Учебники / Computer-Aided Otorhinolaryngology-Head and Neck Surgery Citardi 2002

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FIGURE 9.6 Three-dimensional surface rendered images of MRI data reveal the location of an intracranial mass and its relationship to the soft tissues covering the calvarium. Note the ease of determining the surgical approach from these images.

mary strengths as a modality for use in CAS. While not as good as MR in representing normal soft tissue anatomy and tumor margins, intraoperative CT guidance has been shown to reduce the likelihood of postoperative complications and generally facilitates the surgical procedure by enhancing the surgeon’s perception of the operative field with three-dimensionally reconstructed CT images.

Additional benefits offered by newer developments in CT technology include the potential for real-time imaging, reconstruction, and display offered by fluoroscopic CT, and the flexibility and compatibility with other modalities afforded by mobile CT systems.

9.5.2.1 Real-Time CT Fluoroscopy

Although real-time CT fluoroscopy is most frequently utilized to guide abdominal, thoracic, and other musculoskeletal percutaneous biopsy procedures, a few investigators have used the modality for selected neurointerventional procedures [20]. CT fluoroscopy affords real-time visualization of the needle trajectory from skin entry to target point—prior to this technique, only intermittently obtained static images could be used to depict the relationship of the needle to the targeted lesion. CT fluoroscopy provides continuous visualization of the biopsy procedure, which has been found to significantly reduce both the potential for erroneous needle placement and the length of time needed to perform the procedure with radiographic guidance [21,22].

Katada et al. [21] reported their initial experience with a real-time CT fluoroscopy system used to guide 57 nonvascular interventional procedures, 11 of which were intracranial. The cerebral parenchyma, cerebral ventricles, and hematoma were clearly visualized, and real-time monitoring of the entire process of the puncture could be observed when the location of the lesion allowed the use of a transfrontal approach. When a transparietal approach was necessary, only the arrival of the needle at the target site could be monitored in real time. These investigators determined that both the puncture precision and efficiency of the procedures were improved.

Although CT fluoroscopy is generally recognized to improve the localization and efficiency of biopsy procedures, its significant radiation exposure to both patient and operating room staff remains an issue. Gianfelice et al. [23] reported their experience using a ‘‘spot-check’’ technique that only utilized fluoroscopy at intermittent points during the procedure, including immediately before the needle insertion. This reduced total radiation exposure time to 11 seconds, without compromising the visual benefit of the technique. In a trial that measured radiation exposure resulting from CT fluoroscopy, Nawfel et al. found that modifications in CT scanning techniques similar to those employed by Gianfelice et al., as well as the use of a lead drape placed adjacent to the scanning plane substantially reduced radiation doses [23,24]. Several investigators report the use of a needle holder to reduce the radiation dose absorbed by the physician’s hands; however,

this does limit the tactile feedback from the puncture needle [21,24,25]. Over the past two decades, several advances in the development of robots have occurred. These devices, if trained to have some ‘‘tactile’’ sensation, could have a significant role in the application of CT fluoroscopy. CT coordinates can easily direct the robots and, thus, reduce the radiation dose delivered to the interventional neuroradiologist and staff performing the procedure.

9.5.2.2 Mobile CT

Recently developed mobile CT scanning systems eliminate the need for patient transport to the CT suite if scans are needed during a neurosurgical procedure. These portable CT scanners are equipped with a translating gantry, which permits multislice computed tomographic scans to be obtained of a patient who is positioned in an immobile holder such as an operating room table. The scanner weighs approximately 460 kg and has wheels, which allow for transport to different areas of the hospital. Electrical wall outlets provide a power source and charge the internal battery of the scanner. A computer used to acquire, display, and archive the computed tomographic images is also part of the system [26].

Hum et al. [27] recently reported on their preliminary experience with a mobile CT scanner used intraoperatively for complex craniocervical operations and spinal tumor resections. These investigators found that intraoperative CT scanning facilitated ventral clival and craniocervical decompressions, promoted more complete tumor resections, and verified correct graft and instrument placement before surgical closing. Butler et al. [26] describe their experience with an identical mobile CT scanner, which was utilized during intracranial procedures as well as at the bedside of neuro ICU patients for whom transport is a problem. Although neurosurgical procedures were relatively longer because of the additional time needed to initially position the patient on the table adapter and position the gantry for image acquisition during surgery, the authors found overall efficiency increased as personnel became more accustomed to using the system. They concluded that, in general, the mobile CT scanner improved the efficiency of the procedure. The mobile CT scanner also proved beneficial for bedside imaging of ICU patients receiving mechanical ventilation or invasive monitoring.

