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Navigated Intraoperative Ultrasonography for Brain tumors – technique, utility and benefits in neuro-
oncology –A pictorial essay
Ujwal Yeole, M.Ch, Vikas Singh, M.Ch, Ajit Mishra, M.Ch, Salman Shaikh, M.Ch, Prakash Shetty, M.Ch, Aliasgar
Moiyadi, M.Ch.
Neurosurgery Services, Dept of Surgical oncology, Tata Memorial Centre and Homi Bhabha National Institute,
Mumbai, India. 400012.
1. Ujwal Yeole, M.Ch, Fellow in Clinical Neurosurgical-Oncology.
Email: [email protected]
2. Vikas Singh, M.Ch, Assistant Professor, Department of Neurosurgery.
Email: [email protected]
3. Ajit Mishra, M.Ch, Fellow in Clinical Neurosurgical-Oncology.
Email: [email protected]
4. Salman Shaikh, M.Ch, Fellow in Clinical Neurosurgical-Oncology.
Email: [email protected]
5. Prakash Shetty, M.Ch, Professor, Department of Neurosurgery.
Email: [email protected]
6. Aliasgar Moiyadi, Professor & Chief Neurosurgeon.
Email: [email protected]
ORCID: 0000-0002-4082-2004
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Corresponding author:
Aliasgar Moiyadi,
Professor & Chief Neurosurgeon,
Neurosurgery Services, Dept of Surgical oncology, Tata Memorial Centre and Homi Bhabha National Institute,
Mumbai, India. 400012.
Email: [email protected]
Contact: +91 9324696948
ORCID: 0000-0002-4082-2004
Running title: Navigated Intraoperative Ultrasound.
Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-
for-profit sectors.
Conflict of Interests: The authors declare that they have no conflict of interest.
Acknowledgements: None
Author’s contributions: All authors contributed to the study conception and design. AM conceptualized the paper.
AM, PS and VS were the operating surgeons for the cases on which the paper is based. Material preparation, data
collection and analysis were performed by UY and AM. The first draft of the manuscript was written by UY and
AM and all authors commented on previous versions of the manuscript. All authors read and approved the final
manuscript.
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Navigated Intraoperative Ultrasonography for Brain tumors – technique, utility and 1
benefits in neuro-oncology – A pictorial essay 2
3
Abstract: 4
Intraoperative imaging has become one of the most important adjuncts in Neurosurgery 5
especially in surgery of intra-axial tumors. Navigation and intraoperative MR have their 6
limitations. Intraoperative ultrasound has emerged as a versatile and multifaceted tool for this 7
purpose. With advances in technology of the ultrasound scanners and newer multifunctional 8
probes the potential of IOUS is increasingly being utilized in resection of tumors. Addition of 9
image guidance to IOUS has exponentially increased the power of IOUS. Navigated US (nUS) 10
can now overcome many of the limitations of conventional standalone 2DUS. In this pictorial 11
essay, we outline our technique of performing nUS (both 2D and 3D) during resection of intra-12
axial tumors with illustrated examples highlighting the various steps and benefits of each. 13
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Key words: Intraoperative Ultrasound, Navigated Ultrasound, brain tumors, intraoperative 16
imaging 17
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Introduction: 20
Intraoperative updated imaging has become an important component of most brain tumor 21
surgeries. Conventional image-guided surgery (IGS or neuro-navigation) which relies on 22
preoperatively acquired CT or MR images, is limited by the problem of brain-shift and 23
deformation (once dura is opened and brain is manipulated) rendering such preoperatively 24
acquired “maps” inaccurate and unusable [1]. This has led to the proliferation of intraoperative 25
imaging tools to update and provide “corrected” images. Intraoperative CT (IOCT) is not very 26
useful due to inherent lack of soft tissue resolution and the problem of radiation exposure. 27
Intraoperative MR (IOMR) has been regarded as the gold standard in this context but remains a 28
logistically and financially unattractive option in most neurosurgical settings. Intraoperative 29
ultrasonography (IOUS) has proven to be a useful, easily available and cost-effective tool during 30
neuro-oncological procedures, overcoming the limitations posed by IOMR [2][3]. Standard 2-D 31
grey-scale ultrasound (2DUS) provides rapid, repeated, real-time updates intraoperatively 32
without altering the surgical workflow. Advances in US technology have led to development of 33
newer generation scanners specifically designed for intracranial application. With superior image 34
quality combined with the doppler and angiography as well as the newer US applications like 35
