- Email: [email protected]
Computed Tomography for Oral and Maxillofacial Surgeons. Part I: Spiral Computed Tomography
MacDonald-Jankowski, Li
Asian J Oral Maxillofac Surg 2006;18(1):7-16. SPECIAL CONTRIBUTION
Computed Tomography for Oral and Maxillofacial Surgeons. Part I: Spiral Computed Tomography David S MacDonald-Jankowski,1 Thomas KL Li2 Division of Oral and Maxillofacial Radiology, Department of Oral Biological and Medical Sciences, Faculty of Dentistry, University of British Columbia, Vancouver, Canada, and 2Oral Radiology Unit, Faculty of Dentistry, University of Hong Kong, Hong Kong
1
Abstract Computed tomography, particularly spiral computed tomography, is increasingly available for the investigation of face and jaw lesions. This paper introduces the various types of computed tomography, while concentrating on spiral computed tomography, and covers window level and width, pitch, multislice computed tomography, 3-dimensional reformatting, and the limitations of spiral computed tomography. The indications for an increased need for spiral computed tomography are discussed. The applications of other functions of spiral computed tomography that may be applicable to lesions of the face and jaws are introduced. Key words: Jaw fractures, Jaw neoplasms, Radiology, Tomography, spiral computed
Introduction Radiology has always been about technology, therefore, it necessarily follows that radiology will exploit any technological advances. One such advance of great clinical impact is computed tomography (CT). CT can be divided broadly into fan-beam CT (including spiral CT [SpCT] and its subsets), and cone-beam CT (CBCT) [Figure 1]; the latter will be addressed in part II of this paper. Initially, prohibitive cost confined CT only to the developed world, but now, with so many providers, clinicians based in large medical centres can buy technology that was ‘cutting edge’ only a few years ago for only a fraction of the original cost.
Why Do We Need Computed Tomography? Formerly, clinicians relied on a clinical examination and ‘conventional radiology’ (traditional filmbased imaging) to assess and diagnose lesions affecting the jaw bones. Unfortunately, conventional Correspondence: David MacDonald, Division of Oral and Maxillofacial Radiology, Department of Oral Biological and Medical Sciences, Faculty of Dentistry, University of British Columbia, 2199 Wesbrook Mall, Vancouver V6T 1Z3, BC, Canada. Tel: (1 604) 822 9762; E-mail: [email protected]
© Asian 2006J Asian Oral Maxillofac Association Surg of Oral Vol 18, andNo Maxillofacial 1, 2006 Surgeons.
radiology generally reveals images that lack the sensitivity to display small changes in the bone. Conventional radiology also presents only as a
Computed tomography
Fan beam
Sequential
Cone beam
Spiral
Number of slices?
Single
Multiple
Medical
Dental
Footprint?
From 4 to 64 Medical
Panoramic radiographic unit
Figure 1. The classification and nomenclature of computed tomography.
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Computed Tomography for Oral and Maxillofacial Surgeons
a
b
Figure 2. The 3 components of the computed tomography unit. (a) The gantry, containing the X-ray tube and detectors, and the table upon which the patient lies and is progressively advanced through the gantry; and (b) the control console with monitor. This is separated from the CT unit by a lead wall, including a lead glass window. It is crucial to observe the patient and gantry throughout the entire exposure, in case the exposure needs to be terminated.
2-dimensional (2-D) image the superimposition of all structures within the 3-D volume of bone.
What Are the Basic Construction and Principles of Computed Tomography? The CT unit has 3 main components, as shown in Figure 2. The CT unit itself consists of the gantry (some which may be angled up to 30°) and the patient table (or bed or couch) that moves the patient through the aperture in the gantry (Figure 2a), and the control console (Figure 2b). There are a
currently 2 types of CT unit available: the thirdand fourth-generation units (Figure 3). The former constitutes the vast majority of CT equipment. For the third-generation CT, the X-ray tube and the sensors, which occupy an arc, are fixed in opposing positions within the gantry and rotate as a unit around the patient when in operation (Figure 3a), whereas for the fourth-generation unit, the X-ray tube alone rotates within a complete stationary ring of sensors (Figure 3b). The advantage of the fourth-generation unit is that the sensors have time to recover before being irradiated again.
b
X-ray tube
Fan beam of X-ray photon
Fan of detectors moves with tube
X-ray tube
Fan beam of X-ray photon
Ring of dectors
Figure 3. The 2 types of computed tomography units currently available. (a) The third-generation computed tomography unit permits a fan array of detectors to rotate around the patient in tandem with the rotating X-ray tube — the X-ray beam is fan shaped; and (b) the fourth-generation computed tomography unit only permits rotation of the X-ray tube within the continuous, but stationary, array of detectors — the X-ray beam is fan shaped.
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Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
MacDonald-Jankowski, Li
How Is the Computed Tomography Image Displayed? The display is a digital image reconstructed by the computer as pixels (picture elements), which represent a 3-D block of tissue. The voxel is the pixel size multiplied by the slice thickness (the voxel’s length is from as low as 1 mm in some units to 20 mm). Each pixel is assigned a CT number (see later) representing tissue density. This density is proportional to the degree to which the material within the voxel has attenuated the X-ray beam. The resultant attenuation coefficient of a particular voxel reflects the mean of all tissues within it, the proportion of hard to soft tissues, and the voxel length (slice thickness).
What is Spiral Computed Tomography? SpCT is also known as helical or volume acquisition CT. As Hounsfield’s genius introduced and developed the concept of CT in 1968, that of Kalender introduced SpCT. SpCT violates a previous firm tenet of radiology: the patient should remain motionless during the exposure. Instead, SpCT requires that the patient be moved through the aperture of the gantry during the generation of X-rays by the rotating X-ray head (Figure 4a), creating a helix or spiral of data. This is in contrast to the separate incremental slices, which are stacked like coins, of the conventional sequential slice CT (the original technology of the 1970s and 1980s), a
now frequently called ‘sequential CT’ (SeqCT; Figure 4b).
How Is Spiral Computed Tomography Better than Sequential Computed Tomography? As there is a continuous string of data encompassing a volume of the patient with SpCT, this data can be readily reconstructed to give 3-D images. To achieve the same for SeqCT, the patient would need to undergo a second exposure overlapping with the first exposure; thus doubling the radiation dose. As the data produced by SpCT represents a continuous volume of the patient, it can be readily reconfigured to produce slices in any plane, including the coronal plane. However, the generation of coronal sections by SeqCT would require a re-exposure of the patient through a coronal head position.
