Surg Radiol Anat (2012) 34:701–708 DOI 10.1007/s00276-011-0916-5
ORIGINAL ARTICLE
Anatomical relationship between cranial surface landmarks and venous sinus in posterior cranial fossa using CT angiography Bo Sheng • Furong Lv • Zhibo Xiao • Yu Ouyang Fajin Lv • Jinmu Deng • Yunfeng You • Nan Liu
•
Received: 8 April 2011 / Accepted: 29 November 2011 / Published online: 11 December 2011 Ó Springer-Verlag 2011
Abstract Purpose The purpose of this study was to determine the reliability of applying conventional anatomical landmarks to locate venous sinus in posterior fossa using subtraction computed tomography angiography (CTA) technique. Methods We retrospectively reconstructed transverse sinus (TS), sigmoid sinus (SS), and cranial imaging from 100 patients undergoing head CTA examination. Subtraction CTA data was merged with nonenhanced data and then cranium transparency was adjusted to 50% on threedimensional volume rendering, indicating the anatomical relationship between surface landmarks of cranium and confluens sinuum, TS, and SS. Results CTA technique precisely displayed the anatomical relations between venous sinus in posterior fossa and cranial surface landmarks. The asterion was located directly over the transverse–sigmoid sinus junction (TSST) in 81% cases, inferior to TSST in 15%, and superior to TSST in 4%, mainly distributing on the TS side of TSST, namely the distal-end of TS. Superior nuchal line had complex relation with TS and the line drawn from the zygoma root to the inion (LZI), but failed to represent the location of TS and the trend of LZI. In proximal-end of TS, majority of LZI B. Sheng Furong Lv (&) Z. Xiao Y. Ouyang Fajin Lv N. Liu Department of Radiology, The First Affiliated Hospital of Chongqing Medical University, Chongqing 400016, China e-mail:
[email protected] J. Deng Department of Neurosurgery, The Second Affiliated Hospital of Chongqing Medical University, Chongqing 400010, China Y. You CT Room, The Third Affiliated Hospital of Henan University of Science and Technology, Luoyang 471003, Henan, China
overlapped with TS line. However, most LZI was gradually positioned below TS line as TS moved outwards. Almost half of line drawn from the squamosal–parietomastoid suture junction to the inion and line drawn from the asterion to the inion shared the same trend with TS. Conclusion Subtraction CTA merged into nonenhanced cranial bone with 50% skull transparency provides a feasible method to identify the anatomical relation between venous sinus and surface landmarks of cranium, which is significantly varied among individuals, so it is not accurate to determine venous sinus in posterior fossa merely using surface landmarks. Keywords Cranial suture Transverse Sinuses Sigmoid sinus Computed tomography angiography Digital subtraction angiography Anatomy
Introduction Suboccipital supratentorial approach, retrosigmoid approach, and midline/posterior suboccipital approach have been commonly applied in posterior fossa neurosurgery. Bone windows should be made to transverse sinus (TS) and sigmoid sinus (SS) as close as possible to expose and approach intracranial lesions [6, 17], and to avoid sever complications like hemorrhage induced by sinus injury. Thereafter, preoperative identification of the positions of relevant venous sinus plays a significant role in determining the success of craniotomy. Previous studies mainly employed landmarks on the external surface of cranium to locate the trend of venous sinus and found certain anatomical relationship between them. Nevertheless, most of surgical landmarks and techniques are performed to outline the trend of TS due to
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extreme individual variations. Currently, asterion, superior nuchal line (SNL), line drawn from the squamosal–parietomastoid suture junction to the inion (LSPI) and line drawn from the zygoma root to the inion (LZI) are the main landmarks determining the location of TS. Asterion, the junction of the lambdoid, parietomastoid, and occipitomastoid sutures, plays a vital role in posterior approach surgery. It is regarded as an important landmark during craniotomy and is commonly employed to define the location of transverse–sigmoid sinus junction (TSST) [7, 16, 20, 22, 24]. But more and more scholars noted the significant variations while determining the anatomical relationship between asterion and TSST [12, 13]. The application of SNL as surface cranial landmark of TS remained debatable [1, 18, 21, 27]. Although LZI has been employed to determine the trend of TS [2, 7], at present its reliability [13] remains to be further elucidated. Nowadays, applying bony landmarks into defining venous sinus position lacks individual anatomical information. Due to variations among different patients, the localization of venous sinus may be inaccurate. To avoid sinus injury, the first burr hole should be opened to venous sinus as far as possible during craniotomy, and then gradually enlarge the surgical field towards venous sinus, which increases surgical times and causes unnecessary cranial defects [13]. For the purpose of overcoming this question, neuronavigation technique based upon computed tomography (CT) [3, 10– 13, 26] and magnetic resonance imaging (MRI) [5] has been applied in clinical posterior fossa approach surgery. This study aimed to rapidly and precisely observe the anatomical relationship between venous sinus in posterior fossa and surgical cranial landmarks using subtraction computed tomography angiography (CTA) and threedimensional (3D) reformation techniques for providing references for upcoming posterior fossa surgeries. Meantime, we evaluated and compared the accuracy of applying several vital surface landmarks to locate venous sinus.