contrast-enhanced US (CEUS) and elastosonography, IOUS has become arguably the imaging 36
modality of choice in neuro-oncological surgeries. With minimal alteration in the surgical 37
workflow (unlike MR) and the ease of repeated use, IOUS is an attractive tool. Having said that, 38
2DUS does pose certain limitations essentially related to the problems of orientating a limited 39
field of view and interpreting planar images in unconventional planes. Navigated ultrasound 40
(nUS) utilizing either 2D (n2DUS) or 3D (n3DUS) volumes helps overcome many of the 41
limitations of standalone 2DUS. 42
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We outline our technique of performing n3DUS during brain tumour surgeries describing the 43
technical nuances and discuss with illustrative cases the spectrum of applications and benefits. 44
We have used a commercially available high-end ultrasound scanner bk5000 (® BK Medical, 45
Denmark) digitally integrated with Brainlab KICK navigation with Ultrasound navigation 46
software (®BRAINLAB AG, Munich, Germany) to perform n3DUS in 34 cases. The present 47
paper focusses on the application of this system during brain tumour surgeries. The setup, 48
detailed insonation parameters and operating room workflow are provided in supplementary 49
material as well as figures 1-3. 50
2DUS Acquisition: After completing the craniotomy, curved linear probe with a small footprint 51
(29 x 10 mm) and frequency range of 5-13MHz (Craniotomy N13C5 tansducer, ®BK Medical, 52
Denmark) is brought into the surgical field and a wide transdural 2DUS scan of the entire 53
exposed surface is performed (Fig 4). Adjusting the parameters (depth, gain, time-gain control) 54
helps optimize the images. We repeat the US acquisition after opening the dura again (as dural 55
calcifications may sometimes interfere with optimal insonation). This helps characterize the 56
lesion (extent, echogenicity, internal variability, margins, cysts, calcific shadows) and survey the 57
surrounding anatomical landmarks for better orientation. Falx (linear hyperechoic structure), 58
ventricles (anechoic, bilaterally symmetrical) and choroid plexus (densely hyperechoic tufts 59
within the ventricles) are useful starting points. Performing careful orthogonal scans (in two 60
planes preferably in conventional imaging planes axial, coronal, sagittal -ACS) can aid to 61
generate a mental 3D picture of the entire operative field and help in getting oriented –as shown 62
in figure 5. However, it is often difficult to do this with a 2DUS, (challenges of location and size 63
of craniotomy) unless one is an expert operator. At this point, additional imaging with doppler, 64
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angiography, CEUS or elastosonography can also be performed (though we do not do that 65
routinely) to further characterize the lesion if needed. 66
Navigated 2DUS : n2DUS scan can then be immediately acquired in a pre-calibrated system by 67
just attaching the navigation localizer to the probe (Fig. 3). As the probe is moved in the surgical 68
field, the corresponding 2DUS is visualized in real-time on the navigation screen superimposed 69
on the co-displayed MR scans in the corresponding plane as well as the virtual representation of 70
the probe in the surgical field (Fig. 6). This multimodal image display instantly makes it easier to 71
understand the plane of insonation which may not be a true anatomical plane and helps to 72
manually adjust and realign the US plane in a conventional true ACS plane if needed (Fig. 7). 73
Further, it is easier to identify anatomical landmarks in the US image (especially for a novice 74
who may find it difficult to interpret US image) by correlating with the corresponding MR image 75
which is in the same plane. However, multiplanar US imaging is still not possible with n2DUS 76
unless the probe is rotated in different planes itself. Hence, we prefer performing a 3DUS before 77
commencing resection. (Fig. 8). 78
Navigated 3DUS– After performing a live n2DUS, the probe is swept along the operative field 79
of interest capturing a series of 2DUS images which are reconstructed into a 3D volume (3D 80
sweep). It is important to utilize the entire exposed surface to generate as large a volume as 81
possible. The system permits a single continuous sweep and hence the best sweep which will 82
encompass as much of the representative tumour area as well as surrounding brain is first 83
“practised” on live n2DUS mode, which also ensures that during the actual 3DUS acquisition, 84
there are no line-of-sight problems with the navigation system. The actual sweep is then 85
completed by slowly tilting and translating the probe over the surface of the dura (or brain) from 86
