6.2.3\ Intraventricular Fat Migration

6.2.4\ MRI-Induced Programmable

Valve Setting Alteration

6.2.4.1\ Discussion

Recurrent hydrocephalus in patients with indwelling ventricular shunts is a sign of shunt failure. Of note, high-field-strength MRI can alter the pressure setting of most percutaneous programmable cerebrospinal fluid shunts and may also result in acute hydrocephalus, mimicking shunt malfunction, if the setting is not

checked and reset after the scan (Fig. 6.33). The accumulation of cerebrospinal fluid can lead to ventricular enlargement, unless there is extensive preexisting ventricular scarring that limits ventricular expansion. Enlargement of the temporal horns is among the earliest findings of this complication. Other signs include effacement of the sulci and transependymal flow of cerebrospinal fluid. Hydrocephalus can result in sutural diastasis and enlargement of cranial diameter in children.

Fig. 6.33  MRI-induced programmable valve setting alteration. The patient with a percutaneously programmable cerebrospinal fluid shunt, presented acutely obtunded after undergoing MRI at 1.5T the previous day. The pressure settings were not checked following MRI. The following day, the patient was minimally responsive and was noted that the pressure setting changed from 0.5 to 2.5 on

the Medtronic Strata valve. This change was presumably secondary to the magnetic field. Axial T2-weighted MRI (a) shows no ventricular dilatation. Shunt survey (b) obtained before the MRI shows a pressure setting of 0.5. Axial CT image (c) obtained the day after the MRI shows acute massive hydrocephalus and the subsequent shunt survey (d) shows a pressure setting of 2.5

6.2.5\ Ventricular Loculations

and Isolated Ventricles

6.2.5.1\ Discussion

The formation of loculations of cerebrospinal fluid within the ventricular system can lead to shunt failure. The compartmentalized collection of cerebrospinal fluid can lead to symptoms of hydrocephalus and may be caused by adhesions from prior hemorrhage or infection, for example. CT ventriculography performed by injecting contrast into the shunt catheter can be used to delineate the presence of loculations by the lack of communication of the contrast material with the rest of the ventricular system (Fig. 6.34). Similarly, an isolated or trapped ventricle is an uncommon phenomenon that can occur in the setting of ventricular shunting with adhesion formation and represents a form of focal hydrocephalus. The significance of this complication is that

it can exert mass effect upon surrounding structures. On imaging, disparate sizes of the ventricles are apparent, and contrast does not enter the trapped ventricle on CT ventriculography if the contrast is injected into the other portions of the ventricular system (Fig. 6.35). The level of obstruction is often at the foramen of Monro, but can occur anywhere in the ventricular system. Treatment may consist of ventricular catheter repositioning, septostomy, foramen of Monro reconstruction, or implantation of a catheter into the affected ventricle. Isolated ventricles that are not enlarging can be difficult to differentiate from asymmetric ventricles, which may also be encountered after shunting and do not require treatment. Midline shift and progressive increase in size of the ventricle suggest trapping over simple asymmetry of the ventricles. If there is any doubt, short interval imaging follow-up can be performed.

Fig. 6.34  Ventricular loculation. Axial CT ventriculogram shows the injected contrast confined to a cystic compartment (arrow)

Fig. 6.35  Isolated fourth ventricle. Sagittal CT ventriculogram shows the injected contrast confined to the third and lateral ventricles and a markedly expanded fourth ventricle