RF Energy

Discussion in 'General Electronics Chat' started by Mazaag, Sep 4, 2007.

  1. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    Hi guys,

    I have seen alot of different medical devices that say they use RF energy to perform ablation or lesions using probes that they insert to the target area.... my questions are as follows.. what exactly is "RF Energy" , and how is it possible for it to cause heat (for ablation i'm pressume) using a probe ? and

    secondly, what are the effects of a) Voltage and b) Current on human tissue

    thank you
  2. niftydog

    Active Member

    Jun 13, 2007
    RF = radio frequency. Think "microwave oven" and you're half way there. Microwaves cause water molecules to vibrate, causing friction, which generates heat.

    Voltage by itself doesn't do much to tissue except for maybe slightly polarising molecules, as far as I'm aware. You can be "floating" at very high voltages without any ill effects. But the current is the thing that you want to look out for. Current passing through tissue causes burns, but more importantly, even a tiny current passing through your heart can cause arrhythmia which can kill you.
  3. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    still not 100% sure on that...

    So when we have RF waves for in a medium... do we have voltages and current ? or do we have electrical and magnetic fields? or both ? like does there exist an electrical and magnetic field ( part of the EM wave ) that give rise to voltages and currents in media ? and if so , do these currents give rise to heat ? and is it possible to cause arhythmiya using EM wave radiation ?

    i'm kinda lost lol
  4. niftydog

    Active Member

    Jun 13, 2007
    Well, microwaves are EM waves - as are light, heat, UV, gamma rays etc etc. Electric and magnetic field co-exist - magnetic fields are created by moving electric charges , or in other words, current flow. Current encountering resistance causes heat and voltage.

    As for EM waves causing arrhythmia, well yes they do in a sense, but that's because a current flow of any type is accompanied by EM waves. Are you asking about an external RF energy source powerful enough to generate sufficient current flow in your heart? If so, I think the LEAST of your worries would be arrhythmia! Imagine putting yourself inside a microwave oven and turning it on - would you be more worried about a heart palpitation or being cooked?!
  5. Dave

    Retired Moderator

    Nov 17, 2003
    Whilst true for microwave frequencies, at RF frequencies Joule heating is an increasingly important component for the overall heating because the polar molecules are better able to realign to the applied field.

    Propagating EM waves have both electric and magnetic field components (Maxwell's Equations), and within the workload (tissue) you will experience voltages and currents dependant on the electric and dielectric properties. Heat specifically generated through current flow is Joule heating (as stated above). If you are looking at understanding how EM waves heat biological tissues you should look at a subject called 'Dielectric Heating' -this is a very complex and poorly understood subject, so you should have some interesting questions arise from your study.

  6. beenthere

    Retired Moderator

    Apr 20, 2004
    Wikipedia has an article on the subject - http://en.wikipedia.org/wiki/Cauterization. I don't know where the dividing line between AC and RF has been set, but the 100 KHz of the electrocautery equipment is definitely in the RF region.

    One of the effects that protects the patient is called skin effect. This has nothing to do with animal tissue, but refers to the tendancy of RF currents to stay in the vicinity of the outer layer of the conductor, When things are really high in frequency, you turn the wire inside out and it becomes waveguide.

    I'm not sure that 100KHz is really high enough for significant skin effect, but it may contribute to keeping the current confined to the immediate area where the electrocautery is being performed.
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    I think the RF region begins much lower than 100kHz. IEEE C95.1 is titled
    One of the Omega Navigation System frequencies was 10.2 kHz.
  8. mrmeval

    Distinguished Member

    Jun 30, 2006
    I've had it done to remove a wart. The doctor grounded my hand and then used a probe to burn out the wart. She went pretty deep to. It's just a high frequency, high voltage supply though I'm not sure of the current.
  9. mOOse


    Aug 22, 2007
    beenthere was pointing out that there is no frequency dividing
    line between AC and RF, but that 100k is clearly RF.

    But AC and RF are simply different things, aren't they?
    AC is an alternating current in a conductor which may cause
    em oscilations, which would be RF waves at AC frequency.
    Also, in theory couldn't you have a 100k AC source?
    Is there any special purpose to which it might be suited,
    or is it simply insane?
    Code ( (Unknown Language)):
    2.                         RF Frequency Bands (Hz)
    3.       ------Low Frequency--------------------High Frequency-----
    4.       ELF  SLF  ULF   VLF   LF    MF    HF   VHF   UHF  SHF  EHF
    5. Lower   3   30  300    3k   30k  300k   3M   30M  300M  3G   30G
    6. Upper  30  300    3k  30k  300k    3M  30M  300M    3G 30G  300G
    8. (E)xtremely, (S)uper, (U)ltra, (V)ery, (L)ow, (M)edium, (H)igh
  10. Dave

    Retired Moderator

    Nov 17, 2003
    For what its worth: In Tomography (and dielectric heating applications), we consider RF frequencies to be from 3Hz to 500MHz, and microwave frequencies to from 500MHz to 300GHz (I have seen the RF-microwave boundary stated as 300MHz and 1GHz before). This is by no means a standard, but is the generally accepted ranges stated in publications and literature on the subject.

  11. gmrkiran

    New Member

    Sep 11, 2007
    i want no basics of vhf and uhf