Intracranial implant materialeffects create reporting issues

April 1, 2009

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.

DEVICE CLASSIFICATION

Many common intracranial “medical foreign bodies” may hinder the job of image interpretation. These devices can be considered according to their function.

  • Cerebrospinal fluid shunts. These are used to drain CSF from obstructed ventricular areas and to decrease intracranial pressure. They are mostly composed of radiopaque material, with metallic connectors and tips.

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.

  • Aneurysm clips. The function of these clips is to clamp off the neck or base of an aneurysm. Most are made from steel, tungsten, or tantalum. All are biocompatible and are easily recognized on CT due to metallic artifact. Not all are safe for MRI examinations.7 Steel and tungsten clips are ferromagnetic and are an absolute contraindication to MRI. This is not the case with tantalum, a nonferromagnetic material. It is generally recommended that when there is any doubt about the clip's material composition, then MRI should not be performed.

  • Embolic occlusion devices. Embolotherapy involves the occlusion of blood vessels and vascular spaces by the injection of material under imaging guidance. It can be used to treat a wide variety of clinical conditions, such as aneurysms or arteriovenous malformations.

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).

  • Monitoring devices. Intracranial electrodes can be used to record data. The intracranial pressure monitor, for example, is used to measure pressure surrounding the brain when it is subject to swelling from head trauma, hemorrhage, or brain surgery.10 Monitored patients are normally studied on CT because it is safer, easier, and quicker than MRI.

  • Neurostimulating devices. This group includes electrodes and stimulators used to treat conditions such as epilepsy or Parkinson's disease. Bilateral deep brain stimulators are placed in both thalami in patients with Parkinson's disease or essential tremor. These then use small electric impulses to block pathological brain signals.11 They are usually composed of platinum- iridium contacts and a conductor wire. MRI can be performed safely on patients with these implants. It can also be used to guide implant insertion and to verify lead placement.12

  • Intracranial catheters. The subdural hematoma drainage catheter is the most commonly used intracranial catheter.

It is a gravity drainage device that is placed to reduce hematoma.

13

Other intracranial catheters are used for both infusion and drainage. They are composed mainly of plastic and silicone.

  • Hemostatic or packing materials. Many different materials are used for hemostatic purposes. Some are organic products, such as fibrin or collagen.14,15 These materials can also be used to fill surgical cavities after skull base tumor resection. Fat, bone, and Teflon can be used for this purpose as well. It is important to be aware when this type of material is present because it can mimic the original mass, hindering interpretation of follow-up studies (Figure 3).

  • Oncological devices. A number of new methods have been introduced to treat tumors without affecting normal, adjacent brain tissue.

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

  • Closure materials. These are usually associated with the presence of other intracranial devices. Their function is the postsurgical closure of skin and bone. The most common closure materials are scalp staples, metal plates, and craniotomy fixation plugs (Figure 5). They are normally all metallic and will pose no safety issues during MRI examinations.18

 

DAILY RELEVANCE

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.