RTEC A - WEEK 3

GENERAL SCIENCE REVIEW&

X-RAY PRODUCTION IN THE TUBE

Objectives

• General Science review

• Atomic interactions in the tube

Atomic Models

• BOHR model of the atom.

• Electrons orbit around a nucleus (center)

Notes from Slide 3

• Bohr’s model of the atom.• It looks like a miniature solar system in which the

electrons are orbiting around the nucleus (center) at various energy levels.

• There are many other forms models of the atom, and the most accurate is the quantum mechanics model. But for the purposes of radiology the Bohr model better serves our purpose.

• Atom is the smallest quantity of an element.

ATOM

Notes from Slide 5

• The nucleus is positively charged, small and dense. In a normal atom the number of electrons is equal to the number of protons. The primary particles of the atom are : neutrons, protons and electrons.

• Neutrons are in the nucleus and is electrically neutral. Protons are also in the nucleus but they have a positive charge. Electrons orbit the nucleus and have a negative charge.

Differences in Binding Energy

Notes from Slide 7• Difference in energy levels between the shells. • Takes a 70kvp energy to knock out a K shell electron.

• In this element- which is tungsten. The filament and target material. It has 74 protons.

• Kev get smaller as the shells go out. This tells you the closer you are to the nucleus the more binding energy there is. The binding energy is what it will take to eject an electron from its home. SO in the k shell it would take a minimum energy (kVp) of 69.5 to remove an electron from a k shell.

• The P shell needs a much smaller amount. As it has less binding energy.

• The atoms are very structured. If a k shell electron is empty, it must be filled. As the holes are filled there is more photon energy released. They are always moving into the nucleus because they are negative and the nucleus is positive (law of attraction).

K L M Shells

Notes from Slide 9

• Electrons orbit around nucleus.

• The K,L,M are the shells we are interested in in radiology. Usually the shell of main importance is K shell.

Electrostatic Laws

• Repulsion/attraction– Like charges repel– Unlike charges attract

• Inverse square relationship– Electrostatic force is very strong when objects

are close together – It decreases rapidly as objects separate

Notes from Slide 11

• Like charges repel, unlike charges attract

• Inverse square relationship is much like the inverse square law of x-ray intensity. The electrostatic force is directly proportional to the product of the electrostatic charges and inversely proportional to the square of the distance.

How “X-rays” are created

TO PRODUCE X-RAYS

YOU NEED:

• A SOURCE OF ELECTRONS

• A FORCE TO MOVE THEM QUICKLY

• SOMETHING TO STOP THEM SUDDENLY

How “X-rays” are created

• Power is sent to x-ray tube via cables

• mA (milliamperage) is sent to filament on cathode side.

• Filament heats up – electrons “boil off”

• Negative charge

Notes from Slide 14

• As electron kinetic energy is increased both the intensity (quantity) and energy (quality) of the x-ray beam are increased.

• All electrons have mass, electron kinetic energy is increased by raising the kVp.

• (Kinetic energy is the energy of motion)

• Distance between the filament and the anode target is only about 1cm.

•

How “X-rays” are created

• Positive voltage (kVp) is applied to ANODE

• Negative electrons = attracted across the tube to the positive ANODE.

• Electrons “slam into” anode – suddenly stopped.

• X-RAY PHOTONS ARE CREATED

Notes from Slide 16

• When the projectile electron hit the metal of the atoms of the x-ray tube target, they transfer their kinetic energy to the target atoms.

• The electrons can interact with orbital electrons or the nuclear field of target atoms.

• These interactions result in the conversion of electron kinetic energy into thermal energy (heat -99%) or electromagnetic energy (x-rays- 1%).

Electromagnetic Energy Spectrum

• Spectrum – Continuous range of energy– Although there are precise ranges defined,

they often overlap

• 3 most important to Radiologic technology:– Visible light– X-radiation– Radiofrequency

Notes from Slide 18• EM energy is a continuous range from lower energy AMFM radio to

gamma rays.

• EM travels in the sine wave and oscillates from magnetic to electric fields

• If you get enough energy it can penetrate matter.These generally have shorter wavelengths and increased frequency.

• Each can be described as a bundle of energy consisting of various electric and magnetic fields traveling at the speed of light.

• The photons the EMS differ only in their wavelength and frequency.

Electromagnetic Radiation

• Photon is the smallest quantity of any type of EM radiation– It is a small bundle of energy traveling at the

speed of light– Only visible light is naturally apparent to us

• May be described as wavelike fluctuations of electric and magnetic fields.

Electromagnetic Radiation

• These bundles of electric and magnetic fields travel at the same velocity:– Travel at the speed of light

– 3 x 108 m/s or 186,400 miles per sec

• The Photons of EM radiation differ only in frequency and wavelength

General Characteristics of EMSX-ray photons:

• Have no mass or physical form

• Travel in a linear path (until interaction occurs)

• Dual nature: wave vs. particle

• Unaffected by– electric or magnetic fields– gravity

Notes from Slide 22

• Electromagnetic structure.