What Do the Data Found on the Image Represent? Viewers of SpCT should understand the terms: bone and soft tissue windows, window width (WW) and level (WL), and pitch. Bone and Soft Tissue Windows Bone and soft tissue windows and their widths and levels are expressed in Hounsfield units (HU), which are also called CT numbers. These range from a lower minimum of -1000 HU representing air (fixed point),
b
Figure 4. Spiral and sequential computed tomography. (a) In spiral computed tomography, the rotating X-ray tube describes a spiral as it continuously exposes the patient whose bed is continuously moving through the gantry; and (b) in sequential computed tomography, the rotating X-ray tube can only describe complete loops between incremental movements of the patient’s table through the gantry. Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
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Computed Tomography for Oral and Maxillofacial Surgeons
a
b
Figure 5. Soft tissue and bone windows. (a) The soft tissue window displays cell-rich structures such as the muscles, skin, salivary glands, spinal column, and blood vessels as ‘grey’ structures, whereas the fatty subcutaneous tissues and fascia appear almost as black as the air-filled pharynx and mastoid air-cells — the bony structures appear as homogenous white areas; and (b) the bone window displays the bony structures in such detail that trabeculae could be discerned. Note, that bony structures appear slightly smaller in area than they do in the soft tissue window. The bony window displays soft tissue, but fat is appreciated as a lighter grey shape in comparison to the non-fatty structures.
through 0 HU representing water (fixed point), up to 3000 HU representing dense metal or bone. Bone and soft tissue windows (Figure 5) are 2 of the 3 standard protocols for viewing the data captured by CT; the air window is the third protocol and is used mainly by respiratory physicians. Each of these protocols optimises viewing of tissue types by appropriately adjusting the WL and WW. The soft tissue window for face and jaw lesions is sited close to that of water (0 HU), WL at 40 to 60 HU, and WW at 250 HU, whereas the bone windows for such lesions are WL at 250 to 500 HU and WW 1000 to 2000 HU or greater. The ‘level’ may be defined as equivalent to tuning a radio into the desired frequency, whereas the ‘width’ is equivalent to a filter. The latter can vary greatly depending upon individual radiologists but is no longer a problem to the viewing clinicians with the use of raw data downloaded onto a CD and forwarded to the clinician, who, with the appropriate software, can handle the data to suit his or her own requirements. Pitch Pitch is the tightness of the helix and represents the resolution (detail) that would be visible on reconstruction. As the string of data will be longer for a 10
pitch of 2:1 in comparison for one of 3:1 for a given volume of patient, it follows that the radiation to the patient will be higher, although the detail will be better when viewed on thin reconstructed slices. For severe facial trauma a 1:1 pitch is best.
What Is Multislice Computed Tomography? Multislice CT (MSCT) is a subset of SpCT with up to 64 sets of X-ray tubes and corresponding sensor arrays. This means that the time needed to acquire data from a given volume of patient is correspondingly reduced as the number of slices (sets of X-ray tubes and corresponding sensor arrays) is increased.
What Is the Advantage of Multislice Spiral Computed Tomography? Although MSCT has had a substantial impact in angiography and cardiac imaging, its specific impact for most lesions and non-vascular investigations of the face and jaws is limited. Nevertheless, MSCT is associated with reduced artefacts and better specific resolution of 0.35 mm voxel size. It should be noted the new CBCT, the subject of the next part in the series, has even better spatial resolution, up Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
MacDonald-Jankowski, Li
Volume of tissue of jaw Section
Cuberilles Voxel
Figure 6. Fan-beam computed tomography achieves 3-dimensional reconstruction by dividing the voxel into cuberilles, each with the same attenuation coefficient as the original voxel.
to 0.10 mm voxel size. Baum et al report that MSCT is especially useful for defining the relationship between the tissue and lymph node metastasis and for functional imaging of the hypopharynx and larynx.1
What Is 3-Dimensional Reformatting and Why Is it Required? Each original voxel is divided into cubes, called cuberilles by ‘interpolation’; each cuberille has the same mean ‘attenuation coefficient’ of the original voxel (Figure 6). The need for this interpolation arises because the original voxel’s resolution is best in the axial plane, where the density of pixels is greatest. Only those cuberilles that represent the surface of the object of interest (OI) are projected onto the monitor. The 3-D reconstructions are capable of being rotated to display the reconstruction from any point of view (Figure 7).
Figure 7. The 3-dimensional reconstruction of this postoperative mandible allows a fuller evaluation for definitive reconstruction. Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
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Computed Tomography for Oral and Maxillofacial Surgeons
The OI is broadly defined by, and selected according to, its CT number. By fine adjustment of the former, in addition to supplementary functions such as ‘edit’ with its ‘scalpel’ (a digital freehand tool perfectly analogous to the physical scalpel), exquisite images are possible, particularly if they are assigned different colours. Furthermore, the images can be rotated about any axis to display any surface of the OI both for further edition or definitive viewing. The OI can be copied back into a second 3-D reconstruction of the affected jaw and can be used for treatment planning. Already, it is becoming commonplace for neuroradiologists and neurosurgeons to collaborate to identify cerebral aneurysms, define their extent and associated tissue supplied by their end-arteries, and determine the optimal site for intervention. Furthermore, the 3-D reconstruction can facilitate computerassisted design/computer-assisted manufacturing (CAD/CAM) reconstruction of a face following extensive ablative surgery or severe trauma. Blank and Kalender have précised the principles and issues of virtual images.2
What Are the Limitations of Spiral Computed Tomography? Reduced Resolution in all Planes except the Axial Plane Before we develop this point, we must advise readers that this limitation no longer applies to the mostmodern scanners using 64 slices. Scarfe reported that ‘multiplanar reformatting’ (MPR), especially in the coronal plane, was inadequate for the assessment of severe facial trauma primarily oriented in the axial plane,3 because the spatial resolution is greatest in the axial (XY) plane.4 Hoeffner et al suggested that specific protocols are required for obtaining coronal MPR of data acquired axially.5 Nevertheless, although the spatial resolution of SpCT is poorer than that of conventional radiography, the problem is caused by having isotropic cuberilles, which is intrinsic to most fanbeam types of CT to which SpCT belongs. Only CBCT and the most-modern 64 slice MSCT units avoid this. Streak Artefacts Although streak artefacts can degrade the SpCT image as they do most other imaging modalities, this 12
