physics of diagnostic radiology
• Cho et.al. Digital mobile radiographic apparatus
• Various other websites and ppts.


Portable n mobile unit

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