Soft tissue deficiency in the temporal fossa can produce cosmetic impairment and can result from the transposition of temporalis myofascial flaps and tumor debulking procedures, among other surgeries. The contours of the temporal fossa can be augmented using implants, such as prefabricated porous high-density polyethylene (Fig. 4.3), silicone (Fig. 4.4), and methyl methacrylate (Fig. 4.5). The implants are usually inserted via a hemicoronal approach and can be secured using titanium screws to the underlying bone.

Mohs micrographic surgery is a technique that enables skin neoplasms to be fully resected while maximizing preservation of normal tissues. Mohs surgery consists of removing the tumor via sequential thin sections, which are concurrently examined under the microscope. The process is repeated until no remaining tumor is identified microscopically. When discernible on imaging, the defects characteristically appear as well-defined cavities in the skin and underlying soft tissues (Fig. 4.6). Large defects can be reconstructed using split-thickness skin grafts (Fig. 4.7), flaps, or synthetic materials such as AlloDerm.

Galeal flaps, such as the retroauricular rotation flap, can be used to cover scalp defects as large as 60% of the scalp surface area. Galeal flaps are comprised of fascia, subcutaneous tissue, and vascular components. In the early postoperative period, a remote donor site defect can be apparent, such as with “flip-flop” flaps (Fig. 4.8). Galeal flaps can incur essentially the same complications as other types of flaps, including infection, tumor recurrence, and necrosis, as well as dehiscence and alopecia.

Free flap transfer is used for repairing complex scalp defects in order to provide functional, cosmetic, and structural support when the use of skin grafts, locoregional flaps, and tissue expanders is not feasible. The latissimus dorsi myocutaneous flap is particularly useful for subtotal and total skull reconstruction, in which there is considerable dead space (Fig. 4.9). Latissimus dorsi flaps can be harvested with ribs (myo-osseocutaneous) or combined with titanium mesh for added support. Omental flaps are another option for closing large scalp and cranium defects (Fig. 4.10). These contain mostly adipose tissues and are covered by skin grafts. Other donor tissues for free flap transfer include rectus abdominis muscle flaps, scapular flap, radial forearm flap, and anterolateral thigh flap. Vascular supply is typically obtained via anastomosis to the superficial temporal artery and vein or at times the occipital artery. Complications include delayed flap failure, which requires secondary reconstruction, neck hematoma, venous thrombosis, skull base infection, large wound dehiscence, small wound dehiscence, donor site hematoma and seroma, and cerebrospinal fluid leak.

Most recurrent tumors manifest within the first 2 years following resection and are most common in the tumor bed near the anastomosis. Imaging can detect both locoregional recurrence and the extent of intracranial extension of recurrent tumors (Fig. 4.11). Postoperative imaging can also help in distinguishing tumor recurrence from fibrosis related to the surgery or radiation therapy. Recurrent tumors typically appear as nodular foci of enhancement and have higher T2-weighted signal intensity on MRI than fibrosis.

Burr hole craniostomy is a commonly performed maneuver as part of creating craniotomy flaps, stereotactic biopsy, hematoma decompression, ventricular endoscopic procedures, insertion of ventricular catheters, drains, and electrode insertion. Burr holes are surgical defects that traverse the full thickness of the calvarium created using various drills and can be packed with a variety of materials, such as bone wax and methyl methacrylate and may be covered with a plate (Fig. 4.12). Linear enhancement along the edges of burr holes is commonly observed as vascular granulation tissue forms, thereby potentially mimicking abscesses or neoplasms (Fig. 4.13).