Materials and methods The study was approved by the ethics committee of Chongqing Medical University; written informed consent was obtained in all instances either from the patient or a close family member. Between January and September 2010, we enrolled consecutive patients who underwent CT angiography for suspected intracranial aneurysms and intracranial arteriosclerosis at our institution. Patients, who had cerebral venous sinus diseases and other diseases involving cerebral venous sinuses, and cranial bone malformation, confirmed by clinic and CT, were excluded. The image quality of subtraction CT angiograms of the remaining 100 patients [44 male and 56 female, with a
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mean age of 58 years (range 19–95 years)] was good enough to display vascular structures. All CT angiographic studies were performed [14, 15] using a 64-section scanner (Lightspeed VCT; GE Healthcare, Milwaukee, WI). All patients’ heads were immobilized using a head holder during the examination to minimize motion artifacts. To determine the scan delay, a test bolus of 20 mL of iopromide (Ultravist; Schering, Berlin, Germany) at 370 mg of iodine per milliliter was given at a rate of 4 mL/s. The CT parameters were as follows: pitch 0.531; 0.6-mm section collimation; 0.625-mm reconstruction interval; matrix 512 9 512; 180–240 mm field of view; 100 kV, 300 mA (nonenhanced image); 120 kV, 300 mA (contrast-enhanced image). The scan coverage of the CT examination extended from the first cervical vertebra to the cranial vault. A total of 80 mL of contrast agent was injected through an 18-gauge needle into the antecubital vein by a power injector at a rate of 4 mL/s. The isotropic CTA data were obtained and transferred to a workstation (Advantage for Windows version; GE Medical Systems) for postprocessing. To observe the anatomical relationship between venous sinus in posterior fossa and cranial surface landmarks, the following steps were employed to perform imaging process: first, after both the nonenhanced and contrast-enhanced data were loaded into the memory, the subtraction process was performed by subtracting the nonenhanced data from the enhanced CTA data, which was designated as subtraction CTA. Second, in format of 3D volume rendering (VR) images, subtraction CTA was merged with nonenhanced cranial bone using comp PETCT software. Third, the cranial transparency on images was adjusted to 50% to project cranial venous sinus onto the external surface of cranium (Fig. 1). The anatomical relationship between venous sinus and important surface landmarks was observed at the merge image of 50% transparent skull image and subtraction CTA. The locations of asterion and squamosal–parietomastoid suture junction were defined and marked. Two options were available for determining the anatomical relation between asterion and squamosal–parietomastoid suture junction (Fig. 2): directly observing their locations on the external surface of cranium using VR; Application of maximum intensity projection (MIP) to locate asterion. By rotating 3D angles of merge VR image with 50% skull transparency, the anatomical relationship between asterion and TS/SS was recorded when sight was perpendicular to skull surface at the area of asterion. The anatomical relationship between squamosal–parietomastoid suture junction and TSST was determined using the same method. The center of confluens sinuum was regarded as observation point to define its anatomical relationship with inion. Using Track tool equipped with workstation, lines drawn from inion to root of zygoma, from inion to asterion,
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Fig. 1 Volume rendering image of subtraction CTA and skull: left section indicated enhanced cranial venous vessels (sinus) after removing skull; right section presented the anatomical relationship
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between venous sinus and cranial surface landmarks. Venous sinus was directly projected onto the external surface of cranium when the skull bone transparency was around 50%
Fig. 2 Volume rendering and maximum intensity projection image: the anatomical relationship between asterion and squamosal– parietomastoid suture junction
and from inion to squamosal–parietomastoid suture junction and SNL were marked (Fig. 3). Their anatomical relationship was observed, and the anatomical relationship between TS and surface landmarks of cranium was observed. The distances from proximal-end, middle-segment, and distal-end of TS to TS were measured.
