- Email: [email protected]
Comparing digital and conventional cephalometric radiographs
TECHNO BYTES
Comparing digital and conventional cephalometric radiographs Jason M. Cohen San Jose, Calif With the introduction of digital cephalometry in orthodontics, many clinicians have cephalometric radiographs for a patient taken on 2 different cephalostats. Comparing these images is difficult because of differences in magnification between the 2 units. This article describes how to determine the magnification of a cephalometric radiograph and how to change the dimensions of a digital image to match a conventional image. (Am J Orthod Dentofacial Orthop 2005;128:157-60)
S
uperimposing lateral cephalometric radiographs to determine treatment effects and growth is routine in orthodontics. Ideally, all cephalometric radiographs for a patient would be taken on the same cephalostat. When different units are used, care must be taken to standardize the magnification before making comparisons. The orthodontic community tried to solve this problem by standardizing the distances between the source, the patient’s midsagittal plane, and the film.1 However many conventional and digital cephalostats fail to meet these dimensional standards. Samples from the Bolton and Michigan studies, and from studies from Groningen and London, all have different magnifications.2 Therefore, absolute cephalometric numbers that are not standardized for magnification cannot be compared among samples and cannot be considered accurate, absolute numbers. (Angular measurements are unaffected by uniform magnification.) In the literature, any cephalometric study that uses absolute numbers must include the cephalostat magnification, or the number can be used only in relative terms to that study. The American standard for orthodontics places the patient’s midsagittal plane 60 inches (152.4 cm) from the source and the patient 15 cm from the film.3 By using the following equation, the magnification of a cephalostat can be determined: Magnification ⫽ (source to patient’s midsagittal plane ⫹ patient’s midsagittal plane to film)/source to patient’s midsagittal plane ⫽ 167.4 cm/152.4 cm for conventional cephalostats, ⫽ 1.0984 or 9.84% magnification. In theory, any cephalostat that uses these standardPrivate practice. Reprint requests to: Jason M. Cohen, 2075 Forest Ave, Suite #2, San Jose, CA 95128; e-mail, [email protected]. Submitted, December 2004; accepted without revision, February 2005. 0889-5406/$30.00 Copyright © 2005 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2005.03.017
ized dimensions magnifies lateral cephalometric radiographs by 9.8%. This can be confirmed on any system by using the equation. Requests can also be sent to the manufacturer or the imaging laboratory, if applicable; simply inquire about magnification of the cephalostat. Digital systems are available from many manufacturers, including the Planmeca Group (Planmeca Oy, Helsinki, Finland). The magnification of the Planmeca Promax, as reported in the technical manual, is 13%.4 Magnifications of other systems are probably reported in a similar manner. Another important digital measurement to monitor is the dimensions of the radiograph. The Planmeca Promax allows one to alter the dimensions (ie, 300 mm x 270 mm, 270 mm x 270 mm, 270 mm x 240 mm, and so on). These dimensions must be known to alter the magnification for the most accurate results. For example, a conventional cephalogram with standard 9.8% magnification can be compared with a digital cephalogram with 13% magnification and a size of 300 mm x 240 mm. A software program that allows alteration of the dimensions of a digital image, such as Adobe Photoshop (Adobe, Santa Clara, Calif) is required for this procedure. The following steps describe how to change the dimensions of an image in Photoshop. 1. Determine the magnification of the conventional cephalogram (ie, 9.8%). 2. Determine the magnification of the digital cephalogram (ie, 13%). 3. Determine the dimensions of the digital cephalogram (ie, 300 x 240 mm). 4. Calculate the difference between the magnification of the digital cephalogram and the conventional cephalogram (13% – 9.8% ⫽ 3.2%). 5. Reduce the dimensions recorded from step 3 by the percentage recorded in step 4 by multiplying 157
158 Cohen
American Journal of Orthodontics and Dentofacial Orthopedics August 2005
Fig 1. Digital cephalometric radiograph with Image Size dialog box. Note that dimension on Width measurement has been changed, and Height measurement has automatically been updated.
6.
