Visualisation of needle position using ultrasonography

Transcription

1 doi: /j x REVIEW ARTICLE Visualisation of needle position using ultrasonography G. A. Chapman, 1 D. Johnson 2 and A. R. Bodenham 3 1 Research fellow in Anaesthesia and Intensive Care, 2 SpR in Anaesthesia, 3 Consultant in Intensive Care and Anaesthesia, The General Infirmary at Leeds, Leeds, UK Summary Anaesthetists and intensivists spend a considerable proportion of their working time inserting needles and catheters into patients. In order to access deeper structures like central veins and nerves, they have traditionally relied on surface markings to guide the needle into the correct position. However, patients may present challenges due to anatomical abnormalities and size. Irrespective of the skill of the operator, there is the ever-present risk of needle misplacement with the potential of damage to structures like arteries, nerve bundles and pleura. Repeated attempts, even if ultimately successful, cause patient suffering and probably increase the risk of infection and other long term complications. Portable and affordable, high-resolution ultrasound scanners, has accelerated the interest in the use of ultrasound guidance for interventional procedures. Ultrasound guidance offers several advantages including a greater likelihood of success, fewer complications and less time spent on the procedure. Even if the target structure is identified correctly there is still the challenge to place the needle or other devices in the optimum site. The smaller and deeper the target, the greater the challenge and potential usefulness of ultrasound guidance. As a result of limited training in the use of ultrasound we believe that many clinicians fail to use it to its full potential. A lack of understanding, with regard to imaging the location of the needle tip remains a major obstacle. Needle visualisation and related topics form the basis for this review.... Correspondence to: Dr G. A. Chapman Accepted: 26 September 2005 Historically, ultrasound has been used to guide needle, catheter and guidewire placement by radiologists but it now is being used increasingly by anaesthetists for vascular access, nerve blockade, drainage of pleural or ascitic fluid collections and percutaneous tracheostomy. Ultrasound allows identification of the target and collateral structures and real time guidance to precisely place needles. The ultrasound appearance of vessels, nerves and other structures is well described elsewhere [1, 2]. We believe that many trainees and consultants fail to use ultrasound to its full potential during interventional procedures. A lack of understanding of how to assess the position of invasive devices and in particular the location of the needle tip is a major obstacle. Needle visualisation is essential when inserting needles into tissues, which may be in close proximity to structures such as vessels, nerves and the pleura. Subsequent visualisation of catheters and guidewires within target structures also promotes safe practice and may avoid the need for X-ray verification [3]. The theory and practice of needle visualisation with ultrasound is not well described even in radiology texts [4, 5], and is starting to generate interest in the anaesthetic literature. To obtain the best possible images, a basic understanding of the physics and instrumentation related to medical ultrasound is required. This is extensively covered elsewhere [6 10]. Applied physics Audible sound ranges in frequency from 15 to 20 khz. Ultrasound is defined as any sound with a frequency > 20 khz and for medical imaging this is typically within the range 3 15 MHz. The ultrasound pulse is generated by applying an electric voltage to piezo electric crystals within the probe, which is directed into tissues. The echo 148 Journal compilation Ó The Association of Anaesthetists of Great Britain and Ireland

2 G. A. Chapman et al. Æ Needle position using ultrasonography is produced by reflected sound waves returning to the probe deforming the crystals, to produce an electrical pulse (the echo). Since the speed of sound is relatively constant in soft tissues (1540 m per second), the time taken for the ultrasound pulse to return is proportional to the depth of the structure it was reflected from. This is the pulse echo principle and forms the basis of ultrasound imaging. The amplitude of the echo at various depths is reproduced visually on a grey scale. By convention, no reflection is displayed as black (anechoic) and a strong reflection as white (hyperechoic). Multiple echoes are produced by multiple repeated pulses generated by an array of crystals in the ultrasound probe, allowing a real-time image to be generated. Advanced computing controls all aspects of ultrasound imaging. Tissue interfaces A tissue interface is formed where a tissue of one type abutts onto another. These tissues are said to have different acoustic impedances. At a tissue interface an ultrasound wave can be reflected, refracted or scattered. The same principles apply to both biological tissues and artificial devices. Reflection of a sound wave occurs when it strikes the boundary between two media. Depending upon the angle of reflection, sound waves can return to the transducer probe to provide a signal. Optimum echoes would be provided by ultrasound waves that