A-19
SECTION 10
RAPID FILM SEQUENCE RATINGS
10-1
0.6, 1.0, 1.25 12.5_ (218 mRad) 1 Phase, 10,000 rpm, 50/60 Hz
Rapid sequence rating charts are used because of severe demands on the radio-
graphic tube. In contrast to normal radiography, where cooling is occuring
between exposures, serial radiography does not allow significant cooling of the
focal spot track between exposures. Therefore, tube ratings for serial radiography
must be determined from this chart.
Three variables are considered: % Duty Cycle, Series Duration, kVp x mA.
% DUTY CYCLE =
EXPOSURE TIME* x MAXIMUM FRAME
RATE/SERIES x 100
SERIES DURATION =
TOTAL NUMBER EXPOSURES IN SERIES
MAXIMUM FRAME RATE IN SERIES
Knowing any two, the third can be found from the rapid sequence chart.
EXAMPLE:
– series wanted:
4 film/sec for 3 sec
=20 exp.
1 film/sec for 8 sec
– technique**
80 kVp, 250mA, 0.05 sec.
% duty cycle = 0.05 x 4 x 100 = 20%
(use highest frame rate)
(duty cycle can be found on following chart)
equiv. series duration = 20 exp. total
= 5 sec.
4 films/sec
kVp x mA = 800 x 250 = 20,000
These factors can be transferred to the rating chart that corresponds to the focal
spot size selected. For the example technique we will use the 0.6 mm 12.5° (218
mRad) chart.
Enter the chart at 20,000 for 5 seconds. The intersection of these factors on the
chart is 25% duty cycle, which is well beyond the 20% duty cycle calculated here.
Therefore, this exposure series is safely within the rating of this focal spot.
If the % duty cycle calculated according to the formula above is greater than the %
duty cycle read off the chart, then the technique should not be performed until one of
the factors is reduced – maximum frame rate, total number of exposures, mA or kVp.
* For phototimed exposures such as photospots this represents the setting of
the backup time on the radiographic timer.
** Must not exceed values permitted by Radiographic Rating Chart.
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