Category Archives:Radiography

Understanding The Difference Between kVp and mAs

Leave a comment Quality Control, Radiography

What are KVp and MAs?

To understand the difference between those two, you should first know that kilovolts (kV) and mAs are the two primary controls that we have with an X-Ray tube. They control the amount of radiation and the quality of the radiation beam or the X-ray beam.

In this week’s video, Eric from Olympic Health Physics talks about the difference between kilovolts (kV) and mAs and how each can be used because sometimes these two terms can be confused with each other.

What Does an X-Ray Tube Look Like?

The components that we’re going to be talking about with our X-Ray tube, in particular with kV and mAs is going to be the Heated Filament, the Anode which rotates around an axle or spindle and the Evacuated Chamber, which is where X-Ray production will take place.

Charges: Basic Electrostatic Force

Before we get too far into the operation of the X-Ray tube itself, let’s talk a little bit about charges

  • If we have two positive charges, they will repel from each other. 
  • If we have two negative charges, they will also repel from each other as we see here.
  • If we have two opposite charges, they’re going to be attracted towards each other.

The Anode And Cathode In an X-Ray Tube

If we apply those previous charges to the anode and cathode in an X-Ray tube, the cathode is held at a negative charge and the anode is held at a positive charge. When we put an electron in between the two, that electron is going to be attracted to or pulled towards the anode by the anode because an electron carries a negative charge. 

In conclusion, that electron is going to be pulled towards the anode and at the same time it’s being pulled towards the anode by the positive charges. It’s going to be pushed by the cathode because the negative charges on the cathode are going to repel or push the electron towards the anode.

The X-Ray Tube And The Heated Filament

In our X-Ray tube, the heated filament is going to boil off electrons.

The electrons are going to be created. If the filament is held at a negative charge and our anode is held out of a positive charge, the electrons will flow from the cathode towards the anode. Once they strike the anode, they will create X-Rays. 

We’re not really going into the formation of X-Rays and how they are formed, except to say that the interaction of the electrons with the anode will create two kinds of X-Rays.

How kVp is produced?

To understand how kVp is produced you should first know that within the X-Ray tube we can apply a potential difference.  This is a voltage difference between the cathode or the filament and the anode. This is where we create a potential difference. 

The bigger the voltage difference is or the stronger positive charge that we have on the anode, the stronger negative charge that we have on the cathode. 

The electrons will traverse the gap at a much higher rate. The velocity that the electrons accelerate across the gap of the chamber will be higher and higher as we increase the voltage of the tube. 

So that’s the voltage you’re seeing in the image above. If we have higher and higher energy electrons hitting the anode, the resultant X-Rays will also be higher energy. With higher energy of those, we end up with more penetrating power or the ability for the X-Rays to penetrate thicker and more dense body parts. 

In conclusion, the voltage difference between the cathode and the anode is kVp. 

As we increase the kVp or the voltage difference, we increase the speed at which the electrons traverse the chamber. They impact the anode at a higher and higher energy and create higher energy X-Rays.

If we decrease the voltage, then we will see a decrease in the energy of X-Rays and the X-Rays become less penetrating.

mAs: Introduction

mAs is milliamps seconds. It’s going to be directly proportional to the number of electrons that come off of the filament and are accelerated across the gap.

What exactly does that look like?

We have an X-Ray tube and we have a cathode. The electrons are moving across the tube towards the anode. It will produce a certain number of X-Rays with a certain number of electrons. If we increase the mAs, we increase the number of electrons and the number of X-Ray formed.

Remember that’s kVp, but we’ll increase the actual number of electrons and that’s going to increase the number of X-Rays.

So mAs is a control that’s directly proportional to radiation dose. If we double the mAs, then we’re going to double the radiation dose to the patient. 

What is mAs?

As was mentioned before, mAs is milliamp hour times seconds or times time. We can get rid of the “m” part and what we’re left with is just amps and seconds. 

What is an Amp?

An amp is just a Coulomb per second. 

What's a Coulomb?

A Coulomb is a unit of charge and electrons carry charge.  If we have our amps times seconds, that’s really just coulombs per second time because the seconds cancel out and we’re left with just Coulombs. 

Coulombs is a unit of charge and that’s going to be proportional to the number of electrons, which is going to be proportional to the number of X-Rays. 

So this is how we go from mAs (milliamp seconds) to the number of X-Rays. 

X-Ray Tube: Elements Explanation

Heated Filament

This is where the electrons come off and are accelerated towards the anode over this gap.

The Anode

The anode is going to be held at a positive charge and the voltage difference between the negative cathode and the positively charged anode. The higher that voltage difference is between these two, the faster the electrons will move from the cathode towards the anode and the higher energy the X-Rays will be that we produce. 

 And if we increase the mAs,  it’s going to be increasing the number of electrons that come off of the cathode and are accelerated towards the anode, increasing the number of X-Rays that are generated. 

kVp and mAs

kVp will control the penetrating power of your X-Ray beam or how energetic the X-Rays are and mAs will control the number of X-Rays. 

It is important to remember that If we increase the kVp or we increase the mAs, we’re going to see an increase in radiation dose. 

With increases in mAs, we’ll see better image quality in general. A better image quality because we’ll have less noise in the image. However, we want to weigh or balance the amount of mAs or radiation dose that we give against the resultant image quality

How Much Image Quality Do We Need?

We don’t want to just use as much mAs as possible. We want to be judicious with it because it does contribute to the radiation dose of the patient. 

Finally that was pretty much all you needed to know about the differences between kVp and mAs.

If you have any questions for us, feel free to drop them in the comments below and let us know how we can help. 

