Video Tutorial on Inverse Square Law

This video is using samples from an earlier blog post where my images mysteriously disappeared... I have replaced the images as well as created this video with additional explanations to supplement the original blog post:

Recognizing Motion on Lateral Chest Exams

One of our students brought this to my attention today, and I thought it would be a great example to share here.  The first image was the lateral chest view that I was asked to look at.  The technical factors used were 120 kVp, 32 mAs (320 mA at 0.1 sec) which produced an S# of 459:

Let's zoom in to the lower lungs and diaphragm:

This is one of the reasons it is good to magnify your images (if possible) when you QC your work.  You can tell that there is motion blur in the lung markings, lower ribs, and the diaphragm.  We agreed to repeat the image and we made some changes to the technical factors.  We decided to use 130 kVp at 32 mAs and change the mA and time stations (640 mA at 0.05 sec) which produced an S# of 373.  Here is image 2:

It is obvious that the patient had a larger breath in this time, and we are starting to see more density through the lung bases.  Even without zooming, you can see a large improvement in visualization of lung detail, ribs and diaphragm.  I'm going to window/level adjust and zoom in to compare to the prior zoomed image:

The difference is clearly noted with this magnified image.  Lung markings have clear, crisp detail, as well as well defined diaphragm margins and ribs.  You even notice far less visualization of the thoracic spine - another great example of why we do a breathing technique for T-spine.

I know that it doesn't take long to spot this after a couple of years experience as a technologist, but I have often heard students who say they have difficulty spotting motion on lateral chests.  This is a great example of how motion normally appears - little or no motion toward the apices and motion more exaggerated toward the bases.  Make sure to watch your patients' breathing and keep those time stations low!

New YouTube Channel

I'm excited to announce that I will be starting to post videos here via YouTube.  I only have one as of today, but there will be more to come very soon.  Please take a moment to subscribe to my channel.  I'm also taking suggestions for topics if you have anything you would like to see.  Please keep in mind that I am not planning on placing an entire x-ray program's worth of material here, but I would like to keep it to possibly clarifications of those topics, real-world examples of specific areas in your program, and/or advanced imaging topics - things you may learn after you graduate and obtain more experience.

That being said... my first video is a quick image critique of a portable chest x-ray (note - if you go to my YouTube channel, TopicsInRadiography, you can see the image at full size with much better detail):

How Will Healthcare Reform Affect Us?

To answer this with the information we have available to us right now... I don't know.  Nobody really knows because there is no definitive information about our direction, and it's ever-changing politics and strategies for dealing with it keep us on our toes, nervous and sweating bullets.  

What do we know about issues affecting us as Radiographers?  Medicare reimbursement is expected to decrease, followed by insurance coverage that will follow suit based on what Medicare is doing.  In other words, we will be expected to perform the same job, if not an increase in volume with our aging population, with fewer financial resources.  In order to stay ahead of the wake of financial burden, we are going to be asked to reduce cost... by means of supplies, labor, and waste while improving productivity and efficiency... not to mention the dependence on patient satisfaction for maximum reimbursement.  Quite the task!

I wanted to post this topic because I'm looking for interaction with you.  What is your hospital or health care system doing to plan for this?  Do they have a plan?  Have they begun any changes in practice?  My employer has some ground-breaking initiatives on this side of the country that are measurably making improvements in preparation for these cost-cutting requirements.  It is encouraging, but there is still work to do.  I encourage participation from all levels of the organizational flow chart as we are sometimes blind to perspectives above and below us.  What are your thoughts? 

Exposure Indicators

The current use of CR image plate technology is expanding at a rate that makes it difficult for seasoned technologists to keep up with.  Since radiography programs are now including more digital radiography curriculum, there seems to be a gap between the knowledge that new grads have about these systems, and the knowledge that experienced technologists have.  Some have attended CEU training, or courses that have kept information current, but there is a large population that have not, and who need that gap bridged.  I just wanted to write a quick post about three of the different types of exposure indicators that CR systems are currently using today... if there are other ones not posted here, I personally do not know about them, and would appreciate any info that any readers are willing to share!

Sensitivity Number (S#)

This is probably the most familiar one.  A typical optimum range might be from 200 - 400, although, with all of these examples, every hospital will have a different acceptable range for different exams.  Radiographic density and Sensitivity Number have an inversely proportional relationship.  If I double my mAs, my S# will decrease by 1/2 its original value.  Conversely, if I 1/2 my mAs, my S# will double.

Exposure Index (EI)

A typical optimum range for Exposure Index might be 1700 - 2000.  There is a direct, but not proportional relationship between Exposure Index and radiographic density.  For every double in mAs, the EI should increase about 300.  Every time you 1/2 your mAs, it should reduce the EI by 300.

