Technical Tips -- from the Editor

"30% rule Trips some Students, Techs"

Mike Enriquez, MPA, BSRT, (R) (CT) Director-at-Large, Membership Chair

So what’s in a number? In some cases it is an identity thing. Like 1895, the year that Roentgen witnessed the phenomena of x-ray. Or, if perchance you are a Yankee fan (or hater) you know the significance of the number 27 (as in World Series Championships).

What are some numbers that are significant to radiology besides 1895? How about 37 as in 37 degrees?...as in trauma Towne, relative to the orbitomeatal line (OML) and visualization of the posterior neural arch through the foramen magnum?….(oh, just send the patient to CT!), or, 37 degrees as in the Water’s (Parietoacanthial Projection), with reference to the angle of the OML to the surface of the image receptor? Professional technologists know these things intuitively.

When the discussion turns to technique, there is a special identity assigned to the number 15. That is, as it relates to 15%. In the visual world of conventional film/screen, 15% changes in kVp are the visual benchmark for changes in radiographic contrast and density. And how about the number 30 as it relates to 30%? Isn’t this the benchmark percentage applied to mAs changes to demonstrate a visible difference in optical radiographic density? Yep, sure is.

So what happened at the Student Bowl competition at the San Jose CSRT Annual Conference (AC)? You could have knocked me over with a feather! Now the Student Bowl competition is a great event and it takes a lot of guts for every student who chooses to compete. The 2009 AC Student Bowl drew 75+ anxiously, wide-eyed students to the competition. So many students chose to become involved that it was necessary to ask questions in a group response format in the beginning just to "cull" the competitors. It was during this portion of the competition that the following (paraphrased) question was asked in PowerPoint Format with the following possible answers:

What percentage of change in mAs is necessary to see a visible difference in Density on the film?

5
10
15
30
50 (I think)

An incorrect response to this question eliminated approximately 30 of the student competitors!

I think I know a couple of reasons as to why this occurred. First of all, there is a wide variety of student levels in the competition…from newbie freshmen to graduating student technologists. Second, the clinical experience observed by the students is probably primarily digital. Digital, as in computed radiography (CR) and flat panel imaging (DR). This means that the students probably have not witnessed conventional film/screen .They probably have not had the opportunity to assess differences in technical factors relative to density and contrast. And why? The digital systems "correct" themselves relative to Brightness (density) and Contrast. Can you say "automatic rescaling?" Unless an extreme error has been made, virtually all images created with digital equipment will be satisfactory from a visibility of detail perspective. If the mAs realized during exposure is extremely low, visible quantum mottle may result. If the digital image is obtained with gross overexposure then excessive scatter may be present and viewable. Other than these two exposure extremes obtaining digital images would seem to be automatic and a snap! NOT!

Below is an example of what the students see who are training in completely digital imaging clinical departments:

What’s that? "Variable mAs, Constant kVp".Now appreciate the images. From left to right, from 5 to 80 mAs the density (Brightness) appears constant. What about the dose?

When observing these images there would appear to be no connection between mAs and density!...and, mAs is the primary controlling factor of image density. It is not surprising then that the students who missed the question did so.

Say what you will about the inefficiencies of conventional film/screen . However, the connection between mAs and image density is obvious -- see the images below:

In these images the connection between density changes due to mAs changes is obvious, especially with constant kVp (70).

Evaluating density and contrast is a visual thing when the technology is conventional film/screen. But with digital technology? The professional technologist must consider the relationship between mAs and exposure index. I guess we all had better, "see it to believe it."

Please direct any Questions/Comments directly to the editor at radprof@hotmail.com or 209-384-6125.