one end of the craniotomy to the other along the chosen direction. Continuous feedback from the 87
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live n2DUS screen provides complete control over the quality and content of this 3D acquisition. 88
Within 20-30 seconds, the system computes the 3D volume. Once this is done, the probe is set 89
aside and using the navigation probe the US image depicted on the navigation screen in 90
multiplanar ACS views (co-displayed with MR image) can be used for navigation. (Fig. 8) This 91
3D US-MR image fusion technique greatly improves the ease of orientation and interpretation of 92
US images and compensates for the limited field-of-view of the US. The MR image also provide 93
additional functional information (cortical eloquent areas and tracts) which can be segmented 94
and superimposed on the subsequently acquired US image, thus providing the all-important 95
functional information lacking in US image (Fig. 9). One of the problems with using 96
preoperative MR-based navigation is the loss of accuracy once the dura is opened and CSF is 97
released causing a change in the relative position of the brain and intracranial contents – the 98
phenomenon of brainshift. This brainshift can also be identified using n3DUS and may even be 99
quantified (Fig. 10). Though we use the overlay fMRI and tractography information, we rely on 100
confirmatory intraoperative neuromonitoring using monopolar (under anaesthesia) or bipolar 101
(awake) stimulation during surgeries in all tumours close to eloquent regions. Surgical resection 102
then follows standard micro-neurosurgical principles. We prefer using an en bloc resection 103
strategy for all low grade and most high-grade intra-axial lesions. The IOUS blends seamlessly 104
into the surgical workflow without much discomfort to either patient (if awake) and the surgeon. 105
Resection control scans – As the resection progresses frequent US updates are obtained. Prior to 106
each US scan, the resection cavity is thoroughly rinsed of debris with copious saline and 107
hemostasis achieved meticulously. The table position can be adjusted to ensure that the cavity is 108
as vertical as possible and then filled with adequate saline. It is imperative, always to ensure that 109
the images are of good quality and free of artefacts. Insonation is repeated in the same sequence 110
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as for the pre-resection US scans (live 2DUS sweeps followed by 3DUS acquisition if needed). 111
Occasionally only live 2DUS updates are done to obtain a rapid review of the extent of resection 112
or to check and reorient the progress (Fig. 11). Even with n2DUS, it is important to keep in mind 113
the plane of the previously acquired 2DUS scans to avoid misinterpreting presence or absence of 114
residue, especially when comparing non-coplanar images. (Fig. 12). The challenge is to obtain 115
images in same plane repeatedly when a very small residue needs to be detected. More 116
importantly, once the probe is removed from the operative site, the images are no longer 117
available for review and for finding the exact location of the residue. With n3DUS, once the 118
navigator is brought into the field comparative evaluation of the serial temporally acquired US 119
image can be performed and the exact location of the residue can be pinpointed and further 120
resected with confidence (Fig. 12 and 13). Multiple such 3DUS scans can be repeatedly acquired 121
as the resection progresses. Brainshift across the subsequent US scans can also be appreciated 122
(Fig. 14). Besides the conventional axial, coronal and sagittal views, it is possible to represent 123
the acquired images in a more user-intuitive inline view. The inline views depict the volume in a 124
plane parallel to the plane of surgical view thereby providing more practical orientation. 125
Combining two such mutually orthogonal views is very useful in orientating the operative cavity 126
with respect to the acquired images (Fig. 15). Each 3DUS acquisition takes not more than 1-2 127
minutes overall, adding negligible time to the overall procedure. 128
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Discussion: 131
IGS has become the need of the hour, especially in brain tumours where the importance of the 132
extent of resection is well-established. One of the major reasons for incomplete resections is the 133
inadvertent residue which is missed by conventional surgical examination of the operative cavity 134
[4]. Navigation is of limited value by itself. To maximize the resection, various intraoperative 135
tools are available including fluorescence, ultrasound, and MR of which the IOUS is the most 136
