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Timers are used for big or small body parts ?
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AEC is used for big or small body parts?
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Timers and AEC
| Question | Answer |
|---|---|
| Timers are used for big or small body parts ? | Small |
| AEC is used for big or small body parts? | Big |
| Is a timer or AEC more accurate? | Timer |
| A timer is accurate to what ms ? | 1 ms |
| What images are timers usually used for? | Angiography |
| What does AEC do? | Measures quantity of radiation reaching the IR and terminated the exposure when enough radiation has reached the IR to produce optimal images with decreases patient dose |
| Tube- patient- table- grid- AEC/Timer - IR . What is this? | The path of the photon |
| What are the 2 types of AEC ? | Solid state, and ionization chambers |
| Ionization chambers are ? | Most common |
| Which AEC works on p-n junction principle? | Solid state |
| Decreasing collimation makes light field bigger or smaller? | Bigger |
| Increasing collimation will make the light field bigger or smaller? | Smaller |
| What is there detector in the ionization chamber? | Xenon gas ionization |
| When using AEC what will change with every patient? | Time it takes exposure to terminate |
| Time it takes for AEC circuit to detect and react to the radiation received (usually 10 ms) is called? | Minimum response time |
| What is backup timer? | It is a safety feature . The max time the exposure will go before terminating |
| If the wrong Bucky is selected, or if the wrong chamber is selected what are these examples of? | When the back up timer would be activated |
| What do the AEC density controls adjust? | Amount of radiation required for exposure Increases or decreases OD in IR by 25-30% |
| Density control in AEC increases or decreases OD in IR by how much? | 25-30% |
| When we cannot position anatomy over top of ionization chambers and tech know image will be over /underexposed OR when there is a variance in body type and pathology. Is when we would use what? | AEC density controls |
| What is the density setting for very young/very old/weak person? | -2 |
| What is the density setting for thin, easy to penetrate person? | -1 |
| What is the density control setting for an average/normal build person? | 0 |
| What is the density setting for a very maucalur/large build person? | +1/+2 |
| When should we use KVp? | On large body parts like chest, hip, shoulder |
| Underexposed, light image = not enough radiation reaching the AEC detector = backup timer will kick in 2. If the backup timer allowed exposure to run long enough to compensate for low kVp, image will be OK but patient dose is increased. This will happen w | KVp |
| What should we set the mAs to when using AEC? | Highest possible for focal spot size |
| What happens if we set the mas too short (low)? | Irradiation time is shorter that the minimum response time (IR will get enough exposure before the AEC can detect it and react to it therefore will not terminate when it is supposed to) = image being too dark |
| Time needed for body part = 7ms but machine stays on for 10 ms. This is an example of when we set the ââ too ââ- (low)? | mA, short |
| What should the backup timer for the AEC be set for? | 150-200% of expected manual exposure time |
| What happens if backup timer is set too short? | Prematurely end exposure before optimal density received = image too bright = BUT error |
| What happens if the backup timer is set too long? | Exposure will go on for too long Image = to dark Increased patient dose |
| Unexpected tissue density is present and expected tissue density is absent are causes for problems with? | Subject density and contrast |
| An example of some causes of subject density and contrast problems are? | Fluid in lungs, emphysema, osteoporosis |
| Anatomy if interest must be placed directly over selected chamber. If there is not enough anatomy over chamber what will result? | AEC ends exposure early Image = too light |
| How much current will it take to end exposure if more than 1 chamber is selected? | Voltage from both chambers is averaged to end the exposure Chamber that receives the most irradiation will have more influence |
| 1. Motor driven spin top on IR 2. Use 100 mA, 50 kV, 100 cm SID 3. Select various times: 100 ms, 50ms, 25 ms 4. Take exposure of spin top and measure the arc. What are these steps for? | Checking timer accuracy |
| The formula for actual exposure time ? | Actual exposure time = Arc size / 360 |