can be reduced by metal artefact reducing software.6 Baum et al suggested that a short additional spiral parallel to the body of the mandible reduces artefacts behind the dental arch, and improves the overall diagnostic quality.1 Information Overload Modern SpCT is so versatile that this technology has snatched back much ground previously lost to magnetic resonance imaging (MRI). As busy surgeons infrequently have the time to learn all the nuances of SpCT, it is appropriate now to introduce a new role for radiologists. The demands of SpCT and its exquisite images has projected this imaging specialist into front-line clinical medicine, particularly in neurosurgery, in a role akin to the military analogy of the combat engineer who builds bridges and removes obstacles for the infantry and army so that they can achieve their objectives. Radiologists, armed with an intimate knowledge of a surgeon’s needs, either by protocols or by a personal professional relationship, distils all this information to that necessary for the procedure. For challenging cases, this frequently requires surgeons and radiologists to sit down together to generate images that will be recalled in sequence during the operation, using a monitor or laptop in the operating room, like the ‘story boards’ used to plan movies for television or cinema. This leads to the next point. Intraoperative Imaging in the Operating Theatre During an operation, it may be desirable to obtain more images. So far it has not been possible to make SpCT sufficiently mobile for use in the operating theatre. In part II, we will discover that CBCT has successfully addressed this issue. Low Sensitivity for Identification of Small Tumours Although SpCT has a high specificity for metastatic lesions, which, according to van den Brekel’s review is higher than MRI,7 it has a lower sensitivity. This is largely due to the fact that the necrosis, which is pathognomic for metastasis, is rarely visible in small lesions. Therefore, the sensitivity of SpCT is optimal only for larger lesions with an attendant poorer prognosis. Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
MacDonald-Jankowski, Li
What Are the Indications for an Increased Need for Spiral Computed Tomography? In addition to osseointegrated implants, which have transformed prosthodontics, there is a need to accurately stage carcinoma and to evaluate complex fractures. Malignancy In China, according to Wen et al, the incidence of malignancy, especially squamous cell carcinoma, affecting the maxillofacial region is increasing.8 Schwarz et al reported an incidence of 20 and 23 new cases of nasopharyngeal carcinoma in 100,000 Southern mainland and Hong Kong Chinese people, respectively.9 Due to the proximity of these lesions to the skull base, the ability to achieve acceptable margins may be impossible. Therefore, accurate staging for radiotherapy requires SpCT. Zheng et al reported an increase in mortality from oral cancer in Japanese men.10 Due to the high risk for recurrence, long-term advanced radiological investigation and monitoring is required. Lell et al reported that SpCT is better at displaying postoperative changes and tumour recurrences, whereas MRI’s vaunted ability to differentiate tumours is compromised by oedema following radiotherapy.11 Although CT and MRI are widely used for imaging the primary neoplasm and cervical lymph nodes, van den Brekel commented that, due to the limited accuracy of both modalities with regards to metatasis, only ultrasound-guided fine needle aspiration cytology is more reliable.7 The initial report on the ‘fused (or dual modality) positron emission tomography/CT’ suggests that it has a higher sensitivity and a positive predictive value than CT for detection of the primary tumour in patients with cervical metastases.12 Baum et al advise that the use of the coronal plane is helpful for the base of the skull, orbital floor, palate, and paranasal sinuses, in addition to small tumours crossing the midline of the tongue base and palate.1 Fractures Cusmano et al state that maxillofacial fractures are common and, because of the already complicated Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
Figure 8. The 3-dimensional reconstruction in this patient who was assaulted displays the complexity of the injuries, particularly of the midfacial fractures, prior to surgical reconstruction.
anatomy, there can be difficulty interpreting multiple fractures, which present as overlapping densities with conventional radiology alone (Figure 8).13 Conventional radiology alone should be adequate for demonstrating simple superficial fractures such as those of the nose, zygomatic arch, and body of the mandible. Preda et al also reported that many of their patients with complex maxillofacial fractures benefited from the SpCT’s short scan time because of their multiple-trauma and possible damaged organs that were not yet fully stabilised.14 Roth et al reviewed both the panoramic radiographic and SpCT images of 217 patients.15 These researchers found that SpCT identified more fractures, particularly those of the angle, ramus, and condylar neck, than the panoramic images. Multiplanar CT reconstruction could be useful for evaluation of the feasibility of lag screw osteosynthesis of the fractured condyle.16
What Are the Other Functions of Spiral Computed Tomography? DentaScan In addition to its more usual role of pre-implant planning, a pictorial review displayed this programme’s capacity also to evaluate lesions affecting the jaw, ranging from squamous cell carcinoma to infection.17 Volume Rendering Volume rendering is a technique that uses the concept of ‘opacity’, which quickly reconstructs a 3-D volume 13
Computed Tomography for Oral and Maxillofacial Surgeons
acquired on CT or MRI. The end result is similar to a virtual anatomical dissection and can assist surgical planning for a particular patient. Cavalcanti and Antunes compared volume rendering with surface rendering for 20 patients and found that the former improved visualisation in comparison to the latter.18 It was also more sensitive for the diagnosis of maxillofacial lesions, in particular, those that were primarily intraosseous.
Computer-assisted Design/Computerassisted Manufacturing CAD/CAM technology is such that it can be adopted in any hospital for daily use. A Hong Kong group used a 4-stage process to produce “quantitative osteotomy simulation bone model” that could predict the postoperative appearance with photorealistic quality.21 CAD/CAM can generate 3-D models by laser or by milling.
Colour-coded 3-Dimensional Reformatting Colour-coded 3-D reformatting may be done for extensive lesions,19 by ascribing a separate colour to the lesion, the bone, and adjacent soft tissues. This has been applied to an ameloblastoma in Figure 9.
Computed Tomography Angiography Although Tipper et al reported that the specificity of CT angiography (CTA) for the internal carotid artery a
Navigator This function of ‘perspective volume rendering’ permits virtual antroscopy to evaluate the surface contours of antral lesions (Figure 10), virtual arteroscopy for defining vascular lesions, and virtual pharynoscopy and largynoscopy. This function has been recently applied to the maxillofacial region by Tao et al.20
b
Figure 9. Colour-coded 3-dimensional reformatting displays the extent of the ameloblastoma (represented in red) within the mandible. It has perforated the alveolar bone in 2 places (red). This reconstruction was produced by dissecting out the neoplasm then replacing it within a second reconstruction of the bony mandible.