cost-effective [5]. Not only this, IOUS is mostly available in all setups and is a versatile tool with 137
many applications during tumour surgery [2][3]. By itself, 2DUS is a useful tool during targeting 138
and resection control of tumours. With advances in US technology, the quality of acquired 139
images and functionality of US systems has greatly increased. There remains the issue of 140
subjectivity and the technique is user dependent. Neurosurgeons by training are ill equipped to 141
perform and interpret US scans. Coupled with the unconventional surgical planes during 142
craniotomy exposures, and incomplete (lack of full head) views of the IOUS, there is 143
considerable disorientation, making the learning curve steeper. Also, the wonderful images 144
provided by the ultrasound needs to be memorized by the surgeon as once the US probe is 145
removed from the surgical field, these image are no longer available and the surgeon must rely 146
on the mental reconstruction to proceed with further resection which can be challenging when 147
chasing smaller tumour remnants in proximity of critical and eloquent structures. However, as 148
we have demonstrated in this paper, using navigation and image fusion, the potential benefits of 149
US can be unlocked. 2DUS, n2DUS and n3DUS have incremental benefits as shown in Fig. 16 150
building on the strengths of each component (US and navigation). 151
Image fusion with MR image allows US image to be co-displayed with the MR image. This 152
improves image interpretation (one can use the MR image to learn and understand the US image) 153
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as also overall orientation of the surgical field [6]. Tracking the US probe during the acquisition 154
of the US image intraoperatively is the key step to be able to navigate with the US image. 155
Subsequently some systems (as the one described above) can acquire and reconstruct 3D 156
volumetric US scans which then can be reformatted in any chosen plane of representation such 157
as ACS views. They allow instant re-orientation of the US anatomy in the standard views which 158
makes learning easier in the initial stages of training. n3DUS also permits serial display and 159
comparison of multiple resection control US scans performed sequentially, thereby allowing for 160
accurate assessment of small residual remnants (Fig. 8 and 9). Using nUS, Renovanz et al 161
showed that the sensitivity and specificity of detecting tumour remnants is increased as 162
compared to non-nUS [7]. More such comparative studies are needed to critically analyse the 163
benefits of nUS. Whereas most systems require the preoperative MRI to be co-displayed, some 164
navigated US systems can even use just the 3DUS data for navigation providing a direct-165
navigated US option. In our earlier experience with such a system, we found this to be very 166
convenient especially in situations where a preoperative MR sequence suitable for navigation is 167
not available [8]. This however requires a level of proficiency in using and interpreting US 168
image which may be learnt rather quickly when using the co-display image fusion during the 169
early part of the learning curve. nUS can also demonstrate the brain-shift and is also useful for 170
compensating the same. (Fig. 10 and 14) [6]. Though not always accurate, it is best to continue 171
to rely on the most recently updated US image (and subsequent resection-control scans) for 172
assessing the operative field as they represent the true status and should be relied upon for 173
making intraoperative decisions. Basing intraoperative assessments on the updated US image 174
with the MR image setting up a roadmap is the best way to work with nUS. 175
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At our centre, we have been using IOUS for over 12 years graduated to nUS which we have been 176
using for almost 8 years now. The initial phase of the learning curve is longer, but use of 177
navigated US can substantially shorten this phase. In our experience, IOUS is a very useful tool 178
for all kinds of brain tumours [3,9]. It is particularly useful for targeted biopsies [10] as well as 179
resection of intra-axial tumours [11-14]. Large systematic reviews and meta-analyses have also 180
found IOUS to be a very useful adjunct to improve resections in gliomas [15 -16]. Considering 181
its easy availability and relatively low cost, this is one tool which should be increasingly adopted 182
by neurosurgeons. Acquiring a good US scan remains the cornerstone of this technique and 183
cannot be compensated for by any image fusion. Moreover, post resection artefacts are 184