Results Anatomical relationship among multiple surgical landmarks Referring to MIP in combination with VR image, display rate of asterion was 100%. Majority of asterion was located above LSPI, so the line drawn from the asterion to the inion
(LAI) mainly was distributed above LSPI; meantime, all LAI and LSPI were located above LZI. SNL was an upperheaving or outward-leaning curve, with three types of anatomical relationship with LZI: (1) proximal-end of SNL accompanied by LZI, and middle and distal-end of SNL located below LZI (41%); (2) proximal-end of SNL slightly superior to LZI, middle-segment intersected with LZI, and distal-end of SNL inferior to LZI (31%); (3) middle-segment of SNL positioned above LZI (28%). Therefore, SNL could not represent the trend of LZI. Anatomical relationship between squamosal– parietomastoid suture junction and TSST Approximately 11% of squamosal–parietomastoid suture junction was projected on TSST (left = 15, right = 7).
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the TSST in 15% cases (left = 16, right = 14), and superior to the TSST in 4% cases (left = 5, right = 3). The asterion was located from 6.7 mm inferior to 6 mm superior to the TST. To determine asterion location, the grid tool in workstation was used. Grid frame was designed regarding TSST interior angle as base point; the horizontal and perpendicular line of the grids paralleled TS and SS, respectively. Each grid measured 1 cm 9 1 cm. The asterion located in each grid was shown in Fig. 4. Most asterion was located on the TS side of TSST, namely the distal-end of TS. Anatomical relationship between SNL, LZI, LAI, LSPI, and TS
Fig. 3 3D reformation image: the anatomical relationship between venous sinus and important surface landmarks on cranium
Eighty-nine percent of squamosal–parietomastoid suture junction was located before and above TSST (left = 85, right = 93) and, the distance between it and frontal upper tip of TSST ranged from 0.2 to 19.8 mm (mean ± standard deviation 5.8 ± 3.8 mm).
The anatomical relationship between SNL, LZI, LAI, LSPI, and TS was shown in Fig. 5. The distances from proximal-end, middle-segment, and distal-end of TS to TS were indicated in Table 1, respectively. The results showed that SNL had complex anatomical relation with TS; thus SNL cannot judge the position of TS. LZI in proximal-end TS was mainly distributed in TS line, whereas most LZI was gradually positioned below TS line as TS moved outwards. LSPI and LAI almost shared the same trend with TS. As TS moved outwards, LSPI and LAI approached TS. Anatomical relationship between confluens sinuum and inion
Anatomical relationship between asterion and TSST The variability of the anatomic position of the asterion was confirmed in vivo on VR imaging. The asterion was located directly over the TSST in 81% cases (left = 79, right = 83). However, the asterion was found inferior to
To confirm the anatomical relationship between confluens sinuum and inion, we designed grids centering as inion tip; the perpendicular line of the grids paralleled the line drawn from herringbone point to inion. Each grid was measured 1 cm 9 1 cm (Fig. 6). The position of confluens sinuum
Fig. 4 The anatomical relationship between asterion and transverse–sigmoid sinus junction. Each grid measured 1 cm 9 1 cm. The number represented the percentage of asterion located in each grid
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Fig. 5 Schematic drawing showing the disposition of the superior nuchal line (a), line drawn from the zygoma root to the inion (b), line drawn from the asterion to the inion (c), line drawn from the squamosal– parietomastoid suture junction to the inion (d) in relation to the transverse sinus
Table 1 Position of the related line which above and below the transverse sinus from the line to the transverse sinus (x ± s)
Distance (mm) SNL to TS
Side
Proximal of TS
Middle of TS
Distal of TS
Above TS
8.48 ± 4.60
9.87 ± 5.52
Below TS
7.90 ± 3.47
6.61 ± 3.78
LZI to TS
Above TS
6.43 ± 3.13
3.52 ± 1.73
Below TS
6.98 ± 3.62
6.37 ± 3.53
7.18 ± 3.67
LAI to TS
Above TS
6.64 ± 3.54
5.97 ± 2.79
3.33 ± 1.62
Below TS
6.59 ± 3.72
4.45 ± 2.62
3.73 ± 2.36
LSPI to TS
Above TS
6.09 ± 2.96
5.45 ± 3.19
2.96 ± 1.74
Below TS
7.06 ± 4.05
4.90 ± 3.35
4.42 ± 2.80
10.63 ± 5.12
center was recorded. The number in grid indicated the percentage of the center of confluens sinuum located in each grid. The results showed that the center of confluens sinuum was mainly distributed on the upper right side of inion.
