Applied Radiology

Ultrasound is an imaging modality with a wide variety of uses in thepediatric population. The noninvasive and portable features of ultrasound makeit a particularly attractive imaging option in the infant or child. However,the usefulness of ultrasound in imaging the central nervous system (CNS) islimited because of the difficulty of imaging through the bone of the cranium orvertebrae. As a result, pediatric neurosono-graphic imaging usually is limitedto the neonate or infant because scanning in this specific population can beperformed via the open fontanelle or through the non-ossified posterior aspectsof the lum-bosacral spine. However, with the removal of a portion of theoverlying cranium or vertebral elements in the operating room, sonography againbecomes a viable imaging option. 1-10

We present a series of cases that illustrate and review the various uses ofintra-operative pediatric neurosonography. In particular, the procedure canlocalize and characterize lesions, facilitate biopsy or drainage procedures,and aid in catheter placement. Color Doppler imaging also offers theopportunity of vascular characterization of lesions, an option that isparticularly helpful during embolization procedures.

Preoperative preparation

In order to maximize the efficacy of intraoperative neurosonography,presurgical planning is needed. Review of pertinent preoperative scans andknowledge of the operative plan are important first steps for the radiologistand surgeon. The choice of sonographic equipment to best reach the operativegoal also needs to be determined. Multiple types and sizes of transducers(ranging from 3 to 7 MHz) are available for pediatric sonography. Although thecharacter, size, and location of the lesion in question are certainly importantfactors, often it is the size of the craniotomy, laminectomy, or burr hole thatactually dictates the choice of transducer. Clearly, presurgical planningbetween the neurosurgeon and radiologist is crucial for the successful use ofintraoperative neurosonography.

Linear array transducers offer excellent near-field resolution and areparticularly useful for cranial or spinal lesions that are near the surface.The linear array transducer also is preferred for most intraoperative spinalprocedures. However, this transducer does have a fairly wide aperture,requiring a bony opening of at least 5 cm ´ 1.5 cm. Phased sectortransducers have a smaller aperture (some pediatric probes only measure 1.5 cm´ 1.5 cm), making this transducer particularly attractive when a smallercraniotomy or burr hole will be available. In addition, the phased sectorscanner has the ability to scan a wider field and at a greater depth. Bothtypes of transducers usually are equipped to offer Doppler (color and duplex)as well as conventional realtime, gray-scale imaging.

Operative sonographic technique

Prior to entering the operating room, the ultrasound machine and theselected transducer are cleaned with alcohol. Once in the operating room andafter proper operative exposure has been obtained, the device and its viewingscreen are positioned near the operative site. The radiologist, in sterile gownand gloves, places a sterile cover with sterile coupling gel over thetransducer. An ultrasound technologist assists in the case, adjusting thecontrols of the ultrasound machine and obtaining hard copy images of the case.

Extreme care must be taken during actual transducer contact with the neuraltissue. The neurosurgeon fills the operative site (either cranial or spinal)with sterile saline. This "water bath" acts as a carrier medium forthe sound waves of ultrasound. The transducer is placed into the "waterbath" but care is taken to avoid actual contact with the neural surface,if possible (particularly for spinal tissue). Brain tissue is more forgiving,and very light transducer contact with the cranial neural surface isacceptable.

Although procedures can be performed under direct ultrasound guidance (suchas cyst aspirations or difficult catheter placements), the exact location oftumors or fluid collections are "marked" for surgical exploration.Any sterile surgical instrument can "mark" the location; we havefound that a sterile plastic angiocath works well, particularly in the waterbath surrounding the spinal cord.

The time requirement for each case varies. The US team is "oncall" to the OR and may spend anywhere from 15 to 30 minutes inpreparatory time prior to the 5 to 10 minutes of scanning time. The US teameither waits or returns for follow-up scans after tumor resection or abscessdrainage, which may add an additional 30 minutes.

