2. Introduction
• Transfer of target location from the radiological dataset to the patients’
head is a key step for performing adequately localized craniotomies for
neurosurgical procedures.
• The spherical neurocranium renders orientation difficult and requires a
precise spatial sense
to place adequate freehand convexity craniotomies.
• The precise localization of brain convexity lesions
can be inaccurate due to the oval shape of the skull
and also to unreliable external landmarks.
• For right placement of a burr hole or a small craniotomy for small lesion
• to decrease brain retraction and/or bone resection.
3. Methods
• 1. two methods were
called “double-checked” preoperative localization of
brain lesions.
A). orbito–meatal line, or obtaining a CT scan with
a marker on the scalp.
• uses three
spatial points on the skull convexity and is a simplification
of the method described by Vilela Filho et al.5 These points
are called: height of the lesion, starting point, and ending
point. The height of the lesion is found by calculating the
distance between the orbitomeatal (OM) line and the
chosen slice of the lesion.
4.
5. B). The second method to localize a brain lesion uses both
external auditory meatus as described by Penning. Onthe
picture frame of CT or MR depicting both external meatus a
line is drawn from side to side passing on both external
meatus (Fig 3). This line will cross the scale that is present on
every frame. The same line is then traced on
the chosen slice of the lesion passing at the same level on the
scale.
This line enables the surgeon to figure out if the lesion is
anterior, centered or posterior to the
meatus and calculate its distance. The height of the lesion
is calculated using the first method.
6.
7. • double-checked measurements that take
only two to three minutes to be done, neurosurgeons may
be assured about the correct location of the lesion beneath
the skin and plan a better approach or a minimally invasive
approach to any convexity lesion.
• This technique is not recommended to approach deep-
seated lesions and care must
be taken with very small ones, because even with
the most meticulous calculation some minor difference
may occur.
8. • 2. intraoperative ultrasound
• is a reliable method for visualization of
important brain landmarks during brain mass
lesion surgeries, and ventriculoperitoneal
shunt insertion, and this helps safe surgery,
and is easy to learn how to interpret the
ultrasound image.
9.
10. 3. conventional and frameless stereotaxis
• Craniomapper
• by use of use the external frame
• Frame-based stereotactic
• easy and less-technical localization is mandatory in emergency
surgery and small lesion in routine surgery,
• needs no special training. It is also not costly and can address
the issue, where image guidance/stereotactic systems are
not available.
• It is helpful in localization of brain lesions in
developing countries without high technical facilities.
11. 4.neuronavigation.
become a standard procedure in many
neurosurgical centers, however in most
departments worldwide this tool is
not available due to its high cost.
12. Caliper-based navigation technique
• The calculating part. Prerequisites are
conventional cuboid CT or MRI datasets, any
image viewer, a conventional calculator.
• The marking part. Prerequisite is a suitable
caliper.
With a pencil one can draw the OM line on patient’s skin using a flexible ruler. After it, a parallel line is drawn above the OM line, thus finding the height of lesion on the skull convexity (Fig 1). The next step is to find out the entry point on the oval shape of the skull. This can be accomplished by measuring the midline frontal skull from the chosen slice (starting point), following the shape of the skull with a ruler (dotting every centimeter) until the ending point (Fig 2).
Accuracy can be improved by using intraoperative ultrasound, conventional and frameless stereotaxis and neuronavigation1-4. Neuronavigation has become a standard procedure in many neurosurgical centers, however in most departments worldwide this tool is not available due to its high cost.
Intraoperative ultrasound is a reliable method for visualization of important brain landmarks during brain mass lesion surgeries, and ventriculoperitoneal shunt insertion, and this helps safe surgery, and confident intraoperative decisions for saving the patient, and evaluation of the surgical process. It is easy to learn how to interpret the ultrasound image as well as our familiar CT, and MRI images for encouraging the use of this simple, cheap, and valuable machine.
