Physics Test part 5 Flashcards
Synchronous timer
Found on older single-phase units, they use a small electric motor that rotates at 60 rps.
They are accurate down to 1/20 s and minimum possible exposure is 1/60 s
) Impulse timer
Measures impulses (or pulses) instead of time and are usually paired with synchronous timers in the same unit to measure the short exposure times.
Impulses/120 = time as a fraction.
Electronic timer
Most commonly used in modern equipment, and accurate down to 1/1000 s (1 ms).
Come in 2 basic types:
Resistor/capacitor combination
a capacitor continues to collect a voltage and holds it during the time that it is connected to a voltage source.
Digital (not readout)
Microprocessor linked with a SCR that calculates the length of time the current will need to flow through the x-ray tube.
Mechanical timer
Spring driven timer used in dental and portable x-ray machines.
Not accurate with times less than ½ s.
mAs timer
Monitor the product of mA and time on the secondary side of the step-up transformer.
This simplifies the operation of the machine but takes away the flexibility from the operator.
Anatomically programmed units
A digital microprocessor in the control console is pre-programmed to deliver proper technique for a particular exam.
Operator only has to select kVp, exam, and patient size.
Automatic Exposure Control or AEC
Measures the proper amount of radiation for a particular radiograph and terminates the exposure one this has been reached.
There are 2 main parts to an AEC system:
Sensors or Cells
Detects the x-rays that have passed through the patient and produces a corresponding electric current that is proportional to the quantity of x-rays that are detected.
Photodetector or photocell
this was the first type or sensor developed for AEC systems.
Ion chamber
made of a radiolucent material and placed between the grid and the front of the IR
Solid state detector
most sensitive and expensive, and can placed between the grid and IR.
) Comparator
Receives the current that is sent by the sensor.
A capacitor stores up a voltage as long as the current flows.
Timer Accuracy
Regardless of the type of timer system, accuracy in terminating the exposure is critical.
Digital Timer
records the exact amount of time it is being exposed by x-rays.
Spinning top test
When the top is placed on top of an IR and an exposure taken, a series of dots or a solid arc will appear on the processed image that can be used to determine timer accuracy.
1Φ equipment
dots will appear on image.
½ wave correct number of dots
exposure time x 60
Full-wave correct number of dots
exposure time x 120
3Φ equipment
since x-ray production is constant, dots will not appear in the image but rather a solid arc. Correct arc size = exposure time x 360
Too large an arc
indicate that the timer is too slow in stopping the exposure.
Too small an arc
the timer is stopping the exposure too quickly.
For high frequency units
a digital mAs meter is used to determine the accuracy of the microprocessor circuit in terminating the exposure.
