Xrayfund
6/16/99
Radiographic Technique
- Select a kVp value that will penetrate the part under study
- Select a kVp value that will result in optimal contrast
- Use the 15% rule as needed
- Read about reciprocity in the book
- Using the 15% rule and assuming a "normal" technique of 80kVp and 10mAs look at pt dose (Table 3-1 p49)
- Some equipment ties focal spot size and mA stations
- 100mA or less = small focal spot
- greater than 100mA = large focal spot
- detailed x-ray use small focal spot (extremity and chest)
- use large focal spot on larger body parts to cut down on the exposure time (chance of moving)
- if want to do the small focal spot, lay the pt on the table so to help the pt not to move
- choose the shortest practical time
- w/moving grid use times longer than 0.1s
- w/breathing techniques use times longer than 1 sec (selective motion)
- lateral thoracic—blur ribs while breathing out
- distance does not affect kVp
- smaller parts use smaller kVp
- larger use larger kVp
- chest uses 100+ to blur ribs and see lung fields better
- the inverse square law does affect mAs
- to increase from 40 to 60inches—double mAs (to keep the density the same)—isodense
- to increase 40 to 72 inches—3.3 times the mAs
- 72 inches 20mAs, at 40 need 7mAs to create an isodense film
- longer distances decrease magnification
- longer distances reduce pt exposure
- longer distances lengthen the time of exposure (not longer than 1 sec)
- use grids for parts larger than 12cm
- when using stationary grids use any time of exposure
- be sure to remain w/in focused range of the grid (printed on front of grid)
- projectional distortion—top of ileum are level but b/c of beam, they will be at different levels b/c they are above or below the central ray distorting the image
- technique charts—most do not offer one
- variable kVp charts—not that good
- high kVp w/large body part, contrast goes down, increases scatter
- use kVp at same level—fixed level
- phototiming—stop at the appropriate time
- add on device
- sensors anticipate that they are behind the object that you want to see
- problem—the pt has scoliosis and you are not aware of it,
- need to be a good positioner to get the a part over the sensor
Review for Midterm
Xrayfund
6/16/99
Quality Control
- acceptance testing—making sure that you got what you paid for
- pinhole camera—read captions under the pictures
- make sure equipment is working properly
- make sure grids are all right
- routine performance evaluation
- check out equipment once to twice to as needed depending upon the exact equipment
- processing—should be checked daily if you are going to use it
- error correction
- Mammographic quality standards act of 1992
- NY, TX, FL, MN, CA, etc.
- If you optimize x-ray output
- And optimize film processing
- You will decrease pt dose
- Have fewer repeats
- Reducing your equipment's workload—equipment should last longer
- And decrease your exposure t scattered radiation—decrease the amount of lead that you have to buy
Glossary of terms
- mAs linearity—annual test or a drop off in exposure
- Darkroom fog—like fungus, it is everywhere, any source of fog destroys the film—adds density to film
- Sensitometry—predictable exposure on the film, can't use x-ray machine to do this
- Densitometry—measures final product, transmission of light through
mAs linearity
- To test for linearity you need:
- Radiolucent object
- Stepwedge—Al
- Supertech penetrometer—plastic cylinder w/lead disk in the bottom and calibration film
- Apple or orange—not very good quality control (can't use year after year)
- A cassette loaded w/film
- Two pieces of lead vinyl
- Make sure set the Stepwedge the way you would walk up the steps from above w/11 steps or 21 steps
- Lay the cassette on the x-ray table or floor
- Protect ¾ of the film w/lead vinyl
- Place the object in the unprotected area
- Choose kVp and mAs that will barely penetrate the object
- Approximately 50 kVp, 1 to 10 mAs
- Expose the object at each mA station
- Vary the time so the mAs is always the same
- The result should be a uniform gray image for each mA station
- Uses the supertech penetrometer
- White area that received virtually no light b/c lead
- Look at surrounding area—compare this area on each of the exposures
- To test for you need:
- An 8´
10 cassette loaded w/film
- Eight coins (preferred) or paper clips
- Tape
- Load the cassette into the cassette tray
- Make a minor exposure of the entire film, approximately 50kVp, 1 to 10 mAs
- Collimate to 4"´
4"
- Tape the coins (clips) into the corners of the light field
- Repeat the same exposure
- If the light beam accurately represents the x-ray beam the coins (clips) will be exactly in the corners of the 4´
4 exposure on the processed film
- Federal guideline—agreement b/w light must be w/in 2%
- Totally include or exclude any penny problem w/light beam arrangement
- May need adjustment of the mirror, movement of shutters may be off
- Using paper clips, unbend and put them in the corners of the light field and do the same experiment
- Professional uses brass plate—horizontal and vertical scale printed on it (all are painted w/a leaded painted so leave impression of the film)
- Plastic cylinder—6" tall, bb in lid and bottom plate (directly aligned), take picture of this on the center of the axis, judge if can get the x-ray beam perfectly vertical
- Angle w/project top one away from the bottom one
- Which one the circles will it in—the outer, inner, or outside the out circle
- Misalignment is off about 1.5°
if on the inner circle (6 or 7 mm)
- Larger circle is 3°
off
- There is distortion at the central ray
- Projected to look like it is subluxated
- Asymmetry is the rule for vertebrae
Darkroom Fog
- To test for darkroom fog you need:
- An 8´
10 cassette loaded w/film
- A piece of cardboard, opaque to light
- Make a minor exposure of the entire film
- Approximately 60kVp, 0.1 – 0.2mAs for high ¦
@ 600 speed
- Need an optical density 0.8-2.0
- Go into the darkroom, safelight turned on
- Remove the film form the cassette
- Cover the entire film w/the cardboard