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Three-dimensional imaging of the pterygopalatine fossa
Otolaryngology–Head and Neck Surgery (2007) 136, 1014-1016
CLINICAL TECHNIQUES AND TECHNOLOGY
Three-dimensional imaging of the pterygopalatine fossa Allen O. Mitchell, MD, John F. Alburger, MD, William E. Bolger, MD, Michael I. Frew, MD, and A. Charles Richardson, DDS, Portsmouth, VA
S
inonasal endoscopy has revolutionized the diagnosis and treatment of nasal pathology. Endoscopes allow minimally invasive treatment not previously possible. These techniques have inspired interest in the use of new imaging strategies for surgical planning and guidance.
BACKGROUND AND SIGNIFICANCE The pterygopalatine fossa’s location makes its anatomy particularly challenging to visualize. Unlike the neighboring parapharyngeal space, the pterygopalatine fossa has not undergone three-dimensional CT imaging with anatomic correlation. The efficacy of CT in assessment of the fossa has been established, and high-resolution CT scan has been used to survey dimensions and foramina locations.1-4 Typically, cross-sectional studies have been used to examine the fossa and its anatomy.2,5 In our clinical experience, standard two-dimensional CT projections do not always portray the features of this region in a way that lends itself to endoscopic surgery. Accessibility during surgical dissection as well as the ability to predict whether a direct view might be obtainable from one area to another would be helpful. Finally, the ability to better appreciate a tumor’s three-dimensional characteristics might better help the surgeon to plan for its excision. From the Department of Otolaryngology, Naval Medical Center, Portsmouth, VA. This work was prepared as part of the first author’s official duties as a member of the military service. Copyright protection is not available for any work of the United States Government. The views expressed in this material are those of the authors, and do not reflect the official policy or position of the United States Government, the Department of Defense, or the Department of the Navy. Author disclosures for William Bolger, MD: Gyrus ENT: Advisory
THREE-DIMENSIONAL IMAGING BASICS Three-dimensional imaging can be performed in two ways: surface rendering or volume rendering. Surface rendering can be used to create images that depict the external contours of an object. This modality does not, however, recreate the interior characteristics of the object as volume rendering does. The individual elements that make up a volume are called voxels. In the case of a volume generated from CT data, the voxels have characteristics relating to their radiographic opacity. The process whereby voxels are selected based upon this characteristic is called thresholding and can be used to add or subtract soft tissue or bone elements from a volume. The rendered image may be enhanced with lighting from various angles, and portions of the volume may be removed with “cut planes” to better visualize structures.
THREE-DIMENSIONAL IMAGING PILOT STUDY In a pilot investigation, we studied 15 unpreserved human cadaveric heads with three-dimensional imaging and endoscopic dissection. Approval from our institutional review board was obtained prior to initiating this protocol. The specimens were all axially imaged using a GE HiSpeed Advantage CT Scanner (GE Medical Systems, Milwaulkee, Board, royalties; Acclarent: Advisory Board, minor shareholder; Serim Research Corp: consultant. Presented at the Annual Meeting of the American Academy of Otolaryngology⫺Head and Neck Surgery, New Orleans, LA, September 26⫺29, 1999. Reprint requests: CDR Allen O. Mitchell, MD, Department of Otolaryngology–Head and Neck Surgery, NMCP, 620 John Paul Jones Circle, Portsmouth, VA 23708. E-mail address: [email protected].
0194-5998/$32.00 Published by Elsevier Inc. on behalf of American Academy of Otolaryngology–Head and Neck Surgery Foundation, Inc. doi:10.1016/j.otohns.2007.01.021
Mitchell et al
Three-dimensional imaging of the pterygopalatine. . .
1015
locate V2 and the optic canal. Figure 2 shows a comparison of a volume rendered image of the pterygoid process with the actual endoscopic view. We also noted that the volume rendered finding of a prominent ridge of bone between the foramina, which may also be seen in Figure 2, agreed with the observed findings. A drawback we noted was that the volume imagery was inaccurate in its depiction of the sphenopalatine foramen
Figure 1 Example of threshold manipulation. Top, complete volume. Bottom, same volume after viewing threshold is changed, showing the underlying skull.
WI). The data were processed on a Silicon Graphics Indigo2 High Impact workstation (SGI, Mountain View, CA), using VoxelView and VoxelAnimator (Vital Images Corp, Minnetonka, MN) to render and examine the volumes. By changing the radiographic thresholds, the soft tissue portion of the volume could be removed. Figure 1 shows an example of this technique, called thresholding. The sphenopalatine foramen, the foramen of Vidian nerve, and foramen rotundum were examined. Endoscopic dissection of the heads was then performed, examining these same features, comparing their endoscopic location and appearance to the volume rendered images.
RESULTS The observed relationship of foramen rotundum to that of Vidian nerve was found to be in agreement with the volume reconstruction in all cases. This was thought to be important, because during transnasal dissection, the Vidian nerve was identified medially and used as a reference point to
Figure 2 Pterygoid process of the sphenoid bone. The upper volume-rendered image is compared with the endoscopic view. The open arrow points to foramen rotundum; the solid arrow, to the foramen of the Vidian nerve.