7. 8. 9. 10.
the dimensions and the percentage reduction in magnification (100% – 3.2% ⫽ 96.8%; 300 mm x .968 ⫽ 290.4 mm; 240 mm x .968 ⫽ 232.32 mm). (By changing the dimensions of the digital cephalogram to the new dimensions determined in step 5 (ie, 290.4 x 232.32 mm), the magnifications of the conventional cephalogram and the digital cephalogram will be equal, and they can be superimposed to determine growth or treatment results.) Obtain a JPEG version of the digital cephalogram. Using the Planmeca system with Dimaxis software, export the image as a JPEG. (Remember that, because JPEG is a “lossy” format, repeated file saving will result in degradation of the image in exchange for a smaller file size. Uncompressed image formats, such as bitmaps or TIFs, will be larger files, but image quality will not be sacrificed in the editing process. Try the procedure on your system with a JPEG and a bitmap to observe the significance of the degradation in the software you use.) Open the JPEG in Photoshop. Go to the Image toolbar and select Image Size (Fig 1). Check that cm or mm is selected under Document Size. Make sure Constrain Proportions and Resample are selected. This will cause the height dimen-
sion to automatically be changed when the width measurement is altered, or vice versa. This allows checking whether the measurements are correct. Resample simply maintains the resolution of the image once the dimensions are changed. 11. Go to the width measurement and enter the new calculated width measurement from step 5 (ie, old measurement ⫽ 300 mm; enter new measurement ⫽ 290.4 mm). Make sure your cm and mm are not confused. 12. Confirm that the height measurement has automatically changed to the calculated measurement from step 5. If it matches the calculated number (ie, 240 mm x .968 ⫽ 232.32 mm) within about a millimeter, you can be sure that the conversion worked. If not, confirm the measurements from the previous steps. 13. Print the image or digital cephalogram in Landscape mode from Photoshop. A tracing from this printout can now be used to superimpose with a tracing from the conventional cephalogram (Fig 2). A caveat is how to compare radiographs of different magnifications when using a digital cephalometric software tool like Dolphin Imaging (Dolphin Imaging Systems, Chatsworth, Calif). In Dolphin Imaging, the first 2 points one registers during the cephalometric analysis allow the software to
American Journal of Orthodontics and Dentofacial Orthopedics Volume 128, Number 2
Cohen 159
The simple method to adjust for magnification is as follows:
Fig 2. Example of hand-traced superimposition of digital cephalogram and conventional cephalogram of adult after magnifications were calibrated.
know the actual size of the radiograph. In other words, if the radiograph was originally magnified at 9.8%, the software will know this. If you then try to compare this image with one of a different magnification, the comparison will not be accurate. This can be overcome by using the method described above or by using a slightly less accurate method that does not involve Photoshop.
1. Scan the conventional cephalometric radiograph with a ruler visible into Dolphin Imaging (Fig 3). 2. Scan the digital image, which should already have a ruler visible from the head positioner of the cephalostat. 3. Digitize the conventional image. Dolphin Imaging software will not allow a nondigitized cephalogram for superimpositions because it has no idea how large the cephalogram is without registering its size during the digital analysis. 4. When selecting the registration points for the conventional image, use as large a distance as possible to mitigate the effects of error during landmark identification. Make sure the distance registered matches the distance Dolphin Imaging is expecting. This number can be changed on the initial digitizing screen where the analysis type is selected (Fig 4). (An important distinction needs to be made between the ruler on the conventional cephalogram and the ruler in the digital cephalogram. On the conventional cephalogram, the ruler is added after the x-ray is taken; therefore, it is not magnified, and, when used for calibrating the cephalogram, it will calibrate the cephalogram to the magnification of the cephalostat [ie, 9.8%]. However, the digital cephalogram often has a ruler included during the
Fig 3. Example of conventional cephalogram scanned at 300 DPI with ruler positioned on anterior border.
160 Cohen
American Journal of Orthodontics and Dentofacial Orthopedics August 2005
Fig 4. Digitize Setup dialog box from Dolphin Imaging. Note that Ruler has been selected, and Ruler Length has been entered (DolphinImaging.com).
5.
6.
7.
