March 1, 2014
by Dale A. Miles BA, DDS, MS, FRCD (C), Dip.
INTRODUCTIONFor over 40 years, my academic colleagues in radiology have taught you, your dental assistants and your dental hygienists to use rectangular collimation for intraoral x-ray procedures to minimize the x-ray dose to the patient.1 And for almost 40 years, no dental office has used them. Why? Using the old rectangular collimating devices that we were taught, as pictured in Figure 1, it really is almost impossible to avoid cone cuts and retakes even while using film or digital holding devices called PIDs (position indicating devices). It is hard enough to get most offices, really most dentists, to adopt these film or receptor holder devices for use by their auxiliaries on patients for periapical procedures. The most frequent comment that I hear from dentists’ is “my assistant can possibly get that target with that small rectangle”.
FIGURE 1A. Typical round cones fitted on most existing dental x-ray machines.
FIGURE 1B. Margraf (Jenkintown, PA) rectangular “cones” used in dental schools and dental auxiliary schools for teaching long cone paralleling technique and dose reduction.
FIGURE 1C. RINN Universal Collimator (Dentsply RINN, Elgin, IL).
So, instead of practicing safe radiation hygiene procedures and mastering rectangular collimation technique, almost all dentists in North America continue to use x-ray machines with a large round circular cone, so that their team member can easily “hit the target”. We probably justified the continued use of the large round cylinders because we felt they reduce the number of “retakes” by our staff. This reduction in the number of retakes, and thus reduction in patient dose, was our rationalization for the continued use of old technology.
In today’s modern practice, and with the patient’s increasing concerns about radiation for themselves and their family, especially children, this is no longer acceptable. It is not even recommended.
WHAT DEVICES ARE CURRENTLY AVAILABLE? Although there are several devices, such as the RINN Universal Collimator and the Margraf Rectangular PID (position indicating device), that can collimate the round beam to a rectangular one, none of them actually fix the receptor to the x-ray tube. This is an absolute requirement to be able to use rectangular collimation precisely and routinely. We will discuss such a device, the most contemporary solution to this problem, later in the article.
The average diameter of a round cone is about 2.36 inches (60 mm). Dose reduction using rectangular collimation devices results because the opening for rectangular collimators is 35 x 45 mm. Figure 2 below shows the reduction in surface area to the patient simply by switching to rectangular collimation.
FIGURE 2. Skin surface exposure reduction to the patient from round or cylindrical collimating device (red circle) to rectangular collimating device depicted by the green region.
As early as 2001, the Journal of the American Dental Association published data from its Council on Scientific Affairs reporting the significant reduction achieved by simply adopting rectangular collimation.2 They stated:
“For periapical and bitewing radiography, rectangular collimation should be used whenever possible because a round field beam (as produced by open, round collimators) used with a rectangular image receptor produces segments of the beam circle that are not used in receptor exposure, which causes unnecessary radiation exposure to the patient.
Round collimation delivers three to four times the absorbed radiation of rectangular collimation.”10,13
More recent data published by Ludlow at al.3 is shown in Figure 3 below. As you can see in the table above, there is a tenfold reduction in x-ray dose to the patient simply by using a rectangular collimation device. Obviously, there is no scientific justification to continue to use our round cones.
FIGURE 3. Effective dose reduction achievable by the use of rectangular collimation even while using conventional film. Modified from Ludlow et al. JADA, 2008: 139: 1237-1243.
The good news is that rectangular collimation has now been made simple.3,4 A rectangular collimation product exists, which virtually eliminates angulation errors and retakes that not only add to the frustration of the dentist and office staff but also seriously increases patient x-ray dose. Figure 4 below depicts a device recently introduced to the profession, which finally makes the switch to rectangular collimation procedures simple, efficient and even marketable to the patient. What patient would not elect to go to an office that practices the safest x-ray procedures for themselves or their children?
FIGURE 4A. Tru-Align (Interactive Diagnostic Imaging, Marietta, GA) rectangular collimator attached and ready to use in the office (photo courtesy of Dr. Dale House, Johannesburg, SA).
