X-Rays and Computerized Tomography (CT)

Question 1

What are X-RAYS?

Answer 1

• Electromagnetic radiation with a short wavelength and high energy, typically referred to as either soft or hard in terms of energy
• Often measured in eV (1eV = 1.602*10-19J).
• Due to their high energy they are ionizing and can penetrate deeper than light into the body

Question 2

What is a GRAY, Gy?

Answer 2

A unit used to measure the intensity of an x-ray
1Gy = 1j in 1kg of dry air
measures the absorbed dose, D

Question 3

What is a SIEVERT, Sv?

Answer 3

A unit used to relate the human exposure to radiation
Sv = Gy * Dose
Sv are defined as the dose equivalent, H

Question 4

What is BACKGROUND RADIATION?

Answer 4

The unseen radiation the exists and provides a benchmark for risk

Question 5

What are some SOURCES OF BACKGROUNF RADIATION?

Answer 5

Package inspections, artwork analysis, smoke detectors, water, food, stone, air travel and decay of heavy elements

Question 6

What are SAFE DOSES OF RADIATION?

Answer 6

5-20mSv/year
50mSv/year for occupational

Question 7

What are PHOTONS?

Answer 7

Small particles that comprises waves of electromagnetic radiation

Question 8

How are PHOTONS EMITTED?

Answer 8

Atoms emit photons when they collide with other fast moving particles
* the energy in the collisions excite the atoms causing the electrons to jump up energy levels making them unstable
* the electrons revert to their original level releasing the excess energy in the form of a photon
* for each atom there are many energy transitions, each with a specific energy difference - range of transitions is known as an emission spectrum

Question 9

How are X-rays GENERATED?

Answer 9

• Generated through vacuum tubes (electrons transmitted from cathode to anode gaining kinetic energy)
• impacts at the anode converts the energy into x-rays

Question 10

What is a BREMSSTRAHLUNG?

Answer 10

• The electromagnetic radiation emitted in the form of photons when a charged particle is decelerating upon striking against another charged particle
• AKA braking or deceleration radiation
• Deceleration in a series of collisions, less velocity = less kinetic energy
• Produces a range of energy values, broad series of possibilities for deceleration ∴ broad spectrum

Question 11

What is CHARACTERISTIC RADIATION?

Answer 11

Arriving electrons knock the inner electrons of an atom out of orbit, causing them to be replaced by electrons from the outer shell - for higher electrons, where the energy exceeds the k-shell binding energy.
this causes the release of a photon with an energy equal to the energy difference between levels.
It is material specific hence ‘characteristic’

Question 12

What is a FOCAL SPOT?

Answer 12

In the order of 10s-100s of µm (depending on the resolution) - small focus spots increase the localized heating effect on the anode constraining the x-ray power. This effect can be countered by a rotating anode

Question 13

What are some APPLICATIONS OF X-RAYS?

Answer 13

Package inspections/Artwork analysis/Radiotherapy/Diagnostic imaging

Question 14

How does the BEAM ANGLE and FOCAL SPOT SIZE affect BEAM WIDTH?

Answer 14

The angle of the beam and the size of the focal spots determine the width of the beam
* Focal spot size controlled by applying an electric field to counteract electron-electron repulsion
* The smaller the anode angle the smaller the focal spot of the emitted X-rays

Question 15

How does the DISTANCE FROM THE FOCAL SPOT affect the system?

Answer 15

The intensity decreases with an increased distance from the focal spot at a rate proportional to the square of the distance (inverse square law)
* Impacts the imaging system on two levels:
* Source-to-object distance (radiation safety)
* Source-to-imager distance (image quality)

Question 16

What are the 3 TYPES OF SCATTERING?

Answer 16

• Rayleigh scattering
• Compton Scattering
• The photo-acoustic effect

Question 17

What is RAYLEIGH SCATTERING and when is it used?

Answer 17

• The scattering of photons
• For energies <30keV or high atomic number (z) medias
  of concern in low-energy mammography and micro-CT

Question 18

What is COMPTON SCATTERING and when is it used?

Answer 18

• The scattering of relatively low energy photons emitted from an isotope that pass through the tissue and collide with molecules leading to a change in direction and slight energy loss
• For energies >26keV4
• Dependent on density - independent of atomic number (z)

Question 19

What is the PHOTO-ELECTRIC EFFECT and when is it used?

Answer 19

• When an x-ray is stopped locally and an electron and low energy photon is emitted.
• The probability is proportional to Z^3/E^3
• Generates contrast between bone and soft tissue
• Enables detection
• The basis of several contrast agents

Question 20

Why is ABSORPTION important for X-rays?

Answer 20

• The mass attenuation co-efficient is given as the sum of the attenuation coefficients for each of the scattering effects
• Only high energy x-rays are capable of penetrating the body and reaching the film, most are absorbed (1-4% of the primary radiation is transmitted to the detector)

Question 21

How are x-rays SUPRESSED?