14
Figure 10. This example of virtual antroscopy by use of the ‘navigator’ programme displays a 3-dimensional evaluation of a lesion arising from the roof of the maxillary antrum. Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
MacDonald-Jankowski, Li
approximates that of digital subtraction angiography,22 Teksam et al commented that the presence of small aneurysms may be easier to detect if they are aligned according to the patient’s long axis rather than axially.23 Tomandl et al’s review of the postprocessing of intracranial CTA is relevant for the maxillofacial region.24 Intravenous Contrast For optimal vascular and tissue contrast, Baum et al recommend that 150 mL of contrast medium (CM) be delivered at 2.5 mL/second flow rate with a start delay of 80 seconds.1 It should be appreciated that mild hypersensitivity reactions occur in up to 12.7% with ionic CM and 3.1% with the lower osmolar non-ionic CM, but that the death rates for both are equal at 1 per 100,000 investigations.25 Bettmann has addressed the frequently asked questions on CM-induced allergies, nephropathy, and other risks.26
Conclusion Although much objective work is required to fully evaluate the quality of the predictive aspects of SpCT images, it cannot be denied that SpCT has completely transformed medical imaging. The clinician is provided with a detailed preview of the patient and his or her disease, thus minimising the risk of hidden features complicating both the procedure and its successful outcome. In this way, SpCT has the potential to enhance treatment and procedure planning. The images, when appropriately prepared, should be able to facilitate collaboration between head and neck specialists, who may be called together to treat a lesion in the most effective manner.
Acknowledgements We are grateful to Vicky Earle, medical illustrator/ designer, of the University of British Columbia’s Media Group. We are grateful to Dr Ian Matthew, Chairman of Oral and Maxillofacial Surgery, University of British Columbia, Vancouver, Canada, for permitting use of Figure 8. We are also grateful to Dr Elaine Orpe, Clinical Assistant Professor, Oral and Maxillofacial Radiology, University of British Columbia, Vancouver, Canada, for her review of the manuscript. Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
References 1. Baum U, Greess H, Lell M, Nomayr A, Lenz M. Imaging of head and neck tumors — methods: CT, spiral-CT, multislice-spiral-CT. Eur J Radiol 2000;33:153-160. 2. Blank M, Kalender WA. Medical volume exploration: gaining insights virtually. Eur J Radiol 2000;33:161-169. 3. Scarfe WC. Imaging of maxillofacial trauma: evolutions and emerging revolutions. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005;100 (2 Suppl):75-96. 4. Rosenthal E, Quint DJ, Johns M, Peterson B, Hoeffner E. Diagnostic maxillofacial coronal images reformatted from helically acquired thin-section axial CT data. AJR Am J Roentgenol 2000;175:1177-1181. 5. Hoeffner EG, Quint DJ, Peterson B, Rosenthal E, Goodsitt M. Development of a protocol for coronal reconstruction of the maxillofacial region from axial helical CT data. Br J Radiol 2001;74:323-327. 6. Vannier MV, Hildebolt CF, Knapp RH, Conovor G, Yokoyama N, Wang G. 3D dental imaging by spiral CT. www.erl.wustl.edu/PROJS/Dental/3dd_sCT.html 7. Van den Brekel MW. Lymph node metastases: CT and MRI. Eur J Radiol 2000;33:230-238. 8. Wen Y, Dai X, Wang C, Li L, Fu F, Wang X, Tang X, Liu H, Hua C, Pan J. A retrospective clinical study of 6539 cases of malignant oral-maxillofacial tumors [article in Chinese]. Hua Xi Kou Qiang Yi Xue Za Zhi 2001;19:296-299. 9. Schwarz E, Chiu GK, Leung WK. Oral health status of southern Chinese following head and neck irradiation therapy for nasopharyngeal carcinoma. J Dent 1999;27: 21-28. 10. Zheng Y, Kirita T, Kurumatani N, Sugimura M, Yonemasu K. Trends in oral cancer mortality in Japan: 1950-1993. Oral Dis 1999;5:3-9. 11. Lell M, Baum U, Greess H, Nomayr A, Nkenke E, Koester M, Lenz M, Bautz W. Head and neck tumors: imaging recurrent tumor and post-therapeutic changes with CT and MRI. Eur J Radiol 2000;33:239-247. 12. Gutzeit A, Antoch G, Kuhl H, Egelhof T, Fischer M, Hauth E, Goehde S, Bockisch A, Debatin J, Freudenberg L. Unknown primary tumors: detection with dual-modality PET/CT — initial experience. Radiology 2005;234: 227-234. 13. Cusmano F, Pedrazzini M, Ferrozzi F, Uccelli M, Bassi S, Armaroli S, Mineo F, Pavone P. Cervical spine injuries: diagnostic imaging [article in Italian]. Acta Biomed Ateneo Parmense 2000;71:299-308. 14. Preda L, La Fianza A, Di Maggio EM, Dore R, Schifino MR, Mevio E, Campani R. Complex maxillofacial 15
Computed Tomography for Oral and Maxillofacial Surgeons
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trauma: diagnostic contribution of multiplanar and tridimensional spiral CT imaging [article in Italian]. Radiol Med (Torino) 1998;96:178-184. Roth FS, Kokoska MS, Awwad EE, Martin DS, Olson GT, Hollier LH, Hollenbeak CS. The identification of mandible fractures by helical computed tomography and panorex tomography. J Craniofac Surg 2005;16:394-399. Schneider A, Schulze J, Eckelt U, Laniado M. Lag screw osteosynthesis of fractures of the mandibular condyle: potential benefit of preoperative planning using multiplanar CT reconstruction. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005;99:142-147. Au-Yeung KM, Ahuja AT, Ching AS, Metreweli C. Dentascan in oral imaging. Clin Radiol 2001;56:700-713. Cavalcanti MG, Antunes JL. 3D-CT imaging processing for qualitative and quantitative analysis of maxillofacial cysts and tumors. Pesqui Odontol Bras 2002;16: 189-194. Greess H, Nomayr A, Tomandl B, Blank M, Lell M, Lenz M, Bautz WA. 2D and 3D visualisation of head and neck tumours from spiral-CT data. Eur J Radiol 2000;33:170-177. Tao X, Zhu F, Chen W, Zhu S. The application of virtual endoscopy with computed tomography in maxillofacial surgery. Chin Med J (Engl) 2003;116:679-681. Xia J, Ip HH, Samman N, Wong HT, Gateno J, Wang D,
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Yeung RW, Kot CS, Tideman H. Three-dimensional virtualreality surgical planning and soft-tissue prediction for orthognathic surgery. IEEE Trans Inf Technol Biomed 2001;5:97-107. Tipper G, U-King-Im JM, Price SJ, Trivedi RA, Cross JJ, Higgins NJ, Farmer R, Wat J, Kirollos R, Kirkpatrick PJ, Antoun NM, Gillard JH. Detection and evaluation of intracranial aneurysms with 16-row multislice CT angiography. Clin Radiol 2005;60:565-572. Teksam M, McKinney A, Cakir B, Truwit CL. Multi-slice CT angiography of small cerebral aneurysms: is the direction of aneurysm important in diagnosis? Eur J Radiol 2005;53:454-462. Tomandl BF, Kostner NC, Schempershofe M, Huk WJ, Strauss C, Anker L, Hastreiter P. CT angiography of intracranial aneurysms: a focus on postprocessing. Radiographics 2004;24:637-655. Brockow K, Christiansen C, Kanny G, Clement O, Barbaud A, Bircher A, Dewachter P, Gueant JL, Rodriguez Gueant RM, Mouton-Faivre C, Ring J, Romano A, Sainte-Laudy J, Demoly P, Pichler WJ. Management of hypersensitivity reactions to iodinated contrast media. Allergy 2005;60: 150-158. Bettmann MA. Frequently asked questions: iodinated contrast agents. Radiographics. 2004;24 (Suppl 1):3-10.
Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
Asian J Oral Maxillofac Surg 2006;18(1):7-16. SPECIAL CONTRIBUTION
Computed Tomography for Oral and Maxillofacial Surgeons. Part I: Spiral Computed Tomography David S MacDonald-Jankowski,1 Thomas KL Li2 Division of Oral and Maxillofacial Radiology, Department of Oral Biological and Medical Sciences, Faculty of Dentistry, University of British Columbia, Vancouver, Canada, and 2Oral Radiology Unit, Faculty of Dentistry, University of Hong Kong, Hong Kong
1
Abstract Computed tomography, particularly spiral computed tomography, is increasingly available for the investigation of face and jaw lesions. This paper introduces the various types of computed tomography, while concentrating on spiral computed tomography, and covers window level and width, pitch, multislice computed tomography, 3-dimensional reformatting, and the limitations of spiral computed tomography. The indications for an increased need for spiral computed tomography are discussed. The applications of other functions of spiral computed tomography that may be applicable to lesions of the face and jaws are introduced. Key words: Jaw fractures, Jaw neoplasms, Radiology, Tomography, spiral computed
Introduction Radiology has always been about technology, therefore, it necessarily follows that radiology will exploit any technological advances. One such advance of great clinical impact is computed tomography (CT). CT can be divided broadly into fan-beam CT (including spiral CT [SpCT] and its subsets), and cone-beam CT (CBCT) [Figure 1]; the latter will be addressed in part II of this paper. Initially, prohibitive cost confined CT only to the developed world, but now, with so many providers, clinicians based in large medical centres can buy technology that was ‘cutting edge’ only a few years ago for only a fraction of the original cost.
Why Do We Need Computed Tomography? Formerly, clinicians relied on a clinical examination and ‘conventional radiology’ (traditional filmbased imaging) to assess and diagnose lesions affecting the jaw bones. Unfortunately, conventional Correspondence: David MacDonald, Division of Oral and Maxillofacial Radiology, Department of Oral Biological and Medical Sciences, Faculty of Dentistry, University of British Columbia, 2199 Wesbrook Mall, Vancouver V6T 1Z3, BC, Canada. Tel: (1 604) 822 9762; E-mail: [email protected]
© Asian 2006J Asian Oral Maxillofac Association Surg of Oral Vol 18, andNo Maxillofacial 1, 2006 Surgeons.
radiology generally reveals images that lack the sensitivity to display small changes in the bone. Conventional radiology also presents only as a
Computed tomography
Fan beam
Sequential
Cone beam
Spiral
Number of slices?
Single
Multiple
Medical
Dental
Footprint?
From 4 to 64 Medical
Panoramic radiographic unit
Figure 1. The classification and nomenclature of computed tomography.
7
Computed Tomography for Oral and Maxillofacial Surgeons
a
b
Figure 2. The 3 components of the computed tomography unit. (a) The gantry, containing the X-ray tube and detectors, and the table upon which the patient lies and is progressively advanced through the gantry; and (b) the control console with monitor. This is separated from the CT unit by a lead wall, including a lead glass window. It is crucial to observe the patient and gantry throughout the entire exposure, in case the exposure needs to be terminated.
2-dimensional (2-D) image the superimposition of all structures within the 3-D volume of bone.
What Are the Basic Construction and Principles of Computed Tomography? The CT unit has 3 main components, as shown in Figure 2. The CT unit itself consists of the gantry (some which may be angled up to 30°) and the patient table (or bed or couch) that moves the patient through the aperture in the gantry (Figure 2a), and the control console (Figure 2b). There are a
currently 2 types of CT unit available: the thirdand fourth-generation units (Figure 3). The former constitutes the vast majority of CT equipment. For the third-generation CT, the X-ray tube and the sensors, which occupy an arc, are fixed in opposing positions within the gantry and rotate as a unit around the patient when in operation (Figure 3a), whereas for the fourth-generation unit, the X-ray tube alone rotates within a complete stationary ring of sensors (Figure 3b). The advantage of the fourth-generation unit is that the sensors have time to recover before being irradiated again.
b
X-ray tube
Fan beam of X-ray photon
Fan of detectors moves with tube
X-ray tube
Fan beam of X-ray photon
Ring of dectors
Figure 3. The 2 types of computed tomography units currently available. (a) The third-generation computed tomography unit permits a fan array of detectors to rotate around the patient in tandem with the rotating X-ray tube — the X-ray beam is fan shaped; and (b) the fourth-generation computed tomography unit only permits rotation of the X-ray tube within the continuous, but stationary, array of detectors — the X-ray beam is fan shaped.
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Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
MacDonald-Jankowski, Li
How Is the Computed Tomography Image Displayed? The display is a digital image reconstructed by the computer as pixels (picture elements), which represent a 3-D block of tissue. The voxel is the pixel size multiplied by the slice thickness (the voxel’s length is from as low as 1 mm in some units to 20 mm). Each pixel is assigned a CT number (see later) representing tissue density. This density is proportional to the degree to which the material within the voxel has attenuated the X-ray beam. The resultant attenuation coefficient of a particular voxel reflects the mean of all tissues within it, the proportion of hard to soft tissues, and the voxel length (slice thickness).
What is Spiral Computed Tomography? SpCT is also known as helical or volume acquisition CT. As Hounsfield’s genius introduced and developed the concept of CT in 1968, that of Kalender introduced SpCT. SpCT violates a previous firm tenet of radiology: the patient should remain motionless during the exposure. Instead, SpCT requires that the patient be moved through the aperture of the gantry during the generation of X-rays by the rotating X-ray head (Figure 4a), creating a helix or spiral of data. This is in contrast to the separate incremental slices, which are stacked like coins, of the conventional sequential slice CT (the original technology of the 1970s and 1980s), a
now frequently called ‘sequential CT’ (SeqCT; Figure 4b).
