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attenuate the energy of the ultrasound more than other tissues with more water
content. One can still
find sonographic landmarks along the spine that can be used
to obtain limited view of the anatomy. These landmarks are often used during
interventions, as the operator
finds the way of the needle based on these points.
Ultrasound combined with position tracking is a promising technology that has
recently reached the clinical device market. It allows needle navigation methods
that show the 3D position or projection of the tracked needle relative to the tracked
ultrasound image. This visual aid enhances the accuracy of needle insertions when
the target is directly visible on ultrasound. Some commercial ultrasound machines
recently offer fusion of CT or MR images to real-time ultrasound, which is also a
very promising avenue in computer assisted spine interventions. The real-time
nature of ultrasound combined with the resolution and contrast of other image
modalities may revolutionize image-guided spine interventions, enabling more
procedures to be performed in a minimally invasive way. In this chapter, we will
focus on the tracked ultrasound technology, and show some of its promising
applications that may become routine procedures in the hands of surgeons, anes-
thesiologists, or interventional radiologists.
2 Ultrasound in Spinal Needle Guidance
Ultrasound has been in use for decades in guidance of invasive procedures in the
spine. Although most needle insertion procedures that are commonly performed,
can be completed blindly with knowledge of the anatomy. The procedural dif
culty
of spine interventions has a wide range depending on target structures and indi-
vidual patients. For example, the most common procedure is lumbar puncture,
needle insertion into the spinal canal between two lumbar vertebrae. Lumbar
puncture is generally thought of as a simple procedure that every physician is able
to perform without image guidance or other forms needle guides. However, in
obese patients or degenerative spines, even this procedure can be so dif
cult that it
requires ultrasound or
cult
procedures, such as selective nerve blocks, that are only attempted using CT or MRI
guidance.
The most common use of spine ultrasound is to
fl
fluoroscopic guidance. There are signi
cantly more dif
find vertebral interspaces for
lumbar puncture in dif
cult cases. Ultrasound is helpful when the spine is covered
by thick fat tissue, or when spine pathologies prevent conventional navigation by
palpation. In these cases, ultrasound scanning can be done either before needle
insertion, or during needle insertion to provide real time guidance as the needle
approaches its target. The
first technique uses landmarking. Ultrasound is used
before needle insertion to
find the space between two spinous processes, and
marking it with a pen on the patient
'
is skin. The needle is introduced at the marked
point, which has a high probability of leading to the space between two vertebrae.
In case of the second technique, imaging can be performed simultaneously during
needle insertion too, to provide real time visual feedback on the needle position.
