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TECHNICAL FORUM-- Prosthetic andOrthotic Lab Applications in Medical Imaging Head Immobilization

John C. Goble, PHD
Neal F. Kassell, MD
Mare N. Pilipuf, MD
John M. Berry, RTP

ABSTRACT

A thermoplastic head immobilization system was designed and fabricated at the University of Virginia Medical Center. The department of neurosurgery and the department of prosthetics and orthotics produced a cost-effective head restraint device that is adaptable to a variety of imaging systems. This article highlights the issues involved in material selection, fabrication and the resulting application in MRI imaging studies.

Introduction

Medical imaging modalities (e.g., MRI, CT, SPECT and projection angiography) require head immobilization to reduce motion artifacts, localize pathologic tissue and register the images (to track changes in specific anatomical structure). Motion artifacts cause blurring in the image, thereby reducing the image's ability to accurately depict fine anatomical details. Existing head immobilization systems either require a surgical procedure to bolt a frame to the patient's head (invasive systems) or involve clamping a frame to the patient's head (noninvasive systems)(1,2). The invasive systems include the Brown-Roberts-Wells (BRW) and Cosman-Roberts-Wells (CRW), the Kelly-Goerss (modified Todd-Wells), and the Leksell stereotactic system (3). These systems ensure absolute immobilization during imaging studies and surgical procedures, but they are expensive and can be inconsistent in reproducing patient positions (4,5).

The noninvasive head immobilization systems include the modified Brown-Roberts-Wells (the system is secured to the patient's mouth with orthodontic resin)(4), the Bremen vacuum cap (6), and the Laitinen stereoadapter (2). These systems require less time to apply than invasive systems and have a high degree of reproducibility. Postsurgical studies also are conducted with a high degree of reproducibility (to track the patients' progress over time). The main drawback to these noninvasive systems is the lack of absolute immobilization; for this reason, many neurosurgeons refuse to use them (especially radiosurgeons). In routine imaging studies a noninvasive (nonsurgical) system is preferred to secure the patient's head.

The department of neurosurgery and the department of prosthetics and orthotics at the University of Virginia Medical Center combined efforts to produce a noninvasive head immobilization system for applications in imaging and neurosurgery. The prosthetic and orthotic lab provided the materials and technical expertise for fabrication, and the department of neurosurgery was responsible for the design and application. This system consists of a low temperature thermoplastic facial mask attached to a high temperature thermoplastic head cradle (see Figure 1 ). The head cradle mates with the inner contours of the MRI head coil, which is secured to the examining table. Thermoplastics were chosen for their rigid, durable and nonmagnetic properties. These properties are more desirable than the qualities of thermoset resins or injection molding (7). Thermoset resins are hazardous, more expensive and generally heavier than thermoplastics (8,9). Injection molding is more expensive and time consuming than vacuum-forming thermoplastics.

Methods

To reproduce the inner contour of the MRI head coil, a 9mm (3/8-inch) low-temperature sheet of AquaplastR is drape molded inside the rectangular mortise of the head coil. Petroleum jelly is used as a parting agent between the rigid injection-molded plastic of the head coil and the Aquaplast. A positive plaster model is produced by filling the negative voids of the draped shell (see Figure 2 ). Modifications include adding a 30-degree bevel to the proximal surface of the plaster model to receive a perforated low temperature plastic draped over the subject's facial contours. A 9-mm high-temperature copolymer thermoplastic is then vacuum formed over the mold to serve as the head cradle for the Aquaplast mask. Note that a 9-mm copolymer is chosen to maintain consistency with the 9mm low-temperature drape. A 3-mm (1/8-inch) PeliteR liner is then vacuum formed over this copolymer head cradle to ensure an intimate mate with the head coil (see Figures 3a , 3b , 3c . The beveled surface of the head cradle is drilled and tapped to receive the low-temperature mesh mask. The mesh mask consists of a 9 x 10 1/8-inch perforated sheet of Aquaplast and 9-inch Opti-handles™4. A removable Timo Pad TM5 (foam cervical pad) is placed inside the head cradle and secured with Velcro™ spot attachments.