are reflected back at 90, but this will only be the case for some reflected sound waves. Reflection at other angles may result in a distorted image or artefact. To obtain an optimal image of superficial and deep structures, it is important that not too much of the ultrasound beam is reflected in the superficial tissues (Fig. 1). Reflections from a large smooth surface (e.g. diaphragm or needle) are known as specular reflections. Scattering occurs when the sound wave strikes a small or irregular surface (e.g. erythrocytes) and is reflected in many different directions. Surface size is relative to that of the sound wave and in practice reflection from tissues may be generally regarded as a combination of the above. Some of these scattered sound waves will return to the probe and produce a signal (Figs 2 and 3). Multiple reflections can occur between two strong reflectors and the transducer itself can act as a reflector. This is known as reverberation. These echoes may be sufficiently strong to be detected by the instrument. Reflections that are not real are still displayed and may cause confusion. Multiple reflections can originate between two anatomic reflecting surfaces or walls of a needle, which when in close proximity appear in a form known as a comet tail. This represents a series of closely spaced discrete echoes. When separate echoes cannot be Figure 1 A) When an ultrasound beam is perpendicular to an interface, it will return by the same path. B) If the beam and the tissue boundary are not perpendicular, the reflected ultrasound will travel along a different path. C) When an ultrasound beam is transmitted through an interface between two media of different acoustic impedance, and the beam is not perpendicular to the interface, the ultrasound beam will be refracted (bent). Figure 2 Cross-sectional image of an 18G needle in a water bath. A bright spot in the centre of the display represents the needle tip. An artefact surrounding the tip is caused by scattering of the ultrasound beam; this would be much less easily visible within the heterogeneous appearance of body tissues. Journal compilation Ó The Association of Anaesthetists of Great Britain and Ireland 149

3 G. A. Chapman et al. Æ Needle position using ultrasonography Anaesthesia, 2006, 61, pages The intensity of the reflected sound is portrayed by the brightness of the image. Water, blood cells, fat, liver cells, bile, bile duct walls, blood vessel walls, and connective or fibrous tissue all have densities sufficiently different to create an interface. Needles, catheters and guidewires have markedly different acoustic impedances compared to soft tissues, with the effect that the majority of the ultrasound beam is likely to be reflected and scattered from the tissue device interface. Theoretically, such invasive devices should be easily seen with ultrasound but many other factors are also relevant. Figure 3 A 16G Tuohy needle in longitudinal section at an angle of approximately 45 to the probe with injection of micro bubbles. The shaft of the needle is seen but the machined tip is brighter with an associated artefact due to scattering of the ultrasound beam. identified and the emission of sound from the origin appears continuous, this is known as a ring down artefact. This is caused by a resonance phenomenon associated with the presence of a collection of gas bubbles. Refraction occurs when the transmitted beam is deviated from the path of the incident beam. Refracted sound travels on through the interface to be refracted or reflected at deeper structures. Refraction can lead to distortion or image artefact but this is rarely of clinical significance. Acoustic impedance The acoustic impedance of a tissue is a product of the density of the tissue and the speed of sound in that tissue and is measured in Rayls. The speed of sound in soft tissues is assumed to be constant (1540 m.s )1 ) for ultrasound machine setup and calibration. Were this actually the case, the acoustic impedance would be altered only by a change in the density of the tissues, and the reflection of an ultrasound wave would only occur at the interface of tissues of differing densities. However, the speed of sound varies in different soft tissues ( m.s )1 ). Only small changes in the density of tissues are required to form a tissue interface and so both a change in speed of sound and density of different soft tissues enable them to be sonographically visualised and differentiated. The intensity of sound that is reflected at an interface is determined by the difference between the acoustic impedances of the two tissues. If the difference is small, only a small percentage of sound will be reflected. If the difference is large, most of the sound will be reflected. Attenuation The decrease in signal strength (amplitude) as it passes through a medium is known as attenuation. This may be as a result of absorption, scatter, reflection or divergence of the ultrasound beam. Shadowing If the acoustic impedances of a tissue interface are sufficiently different, then most or all of the ultrasound will be reflected. The area behind this interface will appear black and is termed an acoustic shadow. This shadow, corresponding roughly to the width of the interface, fills the area behind the interface down to the lower border of the image. Hence it is