01 - Basic Physics 2011 Backup

7/30/2019 01 - Basic Physics 2011 Backup

1/13

Radiology Physics

OR: I DIDNT SIGN

UP TO LEARNTHIS STUFF

Chris Ober, DVM,PhD, DACVR

7 February 2011

J ust take a deep breath

Why worry about physics?

Know what the system can give you

Know what the system CANT give you

Recognize errors and know how to correctthem

Understand the importance of radiationsafety

Its on the test, so I might as well teach it

Books to Consider

Thrall. Textbook of Veterinary DiagnosticRadiology. 4th or 5th ed. 2002 or 2007.

Morgan & Silverman. Techniques ofVeterinary Radiography. 4th ed. 1984.

*Bushberg, Seibert, et al. The EssentialPhysics of Medical Imaging. 2nd ed. 2002.

*Masochists only

The Game Plan

X-rays

Generation of X-rays Interaction of X-rays

with matter

Accessory equipment

1st Period

X-rays


with matter

Accessory equipment


2/13

What are X-rays?

Type of electromagnetic radiation

Can act as wave or particle (photon) No mass, no charge

Travel at speed of light

What are X-rays?

Shorter wavelength than visible light

Higher energy than visible light High energy makes them Ionizing Radiation

Ionizing Radiation

Radiation that iscapable of generatingions

Can cause disruptionof molecular bonds

Thus important inradiation safety andradiation therapy

Note: X-rays andgamma rays aredifferent only insource

X: outside nucleus,interaction of high-speed particles

Gamma: withinnucleus due tospontaneous decay

2nd Period

X-rays

Generation of X-rays

Interaction of X-rayswith matter

Accessory equipment

What We Need

Interaction of high-speed charged particles

Particles: electron source High speed: method of accelerating

electrons

Interaction: target for electrons to crashinto

The X-ray Tube


3/13

The X-ray Tube The X-ray Tube

Cathode Negatively charged

Filament made oftungsten

Set into focusing cup

Thermionic Emission Current thru filament

Heat generated

Electrons released(boiled off) in cloudaround filament

Cathode

Number of electrons released is directlyproportional to:

Current across filament (milliamperes =mA)

Exposure time (seconds =s)

mAs =mA x s

10 mAs =600 mA x 1/60 s

10 mAs =100 mA x 1/10 s

Cathode

Number of X-rays produced is directlyproportional to electron number & mAs

Operator selects mAs (or mA and sseparately)

Time selector (s)

mA selector

Cathode

Number of X-rays produced is directlyproportional to mAs

Operator selects mAs (or mA and sseparately)

Time (s) ormAs selector

mA selector

Cathode

Focal Spot sizedetermined byfilament size &

focusing cup Most machines have

2 filaments

Small focal spot:greater detail (spatialresolution)

Large focal spot:routine work (higheroutput capability)


4/13

Cathode

Larger penumbra =more unsharpness

Cathode

The X-ray Tube

Anode

Positively charged

Target made oftungsten (mostly)

Focal Spot

Small area whereinteraction withelectrons occurs

Actual site of X-rayproduction

Anode

Stationary Anode

Fixed target at end ofX-ray tube

Lightweight with fewermoving parts goodfor portables

Limited heatdissipation meanslimited X-rayproduction

Rotating Anode

High speed rotatingdisc interactionsspread over largerarea

Good heat dissipationmeans higher X-rayoutput

Stationary Anode Rotating Anode


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Anode Focal Spot

Target surface angled

relative to the path ofthe electron beam

Larger actual focalspot (interaction area)for better heatdissipation

Smaller effective focalspot for better imagedetail

The X-ray Tube

Envelope

Pyrex containerholding cathode &anode

Vacuum inside

Housing

Lead shieldingenclosing envelope

Window to let X-raysout in the desireddirection

X-ray Production

High potentialdifference (voltage)applied to X-ray tube Generally 40-140 kV

Electrons (-) pulledtoward anode (+)

Value set for kVpdetermines electron energy

photon energy

X-ray Production

Electrons collide withtarget, interacting withtungsten atoms

X-rays are produced

X-ray Generation

Electrons interacting with target produceX-rays in 2 possible ways

Characteristic radiation: Photons ofspecific energies / wavelengths

Bremsstrahlung: Photons of broad energyrange (photons of many wavelengths)

X-ray Production

Electrons with morekinetic energy willproduce X-rays with

greater energy Electron energy

determined bypotential across tube kilovolt peak (kVp)

Operator selects kVp

Maximum X-ray

energy will be equalto kVp, thoughaverage X-ray energyis only about 1/3-1/2kVp


6/13

X-Ray Generation

kVp

X-ray Generation

Only 1% of electrons energy X-rays

Wide range of X-ray energies areproduced (the kVp is the PEAK energy) Very low energy photons cant get out of tube