Certain pitfalls and limitations must be considered in order to optimize theintraoperative sonographic experience. Prior knowledge of the lesion inquestion is clearly vital, and other imaging modalities taken previously needto be studied prior to intraoperative sonography. Knowledge of patientintraoperative positioning is also important. By the time the radiologistenters the operating room, the patient is already draped. Sonographicorientation may therefore be difficult if the radiologist is not aware of thepatient's position under the sterile drapes. Having the proper transduceravailable for the operative site is also crucial. Attempting sonographicguidance with a 5-cm linear transducer through a 1-cm burr hole is futile.Lastly, the importance of having an experienced assistant must be stressed.Because the radiologist must don sterile gown and gloves, an experiencedtechnologist is needed to run the machine and handle technique adjustments,Doppler settings, focal zones, etc. Intraoperative pediatric neurosonographiccases Catheter procedures: cranial-Ventriculoperitoneal (VP) shuntlocalization: Most VP shunts do not require sonographic guidance, butoccasionally US can aid in problem cases.

Case 1: US assistance was requested in this infant with hydrocephaluswhen the plain radiograph of the skull suggested an unusual shunt tip position.Sonographic imaging via the anterior fontanelle demonstrates the tip of thecatheter to be anterior to the frontal horn (figure 1, arrows).

Catheter procedures: spinal- Syringo-subarachnoid shuntplacement: Sono-graphic guidance is important for exact placement of asyringosubarachnoid shunt in children with syringomyelia because multiple shuntpasses is not desirable in the spine.

Case 2: In this child with Chiari-II malformation, a large syrinx waspresent in the upper thoracic spinal cord which markedly decompressed followingsyringosubarachnoid shunt placement (figure 2).

Case 3: This child had a posttraumatic syrinx in the lower cervicalspine. Initial sonographic attempts to localize the syrinx revealed aninadequate operative exposure with bone still covering the majority of thesyrinx. The operative site was further exposed, with subsequent sonographiclocalization of the syrinx and successful placement of the syringosubarachnoidshunt (figure 3).

Lesion localization: cranial lesions- Case 4: Sonography wasused for localization of a cerebral metastatic lesion in this 12-year-oldpatient with melanoma. The lesion could not be seen from the cortical surface.Once it was located by ultrasound, it was surgically resected (figure 4).

Case 5: Sonography was used to locate the tumor portion of amultiseptated cystic astrocytoma in the motor strip. In addition to locatingthe tumor nodule, sonography determined the entry point into the cyst (byfinding the smallest distance from the dural surface to the cyst, therebyminimizing damage to the adjacent motor strip) (figure 5).

Case 6: Small lesions can be difficult to locate surgically. Hence,this 1-cm cavernous hemangioma in the parietal lobe was located sonographicallyto facilitate its removal (figure 6). Lesion localization: spinal lesions-Exact tumor location via sonographic guidance is extremely desirable in thespine to limit surgical time and handling.

Case 7: A cavernous hemangioma in the cervical spinal cord waslocated sonographically for surgical resection (figure 7).

Case 8: Sonography was used to locate a cervical spinal cordastrocytoma for surgical resection (figure 8). Facilitation ofbiopsy/aspiration

Cranial- Sonography can be used not only to locate a cerebralabscess but to guide the aspiration as well. Sonographic imaging followingaspiration also can determine the success of the procedure.

Case 9: A 3-year-old child had an abscess collection (initially seenon CT) that was localized for intraoperative aspiration (figure 9).

Case 10: A 4-year-old girl presented with seizures and fever. A CTscan revealed two fluid-filled cerebral collections (figure 10).

Spinal-Case 11: An 8-year-old girl had a previous resection ofan astrocytoma in the cervical spinal cord. She returned months later withweakness in her arms. MRI examination revealed an enlarged cervical cord but nofocal lesion. Because of the possibility of diffuse infiltration of the spinalcord by tumor, intraoperative biopsy of the cervical spinal cord was plannedwith sonographic guidance. (figure 11).

Vascular characterization: cranial- Case 12: A 6-month-old boyhad a vein of Galen aneurysm which was treated with embolization. During anembolization procedure, sonography with color Doppler was able to assess theocclusion of flow within the lesion (figure 12). AR