Answer 21

The lower energy x-rays can be filtered by a thin sheet of aluminimum

Question 22

What is the HALF-VALUE LAYER (HVL)?

Answer 22

The thickness of aluminium needed to cut the beams intensity in half

Question 23

How does ATTENUATION of the material affect the final image when using X-RAYS?

Answer 23

• an x-ray image is based on the attenuation of the material it passes through
• High attenuation: due to a high density material, i.e., bone appear bright as they absorb more of the x-rays
• Low attenuation: equates to low density matter such as air or fluids, they appear darker

Question 24

What COMPONENTS make up a SIMPLE x-ray system?

Answer 24

• Source
• Detector
• Screens

Question 25

What makes up the SOURCES in an x-ray system?

Answer 25

A high-voltage generator and x-ray tube, filter and collimator (restricts radiation to a specific area)

Question 26

What makes up the DETECTORS in a x-ray system?

Answer 26

A grid allows only primary radiation through, intensifying screens react when hit by x-rays and act as an intermediate detection stage.
film/detector used to record the signal from the screens - fluorescence is captured

Question 27

What are SCREENS in an x-ray system?

Answer 27

• Designed to counter the low absorption rate of film and are made to be highly absorbent of x-ray photons and can be positioned in-front or behind the film
• Have a trade-off as can cause images to blur
• Resolution is measured in line pairs per mm (lp/mm) - a dark line followed by a bright line, the more line-pairs the better the resolution (human eye can’t see >20lp/mm)

Question 28

Why are SCREENS NOT ALWAYS NEEDED?

Answer 28

Not all applications need higher resolution and can lead to unnecessary exposure. very few body parts have a spatial resolution > 1lp/mm

Question 29

What does the FILM/DETECTOR DO?

Answer 29

Converts the x-rays back into electrical signals
* Most detectors are integrating detectors (only consider the net energy - not each photon)
* either by direction detection through ionization or indirect detection through scintillation (flash of light)

Question 30

What is AUTOMATIC EXPOSURE CONTROL (AEC)?

Answer 30

• Similar to digital cameras, a sensor sets the exposure time and the AEC monitors the radiation - typically by using three ion chambers in the sensor plate.
• They must account for film/screen sensitivity and detector sensitivity vs beam energy. As to not over/under -expose the image

Question 31

What are the 3 TYPES OF PROJECTION SYSTEM?

Answer 31

1) Radiography
2) Mammography
3) Fluoroscopy

Question 32

Why is RADIATION EXPOSURE IMPORTANT to consider for x-ray systems?

Answer 32

The risk vs benefit must be considered as we don’t want the patient to experience unnecessary exposure as it can lead to cancer and other mutation of cells.

Question 33

What is COMPUTED RADIOGRAPHY?

Answer 33

An indirect form of digital imaging using a cassette similar in appearance and function to that used in conventional film-screen radiography

Question 34

What are PHOSPHORS?

Answer 34

A solid material that emits light when exposed to radiation, used in x-ray intensifiers to produce afterimages

Question 35

What are STORAGE PHOSPHORS?

Answer 35

• They are capable of absorbing and storing energy from the x-rays. The afterimage effects can be increased for an advantage
• The phosphor fluorescence is detected optically, then amplified and converted into digital signal, the screen can be scanned at a low level.

Question 36

What is TOMOSYNTHESIS?

Answer 36

An advanced form of mammography that produces 3D images using low-dose x-ray systems

Question 37

What is COMPUTED TOMOGRAPHY (CT)?

Answer 37

A diagnostic imaging procedure that takes slices by scanning an area from many angles in each plane. The detail that can be reconstructed is dependent on the number of angles sampled and the sampling coarseness at each angle.

Question 38

How many GENERATIONS of CT machines are there?

Answer 38

4

Question 39

What is a 1ST GENERATION CT MACHINE?

Answer 39

• Single pencil beam
• Single detector
• Translate-rotate scan
• Beam limited to the dimensions of the detector by ‘coning’

Question 40

What is a 2ND GENERATION CT MACHINE?

Answer 40

• An array of 100+ detectors spaced every 5mm
• Fan beam source
• Source and detectors rotated together - no need to translate
• Scan times <10s per slice

Question 41

What is a 3RD GENERATION CT MACHINE?

Answer 41

• Several hundred gantry based, fixed detectors with x-ray tube rotation
• Improved computers + higher output x-ray tubes reduced scan times (<2s per 2mm slice)
• 10-15 exposures permit study of liver/abdomen
• Power supplies that rotate with the x-ray tubes using slip rings avoid inversion every 2 cycles

Question 42

What is a 4TH GENERATION CT MACHINE?

Answer 42

• Table translation with continuous tube assembly rotation
• helical scanning allows for >5 images/s
• CT heart angiography (imaging of vessels)