Stereotactic biopsy is an image-guided procedure that is commonly performed to obtain tissue samples of intracranial lesions. Hemorrhage is one of the most common findings after stereotactic brain biopsy, occurring in up to 9% of cases. Hemorrhage along the biopsy trajectory has a characteristic linear configuration. Small amounts of blood products along the path of the biopsy that may only be discernible on T2* GRE or SWI sequences are usually of no clinical concern and resolve spontaneously. Rather, such findings serve to delineate the path of the biopsy needle and can help account for new neurological deficits (Fig. 5.7). In addition, the blood products left along the biopsy path that are apparent on MRI can also serve as a useful indicator of whether the lesion was appropriately sampled. Nevertheless, some operators prefer to insert a metal marker in the biopsy cavity as a reliable indicator that is visible on imaging (Fig. 5.8). Although off-target biopsy can yield tumor cells if the lesion is an infiltrative tumor, the grade may be underestimated. Ideally, biopsy of the enhancing portion of the tumor with the highest cerebral blood volume (CBV) on perfusion-weighted MRI should be performed. Another uncommon, but notable complication of stereotactic biopsy is tumor seeding (Fig. 5.9). Otherwise, a mild degree of enhancement in the brain parenchyma along the biopsy path is often encountered on early postoperative imaging as an incidental finding that typically resolves within a couple of months (Fig. 5.10).

The space that remains after a tumor is surgically removed is known as the resection cavity. The resection cavity is often lined or packed with hemostatic agents (refer to Chap. 4) and contains variable amounts of cerebrospinal fluid and blood products, especially during the early postoperative period (Fig. 5.11). Oftentimes, resection cavities eventually shrink and collapse, becoming nearly imperceptible (Fig. 5.12), although some cavities stay the same size, particularly if they communicate with the ventricular system.

Variable amounts of tumor may remain adjacent to the cavity depending on whether gross total, near-total, or subtotal resection was performed. The extent of tumor resection depends on several factors, including the location and type of tumor. Tumors that involve eloquent parts of the brain, that are in technically difficult areas to reach, or that involve critical structures, such as cranial nerves or major arteries, can limit the extent of tumor resection. Similarly, it is more difficult to achieve complete resection of infiltrative tumors than well-defined tumors. Ultimately, there is often a trade-off between removing as much tumor as possible versus preserving as much normal tissue and avoiding complications. Comparison with preoperative imaging should be performed when possible to help identify residual tumor.

Surgically induced parenchymal injury, postoperative hemorrhage, and enhancing conditions related to brain tumor surgery and adjunctive treatments are discussed in the following sections.

Vasogenic edema can result from forceful intraoperative retraction, which is sometimes performed in order to access large or deep tumors. The edema may be related to hyperemia of the brain surface after the release of retraction. On imaging obtained during the early postoperative period, retraction-induced vasogenic edema appears as swelling along the path of the retractors (Fig. 5.14). Unlike acute infarction, the vasogenic edema demonstrates elevated diffusivity rather than restricted diffusion.

A peculiar complication related to posterior fossa tumor resections is hypertrophic olivary degeneration, which results from disruption of the dentato-rubro-olivary pathway (Guillain-Mollaret triangle). This phenomenon can occur after surgical resection of cerebellar tumors. If the resection site involves the central tegmental tract, the ipsilateral olivary nucleus is affected, while if the superior cerebellar peduncle is involved, the contralateral olivary nucleus is affected. Thus, bilateral hypertrophic olivary degeneration results from disruption of the central tegmental tract and superior cerebellar peduncle. Tongue fasciculations are characteristic of hypertrophic olivary degeneration. On MRI, hypertrophic olivary degeneration manifests as T2 hyperintensity with or without enlargement of the anterolateral medulla (Fig. 5.15). The differential diagnosis includes ischemia, demyelination, tumor spread, and infection. The lack of enhancement with hypertrophic olivary degeneration may help differentiate this entity from the other possibilities, such as some neoplasms. In addition, most cases demonstrate associated atrophy of the contralateral dentate nucleus or cerebellar cortex. Signal changes on MRI develop approximately 1 month after surgery and can persist for many years. Hypertrophy of the olivary nucleus tends to develop after several months and can resolve after 2–3 years.

