kv, mAs, mA ?

RTE4ME · Joined: 23 May 2008 Posts: 3

Does
64 kv, 200 mA, 1/20 sec
mean
64 kv @ 10 mAs?

And thus

80kv, 600mA, 1/60 sec = 80kv @ 10 mAs ?
78kv, 600 mA, 1/60 sec = 78kv @ 10 mAs?

Leaving the first equation to be the answer to the question as to which factor would be best for upper limb, right? However, the book says 64 @ 6... I know these are "guidelines" but the whole mAs and kv thing confuses me as far as when to raise and lower each....sometimes I feel like I will never "get" this stuff.. Crying or Very sad

Thanks

Student Judd · Joined: 14 May 2008 Posts: 4

I’ve just started my RT classes this spring, so this stuff is new and confusing to me also.

kVp is the chief controlling factor of contrast

mAs is the chief controlling factor of exposure and density

No increase in mAs or density can compensate for inadequate penetration (not enough kVp)

kVp settings usually suggested based on body parts ( hand, chest, abd, ect…) and adjusted because of patient factors. The degree to which the radiation is attenuated depends on tissue characteristics such as cell composition, relative atomic number, thickness, and cell density

15% decrease in kVp will half IR (image receptor) exposure
15% increase in kVp will double IR (image receptor) exposure

That’s kind of my take on all this, then again it’s all very new to me right now.

palatinevelum · Joined: 23 May 2008 Posts: 8

Don't worry, it will all eventually click for you.

Extremities, you want short scale high contrast, and to acheive this you must have a relatively lower kvp. I don't know if you are doing this table top (upper extremity is a little vague), if so there no need for anything over 70, unless the part is huge. Sites I have attended in the past suggested to use 60 kvp for adults, 55 for children.
Now, with your mAs, this affects the overall blackness on the radiograph... extremities do not require alot of mAs. Generally its 1-5 from finger to elbow, then things change when you get to your humerus which is done in the bucky and you need more technique on both ends.

If you pay attention to what you use for say, an ankle, you could use this same technique on what other body part (with similar thickness), ex: I use the same technique generally on "normal" pts on an ankle as I would a forearm.

And the 15% rule is just a pain in the ass that you need to know for the registry... I was taught if you want to change the scale of contrast...
^ 10 kvp and 1/2 mAs
v 10kvp and 2x mAs
Looks exactally the same, rather than pulling 15% out of your ass...

Our teachers were really hard on us about learning technique rather than getting used to aec, i could tell you a few more things if ur interested

Hope it was some help...

cmadams · Joined: 17 Jun 2008 Posts: 2 Location: Kettering, Ohio

Mind if I confuse things a little? You will certainly "get it" because it does "click". One thing to keep in mind, however, when you're actually performing patient exams, is whether you're using film, CR or DR. If you're using CR/DR, you (should) discover that kvp is solely for penetration of body part and mas is for minimal sufficient radiation to adequately expose body part. The contrast/darkening of images can (usually) be taken care of by the imaging processing software and you (or a supervisor, QC tech, etc.) may be able to modify the image (within reason) to compensate. Protocols and policies will vary by department as to how much latitude you are given to do this (sorry, couldn't resist!). What we have to be very careful of re: CR/DR is "dose creep". This is when we aren't really sure of the best technique to use, so we'll "up" the mas a little, just to be sure we get a readable image and don't have to repeat it. BAD IDEA!!! Just hang in there and keep working on it. If you don't currently keep a little notebook of your successful techniques, I suggest you do so. You can jot down body part, patient size, machine used, what worked (and what didn't).
Good luck!
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