not possible to image structures deeper to bone or gas using ultrasound. A needle passing through the ultrasound beam in transverse section produces a shadow, seen as a black stripe, the approximate width of the needle, passing downwards on the display (Fig. 4). This artefact shows where the needle is crossing the beam but does not represent the path of the needle beyond it. If the needle is in longitudinal section no structures will be visualised deep to the length of the reflective element. Enhancement Echoes of interfaces behind or within a structure of low attenuation (low reflectivity) will appear enhanced, hence the request for a full bladder for pelvic examinations. Vessel walls are enhanced due to the presence of blood. This effect can be seen in an agar phantom with a fluid filled channel (Fig. 5) and when the needle or cannula is within a fluid filled space (Fig. 6) [11]. Phantoms A phantom is an object that mimics the properties of biological tissue with respect to sound transmission. Phantoms are used to test ultrasound imaging devices and as a substitute for patients for clinical practice [12, 13]. Phantoms can be homemade from agar, vegetable or meat products [14]. Commercial products consisting of an agar 150 Journal compilation Ó The Association of Anaesthetists of Great Britain and Ireland

4 G. A. Chapman et al. Æ Needle position using ultrasonography Figure 4 Agar phantom with 18G vascular access needle being advanced towards the fluid filled cavity (black hole) in transverse plane. A) The tip of the needle enters the beam and is represented by the white dot. B) As more of the tip and shaft enter the beam there are progressively more artefacts double dot and acoustic shadow. C) Anterior wall of agar fluid interface becomes indented. D) Agar fluid filled tube re-expands and is restored to near normal caliber with the needle tip seen in the fluid filled cavity. Figure 5 An agar phantom with fluid filled channel in longitudinal section. Note enhancement of front and back wall agar fluid interface (bright white line). An air bubble causes sound to be reflected with a bright interface and deep artefact showing mirror image. Figure 6 A 20G IV needle and cannula has been inserted into a solid agar block. The needle was withdrawn and 2 ml water injected through the cannula. The front and back walls of the fluid filled cannula are visible. Journal compilation Ó The Association of Anaesthetists of Great Britain and Ireland 151

5 G. A. Chapman et al. Æ Needle position using ultrasonography Anaesthesia, 2006, 61, pages Figure 7 Different approaches to needle and target structure visualisation. The target structure, i.e. the fluid filled plastic pipe in the agar block (a phantom), can be in transverse (T) or longitudinal (L) section. Needles can be introduced in transverse (t) or longitudinal section (l). block with fluid filled tubes to represent vessels are available (Fig. 7). They are useful to demonstrate needle visualisation and orientation, but are expensive ( ) and deteriorate fairly quickly with repeated use. Unfortunately, the agar block and plastic tubes do not closely mimic the feel or visual effects of real body tissues and vessel walls. Despite such limitations we believe that phantoms or simple water baths should be used prior to attempting needle visualisation in patients. Transducers Transducer design The frequency of sound used by ultrasound probes is a compromise between penetration and resolution. Low frequency transducers allow imaging of deeper structures, but at a lower resolution. High frequency transducers produce higher resolution images, but are only useful for more superficial imaging. Penetration is the ability of ultrasound to travel to depth. High frequency sound waves are attenuated faster than lower frequency sound waves, thus their ability to penetrate tissue is decreased at higher frequencies. Resolution is the ability to discriminate between two points. The resolution of an ultrasound image can be described in three planes axial, lateral resolution and slice thickness (Fig. 8). Axial resolution is the resolution along the direction of the ultrasound beam and is limited by the duration of the ultrasound pulse. As the frequency of the ultrasound increases, the pulse duration shortens, and the axial resolution is improved. Transverse (lateral) resolution is the resolution at right angles to the beam direction and is limited by the bandwidth at the point of reflection. It Figure 8 Transducer resolution is described in three different planes axial, lateral and slice thickness. Slice thickness relates to the width of the beam in the non-imaging plane and governs the thickness of the slice of tissue being imaged. depends upon the probe design and in particular the density of crystals. Lateral resolution is poorer than axial resolution. Slice thickness refers to the out-of-imaging plane beam thickness. This will affect the region perpendicular to the scan plane over which returning echoes will be obtained. Ideally, slice thickness should be as small as possible to maintain image quality. In essence, a needle may appear to be within or adjacent to the desired target structure but lie above or below it. The operator should be aware of 152 Journal compilation Ó The Association of Anaesthetists of Great Britain and Ireland