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Otolaryngology–Head and Neck Surgery, Vol 136, No 6, June 2007
and thin bone of the lateral nasal wall. An often significantly larger foramen was depicted in 12 of 15 cases, and the absence of nasal wall bone was noted in three.
DISCUSSION AND CONCLUSIONS The volume-rendering tools were found to be an accurate modality for examining the foramina of the pterygoid process of the sphenoid. Subjectively, the sphenopalatine foramen was not thought to be accurately depicted, although further study is warranted. An important finding was that thresholding of images could result in inaccurate depiction of thin bone. At the beginning of the study, the investigators had some concerns with regard to the practicality of the three-dimensional imaging. The computer hardware and software available at the time required considerable time to render images, and real-time manipulation was limited. However, the images obtained were found to be quite intuitive to understand and, from a standpoint of “previewing” the relevant anatomy, quite useful. The dissector thought that previewing the three-dimensional images greatly facilitated dissection. Our pilot study serves to support the use of volume rendering to identify key foramina within the pterygopalatine fossa and to highlight its ability to visualize these structures in a minimally invasive manner. It also under-
scores the fact that volume rendering should be used with caution in areas of thin or attenuated bone. The latest three-dimensional imaging tools available allow the clinician to view and manipulate images in realtime. The software is fairly intuitive, and little technical knowledge is necessary for use. The real strength of threedimensional imaging is in its ability to present complicated anatomy from any viewing angle in a minimally invasive manner. We envision a surgeon routinely reviewing and manipulating volume-reconstructed images prior to surgery in the same way that one would review standard twodimensional CT or MR images. We believe that these findings should encourage surgeons to familiarize themselves with available volume rendering techniques, and consider their use for surgical planning.
REFERENCES 1. Nasser JA, Attia EL. A conceptual approach to learning and organizing the surgical anatomy of the skull base. J Otolaryngol 1990;19:114 –21. 2. Daniels DL, Rauschning W, Lovas J, et al. Pterygopalatine fossa: computed tomographic studies. Radiology 1983;149:511– 6. 3. Curtin HD, Williams R, Johnson J. CT of perineural tumor extension: pterygopalatine fossa. AJR Am J Roentgenol 1985;144:163–9. 4. Kim HS, Kim DI, Chung IH. High-resolution CT of the pterygopalatine fossa and its communications. Neuroradiology 1996;38(suppl 1): S120 – 6. 5. Bryce GE, Woodham MB. The cross-sectional osteology of the midface. J Otolaryngol 1988;17:4 –11.
CLINICAL TECHNIQUES AND TECHNOLOGY
Three-dimensional imaging of the pterygopalatine fossa Allen O. Mitchell, MD, John F. Alburger, MD, William E. Bolger, MD, Michael I. Frew, MD, and A. Charles Richardson, DDS, Portsmouth, VA
S
inonasal endoscopy has revolutionized the diagnosis and treatment of nasal pathology. Endoscopes allow minimally invasive treatment not previously possible. These techniques have inspired interest in the use of new imaging strategies for surgical planning and guidance.
BACKGROUND AND SIGNIFICANCE The pterygopalatine fossa’s location makes its anatomy particularly challenging to visualize. Unlike the neighboring parapharyngeal space, the pterygopalatine fossa has not undergone three-dimensional CT imaging with anatomic correlation. The efficacy of CT in assessment of the fossa has been established, and high-resolution CT scan has been used to survey dimensions and foramina locations.1-4 Typically, cross-sectional studies have been used to examine the fossa and its anatomy.2,5 In our clinical experience, standard two-dimensional CT projections do not always portray the features of this region in a way that lends itself to endoscopic surgery. Accessibility during surgical dissection as well as the ability to predict whether a direct view might be obtainable from one area to another would be helpful. Finally, the ability to better appreciate a tumor’s three-dimensional characteristics might better help the surgeon to plan for its excision. From the Department of Otolaryngology, Naval Medical Center, Portsmouth, VA. This work was prepared as part of the first author’s official duties as a member of the military service. Copyright protection is not available for any work of the United States Government. The views expressed in this material are those of the authors, and do not reflect the official policy or position of the United States Government, the Department of Defense, or the Department of the Navy. Author disclosures for William Bolger, MD: Gyrus ENT: Advisory
THREE-DIMENSIONAL IMAGING BASICS Three-dimensional imaging can be performed in two ways: surface rendering or volume rendering. Surface rendering can be used to create images that depict the external contours of an object. This modality does not, however, recreate the interior characteristics of the object as volume rendering does. The individual elements that make up a volume are called voxels. In the case of a volume generated from CT data, the voxels have characteristics relating to their radiographic opacity. The process whereby voxels are selected based upon this characteristic is called thresholding and can be used to add or subtract soft tissue or bone elements from a volume. The rendered image may be enhanced with lighting from various angles, and portions of the volume may be removed with “cut planes” to better visualize structures.
