5
The use of added erbium filtration in intraoral radiography Arthur Z. Ponce, DDS, MA,* William D. McDavid, PhD,** and Olaf E. Langland, DDS, MS,*** Hackensack, N.J., and San Antonio, Texas Using D and E speed film, intraoral exposures were made of a phantom at 70 and 90 kVp, with and without erbium filtration. Measurement of surface exposures indicates substantial reduction when erbium filtration was used. However, there was a visible loss of contrast. The raters agreed that radiographs made with E speed and D speed film, both with and without erbium filtration, were all of adequate diagnostic quality. The radiographs with the highest contrast were consistently preferred by all of the raters. (ORAL SURC ORAL MED ORAL PATHOL 1988;66:513-7) I ncreased awareness of the potential risks associat- ed with the use of x-ray procedures for medical and dental examinations has encouraged investigators to study various methods for reducing exposure with minimal loss of diagnostic yield. Use of additional filtration is one means of decreasing surface expo- sure, but the use of common materials, such as aluminum and copper, alters the quality of the x-ray beam to such an extent that the resultant diminution of subject contrast may affect the diagnostic capabil- ities of the radiographic image. To circumvent this difficulty some investigators have proposed the use of band pass filters, which transmit only the most useful x-ray energies while absorbing other energies at the high- and low-energy ends of the x-ray spectrum. Studies of filtration in diagnostic radiology have prompted the testing of various materials as x-ray filters. The use of gadolinium as a band pass filter in medical radiography and fluoroscopy was theoreti- cally considered by Oosterkamp’ in 196 1. Richards et al.2 advocated the use of a samarium filter (K edge = 46.8 keV) to produce a narrow spectrum in the range of maximum sensitivity of dental film. They found a 33% reduction in patient exposure, as well as improved image quality. Atkins et a1.3studied *Department of Oral Diagnosis, Treatment Planning Radiology and Preventive Dentistry, Fairleigh Dickinson University, Hack- ensack, New Jersey. **Department of Dental Diagnostic Science, The University of Texas Health Science Center at San Antonio, San Antonio, Texas. ***Department of Dental Diagnostic Science, The University of Texas Health Science Center at San Antonio, San Antonio, Texas. the effects of various filter materials on the x-ray beam, using characteristic absorption edges to create a band pass effect. He concluded that the use of gadolinium (K edge = 50.2 keV) filter selectively removed both higher energy photons (more than 50 keV) and lower energy photons (less than 33 keV). This resulted in contrast enhancement for iodine and barium while substantially reducing radiation dose. Villagran et a1.4 studied various heavy elements as possible filters seeking to reduce patient exposure in diagnostic procedures involving excretory urography and lumbar spine studies. These pioneering efforts have been followed by numerous investigations in both medical and dental radiography. Rare earth materials used as filters in medical radiography have included samarium, gado- linium, holmium, erbium, ytterbium, and yttrium.5 The use of rare earth filters in intraoral radiography has also been investigated by dental researchers2v6-‘0 in an effort to maintain adequate diagnostic quality with reduced patient exposure. Investigations in panoramic radiography”-I3 indicate that image qual- ity would not be seriously impaired by use of rare earth filtration in conjunction with rare-earth screen film combinations, but could reduce patient exposure by as much as 70%. TyndalF carried out laboratory investigation of various rare earth filter materials, studying effects on the energy spectra as well as dose reductions. He found that spectral effects were dependent on atomic number, K-edge, and thickness of each filter, with erbium being the most efficient in exposure reduc- tion. The effectiveness of erbium filtration in pan- oramic radiography was demonstrated by Tyndall et a1.‘3,‘4 in other studies. 513

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The use of added erbium filtration in intraoral radiography Arthur Z. Ponce, DDS, MA,* William D. McDavid, PhD,** and Olaf E. Langland, DDS, MS,*** Hackensack, N.J., and San Antonio, Texas

Using D and E speed film, intraoral exposures were made of a phantom at 70 and 90 kVp, with and

without erbium filtration. Measurement of surface exposures indicates substantial reduction when erbium

filtration was used. However, there was a visible loss of contrast. The raters agreed that radiographs made with E speed and D speed film, both with and without erbium filtration, were all of adequate diagnostic

quality. The radiographs with the highest contrast were consistently preferred by all of the raters. (ORAL SURC ORAL MED ORAL PATHOL 1988;66:513-7)

I ncreased awareness of the potential risks associat- ed with the use of x-ray procedures for medical and dental examinations has encouraged investigators to study various methods for reducing exposure with minimal loss of diagnostic yield. Use of additional filtration is one means of decreasing surface expo- sure, but the use of common materials, such as aluminum and copper, alters the quality of the x-ray beam to such an extent that the resultant diminution of subject contrast may affect the diagnostic capabil- ities of the radiographic image. To circumvent this difficulty some investigators have proposed the use of band pass filters, which transmit only the most useful x-ray energies while absorbing other energies at the high- and low-energy ends of the x-ray spectrum.

