2 Radiation safety and physics

What is the role of the cathode in x-ray generation?

The cathode is a coiled wire filament that, when heated by low current (mA), causes electrons to boil off

How does the temperature of the cathode affect electron release?

The higher the temperature (mA), the more electrons are released

What is the role of the electrical current in x-ray generation?

High voltage is applied to the anode, creating a voltage differential between the cathode and anode, allowing electrons to move towards the anode

What happens when electrons interact with the anode?

99% of the interaction produces heat, and 1% produces x-rays

Briefly explain the process of x-ray generation

1. Low current (mA) applied to cathode, causing electrons to boil off the cathode

2. Electrical current (kV) applied to the anode

3. Electrons flow to the anode due to the voltage differential produced by the kV

4. Electrons interact with tungsten

5. X-rays produced

What is the difference between rotating and stationary anodes?

Rotating anodes allow for a larger surface area for heat dissipation, while stationary anodes do not.

What is the difference between a sophisticated machine and portable machine's cost, size, tube current and electrical current?

A sophisticated machine is expensive, larger and has a higher tube current. A sophisticated machine's tube current has a maximum of 150kV, whereas a portable machine is usually <120kV

What is the difference between a sophisticated machine and portable machine's cathode?

In a sophisticated machine, there is more heating of the filament and more electrons are released

What is the difference between a sophisticated machine and portable machine's power supply?

Sophisticated machines have a special power supply, using three-phase power that allows electrons to hit the anode continuously

Portable machines have half or full wave rectification, meaning the electrons hit the anode in bursts

What is the difference between a sophisticated machine and portable machine's anode?

The sophisticated machine has a rotating anode (whirring), allowing larger surface area for heat to dissipate

Portable machines have a stationary anode

What is the difference between a sophisticated machine and portable machine's exposure time, heat and image?

Sophisticated - short exposure time, lower heat produced, crisper margins and less blur

What is the effect of kV on the x-ray beam?

kV determines the average energy of the x-ray beam, equating to its penetrating power

What happens when kV is increased on the x-ray machine?

It increases the electrical current between the cathode and anode, so the electrons will have more energy when they hit the anode, resulting in more high-energy x-rays

What does mA represent in x-ray exposure?

mA represents the amount of x-rays produced from beginning to end

What is the effect of increasing mAs on the x-ray machine?

It increases the current to the cathode filament, causing more electrons to bubble off and produce more x-rays

What is the exposure index in radiography?

It is a number that estimates exposure on the detector, helping to determine underexposure or overexposure

What occurs during overexposure in radiography?

Detector elements are saturated, assigning the maximum pixel blackness value, resulting in a black image that cannot be corrected

What are the two types of technique charts used in practice?

Based on thickness

Rough guide based on size (e.g., small or large dog)

What modifications can be performed with post-processing of DICOM images?

Darkness/density, contrast, magnification, measurement, rotation, and viewing multiple images

What cannot be corrected with post-processing?

Overexposure or underexposure, poor positioning, distortion

How are x-rays produced in the anode of the x-ray tube?

Electrons hit the anode at high speed, slowing down as they pass the nuclei, releasing energy as x-rays

What are the three possibilities for x-rays as they enter the patient?

Pass through (black)

Be absorbed (white)

Scatter

How does the absorption of x-rays differ in various tissues?

Atoms with more electrons absorb more x-rays

Higher atomic number (Z) means more absorption

What is the photoelectric effect?

An x-ray enters an atom, is absorbed, and ejects an electron from the atom

Why are there five radiographic opacities?

The difference in the appearance of the tissues is due to difference in absorption of x-rays due to the photoelectric effect

Tissues that have a higher atomic number (Z), absorb more x-rays and are whiter

How is scatter produced in radiography?

An x-ray interacts with an electron in a patient's atom and is scattered instead of being absorbed

What are the effects of scatter radiation?

It can cause cell damage leading to cancer

It can degrade image quality by creating fog

How can the impact of scatter be reduced?

With a grid placed between the patient and the imaging plate - reduces scatter after it is made and does not have any radiation safety benefit

What materials are grids made from?

Alternating strips of lead (absorbs the scatted radiation) and a spacer material such as foam

List three things that cause distortion

1. The beam is not centred over the area of interest

2. Increased patient/anatomy distance to the imaging plate

3. The anatomy is not parallel to the x-ray beam

How can distortion be prevented?

Center the X-ray

Ensure collimation boundaries are smaller

Keep anatomy close to the imaging plate

Make sure anatomy is parallel to the x-ray beam

What does increased distance from the imaging plate cause?

Magnification and blurry edges

What is movement blur in radiographs?

The radiograph looks blurry and the margins of the anatomy are not well defined

What are the properties of x-rays?

X-rays are invisible, travel in straight lines, penetrate matter, can ionise atoms, and have no mass or charge.

How do x-rays cause damage to atoms?

Ionising radiation ejects an electron from the atom, causing a free radical cascade

What is the biggest risk associated with radiography?

Fluoroscopy is the biggest risk of significant radiation exposure - chronic low dose (especially for cataracts)

What are the possible adverse effects of radiation exposure?

Cancer, birth defects, sterility, and cataracts

What dose of radiation can cause cancer/ birth defects?

Cancer - no threshold, so there is an increased risk with any radiation exposure down to zero

Birth defects due to exposure during pregnancy - >100mSv

Birth defects due to mutations in germ cells -

>1000mSv

What does ALARA stand for?

As Low As Reasonably Achievable

How can you keep radiation exposure ALARA?

Take radiographs only if it is clinically indicated

Take the exposure either behind a lead screen or wearing lead PPE if holding the patient

Only hold the patient if the patient is not able to have sedation (and limit the times the patient has to be held)

No human anatomy in the primary beam

How can radiation exposure be minimised when holding patients for radiography?

Wear lead PPE

Stand as far back as possible

Rotate people holding the patient

Ensure no human anatomy is in the primary beam (even with gloves)

Identify correct use of lead PPE

Lead gown

Thyroid collar

Gloves - fully enclosed

What is the inverse square law?

If you move twice the distance away, you will get a quarter of the exposure

What is the difference between computed radiography (CR) and direct digital radiography (DDR)?

CR requires a plate reader, multiple plates and is slower, while DDR sends images directly to the computer and is faster, but is more expensive

What is the advantage of DDR regarding positioning?

The plate is not moved, allowing for adjustments without repositioning the patient

In CR the plate has to be taken from under the patient, disrupting the positioning

What is the cost difference between CR and DDR?

CR is cheaper to buy and replace plates, while DDR is more expensive