Transcript ARRT Board Preparation

Review
Equipment Operation & Maintenance
ARRT Section Components

30 Questions
Radiographic Equipment (21)
 Evaluation of Radiographic Equipment
& Accessories (9)

2
X-ray Tube Construction
Vacuum diode tube
anode
cathode
3
X-ray Tube Construction
A
C
B
D
E
G
F
WHAT TYPE OF MOTOR IS ROTOR?
The energy of electrons comprising the tube current is
measured in ______________ ?
4
Cathode - Filaments








Negative Electrode
Tungsten with thorium
Filament circuit heats
(>2200°C) with 3-5 amps
Thermionic Emission
Vaporization
Space charge
Saturation current
Focusing cup
5
Filament I vs. mA



Heated filament
emits electrodes
Once emission
starts small  in
filament I = large
 in mA
As kVp rises less
filament I needed
to reach mA
6
Saturation Current/Voltage

Stabilization at
specific mA above
~40 kVp
7
Focusing Cup
8
Anodes - Target
Rotating Anodes
• 2” to 5” disk (focal track)
• Induction motor
Cu
•
W
Cu
W
Speed 3000 to 10000 rpm
Molybdenum or
Graphite base
Other target materials
• Tungsten-rhenium
• Molybdenum
• Rhodium
9
What makes the Anode
spin?

Electromagnetic Induction Motor

How fast does it turn?

______________ rpm
10

A motor is a device that converts

electrical energy into mechanical
energy
11
Line-Focus Principle
Angle (5°-15°)  Effective FS < actual FS
 Beveled anode
 Improves heat capacity

Small angle =  FS

e-


effective

Small FS =  resolution
AHE 
Field coverage 
Heat loading on smaller
anode surface area
12
Anode Angle and Focal Spot Size (1)




Anode angle < range:
7° - 20°
Why are anodes beveled?
1. Line focus principle
(foreshortening of focal
spot length)
Bushberg, et al., The Essential Physics of
Medical Imaging, 2nd ed., p. 108-109.
© UW and Brent K. Stewart PhD, DABMP
13
Anode Angle and Focal Spot Size





1. ↓ <→ ↓ apparent focal spot size (B and C)
Smaller the angle – smaller the effective focal spot
2. ↑ <→ ↑ heat loading:
3. ↑ <→ ↑ field coverage (B and C)
Actual < used trade-off of these factors
Bushberg, et al., The Essential Physics of
Medical Imaging, 2nd ed., p. 108-109.
© UW and Brent K. Stewart PhD, DABMP
1414
Anode Angles and Heat

Smaller the angle = more heat load

Larger angle – less heat load
Different anode angles are used for
different types of equipment:
 Diagnostic vs special procedures

15
Which anode angle has
greater heat load?




ANODE HEEL EFFECT
MORE PRONOUNCED
WITH SMALLER
ANODE ANGLE
(sm anode angle =
larger heel effect)
120% ~75%
difference of intensity
across beam
How does this affect
positioning?
FAT – CAT
HIGHER AT CATHODE
See pg 139 Bushong
17
Which of the following
projections would take
advantage of the anode heel
effect on a hyperstenic patient if the anode was towards the
patient’s head?
A.
B.
C.
D.
AP Thoracic Spine
AP. Lumbar Spine
Lateral Femur
AP FOOT
X-ray Tube Anode Configuration






Tungsten anode disk
 Mo and Rh for
mammography
Stator and rotor make up the
induction motor
Rotation speeds
 Low: 3,000 – 3,600 rpm
 High: 9,000 – 10,000 rpm
Molybdenum stem (poor heat
conductor) connects rotor
with anode to reduce heat
transfer to rotor bearings
Anode cooled through
transmission
Focal track area (spreads heat
out over larger area than
stationary anode
configuration)
Bushberg, et al., The Essential Physics of
Medical Imaging, 2nd ed., p. 107.
© UW and Brent K. Stewart PhD, DABMP
19
19
Heel Effect


Reduction of x-ray beam
intensity towards the
anode side of the x-ray
field
Although x-rays generated
isotropically



Can use to advantage,
e.g., PA chest exposure



Self-filtration by the anode
and the anode bevel
causes
Greater intensity on the
cathode side of the x-ray
field
Orient chest to anode side
Abdomen to cathode side
Less pronounced as SID ↑
Bushberg, et al., The Essential Physics of
Medical Imaging, 2nd ed., p. 112.
© UW and Brent K. Stewart PhD, DABMP
20
20
X-ray Filtration
Filtration: x-ray
attenuation as beam
passes through a layer of
material
 Inherent (glass or metal
insert at x-ray tube port)
and added filtration
(sheets of metal
intentionally placed in the
beam)
 Added filters absorb lowenergy x-rays and reduce
patient dose (↑ beam
quality)
 HVL – half value layer (mm
Curry, et al., Christensen’s Physics of
Diagnostic Al)
Radiology, 4th ed., pp. 89, 91.
© UW and Brent K. Stewart PhD, DABMP

21
21
Voltage generators (power
supply)
22
Miscellaneous Terms
Off Focus Radiation
Protective Housing
Extra-focal Radiation
rebounding e-
Leakage Radiation
<100 mR/hr @ 1 m
23
OFF FOCUS
RADIATION
24
25
26
SHADOW OF
SOMEONE’S
HEAD =
OFF FOCUS
FROM
TUBE
27
28
LEAKAGE RADIATION
may not EXCEED
TUBE
HOUSING
100mR / HR
@ 1 meter
29
Tube Failures & Prevention

