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surgical table, so the navigation system knows where the screws positions are
planned with respect to the patient.
Ultrasound can be tracked using the same tracking system that is used for
surgical navigation. This allows ultrasound to identify landmarks for registration of
the pre-operative plan to the surgical navigation system. The vertebra anatomy
offers many unique surface landmarks, but few are convenient to identify in
ultrasound images. The spinous process is hard to localize with ultrasound because
of the prominent echo signal from the supraspinous ligament. The second closest
structure to the skin that has a face perpendicular to the ultrasound propagation
direction is the set of articular processes. The four articular processes are relatively
easy to
find in ultrasound images, and they surround the vertebra, therefore are
excellent points for landmark registration.
The pre-operative CT can be accurately registered to intraoperative tracking
using the articular processes as landmarks [
19
]. More landmarks can be de
ned to
further reduce the effect of landmark position errors (Fig.
17
), although at the cost if
increasing the total procedure time.
15 Spinal Curvature Monitoring with Tracked Ultrasound
Snapshots
Kyphoscoliosis is a condition with pathological curvatures of the spine. The most
common cause of this condition is a disease called adolescent idiopathic kypho-
scoliosis. It affects 1 individual in 1,000, and is typically discovered in the early
adolescent age. It requires regular monitoring of the pathological curvatures, to be
able to decide on treatment options in time. Spinal curvature measurement may also
be needed during surgery to provide feedback on achieving the surgical plan. Spinal
curvature measurements are currently performed on X-ray images in the clinical
practice. However, regular examinations with X-ray have been linked to an
increased risk of cancer [
20
22
]. Therefore, an alternative measurement method
without ionizing radiation would be ideal for monitoring kyphoscoliosis angles.
In the current clinical practice, measurements are made on X-ray radiographs.
The reader selects two vertebrae that are most angled at the superior and inferior
end of the curvature. A line is drawn on superior end-plate of the superior vertebra,
and on the inferior end-plate of the inferior vertebra. The angle between these lines
is called the Cobb-angle, which is the most common measure of spinal curvatures.
Minor curvature angles can also be de
-
ned besides the most prominent major angle.
However, lots of factors cause variance in the Cobb-angle. The posture of the
patient, the angle of X-ray imaging, and these curvatures are reported to increase
within a day, begin up to 5
°
larger in the afternoon compared to measurements in
the morning [
23
]. Since variability between different readers is reported to be 2
°
-
7
°
even on the same images, spinal curvature differences less than 5
are generally not
considered significant when estimating disease progression [
24
,
25
].
°