6 G. A. Chapman et al. Æ Needle position using ultrasonography the restrictions of 2-D imaging whilst manipulating the needle tip in three planes. The concept of resolution with regards ultrasound is covered in standard radiology texts [15]. Temporal resolution is the ability to locate the position of a moving structure at a specific time point and is dependent on the frame rate. This is important with regards echocardiography, particularly at high heart rates, but is of less consequence to anaesthetists wishing to perform needle-based interventions on relatively immobile structures. Focusing Lateral resolution may be improved by focusing the ultrasound beam where the beam width is reduced by mechanical or electronic means. The beam becomes narrower, improving lateral resolution, but this is restricted to the focal zone settings of the transducer. In the context of needle based interventional procedures in anaesthesia, a small parts vascular linear array probe (7 12 MHz) is typically used. Colour Doppler imaging is useful for identifying blood flow in vessels. It can also show movement of the needle [16] but is not normally used for that purpose. Transducer and display set-up Probe orientation is important to avoid the need to manipulate needles in a mirror image of that displayed on the screen. Touch one side of the probe or move the probe on the patient and observe the image to allow correct orientation. A palpable orientation marker is usually present on the side of transducers, which corresponds to a marker on the display. By convention, when imaging the body in the transverse plane (right angles to the long axis), the right side of the patient corresponds to the left side of the ultrasound image (i.e. the same as looking up from the feet, as is the convention with CT images). When imaging in the longitudinal plane (along the long axis of the patient s body or limb), convention dictates that the left side of the image is orientated towards the patient s head. The top of the display corresponds to the transducer face (in contact with the skin) and the bottom represents the lower extent of tissue visualised by the device. All these settings can be adjusted or reversed electronically and the machine set-up should therefore be checked prior to use. We would suggest when using ultrasound for needle visualisation that the operator sets up the orientation in the correct anatomical orientation as it would be viewed from where the operator is positioned. Needle movements by the operator will then correspond anatomically to those on the display. Advances in technology may make such orientation easier [17]. The depth control is used to adjust the depth of the field of view. If this is set too deep, all areas of interest will be small and compressed into the top half of the image. If too superficial, the structures of interest will not be viewed in their entirety and important collateral structures may not be seen. There are depth markers (cm) on the screen. The depth control should be adjusted until the structures of interest are close to the centre of the screen. The gain control alters the brightness of the image. Gain controls are typically divided into overall, near and far. As the name suggests, overall gain changes the amplification of the whole display. Near gain changes the amplification adjacent to the transducer, the top half of the display. Far gain changes the amplification of the image furthest away from the transducer. A relatively dark image may allow easier identification of the white dots generated by a needle passing through tissues. Sterility Ultrasound gel is required for acoustic coupling between the transducer protective sheath skin interfaces. The gel is also a lubricant that enables the operator to move and manipulate the probe. For interventional procedures this gel needs to be sterile both inside and outside the protective sheath. To use ultrasound to assess needle position, a sterile field is required. Transducers cannot be autoclaved without damage and should be cleaned with disinfectant solutions designed for this purpose. A long sterile plastic or latex sheath is used to cover the probe and cable (Fig. 9). Needle visualisation Considerable real-time scanning experience remains the key factor for successful performance of ultrasoundguided interventions. Nevertheless, useful practice and information can be obtained utilizing a water bath or agar phantom. We illustrate images from such techniques to reduce the background signals and artefacts from body tissues that mask needle imaging, and emphasise the potential benefits of practising on such aids before approaching patients. The images shown here were generated using Sonosite TM Titan and ilook ultrasound machines (Sonosite, Hitchin, Herts., UK), a commercial agar phantom immersed in a water bath, and a water bath alone. Assessment of needle position The importance of real-time assessment of needle position cannot be overstated. Without accurate identification of the position of the needle it is possible that damage to collateral structures may occur, even if the Journal compilation Ó The Association of Anaesthetists of Great Britain and Ireland 153

