Radiology Lecture 1: Radiology Physics and Exposure Settings

X-rays are a form of

Electromagnetic (EM) radiation

EM radiation consists of oscillating _______________ moving through space at right angles to each other

Electric and magnetic fields

What do differences in X-rays depend on?

Wavelength and frequency

All EM radiations are forms of __________. What is the velocity for all forms of EM radiation?

- Light

- Speed of light (c)

Energy carried by the radiation

Photon

Distance between successive peaks in a wave

Wavelength (γ)

What is the wavelength of x-rays?

0.01 to 0.05 nm (very short)

Number of waves that pass a point in a unit of time

Frequency (μ)

What is the frequency of x-rays?

Very high

How are wavelength and frequency related?

Inversely

Energy is proportional to __________. Inversely proportional to _________. X-rays have ________ energy

- Frequency

- Wavelength

- Great

Radiation with enough energy to ionize atoms in absorbing material

Ionizing radiation

X-rays are a form of ________________ and therefore can cause ________________

- Ionizing radiation

- BIologic damage to patients

X-rays have ____ mass and ____ charge

No; no

X-rays are ________ passing through _______ with very _____ wavelengths and _______ frequencies

Waves; space; short; high

X-rays have such great energy that they behave like particle; they can __________

Ionize atoms

What is the X-ray tube?

A vacuum glass tube with cathode filament and anode target

What are the cathode filament and anode target of the X-ray tube made of?

Tungsten

What is the focusing cup of the X-ray tube made of?

Molybdenum

How are x-rays produced?

Step 1=

Thermionic emission; space charge (around filament)

Step 2=

Electrons drawn across x-ray tube at high speed

Potential difference in kilovolts (kV); tube current in milliamperes (mA); focusing cup, electrons focused to a point

Step 3=

Electrons collide with target, converting kinetic energy into EM radiation

Focal spot; kinetic energy; converting kinetic energy into EM; only 1% produce x-rays; primary beam; PID

What is thermionic emission?

Thin tungsten wire is heated by electric current to boil off electrons

Potential difference measured in kilovolts (kV)

Electrons are drawn across tube through the production of positive charge at the anode end of the tube with respect to the cathode end

Electrons moving across the tube from the cathode filament to the anode target produce the ____________, measured in __________

Tube current; miliamperes (mA)

What does the focusing cup do?

Prevents electrons moving across the tube from diverging

- Electrons focused to a point as they move towards anode

What is the point at the anode target that electrons in tube current are focused to?

Focal spot

What kind of energy do moving electrons have?

Kinetic energy

What happens when electrons slow down and/or stop when they hit target?

Kinetic energy is converted into EM

99% of electron interactions at target produce _________, only 1% produce ___________

heat; X-rays

What produces primary beam?

X-rays produced at focal spot of the anode target spread all over; the X-ray that exits through the window forms it

T/F: Cones are used to produce X-rays

False

- Position-indicating devices (PID) are used

Term: The length of time the electrons move across the tube

Exposure time measured in seconds or impulses

Each impulse is _________

1/60 of a second

T/F: X-rays are produced when anode is negative

False

- X-rays only produced in half of cycle when anode is positive with respect to cathode

T/F: X-rays produced continuously

False

- Not produced continuously, but in discrete impulses

Term: The energy that holds electrons in orbit

Binding energy

Binding energies are ________ in elements with more protons

- Give an example

Greater

- Tungsten

What shell has the greatest binding energy? Why?

K shell because they are closest to the nucleus

What happens when electrons encounter anode target?

1. Bremsstrahlung radiation

2. Characteristic radiation

What is Bremsstrahlung radiation?

Electrons "put on the brakes" when they get closer to/reach the nucleus (positive)

- Produces x-rays with a wide range of energy = polyenergetic

What happens when an electron slows down or stops? (Bremsstrahlung)

It converts kinetic energy into EM radiation, including X-ray

T/F: Most x-rays (85-90%) are produced through characteristic radiation

False

- Most from Brehmsstrahlung

- Characteristic radiation makes up 10-15%

X-rays produces through Brehmsstrahlung are ________

Polyenergetic (have multiple energy levels)

In Characteristic radiation, as the electron moves from L to K, it _________ energy equal to the __________

What is the name for this in W?

- Emits

- difference in binding energies

- x-ray range

In characteristic radiation, the energy emitted is ________ of the element

Characteristic

Characteristic radiation is ________

Monoenergetic

Kilovoltage peak (kVp)

Difference in potential or electrical charge b/w cathode and anode

- Peak value

What does kVp control?

energy of x-ray beam

What does high kVp entail?

High energy X-rays

- Darker image

What does low kVp entail?

Low energy X-rays

- Lighter image

What does Milliamperage determine?

Number of x-rays, but NOT the energy level

What does high mA entail?

Darker image

What does low mA entail?

Lighter image

What is the relationship between mA and number of x-rays?

Linear

What does exposure time determine?

Number of x-rays, but NOT the energy level

What does long exposure time entail?

Darker image

What does shorter exposure time entail?

Lighter image

What is the relationship between exposure time and number of x-rays?

Linear

how do you calculate seconds or impulses?

seconds = impluses/60

impulses = seconds x 60

What is mA-s law?

mA-s= milliampere-seconds

- mA and exposure time control the same thing

T/F: If you like the darkness of the image, mA-s should be changed

False

To keep the number of photos constant, if mA is increased, what must happen to time?

Time must decrease

- if mA is decrease, time must increase

Practice:

- Original settings= 10 mA, 0.3 sec= 3 mA-s

- New mA= 7 mA

What exposure time should we use?

Want 3 mA-s=

3 mA-s = 7 mA * X s

X s = 3 mA-s/7 mA = 0.43 s

What causes darker images?

- High kVp (high energy X-rays)

- High mA

- Long exposure time

What causes lighter images?

- Low kVp (low energy X-rays)

- Low mA

- Short exposure time