Matula et al. [28] reported on their experience with mobile CT-guided neuronavigation system used in 20 microsurgery and neuroendoscopy cases. In all cases, the investigators found that the mobile CT system they used provided optimal intraoperative control of brain tumor resections. In 20% of cases, the intraoperative updates of the neuronavigation data sets identified residual tumor tissue that would not otherwise have been identified. In addition, these procedures could be performed on the original CT scanning table, thereby eliminating the difficulty of moving the patient out of the operating room for scanning.

Although the convenience of mobile CT scanners is attractive, this modality does have some limitations. Surgical instrumentation (radiolucent head devices

and cranium pins) that is compatible with intraoperative CT is essential, but such instrumentation is not readily available. When intraoperative CT is contemplated, titanium cranium pins are currently used in lieu of stainless steel pins; although titanium produces fewer artifacts than stainless steel, it does still produce some artifactual signal distortion in images that are coplanar with the pins [26]. Another consideration regarding the use of mobile CT scanners is the additional time that the radiology personnel must devote to their operation and maintenance. In 1999, participants in a comprehensive workshop exploring the technical requirements for computer-aided spinal procedures were less than enthusiastic about mobile CT scanners for spinal surgery, citing reduced image quality, slower acquisition time, and difficult integration with other modalities[19]. Finally, as with any CT system, the significant initial and/or maintenance costs and inevitable radiation exposure to staff and patient remains an issue.

9.5.3 MRI and Intraoperative Image Guidance

The evolution of radiographic modalities and image-processing technologies has made intraoperative image guidance possible through the integration of digitized images with previously described registration/tracking methods. Navigation of surgical instruments is tracked and displayed on 3D images, providing the surgeon with a view of surgical interaction with anatomical structures that would otherwise be ‘‘hidden’’ deep within the brain. This type of image guidance uses three-dimensional reconstructions of CT and/or MR images obtained prior to surgery. The major limitation of these systems is their inability to represent anatomical changes that commonly occur during neurosurgical procedures. The act of opening the skull, as well as the resection or biopsy of brain lesions or leakage of CSF during surgery, may cause significant changes in brain anatomy that are not represented by initially obtained CT or MRIs [29–32]. In addition, CAS systems that use previously obtained images cannot detect the development of potential surgical complications until they become clinically evident. These limitations led to the development of near real-time intraoperative imaging techniques.

Several imaging modalities have been explored for use in intraoperative image guidance, but MRI has become the preferred modality for near real-time intraoperative image guidance. The high spatial and temporal resolution of MRI as well as its superior contrast resolution are essential for intraoperative imaging [31]. In addition, MRI images can be obtained and display in any orthogonal plane [31]. During intracranial procedures, MRI is unmatched in its ability to define soft tissue structures and tumor margins, vascular abnormalities, communication between fluid containing structures, and subtle tissue abnormalities. Until recently, however, the closed magnet systems characteristic of MRI prevented physician access to the patient. This disadvantage, along with the electromagnetic environment and its resultant incompatibility with surgical instruments, precluded

any investigation of potential role of MRI as an intraoperative imaging modality. The development of the mid-field strength, open configuration MRI unit presented new opportunities. Specially designed open MRI systems (Signa SP, GE Medical Systems, Milwaukee, WI) allow neuroradiologists and surgeons to perform interventional and intraoperative procedures with near real-time MR imaging guidance [29]. During operative MR scanning, a series of fast-sequence MR scans are obtained and displayed at external consoles and on two 5-inch (diagonal width) liquid crystal display monitors mounted in the gap of the magnet. In this way, the surgeon may interactively localize, target, and monitor the procedure in near real time. Intraoperative MRI provides dynamic monitoring of anatomical and tissue changes as well as potential complications that may occur as a result of surgery.

Intraoperative MR imaging was first used in biopsy procedures. Black et al. in 1997 [33] reported on their experience with 63 stereotactic biopsies, 16 cyst drainages, 66 craniotomies, 3 thermal ablations, and 3 laminectomies, using a 0.5 T intraoperative MRI system (GE Medical Systems, Milwaukee, WI) in collaboration with Brigham and Women’s Hospital in Boston. This system provided near real-time imaging and tracking through the use of LED-based optical tracking of surgical instruments combined with manipulation of MRI planes. This provided continuous interactive feedback between surgical maneuvers and corresponding image formation [33]. The authors concluded that MRI has distinct advantages over other modalities in localization and characterization of tumor margins. This tracking method is not appropriate for use with flexible instruments such as catheters, guidewires, and flexible endoscopes. When these types of instruments are used, a nonoptical tracking method is appropriate, such as a coilbased tracking method with miniature coil attached to the instruments. For neurosurgical applications, Black et al. concluded that procedures performed under interactive MRI guidance have several advantages, including elimination of the need for fiducial markers and registration, superior capabilities in localization of lesions and dynamic tracking of changes in fluid and tissue compartments, precise plotting of the best trajectory of a surgical approach, as well as dynamic guidance and verification of operative execution, accurate identification of normal structures, and superior tissue characterization.

