Ultrasound Imaging of the Spine for Central Neuraxial Blockade: a Technical Description and Evidence Update

This article describes the anatomy of the spine, relevant ultrasonographic views, and the techniques used to perform the neuraxial blocks using ultrasound imaging. Finally, we review the available evidence for the use of ultrasound imaging to perform neuraxial blocks. Central neuraxial blockade using traditional landmark palpation is a reliable technique to provide surgical anesthesia and postoperative analgesia. However, factors like obesity, spinal deformity, and previous spine surgery can make the procedure challenging. The use of ultrasound imaging has been shown to assist in these scenarios. Preprocedural imaging minimizes the technical difficulty of spinal and epidural placement with fewer needle passes and skin punctures. It helps to accurately identify the midline, vertebral level, interlaminar space, and can predict the depth to the epidural and intrathecal spaces. By providing information about the best angle and direction of approach, in addition to the depth, ultrasound imaging allows planning an ideal trajectory for a successful block. These benefits are most noticeable when expert operators carry out the ultrasound examination and for patients with predicted difficult spinal anatomy. Recent evidence suggests that pre-procedural neuraxial ultrasound imaging may reduce complications such as vascular puncture, headache, and backache. Neuraxial ultrasound imaging should be in the skill set of every anesthesiologist who routinely performs lumbar or thoracic neuraxial blockade. We recommend using preprocedural neuraxial imaging routinely to acquire and maintain the imaging skills to enable success for challenging neuraxial procedures.


Introduction
Central neuraxial blockade (CNB) comprises of epidural or spinal anesthesia or analgesia, is an established technique for surgical anesthesia and postoperative analgesia. The landmark-based technique of the palpation of spinous process with the loss of resistance to saline or air has a high success rate for epidural placement in patients with normal anatomy [1]. Similarly, spinal anesthesia can be performed using landmark technique with needle tactile response and flow of cerebrospinal fluid acting as a clear endpoint. However, factors like obesity, congenital/acquired or age-related altered anatomy, and previous spine surgery can make these procedures technically challenging.
The ultrasound imaging can assist in two unique ways for the placement of central neuraxial block [2]. First, ultrasound scanning prior to skin puncture has been proven to help identify midline and the appropriate interspace, any abnormal anatomy, depth to the epidural space, and planned needle trajectory [3, 4•]. Second, real-time ultrasound-guidance has been described [5]. However, it is currently a cumbersome technique with limited clinical utility and will only be discussed briefly in the article. This article is part of the Topical Collection on Regional Anesthesia This review describes the relevant anatomy, ultrasonographic imaging views, and re-examines the current literature to better understand the role of ultrasound in the placement of neuraxial blockade.

Gross Anatomy and Sonoanatomy of the Lumbar Vertebrae
In-depth knowledge of gross and sonoanatomy of the vertebrae and the vertebral canal is paramount, for understanding neuraxial imaging. Figure 1 shows a typical lumbar vertebra. Each vertebra is made up of a body and arch. The arch is composed of pedicles, a spinous process (SP), lamina, superior and inferior articular processes (APs), and transverse processes (TPs). The vertebral canal is formed by the spinous process and lamina posteriorly, pedicles laterally, and vertebral bodies anteriorly. Within the vertebral canal lie the thecal sac and its contents. The epidural space lies outside the thecal sac within the vertebral canal. The identification of these key anatomical structures in para-sagittal and transverse views enables better performance of ultrasound-guided neuraxial interventions.
The bony structures of the lumbar vertebrae appear as hyperechoic white lines on ultrasound imaging with black acoustic shadowing underneath. Figure 2 shows the bony windows through which the ultrasound beam can pass through and encounter the thecal sac. These are called the interlaminar and interspinous spaces. The interlaminar space is located posterolateral and the interspinous space in the midline posteriorly to the thecal sac. The intervertebral foramina are located laterally from where the spinal nerve roots emerge (Fig. 3).
The ligamentum flavum, epidural space, and posterior dura often appear as single or sometimes double hyperechoic white structure referred to as the posterior complex (PC). The anterior dura, posterior longitudinal ligament, and the posterior aspect of the vertebral body are visible as a single hyperechoic white line referred to as the anterior complex (AC). The anterior and posterior complexes can be visualized in both interlaminar and interspinous views with the thecal sac in between.