Discussion Hamasaki [13] performed head scan data processing by a 3D computed tomographic procedure using the following method by outlining first the bounder of transverse–sigmoid sinus complex onto the external surface of the cranium and second, observing the anatomical relation between those transverse–sigmoid sinus complex and asterion. This method is properly applied to avoid sinus injury in posterior approach surgery. Gharabaghi [10–12] observed the anatomical relation between asterion and TS/ SS on reformation images by using CT threshold technique, which effectively reduced poster operative complications and surgical times during suboccipital approach surgery. Based upon previous findings, our study made improvement in observation technique by merging subtraction CTA into cranial bone and observing anatomical relationship between venous sinus and bony landmarks at
Fig. 6 Grid study of the anatomical relationship between confluens sinuum and inion. The number represented the percentage of confluens sinuum center located in each grid
the setting of 50% transparency of crania. Poor images were obtained and specific data were missed due to the close anatomical relation between venous sinus and cranial
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bone using CT threshold technique. But subtraction CTA displayed enhanced vessel image after removing external skull [14, 15], which presented venous sinus appearance in a 3D, direct and accurate manner. VR was able to reveal cranial structure in a 3D way, and explicitly display root of zygoma, SNL, and inion. However, VR alone cannot precisely locate the positions of all asterion due to wide variations in cranial sutures. Gharabaghi [10, 12] indicated display rate of asterion was 90% using VR alone. MIP presented a clear image of cranial sutures, as well as lambdoidal suture, parietomastoid suture, and mastoid suture. In this study, VR and MIP were properly combined to distinctly display the surface cranial landmarks such as the inion and the asterion. VR image of subtraction CTA merged into 50% transparency skull image explicitly showcased the anatomical relationship between bony structure and TSST (Fig. 2). Asterion may be helpful to identify the TSST, whose location is indispensable for a successful posterior approach surgery. But the reliability of using asterion as a landmark on the cranial surface remains debatable [2, 7, 10, 12, 13, 16, 19, 24]. Ucerler [24] found 87.0% of asterion was located on TSST line, 11.0% below TSST line, and 2% above TSST. Srijit [19] utilized perspective X-ray irradiation method and noted that 91% of asterion was located on TSST line, 7% below TSST line, and 2% above TSST. Mwachaka [16] observed that 80% of asterion was on TSST line, 1.1% above TSST line, and 18.9% below TSST; burr hole was located posterior and inferior to asterion, which could avoid TSST injury. Therefore, the scholars mentioned above believed that asterion could be used to locate TSST position. By contrast, Day [7] found that 63.5% of asterion was on TSST line, 28.5% below TSST line, and 8% above TSST. Gharabaghi [10, 12] revealed that 70% of asterion was on TSST line, 7.5% above TSST line, and 22.5% below TSST by 3D volumetric image-rendering. Bozbuga [2] and Hamasaki [13] found significant variations in asterion distribution and insisted that asterion was unable to precisely locate TSST position. In our study, majority of asterion was projected on the TS side of TSST, namely distal-end of TS. In addition, that 81.0% of asterion was on TSST suggested that the application of asterion to locating TSST was reliable. Burr hole should be chosen inferior to asterion and the distance should be determined depending on the width of TSST during posterolateral approach surgery. Usually the asterion is so difficult to palpate and see, especially in older adults, that this osteological feature is not always a good landmark at surgery. Previous researchers defined SNL as LZI [2, 6, 7, 13, 24]. However, we regarded SNL and LZI as two concepts. First, SNL [18, 21, 23] was defined as the line extending from bilateral inion to the arch ridge of mastoid, from where