• Ionizing EM radiation is usually characterized by the energy of the photon.

• X-ray photons contains considerably more energy than visible light or radiofrequency.

• The energy of the photon is directly proportional to is frequency. As frequency increases: energy increases.

Wavelength and Frequency

• Wavelength is the difference between:– Crest to Crest– Valley to Valley

• Frequency is the number of wavelengths passing a point of observation per second

• Wavelength & frequency are inversely proportional– As Wavelength increases frequency decreases– As wavelength decreases frequency increases

• Frequency and wavelength are closely associated with the relative energy of electromagnetic radiations.

• More energetic radiations have shorter wavelengths and higher frequency.

Wavelength and Frequency

Wavelength

Notes from Slide 26

• A second important characteristic of light waves, and all electromagnetic energy, is wavelength. That is the length of one wave measured from the top of one wave to the top of the next.

The red line may be representative of light, with a longer wavelength. The blue line has a shorter wavelength and may be representative of x-rays.

• Courtesy of Mosby’s Radiography Online. (Elsevier)

Frequency

Notes from Slide 28• Frequency refers to the number of waves that go by a point in 1

second.

• Remember that electromagnetic energy waves all travel at the same speed: the speed of light.

• Imagine two different waves traveling next to each other. You're timing them with a stopwatch and discover that one wavelength of the first wave goes by in 1 second. That's a frequency of one wave per second. The second wave has a wavelength half as long as the first, and because it is traveling at the same speed, two waves will go by in the 1 second. It has a frequency of two waves, or cycles, per second.

• Courtesy of Mosby’s Radiography Online. (Elsevier)

The shorter the wavelength – the higher the frequency

Notes from Slide 30

• Wavelength is the difference between crest to crest. Or valley to valley.

• High frequency or short wavelength is needed to penetrate tissue.

• Frequency how many crests pass per second.

Notes from Slide 32

• Because all electromagnetic energy waves move at the same speed, there is a simple relationship between wavelength and frequency: the longer the wavelength, the lower the frequency. The shorter the wavelength, the higher the frequency

The Electromagnetic Spectrum• X-rays have wavelengths much shorter than

visible light, but longer than high energy gamma rays.

Notes from Slide 34• As frequency increase the waves getter closer together indicating a higher

energy.

• Frequency and wavelength are inversely proportional.

• Right past UV light (sunburn), cannot go all the way through but it does somewhat. (sunburn)

• X-ray is called a photon because it has properties of matter and energy. The scientists are unable to decide if it was matter or energy.

• The only difference between x-rays and gamma rays is their origin. • X-rays are artificially simulated- emitted from an e- cloud of an

atom. In electrical imaging systems.

• Gamma rays come from inside the nucleus of a radioactive atom. Spontaneously from radioactive material.

What is Ionization?

Notes from Slide 36

• Removal or addition of an electron. Electrons always move in a straight line.

• It is removed because another electron moved in and knocked out and replaced.

• X-rays can ionize atoms.

• In an atoms normal state it is electrically neutral, the electrical charge is zero. If an atom has an extra electron or is missing an electron it is said to be ionized.

When an electron is added or removed from the atom- it is

ionized

Notes from Slide 38

• If we lose on it is positive. If we gain one it is negative.

• Loss of an negative electron makes it positive.

Kinetic energy

• Energy of motion

• The electrons KINETIC energy is converted to electromagnetic or PHOTON energy

Notes from Slide 40

• Taking kinetic energy and making it into photon energy.

X-ray production begins at the atomic level Energy (photons) are released when the electron collides with another electron,or passes close to the nucleus of the atom –the change in energy of the shells –produces photons

X-ray Production in the TUBE

INTERACTIONS IN THE TUBE

• BREMS (Bremsstrahlung)

• CHARACTERISTIC

• HEAT

Notes from Slide 44

• 3 interactions with the tubes.

Tube Interactions

• Heat = 99%

• X-ray = 1%

• Bremsstrahlung

(Brems) = 80%

• Characteristic = 20%

Notes from Slide 46

• Characteristic is better for contrast but the patient gets more exposure.

• Brems gets the job done but less exposure to patient.

Bremsstrahlung Radiation

• Heat & Characteristic produces EM energy by e- interacting with tungsten atoms e- of the target material

• Bremsstrahlung is produced by e- passing by closely with the nucleus of a target tungsten atom – the change in direction of the electron – releases a photon of energy

Brem’s Radiation Animation

• http://www.coursewareobjects.com/objects/mrophysics_v1/mod08/0816a.htm

Heat

• Most kinetic energy of projectile e- is converted into heat – 99%

• Projectile e- interact with the outer-shell e- of the target atoms but do not transfer enough energy to the outer-shell e- to ionize

Notes from Slide 49• Projectile electron from cathode to anode

• Not enough energy to kick a an electron out of its shell…but it excites the atom.