Sonographic Views of the Lumbar Vertebrae
Most commonly, a curvilinear low-frequency (2-5 MHz) ultrasound transducer is utilized for neuraxial scanning with the patient in the sitting or lateral decubitus position. This transducer allows deeper penetration and wider viewing of deeper structures through the bony windows. The authors recommend pre-procedural scanning routinely for all patients prior to spinal anesthesia. This may allow the clinicians to get comfortable with spine imaging in patients with normal anatomy and may improve success in more challenging scenarios.   5 show the transducer orientations during the sagittal and transverse systematic scanning that is performed for the correct location of intervertebral level, location of the midline, measurement of the depth to the epidural space, and identification of other relevant structures. Conventionally, three para-sagittal and two transverse views are performed for complete neuraxial scanning.

Para-Sagittal Transverse Process View
The ultrasound transducer is placed in a para-sagittal plane a few centimeters lateral of midline as shown in Fig. 4, probe position A and Fig. 6. The surface of the transverse processes are seen as round hyperechoic outlines with deeper hypoechoic shadows as dark finger-like projections, as shown in Fig. 7. This is described as a "trident sign." The psoas major muscle is seen between these hypoechoic shadows.
The para-sagittal scanning can be used for ascertaining the exact vertebral level before the procedure. In this method, the ultrasound transducer is placed over the sacrum to identify the L5 transverse process and L5-S1 intervertebral space (Figs. 8 and 9). The transducer is then slid cranially to identify the respective L5-L4, L4-L3, and L3-L2 interspaces.

Para-Sagittal Articular Process View
The probe is then moved medially until a continuous white hyperechoic line with "camel humps" is seen, indicating the facet joint's articular processes ( Fig. 4 probe position B). It is difficult to see any neuraxial structures in this view as the bone is continuous and does not permit ultrasound signals beyond the articular processes ( Fig. 10).

Para-Sagittal Interlaminar (Oblique) View
From the para-sagittal articular process view, the probe is tilted medially toward the median sagittal plane to bring the lamina into view (Fig. 4 probe position C). This view can also be referred to as a para-sagittal oblique view. The sloping lamina appears as white hyperechoic lines described as a "sawtooth" or "horsehead" pattern ( Fig. 11). The gaps represent the interlaminar spaces through which the posterior and anterior complexes are visualized. This is the most important window in sagittal scanning to identify interspaces. The appropriate interlaminar space is identified and marked on the skin using this view. This view is also helpful in identifying the open spaces for a paramedian approach to neuraxial anesthesia. If no open windows are noticed in the midline, but adequate windows are observed in the para-sagittal interlaminar scan, the clinician could directly start with a para-median approach.

Transverse Spinous Process View
After identification of the appropriate interspace using the para-sagittal interlaminar view, the probe is turned 90°to obtain a transverse spinous process view (Fig. 12). The tip of the spinous process is identified as a white hyperechoic line with acoustic shadowing beneath it with a sloping lamina seen laterally (Fig. 13). This is the key view for the identification of midline (Fig. 5 probe position A) and the interspinous spaces between the consecutive spinous processes in obese patients. We recommend using M-mode line for determining the exact midline in this view (Fig. 14).

Transverse Interspinous View
After identification of the spinous process, the probe is either moved cephalad or caudad to the interspinous space ( Fig. 5 probe position B). This view, also known as the transverse interlaminar view, allows for visualization of the posterior and anterior complexes along with articular and transverse processes laterally (Fig. 15). The depth of the posterior complex from the skin can be noted in this view and is useful for guiding epidural placement. The angle of the probe required to visualize posterior and anterior complexes in this view facilitates the angle of incidence for needle entry for successful neuraxial placements. After identification of posterior and anterior complexes, the ink markings are made in horizontal and vertical directions are joined together to mark the entry point for neuraxial procedures (Figs. 17 and 18). The intrathecal space is seen as hypoechoic space between the posterior and anterior complexes. An un-obstructed interlaminar space is a space where both the posterior and anterior complexes can be clearly visualized. The widest, unobstructed interspace can be used for access to the neuraxis. This is done by sliding the ultrasound transducer caudad and cephalad in the transverse interspinous process view. Maintaining the visibility of the anterior complex for a larger distance indicates a wider interspinous space. Although multiple variations of the scanning technique have been described in the literature [2,4], core principles are the same. The following stepwise approach is utilized by the authors.

