Review

Equipment Operation & Maintenance

ARRT Section Components 30 Questions

Radiographic Equipment (21) Evaluation of Radiographic Equipment

& Accessories (9)

Radiographic Equipment

X-ray Tube ConstructionVacuum diode tube

cathodeanode

Radiographic Equipment

X-ray Tube Construction

GF

ED

C

A

B

WHAT TYPE OF MOTOR IS ROTOR?

The energy of electrons comprising the tube current is measured in ______________ ?

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

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

Saturation Current/Voltage Stabilization at

specific mA above ~40 kVp

Focusing Cup

Anodes - Target

Cu

W

CuW

Other target materials• Tungsten-rhenium• Molybdenum• Rhodium

Molybdenum or Graphite base

Rotating Anodes• 2” to 5” disk (focal track)• Induction motor

• Speed 3000 to 10000 rpm

What makes the Anode spin? Electromagnetic Induction Motor

How fast does it turn?

______________ rpm

A motor is a device that converts

electrical energy into mechanical energy

Line-Focus Principle Effective FS < actual FS

Beveled anode Improves heat capacity

Small angle = FS Small FS = resolution AHE Field coverage Heat loading on smaller

anode surface area

Angle (5°-15°)

e-

actualeffective

© UW and Brent K. Stewart PhD, DABMP© UW and Brent K. Stewart PhD, DABMP

Anode Angle and Focal Spot Size (1)Anode Angle and Focal Spot Size (1)

Anode angle < range:Anode angle < range: 7° - 20° 7° - 20°

Why are anodes beveled? Why are anodes beveled? 1. Line focus principle 1. Line focus principle

(foreshortening of focal (foreshortening of focal spot length) spot length)

Bushberg, et al., The Essential Physics of Bushberg, et al., The Essential Physics of Medical Imaging, 2nd ed., p. 108-109.Medical Imaging, 2nd ed., p. 108-109.

© UW and Brent K. Stewart PhD, DABMP© UW and Brent K. Stewart PhD, DABMP1414

Anode Angle and Focal Spot Size Anode Angle and Focal Spot Size

1. ↓ <→ ↓ apparent focal spot size (B and C) 1. ↓ <→ ↓ apparent focal spot size (B and C) Smaller the angle – smaller the effective focal spotSmaller the angle – smaller the effective focal spot 2. ↑ <→ ↑ heat loading: 2. ↑ <→ ↑ heat loading: 3. ↑ <→ ↑ field coverage (B and C) 3. ↑ <→ ↑ field coverage (B and C) Actual < used trade-off of these factorsActual < used trade-off of these factors

Bushberg, et al., The Essential Physics of Bushberg, et al., The Essential Physics of Medical Imaging, 2nd ed., p. 108-109.Medical Imaging, 2nd ed., p. 108-109.

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

Which anode angle hasgreater 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

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. AP Thoracic Spine B. AP. Lumbar Spine C. Lateral Femur D. AP FOOT

© UW and Brent K. Stewart PhD, DABMP© UW and Brent K. Stewart PhD, DABMP1919

X-ray Tube Anode ConfigurationX-ray Tube Anode Configuration Tungsten anode disk Tungsten anode disk

Mo and Rh for Mo and Rh for mammography mammography

Stator and rotor make up the Stator and rotor make up the induction motor induction motor

Rotation speeds Rotation speeds Low: 3,000 – 3,600 rpm Low: 3,000 – 3,600 rpm High: 9,000 – 10,000 rpm High: 9,000 – 10,000 rpm

Molybdenum stem (poor heat Molybdenum stem (poor heat conductor) connects rotor conductor) connects rotor with anode to reduce heat with anode to reduce heat transfer to rotor bearings transfer to rotor bearings

Anode cooled through Anode cooled through transmission transmission

Focal track area (spreads heat Focal track area (spreads heat out over larger area than out over larger area than stationary anode stationary anode configuration)configuration)

Bushberg, et al., The Essential Physics of Bushberg, et al., The Essential Physics of Medical Imaging, 2nd ed., p. 107.Medical Imaging, 2nd ed., p. 107.

© UW and Brent K. Stewart PhD, DABMP© UW and Brent K. Stewart PhD, DABMP2020

Heel EffectHeel Effect

Reduction of x-ray beam Reduction of x-ray beam intensity towards the intensity towards the anode sideanode side of the x-ray of the x-ray field field

Although x-rays generated Although x-rays generated isotropicallyisotropically Self-filtration by the anode Self-filtration by the anode

and the anode bevel and the anode bevel causes causes

Greater intensity on the Greater intensity on the cathode side of the x-ray cathode side of the x-ray field field

Can use to advantage, Can use to advantage, e.g., PA chest exposure e.g., PA chest exposure Orient chest to anode side Orient chest to anode side Abdomen to cathode side Abdomen to cathode side

Less pronounced as SID ↑Less pronounced as SID ↑Bushberg, et al., The Essential Physics of Bushberg, et al., The Essential Physics of Medical Imaging, 2nd ed., p. 112.Medical Imaging, 2nd ed., p. 112.