Studies of filtration in diagnostic radiology have prompted the testing of various materials as x-ray filters. The use of gadolinium as a band pass filter in medical radiography and fluoroscopy was theoreti- cally considered by Oosterkamp’ in 196 1. Richards et al.2 advocated the use of a samarium filter (K edge = 46.8 keV) to produce a narrow spectrum in the range of maximum sensitivity of dental film. They found a 33% reduction in patient exposure, as well as improved image quality. Atkins et a1.3 studied

*Department of Oral Diagnosis, Treatment Planning Radiology and Preventive Dentistry, Fairleigh Dickinson University, Hack- ensack, New Jersey. **Department of Dental Diagnostic Science, The University of Texas Health Science Center at San Antonio, San Antonio, Texas. ***Department of Dental Diagnostic Science, The University of Texas Health Science Center at San Antonio, San Antonio, Texas.

the effects of various filter materials on the x-ray beam, using characteristic absorption edges to create a band pass effect. He concluded that the use of gadolinium (K edge = 50.2 keV) filter selectively removed both higher energy photons (more than 50 keV) and lower energy photons (less than 33 keV). This resulted in contrast enhancement for iodine and barium while substantially reducing radiation dose. Villagran et a1.4 studied various heavy elements as possible filters seeking to reduce patient exposure in diagnostic procedures involving excretory urography and lumbar spine studies.

These pioneering efforts have been followed by numerous investigations in both medical and dental radiography. Rare earth materials used as filters in medical radiography have included samarium, gado- linium, holmium, erbium, ytterbium, and yttrium.5 The use of rare earth filters in intraoral radiography has also been investigated by dental researchers2v6-‘0 in an effort to maintain adequate diagnostic quality with reduced patient exposure. Investigations in panoramic radiography”-I3 indicate that image qual- ity would not be seriously impaired by use of rare earth filtration in conjunction with rare-earth screen film combinations, but could reduce patient exposure by as much as 70%.

TyndalF carried out laboratory investigation of various rare earth filter materials, studying effects on the energy spectra as well as dose reductions. He found that spectral effects were dependent on atomic number, K-edge, and thickness of each filter, with erbium being the most efficient in exposure reduc- tion. The effectiveness of erbium filtration in pan- oramic radiography was demonstrated by Tyndall et a1.‘3,‘4 in other studies.

513

Page 2: The use of added erbium filtration in intraoral radiography

5 I 4 Ponce, M&avid. and Langland Oral Surg October I988

Fig. 1. Phantom used for the evaluation.

Erbium (K edge 57.5 keV) has been found to be an efficient filter of lower-energy photons (less than 30 keV),3 as well as higher-energy photons (more than 58 keV). The results of other studies of erbium filtratiorQ5~ I6 were followed by investigations by Wesenberg et a1.,17 who carried out clinical evalua- tion of erbium filtration to quantify dose reduction. Reductions in skin dose were found to range from 57% at 50 kVp to 36% at 80 kVp to 37% at 120 kVp. The results of this study also indicated a slight improvement in radiographic contrast when used with high-speed rare-earth screen film combina- tions.

In 1986, Reactor Experiments, Inc.* began to market erbium filters specifically designed for intraoral radiography. Two filters are available. One filter (Cat. no. 829B) is fabricated as a low-density filter and is used with the slower Kodak Ultraspeed (or equivalent) D speed film. The second filter (Cat. no. 825B) is manufactured with a higher density than the first filter and is used with the fast Kodak Ektaspeed (or equivalent) E speed film. These filters are designed to be used along with the filtration already present in the x-ray machine. The filters are inserted in the beam path over the collimator at the x-ray tube. The manufacturers claim that these filters reduce patient dosage up to 50% without loss of radiographic quality.