The Aquaplast mask is formed to the patient's head using the following procedures: First, the patient lies supine on an examining table with his or her head in the head cradle. Next, the perforated sheet of Aquaplast is heated in a 140-degree Fahrenheit water bath. Liquid soap is added to the bath to prevent the Aquaplast from sticking to the patient. When the mesh becomes transparent, it is removed from the bath and blotted with absorbent towels. The softened material is then temperature-tested for patient comfort.

When the Aquaplast is a suitable temperature, it is centered over the patient and pulled from the chin toward the forehead. The Opti-handles attached to the mask are then secured to the beveled surface of the head cradle with nylon screws (see Figure 4 ). The facial landmarks, including the eye cavity, the bridge of the nose and the chin, are contoured by hand while the Aquaplast is still pliable to ensure an intimately formed mask. Cure time is seven to 10 minutes, and shrinkage is limited to less than 2 percent.

When the Aquaplast has cured, the mask is removed and trimlines are marked and cut out for patient comfort. Also portions of the mask around the eyes and mouth can be removed for intubation, tracheotomy or claustrophobia (10).

Results

The head cradle and Aquaplast system produced MRI images of excellent quality. A sample of 20 subjects was tested, and both subjects and medical personnel preferred this system over other head immobilization devices. Subjects described the experience as relaxing and the mask as similar to a second layer of skin. Painful pin-point pressure spots were eliminated by the even distribution of pressure, and claustrophobic subjects felt more at ease when portions of the mask were cut away, especially the eyes and mouth. Cutting out portions of the mask did not affect the quality of the images.

Discussion

By combining the efforts of neurosurgery and prosthetics and orthotics, a superior noninvasive head immobilization device was designed and fabricated. This head holder offers many advantages over existing systems. The Aquaplast mask is a nonsurgical procedure and accommodates all head sizes. This system is less expensive than existing head holders and requires less time to apply to the subject, further reducing cost and patient discomfort.

The mask also can be used in follow-up studies to duplicate the positioning of the patient's head, which allows the physician to track the patient's progress over time (11-13). Subjects were successfully immobilized, and motion artifacts were dramatically reduced. Although motion is sufficiently reduced for imaging, the system is inadequate for surgery, which requires absolute immobilization. Another potential disadvantage is that only trained personnel can properly mold a mask.

Future applications include thermoplastic adapters for other imaging modalities: CT, SPECT and projection angiography (see Figure 5 ). Each of these imaging modalities has different advantages. This system allows neurosurgeons to incorporate all or a combination of these modalities in their diagnoses (14,15). For example, MRI highlights soft tissue, and projection angiography highlights vascular structures.

By superimposing these two images, neurosurgeons have the information from both modalities at their disposal. Neurosurgeons can also localize tumors and other vascular malformations in each of the imaging modalities by attaching reference markers (fiducials) to the Aquaplast mask (16). These presurgical tasks improve the efficiency of the surgical procedure.

The use of the thermoplastics is rapidly increasing in commercial and medical fields. This increased usage requires technical training in the application of thermoplastics. As demonstrated with this project, it is advantageous to increase interaction among medical center departments to promote new medical technologies.


MARC N. PILIPUF, ME, Department of Neurological Surgery, Department of Biomedical Engineering, 853 W. Main St., Charlottesville, VA 22903.

JOHN M. BERRY, RTP, is a prosthetics-orthotics assistant in the department of prosthetics and orthotics at the University of Virginia Health Sciences Center in Charlottesville, Va.

JOHN C. GOBLE, PhD, is an assistant professor in the Department of Neurological Surgery in Charlottesville, Va.

NEAL F. KASSELL, MD, is a distinguished professor and bice chairman at the department of neurological surgery at the University of Virginia Medical Center.

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