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The most important limitation of ultrasound and TUSS guidance in the spine is
limited visibility of bone structures in ultrasound images. Visual enhancement of
the spine could be achieved by fusion of a previous CT image to the tracked
ultrasound [ 17 ], however, ultrasound-only procedures are preferred to reduce
radiation risks and cost. Vertebra visibility could be improved in the needle navi-
gation display by
fitting a deformable general vertebra shape model to automati-
cally detected bone contours [ 18 ]. Although shape model
fitting is still in the
experimental phase, and will likely have limitations in certain pathological cases, it
may greatly enhance the potentials in ultrasound guidance in the spine.
13 Spinal and Epidural Anesthesia with Tracked
Ultrasound Snapshots
Spinal and epidural anesthesia are similar procedures; the needle is just pushed a
little further in case of spinal anesthesia. Both are performed to numb the lower
body for surgery while the patient remains awake. These procedures are preferred
over general anesthesia, having lower risks and the contributing to faster recovery
after surgery. Spinal and epidural needles are both placed in the spinal canal. Spinal
anesthesia is injected inside the dura sac, where the medicine takes effect imme-
diately, and is usually used in shorter and simpler procedures. Epidural injections
are placed just outside the dura sac. A catheter can be left in the epidural space to
provide continuous administration of medicine for longer procedures. From the
needle guidance point of view, the needle should be similarly navigated in the
spinal canal between two lumbar vertebrae in both cases (Fig. 15 ).
Spinal and epidural anesthesia is routinely performed without image guidance, as
the vertebral interspaces are palpable in the average patient. However, some path-
ological conditions may cause the narrowing of the interspaces, making it dif
cult or
impossible to lead a needle to the spinal canal. In less severe cases, conventional
ultrasound may help identify the interspaces where needle insertion can be attempted
with higher probability of success, but in extreme cases, only a CT image-based
guidance may provide enough information for needle navigation. Tracked ultra-
sound offers the accuracy of CT-guided navigation, using a pre-operative CT image,
registered to the patient using landmarks visible on ultrasound images.
The most intuitive display for needle navigation is when vertebrae and the
needle are represented with surface models. Surface models can be generated from
CT images using a threshold-based segmentation, but pathological spines may
require manual slice-by-slice contouring, especially in the presence of metallic
implants. The CT-derived surface models can be registered to the needle navigation
coordinate system using landmark points. The landmarks should be rigidly
xed to
the vertebrae, and should be easy to identify on ultrasound images. Natural land-
marks can be the facet joints, or transverse processes. In case of implanted vertebral
screws, the screw heads are excellent landmarks (Fig. 16 ).
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