troublesome and need to be minimized for accurate interpretation. [17]. Despite the challenges, 185
the learning curve can be quickly climbed with dedicated training and the frequent use of 186
navigated ultrasound. Interpretation of the post-resection US scan remains a key factor in its role 187
as a tool for resection control. Diagnostic accuracy of US compared to postoperative MRI [18] as 188
well as pathological tumor confirmation [19] is reported between 60-100%. However, 189
comparison across studies is difficult due to lack of uniform criteria of US residue and the gold 190
standard in all studies. 191
192
Conclusion: Navigated ultrasound is a useful addition to the intraoperative-imaging 193
armamentarium of the neurosurgeon. It combines the benefits of real-ime US imaging with 194
powerful navigation technology and is a versatile and multi-functional adjunct. With experience 195
in US image acquisition and interpretation if can be a potentially useful tool for surgery of brain 196
tumors. 197
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Figure Captions 268
Figure 1. Operation theatre layout (a) – note the position of the US scanner in relation to the 269
navigation system and its proximity to the operating filed. Covering the control panel with a 270
sterile drape (b) allows direct control by the surgeon. 271
Figure 2. Navigated Ultrasound – (a) the probe which we use for performing nUS. (b) Localizer 272
with reflective spheres is fixed in a predetermined position onto the US probe. This is calibrated 273
to the navigation system allowing the US images to be registered in the same frame of reference 274
as the MRI scans previously registered on the navigation. 275
Figure 3. Positioning of the patient during US-guided surgery. It is essential to plan the position 276
in such a way that the conditions can be optimized for performing a post-resection scan bearing 277
in mind to have a cavity which can contain saline to allow a good acoustic coupling. The aim is 278
to keep the operating surface as flat (parallel to the floor) as is possible. (a) a right frontal 279
craniotomy and (b) a right paramedian suboccipital craniotomy. With careful planning an 280
optimal US scan can be obtained for virtually any cranial exposure. 281
Figure 4. 2D ultrasound – The same lesion is insonated with 2 different probes – (a) a high 282
frequency linear, 6-15MHz hockey-stick probe providing exquisite anatomical details of the 283
superficially located tumor with poorer delineation of deeper structural details; (b) a curvilinear, 284
5-13MHz probe showing a panoramic view which aids in the image orientation also.[high 285
resolution imgaes of both the US probe images are available in supplementary material 2 and 3 286
respectively] 287
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Figure 5. 2D ultrasound – (a) attempting to obtain a true axial image in the ultrasound (b). 288
Corresponding MR image in (c). It may not always be feasible to obtain such true anatomical 289
planes depending on the craniotomy aperture and location. 290
Figure 6. Navigated 2DUS - Screenshot of the real-time navigated 2D ultrasound from the 291
navigation system. The right panel shows the actual US image superimposed on the 292
corresponding MR plane. This screen is “live” and depicts the real-time moving US image as one 293
sweeps the operative field, exactly duplicating the US scanner screen. The left panel shows the 294
plane of insonation represented in the conventional axial, coronal, sagittal MRI images. Note that 295
the plane of insonation is not in a true anatomical plane as is the case in most surgical procedures. 296
Tumor which appears hypointense on the T1 MRI images and hyperechoic on US is outlined 297
with white dashed line. 298
Figure 7. Using Navigated 2D US to align the real-time 2D US image in a true anatomical plane. 299
(a) True Sagittal image and (b) True coronal image; in a case of left frontal high-grade glioma. 300
Figure 8. Navigated 3D Ultrasound – Screenshot after acquiring the 3DUS in the same case as 301
Figure VII. Using the navigator tool (depicted virtually as the green line with cross hairs) the US 302
images can now be seen in all 3 conventional ACS planes along with the corresponding MR 303
images. This screen layout can be customized in multiple display formats. 304
Figure 9. MR-US image fusion: Functional information can be outlined on the MR images. In 305
(a) Motor cortex in orange, corticospinal tracts in blue and the tumor itself in red have been 306
segmented. In (b), the inferior fronto-occipital tract has been outlined in green and co-displayed 307
with the US images. 308