Failure Causes




Anode pitting/cracking
after single excessive
exposure
Bearing damage from
numerous long
exposures
Vaporization of the
filament
Prevention



Warm up tube before
use
Avoid “boost & hold”
exposures if possible
Use acceptable levels of
exposure (e.g. tube
rating charts)
• Filament break
• Coating of glass
envelope with tungsten
30
Tube Rating Charts (mA)
31
Tube Rating Charts (kVp)
32
Heat Units


HU – measure of thermal energy applied
to the x-ray tube from an exposure
Formula based on generator
HU1Ø
= kVp x mAs
HU3Ø6p = kVp x mAs x 1.35
HU3Ø12p = kVp x mAs x 1.41

If multiple, consecutive exposures made,
HUT
= HU x #exposures
33
Anode Cooling Chart
34
Name 3 types of rectifiers
35
Rectification



 AC to DC
Keeps e- flowing
from cathode to
anode
Uses rectifying
bridge between
HVT & tube
tube
HVT
AC
bridge
DC
tube
HVT
36
SOLID STATE - DIODES
– Semi Conductor

Allows current to
only flow in one
direction

Most common type
used in rectifiers
37
AEC

Operation




film
Fluorescent
screen
PMT
Exit
Select sensor(s)
Select density level
Set back-up time
Set kVp
Ion chambers
film
Exposure
Switch
vs.
Entrance
38
Beam Restrictors
Collimator
Cone
Diaphragm
39
Power
Source
autotransformer
Important Circuit Elements
HV
subcircuit
X-ray
Tube
Filament
subcircuit
40
41
X-ray Generator
cathode
anode

Transformers

F



HV (step-up)
Filament (step down)
Rectification
Connection to tube
diodes
HV
42
Transformers

Autotransformer



Step-up or step-down
Variable TR
Controls kV by varying
V sent to HVT


HV




Step-up
Fixed TR > 1
VS > VP
 V to kV
(500 to 1000)
Filament




Step-down
Fixed TR < 1
IS > IP (VS<VP)
I in filament to cause
e- emission
43
44
Transformer Review



Turns Ratio

NS
NP
Transformer Law
N S VS I P


N P VP IS
Power
Step Up



TR>1
V
I
Step Down


TR<1
V
I
PP  PS  VP I P  VSIS
45
46
Autotransformer – Self
Induction



There is only one wire –
but works like when
there are 2 wires =
The windings are used
as the primary and
secondary coils
The induced voltage
varies on where the
outside wires are
connected (KVP Taps)
47
TRANSFORMER FORMULAS
(STEP UP OR DOWN)





V = voltage
N = # turns
p = primary
s = secondary
I = current

Vp = N p
Vs
Ns

Vp = I s
Vs
Ip

Np = I s
Ns
Ip
48
Transformer Efficiency

By design




Shell – most
Closed core
Open core
Air core



Ideal – no loss
Reality best = ~95%
induction
Loss due to

Cu resistance
•  wire diameter

Eddy currents
• Laminate core

Hysteresis
•  core mag. perm.
49
Rectification



 AC to DC
Keeps e- flowing
from cathode to
anode
Uses rectifying
bridge between
HVT & tube
tube
HVT
AC
bridge
DC
tube
HVT
50
Phase, Pulse & Frequency
Type
P
C
P
S
# of
rectifiers
% ripple
1(self)
1
60
0
100%
1(half)
1
60
1 or 2
100%
1(full)
2
120
4
100%
36p
6
360
6 or 12
12-15%
312p
High
Freq.
12
720
10kHz
12
3-5%
Wave form
+
+
+
+
-
+
-
<1%
51
GENERATOR



THAT CREATE AN ALTERNATING
CURRENT ARE CALLED:
AN ALTERNATOR –
What does the generator do? CONVERT
MECHANICAL ENERGY INTO
ELECTRICITY
52
53

WHAT MEASURES ELECTRIC
POTIENTAL = VOLT

CURRENT =

ELECTRIC CIRCUIT IS THE
PATHWAY FOR ELECTRIC
CURRENT
AMP
54
What does a MOTOR due
Converts ___________ energy
 To
 _______________ energy

55
OHMS LAW
V = IR
V
= POTIENTAL
 A = AMPS (CURRENT)
 R = RESISTANCE (OHMS)
56
OHM’S LAW:
V = Potential difference in volts
 I = Current in amperes
 R = Resistance in ohms ()


V= IR
I =V/R
R=V/I
57
Simple SERIES Circuit
58
Series Circuit Rules:
Current: IT = I1 =I2 =I3
 Voltage: VT = V1 + V2 +
 Resistance:
 RT = R1 + R2 + R3

V3
59
Parallel Circuit Rules
Current:
IT = I1 + I2 + I3
 Voltage:
VT = V1 = V2 = V3
Resistance:
 1
1
1
1
 RT = R1 + R2 + R3
 (REMEMBER TO FLIP SIDES RT/1

60
61
TYPES OF EQUIPMENT

FIXED

MOBILE

DEDICATED

MAMMO , CHEST, HEAD
62
63
64
EXPOSURE
SWITCH
TIMER
65
RHEOSTAT

VAIRIABLE REISITOR

regulate the amount of
resistance in a circuit
66
67
What makes the Anode
spin?

Electromagnetic Induction Motor

How fast does it turn?

______________ rpm
68