Ultrasound imaging of the thoracic spine can be challenging due to peculiarities in its anatomy. The osseous framework of the thoracic spine makes up for a narrow acoustic window with limited ultrasound visibility of the spinal canal and neuraxial structures. ^1,2 Ultrasound visibility of the thoracic spine also varies depending on the plane ¹ of the ultrasound imaging and which part of the thoracic spine is being imaged. ¹ Ultrasound visibility progressively decreases as one moves up the thoracic spine. ¹ Currently data are limited on the use of ultrasound to guide or assist thoracic epidural injections. ^3,4 This chapter briefly outlines the anatomy, the technique of ultrasound imaging, and sonoanatomy of the thoracic spine relevant for thoracic epidural injection.

The thoracic spine is made up of a column of 12 vertebrae (Fig. 7–1) that makes up the midsection of the vertebral column. The thoracic vertebrae are identified by the presence of articular facets on the lateral surface of the vertebral bodies for articulation with the head of the ribs (Figs. 7–1 to 7–4). There are also facets on the transverse processes of all, except the 11th and 12th vertebrae, for articulation with the tubercle of the ribs (Fig. 7–1). The thoracic vertebrae are intermediate in size between the cervical and lumbar vertebrae, with the lower thoracic vertebrae being a lot larger than the upper thoracic vertebrae (Fig. 7–4) and the upper thoracic vertebrae (T1–T2) being similar in size to the cervical vertebrae (Fig. 7–2). The thoracic spine has a primary curvature, which is concave anteriorly, but also has a lateral curvature that is slightly concave to the left, most likely from greater use of the right upper extremity and pressure from the aorta.

Typical Thoracic Vertebrae

The 2nd to 8th thoracic vertebrae are considered typical thoracic vertebrae (Figs. 7–2 and 7–3), whereas the remaining five vertebrae (1st, 9th, 10th, 11th, and 12th) are atypical as they have certain unique features. The body of a typical thoracic vertebra is heart-shaped (Fig. 7–3) with its anteroposterior and lateral diameters being roughly the same (Fig. 7–3). Also the distance between the two lamina of the vertebra is greater than the width of the vertebral body (Fig. 7–3). ⁵ On either side of the vertebral body are two costal (superior and inferior) facets (Fig. 7–3). The superior costal facets are larger, located on the superior border of the vertebra near the pedicle, and articulate with the head of the numerically identical rib (Figs. 7–2 and 7–3). The inferior costal facets are smaller in size, they are located near the inferior border of the vertebra and in front of the inferior vertebral notch, and they articulate with the next lower rib. The spinal canal is relatively small and circular (Fig. 7–3) and contains the spinal cord and meninges.

The pedicles of the thoracic vertebra are short and directed backwards (Fig. 7–2). The superior vertebral notch is shallow, whereas the inferior vertebral notch is large and deep (Figs. 7–1 and 7–5). The laminae are broad and thick, overlap the one from the adjacent vertebrae (Fig. 7–6), and are connected to the pedicle anteriorly (Fig. 7–5). The interlaminar spaces are also narrow, and using ultrasound they measure approximately 0.9 cm at the lower thoracic spine to 0.8 cm and 0.6 cm at the mid- and upper thoracic spine, respectively. ¹ The transverse processes are large and are directed laterally and backwards (Figs. 7–3 and 7–6) from the junction of the lamina and pedicle (Fig. 7–5). The costal facets on the anterior surface of the transverse process of the upper six vertebrae are concave (Fig. 7–3), facing forward, and articulate with the tubercle of the corresponding rib. The inferior articular processes are fused to the laminae, and their articular facets are directed forwards and slightly downwards and medially (Fig. 7–5). The superior articular processes in contrast project from the junction of the pedicle and laminae and are directed backwards and slightly laterally and upwards. The articulation of the rib to the transverse process anteriorly results in the neck of the rib being hidden anteriorly by the transverse process at the vertebral levels T1 to T4, but from there on until T9 the neck of the rib progressively projects above the transverse process. ⁵ The spinous processes are long and directed backwards, downwards (Figs. 7–1, 7–6 and 7–7), and often slightly obliquely. Therefore even in a perfectly normal spine, the tips of the spinous processes may be slightly deviated from the midline (ie, paramedian in location, Fig. 7–7). The spinous processes are longest between T2 and T9 levels and overlap each other like “tiles on a roof.” This creates an acute angle for epidural needle insertion or insonation of the ultrasound beam if one were to do so through the midline. The spinous processes are less oblique above T2 and below T9. The spinous processes of T11 and T12 are directed backwards as with the lumbar spinous processes. The orientation of the T10 spinous process varies, with it being only slightly caudally directed to resemble that of the T11 and T12.

The ligamentum flavum is attached to the upper border and the upper part of the anterior surface of the laminae. The transverse process gives attachment to the following ligaments (Fig. 11-3): (i) lateral costotransverse ligament at the tip, (ii) superior costotransverse ligament to the lower border, (iii) the inferior costotransverse ligament to the anterior surface, (iv) intertransverse ligament to the superior and inferior borders, and (v) the levator costae to the posterior surface (T1–T11). The spinous processes give attachment to the supraspinous and interspinous ligaments. Also the superior and inferior borders of the vertebral bodies give attachment in front and behind to the anterior and posterior longitudinal ligaments, respectively. There are also several muscles attached to the spine of the thoracic vertebrae, including the latissimus dorsi, trapezius, rhomboids, and many deep muscles of the back.

Gross Anatomy of the Upper Thoracic Spine (T1–T4)

Figs. 7–8 and 7–9

Computed Tomography Anatomy of the Upper Thoracic Spine (T1–T4)

Figs. 7–10 to 7–13

Magnetic Resonance Imaging Anatomy of the Upper Thoracic Spine (T1–T4)

Figs. 7–14 to 7–17

Gross Anatomy of the Midthoracic Spine (T5–T8)

Figs. 7–18 and 7–19

Computed Tomography Anatomy of the Midthoracic Spine (T5–T8)

Figs. 7–20 to 7–23

Magnetic Resonance Imaging Anatomy of the Midthoracic Spine (T5–T8)

Figs. 7–24 to 7–27