How Is Spiral Computed Tomography Better than Sequential Computed Tomography? As there is a continuous string of data encompassing a volume of the patient with SpCT, this data can be readily reconstructed to give 3-D images. To achieve the same for SeqCT, the patient would need to undergo a second exposure overlapping with the first exposure; thus doubling the radiation dose. As the data produced by SpCT represents a continuous volume of the patient, it can be readily reconfigured to produce slices in any plane, including the coronal plane. However, the generation of coronal sections by SeqCT would require a re-exposure of the patient through a coronal head position.
What Do the Data Found on the Image Represent? Viewers of SpCT should understand the terms: bone and soft tissue windows, window width (WW) and level (WL), and pitch. Bone and Soft Tissue Windows Bone and soft tissue windows and their widths and levels are expressed in Hounsfield units (HU), which are also called CT numbers. These range from a lower minimum of -1000 HU representing air (fixed point),
b
Figure 4. Spiral and sequential computed tomography. (a) In spiral computed tomography, the rotating X-ray tube describes a spiral as it continuously exposes the patient whose bed is continuously moving through the gantry; and (b) in sequential computed tomography, the rotating X-ray tube can only describe complete loops between incremental movements of the patient’s table through the gantry. Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
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Computed Tomography for Oral and Maxillofacial Surgeons
a
b
Figure 5. Soft tissue and bone windows. (a) The soft tissue window displays cell-rich structures such as the muscles, skin, salivary glands, spinal column, and blood vessels as ‘grey’ structures, whereas the fatty subcutaneous tissues and fascia appear almost as black as the air-filled pharynx and mastoid air-cells — the bony structures appear as homogenous white areas; and (b) the bone window displays the bony structures in such detail that trabeculae could be discerned. Note, that bony structures appear slightly smaller in area than they do in the soft tissue window. The bony window displays soft tissue, but fat is appreciated as a lighter grey shape in comparison to the non-fatty structures.
through 0 HU representing water (fixed point), up to 3000 HU representing dense metal or bone. Bone and soft tissue windows (Figure 5) are 2 of the 3 standard protocols for viewing the data captured by CT; the air window is the third protocol and is used mainly by respiratory physicians. Each of these protocols optimises viewing of tissue types by appropriately adjusting the WL and WW. The soft tissue window for face and jaw lesions is sited close to that of water (0 HU), WL at 40 to 60 HU, and WW at 250 HU, whereas the bone windows for such lesions are WL at 250 to 500 HU and WW 1000 to 2000 HU or greater. The ‘level’ may be defined as equivalent to tuning a radio into the desired frequency, whereas the ‘width’ is equivalent to a filter. The latter can vary greatly depending upon individual radiologists but is no longer a problem to the viewing clinicians with the use of raw data downloaded onto a CD and forwarded to the clinician, who, with the appropriate software, can handle the data to suit his or her own requirements. Pitch Pitch is the tightness of the helix and represents the resolution (detail) that would be visible on reconstruction. As the string of data will be longer for a 10
pitch of 2:1 in comparison for one of 3:1 for a given volume of patient, it follows that the radiation to the patient will be higher, although the detail will be better when viewed on thin reconstructed slices. For severe facial trauma a 1:1 pitch is best.
What Is Multislice Computed Tomography? Multislice CT (MSCT) is a subset of SpCT with up to 64 sets of X-ray tubes and corresponding sensor arrays. This means that the time needed to acquire data from a given volume of patient is correspondingly reduced as the number of slices (sets of X-ray tubes and corresponding sensor arrays) is increased.
What Is the Advantage of Multislice Spiral Computed Tomography? Although MSCT has had a substantial impact in angiography and cardiac imaging, its specific impact for most lesions and non-vascular investigations of the face and jaws is limited. Nevertheless, MSCT is associated with reduced artefacts and better specific resolution of 0.35 mm voxel size. It should be noted the new CBCT, the subject of the next part in the series, has even better spatial resolution, up Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
MacDonald-Jankowski, Li
Volume of tissue of jaw Section
Cuberilles Voxel
Figure 6. Fan-beam computed tomography achieves 3-dimensional reconstruction by dividing the voxel into cuberilles, each with the same attenuation coefficient as the original voxel.
to 0.10 mm voxel size. Baum et al report that MSCT is especially useful for defining the relationship between the tissue and lymph node metastasis and for functional imaging of the hypopharynx and larynx.1
What Is 3-Dimensional Reformatting and Why Is it Required? Each original voxel is divided into cubes, called cuberilles by ‘interpolation’; each cuberille has the same mean ‘attenuation coefficient’ of the original voxel (Figure 6). The need for this interpolation arises because the original voxel’s resolution is best in the axial plane, where the density of pixels is greatest. Only those cuberilles that represent the surface of the object of interest (OI) are projected onto the monitor. The 3-D reconstructions are capable of being rotated to display the reconstruction from any point of view (Figure 7).
