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Medical Anyone Know A Lot About Medical Physics?

  1. Aug 13, 2011 #1
    I am taking my radiology board exam on Sept 7, and I need some urgent help with certain concepts if the help can be found. Concepts I am struggling with include but definitely are not limited to:

    1. transformers (step-up / step-down)
    2. spatial resolution
    3. image formation in digital x-ray

    I can be as specific as required to ask intelligible questions. Unfortunately, my residency does not have the option of taking a medical physics course, so everything is self-taught. I am struggling to say the least!

    I have 4 books I am using to study, but still my understanding is weak. Hoping to find anyone who can explain anything in medical physics...

    Thank you in advance,
  2. jcsd
  3. Aug 14, 2011 #2


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    Staff: Mentor

    For -1- http://en.wikipedia.org/wiki/Transformer

    The equations part-way down that page should help.

    For -2-, you must have some idea what spatial resolution is. Please explain it to us as you understand it...
  4. Aug 15, 2011 #3
    Entire textbooks have been written on digital imaging so it would be very helpful if you could ask specific questions.

    Spatial resolution is so omnipresent in your field that some additional context or questions would help there as well.

  5. Aug 16, 2011 #4
    Dear Berkeman and EricVT,

    Thank you very much for offering help with physics! I am in over my head with chaos since I am trying to study for this exam and work full time. Your help is soooo appreciated. :)

    OK. Yes, I do know a good bit about the topics I mentioned in my first thread, BUT this is my second go-round at trying to pass the preliminary written exam. The exam format has changed; thus, it no longer suffices to memorize the physics. One must have an integrated understanding of many things. I have read the gazillions of facts in the textbooks, but it seems that some of the "big pictures" are still lost on me. I am so happy to find physics whizzes who may be able to bestow this bigger picture on me.

    I am currently working through Huda's Review of Radiologic Physics to help organize my thoughts. I will use that book to guide my systematic approach to asking questions.

    ***On the topic of saturation current / saturation voltage: Why is 40kV some magic number for causing all the electrons to leave the thermionic emission cloud? At what mA is this relevant? What about higher kVp settings with higher mA settings? Do all the electrons leave the thermionic cloud then, too (I'm assuming no unless the conditions are "just so")? I have seen a graph of this, but for the life of me, I cannot interpret it because there are numbers everywhere on it. My brain freezes. Why is this relevant? Afterall, am I right in thinking that we adjust techniques on the x-ray machine in response to image quality or lack thereof--not because of some magical thermionic cloud number?***

    More questions to come tomorrow. Hopefully, my replies will be less wordy. Gotta go read.

    Thank you!
  6. Aug 16, 2011 #5
    Are you looking for a qualitative explanation of thermionic emission and how cathode current, tube current, and tube voltage are tied together in different limiting states? Or do you feel like you understand the theory and just want help with this one question?

    Either way I will try to help in the morning.
  7. Aug 16, 2011 #6
    Ok. Specifically, the following paragraph from Bushberg reads:

    “The existence of the space charge cloud shields the electric field for tube voltages of 40 kVp and lower, and only a portion of the free electrons are instantaneously accelerated to the anode. When this happens, the operation of the x-ray tube is space charge limited, which places an upper limit on the tube current, regardless of the filament current (see the 20kVp and 40kVp curves in Fig 5-9).”

    I now understand this chart a bit better. I do see specific kVp, tube current (mA), and filament (A) numbers that correlate to one another. However, I do not understand:

    1. How/why does the space charge cloud shield the electric field?
    2. Why does the space charge limitation only seems to occur between 20-40kVp?
    3. Unrelated: Why is the tube current is so much less than the filament current?

    Tube current is in mA (variable) and filament current is 3-5 A. What should I re-read to clue in on this difference?

    Thanks a million!
  8. Aug 17, 2011 #7
    1. Electrons are accelerated from the cathode (filament) to the anode via coupling of the electromagnetic fields of the electrons and the anode. If you have only 1-2 electrons then they can couple easily with the field created by the anode and are accelerated promptly.

    If you have MANY electrons then the ones nearest the anode can couple easily to the anode's field and be accelerated right away. However, that coupling causes the anode's field to be distorted for the electrons that are further away (the ones near the filament), and they cannot couple as easily.

    You also have to consider that the electrons repel one another. Those nearest the anode and repelled even closer toward the anode, and those closer to the cathode are repelled closer toward the cathode.

    This combination of shielding from the anode's EM field and the electron-electron repulsion means that when the space near the cathode is saturated with electrons your tube current is space-charge limited. For a lower, fixed tube voltage you will not increase tube current by increasing filament current. Those additional boiled off electrons will simply stay in the charge cloud for a while because they are shielded from the EM field of the anode by the other electrons.

