6.1\ Types of Procedures

6.1.1\ External Ventricular Drainage

6.1.1.1\ Discussion

External ventricular drains (EVD) are used for a variety of purposes, including temporary decompression of an enlarged ventricular system and acute hydrocephalus from tumor obstruction in order to better define the resection or following subarachnoid hemorrhage. An EVD catheter is

inserted into the ventricular space via a transcranial approach after creating a burr hole along the coronal suture at the mid-pupillary line or secondarily along the parieto-occipital junction one-­ third of the way from the ear to the vertex. Imaging may be performed to assess the status of the ventricular system, as well as to evaluate for complications, which include infection, hemorrhage, excess drainage, catheter obstruction, cerebrospinal fluid leak, and malpositioning, which may require repositioning.

6.1.2\ Ventriculoperitoneal

(VP) Shunts

6.1.2.1\ Discussion

VP shunting consists of diverting cerebrospinal fluid from an intracranial compartment to the peritoneum via a catheter and is commonly performed to treat hydrocephalus. VP shunt devices consist of a ventricular catheter, valve, and a distal catheter (Figs. 6.1 and 6.2). The catheter portion of a VP shunt is composed of extruded Silastic tubing impregnated with a radiopaque material, such as barium, in order to confer conspicuity on radiographic studies. Integrated reservoirs can also be added to the proximal shunt catheter, which enables percutaneous access and testing of the shunt system. The built-in reservoirs are usually positioned within the subgaleal space (Fig. 6.3).

Programmable valves contain radiopaque chiral markers that enable the valve opening pressure setting or performance level to be determined radiographically (Figs. 6.4, 6.5, 6.6, and 6.7). Some models have devices that allow these settings to be determined without radiographs. Antisiphon devices are also incorporated into some models in order to prevent cerebrospinal fluid overdrainage, when the patient is upright. While programmable shunts are generally MRI compatible up to 3T, there is a potential risk for inadvertent change of settings during MRI scanning. Thus, it is imperative to verify the settings following MRI. The pressure settings can be adjusted noninvasively using a magnetic tool. Furthermore, recent innovations have made available programmable valves that are resistant to environmental magnetic influences.

Gliosis often forms around the ventricular shunt catheter tract, but generally does not have clinical significance. The gliosis typically appears as circumferential low attenuation on CT and high signal on T2-weighted MRI measuring up to several millimeters in thickness (Fig. 6.8).

Up to one-third of VP shunts fail within 1 year of placement, and shunt revision is necessary in

up to 70–80% of patients during their lifetime. Overall, programmable VP shunts have a similar failure rate as standard shunts. However, the pressure adjustment capability of programmable VP shunts leads to patient improvement in over 50% of cases. Complications include the following, for which examples are depicted later in this chapter:

•\ Infection (most common: 5–47%)

•\ Obstruction (usually proximal: emergency condition due to resulting increased ICP)

•\ Subcutaneous cerebrospinal fluid collections •\ Catheter disconnection/migration/retraction

(anywhere from mouth to anus!) •\ Incisional hernia

•\ Bowel obstruction/volvulus •\ Viscus perforation

•\ Cerebrospinal fluid pseudocysts •\ Conduit for metastatic spread

Imaging plays an important role in evaluating patients with VP shunts. Radiographic shunt series are commonly performed as an initial screening for suspected shunt failure. However, these studies are less sensitive than cross-­ sectional imaging modalities. Nuclear medicine shunt studies are uncommonly performed but can be used to assess for shunt patency. Radiotracer, usually Tc-99m DTPA or In-111 DTPA, is injected into to the reservoir (Fig. 6.9). Normally, the radiotracer material spills freely throughout the peritoneal cavity. A focal collection of radiotracer suggests the presence of a pseudocyst. Reflux may normally occur into the ventricles and the reservoir emptying half-time of less than 10 min, although this may vary depending on the type of shunt. A similar concept for evaluating shunt patency is the “shuntogram,” which involves injection of contrast material into the shunt valve, and tracking the flow of the contrast via serial radiographs of the cranial, chest, and abdominal components of the shunt system is obtained over the course of approximately 15 min.

Fig. 6.2  Shunt series. Selected radiographs (a–c) show the proximal portion of the shunt catheter overlies the lateral ventricle (arrow); exits through a burr hole; tunnels into the subcutaneous tissues of the head, neck, chest, and

abdomen (arrow); and terminates within the peritoneal cavity (arrow). Radiolucent portions (encircled) of the shunt should not be mistaken for discontinuities

Fig. 6.3  Delta 1.5 valve VP shunt. Lateral skull radiograph (a) and 3D CT (b) images demonstrate the reservoir component (arrows) of the VP shunt containing performance level markers. Axial T2-weighted (c) and coronal

post-contrast T1-weighted (d) MR sequences show the cerebrospinal fluid-filled reservoir (arrows) positioned in the subgaleal space

Fig. 6.5  Strata valve programmable shunt. Lateral radiograph (a) with magnified view (inset) of the VP shunt valve (encircled). The pressure setting can be read on the

radiograph, but not on the axial CT image (b). The magnetic components of the programmable shunt produce extensive susceptibility artifacts on MRI (c)

Fig. 6.8  Catheter-associated gliosis. Axial FLAIR MRI shows circumferential high signal surrounding the shunt catheter tract (arrow)

Fig. 6.9  Patent shunt catheter depicted on a nuclear medicine shunt study. Sequential 99mTc DTPA shunt images obtained over a 30-min period after injection of radio-

tracer into the ventricles via a shunt catheter show unimpeded passage of radiotracer from the ventricular system into the peritoneal cavity