What follows are a few pointers for image optimisation, and the avoidance of common pitfalls. Machine setting instructions relate predominantly to the Sonosite equipment range, however, the priniciples can be applied to most other portable machines. Inevitably, experience is required to acquire satisfactory images and develop the necessary hand-probe-eye coordination to perform safe and effective blocks.
Chose a probe fulfilling the following requirements:
1. Highest resolution to achieve the required tissue penetration. The operating frequency of commonly available probes ranges from 4-15 MHz. Higher operating frequencies give better resolution but more limited tissue penetration. For structures < 3 cm in depth, use a probe with an operating frequency > 7 MHz (e.g. 10-15 MHz). For structures > 4 cm in depth, use a probe operating at < 7 MHz (e.g. 4-7 MHz).
2. A probe shape which will facilitate skin-probe contact and needle access to the intended skin puncture site.
For most blocks, a high frequency (e.g. 10-15 MHz) wide (6 cm) linear probe is used because it gives good resolution up to depths of 4 cm, and gives a relatively wide field of view (Fig. 1). However, in concavities (e.g. infraclavicular deltopectoral groove, immediately anterior to the achilles tendon) (Fig. 2) and when the needle has to negotiate bony prominences (e.g. the clavicle during infraclavicular block), a narrow footprint probe is more appropriate. The most commonly used narrow probes are a narrow curvilinear probe (e.g. Sonosite C11) or a narrow linear probe (e.g. Sonosite hockey stick).
Fig. 1. Probes commonly used for nerve blocks. From left: L38 (linear, 6 cm footprint, 10-15 MHz), Hockey Stick (linear, narrow footprint, 10-15 MHz), C11 (curvilinear, narrow footprint, 5-8 MHz).
Fig. 2. Narrow foot print curviinear probe. Even though the target structure (sural nerve) is < 3cm deep and would therefore suit a high frequency probe, a curvilinear (low frequency) probe is a better choice because of better skin contact in the concavity anterior to the achilles tendon.
Ultrasound machine orientation
It is preferable to have the machine directly in front of the operator so operator head/neck turning is not required for ultrasound screen visualisation. This is particularly important when using the in-plane technique, where frequent checks need to be made of probe-needle alignment so the needle shaft does not inadvertently drift out of the path of the (narrow) ultrasound beam. When the ultrasound machine is positioned directly in front of the operator, the eyes can readily switch between needle/probe (below) and ultrasound screen (above) without the need for operator head movement.
Don’t get too preoccupied with all the controls and features of the machine. Significant machine adjustments can be made, but only a few are essential to performing safe and effective ultrasound-guided blocks.
Before block commencement, set the following:
1. Res(olution) /Gen(eral) /Pen(etration) – in our experience this setting has only a limited effect on image optimisation. The setting adjusts the predominat frequencies sent out by the probe: "Gen" sends out all frequencies; "Res" the frequencies at the higher end of the probe's operating frequency; "Pen" the frequencies at the lower end. If the objective is to focus on the superficial parts of the US image, then set to "Res". If the objective is to focus on the deeper parts of the US image, set to "Pen".
2. “MB” to On.
3. “Examination” type to either Nerve (Nrv) or Vascular (Vas).
After placing the probe on the patient adjust:
1. Gain: to optimise contrast between the target to be visualised (nerve, vessel or fascia) and the surrounding structures. The “AutoGain” function can be useful for setting the appropriate gain setting. Just as important as this setting is ensuring an appropriate ambient lighting level – “comfortably dim”. With too many lights on, the ultrasound image will be difficult to see; too dark and needle/probe/patient visualisation will be difficult. Avoid direct light streams coming from a window, which may cause glare on the screen and difficulty with image optimisation. Finally, in our experience, the separate “near” and “far” field gain controls are of limited value.
2. Depth: so the target structure is near the centre (e.g. interscalene, median nerve) or deeper third (e.g. infraclavicular, sciatic) of the ultrasound screen. If depth is set too high (deep), the target structure will appear small.