Several other investigators have also obtained good results using similarly designed intraoperative MRI units. Knauth et al. [34] found that a more complete resection of intracranial lesions could be accomplished using intraoperative MRI guidance. Samset and Hirschberg [35] reported favorable results using intraoperative MRI equipped with an optical tracking technique as well as an additional surgical navigation system. These authors found that the combination of these two technologies yielded better results than the use of either by itself. Real-time MRI images may have limited contrast resolution and/or a poor signal-to-noise ratio, and they are only available in one plane. On the other hand, intraoperative

MRI affords a continuous display of the operative procedure. Since the images are continually updated, they depict soft tissue changes, including tissue edema, brain shift, and even early intraoperative surgical complications. The surgeon can use this information to identify incipient problems and take appropriate corrective actions. Of course, repetitive intraoperative MRI images also serve to monitor the progress of the procedure. Surgical navigation devices support the 3D reconstruction of high-quality images in any orthogonal plane, and the combination of the two systems (intraoperative MRI and surgical navigation) provides mapping between the image space and the surgical space, which makes patient reregistration in the field unnecessary when images are updated [35].

Intraoperative real-time MR scanning, although not currently available for guidance of spinal procedures, does hold future potential for reducing the invasiveness and improving the general outcome of these surgeries. The vertical gap configuration of open magnet units allows the patient to sit upright, raising the possibility of new modes of radiographic evaluation and dynamic spinal surgery [33].

Although the initial experiences with intraoperative MRI are exciting, the challenges of realistically incorporating this complex technology into surgical practice cannot be ignored. One of the major issues with the use of an MR magnet in the operating room is the necessity for surgical instruments that are compatible with the magnetic field. Titanium and ceramic probes, forceps, headholders, and other tools have been developed and continue to evolve in order to meet this need; however, even the use of these nonmetallic materials does not completely eliminate the presence of artifacts [31].

All authors stress that state-of-the-art magnet design, high-resolution image quality, and incorporation of navigational methods are critical for the optimal utilization of intraoperative MRI. If these criteria are not met, the benefit derived from intraoperative MRI will be diminished, and it becomes questionable as to whether the significant cost and effort involved in the implementation of intraoperative MRI is justified. Tronnier et al. [30] point out the necessity for an analysis of the cost-effectiveness of implementing intraoperative MRI systems. Costanalysis methods should include duration of surgery, maintenance costs of the system, and the length of hospital stay as well as patient variables (namely, life tables relating to both length and quality of life).

Accuracy issues for this technology should not be dismissed. One study has even reported systematic error with the use of MRI in stereotactic procedures due to spatial distortion resulting from magnetic field warping, susceptibility artifacts, and chemical shift effects [36]. To address these issues, the authors developed a method of image fusion between MRI and CT using a chamfer matching technique, which resulted in more precise stereotactic localization of anatomical structures.

9.6 IMAGE RESOLUTION

The ability of the computer hardware/software program to maintain a high degree of resolution when reconstructing and manipulating images is vital to the utility of any CAS system. Image resolution determines a lower boundary on a system’s accuracy; that is, system accuracy cannot exceed the resolution of the scan data set. On a practical level, system accuracy is the net result of several factors.

Image resolution in these systems may be classified into two types. Understanding the subtle differences between in-plane and out-of-plane resolution is critical for a better understanding of CAS technologies, image resolution, and system errors.

In-plane resolution, which reflects the x and y dimensions of individual pixels, is determined by imaging parameters as well as hardware limitations. Typically, MRI array sizes are 256 256, with a field of view (FOV) from as little as a few centimeters up to 40 cm. For head and neck imaging, approximately 25 cm is typical for a FOV. This FOV results in a pixel size of approximately 1 mm 1 mm. For the typical CT scan, a pixel matrix size of 512 512 is common, and similar FOV boundaries are employed. These parameters lead to a corresponding reduction in pixel dimensions. Although it is possible to achieve decreases in pixel size, this objective cannot be accomplished without greater imaging scan time, greater radiation exposure (CT only), and/or decreased tissue contrast; however, the above-mentioned parameters have been found to be adequate for stereotactic procedures.

Out-of-plane resolution has also been referred to as slice thickness. This type of resolution is usually less than that of in-plane resolution, with slice thickness of 1.5–3 mm common in MRI and 3–4 mm in CT. These measurements can be reduced, but only at the cost of greater scanning times, greater radiation exposure (CT only), and/or decreased tissue contrast. Both in-plane resolution and out-of-plane resolution are important because it is necessary that the slice spacing be the same as or less than the slice thickness to achieve the best 3D data set for stereotactic procedures.

Another important consideration is software interpolation between pixels; this process can produce larger pixel arrays with smaller dimensions both inplane and out-of-plane. Such algorithms must be used with caution because the computer will make an educated guess at the derived pixel values. The computer then smoothes out sharp edges that are representative of actual anatomy—this smoothing may lead to a decrease in resolution or a less accurate depiction.