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trapezius and partial cleidomastoid muscle originated. SNL corresponded with inferior nuchal line (INL). However, LZI represented a line connecting the peak points of root of zygoma and inion. This study found SNL was an upper curve. Irrespective of whether SNL intersected with LZI or not, where the intersect position was, if any, and the curvature of SNL collectively determined the complexity of anatomical relationship between SNL and LZI: (1) proximal-end of SNL basically shared the similar trend with LZI; (2) middle-segment of SNL could be either above or below LZI; (3) majority of distal-end of SNL was located below LZI. Thereafter, SNL and LZI should be strictly discriminated. Moreover, the viewpoint that SNL can be used to locate the position of TS similar to LZI lacks accuracy. Day [6] conducted an experiment using fixed skull specimen and found that LZI could be used to locate the lower margin of TS. Bozbuga [2] suggested that LZI was almost parallel to and below the lower margin of TS. However, Hamasaki [13] observed significant individual difference existed regarding the anatomical relationship between LZI and TS using volumetric 3D image-rendering and concluded that LZI was unsuitable to locate TS. The authors in this study observed that 70% LZI was projected on TS line in proximal-end of TS; 46% LZI was projected on TS line in middle-segment of TS; in distal-end of TS, 24% LZI was located on TS line, and 76% below TS. In the proximal-end of TS, most LZI was located on TS line, and as TS moved outwards, LZI moved below TS line accordingly. In distal-end of TS, LZI was located on or below TS line. Consequently, it is not accurate to apply LZI to display TS trend. TS roughly showcased a trend shaping like S [1, 27], with the middle-segment presenting a line trend which was gradually climbing from inside to outside. However, SNL was shaped like an upper curve in some degrees. Thus, it is not reasonable to utilize SNL to outline the trend of TS, simultaneously indicating that SNL is unable to function as surface surgical landmark of TS. Avci [1] found that the upper margin of SNL was below the lower margin of TS within a range of 1.5–14 mm. Roberts [18] suggested that SNL did not precisely reflect the location of TS. Suslu [21] revealed the same trend shared by INL and the upper margin of TS, with SNL superior to INL. Thus SNL was not a suitable landmark for representing TS. Ziyal [27] considered that the attachment points of half spinal muscle were accurate and reliable surface surgical landmarks when locating TS and confluens sinuum, rather than SNL and inion. According to the findings in this study, we draw a conclusion that SNL cannot be regarded as a surface landmark of TS trend because both SNL and TS were significantly different in terms of shape and location, which was consistent with the findings found by other scholars [9, 17, 27]. Moreover, the meeting point of bilateral
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SNL–inion is not a reliable landmark of proximal-end TS. Ebraheim [8] found inion was located within the range of 7.3 mm above and 6.5 mm below inion. The results in this study indicated that inion was generally located on bottom left side of the center of confluens sinuum, reminding the surgeons to realize that majority of the center of confluens sinuum was located on the upper right side of inion rather than below inion while searching for the location of confluens sinuum. In this study, we found that almost half LAI and LSPI displayed the same trend with TS, whereas LAI is more suitable to represent the trend of TS compared with LSPI because LAI is more inclined to approaching TS as TS moved outwards. Ribas [17] noted that asterion and midpoint of LAI were mainly distributed in the lower part of TS. Therefore, although both LAI and LSPI have good correlation with TS, the condition that LAI and LSPI are not accompanied by TS indicates both LAI and LSPI are not appropriate landmarks. Among the four lines mentioned in this study, SNL is the least reliable landmark to determine the location of TS. In addition, significant variations exist in terms of the anatomical relationship between LZI and TS. In proximalend of TS, LZI was distributed within a limited range from TS, whereas in distal segment, LZI was mainly located below TS. The location of most LAI was higher than LSPI, and both LAI and LSPI were all above LZI, which enables LZI and LSPI to share the same trend with TS. To sum up, LZI and LAI are more reliable landmarks representing the trend of TS compared with SNL and LSPI. However, these lines are not able to precisely locate TS due to significant individual difference. How to accurately locate the bone window plays a vital role during surgery. Bone window should be adjacent to lower margin of TS