• Excitation is the release of heat

• 99% of the kinetic energy is converted into heat.• This occurs when the projectile electron interacts with the outer-

shell electron of the target. But they do not have enough force to ionize them. Therefore the electrons have an “excitation” phase in which heat is produced.

• Heat is directly proportional to increasing the x-ray tube current. Heat also increases when increasing kVp.

Heat

8 p+ + 8e- = neutral atom

1. Projectile electrons from cathode

2. Pass by the electrons in the target

3. Causing the electrons to vibrate (excitation)

4. Excitation produces small amounts of heat

e

e

HEAT

Heat is an excitation rather than an ionization

Notes from Slide 53

• Not enough energy to ionize….just excite.

Bremsstrahlung

German

word meaning

slowed-down

or braking

radiation

Notes from Slide 55

• This occurs when a projectile electron loses some of its kinetic energy as a result of interacting with the nuclear field of the atom.

• The kinetic energy of the projectile electron is converted to electromagnetic energy (x-ray photons).

Notes from Slide 57• The projectile electron has great kinetic energy as it approaches the nucleus.

Because the nucleus has a positive charge and the electron has a negative charge, there is an electrostatic attraction between them. This pulls the electron closer to the nucleus, even though its momentum continues to carry it forward in a bending line. It loses kinetic energy due to the acceleration as its path changes. This is the energy that is emitted as an x-ray photon.

• Courtesy of Mosby’s Radiography Online. (Elsevier)

• The projectile electron completely avoids the electrons of the target atom. It is the close proximity to the positive nuclear field of the nucleus that it interacts with that creates a braking or slowing down of the electron and a x-ray photon.

• The closer the projectile e- gets to the nucleus the more it is influenced by the field of the nucleus.

Notes from Slide 59• Incoming electron is electrons coming in (straight line).

• Photons when they exit the tube ( wavy).

• Straight line- has to be happening in the tube.

• Wavy line is an x-ray photon in the sine wave form.

• As the projectile e- passes by the nucleus it is slowed down and changes its course. This change in direction causes a loss of kinetic energy. This loss of kinetic energy reappears as an x-ray photon.

• The e- can lose all of it’s energy or a fraction of it’s energy depending how close it is to the nucleus.

Bremsstrahlung Radiation

Notes from Slide 61• If the projectile electron entering an atom in the metal of the anode

does not strike any of that atom's electrons, it may continue toward the center of the atom and come near the nucleus. Remember that the electron has a negative charge and the nucleus has a positive charge. Therefore the passing projectile electron is attracted to the nucleus. This attraction slows the electron down as it passes the nucleus and alters the direction of the electron's path as the nucleus "pulls" on the electron.

The slowing of the electron means that it loses kinetic energy—and this energy takes the form of a photon of x-ray energy being released.

• Courtesy of Mosby’s Radiography Online. (Elsevier)

• Brem’s accounts for 80% of the photons as it can be produced by any projectile e-. For this reason in diagnostic range, most x-rays are Brem’s radiation.

6464

X-ray Photons – BREMS

Creates a polychromatic spectrum – x-rays of different energies

Energy (photons) are released when the e passes close to the nucleus, then

changes direction

BREMS RADIATION

• Electron• Passes by

nucleus• Changes

direction• Energy

released as a PHOTON

Brem’s Radiation Animation

• http://www.coursewareobjects.com/objects/mrophysics_v1/mod08/0816a.htm

Characteristic Radiation

• Projectile e- with high enough energy to totally remove an inner-shell electron of the tungsten target

• All tube interactions result in a loss of kinetic energy from the projectile e-

• Characteristic x-rays are produced when outer-shell e- fills an inner-shell void

CHARACTERISTIC (in tube)• Electron hits inner

shell e in orbit – knocked out & creates a hole

• Other E’s want to jump in

• Energy released as PHOTONS

Notes from Slide 69

• Energy is created when the spaces in the shells are filled. The electron energy is converted in photon energy.

• With this interaction you get much more photons created, much more damaging to patients tissue. The only photons that get tot the patient are those in the K shell.

• It is called

characteristic

because it is

characteristic of

the target element

in the energy of

the photon

produced

Characteristic Radiation Animation

• http://www.coursewareobjects.com/objects/mrophysics_v1/mod08/0808a.htm

Tungsten Atom

Notes from Slide 73

• As this chart shows, the energy of x-rays that results from electrons farther from the atom's nucleus is greatly diminished. These x-rays have no value for diagnostic imaging, although they can still have effects within the body. Modern x-ray equipment is designed to minimize such low-energy x-rays. This is why a transformer is used in the x-ray machine to boost the voltage high enough to produce higher-energy x-rays.

• Courtesy of Mosby’s Radiography Online. (Elsevier)