Chronic hemorrhage after surgery can result in superficial siderosis and mainly occurs when there is a cystic cavity that contains friable vessels or residual/recurrent tumor (Fig. 5.18). Hemosiderin deposits can coat remote leptomeningeal surfaces, particularly the cerebellum and brainstem. On CT, superficial siderosis can appear as a mildly hyperattenuating coating of these structures, which may also become atrophic as a result. MRI is more sensitive for depicting hemosiderin deposits, which appear as very low signal intensity on all sequences. Blooming artifact on T2* GRE or SWI sequences accentuates the lesions. The significance of superficial siderosis is that it may cause symptoms such as ataxia and deafness.

Intrathecal chemotherapy can lengthen survival and alleviate symptoms in patients with widespread leptomeningeal metastases. The two primary means of delivering intrathecal chemotherapy are Ommaya reservoirs and repeat lumbar puncture. Ommaya reservoirs are implanted in the subcutaneous tissues of the scalp and contain a pump mechanism for drug delivery agents into the ventricular system through an intraventricular catheter (Fig. 5.24). Ommaya reservoirs offer many advantages over repeat lumbar punctures, including greater patient comfort, diminished risk for patients with thrombocytopenia, more consistent drug levels, and possibly greater clinical efficacy. Tumor cyst devices are similar to Ommaya shunts, but are used to inject chemotherapeutic agents directly into tumors.

Infection is a major complication of Ommaya catheter placement. The incidence of Ommaya-associated infection is 15% within the first year of placement (range 2–23%). Staphylococcus aureus and Staphylococcus epidermidis are the most common causative organisms. Manifestations of catheter-associated infection range from meningitis to abscess, for which imaging is useful for identifying fluid collections surrounding the catheter (Fig. 5.25). Debris in the fluid and enhancement helps differentiate infection from hygromas (simple fluid collections), which can also occur with Ommaya catheter placement. Management consists of antibiotic therapy and possible hardware removal and debridement depending on the extent of the infection.

Focal brain necrosis due to chemotherapy extravasation secondary to Ommaya reservoir catheter obstruction is rare, with an incidence of 0.6% of patients. This condition is caused by displacement of the catheter tip into the brain parenchyma. Imaging demonstrates circumferential areas of necrosis surrounding the retracted Ommaya catheter, manifesting as patchy enhancement, high T2 signal, and restricted diffusion, representing cytotoxic edema (Fig. 5.26). A unique and serious complication of methotrexate extravasation is progressive leukoencephalopathy. This entity involves the white matter diffusely and can be either hemorrhagic or nonhemorrhagic.

Cerebrospinal fluid cysts can sometimes form around Ommaya catheters and may be caused by distal shunt obstruction, although this complication can also occur when the catheter is appropriately positioned, with or without hydrocephalus. The pericatheter cysts do not have perceptible walls or rim enhancement, but may have surrounding edema. Although the cysts may be asymptomatic, it is important to evaluate for predisposing factors that could be addressed, such as malpositioning of the Ommaya catheter (Fig. 5.27).

Chemotherapy wafers, such as carmustine implants (Gliadel), are sometimes implanted in the surgical bed after malignant brain neoplasm resection. The wafers are biodegradable sheets of polymers that are impregnated with the chemotherapy agent. Initially, the wafers appear as hypointense linear structures on T1- and T2-weighted MRI sequences, but they can change in signal intensity characteristics over time (Fig. 5.28).

