A Device to Provide For Rotation of the Neck to Facilitate Functional CT and MR Imaging
Team Members:
Steven Pauls
Dana Nadler
Biomedical Engineering Design 300/200
University of Wisconsin-Madison
October 10, 2003
Advisor: Robert Radwin
Client: Victor Haughton, M.D.
Abstract
Current head support systems used in CT and MRI scanners provide stability to a patient’s head and neck for imaging in a single position. The cradles used do not allow for rotation about the axis passing through the spine, or for flexion and extension about a coronal axis passing through the center of the head. The purpose of this project is to design a dynamic head support cradle that would allow for rotation of the head about the aforementioned axes in order to allow researchers to image the head and neck in a variety of positions. Several preliminary designs have been developed and researchers have chosen one design to pursue. The design will eventually lead to a fully functional prototype which will be tested in the future.
Design Problem
The goal of this project is to create a head support device that allows for rotation of the head and neck about two different axes. The rotation about each axis should be independent of the rotation about the other, and the degree of rotation should be both measurable and reproducible. Ideally the device should be compatible with both MR and CT scanners, and should be easy to manually adjust.
Background Information on Magnetic Resonance Imaging
Magnetic Resonance Imaging (MRI) is an extremely helpful tool in the medical industry. An MR scanner can display small differences in tissue density and it is especially advantageous over other imaging techniques because of its extraordinary ability to show contrast in soft-tissues (Banna, 411) (see Figures 1 below). The scans are painless and have no known side effects (RadiologyInfo.org). Commercial scanners have been utilized since the early 1980’s and can be very effective in diagnosing several different types of medical problems. MR images can be used to detect many different injuries including head trauma, aneurysms, strokes, and brain tumors. The contrast available in MR images also makes the imaging technique useful in evaluating joints, bones and other soft-tissues.
Figure 1: A MRI scan of the brain (www.gemedicalsystems.com).
MR imaging works by utilizing a large magnet to produce an extremely strong magnetic field (Macovski, 63.2). A consequence of this fact is that no ferromagnetic materials can be placed in or near the scanner. Scans typically range from 1 to 10 minutes in duration (Macovski, 63.1). There are also new imaging techniques that are faster and can scan in less than 50 ms. The patient must lie totally still during the entire scan and must be able to relax. This is extremely important since any movement by the patient could cause the image to be distorted.
Background Information on Computed Axial Tomography
Computed Tomography (CT) allows physicians to obtain high-quality cross-sectional images of structures within the body. CT images are put together from a large number of small cross-sectional x-ray measurements that are taken from the patient. Adding these cross-sectional views together allows for imaging in three dimensions (Cunningham, 62.1).
CT scans can take approximately one half hour to one and a half hours for a specific part of the body. The accuracy of this type of imaging is dependent on total immobilization of the patient (Banna, 88). The scans can be used for various situations and on various parts of the body, and serve as an excellent diagnostic tool. CT scans may be used for analysis of head trauma, spine structure, tumor sites, osteoporoses, or specific anatomy (RadiologyInfo.org). Scans can also be used to assist in the guidance or placement of instruments or treatments. Patients must have no metallic material on them while having a CT scan because it can interfere with the image clarity.
There are low risks involved in CT scans. The worst possible adverse affect usually comes from material injected into the patient to help with picture contrast. These materials are commonly iodine-based. There is also minimal radiation exposure during these procedures (RadiologyInfo.org).
Background Information on Anatomy, Basic Biomechanics, and Range of Motion of the Head and Neck
The neck consists of intricate vertebrae that have articulations which are called amphiarthrodial joints. These joints allow movement in three different planes. The overall structure of the spine allows for a large variety of movements. Two of the most important joints are found in the neck where the spinal column joins with the head. The atlantooccipital joint is between the first cervical vertebra (the atlas) and the occipital bone of the head. This joint allows for movements of the head in the sagittal plane (flexion, extension) and the frontal plane (lateral bending) (Nordin, 96). The first 10º-15º of flexion occurs at this joint, but further flexion occurs in all cervical vertebrae (Bannam, 19). The atlantoaxial joint is between the atlas and the odontoid process of the head, and allows the head to rotate in the transverse plane (Nordin, 96). The first 40º-50º of head rotation occur at this joint, while further rotation takes place in the facet joints in the lower cervical spine(Banna, 19). The movements of the neck are controlled and coordinated by a large, complex group of muscles that are beyond the scope of this background information. It is, however, important to note that there is considerable variation between people in the range of motion possible in their neck (Hay, 57).
The vertebrae of the cervical spine contain large openings in the bone called foramina. These allow passage of arteries, nerve roots, and the spinal cord. These spaces will shift and change when the neck is moved into different positions. The goal of this project is to create a device that could allow doctors to investigate the changes in these spaces by allowing the head to be supported in a variety of different positions during an MR or a CT scan. Our device should confine the neck and head to a single specific movement at a time. This includes rotation isocentric about the spinal axis and flexion or extension about the coronal plane (figure 2).
Figure 2: Head and neck model: (A) Nuetral position (B) Full extension (C) Full rotation
(Vasavada, 413).
Current Devices Used and the Need for a Device to Rotate the Head and Neck Accurately and Efficiently in MR and CT Imaging
Currently there are two very crude head cradles in use in MR and CT imaging machines (Figure 3). Both head cradles are designed solely for the purpose of supporting the head of the patient. Neither restricts movement of the head and neck about any axis, however, they are not designed to allow for any motion either since patients must lay extremely still during the imaging process. Although our proposed device would allow for movement, it will lock in place for the scanning procedure and discourage patient movement.