8.
x-ray; therefore, the ruler is already magnified by the magnification of the cephalostat [ie, 13%]. Thus, calibrating the digital cephalogram by using the ruler from the head positioner will actually calibrate the cephalogram to 0% magnification.) Use the concept from the previous example but with a different application. To change the digital cephalogram to match the conventional cephalogram, the digital cephalogram dimensions must be decreased by 3.2%, if we use the numbers from our previous example. However, Dolphin does not allow the dimensions to be changed but it does allow us to register the size of the cephalogram by using the ruler included. Therefore, we must actually register 9.8% more on the ruler from the digital cephalogram to simulate a 3.2% decrease in the size of the digital cephalogram because the ruler on the digital cephalogram is included during the x-ray. When asked to enter the distance that will be used to calibrate the size of the cephalogram, once again, use as large a distance as possible (ie, ruler length ⫽ 30 mm) (Fig 4). Calculate 9.8% more then the calibration distance entered (eg, original distance ⫽ 30 mm, new distance 30 mm x 1.098 ⫽ 32.94 mm). Remember, the digital cephalogram ruler is already magnified by 13%. Therefore, if you select 30 mm, from this example, the cephalogram will be calibrated to 0% magnification. Thus, 32.94 mm must be selected to simulate a magnification of 9.8% for the digital cephalogram. When asked to select the calibration points, select the distance 32.94 mm, from this example, and the cephalogram should match the dimensions of the conventional cephalogram for digital superimpositions (Fig 5). Because the converted number is sometimes a
Fig 5. Exaggerated example of first 2 registration points selected on ruler included on cephalogram (DolphinImaging.com).
fraction of a millimeter and we must rely on landmark identification to select the calibration distance in Dolphin Imaging, this method is less accurate than the first technique. If your cephalograms are automatically brought into Dolphin Imaging, the second technique is probably your only option. However, if you want it to be exact, you should use the actual image of the cephalogram, which Dolphin Imaging stores in its Working directory, and then convert the dimensions of the digital cephalogram to match the magnification of the conventional cephalogram. Then simply scan the new image into Dolphin and use the same distance for calibration points for both cephalograms. Because the magnification of the digital cephalogram would have already been converted to match that of the conventional cephalogram, using the method described for the Dolphin imaging software, the cephalogram magnifications should match. REFERENCES 1. Thurow RC. Fifty years of cephalometric radiography. Angle Orthod 1981;51:39-40. 2. Dibbets JM, Nolte K. Effect of magnification on lateral cephalometric studies. Am J Orthod Dentofacial Orthop 2002;122:196-201. 3. Proffit W, Fields H. Contemporary orthodontics. 3rd ed Saint Louis: Mosby; 2000. 4. Planmeca Promax technical specifications. Helsinki, Finland; 2004. p. A-5.
Comparing digital and conventional cephalometric radiographs Jason M. Cohen San Jose, Calif With the introduction of digital cephalometry in orthodontics, many clinicians have cephalometric radiographs for a patient taken on 2 different cephalostats. Comparing these images is difficult because of differences in magnification between the 2 units. This article describes how to determine the magnification of a cephalometric radiograph and how to change the dimensions of a digital image to match a conventional image. (Am J Orthod Dentofacial Orthop 2005;128:157-60)
S
uperimposing lateral cephalometric radiographs to determine treatment effects and growth is routine in orthodontics. Ideally, all cephalometric radiographs for a patient would be taken on the same cephalostat. When different units are used, care must be taken to standardize the magnification before making comparisons. The orthodontic community tried to solve this problem by standardizing the distances between the source, the patient’s midsagittal plane, and the film.1 However many conventional and digital cephalostats fail to meet these dimensional standards. Samples from the Bolton and Michigan studies, and from studies from Groningen and London, all have different magnifications.2 Therefore, absolute cephalometric numbers that are not standardized for magnification cannot be compared among samples and cannot be considered accurate, absolute numbers. (Angular measurements are unaffected by uniform magnification.) In the literature, any cephalometric study that uses absolute numbers must include the cephalostat magnification, or the number can be used only in relative terms to that study. The American standard for orthodontics places the patient’s midsagittal plane 60 inches (152.4 cm) from the source and the patient 15 cm from the film.3 By using the following equation, the magnification of a cephalostat can be determined: Magnification ⫽ (source to patient’s midsagittal plane ⫹ patient’s midsagittal plane to film)/source to patient’s midsagittal plane ⫽ 167.4 cm/152.4 cm for conventional cephalostats, ⫽ 1.0984 or 9.84% magnification. In theory, any cephalostat that uses these standardPrivate practice. Reprint requests to: Jason M. Cohen, 2075 Forest Ave, Suite #2, San Jose, CA 95128; e-mail, [email protected]. Submitted, December 2004; accepted without revision, February 2005. 0889-5406/$30.00 Copyright © 2005 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2005.03.017
