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QuestionAnswer
What are the three purposes of a phototimer? Achieving more consistent film densities, reducing repeat rates, and reducing patient exposure
The phototimer operates based upon the ability of radiation detection devices to convert radiant energy into what? An electrical current
Where is the phototimer located? behind or under the bucky tray
What are the two most common types of phtocells? Ion chamber and photo multiplier tube
What induces an electrical current when gas atoms are ionized by impinging x-rays? ion chamber/ this frees electrons which are attracted to a positively charged anode
What photocell uses a fluorescent screen to convert x-rays to light. The light strikes a photocathode & induces a current to charge an electromagnet. The magnet then pulls the switch & terminates the exposure. photo multiplier tube
What device holds or stores electrical charges? Capacitor
What device sets a maximum charge which the capacitor may hold? Thyristor
When the charge in the capacitor reaches the max setting, what allows the charge to be discharged thrrough the circuit to terminate the exposure? Thyristor
Where is the density control knob located? On the console
When should the density control knob be used? As long as the system is working
With the density control knob there will be how much of a reduction in density for each -1 step? 25%
With the density control knob there will be how much of a increase in density for each +1 step? 25%
Where is the paddle ion chamber used? portables
Phototimers should never be used on anatomy that is what? Too small or narrow to completely cover the photocell(film will be to light if you do)
If the phototimer is not centered to the part what happens to the film? it will be to light
Phototimers should not be used on what? complex contrasty anatomy such as sinuses
With phototimers the x-ray field must be collimated to what? Anatomy of interest
What will excessive scatter do with the phototimer? shut it off too soon and the film will be to light
Why should the correct photocell or combination of photocells be used? So that the density levels are not averaged out
Phototimers should not be used on what type of anatomy? That which is peripheral in relation to the x-ray beam
What should never be compromised for AEC? Proper positioning
Why should AEC not be used on repeat exams and what is the exception? Because the same error will be made unless you know what caused improper density the first time.
What cell should used for a pelvis or PA chest? off center cell
What cell should be used for a spine and most body parts? Center cell
What cell should be used for the heart? ALL
The percentage of accuracy in repeated exposures should not be no more than what? +/-10%
The percentage of linearity between phototimers in different room should not be more than what? +/-20
When sufficient intensity of light or x-rays is detected what is automatically increased or decreased as needed to maintain brightness? kVp
Where is the brightness control knob located? fluoro tower
What are the techniques that are pre-programmed into the control panel? programmed exposures
What is the advantage of programmed exposures? simplicity
What is the disadvantage of programmed exposures lack of flexibility
What is the response time needed for the shortest possible exposure with an AEC? 1/60 sec
Increasing kVp will do what to exposure time? decrease it
Decreasing SID will do what to exposure time? decrease it
Increasing mAs will do what to exposure time? decrease it
What is the back up timer? Maximum exposure time allowed/limit exposure time should there be an error
What is the maximum exposure time that is built into every machine? 2 sec
According to government regulations a maximum phototimed exposure shall not exceed? 800mAs/ tube limit
For repeatability test what should you get? a phantom, two exposures and measure density with a densitometer
What is used to measure the patient? Calipers
How does a phototimer work? It converts radiant (x-ray) energy into an electric current. When it receives enough it terminates the exposure
What are the 3 needs for Standardization of technique Consistent Quality, Reduced Patient Exposure and, Reduced Operating Costs
What are the 3 needs for Standardized EXPOSURE Charts new technologists, students, and to help with trouble shooting
What are the three phases to standardization? exposure factor, projections and processing procedures
List the 7 criteria of a Satisfactory Radiograph 1Translucent Densities 2)Silver deposits on the image 3)Part fully penetrated 4)mAs factor for best overall density 5)Differentiating contrast between all densities 6)Details not obscured by scatter 7)Max sharpness/true shape
Explain how to adapt a chart from one institution to another: Need 3 phantoms (Skull, Chest, Extremity); Use optimum kV & old technique in new hospital; Go Up or Down w/ mAs Apply same mAs % difference to all similar exams; Repeat process for each part (skull, Extremity, Chest)
What are the two most common types of charts? fixed kVp and variable kVp
Why is the fixed kVp called optimum? you will use just enough kVp to penetrate the part(most oftened used mAs varies kVp is constant)
On a variable kVp chart, kVp is varied according to what? thickness of the part
What are the steps to creating a Fixed kVp chart? 1st Step: Separate anatomical parts into Small, Medium, Large(85% of patients) 2nd Step: Get phantom & make 3 exposures to determine optimum density 3rd Step: Small patient reduce mAs 30%/ large increase mAs 30% 4th step: Repeat for chest & extremities
Exposure time or mA may have to be increased meaning more exposure to the patient and a chance of motion is a disadvantage of what? fixed kVp
How do you use a Variable kVp Chart? 2 kVp change for every cm change from average; Measure part+double it+add 40= new kVp for that part & select mAs from a guide
For every cm change how much is kVp changed? increased by 2
What are 4 advantages of Fixed kVp charts? 1)kVp tends to be higherthan in variable 2)patient receives lower exposure 3)greater latitude for error 4)contrast more consistent
How much change in mAs does there have to be to have a visible change in density? 30%
With the conversion chart from an adult, mAs is multiplied by what for infants(birth to 2)? .25
With the conversion chart from an adult, mAs is multiplied by what for preschool(2 to 6 years)? .50
With the conversion chart from an adult, mAs is multiplied by what for school age(6 to 12 years)? .75
With the conversion chart from an adult, mAs is multiplied by what for teenagers(>12)? nothing
Conversion for plaster cast for dry, small extremity? 2x's mAs or +15% kVp
Conversion for plaster cast for wet, small extremity? 3x's mAs or +22% kVp
Conversion for plaster cast for dry, large extremity? 3x's mAs or +22% kVp
Conversion for plaster cast for wer, large extremity? 4x's mAs or +30% kVp
Conversion for plaster cast for half cast? +50% mAs or +8% kVp
Conversion for plaster cast for pure fiber glass or air splint? No change
For soft tissue what is the kVp and mAs set? reduce kVp 15% and maintain same mAs
Created by: atesta0824