1.
A sitting position is preferable for the procedure. Alternatively, a lateral decubitus position is acceptable.

A curvilinear low-frequency transducer (2-5 MHz) is
selected, and ultrasound gel is used as a coupling medium. 3. The screen depth is set to 9-11 cm and adjusted after initial assessment. 4. The scanning is started in the para-sagittal transverse process view. The transverse processes are identified as "trident sign." (Fig. 7) 5. The transducer is slid medially to obtain a parasagittal articular process view identified as "camel humps" (Fig. 10) 6. The transducer is then tilted medially to obtain a parasagittal interlaminar (oblique) view. The laminae appear as a "sawtooth" or "horsehead" and, medially, the posterior complex, anterior complex, and thecal sac are subsequently identified. (Fig.11) 7. The interlaminar spaces (acoustic windows) are counted up from the sacrum ( Fig. 9) in the para-sagittal interlaminar view, and the L3-4 level is marked (Fig. 16).  The Utility of a Pre-Procedural Scan The following information could be obtained by use of a preprocedural ultrasound scan of the spine:

Thoracic spine
The upper thoracic (T1-T4) and lower thoracic (T9-12) vertebrae have similar geometry to cervical and lumbar vertebrae and amenable for US scanning (Fig. 19). The mid-thoracic (T5-T8) vertebrae have extreme inferior angulation of spinous process and pose technical challenges for ultrasound scanning (Fig. 20).
The para-sagittal windows can be obtained by beginning laterally with identification of ribs and pleura, then moving medially with identification of transverse process, articular process, and lamina. The para-sagittal interlaminar view (Figs. 21 and 22) is used to locate the interlaminar space as a marking point for the neuraxial procedure. The transverse views (Figs. 23 and 24) are challenging to obtain in the midthoracic spine as the transverse interspinous windows are narrow here. The presence of a rib marks the junction of the T12 and L1 vertebra. The 12 th rib can be identified to locate the T12 vertebra, and the counting-down approach can be used to locate accurate lumbar intervertebral levels, or the countingup approach can be used to locate the correct thoracic intervertebral level. Alternatively, the correct level can be determined by counting down from the T1 level, after locating the first rib.
Current Evidence for the Use of Ultrasound Imaging of the Spine to Facilitate CNB There are numerous systematic and narrative reviews that have synthesized the evidence on the role of ultrasound in the placement of neuraxial blocks in various patient populations. The important findings from each of these reviews are listed chronologically in Table 1.
The 2013 meta-analysis by Shaik et al. evaluated ultrasound imaging for lumbar punctures and epidural catheterizations [19]. They included 14 randomized controlled trials (RCTs) with 1334 subjects and found that ultrasound imaging reduced the risk of failed procedures (risk ratio 0.21, 95% confidence interval 0.10 to 0.43, p < 0.001). The number needed to treat (NNT) to prevent one failed procedure was sixteen. After subgroup analysis, they found lower risk of both failed lumbar puncture (RR 0.19; 0.07 to 0.56; p = 0.002) and epidural catheterization (RR 0.23; 0.11 to 0.65; p = 0.005) The authors found reasonable evidence for the accuracy of ultrasound imaging to identify lumbar intervertebral spaces compared to landmark palpation alone. An excellent correlation was found between ultrasound-measured depth and needle insertion depth to the epidural or intrathecal space. The ultrasound guidance resulted in a reduced failure rate with a risk reduction of 79% (RR 0.21) for failed lumbar puncture or epidural catheterization. The ultrasound use reduced the risk of traumatic procedures and total needle redirections required for success (MD − 1.00; p < 0.001).
In 2017, Elgueta et al. performed a narrative review with a slightly wider scope assessing the evidence for the role of ultrasound in the neuraxial blockade that included the pediatric and chronic pain patient populations [16]. They found that when compared to conventional palpation of landmarks, ultrasound preprocedural scanning results in fewer needle passes/insertions and skin punctures for neuraxial blocks in  [21]. A single experienced investigator performed all ultrasound examinations and skin markings. They found a significantly higher first-attempt success rate for the ultrasound group (87.5%) vs. the control group (52.5%) (p = 0.001). Additionally, the ultrasound group had fewer number of skin punctures (1.2 vs. 3.6, p < 0.001) and fewer cases requiring > 10 needle passes (1 vs. 17, p < 0.001).
In 2020, Park et al. examined the role of pre-puncture ultrasound scanning for placement of spinal anesthesia in patients with abnormal spinal anatomy [22]. In this RCT, all patients were undergoing elective orthopedic surgery under spinal anesthesia with a set of defined lumbar spine abnormalities. These abnormalities included mild to severe lumbar scoliosis with a Cobb angle ≥ 10°, or a history of lumbar surgery involving L2-L5 vertebra. The control group utilized classic anatomical landmarks for placement. The authors found the ultrasound group had a successful dural puncture with the first attempt (one skin puncture) in 90.9% vs. 40.9% in the control group (p < 0.001). In terms of needle passes, the ultrasound group had 1 st pass success in 50.0% vs. 9.1% in the control group (p < 0.001). While the actual needling time in the ultrasound group was 38 s compared to 118 s in the landmark group, the total procedure time for both groups was roughly the same 146 s vs. 141 s.

