Exposure Timer And Switches

Exposure Timer And Switches Manish Paudel Roll No:31 B.Sc.MIT

Timers • The device used to control the length of xray exposure with low powered x-ray sets. exposure time being mostly over 1 sec .Small inaccuracies will not be a significant proportion of these intervals,so timing mechanism is simple one .

• But with high powered x-ray sets ,exposure time is very short and minimum time interval i.e . >0.01 sec or less in some cases.

• So,the timer should be with high degree of accuracy. • The basic exposure timers are : Mechanical Timer (obsolete) Synchronous Timer Electronic Timer Automatic Exposure Control(AEC) Pulse Counting Timer

Mechanical Timer • In older xray equipment ,small portable and dental sets had mechanical or clockwork timer.In such timer a spring is provided with a motive power for a timer movement.

Synchronous Timer • These are the type of mechanical timers ,have an electric motor called synchronous motor which is accurate than clockwork timer. • This is designed to drive a shaft at precisely 60rev/sec.

Electronic Timers • These are the most sophisticated ,most complicated and the most accurate XR exposure timer. • They consist of complex circuitary based on the time required to change a capacitor through a variable resistance.

• These allow wide range of time intervals to be selected and are accurate to intervals as small as 1ms. • Because they withstand rapid serial exposures they are particularly suitable for angiointerventional procedures.

• The main basis of ET is charging and discharging of a capacitor .The length of the xray exposure is determined by the time required to charge or discharge a capacitor through a selected resistance.

A charging capacitor • The exposure button starts the exposure and also starts charging the capacitor . • Exposure terminates when the capacitor is charged to a value necessary to turn on the associated electronic circuit. • The exp. time is determined by the length of the time to charge the capacitor.

• This time can be varied by varying the value of resistance in the charging circuit. • If the resistance is low the capacitor take shorter time to charge thus giving low exposure time.

A discharging capacitor • The capacitor is fully charged before the exposure .When the exposure button starts the charged capacitor begins to discharge the selected resistance .When the voltage has fallen to the specific value the exposure stops.If resistance decreases the capacitor discharges in shorter time.

AEC • Called Automatic Exposure Control • Accurate than electrical and mechanical. • AEC was developed to correct the errors by the radiology technologists under the exposure factors . • They measure the amount of radiation required to produce correct exposure for a radiographic exam.

• It is first necessary to select the kvp that will produce satisfactory penetration of the part to be examined.Once kvp is selected either the technologist or photo timer must select the mAs that will produce the proper exposure.

• An automatic timer overcomes the difficulties of varying absorption in the patient because the exposure is terminated only when the film has that dose of radiation necessary to give it required range of densities after processing.

• If the timer is properly used and is functioning correctly all radiograph should receive standarized exposure and be correctly exposed. • The essential element in phototimer is a device that can detect radiation and in response to this produce a small electric current.

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There are 3 such devices which are : 2. Photomultiplier Detector 3. Ionization Chamber Detector 4. Solid State Detector

Photomultiplier Detector • A photo multiplier tube is a vacuum tube consisting of photocathode ,several intermediate electrodes called dynodes and an anode.When light strike on the photocathode ,it emits photoelectrons in no . proportional to the intensity of light.

• The intermediate electrons are coated with material that emits secondary electrons when stuck by another electron . • The electrons are accelerated by the positive potential from one dynode to next with each dynode giving rise to more electrons.

• The final no of electron collected at the anode represents the output current and is proportional to the intensity of light that stuck the photo cathode.

• Photomultiplier detector is the most common type of AEC .The detector is made up of Lucite and is covered with black paper except for an area which is used as detector field for X-ray.

Ionization Chamber Autotimer • IC are almost used as entrance type autotimer. i.e.xray reaching the film passes through the IC. • An IC consists of 2mm thin parallel sheets of Al and Pb foil with gas filled in between the plates which is ionized by radiation.

• Prior to the exposure charge is placed on the parallel plate by applying voltage across them. • When the gas is ionized by radiation the – ve ion moves towards the +ve plate and viceversa.

• When ions arrive at the plate they neutralise the part of charge that has been placed on the plates. • When the voltage has been reduced to previously determined value ,an electronic circuit will activate that will terminate the exposure.

Solid State Autotimer • A variety of SSA is used in the marked today that operates on the basis of radiation producing ionization in or near the receptor. • This offers the advantage of small size,almost no X-ray beam absorption and a consistent rapid response.

Pulse Counting Timer • In this type,timing is accomplished by counting the pulse of a regular periodic voltage.When it is necessary the exposure of few ms.,a convenient way is to measure such a time would be to count voltage pulses of high frequency.

• The higher the frequency ,the more the pulses that are counted for a given exposure period. • The higher frequency has a tendency to reduce the error in the time period .So,this device is used to control the time of short exposure technique.

mAs timers • An electronic timer – T he technologist selects the mAs and when the desired mAs is reached the exposure is terminated

Switches • It is a control device designed to interrupt the power automatically when pressure by the operator's hand or foot is released. The purpose is to prevent accidental continuing exposure of the patient to radiation.

A primitive X-ray control panel with analog switches.

Latest digital control panel with electronic timer and integrated switches

Switches contd.. •

Based on the location on the circuit of the transformer they are fitted they are divided into two types.These are : 2. Primary and 3. Secondary

Primary Switching 1. 2. iii. iv.

Electromechanical Contractors Electronic Gas filled switches or thyratrons Silicon Controlled Rectifier(SCR) or thrysistors

Electromechanical Contractors • These are spring loaded so that they close firmly and without bounce,the spring acting strongly to contact pieces together.These contact to close a circuit and remove contact to open the circuit. • These should be heat resistant and also the springs should be strong.

• Nowadays these are replaced by the electronic switching.

Thyratrons • These are gas filled switches used in the primary circuit of the xray generator circuit. • These are cold cathode type and take certain time for flow and are good at high voltages.

Thyristors • Suitable for switching big current ie can be made to switch between 250 A at 500 V so can be used in the primary circuit of the xray generators.Mostly used nowadays .

Secondary Switching Circuit •

These are fitted within the high tension circuit . • These must be wholly electronic and they must be vacuum valve devices.These are mainly: 3. Triode Valve 4. Grided X-ray tubes

Triode Valve • These are mainly used to Switch the circuit and may also act to stabilize the kilovoltage. • These triode valves are designed to act as electronic switches operating at high kilovoltages and they directly close at teh start the exposure and open at the end of the exposure.

• The grid is responsible for the switching mechanism ie the voltage in the grid. • When grid is sufficiently negative with respect to the cathode ,no electron can pass throgh it causing open circuit. • When voltage in the grid is decreased the valve is conductive ie the switch is open.

Grided X-ray tubes • In this case a control grid is kept in between the anode and cathode of the x-ray tube making the x-ray tube itself as a switch. • At the beginning of the exposure it provides a continuous pathway for its own flow of electrons by becoming conductive through change in negative bias voltage in the grid.

• At the end of the exposure it becomes non conductive and again through the grid bias voltage ,it instantaneously opens the high tension circuit and the exposure stops. • It is the method of exposure switching that gives precise control for short exposures of a few milliseconds only.