and posterior margin of SS as close as possible to shorten distance in target region and facilitate the operation. But sinus injury might be induced by surpassing boundary line. Meanwhile, bone window should be made as small as possible under clear exposure field to minimize pain. Due to anatomic variation, there is no absolutely accurate method to locate venous sinus merely by bony landmarks. Additional preoperative assistance should be provided to explicitly locate venous sinus. Vrionis [25] suggested digital subtraction angiography or magnetic resonance venography examinations for the patients with asterion meningioma to better identify the position of TSST and provided individual operation scheme to each patient. MRI for neuronavigation could determine the anatomical relationship between asterion and TSST and reduce the unnecessary skull resection [5]. Gharabaghi [10–12] and Hamasaki [13] analyzed the anatomical relation between asterion and venous sinus using 3D CT and then applied it to operation, which effectively
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eliminated postoperative complications and shortened surgical times. 3D reformation technique introduced in this study is properly applied in clinical condition because it can clearly identify the anatomical relationship between confluens sinuum, TS, SS, and surface cranial landmarks and further provide guidance for individual posterior fossa approach surgery. The conclusions derived from normal adult data cannot be transferred to children and the patients with sinuses under some pathologies [4]. But the 3D imaging reformation technique was employed to accurately and rapidly observe the anatomical relationship between venous sinus in posterior fossa and cranial surface landmarks, providing valuable reference regarding individual anatomical data for children and the patients under pathological conditions. In this investigation, we clearly observed the anatomical relationship between venous sinus and skull bone from multiple angles and selected the optimal point. However, we have to admit that certain artificial factors may occur when determining the anatomical relationship between TSST and asterion, and describing the positions of asterion and confluens sinuum projected onto cranial bone on the grids. Subtraction CTA in this study is easily affected by head movement, and even slight movement may cause the failure of subtraction. Plus, incomplete removal of cranial bone also has a negative influence on the observation of venous sinus. Subtraction CTA technique requires two scans including conventional and enhanced scans, which relatively increases the radiation dose to patients. To counteract harm caused by excessive radiation, this study attempted to optimize the scanning technique, but it should be further elucidated extensively and intensively. Error analysis was not performed in our study, but Gharabaghi [11] shared similar methods with us in terms of determining the anatomical relationship between venous sinus and asterion with a registration error of 1.4 mm. This study aims to introduce a novel method to evaluate the reliability of using landmarks on the cranial surface to locate intracranial venous sinus. This technique can not only observe the anatomical relationship between venous sinus and surface cranial landmarks, but also identify the anatomical relation between lines usually applied in clinics and venous sinus.
Conclusions Asterion can be used to determine the position of TSST. SNL is unsuitable for serving as surface landmark of TS. However, inion and the lines drawn from inion to root of zygoma, asterion, and squamosal–parietomastoid suture junction have certain anatomical relationship with TS, respectively, but which obviously varied depending upon
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different individuals. Thus, it is unlikely to precisely locate the position of deep parts sinus by using surface landmarks. In practice, preoperative imaging assistance is required for accurate localization. In this retrospective study, we employed imaging reformation technique to observe the anatomical relationship between venous sinus and surface surgical landmarks of cranium rapidly and accurately, providing valuable evidence for potentially reducing operation times and surgical complications. Conflict of interest The authors do not have any further financial interest in the subject or materials under discussion.
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