FIGURE 4B. Tru-Align showing the receptor holder and Tru-Align connected. The plastic receptor holder with a receptor are first placed in the oral cavity and the Tru-Align, attached to the x-ray tube, is a brought up to the rectangular holder and connected. When coupled, LED lights light up on the top of the unit to indicate the device is ready. In Canada, true align is distributed exclusively in Canada by Clinical Research Dental Supplies (London, Ontario).
Obviously, the most important reason to adopt rectangular collimation technology is to reduce x-ray dose to all of our patients, especially children. In addition, however, the latest guidelines from the NCRP (National Council on Radiation Protection), the FDA and the ADA contain very strong statements, labeled “shall” statements in their latest guidelines about X beam size.4
NCRP report #145 states:
“Equipment designed for use with intraoral image receptors shall be capable of providing rectangular collimation to appro
ximate the dimensions of the image receptor. The linear dimensions of the beam in each axis should not exceed those of the receptor by more than two percent of the source-to-image receptor distance. This collimation may be inherent in the x-ray machine position-indicating device.”
In Canada, according to the 1999 Health Canada publication “Radiation Protection in Dentistry-Recommended Safety Procedures for the Use of Dental X-Ray Equipment-Safety Code 30”,5 there is currently a less stringent requirement for beam diameter. This report states:
“Beam limiting device–The primary radiation beam must be collimated in size at the end of the applicator to a circle not more than 7 centimetres in diameter, or a rectangle of area not more than 38.5 cm2.”
The Canadian Dental Association’s position published in 2005 states the requirement for collimation slightly differently. The Board of Directors of the CDA stated in their document, “Control of X-Radiation in Dentistry”:6
10. The x-ray beam for radiographs should be collimated so that it irradiates only the minimum area necessary for the examination.
In addition to being a good radiation hygiene practice and protecting our patients, guidelines which eventually will become state and provincial mandates are already in effect. Since it is likely that we will all adopt these guidelines in practicing safe x-ray procedures, finding the right technology and beginning the transition to its use is just common sense.
CONCLUSIONIt is not whether or not dentists in North America will finally adopt and use rectangular collimation, it is simply when. The more quickly they begin to use this x-ray dose reduction technology in their office, the faster they can demonstrate to their patients that they are practicing the safest dental x-ray procedures possible. OH
Dr. Miles is an adjunct Professor of Oral and Maxillofacial Radiology at the Arizona School of Dentistry & Oral Health and the University of Texas, San Antonio. He is a Diplomate of both the American Board of Oral and Maxillofacial Radiology and the American Board of Oral Medicine. He is in full-time practice of Oral and Maxillofacial Radiology in Fountain Hills, AZ. he is the co-inventor of Tru-Align.
Oral Health welcomes this original article.
1. National Council on Radiation Protection and Measurements. Dental X-Ray Protection. Bethesda, MD: NCRP; 1970. NCRP Report No. 35.
2. ADA Council on Scientific Affairs: an update on radiographic practices: information and recommendations. JADA, Vol. 132, p. 234, 2001
3. Ludlow JB, Davies-Ludlow LE, White SC. Patient risk related to common dental radiographic examinations: The impact of 2007 International Commission on Radiological Protection recommendations regarding dose calculation. Journal of the American Dental Assoc 139:1237-1243; 2008.
4. Radiation Protection in Dentistry: Recommendations of the National Council on Radiation Protection and Measurements, report #145; December, 2003.
5. Health Canada, “Radiation Protection in Dentistry-Recommended Safety Procedures for the Use of Dental X-Ray Equipment-Safety Code 30, 1999 http://www.hc-sc.gc.ca/ewh-semt/pubs/radiation/99ehd-dhm177/index-eng.php
6. CDA position on Control of X-Radiation in Dentistry, 2005 http://www.cda-adc.ca/_files/position_statements/xradiation.pdf
4. Miles DA: Radiation Minimization — “Are We There Yet?” Dental Town, July 2013, pp. 24-30.
5. Miles DA: Integrate New X-Ray Technology into Your Practice. Inside Dentistry, September 2012, In Practice (Technology Section), pp. 74-77.
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