The presence of wafers does not alter the pattern of tumor recurrence. Perfusion MRI is particularly useful to monitor the treatment effects and differentiate these from recurrent neoplasm. The presence of foci with elevated CBV suggests tumor recurrence. MR spectroscopy can also be useful for monitoring tumor response to chemotherapy wafers. For example, it has been noted that increased peritumoral NAA/Cr and decreased peritumoral Cho/NAA compared with normal brain tissue by 3–5 weeks suggest treatment response.

Local radiation therapy can be administered for treatment of brain tumors via brachytherapy (interstitial) seed implantation. The seeds contain radioactive isotopes, such as I-125, and can be implanted temporarily (for approximately 1 week) or permanently. Temporary seeds are typically introduced and removed using plastic catheters passed through burr holes, while permanent seeds are implanted in the tumor/surgical bed. The seeds appear as tiny metallic cylinders on CT or signal voids on MRI (Fig. 5.29).

The GliaSite radiation therapy system is used to administer intracranial brachytherapy for brain tumor treatment. The system is a catheter-based device that consists of an infusion port on one end and a double balloon on the other. Positioning markers are also included along the length of the catheter. The balloon, which contains the radioactive isotope solution, is positioned within the surgical cavity. The filled balloon is hyperattenuating on CT and displays fluid signal on MRI (Fig. 5.30). Normally, there can be enhancement in the tissues surrounding the balloon. The catheter and its position markers are also visible on both modalities. However, the balloon and surrounding tissues are better assessed on MRI, particularly when there is tumor recurrence. Perfusion MRI is especially helpful for evaluating enhancing lesions in the tumor bed, whereby elevated rCBV suggests recurrence of high-grade tumor.

Prefrontal lobotomy (leucotomy) is a now obsolete procedure that was introduced in 1935 as a treatment option for psychiatric illnesses, such as schizophrenia. The procedure essentially consists of ablating the frontal lobe white matter tracts using a probe-like device known as the leukotome via a transorbital or transcranial approach. This produces the appearance of band-like cavitary lesions in the frontal lobe white matter (Fig. 5.31). On MRI, FLAIR sequences show a hyperintense rim of gliosis surrounding the cavitary defects. Focal atrophy of the frontal lobe and corpus callosum is common and often pronounced. High-attenuation foci on CT and susceptibility effects on MRI can be observed along the lobotomy margins, which correspond to residual Pantopaque used for visualization of the lobotomy plane during the operation.

Pallidotomy is a procedure that can be performed in Parkinson’s disease patients who do not experience adequate symptom relief from medical therapy. The surgery consists of introducing probes via frontal burr holes for ablation of the posteroventral portion of the globus pallidus interna (Fig. 5.32). The goal of the procedure is to interrupt excessive inhibitory output from the basal ganglia. On CT, the pallidotomy lesions appear as hypoattenuating foci of encephalomalacia that become more pronounced over time. On MRI, acute pallidotomy lesions are usually hyperintense centrally on T1 and hypointense centrally on T2 due to hemorrhage surrounded by a rim of T2 hyperintensity and hypointensity on T1 and GRE, which represents edema. Restricted diffusion due to focal cytotoxic edema can also be encountered. Eventually, the lesion-edema complex evolves into a smaller focus of low T1 signal and high T2 signal. Lesion sizes can be variable depending upon technique implemented.

Cingulotomy is a form of psychosurgery that is used to treat conditions, such as intractable obsessive-compulsive disorder. The procedure can be performed in a minimally invasive manner via thermal ablation. This process results in necrosis of the surrounding brain tissue, which appears as concentric rings of signal abnormality (Fig. 5.33). There can be T1 hyperintensity due to petechial hemorrhage, as well as T2 hyperintensity from edema and restricted diffusion due to ischemia during the early postoperative period, which then evolves over time. Diffusion tensor imaging is also useful for confirming interruption of the cingulum. In particular, the dorsolateral region of the cingulotomy lesion is associated with improved behavior.