Figure 3: The current head cradle used in MRI. The patient lays on his/her back and simply rests the head on the cradle. This cradle does not allow for rotation of the head or neck in any direction.
Our search of current literature turned up no head cradle designs that are in competition with the product that we aim to create. However, there are designs for other parts of the body that have a similar application relative to the site on the body in which they are used. One such design was created by a biomedical engineering group in the spring of 2001 at the University of Wisconsin – Madison. There goal was to design a device that would create graded pelvic rotation about an axis passing through the spine (Asti, 1). Their solution was to rest rollers on the bench already used in an MR scanner. A backboard rests between the rollers and rocks back and forth to achieve the desired pelvic rotation in the desired direction (Figure 4a, b). Although this device is not in direct competition with our potential design, it is important to note that there are other devices which have been developed with similar applications focused on different parts of the body.
Figure 4a: End view of the device designed to create graded pelvic rotations. The rollers underneath the bench are unidirectional and are responsible for rotation in this design (Asti, 14).
Figure 4b: Final drawing of the pelvic rotation device previously designed (Asti, 15).
Even though CT and MR scanners can present extremely clear and detailed pictures in an anatomical sense, each is very limited from a physiological standpoint due to the fact that the images are only still frames. A device to accurately and efficiently “lock” the head and neck into various positions, other than the comfortable straight resting position, could prove very useful in helping to explain physiological changes that could occur during motion. Changing the position of the head and neck would allow for comparisons of spinal anatomy during those movements. Our client, Dr. Victor Haughton, specifically plans to use a head and neck rotation device in order to research physiological changes that occur both specifically during flexion and extension of the neck and during isocentric rotation about the spinal axis.
Although the interest in building a head and neck rotation device is rooted in Dr. Haughton’s research, such a device, if built, could have a much broader range of use. According to our client, the full advantages of using such a device have not been realized partially because such a device does not currently exist. A head and neck rotation device for use in MR and CT imaging could eventually help radiologists to diagnose patients that have undergone serious trauma, or even those with unusual neck pains that otherwise go undiagnosed. Much more research on such a device is needed before the potential uses can be completely realized. Eventually our client envisions this device as something that could be used daily in a large hospital setting.
Design Constraints
After meeting with Dr. Haughton, a precise set of design constraints was drawn up. As mentioned before, the device must be able to rotate the head and neck isocentrically around the axis passing through the spine. It also must be able to flex and extend the head about the coronal axis that passes through the head just below each ear and crosses the atlantooccipital joint (where the cervical spine meets the skull). The device should be able to secure the head in place at a variety of different angles, and more importantly should allow researchers to reproduce those angles if needed. Therefore, the device must be calibrated and degrees of rotation marked in a clear manner to simplify use for technicians. The device should be relatively light and portable enough that a slight technician could still operate it. Although an aesthetically appealing device would be ideal, looks can be sacrificed for function in the early design stages.
Patient concerns are also an important part of this design project. One of the main concerns of patients undergoing either a CT or an MR scan is claustrophobia. Our designs should avoid having anything directly in front of a patients face. Another important aspect to consider from a patient standpoint is comfort. Padding inside of a solid head support should be considered if possible.
As far as design constraints, we must also keep in mind that our product has the potential to be used in both CT and MR scanners. It is our goal to create a device that is compatible with both machines. This strictly limits the materials available for use in this project. Materials must be radiopaque (do not absorb x-rays) and non-ferromagnetic. In addition, metal materials of any type should be avoided if possible due to the fact that they interfere with clarity of the images produced in CT images. Polyvinyl chloride was suggested as a material to use by the client, and was also the material that was used by the group that designed the lower lumbar device pictured earlier (Asti, 15). Also, in order for our device to be compatible with both CT and MR machines, it must be small enough in size to fit into the bore of each scanner. In order to simplify the project, it is desirable that the device be able to be mounted on the bench that is currently used in each machine. Beyond this, a more complete list of the physical and operational characteristics can be found in the product design specification (Appendix 1).
Although not a specific requirement of the design, the ability to accommodate kyphotic patients would be beneficial for future studies using CT and MRI. Kyphosis is a spinal deformity that can be caused by developmental problems, trauma, or disease (Banna, 428). Research into the area of kyphosis would need to be done to adequately integrate this type of patient handling, and this requirement will be pursued if time allows.
Design Alternatives
After several brainstorming sessions, we came up with several preliminary solutions to the problem we were presented. Each solves the problem in a unique manner. Ultimately, we settled on one design to pursue in our future work.
Design #1 – The Roller Design
The first design proposed sought to build upon the ideas used to construct the device used to rotate the pelvis (see above description of Spring 2001 project). The past project allowed for rotation about the spinal axis. Our proposed solution involved universalizing the concept of the unidirectional rollers (Figure 4 above) for use in the head and neck area by constructing rollers that are allow for multidirectional rotation. Rollers for our proposed design would be spherical in shape to allow for the desired multidirectional rotation about a number of axes. The grid of rollers would rest on the bench already used in both the MR and CT machines. The actual head cradle would likely be made of PVC and rest on a grid of the spherical rollers mentioned above. The cradle would “roll” on the grid in the desired direction creating rotation about either the coronal or the spinal axis.