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Our team is dedicated to ensuring that your facilities’ radiation safety program functions in accordance to regulatory standards, sound radiation safety principles, and most importantly serves to protect staff, patients, and the general public from the hazards associated with ionizing radiation. To this end, Olympic Health Physics can provide a physicist to your organization to fulfill the Duties of the Radiation Safety Officer. To learn more, check out our RSO Services or click the link below. 

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Veterinary Radiation Shielding Design for X-Ray

Leave a comment Radiography, Veterinarian
Veterinary Shielding Design for X-Ray

In this post, we’re going to be talking about veterinary radiation shielding designs for X-Ray.

Watch the video below on our YouTube channel as Eric walks you through what a veterinary radiation shielding design is for, what information is required to prepare a design, and the calculations used to create it.

What Are We Shielding in Veterinary Medicine?

When creating a radiation shielding design for veterinary medicine, we’re shielding the x-ray units that are installed in veterinarian hospitals. Oftentimes, we will also shield dental spaces, although we aren’t talking about dental today.

Why Do We Need Shielding At All?

The reason that we need shielding is so that we can demonstrate that we are not exposing members of the public or employees to radiation unnecessarily. We want to create a shield around the x-ray unit and the room that the x-ray unit is in to ensure that the radiation produced stays in that space. This includes either primary radiation or scatter radiation.

Factors Required For Shielding a Veterinarian Radiographic Space

What are the factors that we need to consider when we’re shielding a veterinarian radiographic space?

There are actually a number of factors. All of the information that we need comes from a guidance document called NCRP, Report Number 148, which is Radiation Protection in Veterinary Medicine. The appendix walks us through exactly how to do a shielding design. The pieces of information that we need are:

  • The maximum operating Kv (max kVp). This is the maximum operating Kv that you’re going to use clinically.
  • The average mAs that you’re going to use clinically
  • How many patients per week on average are going to receive x-ray imaging

What is Workload?

Using the three values from above, we can determine the workload or how much radiation is going to be generated within the radiology space for a given week. We then convert that to milliamp minutes per week to get the workload.

What is the Use Factor?

We also need to know the use factor. The use factor is how often the x-ray tube is pointed or radiation is incident on a particular barrier. Oftentimes if you’re in a single story building, there’s no occupancy above or below the x-ray space. Then we only consider scatter radiation. If you have a multiple story building and you are directing the useful beam either into the floor or into a wall, then we need to consider the use or how often it’s going to be pointed at a particular wall to understand the primary radiation. In most veterinarian applications, the x-ray tube is just pointed at the floor. Most veterinarian hospitals are in a single story building, or if it isn’t in a single story building, the x-ray will be on the first floor with nothing below. In those cases we can just assume scatter radiation, which is a little bit easier to deal with from a shielding perspective.

What is the Occupancy Factor?

In addition to the workload and the use factor, we also need to know the occupancy factor. The occupancy factor is a description of what is in the surrounding areas that are around the x-ray room. For example, is there a hallway on the other side or is it a office, a bathroom or is it an outside exterior to the building? We need to know what is around the x-ray room so that we can apply the correct occupancy factor.

A Floor Map or Blueprint

The best way for us to do this is to get a floor map, and that floor map should be to scale so that we know what the distances are from the x-ray unit itself to each of the different barriers. With that floor map, we can measure those distances and we can also determine what is on the other side of each wall.

From there, we need to know a description of what is in each barrier that is already in the room. Are we talking about just standard gypsum wallboard? Are we talking about exterior walls with concrete masonry unit blocks? Concrete blocks? What is in the walls already? Has the room previously been shielded or is there already maybe lead in the walls?

We also want to know this information before we get started on the shielding design. Oftentimes we can get that information if it’s an older building using an as-built blueprint from the architect or potentially we could use a previous shielding design that was done in the past.

The Shielding Design Calculation

From all of the information gathered above, we calculate the shielding design using the following formula, where each value equals:

W = Workload

U = Use Factor

T = Occupancy Factor

We multiply those numbers together and divide by PD squared.

P = Design Goal. The design goal is the amount of radiation dose that we’re allowed to go through the wall.

D = Distance From the X-Ray Source to the Barrier

Putting it All Together

Once we complete the calculation using the formula, we can use this information to go back to a lookup table in our guidance documents and NCRP Report Number 148. We look up that value against the max operating voltage, the max kVp, and determine the amount of shielding that needs to be put into the barrier for a veterinarian x-ray.

Oftentimes we find that we can shield the room with simple drywall or perhaps additional layers of or thicker drywall or gypsum wallboard. There are some applications where we need to put lead into the walls because it would be too much drywall stacked on top of each other.

Height of Shielding

When we prescribe any kind of shielding, it needs to extend from the floor to a height of seven feet, and any barrier penetrations for things like outlets or duct work would need to be backed or somehow stopped with the equivalent material that’s prescribed in the rest of the wall.

Additionally, for veterinarian x-ray installations, we oftentimes don’t see control rooms. This is standard practice in veterinarian medicine. The last thing to know about veterinarian shielding designs is that even though we may not need to put lead in the walls, oftentimes the actual shielding design is going to be required by a regulatory agency, normally a state.

If you’re installing a new x-ray unit in a veterinarian suite, it’s important to understand the actual requirements for producing a radiation shielding design for veterinarian spaces.

Conclusion

If you have questions about radiation shielding design for veterinarian applications, feel free to reach out.

We’ll be happy to review your particular situation and give you ideas on the best way that you can shield your x-ray room and make sure that you’re keeping your department, your clinic, or your hospital safe for your staff and your visitors.

If you require assistance with a veterinary radiation shielding design, including CT and dental, please reach out or click here to learn more about our shielding design services. You can also always reach out to us if you have questions or want more information on why you should partner with us. 

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