Ultrasound has emerged as a valuable tool in anesthesia practice, “unblinding” many of what were once “blind” procedures. Because ultrasound is a radiation-free imaging tool, it has gained popularity for patient management in many areas of medicine. In anesthesia practice, it has become important for line placement1 and provides some distinct advantages in placement of regional anesthesia.2 The educational benefits of ultrasound imaging for teaching regional anesthesia have also been elucidated.3,4
Several studies conducted to assess the effectiveness of spinal ultrasound for epidural anesthesia have shown that an ultrasound-guided neuraxial approach can reduce the number of attempts and the procedure time while at the same time increasing the block success rate.6, 7, and 8 These studies have provided a foundation for National Institute of Clinical Excellence (NICE) guidelines related to ultrasound imaging to facilitate instrumentation of the epidural space.9 The benefits of routine ultrasound imaging for intrathecal anesthetic placement are less clear.In obstetric anesthesia, central neuraxial blockade analgesia is the most common anesthetic technique. The availability of ultrasound has naturally extended interest in this modality from peripheral nerve blocks to facilitating spinal/epidural techniques in obstetrics. Successful use of spinal ultrasound to aid in the placement of epidural and spinal injection in obstetric anesthesia was first reported in 1984.5 Recently, as ultrasound machines with higher quality images have become more affordable and more commonly available, their popularity has increased further.
Ultrasound imaging is based on high-frequency sound waves that are transmitted and received by a transducer (1-20 MHz). The transducer detects both the intensity of the echo and the time required to travel back to the source, which enables the calculation of the distance of the reflecting interface. Different layers of tissue produce a separate reflection of the ultrasound signal. At each interface, some of the wave is reflected back and detected by the transducer. The proportion of reflected to transmitted wave depends on acoustic impedance of tissues forming the interface. Bone reflects the majority of the energy, so few structures beyond bone can be visualized. Ultrasound examination of spine is challenging because the area of interest is deep and shielded by a complex, articulated cage of bones. For these reasons, images of the spinal structures are best observed with a low-frequency, curved ultrasound probe (2-5 MHz). Although higher frequency allows higher image resolution, the low frequency ultrasound beam provides deeper penetration, at the expense of image resolution.
Two acoustic windows are effective for lumbar spine sonographic assessment: one seen on the transverse approach and one seen on the longitudinal paramedian approach. The information from these two scanning planes complements each other.
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