The purpose of this study was to investigate the reduction in surface exposure using these two com- mercially available erbium filters and to study their

*Reactor Experiments, Inc., San Carlos, California.

clinical usefulness in terms of practical considera- tions. such as exposure time and the acceptability of the resulting images for diagnostic purposes.

METHODS

A Gendextt 1000 Intraoral X-ray Unit was used for all the test exposures. This machine has kVp variable from 60 to 90 kVp, mA options of 10 and 15 mA, and a total filtration of 2.7 mm aluminum equivalent as stated by the manufacturer. D- and E-speed film was used* and the resulting radio- graphs were developed in an automatic processor? according to the manufacturer’s instructions.

A series of preliminary radiographs was made using a phantom consisting of a mandible and a step wedge embedded in plastic (see Fig. 1). It was determined that an acceptable radiograph could be produced when the optical density of the first step was around 1 .O. Timer settings were then determined that resulted in a density of 1 .OO + 0.05 for this step using D and E speed film at 70 and 90 kVp with and without the two erbium filters marketed by Reactor Experiments, Inc. The resulting radiographs were then read with a densitometer in order to arrive at indices of contrast for the various radiographic techniques. The index of contrast was simply the difference of recorded optical density for the first and eighth steps. Relative values of surface exposure were then determined for the various techniques using an mdh model 10 15 x-ray monitor and a 10-5X probe.$

It was apparent from the results of the preceding tests that while significant reductions in surface exposure could be achieved using the erbium filters, there was likewise a considerable reduction in con- trast when these filters were used. The next step was to determine whether this loss of contrast would be considered clinically acceptable. In order to do this, the preceding exposure factors were used to radio- graph a Dxttr phantomj Multiple exposures of the upper molars, the lower molars, and the upper incisors were made using the various techniques (eight combinations of 70 or 90 kVp, D- or E-speed film, and filtered or unfiltered beams). Since the high-density erbium filter required exposure times that were believed to be impractical (1 set or more),

*Eastman Kodak Company, Rochester, N.Y. TPhilips Dental Systems, Stamford, Connecticut. VGendex Co. Milwaukee, Wis. §mdh Industries, Monrovia, Calif. Ipxttr-Dental X-ray Teaching and Training Replica, Rinn Corp., Elgin, III.

Page 3: The use of added erbium filtration in intraoral radiography

Volume 66 Number 4

Use of erbium jiltration in intraoral radiography 5 15

Table 1. Results of exposure/contrast study at 90 kVp

Film

type

D D D E E E

Added filter

None Low-density High-density None Low-density High-density

HVL Density Contrast (mm Al) (step 1) (step R-step 1)

2.9 I .03 1.22 4.2 0.97 0.93 5.4 1.01 0.84 2.9 0.97 0.74 4.2 1.01 0.65 5.4 1.03 0.64

Exposure at cone tip

WV

332 224 199 163 130 106

Relative surface exposure

1 .oo 0.67 0.60 0.49 0.39 0.32

Table II. Results of exposure/contrast study at 70 kVp

Film Added HVL Density Contrast

We jilter (mm Al) (step 1) (step R-step I)

D None 2.3 1.03 1.25 D Low-density 2.5 1.02 1.14 D High-density 4.4 0.99 0.98 E None 2.3 0.96 0.89 E Low-density 2.5 1 .oo 0.80 E High-density 4.4 I .02 0.66

Exposure at cone tip

WV

416 309 220 225 147 110

Relative surface exposure

1 .oo 0.74 0.53 0.54 0.35 0.26

only the low-density erbium filter was used for this portion of the study. The radiographs were then arranged in masked pairs representing the same anatomic site and shown to ten viewers who were faculty members and postdoctoral students in the Department of Dental Diagnostic Science at the University of Texas Health Science Center at San Antonio. For each pair of radiographs, the viewers were asked which one they would prefer to use for diagnostic purposes. If there was no preference, they were asked to state this fact rather than choosing. The viewers were given no information about the objectives of the study.

Responses were scored I, 2, or 3 for each pairing and each radiographic projection. For example, an upper molar pair for the filter versus no-filter com- parison was scored as a 2 if the filter was preferred, a 3 if no filter was preferred, or a 2 if there was no preference. An average score was then computed for each rater and each pairing from the three scored radiographic projections. The range of values for the average score was 1 to 3. If the distribution of the 10 raters’ scores computed for a pairing was not differ- ent from a normal distribution with a mean of 2 and a standard deviation of 0.6, then the raters were unable to decide which of the two techniques was better. This distribution comparison was made using the Kolmogorov-Smirnov (K-S) Goodness of Fit Test.16 A significant K-S p value (<O.Ol) indicated

that the raters strongly preferred one technique over another.