Pixel intensity level errors can also affect accuracy. Some pixels can be read as higher or lower than their actual value due to image noise. This issue can be addressed by signal averaging, but this technique risks increased imaging time or decreased resolution. A much greater problem is partial volume averag-

ing, which occurs when two or more substances with different imaging characteristics occupy the same voxel (the imaging data set’s unit volume that corresponds to each pixel in the displayed image). The resulting voxel value is then an average of all the intensities produced by each substance, weighted by the percentage of the voxel they occupy. These averaged voxels can cause errors during 3D reconstruction because they are classified into incorrect tissue categories. This incorrect categorization affects both the reconstructed boundaries and volumes of affected structures [37].

9.7 ACCURACY

Stereotactic neurosurgical procedures demand a very high standard of accuracy. Surgeons must be able to rely on the information provided by the system to guide them with precision. Localization errors could have quite a drastic effect on the outcome of such procedures. Thus, it is imperative to have a clear understanding of the nature and sources of errors in these systems.

The two principal terms to describe accuracy, bias and skew, are described by Maciunas et al. [38] Bias is the average measure of how well a device reaches a desired target point in space. An unbiased device will have errors that measure symmetrically around the correct target point; that is, its errors will have a mean close to the true value. The measure of the spread or a number of trials around their average value is defined as precision. A precise device will have errors that are clustered close to some center point [37]. Statistically speaking, precision is analogous to standard deviation, such that lower precision will produce a higher standard deviation in measured values. Both these terms are encompassed by the term accuracy. A device must be both unbiased and precise to be accurate.

There is also a distinction between the accuracy of the stereotactic device itself and the accuracy obtained when the device is used during the surgical procedure. This has been defined as the difference between mechanical accuracy and application accuracy [9]. While mechanical accuracy depends solely on the quality of the apparatus construction and refers to the device’s ability to reach a known location within its area of operation, application accuracy is a measure of the device as it is used in a real-world setting. This, then, encompasses such contributing factors as imaging error, reconstruction error, point selection error, registration error, perception error, and movement error, as well as errors associated with the localizing device [37]. Application accuracy is more significant than mechanical accuracy because it better represents the magnitude of errors likely to be encountered during surgery. Device manufacturers, however, often report on mechanical accuracy when describing the accuracy of their particular system.

When the accuracy figures associated with a particular system are reported, physicians should demand that the 50th percentile accuracy figure as well as the

95th percentile accuracy figure be presented. This would prevent authors from simply expressing their very best accuracy data, which is usually achievable in less than 5% of their patient population.

9.8 SOURCES OF ERROR

To detect and compensate for sources of errors when using stereotactic devices, a thorough knowledge of potential pitfalls is necessary. This knowledge is also helpful in designing protocols and procedures that will minimize the effects of these errors.

Factors that are intrinsic to the imaging data set may greatly influence surgical navigation accuracy. A study by Galloway et al. showed that CT slice thickness greatly influences the accuracy of each of the four most commonly used frame stereotactic systems [9]. Higher application accuracies are obtained when thinner CT slices are used. It seems reasonable to anticipate that these results may be extrapolated to MRI. Geometric distortions caused by scanner hardware or software can lead to errors in the acquired dataset. However, proper calibration and maintenance of the scanner will minimize this problem. Other factors, such as patient movement or artifacts from foreign objects (metal fillings or a prosthesis), can also degrade the quality of images and affect accuracy.

Another source of error is the 3D reconstruction process. Any chosen method involves some subjective way of evaluating the boundaries of structures to be included in the 3D objects. The process of object creation and display requires smoothings and interpolations (typically of the software) and can cause inaccuracies. Thus, discrepancies between real and reconstructed objects and structures can occur [39]. These types of errors can affect a stereotactic system in two ways. First, position identification inaccuracies on the reconstruction during registration could result in errors in the data fit to the patient. In addition, ambiguities in display of the 3D data on the screen may make the viewer perceive the location incorrectly even if the system may correctly identify the location of the probe tip. The use of the original cross-sectional radiographic information as well as the multiplanar reconstructions can reduce these ambiguities.

The data derived from the localizing device are also subject to several factors, including changes in temperature, aging, bending of the pointing device, bending or maladjustment of the sensor array, and poor lighting or acoustic conditions that can influence accuracy. This is particularly true of surgical instruments that are used as pointing devices, because these instruments undergo constant use and stress. Periodic checks on these instruments are mandatory. In the case of endoscopes used as localizing pointers, this problem can be more complex because the actual localizing point is arbitrary. Using the end of the scope as the stereotactic localizer will show where the lens is; however, that may not necessarily represent the area of interest because the FOV provided is 1–1.5 cm distal