7 G. A. Chapman et al. Æ Needle position using ultrasonography Anaesthesia, 2006, 61, pages Figure 9 Sterile disposable needle guides mounted over sterile plastic sheath and ultrasound probes. Guides can be aligned in transverse (A) or longitudinal configuration (B). CISCO guides and Sonosite probes were used. target structure has been correctly visualised using ultrasound before starting and the initial starting position of the needle is correct. We would emphasise that for vessel puncture and major nerve trunk blockade there will be at least three major structures in close proximity in the neurovascular bundle, i.e. an artery, vein and nerve. You should not contemplate needle placement until you have made every effort to visualise all three of these structures. Unfortunately, there is no universally accepted nomenclature to describe the needle position within the ultrasound beam. We describe the needle as being longitudinal when this is in line with the long axis of the ultrasound probe. In transverse, the needle is inserted adjacent to the longest side of the probe and is seen in relation to the short axis of the probe. A number of factors influence assessment of needle visualisation [18]. Larger needles are more easily seen than smaller needles, particularly in cross-section. Smaller gauge needles may produce fewer artefacts. Needles placed perpendicular to the beam are easier to visualise than needles placed parallel or at a less acute angle to the beam. The needle tip can often be visualised even when the shaft cannot. This is because the needle tip, with its machined cutting bevel, has an irregular surface. The portion of the beam that interacts with this interface is scattered in all directions. Some of the beam will therefore be reflected back to the transducer, even when the shaft of the needle is nearly parallel to the ultrasound beam (Fig. 2). The principle of scattering can also be utilised to improve visualisation of the shaft of the needle. The inner or outer aspect can be scratched or roughened, increasing scattering and producing an echo [14, 15]. Teflon coating or a guidewire trocar through the needle may improve visualisation. Extra reflective needles specifically designed for ultrasound are commercially available. There is debate as to how useful such modifications are in clinical practice [21]. Most needles are sufficiently visible sonographically providing the correct alignment is maintained. Introducing the needle in a short in-and-out or sideto-side motion causes deflection of the adjacent soft tissues and makes the trajectory of the needle more discernible within the otherwise stationary field. This may be likened to the analogy of finding a moving needle as opposed to a static needle in a haystack. When using a focused transducer, needles are seen best when within the focal zones. Rocking the transducer into the path of the needle, if it has deflected out of the image plane, will improve needle visualisation. When the needle shaft crosses the ultrasound beam an acoustic shadow is seen. Injection of fluid has been shown to enhance needle and catheter tip visualisation [11]. This may be useful in nerve blockade where it is helpful to confirm the site and spread of local anaesthetic solution. Injection of solutions, containing micro bubbles, from the needle into a fluid filled vessel can be seen with ultrasound and this is the basis of one type of ultrasound contrast media [22, 23]. This may be used to identify central venous catheter tips in the central veins using trans-oesophageal echocardiography. Any solution injected from a syringe will have some micro bubbles in it (Fig. 3). These can be dramatically increased by agitation or repeated injection between two syringes with a nearly closed 3-way tap. 154 Journal compilation Ó The Association of Anaesthetists of Great Britain and Ireland

8 G. A. Chapman et al. Æ Needle position using ultrasonography Injection of air or micro bubbles in solid tissues is not helpful because the acoustic shadow they produce spoils further visualisation of deeper structures. This may be relevant with ultrasound-guided injection of local anaesthetic solutions. Pre-warming of local anaesthetic solutions may help prevent micro bubble formation as cold solutions warm up to body temperature. It is sensible to use fluid filled needles and the minimum number of syringes necessary to avoid inadvertent air deposition in tissues. Guidance techniques There are two main ways that needles can be guided through the tissues direct and indirect. Indirect ultrasound guidance Indirect ultrasound guidance involves identification of the structure to be localised prior to puncture. The skin is marked and the angle and depth of the needle course estimated. This method is likely to be safer than pure landmark techniques as it identifies the approximate position of a suitable target structure, e.g. a large patent and non-thrombosed vein. However, it is only really suitable where there is a widespread superficial collection to be drained, for example ascitic fluid or a pleural effusion, which do not require precise needle placement. Even in these circumstances the use of ultrasound to guide the needle will avoid collateral structures, e.g. the epigastric intercostal vessels or the bowel. Many users of ultrasound deliberately or unknowingly do not progress beyond this approach due to lack of expertise in needle visualisation. Direct ultrasound guidance Direct ultrasound guidance