In order to improve ultrasonic needle visibility, manufacturers indented the surface of needles with angular depressions. Studies of single injection and Tuohy needles in Thiel embalmed human cadavers [2, 4] and patients have shown improved visibility of needle tip and needle shaft for both in-plane (Fig. 16.1) out-of-plane needle insertion. In a study conducted on the soft embalmed Thiel cadaver [2] the median (IQR) proportions of visible needle tips were 83 % (67–83 %) for the echogenic Tuohy needle, 75 % (67–83 %) for the echogenic single-shot needle and 33 % (33–46 %) for the smooth-surfaced Tuohy needle (p < 0.001). Median (IQR) needle shaft visibility (between 1 and 5) was rated as 4.0 (3.5–4.7) for the echogenic Tuohy needle, 4.0 (3.8–4.5) for the echogenic single-shot needle and 3.0 (2.4–3.3) for the smooth-surfaced Tuohy (p = 0.015).

Improved overall visibility can be explained by the circumferential placement of reflectors, in contrast to standard needles which scatter beams randomly and transmit them poorly back to the transducer [5]. Clinical experience confirms cadaver findings, although pain associated with needle movement can be problematic, and poor visibility persists when using high angles in-plane and narrow angles out-of-plane are used.

The development of needle guidance systems has been driven by the need for accurate cancer biopsy [6] and diagnosis, particularly in liver and prostate cancer. For surgical procedures, including laparoscopy, ultrasound based systems using electromagnetic and/or robotic guidance [7, 8] have been shown to be more accurate than freehand targeting of lesions.

Perioperative interventional ultrasound procedures are also complex. The 3D alignment of the needle and position of the needle tip relative to the target is difficult to calculate, and using, hydrolocation, the incremental injection of small aliquots of local anaesthetic, tissue anatomy becomes distorted and the target moves. With these difficulties in mind, a needle guidance system (Sonix GPS, Ultrasonix, Vancouver, Canada) has been developed. The equipment consists of a transmitter and two sensors – at the needle tip and in the transducer – allowing real-time tracking in three dimensions. Before conducting the block, the sensor needle is positioned either in-plane or out-of-plane to the ultrasound transducer at an angle aiming for the target. From this information a white trajectory is plotted on the screen. During needle insertion, guidance is optimized with the aid of coloured orientation bars (where green represents good orientation and red represents poor orientation) and a schematic diagram of needle transducer alignment (Fig. 16.2). The advantages of this system are that the optimal needle orientation is decided before insertion, potentially reducing the number of needle passes, the needle tip is seen irrespective of plane or angle, and that it is applicable to linear and curvilinear transducers. Two clinical studies have reported on performance of spinal anaesthesia using electromagnetic guidance. Successful spinal anaesthesia was achieved in between 70 and 90 % of patients [9, 10], but on the first pass of the needle only in between 35 and 57 % of patients. These results show the potential utility of a GPS system, but one must bear in mind that these results were obtained in patients with BMI <35 who had no history of back surgery or spinal deformation – a low risk group! It is crucial, when introducing new technology to the market place in any speciality, that efficacy and side effects are investigated on a predefined population that encapsulates the population of patients with most to gain and/or at high risk of full or partial technical failure. Comparison between low risk and high risk groups is called stratification and offers a more precise evaluation of efficacy and side effects [11].

The GPS system was also evaluated in 26 medical students who conducted 30 simulated ultrasound nerve blocks on a pork simulator [12]. Students were randomised into GPS tracking or control groups, and block success or failure recorded. The GPS group had fewer attempts and conducted blocks in a shorter length of time compared to those using standard B-Mode ultrasound. It would appear that the GPS system may have benefits to training novice practionners.

Knowledge of real-time tissue properties during needle movement could potentially improve the accuracy of regional anaesthesia and cancer biopsy. The optical characteristics of tissues, measured using spectroscopy, have been investigated as a means of differentiating between different tissues and may indicate the position of the needle tip. Integrated optical fibres transmit near infra red light from the tip of the needle or introducer. Spectroscopic analysis of reflected light estimates the scattering coefficient and the absorption coefficient of tissue such as haemoglobin, oxygenated haemoglobin, water and adipose tissue. Each tissue has a characteristic narrow absorption peak; oxyhaemoglobin absorbs light in the visible spectrum, whereas lipid and water absorb light in the near-infrared spectrum.

Experiments conducted in pigs have identified the ligamentum flavum and epidural space [13] and transitions from muscle to nerve in the axillary region [14]. Proximity to peripheral nerves was associated with higher lipid and lower hemoglobin values in pigs [14] and patients [15]. Future benefits may include detection of tissue vascularity [16] and reduction in accidental intravascular injection.