THREE-DIMENSIONAL IMAGING PILOT STUDY In a pilot investigation, we studied 15 unpreserved human cadaveric heads with three-dimensional imaging and endoscopic dissection. Approval from our institutional review board was obtained prior to initiating this protocol. The specimens were all axially imaged using a GE HiSpeed Advantage CT Scanner (GE Medical Systems, Milwaulkee, Board, royalties; Acclarent: Advisory Board, minor shareholder; Serim Research Corp: consultant. Presented at the Annual Meeting of the American Academy of Otolaryngology⫺Head and Neck Surgery, New Orleans, LA, September 26⫺29, 1999. Reprint requests: CDR Allen O. Mitchell, MD, Department of Otolaryngology–Head and Neck Surgery, NMCP, 620 John Paul Jones Circle, Portsmouth, VA 23708. E-mail address: [email protected].
0194-5998/$32.00 Published by Elsevier Inc. on behalf of American Academy of Otolaryngology–Head and Neck Surgery Foundation, Inc. doi:10.1016/j.otohns.2007.01.021
Mitchell et al
Three-dimensional imaging of the pterygopalatine. . .
1015
locate V2 and the optic canal. Figure 2 shows a comparison of a volume rendered image of the pterygoid process with the actual endoscopic view. We also noted that the volume rendered finding of a prominent ridge of bone between the foramina, which may also be seen in Figure 2, agreed with the observed findings. A drawback we noted was that the volume imagery was inaccurate in its depiction of the sphenopalatine foramen
Figure 1 Example of threshold manipulation. Top, complete volume. Bottom, same volume after viewing threshold is changed, showing the underlying skull.
WI). The data were processed on a Silicon Graphics Indigo2 High Impact workstation (SGI, Mountain View, CA), using VoxelView and VoxelAnimator (Vital Images Corp, Minnetonka, MN) to render and examine the volumes. By changing the radiographic thresholds, the soft tissue portion of the volume could be removed. Figure 1 shows an example of this technique, called thresholding. The sphenopalatine foramen, the foramen of Vidian nerve, and foramen rotundum were examined. Endoscopic dissection of the heads was then performed, examining these same features, comparing their endoscopic location and appearance to the volume rendered images.
RESULTS The observed relationship of foramen rotundum to that of Vidian nerve was found to be in agreement with the volume reconstruction in all cases. This was thought to be important, because during transnasal dissection, the Vidian nerve was identified medially and used as a reference point to
Figure 2 Pterygoid process of the sphenoid bone. The upper volume-rendered image is compared with the endoscopic view. The open arrow points to foramen rotundum; the solid arrow, to the foramen of the Vidian nerve.
1016
Otolaryngology–Head and Neck Surgery, Vol 136, No 6, June 2007
and thin bone of the lateral nasal wall. An often significantly larger foramen was depicted in 12 of 15 cases, and the absence of nasal wall bone was noted in three.
DISCUSSION AND CONCLUSIONS The volume-rendering tools were found to be an accurate modality for examining the foramina of the pterygoid process of the sphenoid. Subjectively, the sphenopalatine foramen was not thought to be accurately depicted, although further study is warranted. An important finding was that thresholding of images could result in inaccurate depiction of thin bone. At the beginning of the study, the investigators had some concerns with regard to the practicality of the three-dimensional imaging. The computer hardware and software available at the time required considerable time to render images, and real-time manipulation was limited. However, the images obtained were found to be quite intuitive to understand and, from a standpoint of “previewing” the relevant anatomy, quite useful. The dissector thought that previewing the three-dimensional images greatly facilitated dissection. Our pilot study serves to support the use of volume rendering to identify key foramina within the pterygopalatine fossa and to highlight its ability to visualize these structures in a minimally invasive manner. It also under-
scores the fact that volume rendering should be used with caution in areas of thin or attenuated bone. The latest three-dimensional imaging tools available allow the clinician to view and manipulate images in realtime. The software is fairly intuitive, and little technical knowledge is necessary for use. The real strength of threedimensional imaging is in its ability to present complicated anatomy from any viewing angle in a minimally invasive manner. We envision a surgeon routinely reviewing and manipulating volume-reconstructed images prior to surgery in the same way that one would review standard twodimensional CT or MR images. We believe that these findings should encourage surgeons to familiarize themselves with available volume rendering techniques, and consider their use for surgical planning.
REFERENCES 1. Nasser JA, Attia EL. A conceptual approach to learning and organizing the surgical anatomy of the skull base. J Otolaryngol 1990;19:114 –21. 2. Daniels DL, Rauschning W, Lovas J, et al. Pterygopalatine fossa: computed tomographic studies. Radiology 1983;149:511– 6. 3. Curtin HD, Williams R, Johnson J. CT of perineural tumor extension: pterygopalatine fossa. AJR Am J Roentgenol 1985;144:163–9. 4. Kim HS, Kim DI, Chung IH. High-resolution CT of the pterygopalatine fossa and its communications. Neuroradiology 1996;38(suppl 1): S120 – 6. 5. Bryce GE, Woodham MB. The cross-sectional osteology of the midface. J Otolaryngol 1988;17:4 –11.