© UW and Brent K. Stewart PhD, DABMP© UW and Brent K. Stewart PhD, DABMP2121

X-ray FiltrationX-ray Filtration

Filtration: x-ray attenuation Filtration: x-ray attenuation as beam passes through a as beam passes through a layer of material layer of material

Inherent (glass or metal Inherent (glass or metal insert at x-ray tube port) insert at x-ray tube port) and added filtration (sheets and added filtration (sheets of metal intentionally of metal intentionally placed in the beam) placed in the beam)

Added filters absorb low-Added filters absorb low-energy x-rays and reduce energy x-rays and reduce patient dose (↑ beam patient dose (↑ beam quality) quality)

HVL – half value layer (mm HVL – half value layer (mm Al)Al)

Curry, et al., Christensen’s Physics of Curry, et al., Christensen’s Physics of Diagnostic Radiology, 4th ed., pp. 89, 91.Diagnostic Radiology, 4th ed., pp. 89, 91.

Voltage generators (power supply)

Miscellaneous Terms

Leakage Radiation<100 mR/hr @ 1 m

Extra-focal Radiationrebounding e-

Off Focus Radiation Protective Housing

OFF FOCUSRADIATION

SHADOW OFSOMEONE’S

HEAD = OFF FOCUS

FROM TUBE

LEAKAGE RADIATIONmay not EXCEED

TUBE HOUSING 100mR / HR

@ 1 meter

Tube Failures & Prevention

Failure Causes Anode pitting/cracking

after single excessive exposure

Bearing damage from numerous long exposures

Vaporization of the filament

• Filament break• Coating of glass

envelope with tungsten

Prevention Warm up tube before

use Avoid “boost & hold”

exposures if possible Use acceptable levels of

exposure (e.g. tube rating charts)

Tube Rating Charts (mA)

Tube Rating Charts (kVp)

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

Anode Cooling Chart

Name 3 types of rectifiers

Rectification AC to DC Keeps e- flowing

from cathode to anode

Uses rectifying bridge between HVT & tube

HVT

tube

HVT

tube

AC DCbridge

SOLID STATE - DIODES – Semi Conductor

Allows current to only flow in one direction

Most common type used in rectifiers

AEC Operation

Select sensor(s) Select density level Set back-up time Set kVp

PMT

ExposureSwitch

filmfilm

Ion chambers

Fluorescentscreen

Exit vs. Entrance

Beam RestrictorsCollimator Cone

Diaphragm

Important Circuit Elements

PowerSource

HVsubcircuit

Filamentsubcircuita

uto

tran

sfo

rmer

X-rayTube

X-ray Generator Transformers

HV (step-up) Filament (step down)

Rectification Connection to tube

HV

F

diodes

anodecathode

Transformers HV

Step-up Fixed TR > 1 (500 to 1000)

VS > VP

V to kV

Filament Step-down Fixed TR < 1 IS > IP (VS<VP) I in filament to cause

e- emission

Autotransformer Step-up or step-down Variable TR Controls kV by varying

V sent to HVT

Transformer Review Turns Ratio

Transformer Law

Power

Step Up TR>1 V I

Step Down TR<1 V I

P

S

N

N

S

P

P

S

P

S

I

I

V

V

N

N

SSPPSP IVIVPP

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)

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

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.

Rectification AC to DC Keeps e- flowing

from cathode to anode

Uses rectifying bridge between HVT & tube

HVT

tube

HVT

tube

AC DCbridge

Phase, Pulse & Frequency

Type# of

rectifiers% ripple

Wave form

1(self) 1 60 0 100%

1(half) 1 60 1 or 2 100%

1(full) 2 120 4 100%

36p 6 360 6 or 12 12-15%

312p 12 720 12 3-5%

HighFreq.

10kHz <1%

CP

SP

+

-

+

-

+

-

+

-

+

-

GENERATOR

THAT CREATE AN ALTERNATING CURRENT ARE CALLED:

AN ALTERNATOR –

What does the generator do? CONVERT MECHANICAL ENERGY INTO ELECTRICITY

WHAT MEASURES ELECTRIC POTIENTAL = VOLT

CURRENT = AMP

ELECTRIC CIRCUIT IS THE PATHWAY FOR ELECTRIC CURRENT

What does a MOTOR due Converts ___________ energy To _______________ energy

OHMS LAW V = IR

V = POTIENTALA = AMPS (CURRENT)R = RESISTANCE (OHMS)

OHM’S LAW:

V = Potential difference in volts I = Current in amperes R = Resistance in ohms ()

V= IR I =V/R R=V/I

Simple SERIES Circuit

Series Circuit Rules:

Current: IT = I1 =I2 =I3 Voltage: VT = V1 + V2 + V3

Resistance: RT = R1 + R2 + R3

Parallel Circuit Rules Current: IT = I1 + I2 + I3 Voltage: VT = V1 = V2 = V3Resistance: 1 1 1 1 RT = R1 + R2 + R3 (REMEMBER TO FLIP SIDES RT/1

TYPES OF EQUIPMENT FIXED

MOBILE

DEDICATED MAMMO , CHEST, HEAD

EXPOSURE SWITCH

TIMER

RHEOSTAT VAIRIABLE REISITOR

regulate the amount of resistance in a circuit

What makes the Anode spin? Electromagnetic Induction Motor

How fast does it turn?

______________ rpm