Although clinically appealing, several issues need to be resolved before commercialization. Adipose tissue stores vary between patients and with age, sex and BMI; spectral change from muscle to nerve is more acute than adipose tissue to nerve; and false positive results may arise from blood accumulation at the needle tip due to repeated injections. Spectral analysis may also be altered by transducer compression, reducing the distance between the skin and nerve target, and needle angulation. Future clinical trials will establish diagnostic accuracy by measuring the sensitivity and specificity of the test compared to traditional loss of resistance techniques.

Optical coherence tomography (OCT) is a laser based imaging technology already used clinically to image retinal disease. Resolution is remarkable down to between 10 and 30 μm. However, backscatter and absorption of the laser beam limit imaging to the first 2–3 mm of tissue. Incorporation of OCT into an epidural needle [17–19] has the potential to Image anatomy in deep spaces with very high resolution, delineate anatomical boundaries and reconstruct images in 3-D.

Vibration of needles at ultra-high frequencies has been investigated as a means of visualising interventional needle. Actuation of peizo-electric crystals up to 20,000 Hz has recently demonstrated full visibility of needles using colour Doppler ultrasound (Fig. 16.3). Initial cadaver work have shown improved visualisation along the length of needle and at the needle tip at any angle, tissue depth and plane relative to the ultrasound transducer [20]. Added advantages include reduced force of tissue penetration and less deflection.

As ultrasound waves pass through tissue, acoustic intensity diminishes as energy is attenuated within tissue. The greater the distance travelled and the higher the transducer frequency the more energy is lost. Attenuation of ultrasound energy occurs through absorption, scattering, reflection or refraction of ultrasound. Reflection and refraction of ultrasound waves occurs at the interface of tissues with different acoustic impedance, the product of density (σ) and speed of sound of acoustic waves (c). The greater the relative difference in impedance, the greater the reflection and the less the transmission of ultrasound waves [34]. The brightness of the image on the screen is dependant on the amount of acoustic energy returning back to the transducer; surfaces such as periosteum, tendons, pleura are hyperechoic because they reflect more energy, whereas interscalene nerve roots, blood and fluids are hypoechoic because they reflect less energy. Thus, in current clinical practice, linear array high transducer frequencies between 8 and 15 MHz are recommended order to image superficial anatomy whereas to image deeper structures convex low frequency 2–6 MHz transducers are used, albeit with less resolution.

In basic science, there is a need to scan small animal models in the fields of developmental biology, genomics and cancer in order to characterize anatomy. Both CT and MRI provide high quality images but are expensive and cumbersome. In order to observe changes in tissue configuration, a high resolution of 20–100 μm is needed, corresponding to ultrasound frequencies between 150 and 30 MHz. Mechanical single element transducers with a capacity to generate frequencies up to 100 MHz have traditionally been used, but are now being replaced by linear transducer arrays with greater functionality. As well as investigating the molecular basis of disease, micro-ultrasound will, in future, be used clinically to characterize the ultrasound signatures of organs such as breast, prostate and skin in order to detect and treat pre-cancerous tissue changes.

Application of micro-ultrasound to UGRA may not be immediately obvious until one recognizes that, although the widespread application of B-Mode ultrasound to UGRA has improved the efficacy of nerve block, the incidence of transient neuropraxia has remained unchanged compared to peripheral nerve stimulation techniques [35]. As described above, the greater the distance between transducer and target, the greater the energy attenuation and the poorer image resolution. Experimentation with different shaped transducers using a wide range of frequencies is unlikely to resolve this clinical dilemma as obesity levels are rising within Western economies. One solution is to develop active ultrasound needles with peizo-electric crystals at the tip in order to more accurately identify relevant anatomy and target local anaesthetic injection. For clinical application we would propose using a combination of an active needle with a traditional ultrasound transducer and two images. The needle would be guided to, for example, within 1–1.5 cm of the target nerve, then the micro-image activated, nerve and connective tissue identified and the needle tip guided precisely to its destination. Two studies have been published on the application of microultrasound in regional anaesthesia. Scanning fresh cadaver nerves using 30 MHz frequencies, laboratory images [36] were able to delineate the epineurium, the size of the nerve, the number and size of fascicles, as well as visualise needle insertion and intraneural tissue trauma. Ultrasound images showed fascicles down to 0.5 mm diameter (Fig. 16.7), and splitting or rotation of fascicles when a needle was inserted into a nerve. In the second study [33], a single crystal 40 MHz ultrasound transducer was placed within an 18 g Tuohy needle in order to locate the epidural space of experimental pigs. All spaces were successfully identified and catherised using the paramedian approach in five pigs. Injected contrast confirmed anatomical spread.