Implanted medical devices such as neurostimulators,cardiac pacemakers, cochlear implants, and infusionpumps have become common.
Implanted medical devices such as neurostimulators, cardiac pacemakers, cochlear implants, and infusion pumps have become common. Many are now used in patients with head injuries or brain disease.
Such in situ devices can sometimes hinder radiological image interpretation, and radiologists need to be aware of the material composition of new products. Knowledge of the specific name of each device is not important. It is, however, necessary to recognize the presence of a device, its position and function, and be aware of any complications related to its use (see table).1 It is also vital to know the device's compatibility with a magnetic field for MRI studies.2 A product is considered safe for use in a magnetic field if it has been shown to present no additional risk to the patient or other individuals through displacement or deflection force, torque or rotational force, and/or induced heating.3
Intracranial devices can be grouped in many ways, according to their location, function, or material. Radiologists would benefit from a classification of implants as either metallic or nonmetallic. The first group includes electrodes, clips, and coils. Nonmetallic implants include adhesive particles, treatment materials, and shunts. Some devices contain more than just one material, so a single device may fall into both categories.
Purely metallic devices will always produce artifact on CT or MRI. Artifacts facilitate device recognition, but they also reduce image quality and sometimes prevent the study of nearby areas or even areas more distant from the implant material.
Nonmetallic devices may be composed of many different materials. The main problem with these devices concerns their radiological behavior on CT (density) or MRI (signal) and their subsequent recognition. The most commonly used nonmetallic devices are easy to recognize. Some devices are minimally radiopaque or even radiolucent, making them difficult to see on radiography or CT. Certain materials used for hemostasis or filling can also be confused with pathological conditions.
Many common intracranial “medical foreign bodies” may hinder the job of image interpretation. These devices can be considered according to their function.
CSF shunt components include a proximal catheter, reservoir, valve, and distal catheter (Figure 1A). Excess fluid is most commonly drained to the peritoneum. The proximal segment of these shunts should normally be in the ventricles, near the foramen of Monro.4 The distal part of the catheter is placed inside the subcutaneous tissue of the head and neck when the CSF shunt goes to the peritoneum.
Other ventricular shunts are external and do not terminate in the peritoneal cavity. They are less permanent forms of CSF drainage and can be differentiated radiologically because the distal catheter is placed outside the skin.
CSF shunts are subject to multiple complications, the most frequent of which is improper placement (Figure 1B).5 Other complications include shunt obstruction, central nervous system infection, associated parenchymal or subdural hematomas (Figure 1C), and excessive CSF drainage. Peritoneal complications are normally related to poor shunt function.6Most patients also have brain parenchymal changes related to the introduction and replacement of shunts, such as gliosis, pneumoventricle, ventricular morphology changes, cystic formation, and porencephalic areas.
It can also be used for the preoperative devascularization of hypervascular lesions. Occlusion devices include coils, particles, and adhesives. A coil is a permanent embolic agent composed of stainless steel or platinum.
Coils are deployed into the lesion via a catheter with a guidewire or coil pusher. The coils come in a variety of lengths, configurations, and diameters. Choice of the correct size is vitally important to prevent coil migration or perforation of the target vessel.8
Platinum coils are MRI-compatible.
Stainless steel coils are not compatible because they are ferrous and are affected by the magnetic field. Benchtop testing on stainless steel embolization coil samples has shown high degrees of torque and deflection. Therefore, stainless steel embolization coils have a designation of “MR unsafe.”
Liquid adhesive or glue is used to treat arteriovenous malformations and fistulas with a high flow rate.9 Glues contain a mixture of different substances, such as butyl cyanoacrylate, contrast material, and tantalum powder, and they are deployed via a catheter by syringe injection. This mixture polymerizes quickly on contact with the blood, extending distally into small vessels that will be occluded permanently (Figure 2).
It is a gravity drainage device that is placed to reduce hematoma.
Other intracranial catheters are used for both infusion and drainage. They are composed mainly of plastic and silicone.
Among these, the behavior of anticancer carmustine wafers is important. Carmustine appears on CT as an area of high density surrounded by air, which is due to disintegration of the wafer. It is seen on all MRI sequences as a region of low signal intensity (Figure 4).16
Disintegration of a carmustine wafer can be confused with a pathological collection of air, such as pneumoencephalus or abscess. Increased edema, peripheral enhancement, and heterogeneous density or signal are all indicative of an inflammatory process. Diffusion weighted MRI can help differentiate an abscess from an implant disintegration.
An abscess will restrict water movement. Consequently, it will be bright on diffusion-weighted imaging. Other oncological devices include intracranial brachytherapy catheters that are made of plastic or Teflon. These have a low radiodensity when empty because they are filled with air.17
It is not uncommon to come across intracranial implants during routine radiological studies. Radiologists must consequently be prepared to interpret brain CT and MRI examinations containing different implant materials. It is also mandatory to know the compatibility of different materials with a magnetic field. The majority of brain devices used today are MR-compatible. Radiology personnel are nonetheless urged to use an updated list of device compatibilities for different magnetic fields.
Unstable patients are generally studied with CT, not MRI. CT environments are safer than MRI suites, and examinations are shorter. CT is also more readily available and cost-effective. MRI can, however, provide useful additional information about certain brain devices, such as hemostatic or packing materials, which are more difficult to evaluate on CT. The fat-packing material used in parasellar surgery, for example, is defined perfectly from its signal on different MRI sequences.19 Bone implants can also be characterized using T1- and T2-weighted sequences. Some complications associated with devices, such as abscess or cerebritis, are well documented on MRI. These should be studied with MRI rather than with CT, especially in difficult or subtle cases.
The discussion of intracranial medical devices can be simplified by classifying them according to their function. We strongly recommend working together with neurosurgeons, however, especially in less obvious cases.
The makes and models of devices favored for intracranial applications are ever-changing. The important thing is to be able to recognize when an implant is present. Radiological reports should include the type of device, its exact location, and information about any associated complications. The radiological follow- up of patients with intracranial devices depends on the clinical setting, the MR compatability of the material, and the reasons for the examination.