X-ray Physics

In a nutshell

By Dr. Jill Davis

What you need to know: The tube – how x-rays are produced The body – how x-rays interact with the body The image – how x-rays interact with film Film processing

What are X-rays? Made of photons

Travel at speed of light Travels in a straight line

Has no mass nor charge (cannot be focused by magnets)

X-ray beam has a mix of energies Maximum energy in a beam = kVp Diagnostic X-ray range 20-150 kVp

What are X-rays?

The X-ray tube

The X-ray tube parts: Cathode (-)

Filament made of tungsten

Anode (+) target Tungsten disc that

turns on a rotor

Stator motor that turns the

rotor

Port Exit for the x-rays

X-ray production Push the “rotor” or

“prep” button Charges the filament –

causes thermionic emission (e- cloud)

Begins rotating the anode.

Push the “exposure” or “x-ray” button e-’s move toward anode

target to produce x-rays

Hitting the target e-’s hitting the target creates x-rays two

different ways: Characteristic x-rays – are due to the material the

e-’s hit (tungsten). Only occurs above 70 kVp Bremsstrahlung (braking) x-rays – due to slowing

down of e- beam. < 70 kVp – 100% of X-rays are of this type > 70 kVp – 85% of X-rays are of this type

Characteristic

Bremsstralung

Anode Heel Effect

Exposure Factors: kVp – kilovoltage peak mA – miliamps (current) s – seconds (duration of exposure) mAs – product of mA and s

Exposure factors are set by radiographer

X-ray Quality vs. Quantity Quality = penetrating power / energy Quantity = # of X-rays in beam

↑kVp = ↑ speed of e- = ↑ quality ↑ kVp = efficiency of x-ray production = ↑

quantity ↑ mA = more e- hit target = ↑ quantity ↑s = longer exposure time = ↑ quantity

What you need to know: The tube – how x-rays are produced The body – how x-rays interact with the

body The image – how x-rays interact with film Film processing

Interactions in the Body: Three things can happen to x-rays as they hit

the body: Absorption (photoelectric effect) – x-ray is

absorbed by tissues – does not contribute to image.

Scatter (Compton effect) – contributes to “fog” Transmission – penetrates through body to hit

radiographic film.

Interactions in the Body

Problem: Only x-rays of sufficient energy (quality) can

transmit through body to create an image. Low energy x-rays don’t contribute to the image, but

add to patient radiation dose. Also, different thicknesses, and composition of body

parts will determine amount of x-ray penetration. Therefore we need to reduce low energy (low

quality) x-rays, but at the same time have the right quantity of x-rays hitting the body part.

Filtration How we fix the problem is with filtration Three kinds of filtration:

Inherent – due to tube housing, insulation, etc. Added – aluminum shielding that blocks low

energy x-rays. Special – used to image body parts that have

varying thickness or density. Filtration is measured in terms of “half-value

layer”

“Special Filtration”

What you need to know: The tube – how x-rays are produced The body – how x-rays interact with the body The image – how x-rays interact with film Film processing

Image Quality

Density Controlling Factors:

mA and s ↑mAs = ↑quantity of photons reaching film =

↑density

Density Influencing factors:

↑kVp = ↑quality (penetration) = ↑density ↑SID (source-image distance) = ↓density

Due to inverse square law – intensity of x-ray is inversely proportional to the square of the distance from source.

↑OID (object-image distance) = ↓density Grids (discussed later) = grids ↓density ↑Film/screen speed = ↑density ↑body part thickness = ↓density ↑filtration = ↓density

Density and kVp

Density and SID

Image Quality

Contrast ↑contrast = short scale = more black and

white (less detail) ↓contrast = long scale = mores shades of grey

(more detail)

Contrast Controlling factor kVp

↑kVp = ↓ contrast (more shades of grey)

Contrast Influencing factors:

Grid –↓fog (scatter) = ↑contrast Collimation – narrow collimation = ↓ scatter =

↑contrast Anatomic part – variation in tissue density visible

on film What are the 5 tissue densities?