Craniotomy consists of opening the cranial vault by removing a bone flap during the course of surgery and replacing it at the completion of the procedure, as opposed to craniectomy in which the bone is removed and not replaced. Once the skull is exposed by raising the overlying scalp and pericranial flap, burr holes are drilled, from which the bone flap is created using a saw or drill. The bone flap is set aside during the procedure and replaced upon completion of the surgery. Some of the standard types of craniotomy include the following:

Skin staples are often used to close the scalp flap after surgery and can be present on imaging during the early postoperative period (Fig. 4.14). A variety of devices and methods are available to secure cranial bone flaps following craniotomy. The most commonly used are microfixation plates or clamps (Fig. 4.15). Microfixation plates are often composed of titanium, which produces minimal streak or susceptibility artifact. Complications related to the presence of microfixation plates and screws, such as transcranial migration, are uncommon, except in young children, where absorbable hardware or suture may be used. Stainless steel wires threaded across the craniotomy margin to the bone plates is no longer performed in developed countries, but may still be encountered on imaging (Fig. 4.16).

Hinge craniotomy is an alternative to traditional decompressive craniectomy and allows swollen brain parenchyma to expand extracranially while avoiding some of the complications associated with cranial revision. The bone flap is left attached on one side to the scalp soft tissues, usually the temporalis muscle. This results in the appearance of an outwardly displaced bone flap on CT (Fig. 4.17).

The normal imaging appearance of the craniotomy site can evolve over time. In the early postoperative period, the bone flap margins are sharp and should align precisely with the rest of the skull, unless a craniectomy was done to provide more surgical accessibility. The extent of dural enhancement that normally occurs after craniotomy ranges considerably. Dural enhancement is present in the majority of patients following cranial surgery and can be seen within the first postoperative day (Fig. 4.18). The presence and degree of enhancement are largely dependent on the time elapsed since the time of surgery and the types of substitutes used during closure, which in some cases can persist indefinitely. Dural enhancement tends to be apparent earlier and lasts longer with gadolinium-based contrast than with iodine-based contrast. Granulation tissue also normally forms along the edges of the bone flap, which manifests as linear enhancement that is often visible on MRI and often accompanies dural enhancement. This type of enhancement usually persists up to 1 year following craniotomy.

Pneumocephalus is the presence of intracranial air and is an expected finding after recent cranial surgery. Indeed, virtually all patients exhibit some degree of pneumocephalus during the immediate postoperative period. Pneumocephalus can be identified by air attenuation on CT and signal voids on MRI (Fig. 4.19). Regardless of the location, pneumocephalus normally resolves within 3 weeks of surgery.

During the early postoperative period, typical changes also occur in the soft tissues overlying the craniotomy site, including temporalis muscle swelling (Fig. 4.20), which likely represents edema due to manipulation during surgery.

Intraoperatively fashioned acrylic cranioplasty is composed of methyl methacrylate resin that can be molded to a desired shape on the surgical field. The material tends to appear heterogeneous on CT and low signal on MRI (Fig. 4.21). The material often contains multiple foci of air due to the exothermic reaction that occurs when formed. The trapped bubbles should not be confused with infection. The methyl methacrylate at times can migrate prior to hardening, the appearance of which can potentially mimic hemorrhage on CT.

Preformed acrylic methyl methacrylate plates are specially molded to fit individual craniectomy defects using a computer-aided design system and 3D CT data. These flaps are usually secured to the adjacent calvarium using titanium plates and are thus continuous with the calvarium. On CT, preformed acrylic plates demonstrate homogeneous intermediate attenuation, around 100 HU (Fig. 4.22). Unlike the acrylic plates prepared intraoperatively, the preformed plates do not contain air bubbles. However, the preformed acrylic plates contain holes that are drilled to promote tissue ingrowth and leave a pathway to prevent accumulation of fluid in the epidural space.

Hydroxyapatite cement paste is sometimes used to close off small gaps in the calvarium. It can be molded to match the particular anatomy and can be applied in conjunction with other materials, such as titanium mesh. The cement appears as homogeneously hyperattenuating on CT, similar in attenuation as natural bone (Fig. 4.23). On MRI, the hydroxyapatite appears as a signal void.

Titanium mesh is commonly used as a cranioplasty material. These plates generally produce good cosmetic results and minimal discomfort. Titanium produces minimal streak artifact on CT (Fig. 4.24). Solid titanium plates are now infrequently used but may be observed on follow-up imaging (Fig. 4.25).