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Figure 10. Brain-shift demonstrable on the nUS. Screenshot from the navigation system shows 309
the tumor segmented in red based on MRI images. Motor cortex is depicted in orange. Bottom 310
panel centre image shows the US image and the tumor now segmented in blue overlapping with 311
the MR segmented tumor (red). Shift is calculated manually in different directions and is 312
appreciably variable in all of them. Importantly this information can be used to infer the new 313
position of the motor cortex even if the US cannot define the motor cortex itself. 314
Figure 11. Resection control scans using serial real-time 2D grey scale US images. (a) Pre-315
resection (b) Intermediate, and (c) Post-resection imaging. Note the good image resolution of the 316
tumor mass as well as the surrounding anatomical details. However, also note that each of the 317
images is in a different 2D plane, thereby making direct comparison with the preceding images 318
challenging. 319
Figure 12. Resection Control Scans using n2DUS vs n3DUS - Left column depicts serially 320
acquired n2DUS images (a-c). Note the differing planes of each 2DUS as shown in the 321
corresponding MR cuts. In the same patient, (d) after acquiring a 3DUS, serial scans can be 322
depicted side-by-side and visualized in the same planes allowing easier comparison. 323
Figure 13. Navigated 3DUS (n3DUS) showing preoperative MRI (upper row), pre-resection US 324
(middle) and post-resection US images (bottom row). 325
Figure 14. Demonstration of progressive brain shift – Top row shows the pre-resection 3DUS in 326
ACS views. The tumor is outlined in orange. Middle row shows the same US superimposed on 327
the preoperative MRI. The brain-shift is obvious. Bottom row shows the pre-resection US 328
segmented tumor and the post-resection updated 3DUS showing the cavity which is displaced in 329
all three planes with respect to the pre-resection US. 330
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Figure 15. In-line views. Left insular glioma. Orthogonal inline views of MR are on leftmost 331
column. Central column shows coregistered pre-resection orthogonal US images and the right-332
most column is the post-resection US images in the same orthogonal planes. The vertical green 333
line corresponds to the direction of the surgeon’s operative field of view. Tumor which appears 334
hyperechoic on US is outlined with white dashed line in the pre-resection (central panel) as well 335
as post-resection (right panel). Note the residual tumor seen along with posterior enhancement 336
artefacts in the right panel. 337
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Figure 16. Multimodal US-based intraoperative image guidance. Clockwise starting from the top 339
left corner (Navigation) till the bottom left corner (navigated 3-D US) there is an incremental 340
benefit of the US. The benefits (green boxes) and limitations (orange boxes) of each of the 341
modalities is highlighted. (IOMR- intraoperative MR Imaging). 342
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Figure 1
Operation theatre layout (a) – note the position of the US scanner in relation to the navigation system and its proximity to the
operating filed. Covering the control panel with a sterile drape (b) allows direct control by the surgeon
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Figure 2
Navigated Ultrasound – (a) the probe which we use for performing nUS. (b) Localizer with reflective spheres is fixed in a
predetermined position onto the US probe. This is calibrated to the navigation system allowing the US images to be registered
in the same frame of reference as the MRI scans previously registered on the navigation.
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Figure 3
Positioning of the patient during US-guided surgery. It is essential to plan the position in such a way that the conditions can be
optimized for performing a post-resection scan bearing in mind to have a cavity which can contain saline to allow a good
acoustic coupling. The aim is to keep the operating surface as flat (parallel to the floor) as is possible. (a) a right frontal
craniotomy and (b) a right paramedian suboccipital craniotomy. With careful planning an optimal US scan can be obtained for
virtually any cranial exposure.
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Figure 4
2D ultrasound – The same lesion is insonated with 2 different probes – (a) a high frequency linear, 6-15MHz hockey-stick probe
providing exquisite anatomical details of the superficially located tumor with poorer delineation of deeper structural details; (b)
a curvilinear, 5-13MHz probe showing a panoramic view which aids in the image orientation also.