involves the use of ultrasound to visualise the needle in real-time as it traverses through the tissues and to guide it to precisely penetrate or lie alongside the target structure. Collateral structures can be deliberately avoided. Direct ultrasound guidance may be divided into two broad techniques. Needle guidance devices. Needle guides direct the needle in a predetermined direction to various depths from the transducer surface, depending on the selected angle of the guide relative to the transducer [24 26]. The guides vary between manufacturers and may be a fixed part of the transducer, or detachable, sterile, single use plastic or reusable metal devices. Fixed guides will lie within the sterile sheath. Sterile, detachable guides are attached onto the probe over the outside of the sterile sheath either in the transverse or the longitudinal plane (Fig. 9). Guidance lines can be generated electronically on the display to show the approximate path of the needle. The probe is then adjusted so that the target structure lies within the guidance lines. Different predetermined depth guides will be required for needle insertion in the transverse plane, as the needle will cross the ultrasound beam at a predetermined depth. The appropriate guide is chosen after measurement of the depth of the target structure from the display. Needle guides tend to be costly but do help the inexperienced user and produce valuable insight into the concepts of different approaches to needle visualisation. These guides may be particularly useful to direct needles into deeper structures like the kidney. Longer than standard needles may be required to compensate for the less acute angle of trajectory and the section of needle held in the guide. The direction of the bevel and the flexibility of the needle shaft may alter the trajectory, particularly with long needles. The target structure is identified and positioned within the guidance lines on the display and the probe held still. The depth from skin can be estimated or formally measured on the display. The needle is clamped in the guide and passed through tissues either to a predetermined depth or until needle tip position in the target structure can be identified by direct visualisation or other means (e.g. aspiration of blood). Free hand puncture. Many experienced operators prefer the free hand approach in which the needle is inserted through the skin directly into the ultrasound beam without the use of a guide [14, 27]. This provides greater flexibility without having to use accessory instruments and allows for subtle adjustments compensating for improper trajectory or patient movement. The operator holds the transducer with one hand and inserts the needle with the other. The target structure may be imaged in longitudinal or cross-section. The needle may be inserted parallel to the transducer (longitudinal) or perpendicular (transverse) to it. The choice will depend on the applied anatomy, desired direction of needle insertion, ease of visualisation of structures with ultrasound, and operator skill and experience. For example, a cross-sectional view of central veins may be easier to interpret, and give a better view of collateral structures such as the artery or chest wall. Machine operator dynamic interaction Real-time scanning techniques produce a rapid series of images which are displayed sequentially to depict motion, giving a real-time 2-D image of a 3-D structure. With the introduction of the needle an operator machine feedback loop is formed, and, with practice, control of the probe and needle is accomplished. Novice operators may well Journal compilation Ó The Association of Anaesthetists of Great Britain and Ireland 155

9 G. A. Chapman et al. Æ Needle position using ultrasonography Anaesthesia, 2006, 61, pages feel they need a third hand to control probe, needle and syringe simultaneously. Self-aspiration devices (to identify penetration of a fluid filled target structure) may free up the hands and allow easier direct control of the needle. The operator is required to concentrate on the image on the screen as well as on the patient. Hence it is important to have the patient s anatomical area of interest and the ultrasound screen in the same line of vision and to have the correct anatomical orientation between the two (see above). Usually this means placing the display at eye level on the opposite side of the patient to where the operator is positioned. Techniques of needle visualisation The importance of ultrasound-guided insertion of central lines for vascular access is well recognised [28, 29]. A recent review has illustrated the techniques of ultrasound used during nerve blockade [30]. Irrespective of the technique that is being performed, some verification of needle tip placement in the target structure is required, e.g. aspiration of blood or fluid, or re-opening of the vein following collapse due to the pressure of the advancing needle. The needle may appear to be in the vessel sonographically but still be covered by adherent tissues due to the relatively blunt tip of large needles. The same considerations apply during regional nerve block techniques when relatively blunt needles need to penetrate the fascial sheath encasing the neurovascular bundle. Confirmation of position could be established by the click as the