Figure 7. The 3-dimensional reconstruction of this postoperative mandible allows a fuller evaluation for definitive reconstruction. Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
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The OI is broadly defined by, and selected according to, its CT number. By fine adjustment of the former, in addition to supplementary functions such as ‘edit’ with its ‘scalpel’ (a digital freehand tool perfectly analogous to the physical scalpel), exquisite images are possible, particularly if they are assigned different colours. Furthermore, the images can be rotated about any axis to display any surface of the OI both for further edition or definitive viewing. The OI can be copied back into a second 3-D reconstruction of the affected jaw and can be used for treatment planning. Already, it is becoming commonplace for neuroradiologists and neurosurgeons to collaborate to identify cerebral aneurysms, define their extent and associated tissue supplied by their end-arteries, and determine the optimal site for intervention. Furthermore, the 3-D reconstruction can facilitate computerassisted design/computer-assisted manufacturing (CAD/CAM) reconstruction of a face following extensive ablative surgery or severe trauma. Blank and Kalender have précised the principles and issues of virtual images.2
What Are the Limitations of Spiral Computed Tomography? Reduced Resolution in all Planes except the Axial Plane Before we develop this point, we must advise readers that this limitation no longer applies to the mostmodern scanners using 64 slices. Scarfe reported that ‘multiplanar reformatting’ (MPR), especially in the coronal plane, was inadequate for the assessment of severe facial trauma primarily oriented in the axial plane,3 because the spatial resolution is greatest in the axial (XY) plane.4 Hoeffner et al suggested that specific protocols are required for obtaining coronal MPR of data acquired axially.5 Nevertheless, although the spatial resolution of SpCT is poorer than that of conventional radiography, the problem is caused by having isotropic cuberilles, which is intrinsic to most fanbeam types of CT to which SpCT belongs. Only CBCT and the most-modern 64 slice MSCT units avoid this. Streak Artefacts Although streak artefacts can degrade the SpCT image as they do most other imaging modalities, this 12
can be reduced by metal artefact reducing software.6 Baum et al suggested that a short additional spiral parallel to the body of the mandible reduces artefacts behind the dental arch, and improves the overall diagnostic quality.1 Information Overload Modern SpCT is so versatile that this technology has snatched back much ground previously lost to magnetic resonance imaging (MRI). As busy surgeons infrequently have the time to learn all the nuances of SpCT, it is appropriate now to introduce a new role for radiologists. The demands of SpCT and its exquisite images has projected this imaging specialist into front-line clinical medicine, particularly in neurosurgery, in a role akin to the military analogy of the combat engineer who builds bridges and removes obstacles for the infantry and army so that they can achieve their objectives. Radiologists, armed with an intimate knowledge of a surgeon’s needs, either by protocols or by a personal professional relationship, distils all this information to that necessary for the procedure. For challenging cases, this frequently requires surgeons and radiologists to sit down together to generate images that will be recalled in sequence during the operation, using a monitor or laptop in the operating room, like the ‘story boards’ used to plan movies for television or cinema. This leads to the next point. Intraoperative Imaging in the Operating Theatre During an operation, it may be desirable to obtain more images. So far it has not been possible to make SpCT sufficiently mobile for use in the operating theatre. In part II, we will discover that CBCT has successfully addressed this issue. Low Sensitivity for Identification of Small Tumours Although SpCT has a high specificity for metastatic lesions, which, according to van den Brekel’s review is higher than MRI,7 it has a lower sensitivity. This is largely due to the fact that the necrosis, which is pathognomic for metastasis, is rarely visible in small lesions. Therefore, the sensitivity of SpCT is optimal only for larger lesions with an attendant poorer prognosis. Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
MacDonald-Jankowski, Li
What Are the Indications for an Increased Need for Spiral Computed Tomography? In addition to osseointegrated implants, which have transformed prosthodontics, there is a need to accurately stage carcinoma and to evaluate complex fractures. Malignancy In China, according to Wen et al, the incidence of malignancy, especially squamous cell carcinoma, affecting the maxillofacial region is increasing.8 Schwarz et al reported an incidence of 20 and 23 new cases of nasopharyngeal carcinoma in 100,000 Southern mainland and Hong Kong Chinese people, respectively.9 Due to the proximity of these lesions to the skull base, the ability to achieve acceptable margins may be impossible. Therefore, accurate staging for radiotherapy requires SpCT. Zheng et al reported an increase in mortality from oral cancer in Japanese men.10 Due to the high risk for recurrence, long-term advanced radiological investigation and monitoring is required. Lell et al reported that SpCT is better at displaying postoperative changes and tumour recurrences, whereas MRI’s vaunted ability to differentiate tumours is compromised by oedema following radiotherapy.11 Although CT and MRI are widely used for imaging the primary neoplasm and cervical lymph nodes, van den Brekel commented that, due to the limited accuracy of both modalities with regards to metatasis, only ultrasound-guided fine needle aspiration cytology is more reliable.7 The initial report on the ‘fused (or dual modality) positron emission tomography/CT’ suggests that it has a higher sensitivity and a positive predictive value than CT for detection of the primary tumour in patients with cervical metastases.12 Baum et al advise that the use of the coronal plane is helpful for the base of the skull, orbital floor, palate, and paranasal sinuses, in addition to small tumours crossing the midline of the tongue base and palate.1 Fractures Cusmano et al state that maxillofacial fractures are common and, because of the already complicated Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
Figure 8. The 3-dimensional reconstruction in this patient who was assaulted displays the complexity of the injuries, particularly of the midfacial fractures, prior to surgical reconstruction.
anatomy, there can be difficulty interpreting multiple fractures, which present as overlapping densities with conventional radiology alone (Figure 8).13 Conventional radiology alone should be adequate for demonstrating simple superficial fractures such as those of the nose, zygomatic arch, and body of the mandible. Preda et al also reported that many of their patients with complex maxillofacial fractures benefited from the SpCT’s short scan time because of their multiple-trauma and possible damaged organs that were not yet fully stabilised.14 Roth et al reviewed both the panoramic radiographic and SpCT images of 217 patients.15 These researchers found that SpCT identified more fractures, particularly those of the angle, ramus, and condylar neck, than the panoramic images. Multiplanar CT reconstruction could be useful for evaluation of the feasibility of lag screw osteosynthesis of the fractured condyle.16
What Are the Other Functions of Spiral Computed Tomography? DentaScan In addition to its more usual role of pre-implant planning, a pictorial review displayed this programme’s capacity also to evaluate lesions affecting the jaw, ranging from squamous cell carcinoma to infection.17 Volume Rendering Volume rendering is a technique that uses the concept of ‘opacity’, which quickly reconstructs a 3-D volume 13
Computed Tomography for Oral and Maxillofacial Surgeons
acquired on CT or MRI. The end result is similar to a virtual anatomical dissection and can assist surgical planning for a particular patient. Cavalcanti and Antunes compared volume rendering with surface rendering for 20 patients and found that the former improved visualisation in comparison to the latter.18 It was also more sensitive for the diagnosis of maxillofacial lesions, in particular, those that were primarily intraosseous.
Computer-assisted Design/Computerassisted Manufacturing CAD/CAM technology is such that it can be adopted in any hospital for daily use. A Hong Kong group used a 4-stage process to produce “quantitative osteotomy simulation bone model” that could predict the postoperative appearance with photorealistic quality.21 CAD/CAM can generate 3-D models by laser or by milling.
Colour-coded 3-Dimensional Reformatting Colour-coded 3-D reformatting may be done for extensive lesions,19 by ascribing a separate colour to the lesion, the bone, and adjacent soft tissues. This has been applied to an ameloblastoma in Figure 9.
Computed Tomography Angiography Although Tipper et al reported that the specificity of CT angiography (CTA) for the internal carotid artery a
Navigator This function of ‘perspective volume rendering’ permits virtual antroscopy to evaluate the surface contours of antral lesions (Figure 10), virtual arteroscopy for defining vascular lesions, and virtual pharynoscopy and largynoscopy. This function has been recently applied to the maxillofacial region by Tao et al.20
b
Figure 9. Colour-coded 3-dimensional reformatting displays the extent of the ameloblastoma (represented in red) within the mandible. It has perforated the alveolar bone in 2 places (red). This reconstruction was produced by dissecting out the neoplasm then replacing it within a second reconstruction of the bony mandible.