ized dimensions magnifies lateral cephalometric radiographs by 9.8%. This can be confirmed on any system by using the equation. Requests can also be sent to the manufacturer or the imaging laboratory, if applicable; simply inquire about magnification of the cephalostat. Digital systems are available from many manufacturers, including the Planmeca Group (Planmeca Oy, Helsinki, Finland). The magnification of the Planmeca Promax, as reported in the technical manual, is 13%.4 Magnifications of other systems are probably reported in a similar manner. Another important digital measurement to monitor is the dimensions of the radiograph. The Planmeca Promax allows one to alter the dimensions (ie, 300 mm x 270 mm, 270 mm x 270 mm, 270 mm x 240 mm, and so on). These dimensions must be known to alter the magnification for the most accurate results. For example, a conventional cephalogram with standard 9.8% magnification can be compared with a digital cephalogram with 13% magnification and a size of 300 mm x 240 mm. A software program that allows alteration of the dimensions of a digital image, such as Adobe Photoshop (Adobe, Santa Clara, Calif) is required for this procedure. The following steps describe how to change the dimensions of an image in Photoshop. 1. Determine the magnification of the conventional cephalogram (ie, 9.8%). 2. Determine the magnification of the digital cephalogram (ie, 13%). 3. Determine the dimensions of the digital cephalogram (ie, 300 x 240 mm). 4. Calculate the difference between the magnification of the digital cephalogram and the conventional cephalogram (13% – 9.8% ⫽ 3.2%). 5. Reduce the dimensions recorded from step 3 by the percentage recorded in step 4 by multiplying 157
158 Cohen
American Journal of Orthodontics and Dentofacial Orthopedics August 2005
Fig 1. Digital cephalometric radiograph with Image Size dialog box. Note that dimension on Width measurement has been changed, and Height measurement has automatically been updated.
6.
7. 8. 9. 10.
the dimensions and the percentage reduction in magnification (100% – 3.2% ⫽ 96.8%; 300 mm x .968 ⫽ 290.4 mm; 240 mm x .968 ⫽ 232.32 mm). (By changing the dimensions of the digital cephalogram to the new dimensions determined in step 5 (ie, 290.4 x 232.32 mm), the magnifications of the conventional cephalogram and the digital cephalogram will be equal, and they can be superimposed to determine growth or treatment results.) Obtain a JPEG version of the digital cephalogram. Using the Planmeca system with Dimaxis software, export the image as a JPEG. (Remember that, because JPEG is a “lossy” format, repeated file saving will result in degradation of the image in exchange for a smaller file size. Uncompressed image formats, such as bitmaps or TIFs, will be larger files, but image quality will not be sacrificed in the editing process. Try the procedure on your system with a JPEG and a bitmap to observe the significance of the degradation in the software you use.) Open the JPEG in Photoshop. Go to the Image toolbar and select Image Size (Fig 1). Check that cm or mm is selected under Document Size. Make sure Constrain Proportions and Resample are selected. This will cause the height dimen-
sion to automatically be changed when the width measurement is altered, or vice versa. This allows checking whether the measurements are correct. Resample simply maintains the resolution of the image once the dimensions are changed. 11. Go to the width measurement and enter the new calculated width measurement from step 5 (ie, old measurement ⫽ 300 mm; enter new measurement ⫽ 290.4 mm). Make sure your cm and mm are not confused. 12. Confirm that the height measurement has automatically changed to the calculated measurement from step 5. If it matches the calculated number (ie, 240 mm x .968 ⫽ 232.32 mm) within about a millimeter, you can be sure that the conversion worked. If not, confirm the measurements from the previous steps. 13. Print the image or digital cephalogram in Landscape mode from Photoshop. A tracing from this printout can now be used to superimpose with a tracing from the conventional cephalogram (Fig 2). A caveat is how to compare radiographs of different magnifications when using a digital cephalometric software tool like Dolphin Imaging (Dolphin Imaging Systems, Chatsworth, Calif). In Dolphin Imaging, the first 2 points one registers during the cephalometric analysis allow the software to
American Journal of Orthodontics and Dentofacial Orthopedics Volume 128, Number 2
Cohen 159
The simple method to adjust for magnification is as follows:
Fig 2. Example of hand-traced superimposition of digital cephalogram and conventional cephalogram of adult after magnifications were calibrated.
know the actual size of the radiograph. In other words, if the radiograph was originally magnified at 9.8%, the software will know this. If you then try to compare this image with one of a different magnification, the comparison will not be accurate. This can be overcome by using the method described above or by using a slightly less accurate method that does not involve Photoshop.
