Mobiles and mammography

Question 1

3 reasons why a patient may need a mobile x-ray

Answer 1

1) too ill to come down to the department
2) May be in an isolation unit- cannot travel
3) May have equipment attached to them that cannot be transported

Question 2

examples of where a mobile x-ray is necessary

Answer 2

• ITU/ICU/HDU
• SCBU
• Theatre
• Resus
• Cardiac care unit

Question 3

name the 9 components of a mobile x-ray machine

Answer 3

1) Pedestal
2) Control panel; Kvp;ma;time
3) wheels
4) Adjustable arm
5) x-ray tube housing
6) Power cable
7) applicator
8) handle
9) base

Question 4

what is the target-rotating anode made of?

Answer 4

tungsten-rhenium

Question 5

what design is the tube head

Answer 5

‘tank head’ design

Question 6

what are the two reasons for it being mounted on a pivot joint?

Answer 6

makes it;

flexible and manoeuvrable

Question 7

What is the purpose of the telescopic arm? (3 things)

Answer 7

1) holds tube housing
2) carries power cable
3) allows for extendable ‘reach’

Question 8

what two things does the pedestal for attachment to telescopic arm do

Answer 8

1) gives stability

2) allows varying height of tube

Question 9

what does the control panel allow you to change

Answer 9

exposure factors; KvP and mAs

Question 10

Why is the base weighted

Answer 10

to stabilise to the centre of gravity

Question 11

what other 4 things does the machine have to aid movement?

Answer 11

1) wheels to allow movement/motor assisted
2) handle with automated locking device/wheel break when released
3) Collision sensors
4) Power indicator on console

Question 12

what are the function of the base (6 things)

Answer 12

1) stability
2) houses a large battery
3) Contains step-up/down HV transformers
4) provides a rectifier system
5) contains an oil coolant
6) Made of grounded steel with fibreglass cover to provide durability

Question 13

How does the battery work (4 points)

Answer 13

1) stores electrical energy taken from the mains supply when the unit is plugged in-provides a constant potential
2) Power indicator shows 100%/FULL when fully charged
3) can be used under 100%
4) discharges some of its energy during an exposure- independent of the mains

Question 14

What is the purpose of the step-up/down transformers? (4 points)

Answer 14

1) as in a static x ray tube, voltage from the battery needs adjustments before it reaches the tube
2) Step-up transformer increases the voltage to cathode/anode creating high potential across the tube to allow photon production
3) Step-down transformer decreases voltage to the filament to allow thermionic emission without damage to the filament
4) modern mobile units usually have a choice of two focal spot sizes

Question 15

why is a rectifier needed?

Answer 15

as electrical supply from the mains is an alternating current. If left like then photon production is inefficient

Question 16

what is half wave rectification

Answer 16

when a direct current is needed to maintain constant flow of photons in beam

Question 17

what is single phase full wave rectification

Answer 17

when a double electrical current is needed

Question 18

what is 3 phase full wave rectification

Answer 18

this is the ideal current ‘smooth’

Question 19

how does a theatre C-arm work? (5 points)

Answer 19

1) still a mobile x-ray unit with additional functionality
2) isocentric arm-i.e. rotates around a point in air
3) Image capture using an intensifying screen
4) CANNOT be used independently of mains power supply
5) Allows DYNAMIC SCREENING i.e. live images

Question 20

name 5 additional features of a theatre C-arm

Answer 20

1) PULSED screening available to reduce patient dose
2) Stationary or rotating anode depending on manufacturer
3) Digital functionality
4) Instant image display
5) Instant hard copy available- mini laser printer

Question 21

what is the aim of a mammography unit

Answer 21

to image soft tissue only- design of the equipment must allow for this functionality

Question 22

what is needed due to the low mass attenuation of soft tissue

Answer 22

photons with very low energy

Question 23

why is a high speed rotating anode made of a high atomic number (tungsten-rhenium) no good for a mammography unit?

Answer 23

as it produces relatively high energy photons

Question 24

what kvP range does mammography units use

Answer 24

20-45 kV

Question 25

what is the problem with a low kV range being used for mammography units?

Answer 25

1)thermionic electrons do not have enough energy to cross to the anode which produces a beam of low quality- low intensity

Question 26

what is the problem with using a longer exposure time for mammography

Answer 26

this increases movement unsharpness

Question 27

what is the problem with breast tissue

Answer 27

has very little subject contrast

Question 28

What 4 things resolve these problems with a mammography units?

Answer 28

1) shorter distance between anode and cathode
2) short SID of 45-60cm
3) use a fine focal spot size of 0.4 to 0.6mm (or less)
4) use a rotating anode of molybdenum

Question 29

what SID is used for mammography units

Answer 29

45-60cm

Question 30

why is a focal spot size of 0.4 to 0.6mm (or less) used in mammography units

Answer 30

reduces geometric unsharpness

Question 31

what is the rotating anode made of in a mammography unit

Answer 31

molybdenum

Question 32

what does a molybdenum rotating anode produce

Answer 32

a MONOENERGETIC beam when filtered