Tissues that are more echogenic than brain tissues appear more white on the screen, and the less echogenic appear more dark. For delineation of the best image of the brain, and the mass lesion; the following factors must be applied: – Position of the patient head allows the lesion, and the resection cavity are upward to permit saline stay in the resection cavity, and prevent air artifact. In posterior fossa surgery in sitting position elongated glove gel pad was used to fill the surgical cavity. – Selecting the proper ultrasound probe size, and frequencies. – Gel connection without air bubbles inside the glove. – Adjusting the scanner brightness, depth, gain compensation, focus, and frequency for best images. – Cottonoids, and spatula should be removed, and blood washing by saline during imaging.
easy and less‑technical localization is mandatory in emergency surgery and small lesion in routine surgery, where such facilities are not available. A similar rapid, simple and inexpensive CT technique has been developed for marking the scalp and lateral skull radiograph of patients with small cerebral convexity lesions.[1] It is safe to use the frame and needs no special training. It is also not costly and can address the issue, where image guidance/stereotactic systems are not available. We need to try it with other small lesions like brain tumor and in a number of cases prior to judging its efficacy. It is helpful in localization of brain lesions in developing countries without high technical facilities.
Calculating part When the screen position of the cursor is not changed while scrolling through the slices, then the line a “travelled” by the cursor from TS to LS is vertical to all slice planes. Since the line b is within the slice orientation, a is vertical to b. Hence, the distance between L and T is pa2 þ b2,according to Pythagoras’ theorem. (Fig 8) It is of no importance whether the data set is exact in terms of axial, sagittal or coronal orientation or oblique (e.g. rotated head in obscured patients in emergency scans). Any point identifiable on both imaging and the surface of the patient’s head can be used as landmark (e.g. bregma, inion, tuber parietale, occipital protuberance, palpable bone lesions). The use of nasion and porus has the advantages of easy localization, solid placement of the caliper with minimization of scalp shift over the skull while marking, and the fact that for most convexity areas, the two circles will intersect in an obtuse angle. This all offers the best accuracy for the presented technique. However, it is mandatory that the dataset used for the calculation is cuboid. Some CT scanner can tilt the gantry, which is employed to modify the field of acquisition to reduce irradiation to the eyes during head CTs. If such an acquisition technique is used, then an additional cuboid dataset must be reconstructed at the CT. This can be done without new acquisition and irradiation. The gantry angle can be seen in the scout of the corresponding dataset (see Fig 9). Marking part The calculation part has delivered the lengths of the straight lines between two identifiable landmarks and the target. Geometrically, in a three-dimensional space, the position of a point (in our case the landmark L) and a length (in our case the distance from L to T) defines a ball. The target T can be anywhere on the ball’s surface. Calculating part at the image viewer (Fig 2) STEP 1 Define the target (T) in the image viewer software. T will be the center of the desired craniotomy correctly projected onto the skin surface for the respective lesion (see chapter troubleshooting for the correct choice of a target). Memorize the running number of the respective target slice (TS). STEP 2 Scroll to the slice of the first landmark (L), nasion. Memorize the respective landmark slice (LS). STEP 3 The cursor function of the applied image viewer is set to distance measurement. Within the landmark slice (LS), the cursor is placed on L and will not be moved from this cursor position until step 4. STEP 4 Scroll back to TS without moving the cursor on the screen surface. The number of scrolled slices are multiplied by the slice thickness to assess the distance “travelled” by the cursor between LS and TS. This is distance a. Within TS, measure the distance from the priorly unchanged cursor position to T. This is distance b. The distance from L to T is ffiffiffiffiffiffiffiffiffiffiffiffiffiffi pa2 þ b2 (Pythagoras’ theorem). Perform the same steps for the second landmark, porus acusticus externus, referred to as porus in the following. The distances from nasion to the target and from porus to the target have thereby been assessed. Note that some image viewer offer automated distance measurement between points in different slices, rendering this part much faster.
Marking part on the head of the patient (Fig 3) STEP 5 The two respective distances are set on a caliper and circles are drawn for each landmark distance in the approximate target area on the patient’s head. STEP 6 The intersection of the two circles marks the target. If in doubt, a third landmark distance (e.g. bregma-target) can be calculated and used for control.
