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Homework Help: X-ray tube output

  1. Apr 26, 2016 #1
    1. The problem statement, all variables and given/known data
    (The question is in the image attached)

    (c) An X-ray unit in the cardiac catheter labs has an output of 80µGy/mAs at 1 meter when the unit is set at 100 kVp. After adding 0.3mm Cu to the beam the photon intensity is reduced to a quarter and tends to vary in proportion to kV3 (i.e. kV to the power of 3). What would be the output rate at 80cm from the tube focus for a
    continuous beam at 80 kV p and 5 mA ?
    (d) The same unit as in c) is now used for an acquisition run. The protocol uses 15 frames per second with a 10ms pulse width at 100kV without the copper filtration and a 20 x 20cm square field. The detector distance is 1m from the tube. If the measured output rate at 1m is 900 µGy/s what is the peak tube current of each pulse? What DAP reading in cGycm2 would be measured in 10 seconds of acquisition?

    2. Relevant equations
    none relevant
    3. The attempt at a solution
    Please can someone confirm if my answers are correct
    (c) 80 μG/mAs
    (Working: 20 * (1/0.8)2 * (80/100)3 * 5)

    Current = 750 mA
    DAP reading = 360 cGycm2

    Thank you

    Attached Files:

    Last edited by a moderator: Apr 26, 2016
  2. jcsd
  3. Apr 28, 2016 #2
    first part looks good.

    Second part in order to achieve an output of 900uG/sec when the output is 80yGy/mas you need how many ma on average for continuous operation to achieve that output? Since the the tube operates only a fraction of the time (duty cycle is what?) the current must be higher than the average right?
  4. Apr 28, 2016 #3
    If I have 900 uGy/sec when I have 80 uGy/mAs and the machine is on for 150ms in total over a 1 second period
    Then I have
    80 * mA per second = 900
    Therfore mA = 11.25 mA

    BUT as the current is not always on the actual mA is

    mA * 15 * 10 * 10-3 = 11.25 SO
    Answer = 75 mA

    But for part (ii)
    If output is 900 uGy/sec and I run the machine for 10 seconds then I have a dose of 9000 uGy
    With an area of 400 cm2 I have a DAP of 9000 uGy * 400 = 3.6 Gy cm^2 = 360 cGycm^2

    So I got the mA wrong initially but the DAP correct.
    Is my answer now correct?
    Last edited: Apr 28, 2016
  5. Apr 29, 2016 #4
    looks good
  6. Apr 29, 2016 #5
    Thank you for your help :)
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