needle enters the sheath, paraesthesia, visualisation of the needle tip in the tissues, a positive low threshold signal from a nerve stimulator or visualisation of injected local anaesthetic in the area of interest. Transverse and longitudinal approaches are described and illustrated below. Transverse In the transverse approach the needle is inserted steeply down, nearly parallel to the ultrasound beam. Typically, in vascular access the vessels are viewed in cross-section, enabling adjacent structures to be easily visualised, but this approach gives poorer visualisation of the needle compared to the longitudinal approach. This is because the angle of approach of the needle is by necessity more parallel to the ultrasound beam and only one short segment of the needle is visible as an echogenic area on the display. What is thought to be the tip of the needle may actually be the needle shaft. The position of the needle tip can be established by rocking the transducer back and forth or by withdrawing the needle slightly and re-aligning it in a more vertical plane. The needle tip is seen as a highly echogenic (white) spot which disappears immediately the transducer is angled distal to it. Despite the above disadvantages, a study on residents who attempted to cannulate a synthetic arm showed that novice ultrasound users obtain vascular access much faster Figure 10 Cross-sectional images of right infraclavicular axillary subclavian vein and artery. An 18G vascular access needle is introduced in the cross-sectional plane. A) The tip of the needle has just entered the beam (white dot) arrow. B) More of the tip and distal shaft is now in the beam nearest to the vein. Note the reverberation artefact and acoustic shadow. The total depth of field is 4.6 cm. The acoustic shadow and artefact may be mistaken for the needle within the vein but the needle tip can be seen as a bright spot immediately outside of the vein wall. On passing a needle through the various layers, a bowing of the tissue is seen. C) Further needle advancement compresses the anterior vein wall. An 18G vascular access needle is relatively blunt; as the tip abuts the blood vessel its wall indents. D) Finally, the needle passes into the vein and the vein re-opens. The needle tip may be seen within the lumen of the vein and can be seen to move from side to side in the vessel lumen as the shaft is rocked from side to side. Correct positioning is then confirmed by free aspiration of blood. 156 Journal compilation Ó The Association of Anaesthetists of Great Britain and Ireland

10 G. A. Chapman et al. Æ Needle position using ultrasonography using a short axis (transverse) approach than using a long axis (longitudinal) approach [30]. Illustration of this approach is difficult with still paper images because the approach relies on the dynamic interaction between the operator and ultrasound probe (Figs 4, 10). Longitudinal The longitudinal view allows much better needle visualisation, as the needle is more perpendicular to the ultrasound beam and multiple dots build up the image of the needle (Figs 11 13). Practice is required to keep the needle precisely within the image plane as any small Figure 13 A 16G Tuohy needle and epidural catheter through the needle in agar fluid phantom seen in longitudinal plane from the right. Air bubbles are visible in the catheter. Note the reverberation artefact below the needle. movement out of the ultrasound beam results in the loss of the image of the needle. Care must also be taken to avoid the probe inadvertently moving away from the target structure. The position of the needle is confirmed by other means. Conclusions Figure 11 Agar phantom with fluid filled cavity in longitudinal section. An 18G vascular access needle introduced at about 30 to the longitudinal plane with the bevel facing downwards. Only the front wall of the needle is visualised. Note the acoustic shadow (no back wall of needle visible, some loss of definition and darker agar seen below the needle) and bubbles in agar (white spots). The operator s skill in aligning the ultrasound probe and needle is probably the most important variable influencing needle visibility [18]. Time spent understanding and practising the applied physics of needle visualisation with ultrasound will be repaid many times over when performing invasive procedures. Acknowledgements We thank Dr J. A. Evans (Senior Lecturer in Medical Physics) and Dr M. J. Darby (Consultant Radiologist) for reviewing this paper and Medical Photography for help with the illustrations, all based in Leeds. Dr Chapman is in receipt of a research fellowship from the Association of Anaesthetists of Great Britain and Ireland. References Figure 12 The same preparation as Figure 11. A J-wire has been passed through the needle into a fluid filled cavity. 1 Beekman R, Visser LH. High-resolution sonography of the peripheral nervous system a review of the literature. European Journal of Neurology 2004; 11: Marhofer P, Greher M, Kapral S. Ultrasound guidance in regional anaesthesia. British Journal of Anaesthesia 2004; 26: Maury E, Guglielminotti J, Alzieu M, Guidet B, Offenstadt G. Ultrasonic examination: an alternative to chest radio- Journal compilation Ó The Association of Anaesthetists of Great Britain and Ireland 157

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