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Figure 5
2D ultrasound – (a) attempting to obtain a true axial image in the ultrasound (b). Corresponding MR image in (c). It may not
always be feasible to obtain such true anatomical planes depending on the craniotomy aperture and location.
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Figure 6
. Navigated 2DUS - Screenshot of the real-time navigated 2D ultrasound from the navigation system. The right panel shows the
actual US image superimposed on the corresponding MR plane. This screen is “live” and depicts the real-time moving US image
as one sweeps the operative field, exactly duplicating the US scanner screen. The left panel shows the plane of insonation
represented in the conventional axial, coronal, sagittal MRI images. Note that the plane of insonation is not in a true anatomical
plane as is the case in most surgical procedures. Tumor which appears hypointense on the T1 MRI images and hyperechoic on
US is outlined with white dashed line.
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Figure 7
Using Navigated 2D US to align the real-time 2D US image in a true anatomical plane. (a) True Sagittal image and (b) True
coronal image; in a case of left frontal high-grade glioma.
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Figure 8
Navigated 3D Ultrasound – Screenshot after acquiring the 3DUS in the same case as Figure VII. Using the navigator tool
(depicted virtually as the green line with cross hairs) the US images can now be seen in all 3 conventional ACS planes along
with the corresponding MR images. This screen layout can be customized in multiple display formats.
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Figure 9
MR-US image fusion: Functional information can be outlined on the MR images. In (a) Motor cortex in orange, corticospinal
tracts in blue and the tumor itself in red have been segmented. In (b), the inferior fronto-occipital tract has been outlined in
green and co-displayed with the US images
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Figure 10
Brain-shift demonstrable on the nUS. Screenshot from the navigation system shows the tumor segmented in red based on MRI
images. Motor cortex is depicted in orange. Bottom panel centre image shows the US image and the tumor now segmented in
blue overlapping with the MR segmented tumor (red). Shift is calculated manually in different directions and is appreciably
variable in all of them. Importantly this information can be used to infer the new position of the motor cortex even if the US
cannot define the motor cortex itself.
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Figure 11
Resection control scans using serial real-time 2D grey scale US images. (a) Pre-resection (b) Intermediate, and (c) Post-
resection imaging. Note the good image resolution of the tumor mass as well as the surrounding anatomical details. However,
also note that each of the images is in a different 2D plane, thereby making direct comparison with the preceding images
challenging
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Figure 12
Resection Control Scans using n2DUS vs n3DUS - Left column depicts serially acquired n2DUS images (a-c). Note the differing
planes of each 2DUS as shown in the corresponding MR cuts. In the same patient, (d) after acquiring a 3DUS, serial scans can
be depicted side-by-side and visualized in the same planes allowing easier comparison.
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Figure 13
Navigated 3DUS (n3DUS) showing preoperative MRI (upper row), pre-resection US (middle) and post-resection US images
(bottom row).
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Figure 14
Demonstration of progressive brain shift – Top row shows the pre-resection 3DUS in ACS views. The tumor is outlined in
orange. Middle row shows the same US superimposed on the preoperative MRI. The brain-shift is obvious. Bottom row shows
the pre-resection US segmented tumor and the post-resection updated 3DUS showing the cavity which is displaced in all three
planes with respect to the pre-resection US.
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Figure 15
In-line views. Left insular glioma. Orthogonal inline views of MR are on leftmost column. Central column shows coregistered
pre-resection orthogonal US images and the right-most column is the post-resection US images in the same orthogonal planes.
The vertical green line corresponds to the direction of the surgeon’s operative field of view. Tumor which appears hyperechoic
on US is outlined with white dashed line in the pre-resection (central panel) as well as post-resection (right panel). Note the
residual tumor seen along with posterior enhancement artefacts in the right panel.
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Fig 16
Multimodal US-based intraoperative image guidance. Clockwise starting from the top left corner (Navigation) till the bottom left
corner (navigated 3-D US) there is an incremental benefit of the US. The benefits (green boxes) and limitations (orange boxes)
of each of the modalities is highlighted.
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