    This can be overcome by increasing the tube voltage so that the anode's field can couple to all the electrons easily even when emissions are rapid. Therefore, for sufficiently high tube voltages you can increase the tube current by increasing the filament current -- those additional electrons boiling off are promptly accelerated to the anode and contribute to an increase in tube current. In this case your tube current is (filament) emissions-limited instead of space charge limited.

    2. Tube voltage, tube current and filament current are all tied together through those complex looking graphs you see. The specific values depend on many things such as tube design, filament size, and filament material. You are space-charge limited in those low tube voltage regions because the anode's EM field is insufficient to couple to the EM field of all the electrons in the space charge cloud and therefore the tube current is limited by the space-charge and not by the emission rate.

    I personally can't give you a precise mathematical model of these relationships that will make a specific number such as 20-40 seem obvious. I think knowing the ranges of appropriate values for x-ray tube operation and having an understanding of the fundamentals behind those numbers is more important, anyways.

    3. Just to be sure we are on the same page: filament current refers to the current through the tungsten (or equivalent) wire that serves as the cathode. The filament current causes heating of the wire and thermionic emission of electrons is the result.

    Tube current is the current created by the electrons being accelerated from the cathode to the anode. Those are the electrons that were boiled off of the wire and are now being accelerated into the anode.

    You have to send significant amounts of current through the wire to get sufficient heating for thermionic emission to take place. Though it doesn't work exactly like this would make it sound, you can think of it like needing to send 1000+ charge carriers through the filament wire in order to get emission of 1 electron. Those 1000+ charge carriers moving through the filament wire contribute to the filament current value (~A) while only the single emitted electron can contribute to the tube current values (~mA).

    Hope that helps somewhat and I am willing to clarify anything that didn't turn out very clear.

  9. Aug 18, 2011 #8

    I read your reply yesterday, but I am up to my neck in cases this week. Thank you very much for wonderful explanations of my first 3 questions. I am studying with 2 other radiology residents, and it's the blind leading the blind with nitty gritty physics concepts. Your help is sooooo appreciated. My next set of questions are to follow later this evening when I get them clarified enough to ask.

    Many thanks,
  10. Aug 23, 2011 #9
    Auger electrons

    Dear Eric,

    Thank goodness I am now off clinical duty and in my office studying as I enter the home-stretch toward my exam!

    My next question is about Auger electrons:

    Bushberg states: "An electron cascade does not always result in the production of a characteristic x-ray. A competing process that predominates in low Z elements is Auger electron emission...The probability that the electron transition will result in the emission of a characteristic x-ray is called the fluorescent yield. Thus, 1-fluorescent yield is the probability that the transition will result in the ejection of an Auger electron...The K-shell fluorescent yield is essentially zero (<1%) for elements Z<10 (esp. soft tissues), about 15% for calcium (Z = 20), about 65% for iodine (Z = 53), and approaches 80% for Z >60."

    Specifically, the wording of the sentences regarding fluorescent yield is too abbreviated and does not help me “read between the lines”. Can you please explain the probability of fluorescent yield in these lower Z atoms? I can memorize those percentages readily, but I do not understand how those numbers were generated to be able to answer a more complex question. I know I will be asked about this.

    Thank you,
  11. Aug 24, 2011 #10
    As you said, after an electron cascade you have two competing processes for energy release, characteristic x-rays and Auger electrons.

    Auger electrons tend to be ejected from a shell that is very close (the same or adjacent) to the shell that the atom's electron "fell in" from to fill the original vacancy. That is, if you have a K shell vacancy and an L shell electron filled that vacancy, the Auger electron will almost always be ejected from the L shell (LI, LII, LIII may all be possible).

    A semi-empirical result is that Auger electrons emission is favored when the difference between energy states of the involved shells is small. Difference in energy states grows as atomic number (Z) grows. As an example, consider the numbers below:

    Oxygen K shell binding energy = 543.1 eV
    Oxygen LI shell binding energy = 41.6 eV
    Difference = 501.5 eV

    Lead K shell binding energy = 88005 eV
    Lead LI shell binding energy = 15861 eV
    Difference = 72144 eV

    Therefore you get the result that you posted in that Auger electrons are more prevalent for low Z (low energy difference) atoms compared to high Z (high energy difference) atoms.

    Fluorescence yield "picks up" where Auger electron emission falls off as atomic number increases, so you get increasing fluorescence yield % as Z increases, like in the example numbers you posted. See the attached image below.

    Hope that helps somewhat. I'm not a quantum mechanics expert, and you would probably have to get your hands dirty in something like that to really get a complete understanding of why one thing is favored over another and how that changes with atomic number.

    All I can really offer is a qualitative explanation and some empirical results.

    Attached Files:

  12. Aug 25, 2011 #11
    Re:Auger Electrons


    Thank you very much for the reply. I remember being asked a question on the exam last year about why Auger vs X-Rays but I had no idea. Your explanation more than suffices as the examiners are not quantum physicists either. I have never seen that graph before showing the propensity for one condition over the other. :) It is printed and stuffed in my book. Back to cramming...

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