1. Scan the conventional cephalometric radiograph with a ruler visible into Dolphin Imaging (Fig 3). 2. Scan the digital image, which should already have a ruler visible from the head positioner of the cephalostat. 3. Digitize the conventional image. Dolphin Imaging software will not allow a nondigitized cephalogram for superimpositions because it has no idea how large the cephalogram is without registering its size during the digital analysis. 4. When selecting the registration points for the conventional image, use as large a distance as possible to mitigate the effects of error during landmark identification. Make sure the distance registered matches the distance Dolphin Imaging is expecting. This number can be changed on the initial digitizing screen where the analysis type is selected (Fig 4). (An important distinction needs to be made between the ruler on the conventional cephalogram and the ruler in the digital cephalogram. On the conventional cephalogram, the ruler is added after the x-ray is taken; therefore, it is not magnified, and, when used for calibrating the cephalogram, it will calibrate the cephalogram to the magnification of the cephalostat [ie, 9.8%]. However, the digital cephalogram often has a ruler included during the
Fig 3. Example of conventional cephalogram scanned at 300 DPI with ruler positioned on anterior border.
160 Cohen
American Journal of Orthodontics and Dentofacial Orthopedics August 2005
Fig 4. Digitize Setup dialog box from Dolphin Imaging. Note that Ruler has been selected, and Ruler Length has been entered (DolphinImaging.com).
5.
6.
7.
8.
x-ray; therefore, the ruler is already magnified by the magnification of the cephalostat [ie, 13%]. Thus, calibrating the digital cephalogram by using the ruler from the head positioner will actually calibrate the cephalogram to 0% magnification.) Use the concept from the previous example but with a different application. To change the digital cephalogram to match the conventional cephalogram, the digital cephalogram dimensions must be decreased by 3.2%, if we use the numbers from our previous example. However, Dolphin does not allow the dimensions to be changed but it does allow us to register the size of the cephalogram by using the ruler included. Therefore, we must actually register 9.8% more on the ruler from the digital cephalogram to simulate a 3.2% decrease in the size of the digital cephalogram because the ruler on the digital cephalogram is included during the x-ray. When asked to enter the distance that will be used to calibrate the size of the cephalogram, once again, use as large a distance as possible (ie, ruler length ⫽ 30 mm) (Fig 4). Calculate 9.8% more then the calibration distance entered (eg, original distance ⫽ 30 mm, new distance 30 mm x 1.098 ⫽ 32.94 mm). Remember, the digital cephalogram ruler is already magnified by 13%. Therefore, if you select 30 mm, from this example, the cephalogram will be calibrated to 0% magnification. Thus, 32.94 mm must be selected to simulate a magnification of 9.8% for the digital cephalogram. When asked to select the calibration points, select the distance 32.94 mm, from this example, and the cephalogram should match the dimensions of the conventional cephalogram for digital superimpositions (Fig 5). Because the converted number is sometimes a
Fig 5. Exaggerated example of first 2 registration points selected on ruler included on cephalogram (DolphinImaging.com).
fraction of a millimeter and we must rely on landmark identification to select the calibration distance in Dolphin Imaging, this method is less accurate than the first technique. If your cephalograms are automatically brought into Dolphin Imaging, the second technique is probably your only option. However, if you want it to be exact, you should use the actual image of the cephalogram, which Dolphin Imaging stores in its Working directory, and then convert the dimensions of the digital cephalogram to match the magnification of the conventional cephalogram. Then simply scan the new image into Dolphin and use the same distance for calibration points for both cephalograms. Because the magnification of the digital cephalogram would have already been converted to match that of the conventional cephalogram, using the method described for the Dolphin imaging software, the cephalogram magnifications should match. REFERENCES 1. Thurow RC. Fifty years of cephalometric radiography. Angle Orthod 1981;51:39-40. 2. Dibbets JM, Nolte K. Effect of magnification on lateral cephalometric studies. Am J Orthod Dentofacial Orthop 2002;122:196-201. 3. Proffit W, Fields H. Contemporary orthodontics. 3rd ed Saint Louis: Mosby; 2000. 4. Planmeca Promax technical specifications. Helsinki, Finland; 2004. p. A-5.