Porous polyethylene implants are also suitable for covering cranial defects. This type of implant is also custom created for each patient using a computer-aided design system and 3D CT data. The material enables soft tissue and bone ingrowth and displays low attenuation on CT and low signal intensity on T2- and T1-weighted MRI sequences (Fig. 4.26).

Synthetic bone grafts that are biocompatible have been developed for cranioplasty. For example, Bioplant HTR Synthetic Bone is a microporous composite of poly(methyl methacrylate) (PMMA), polyhydroxyethylmethacrylate (PHEMA), and calcium hydroxide. On CT, the HTR cranioplasty appears as heterogeneous with an overall attenuation that is greater than soft tissue but lower than bone (Fig. 4.27).

Autologous bone grafts can also be used for cranioplasty. These are often used in the form of split calvarial grafts in which the inner table is separated from the outer table in order to increase surface area for maximal coverage of a craniectomy defect (Fig. 4.28).

Several options are available for storing autologous bone flaps after decompressive craniectomy for delayed autocranioplasty. Often, bone flaps are stored in a sterile freezer. This approach preserves the bone flap very well, providing good cosmetic results. An alternative is subcutaneous storage, which allows the bone flap to stay with the patient and may decrease the risk of devitalization and/or infection of the flap. Bone flaps can be kept fresh by implanting them into subcutaneous pockets in the abdomen and may be encountered on imaging (Fig. 4.29). Bone grafts stored in this manner tend to undergo remodeling.

Decompressive craniectomy is performed in order to decrease intracranial pressure when medical management alone is insufficient and is most commonly used in the setting of traumatic brain injury, subarachnoid hemorrhage, intraparenchymal hemorrhage, and cerebral infarction. The procedure consists of removal of portions of the skull, which is not replaced during the procedure. Hemicraniectomy is performed when one side of the brain is affected and usually entails removal of bone in the frontoparietotemporal region. On the other hand, bilateral (bifrontal) craniectomy is performed when both sides of the brain are affected and consists of removing the calvarium of the anterior cranial fossa to the coronal sutures.

Decompressive craniectomy has characteristic imaging findings. Following craniectomy, the galea aponeurotica becomes juxtaposed against the dura. Scar tissue then forms between these two layers, which creates the meningogaleal complex beneath the subcutaneous tissues. Normally, this complex measures between 2 and 6 mm in thickness. On CT, the meningogaleal complex is slightly hyperattenuating, while the appearance on MRI is variable depending on the degree of hypervascular tissue that is incorporated (Fig. 4.30). Smooth, uniform enhancement is expected on CT and MRI.

Traditional management of encephaloceles consists of resecting the abnormal herniated brain tissue and closing the wound via primary intention if there is sufficient skin available (Fig. 4.37), with the goal of avoiding new deficits related to extended surgery. Cranioplasty can be performed at the same time as the primary surgery or at a later time, using materials such as methyl methacrylate, hydroxyapatite bone cement, demineralized bone matrix, and autologous grafts, which can be harvested from the adjacent calvarium, for example (Fig. 4.38). If a substantial amount of functional brain tissue has herniated through the encephalocele sac, expansile cranioplasty can be performed. This technique consists of reconstructing the calvarial defect with autologous bone graft harvested from the adjacent parietal region. This approach effectively enlarges or extends the intracranial cavity to encompass the herniated brain tissue.

Several types of topical absorbable hemostatic agents are available for neurosurgical procedures, including cellulose-, gelatin-, and collagen-based agents and thrombin and fibrin glue.

Oxidized regenerated cellulose, such as Surgicel, is available in the form of a fabric that can be used to line the margins of resection cavities or packed tightly to control a more focal source of bleeding. Implanted oxidized cellulose has been reported to mimic abscesses and masses on postoperative imaging. On CT, oxidized cellulose often displays low attenuation, and on MRI, it usually shows low signal on T2, but variable T1 signal (Fig. 4.40). Sometimes, the presence of high T1 signal can potentially mimic residual tumor on contrast-enhanced images. The hemostatic agent ultimately resorbs over the course of months.