COLLIMATION PROBLEMS  A couple of common problems occur regarding collimation.  The function of the collimator is to project a field of light that is co-incident with the radiographic field and to give the technologist a method of controlling the size of the radiographic field.  A backup field size is available by using the scale printed on the collimator surface in case the light bulb has burned out.  There may be several scales printed allowing field sizes based on a 36", 40" and 72" SID (Source to image distance).  Let's examine some of the common problems. 

COLLIMATION ALIGNMENT (an often confusing subject)

LIGHT FIELD AND X-RAY BEAM ARE NOT CO-INCIDENT  Your State radiation protection agency will generally examine this issue.  In general, a light field must be within 2% of the SID (source to image distance) total, along both edges of the x-ray field when the beam is perpendicular.   An internal mirror in the collimator reflects the light beam onto the x-ray field and is adjustable.  Various collimators have different methods of adjusting.   Replacing the bulb can knock the light field off of the x-ray beam if anything moves during replacement.  Generally a service call is required to correct this problem, but be very careful when replacing the light bulb in your collimator.  NOTE;   If you process a film from the grid cabinet (under the table top) and compare it to the light field on the table top the sizes of the projected x-ray image will be different.  That is because the x-ray beam is enlarging as it gets further away from the tube so the result is to be expected.   The serviceman (person for those politically correct) will adjust the collimator so that the light field and beam are co incident and this accuracy is enforced by your State Radiation Dept. inspection folks.  

X-RAY BEAM IS NOT CENTERED IN THE IMAGE RECEPTOR This problem is a function of the tube beam alignment versus the image receptor at least in the transverse (in and out) direction.  Adjusting the tubearm or tube mount or grid cabinet (all depends on the manufacture and design) will usually correct this problem and allow the system to meet accuracy specifications as determined by your State.   Generally the longitudinal (up and down the table or left and right) adjustment is controlled by the tube rotation adjustment and can be set by the operator.  Usually your eyes can maintain this accuracy however better certified collimators include an adjustable light beam that will intercept the tray handle and allow for accurate beam placement.  An error of up to 2% of SID is generally allowed for beam versus image receptor. 

BEFORE YOU CALL YOUR SERVICEMAN FOR ALIGNMENT...CONSIDER THE FOLLOWING:  Most of the time we find that so called alignment problems are not caused by the x-ray itself.  It is not unusual to have some slight misalignment due to cumulative tolerances.  Possible sources for cumulative error include: slack in a rotational tubestand even though it may be in its detented position;  slack in a transverse tubearm assy;  general position of the cassette tray in the grid cabinet (both directions);  relative positioning of the cassette in the tray;  or the mounting brackets or bearings that hold the grid cabinet under the table.  IF YOU HAVE A TRUNNION MOUNT OF YOUR X-RAY TUBE VERSUS THE MORE COMMON PORT MOUNT, YOU HAVE ALMOST GUARANTEED SOME ALIGNMENT ERROR WHICH IS VIRTUALLY UNAVOIDABLE.  GENERALLY HOSPITALS, RADIOLOGY AND SOME ORTHOPEDIC CLINICS WILL USE TRUNNION MOUNTED TUBES.

HOW TO ALIGN A TYPICAL COLLIMATOR  Generally you should be a qualified x-ray serviceman before you attempt this procedure, but for the benefit of the brave (or reckless) the following procedure has served us for many years.  Before you do anything, you should ascertain how to adjust your specific collimator.  You may email Rduncan@leswilkins.com for detailed instructions on how to adjust collimators made by Machlett, Eureka, Summit or Universal x-ray.  We would need the specific model you are attempting to adjust.

1.  Verify that the x-ray field is in the center of your image receptor.  After verifying that things are aligned as well as possible, insert a 10 x 12 cassette in the grid cabinet and adjust the light field to leave at least an 1-1/2" to 2" border around the film edge.  Place a coin in the center of the light field in the middle of the crosshairs and scotch tape into position.  If you have a float top table, you must energize the electric locks and not turn them off during the entire alignment process.  NOTHING MOVES or you have to start over!   Place 2 other coins in an odd shaped triangle so you can orient your processed film in exactly the same position as the cassette.  I generally use a dime in the middle and other denominations for the other coins.  Take a light film, typically 55 kvp, 2.5 mas and process it.  Now center the middle coin and orient the film properly.   The x-ray field should be a light gray and must be in the center of the film (within 2% of SID source to image distance).  So at 40" SID you are allowed an error in both directions of up to .8".

If the location of the gray (actual x-ray field) is within tolerances fine.  If not, the tube arm or tube itself must be adjusted to compensate for the in and out (transverse) alignment.  Up and down the table (longitudinal error) may be compensated in the tube rotation adjustment or any coupling device used to fix the grid cabinet to the tubestand, or any light indicator in the collimator that intersects the middle of the tray handle.  Consult your owner manual for the collimator if necessary.

BEFORE ANY ADJUSTMENTS ARE MADE, HOWEVER, ensure that the light field from the collimator is co incident with the x-ray field.  With the film in position over the center coin, make transverse and longitudinal adjustments to the light field of the collimator so that the light field falls on top of the gray x-ray field within 2% of SID.  Now it is relatively easy to use the light field as a guide to center over the film.  Take a ruler and draw two diagonal lines (a St Andrew's cross) from the corners which will locate the true center of the film.  Mark the true center with a fine felt tip pen.  Put the collimator crosshairs on the "new" true center and put the dime at the new location.   Repeat exposing another film and verify that the adjustments corrected the alignment problem.  Generally two or three films will be sufficient to ensure correct alignment.

Now the collimator light field is aligned with the x-ray field which is aligned with the image receptor and everyone, including the State inspectors are happy. 

BULB LIFE  Collimator bulb life has improved in the past years; however in general we have high wattage bulbs (hot) in very confined spaces which is not conducive to long life.  Newer collimators will often go for several months without a bulb change.  Older collimators such as the Duocon I or Videx use a bulb that typically lasts a few weeks.  They are not certified for human use, so these are only found (with rare exception) in veterinary offices.  The following common collimators use the noted bulbs which are generally found at your x-ray supply company (or us..we stock them).

FILTERS AND FILTRATION  Filters fulfill the need to change the x-ray beam to a more useful and safe condition.  X-ray tubes are required by federal law to have "inherent" filtration and additional filtration is desirable to limit the harmful "soft" low kV x-rays.  All x-ray exposures (even at higher kV levels) contain some photons of low energy.  These "soft" x-rays will not penetrate a patient and contribute to the useful image and only serve to increase the dose absorbed by the patient.  Filters will block these harmful rays and only allow the higher energy useful rays to pass through to the film.  Contrast media such as barium sulfate is in reality a type of filter.  In general we offer filtration that the technologist may add to the outside of the collimator in order to make the beam more uniform.  Filtration to harden the beam is usually in the form of a piece of aluminum inserted between the tube and the collimator, or it may be in the collimator itself, laminated to the mirror.  Add on filters such as wedge filters and tracks are available from your dealer or contact us if you would like a quotation.  For Chiropractors or Orthopedic surgeons using full spine radiology a wedge type filter is very desirable or a split screen technique may be used to help balance out the beam between lumbar and cervical regions.  Additional information on filtration is available from textbooks dealing with fundamentals of radiology.

Comments about non image forming radiation detected outside of the collimated field are noted on the companion page Scatter Radiation (press the toolbar button at the top of this page)

last updated 12/03/08

Last Updated 05/23/08

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