Log Mean Exposure (LgM or LME)

A typical optimum range for Log Mean Exposure is around 1.9 - 2.1.  This one closely resembles the H&D curve representation of log relative exposure in the x-axis.  There is a direct, but not proportional relationship between LME and radiographic density.  For every double in mAs, the LME will increase by 0.3.  Every time the mAs is cut in 1/2, the LME is reduced by 0.3.

Knowing how to correct for insufficiencies in technical factors is the first step to producing diagnostic images using CR/DR systems.  Hopefully, we can all help each other to understand these principles and continue the efforts to improve quality and radiation protection standards.

WikiRadiography

I would really like to see more people getting involved in this site:

http://www.wikiradiography.com/

There are resources from all over the world, discussion forums, images and lots of resources for anything related to Radiography.

Inverse Square vs. Direct Square

Learning about the inverse square law and the direct square law can be quite confusing at first. Once you learn the formulas and dust the cobwebs off of your algebra skills to be able to solve for the variable using the formulas, knowing WHEN to use WHICH formula is sometimes the biggest challenge you will face. In order to determine this, we need to know what kind of information the question is asking you for. Let's take a close look at each formula:

Inverse Square Law states: "The intensity is inversely proportional to the square of the distance."

Notice that the value for original intensity (I1) is in the numerator, and the value for the original distance (D1) is in the denominator, thus it is "inversely proportional to the square of the distance."

Use this formula when the problem asks you to solve for a unit of radiation intensity, dose, or exposure. Also, remember that radiation "intensity" is not measured in units of mAs, so if the question is asking you for a mAs value, this is not the formula for you. Units of radiation exposure or radiation dose are required for this formula (R - Roentgen, mR - milliRoentgen, rad, rem, Gy - gray, or Sv - Seivert).

Still confused? This simple tip could save you... you should now already have the fundamental knowledge that radiation intensity will decrease as the distance from its source increases. So look at your distance values: If the distance increases, then I2 should be a smaller number than I1. The opposite is true as well; if the distance decreases, the intensity will be stronger, and I2 will be a larger number than I1. This is important to remember when we discuss the direct square law:

Direct Square Law / Density Maintenance Formula:

Two main differences with this formula are: Instead of radiation intensity, we are using mAs values. Also, the original mAs and the original distance are both in the numerator - "direct" vs. "inverse." We need to be using this formula when the question asks for a mAs value.

*** side note: You know that radiation exposure is directly proportional to mAs. In other words, if I double my mAs value, the radiation exposure value will double. Remember, these two units are distinct and separate, but related to one another.

After performing a few practice problems, you may notice that as the distance increases, the mAs value will increase. This is due to the "direct" relationship. As the distance from the radiation source to the image receptor increases, the mAs required to maintain density (density maintenance formula) will increase. So, if your D1 value is smaller than your D2 value, then your mAs1 value should be smaller than your mAs2 value. Conversely, if your D1 value is larger than your D1 value, then your mAs1 value needs to be larger than your mAs2 value.

What if the values presented in the question provide units of radiation exposure/intensity/dose AND mAs values? Don't panic... just find out what the question is asking for, and apply what we have discussed.

Example: A radiographic exposure of the chest was taken at a distance of 72" using 10 mAs and had an exposure of 50 mR. What would the exposure be at a distance of 80"?

The question is asking "What would the exposure be ...?" Key word: exposure. This is your key term that determines we are looking for a unit of radiation intensity. First, fill in your variables:

I1 = 50mR
I2 = "x" or unknown
D1 = 72"
D2 = 80"

I trust your ability to solve once the equation is set up properly ;-) Remember that your distance is increasing, so your value for I2 should be smaller with this formula (this is how you can tell if you forgot to invert the distances - it will be a larger value if you forget).

Example: A radiograph of the knee produced 100mR of exposure when 70 kVp and 10 mAs was used at 40". What new mAs would be required at a distance of 60 to maintain density"?

Key term: "What new mAs...to maintain density?" This one screams, "Density maintenance formula!!!"

Fill in your variables and solve:

mAs1 = 10
mAs2 = x
D1 = 40"
D2 = 60"

Your mAs2 value should be greater than mAs1 because your distance is increasing.

On a final note - if you have time, double-check your work by plugging in your answers into the original equation. Don't forget to square your distances, and reduce fractions into the lowest terms before squaring to save you from having large numbers to deal with.

Looking for tips on success through Radiography school?  Check out my book coming Summer 2012... more info HERE.