RESULTS

The results of the exposure/contrast study are presented in Tables I and II. Clearly, substantial reductions in surface exposure can be obtained using the erbium filters, but at the expense of reduced contrast, as measured by the difference between densities in the step-wedge images. This is consistent with the increased half-value layers that accompa- nied the addition of the erbium filters. Despite the band pass properties of the erbium filtration, the net effect of the additional filtration is still to substan- tially raise the effective energy of the beam. This leads to the observed properties of reduced surface dose and contrast.

Diminished contrast is not necessarily undesirable. For certain diagnostic tasks it may even be prefera- ble. In order to evaluate the subjective reactions of dental radiologists to the various techniques included in this study, a representative sample of viewers was asked to compare various pairs of radiographs repre- senting different anatomic regions. Statistical analy- sis was then performed in order to evaluate the results. The outcome of this exercise is shown in Figure 2. An analysis of the results leads to the following observations:

1. Subjective preference clearly favored high-con-

Page 4: The use of added erbium filtration in intraoral radiography

515 Ponce, MeDavid, and Langland

R. 70 kVp

Ll- E-

D’

6. 90 kVp

x + E’

D- E-

ll+ E+ Fig. 2. Comparison of D- and E-speed film with (+) and without (-) the low-density erbium filter. > indicates significant preference @ < 0.01).

trast images. In all cases in which a preference was expressed it was a high-contrast image that was chosen in preference to an image with less contrast.

2. For high and low kVp, with and without additional erbium filtration, D-speed film was preferred to E-speed film. The preference became more marked at higher kilovoltages.

3. When D-speed film was used, there was no preference between conventional images and images produced with added erbium filtration (surface exposure reduction of 26%) at lower kilovoltages. This is probably because the D- speed film contrast is relatively high, with and without the filter. At higher kVp, the conven- tional images were preferred.

4. When E-speed film was used, there was a preference for conventional images over images produced with added erbium filtration at lower kilovoltages. At higher kVp, there was no preference, probably because the E-speed film contrast is relatively low, with and without the filter.

5. By combining a change to E-speed film with the use of erbium filters, it is possible to achieve a dramatic reduction of surface exposure on the order of 65% to 70%. At both high and low kilovoltages, however, the conventional D-

Oral Surg October 1988

speed images were preferred to the low-dose images.

DISCUSSION

The use of erbium filters, in addition to the existing filtration in dental x-ray units, can lead to a substantial reduction in surface exposure to the patient. The low-density filter gave radiographs equivalent to those without erbium when D-speed film was used at 70 kVp. This was accompanied by a 26% reduction in exposure. At 90 kVp, the low- density filter gave an equivalent radiograph with E-speed film at 90 kVp. In this case, the exposure with the erbium filter was 20% lower than that with the aluminum filter only.

Combined with the use of E-speed film, the high-density filter can make it possible to produce radiographs with as little as 25% of the surface exposure associated with D-speed film and the usual aluminum filtration. This is a dramatic decrease in patient exposure and dentists who are concerned with radiation protection would be well advised to consider the adoption of these techniques. There are, however, a number of drawbacks to the use of the high-density filter. This filter reduces the intensity of the beam considerably so that relatively long expo- sure times are required in order to yield radiographic images of acceptable quality. This may lead to problems with patient motion and may not prove acceptable for routine clinical use. In addition, the combination of heavy filtration with the use of E-speed film leads to radiographs that have marked- ly reduced contrast. This, in general, appears to be a disadvantage, since clinicians seem consistently to prefer images with greater contrast. This is consis- tent with a previous study by Ponce et al.‘O in which additional aluminum, gadolinium, and yttrium filters were evaluated in conjunction with E-speed film. While the resultant radiographs were considered diagnostically adequate, the viewers expressed a preference for conventional, high-contrast (and, it may be added, high-dose) radiographs. In the present study, the viewers also expressed a general acceptance of the low-dose images. They were con- sidered of acceptable diagnostic quality and were in no case rejected out of hand as being in any way unsatisfactory for diagnostic purposes. When forced to choose, however, between pairs of radiographs covering the same anatomic regions, the viewers in general opted for the images exhibiting greater contrast.