Air, Fat, Water/Tissue, Bone, Metal

Image Quality

Recorded Detail The “sharpness” of structural lines in the

image Geometric unsharpess Image receptor unsharpness Motion unsharpnesss

Geometric Unsharpness ↑SID = ↓divergence of rays = ↓ unsharpness ↑OID = ↑divergence of rays = ↑ unsharpness

Penumbra = geometric unsharpness along the edges of the film.

Image receptor unsharpness ↑film/screen speed = ↓detail = ↑unsharpness

Motion Unsharpness ↑ motion of patient, image receptor, or tube =

↑unsharpness Prevention of motion unsharpness:

↓ exposure time Patient instruction (i.e. hold breath) immobilization

Image Quality

Distortion Size Distortion

↑OID = ↑size distortion (magnification) ↑SID = ↓size distortion

Shape distortion Occurs when anatomical part is not parallel to the

image receptor (elongation or foreshortening) Reduced by proper patient positioning and/or

tube tilt.

Collimation Is located under the port of the X-ray tube. Has a light in it for radiographer to see where

x-rays would hit the patient Purpose- restricts beam

↓ patient dose ↓scatter (↑contrast)

Collimation should be visible on a minimum of three sides of the film

Grids Part of the “bucky” that hold the film cassette Reduces scatter radiation that hits film Grid is made of lead strips

Grid ratio – height/width of interspace Hitting prep button causes grid to vibrate to

blur out grid lines (doesn’t show up on film)

What you need to know: The tube – how x-rays are produced The body – how x-rays interact with the body The image – how x-rays interact with film Film processing

Film Photographic film has several layers:

Supercoat – protective covering Emulsion – is radiation and light sensitive

Made of silver halide and gelatin Base – plastic; for stability

Film is available in different “speeds” just like 35 mm camera film: the faster the speed, the less radiation is needed to produce an image.

Image formation Latent image – invisible image caused by

light or radiation exposure Manifest image – shows up after film is

developed

Intensifying screen Is located in the cassette that film is placed inside of. Screen contains “phosphors” – that fluoresces when

exposed to x-rays. Purpose – screens amplify x-rays that hit the film so

you need a lot less mAs to produce an image . Drawback – lose some recorded detail Screens also come in different “speeds” – i.e. the

degree to which it fluoresces upon exposure.

Film Processing May become obsolete as the industry moves

to digital Steps of processing (automatic)

Developer – converts latent image to manifest image (22 sec)

Fixer – acetic acid Wash- water removes residual chemicals Dry – blow dryer in the processor

Radiation Dosimetry - definitions Roentgen – unit of radiation that will liberate

a charge of 2.58 x10(-4) coulombs per kilogram of air.

Coulomb – unit of electrical charge RAD = radiation absorbed dose – 1 rad is

equal to the radiation necessary to deposit 100 ergs (unit of energy) in 1 gram of irradiated material SI unit: 1 gray = 100 RAD

Radiation Dosimetry - definitions REM – rad equivalent man – is the unit of

absorbed dose equivalent; is a measure of the biological effect of radiation. SI unit: 1 sievert = 100 REM

Radiosensitivity Radiation damages DNA Tissues that are sensitive to radiation are:

High – lymphocytes, spermatogonia, erythroblasts, intestinal crypt cells

Intermediate- endothelial cells, osteoblasts, spermatids, fibroblasts

Low – muscle cells, nerve cells, chondrocytes. Rule of thumb – the cells that proliferate more

are more sensitive

Positioning Tips SID (aka FFD or TFD)

Is either 40” or 72” Think 40” for all except FS, lat or oblique C-sp

(air gap), P-A chest Tube Tilt

For every 5° tube tilt, move tube 1” closer When to use tube tilt – to reduce shape

distortion Example – A-P lower cervical 15° cephalad = 37”

Positioning Tips Central Ray

Generally aim at middle of anatomy you want to see.

Film Size – small film for small part 8 x 10, 10 x 12, 14 x 17, 14 x 36

Collimation – how much do you restrict beam? Collimation – visible on film at least 3 sides Include anatomy you want to see

Positioning Tips Ten day rule

For females of childbearing age X-rays not taken after 10 days of start of menstrual

period. Shielding / filters

Gonadal shield any A-P view that includes the pelvis Lead apron over body parts not to be visualized

(extremity views) Filters – wedge filters (example, for A-P FS, wedge over

the superior half of spine.

You’re done!