| What is the SC 35 standards for timer accuracy? | Values cannot deviate from selected value by more than 10% + 1ms |
| Timers help reducing the chance of ? | Overloading the tube |
| Different test varying mA, KVp, field size, conscience of detectors can all be used for what? | Checking for consistency of exposure with AEC |
| You are performing a lateral chest X-ray on a patient but you forget to change the control panel from PA chest to lateral chest so the 2 outer chamber are selected instead of the middle one when you take exposure. How will AEC respond? What will the image | Radiation will reach outer chambers quick because there is not enough anatomy covering them so exposure terminates quickly. Image brightness will be OK, but may be too light with quantum noise Exposure # will indicate insufficient exposure |
| Imaging the abdomen but the middle chamber is activated Imaging a PA chest but lateral is selected- active chamber is beneath sternum Would terminate linger that requires. What is happening here? | The active chamber is beneath thicker tissue that the one being imaged |
| Performing an AP L-spine. Collimation is open to size of a 35x43 instead of collimating to the size of the spine. AP t-spine- is similar to l-spine Brighter and low contrast. What is this causing? | Inadequate collimation causes excessive scatter radiation to reach AEC chamber causing an exposure that is too short |
| When should electronic timers been used instead of AEC? | AP knee of child is smaller that an AP adult Axial clavicle- is smaller plus angle of time causes part of AEC to not be covered by anatomy AP humerus- if not properly cemeteryâs then chamber may not be fully covered |
| Think if collimation as making something smaller, so when you are increasing collimator you are increasing how small it is, and when you are decreasing collimator you are decreasing how small it is | |
| For any given radiographic exam, the number of X-rays that reach the IR is directly related to both the X-ray tube current and the time that the X-ray tube is energized. T or F? | True |
| What kind of timer will terminate an exposure after a prescribed time, usually approx. 6 s. Thus it is not possible for any timing circuit to continuously irradiate a patient for an extensive period? | Guard timer |
| Which is the most accurate of the X-ray exposure timers ? | Electronic timers |
| Which timer allows a wide range of timer intervals to be selected and are accurate to intervals as small as 1 ms? | Electronic timers |
| The product of mA and time -mAs- determines the total number of X-ray emitted and therefore the exposure of the IR? T or F? | True |
| What kind of timer monitors the product of mA and exposure time and terminates exposure when the desired mAs value is attained? | mAs timer |
| The AEC is a device that measures the quantity of radiaiton that reaches the IR? T or F | True |
| Does an AEC automatically terminate the exposure when the IR has recorded the required radiation intensity? | Yes |
| The AEC s radiolucent so that it will not interfere with the radiographic image? T or F?? | True |
| The AEC requires particular care, especially in exams that use low KVp such as mammography, because of the varying tissue thickness and composition the AEC may not respond properly at low KVp. T or F? | True |
| Solid state radiation detectors are now used for exposure-timer checks? T or F ? | True |
| According to SC 35 how often should AEC be tested? | Annually |
| AEC must respond within 1/60 of a second at 5 mAs for what? | Minimum response time |
| Current time product limit = back up timer states that the X-ray tube voltage is ââ or more, and the time product doesnât exceed âââ per irradiation? | 50kV, 600 mAs |
| To common AEC errors that result in exposure errors? | Control panel selection error, positioning error, to little collimation |
| What is the advantage of using AEC? | Allows for variations in tissue thickness and compensates for mA in tube correct |
| What is the purpose of using AEC? | Providing consistent optical density on the image |
| What is the purpose of AEC density controls? | To increase or decrease OD in the IR by 25-30% |
| AP of the right knee need what chamber ? | Middle chamber |
| PA chest needs what chamber ? | 2 outside chambers |
| AP of the left hip needs what chamber? | Centre chamber |
| AP of the T spine needs what chamber? | Middle chamber |
| Lateral is the L spine needs what chamber? | Middle chamber |
| Upright abdomen needs what chamber? | 1 outside, a middle |
Created by:
Em17