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Figure 10. This example of virtual antroscopy by use of the ‘navigator’ programme displays a 3-dimensional evaluation of a lesion arising from the roof of the maxillary antrum. Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
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approximates that of digital subtraction angiography,22 Teksam et al commented that the presence of small aneurysms may be easier to detect if they are aligned according to the patient’s long axis rather than axially.23 Tomandl et al’s review of the postprocessing of intracranial CTA is relevant for the maxillofacial region.24 Intravenous Contrast For optimal vascular and tissue contrast, Baum et al recommend that 150 mL of contrast medium (CM) be delivered at 2.5 mL/second flow rate with a start delay of 80 seconds.1 It should be appreciated that mild hypersensitivity reactions occur in up to 12.7% with ionic CM and 3.1% with the lower osmolar non-ionic CM, but that the death rates for both are equal at 1 per 100,000 investigations.25 Bettmann has addressed the frequently asked questions on CM-induced allergies, nephropathy, and other risks.26
Conclusion Although much objective work is required to fully evaluate the quality of the predictive aspects of SpCT images, it cannot be denied that SpCT has completely transformed medical imaging. The clinician is provided with a detailed preview of the patient and his or her disease, thus minimising the risk of hidden features complicating both the procedure and its successful outcome. In this way, SpCT has the potential to enhance treatment and procedure planning. The images, when appropriately prepared, should be able to facilitate collaboration between head and neck specialists, who may be called together to treat a lesion in the most effective manner.
Acknowledgements We are grateful to Vicky Earle, medical illustrator/ designer, of the University of British Columbia’s Media Group. We are grateful to Dr Ian Matthew, Chairman of Oral and Maxillofacial Surgery, University of British Columbia, Vancouver, Canada, for permitting use of Figure 8. We are also grateful to Dr Elaine Orpe, Clinical Assistant Professor, Oral and Maxillofacial Radiology, University of British Columbia, Vancouver, Canada, for her review of the manuscript. Asian J Oral Maxillofac Surg Vol 18, No 1, 2006
References 1. Baum U, Greess H, Lell M, Nomayr A, Lenz M. Imaging of head and neck tumors — methods: CT, spiral-CT, multislice-spiral-CT. Eur J Radiol 2000;33:153-160. 2. Blank M, Kalender WA. Medical volume exploration: gaining insights virtually. Eur J Radiol 2000;33:161-169. 3. Scarfe WC. Imaging of maxillofacial trauma: evolutions and emerging revolutions. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005;100 (2 Suppl):75-96. 4. Rosenthal E, Quint DJ, Johns M, Peterson B, Hoeffner E. Diagnostic maxillofacial coronal images reformatted from helically acquired thin-section axial CT data. AJR Am J Roentgenol 2000;175:1177-1181. 5. Hoeffner EG, Quint DJ, Peterson B, Rosenthal E, Goodsitt M. Development of a protocol for coronal reconstruction of the maxillofacial region from axial helical CT data. Br J Radiol 2001;74:323-327. 6. Vannier MV, Hildebolt CF, Knapp RH, Conovor G, Yokoyama N, Wang G. 3D dental imaging by spiral CT. www.erl.wustl.edu/PROJS/Dental/3dd_sCT.html 7. Van den Brekel MW. Lymph node metastases: CT and MRI. Eur J Radiol 2000;33:230-238. 8. Wen Y, Dai X, Wang C, Li L, Fu F, Wang X, Tang X, Liu H, Hua C, Pan J. A retrospective clinical study of 6539 cases of malignant oral-maxillofacial tumors [article in Chinese]. Hua Xi Kou Qiang Yi Xue Za Zhi 2001;19:296-299. 9. Schwarz E, Chiu GK, Leung WK. Oral health status of southern Chinese following head and neck irradiation therapy for nasopharyngeal carcinoma. J Dent 1999;27: 21-28. 10. Zheng Y, Kirita T, Kurumatani N, Sugimura M, Yonemasu K. Trends in oral cancer mortality in Japan: 1950-1993. Oral Dis 1999;5:3-9. 11. Lell M, Baum U, Greess H, Nomayr A, Nkenke E, Koester M, Lenz M, Bautz W. Head and neck tumors: imaging recurrent tumor and post-therapeutic changes with CT and MRI. Eur J Radiol 2000;33:239-247. 12. Gutzeit A, Antoch G, Kuhl H, Egelhof T, Fischer M, Hauth E, Goehde S, Bockisch A, Debatin J, Freudenberg L. Unknown primary tumors: detection with dual-modality PET/CT — initial experience. Radiology 2005;234: 227-234. 13. Cusmano F, Pedrazzini M, Ferrozzi F, Uccelli M, Bassi S, Armaroli S, Mineo F, Pavone P. Cervical spine injuries: diagnostic imaging [article in Italian]. Acta Biomed Ateneo Parmense 2000;71:299-308. 14. Preda L, La Fianza A, Di Maggio EM, Dore R, Schifino MR, Mevio E, Campani R. Complex maxillofacial 15
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Yeung RW, Kot CS, Tideman H. Three-dimensional virtualreality surgical planning and soft-tissue prediction for orthognathic surgery. IEEE Trans Inf Technol Biomed 2001;5:97-107. Tipper G, U-King-Im JM, Price SJ, Trivedi RA, Cross JJ, Higgins NJ, Farmer R, Wat J, Kirollos R, Kirkpatrick PJ, Antoun NM, Gillard JH. Detection and evaluation of intracranial aneurysms with 16-row multislice CT angiography. Clin Radiol 2005;60:565-572. Teksam M, McKinney A, Cakir B, Truwit CL. Multi-slice CT angiography of small cerebral aneurysms: is the direction of aneurysm important in diagnosis? Eur J Radiol 2005;53:454-462. Tomandl BF, Kostner NC, Schempershofe M, Huk WJ, Strauss C, Anker L, Hastreiter P. CT angiography of intracranial aneurysms: a focus on postprocessing. Radiographics 2004;24:637-655. Brockow K, Christiansen C, Kanny G, Clement O, Barbaud A, Bircher A, Dewachter P, Gueant JL, Rodriguez Gueant RM, Mouton-Faivre C, Ring J, Romano A, Sainte-Laudy J, Demoly P, Pichler WJ. Management of hypersensitivity reactions to iodinated contrast media. Allergy 2005;60: 150-158. Bettmann MA. Frequently asked questions: iodinated contrast agents. Radiographics. 2004;24 (Suppl 1):3-10.
Asian J Oral Maxillofac Surg Vol 18, No 1, 2006