3. Mode: to 2D. The colour doppler and colour power doppler function can be useful for differentiating vascular from non-vascular structures. Effective use of this mode requires the probe to be orientated partly in the direction of the blood flow/vessel rather than exactly 90 degrees perpendicular. A useful technique to facilitate doppler enhanced flow is "compress and release": press firmly on the skin to obstruct the vein and then release this pressure - the subsequent increase in venous flow should be readily visualised.
Due to the slippery nature of ultrasound gel, a common fault made by novices is inadvertent probe drift across the skin. It is a good idea to stabilise the probe in position by pressing the wrist, side of the hand (holding the probe) or the little finger against the patient (Fig. 3).
Fig. 3. Probe stabilisation (in this case through use of the little finger).
Central to optimal ultrasound imaging is probe orientatation 90 degrees to the target structure, so the ultrasound beam emitted and subsequently reflected will be received by the probe and therefore projected on to the ultrasound screen. This 90 degree orientation requires a combination of the following probe movements:
1. Rotation (around the probe’s long axis) - so the ultrasound beam has a transverse orientation to the nerve (i.e. making a "square cut"). Rotation is particularly useful during the in-plane technique to re-align the probe with the needle shaft because once the needle is located deep within the muscle, needle re-angulation can be difficult. Probe rotation in this setting usually does not significantly compromise short axis nerve visualisation.
2. Tilting (back and forward) - so the emitted ultrasound beam approaches the target structure exactly perpendicular. Tilting is also important during colour doppler blood flow detection as the probe needs to be angled to some degree in the direction of flow.
3. Tilting (sideways) – to maximise probe-skin contact.
Left-Right probe orientation: Immediately after placing the probe on the skin, check probe orientation by sliding laterally left to right to left. We typically align the right side of the US screen to the right side of the patient (or the patient's left side if facing caudad to cephalad during lower limb blocks)
Short extension tubing vs. syringe connected direct to needle hub
Many anesthesiologists experienced in ultrasound-guided regional anesthesia advocate performing blocks with a short piece of extension tubing between needle hub and syringe. This is often called the “immobile needle technique”. Because the needle hub is not directly connected to the operator’s hand, the needle is suspended freely and said to be more stable during LA injection, particularly if sudden patient movement occurs. This was commonly an issue for blocks performed using the paresthesia technique where sudden patient movement was an issue. We rarely use this technique and prefer to have the LA or dextrose filled syringe attached directly to the needle hub. This is because the procedure is less assistant dependent as the operator can both inject and aspirate independently. It therefore requires one less step (operator assistant communication) for feedback during LA/dextrose aspiration/injection.
Needle tip visualisation
We prefer a large calibre needle (e.g. 18G) with a Tuohy tip. When using the in-plane technique, larger calibre needles are more readily visualised. For both the in-plane and out-of-plane techniques, needle tip approximation is facilitated by use of a blunt needle tip, which results in tissue displacement on needle advancement. Needle tip approximation can also be facilitated by injection of 1-2 mL aliquots of LA/saline/dextrose. With the In-plane technique, in which it is possible to visualise the entire needle shaft and tip, try to align the needle as perpendicular as possible to the direction of the US beam as this will improve needle shaft visibility (more waves reflected back to the transducer). For deeply situated target structures, achieving a needle angle perpendicular to the US beam requires a skin puncture site as far way from the probe as is practicable.
For the out-of-plane technique, a blunt needle is imperative as needle tip approximation is dependent on observation of tissue displacement +/- observation of small aliquots of injectate.
Consider using an echogenic needle. These are now available in a range of calibres and tip configurations e.g. Pajunk echogenic Tuohy needle.
Local anesthetic visualisation
LA spreading immediately adjacent to a fascial layer appears hypoechoic, generally with a clear demarcation between the surrounding structures e.g. between fascia and nerve or between 2 fascial layers. When LA injectate appears hyperechoic, intramuscular injection should be suspected.
Machine and probe care
1. Do not allow the screen to be cleaned with solvent based cleaner. They may cause troublesome smudges and may even cause surface damage.
2. Do not allow the probe to come into contact with iodine based antiseptics, which will cause staining over time. Non-tinted alcoholic chlorhexidine or clear disinfectant wipes will not cause probe discolouration.