It is an unfortunate fact that techniques for dose reduction more often than not lead to radiographs that are-at least at first glance-less acceptable to

Page 5: The use of added erbium filtration in intraoral radiography

Volume 66 Use of erbium filtration in intraoral radiography 517 Number 4

the clinician. The value of the radiographic examina- tion, however, lies not in the aesthetic value of the images produced, but in their usefulness as an aid in the diagnostic process. Further studies are required that would demonstrate whether or not diagnostic capability is compromised by the introduction of these and other dose reduction techniques.

We wish to thank Mr. Warren J. Heiman and Mr. Don Eldred of Reactor Experiments, Inc. for supplying the filters used in this study, as well as for providing technical data and reviewing the manuscript. We also wish to thank John Schoolfield for assistance with the statistical analy- sis.

REFERENCES

1.

2.

3.

Oosterkamp IW. “Monochromatic x-rays for medical fluoros- copy and radiography?” Medicamundi 1961;7:68-77. Richards AE, Barber GL, Badger JD, Hale JD. Samarium filters for dental radiology. ORAL SLJRG ORAL MED ORAL PATHOL 1970;29:704-15. Atkins HL, Fairchild RG, Robertson JS, Greenberg D. Effect of absorption edge filtration on diagnostic x-ray spectra. Radiology 1975;li5:431-7.

4. Villaeran JE. Hobbs. BB. Tavlor KW. Reduction of natient , d

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exposure by use of heavy elements as radiation filters in diagnostic radiology. Radiology 1978;127:249-54. Tyndall DA. Spectroscopic analysis and dosimetry of diagnos- tic beams filtered by rare earth materials. ORAL SURG ORAL MED ORAL PATHOL 1986;62:205-I 1. Duckworth JE, Webber RL, Youmans H, Fewell TR. The effects of spectral distribution on x-ray image quality. ORAL SURG ORAL MED ORAL PATHOL 1981;52:314-20. Gelsky DE, Baker CG. Energy selective filtration of dental x-rav beams. ORAL SURG ORAL MED ORAL PATHOL 198 1; 52:565-7. Huen A, Sternglass EJ, Mazzocco DM, Fisher WG: Quasi- monoenergetic radiation for dental radiography. J Dent Res 1976;55:148.

9. Mauriello S, Washburn D. Effects of gadolinium filtration on patient exposure and image quality. J Dental Res 1984; 63(Abstr):333.

10. Ponce AZ, McDavid WD, Underhill TE, Morris CR. Use of E-sneed film with added filtration. ORAL SURF ORAL MED ~R;L PATHOL 1986;61:297-9.

11. Ponce AZ, McDavid WD, Lundeen RC, Morris CR. Adapta- tion of the Panorex II for use with rare-earth screen-film combinations. ORAL SURG ORAL MED ORAL PATHOL 1986; 61:645-8.

12. Ponce AZ, McDavid WD, Lundeen RC, Morris CR. Kodak T-Mat G Film in rotational panoramic radiography. ORAL SURG ORAL MED ORAL PATHOL 1986;61:649-52.

13. Tyndall DA, Washburn DB. Rare-earth filters in panoramic radiography: a means of reducing patient dose without compromising image quality. Dentomaxillofac Radio1 1986; 15:19-26.

14. Tyndall DA. Rare earth filtration: spectral distributions, exposure reduction and image quality effects for panoramic radiography. Dentomaxillofac Radio1 1987;16:23-7.

15. Previtke RG Jr. Erbium filtration in iodine contrast studies. Radio1 Technol 1982;53:399-405.

16. Chakera TMH, Fleay RF, Hensen PW, Cole SN. Dose reduction in radiology using heavy metal foils. Br J Radio1 1982;55:853-8.

17. Wesenberg RL, Amundson GM, Fleay RF. Ultra low-density routine radiography utilizing a rare-earth filter. Presented at 69th Scientific Assembly and Annual Meeting of the Radio- logical Society of North America. Chicago, Ill.: Nov. 13-18, 1983.

18. Siegel, Sidney. Nonparametric statistics for the behavioral sciences. New York: McGraw-Hill, 1956:47-52.

Reprint requests to:

Dr. William D. McDavid Department of Dental Diagnostic Science School of Dentistry The University of Texas Health Science Center at San Antonio San Antonio, TX 78284