Imaging modalities
Preoperative evaluation with anatomical and functional imaging techniques
Anesthetic considerations for intraoperative imaging
Neuronavigation
Contrast media
Stereotactic surgery
Frame-based and frameless
Anesthetic considerations
Stereotactic radiosurgery
Anesthetic considerations
SUMMARY
1. The anesthesiologist needs to have an understanding of the various imaging modalities used in the preoperative preparation of patients as well as the anesthetic considerations when these modalities are used in the operating room.
2. Stereotactic surgery, especially frame-based, provides a challenge for the anesthesiologist because of concerns about airway management.
3. Stereotactic radiosurgery, such as gamma knife therapy in children, requires the provision of anesthesia in remote imaging locations and interaction with a multidisciplinary team.
INTRODUCTION
The imaging of patients who have neurologic disorders plays an important role in their diagnosis, treatment, and outcome. Anatomical imaging with CT and MRI provides structural and pathologic information. Functional imaging with systems such as positron emission tomography (PET) and functional MRI provide neurophysiologic and metabolic information. Preoperative brain imaging and mapping of eloquent function such as speech, motor, and sensory are useful adjuncts in surgical planning. Frame-based and frameless stereotactic surgery and radiosurgery are used for accurate localization and/or safe treatment of deep intracranial structures and lesions. With new developments in imaging technology and computerization, there has been a move to integrate real-time imaging with surgery in the operating room. These systems are proving beneficial as preoperative imaging and mapping may fail to account for brain shifts that can occur during surgery as a result of patient positioning, cerebrospinal fluid drainage, diuretics, and resection.
I. IMAGING MODALITIES
A. Computed tomography provides cross-sectional images of the body using mathematical reconstructions based on x-ray images taken circumferentially around the subject. The images are combined by a computer into a single picture. Contrast dye is used to enhance lesions. CT angiography is used in the diagnosis of abnormalities of the cerebral vasculature thus avoiding direct invasive angiography. Images from CT scanning can also be fused with scans from other modalities, such as PET.
1. Intraoperative CT allows for direct imaging during surgery via several different systems.
a. A fixed CT unit is integrated with the operating room table. During surgery, both the patient and the CT unit may be completely covered with sterile drapes, limiting access to the patient.
b. A mobile CT scanner is situated in the operating room and glides on floor rails to the patient when a scan is to be performed.
c. The combining of CT with a neuronavigation system will improve accuracy as the images used for reference can be obtained in the operating room. The navigation can also be updated at any time during surgery.
d. Image-guided CT fluoroscopy provides real-time imaging for specific intraoperative procedures such as localization of the trigeminal nerve for the treatment of neuralgia.
2. Anesthetic considerations
a. During diagnostic CT scanning, patients usually do not need anesthesia or sedation, except in special situations such as an uncooperative or pediatric patient. If required, the principles of anesthesia outside of the operating room must be followed. The anesthetic technique may be either general anesthesia or conscious sedation, depending on the needs of the patient.
b. All personnel are at risk from exposure to high levels of radiation during scanning. Video or remote monitoring of the patient from outside of the CT suite is usually used.
c. In the operating room, the overall anesthetic management is based on the needs of the surgical procedure and patient.
d. Access to the patient, especially the airway, may be limited during the procedure and even more so during imaging. Appropriate placement of monitors and lines is required, especially when a mobile CT unit is used.
e. During CT scanning in the operating room, all personnel should be a safe distance away and must wear proper radiation shielding (lead aprons) and use mobile screens. If possible, one should use video or remote monitoring of the patient outside of the operating room.
B. Magnetic resonance imaging is a non-invasive, radiation-free diagnostic procedure that uses a powerful magnetic field and intermittent radiofrequency waves. MRI produces greater contrast between soft tissues, making it is useful for neurologic imaging. Intravenous contrast is frequently given to enhance the appearance of the lesion. MRI angiography is also used to diagnose abnormalities of the cerebral vasculature.
1. Intraoperative MRI: The merger of an operating room and MRI suite has been well accepted and is being used more frequently for neurosurgical procedures. This allows for real-time imaging during surgery (discussed in Chapter 13).
a. Anesthetic considerations
1. General anesthesia or conscious sedation is administered for diagnostic MRI scanning for patients who are unstable, uncooperative, claustrophobic, and pediatric.
2. Safety concerns when providing anesthesia in an MRI environment are the same whether it is in a diagnostic suite or an operating room.
3. A magnetic field is constantly present and extends beyond the magnet. The strong attraction of ferromagnetic objects to the magnetic field can cause them to become projectiles that may harm patients or personnel.
4. All anesthesia and monitoring equipment must be MRI compatible.
5. Contraindications to being in the MRI environment for the patient and all personnel must be considered.
6. The overall anesthetic management including type of anesthetic and requirements for monitoring will depend on the surgical procedure and the patient.
C. Neuronavigation is computer-assisted technology that is used in the operating room to localize and visualize intracranial structures and pathology during surgery. Neuronavigation uses a set of images obtained preoperatively either by CT or MRI.
1. Before imaging, external fiducial adhesive skin markers are placed on the patient’s scalp or a set of marks is located in the reticule of an optical instrument. These markers are then used as points of reference. In the operating room, the orientation of these markers is used to register the computer containing the preoperative images. During the procedure, the imaging can show the relationship of the planned surgical incision or resection of the lesion to the previously obtained images. The main indications for neuronavigation are for the targeting of small and deep intracranial lesions, but neuronavigation is widely used now for many neurosurgical procedures. The addition of an intraoperative 3-dimensional ultrasound may also enhance the quality of neuronavigation by adding information about brain shift and tumor remnants.
2. Anesthetic considerations: Neuronavigation itself does not impact the actual anesthetic management of the patient, but it may prolong the duration of the anesthetic, especially before the initial surgical incision when planning for the procedure is conducted with the aid of navigation. Stimulation is minimal at this time and the anesthetic depth needs to be adjusted accordingly.
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