Moderately low energy photons can get out,but are diagnostically worthless, so filters areused to absorb them just outside the window

Energy not used in primary X-ray beam isconverted to heat in tube

X-ray Generation

Heat can kill the tube Oil housing helps dissipate heat

Rotating anode helps dissipate heat

Two-step exposure

Warm up high-output tubes

Overheating will Burn out filament

Pit anode

Cause metal deposits on envelope

Cost $$$

X-ray technique

Technique =combination of kVp and mAsused to make a radiograph

Values determine overall blackness aswell as overall contrast of image

Well get to this in more detail in a coupleof lectures

3rd Period

X-rays


with matter

Accessory equipment

Interaction with Matter

Photons have 3choices:

Pass through(Transmission)

Deposit all of theirenergy (Absorption)

Be deflected (Scatter)


7/13


Photons have 3

choices:




Photoelectric effect


Photons have 3

choices:




Compton scatter


Determinants of Transmission High energy photonmore transmission

Dense material less transmission

High atomic number (Z) material lesstransmission

Thicker material less transmission

Transmitted photons result in imageformation Transmitted photons turn film black

The Noble Step Wedge

High Energy Low Energy Atomic Number and Density

Basis of 5 basicradiopacities(remember those?)

Air

Fat

Fluid / Soft Tissue

Bone / Mineral

Metal


8/13

Atomic Number and Density

MetalFat

ST

Mineral

Gas

Low Z High Z

Thin vs. Thick Material Thick Thin


Absorption of X-ray photons =radiationexposure (important for radiation safety)

Absorption generation of small amountof heat

Differential photon absorption/transmissionof various structures is what produces thediagnostic image

Scatter Radiation

Degrades image(photons dont conveyinfo, as we dont know

where they camefrom)

Adds to personnelexposure

Unavoidable


9/13

Scatter Radiation

Lots of it Not very much of it

Minimizing Scatter

Make radiographedpart thinner

Collimate

Use (antiscatter) grid

Use air gap technique

Making Part Thinner

Less material for photons to pass through=fewer opportunities for ricochet

Commonly used in mammography

Radiolucent paddleused to displaceother structures

e.g. squish abdomen, get intestine out of way

Must decrease mAs to account fordecreased thickness

Making Part Thinner

Making Part Thinner Air Gap Technique

Space betweenpatient and film

Scatter photons more

likely to miss the film Extra point try vs.

50-yd. field goal

Rare in vet med

Distance between tubeand patient must alsobe large


10/13

Air Gap Technique

Space between

patient and film Scatter photons more

likely to miss the film

Extra point try vs.50-yd. field goal

Rare in vet med

Distance between tubeand patient must alsobe large

The OT

X-rays

Generation of X-rays

Interaction of X-rayswith matter

Accessory equipment

Collimators

X-rays are emitted from target in alldirections

Lead housing blocks most photons

Remainder go thru window, but even thissmall beam is wider than is needed for mostpurposes

Use a collimator

Restrict X-ray beam to area of interest

Collimators

Collimators Collimators

Original collimators were lead cones

We still use the phrase cone downwhenreferring to collimation

Now sets of lead shutters variableaperture much more versatile

Required by OSHA for radiation safety

Improves image quality by reducingscatter radiation


11/13

Collimators

Cone Collimator Fixed Aperture

Lead Shutters Variable Aperture

Collimators

Collimators

Used to minimize scatter

Fewer photons passing through material atthe edges means less opportunity to deflectinto the area of interest

Must walk fine line

Only include what you need in the field, butmake sure the study is complete

Always collimate at least a little bit

Collimators

No Collimation+

No Patient IDLabel

=

Grids

Plates with thin lead strips alternating withradiolucent material

Placed between patient and film Decreases scatter reaching film DOES NOT decrease scatter reaching staff

Generally located in table or Bucky tray, butcan have loose varieties

Grid absorbs many of the scatter photons,but also some primary photons Exposure technique must be increased


12/13

Grids

Focused grids most common

Lead strips angled to match X-ray beamdivergence

Distance from tube to grid must match gridsfocal distance

Grids can be unfocused (parallel strips)

Grids

Grids generally not used if part thickness


13/13

Trouble with Grids

Need to increase exposure technique

Often 3-5x higher than without grid Grid lines visible on image

Distracting

Use Potter-Bucky mechanism (tray in table) tooscillate grid during exposure and blur lines

DR systems may have software-based gridline suppression

Trouble with Grids

Damaged grid strips will be visible on

image Poor alignment of grid and X-ray beam will

cause various types of grid cutoff artifact

Grid Artifacts

Upside-Down Focused Gr id Off-Level Focused Gr id

What Have We Learned?

You dont have to be Einstein tounderstand how X-rays are produced

Scatter is not our friend

The collimator and grid, however, ARE ourfriends

01 - Basic Physics 2011 Backup

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