Biomedical Engineering Reference
In-Depth Information
remain fixed to the patient's skull from imaging to surgery, and the bulky
device hampers any kind of complex surgical approach. These factors limit
the application of stereotactic frames primarily to biopsies and placement of
cannulae or electrodes.
12.2.2.2
Head Clamp
The most common method of immobilization recommended by commercial
systems is to attach a tracker to a head clamp, such as the Mayfield head
clamp (Ohio Medical Instrument Company Inc., Cincinnati, Ohio). These
clamps are often used in neurosurgery and have the advantage over stereo-
tactic frames that they are not required during imaging and allow much
freer access to the patient for open surgery. With a Mayfield clamp or similar
device, a process of registration is required at the beginning of the proce-
dure. A tracking device can be attached to the clamp when the patient is
positioned to track any movement of the clamp and, hence, the head during
the procedure.
12.2.2.3
Molded Devices
An alternative method to screwing a device firmly into the bone of the
patient's skull during imaging and therapy is to use a patient-specific mold
that can be accurately removed and replaced. In radiotherapy, for example,
a cast of the patient's head is taken and a plastic mold is made. This device
is used to ensure that the patient is immobilized and placed correctly in a
simulator room where the images are taken that will be used to plan the treat-
ment. The same mold is then used to position the patient in the treatment
room.
The accuracy of guidance using these devices is determined by the repeat-
ability and stability of the patient position with respect to the mold. A head
mold covers the skin of the face and accuracy is therefore limited by move-
ment of the patient's skin. Such masks are generally considered to give ther-
apy accurate to 3 to 5 mm.
16
For surgery guidance, a significantly higher
accuracy is needed. Blocks molded to fit the patient's upper teeth have been
proposed as a more accurate method.
17, 18
Such blocks allow free movement
of the patient's head, which is desirable in ENT surgery, and also place the
tracker close to the target volume. Target registration errors of 1 to 2 mm RMS
have been achieved with such devices.
18
In spinal surgery, a mold of the relevant vertebra may be made by stero-
lithography from a CT scan of the patient. The desired trajectory of pedicle
screws in the spinous processes can be marked as part of the planning pro-
cess and, guide holes made in the prototyped block. During surgery this
block is simply clipped into place on the correct vertebra and the screws
inserted through the guide holes.
19