Real-Time Ultrasound Guidance for Neuraxial Procedures
One of the main drawbacks of pre-puncture scanning is that the actual procedure is still "blind." Additionally, pre-puncture scanning provides the operator with measurements at a specific time with the patient in a specific position. These measurements become inaccurate with patient movement, needle insertion, distortion of tissue, and needle angle adjustment [25]. With the application of realtime ultrasound, active needle tracking allows experienced providers to visualize their needle as it travels the tissue layers. Adjustment of needle trajectory, as well as potential confirmatory tip location, can be done with realtime imaging. Furthermore, active scanning allows for readjustment without having to remap when a patient moves from their pre-scanned position. Recent studies testing the feasibility and success of real-time ultrasound for neuraxial block placement have been promising, and the advances in transducer technology have improved the quality of the images acquired. The real-time ultrasound guidance for midline neuraxial blockade is complicated by the acoustic shadows from the vertebrae. The paramedian longitudinal approach provides superior quality images compared to classical ultrasound planes used     for preprocedural "mapping." Due to the possibility of neurotoxicity of ultrasound gel, saline is commonly used as a coupling medium for real-time imaging, reducing image quality [26]. Currently, real-time ultrasound scanning is a cumbersome and challenging technique with limited clinical utility.

Advances
Technological improvements in ultrasound imaging have been dramatic over the last 20 years, and includes experiments with three-dimensional (3D) and four-dimensional (4D) imaging [27]. A real-time 3D ultrasound imaging in conjunction with innovative needle-guide for midline epidural needle insertion technique has been described in animal models [28]. Further, the feasibility of using 4D ultrasound on cadavers for real-time epidural placement found that 4D ultrasound has the potential to improve operator orientation of the neuraxis but at the expense of decreased resolution, frame rate, and needle visibility [29]. The incorporation of a new signal processing technology to enhance bone imaging and including 3D navigation has led to production of newer devices to locate the ideal puncture site [30]. Artificial intelligence integration into ultrasound machines was recently reported [31•, 32]. Machine learning and advances in ultrasound technology have already proved to be successful in the field of cardiac imaging [33]. We can therefore expect to see something similar that would prove to be superior to current technology and increase the image quality and ease of performing real-time ultrasound-guided neuraxial anesthesia in future.

Conclusion
Neuraxial ultrasound imaging should be in the skill set of every anesthesiologist who routinely performs lumbar or thoracic neuraxial blockade. Evidence for the clinical benefits of ultrasound "mapping" prior to performing neuraxial blocks is mounting. Preprocedural imaging of the neuraxis minimizes the technical difficulty of spinal and epidural placement. It helps to accurately identify the midline, vertebral level, intervertebral space, and predicts the depth to epidural and intrathecal spaces. Ultrasound imaging also provides information about the best angle and approach for a successful block. Compared to landmark techniques, ultrasound use results in fewer needle passes and skin punctures in both obstetric and non-obstetric surgical patients. Studies consistently show that the use of ultrasound increases the success rate and ease of neuraxial block performance. These benefits are most evident when experienced operators perform the ultrasound examination and for patients with predicted difficult spinal anatomy. Evidence suggests that ultrasound usage for neuraxial procedures reduces the risk of traumatic procedures and, thus, may increase safety. The ultrasound equipment with curvilinear probes are readily available, especially in obstetric units, as they are used for fetal scanning and monitoring. Neuraxial ultrasound scanning is a non-invasive procedure with the only possible downside that it may increase the procedure-related time by approximately 2 min. However, the evidence related to increased procedural time is conflicting and may depend on the clinical situation. We recommend using preprocedural neuraxial ultrasound imaging routinely to acquire and maintain this unique skillset. This will aid clinicians to deal with challenging neuraxial procedures when required.