Section 23 Ultrasound

23.1 Emergency department ultrasound 712

23.1 Emergency department ultrasound

1 Ultrasound examination, interpretation and clinical correlation should be available in a timely manner 24 h a day for emergency department patients.

2 Emergency physicians providing emergency ultrasound services should possess appropriate training and hands-on experience to perform and interpret limited bedside ultrasound imaging.

3 Ultrasound imaging by emergency physicians is useful for at least the following clinical indications: traumatic haemoperitoneum, abdominal aortic aneurysm, pericardial fluid, ectopic pregnancy, vascular access, therapeutic diagnostic tests and evaluation of renal and biliary tract disease.

4 Continued research is required in the area of ultrasound imaging and any other known or evolving bedside imaging techniques and modalities.

5 Emergency medicine training programmes should provide instruction and experience in bedside ultrasound imaging for their trainees.

6 The Australasian College for Emergency Medicine supports the use of bedside ultrasound by emergency physicians, as does the American College of Emergency Physicians and the College of Emergency Medicine in the UK.

Background

Clinical ultrasound followed developments in the use of sonar, where the principle that sound waves could be used to locate objects was developed. Initially ultrasound machines were large and cumbersome, but advances in technology have improved image quality while reducing machine size, so that today small machines are able to produce high-quality images. As a result of this improved technology, ultrasound is now available to clinicians and can be performed at the bedside of patients. Although clinician-performed ultrasound has occurred in Europe and Japan for many years, and in the field of obstetrics and gynaecology worldwide, it is a relatively new development in Australasia in emergency departments (EDs).

Clinician-performed ultrasound has a different approach to formal diagnostic ultrasound, such as that performed in radiology departments. Clinician-performed ultrasound is generally limited in scope and targeted to answering a specific question (such as ‘Is there an abdominal aortic aneurysm?’), rather than providing a full assessment of an anatomical area. In this regard, it is often viewed more as an extension of the clinical examination than a technique that competes with other imaging techniques (including formal ultrasound).

The Australasian College for Emergency Medicine supports the use of bedside ultrasound by emergency physicians,¹ as does the American College of Emergency Physicians and the College of Emergency Medicine in the UK.^2,³ It is expected that with increasing experience the range of conditions for which ultrasound is used in the ED will increase.

Basic physics of ultrasound

Sound waves are mechanical waves that transmit energy through the vibration of particles. Ultrasound waves are defined as those that are above the usual range of human hearing (20–20 000 Hz). Current diagnostic ultrasound machines are based on the pulse–echo principle, using pulses of sound waves at frequencies of 2–15 MHz that are reflected back. Processing of these reflected echoes creates the ultrasound data and image.⁴

The ultrasound transducer converts electrical impulses into pulses of sound (via the piezoelectrical effect) which are then directed into the body. As the sound wave travels through tissue, it gradually loses energy, termed ‘attenuation’. The degree of attenuation differs for different tissues and is also dependent on the frequency of the pulse wave. On reaching a tissue interface, some of the energy is reflected back as an echo, due to the differences in acoustic impedance (gel or other coupling material is used to minimize reflection at the probe/skin surface). This reflected echo then travels back through tissue, undergoing further attenuation, until it reaches the transducer, which converts the energy back to an electrical impulse, which is then amplified and processed. The time taken for the pulse wave to travel to the tissue interface and back is converted into distance using the average speed for sound in tissue. The intensity of the returning wave determines the brightness of the displayed pixel. The returning pulses from the different reflecting surfaces along the path of the ultrasound beam generate a single line of the ultrasound image. The ultrasound beam is steered across the field to generate the multiple lines of information that then form the two-dimensional image (termed ‘B mode’, for brightness modulation). Alternatively, if the direction of the beam is kept constant and the changing surfaces are mapped over time then an M mode image is generated.

The degree of attenuation is dependent on the frequency of the sound wave, so higher frequency pulses undergo greater attenuation. They also have shorter wavelengths, which improves the resolution of the ultrasound beam (the ability to distinguish two separate objects close together). This leads to one of the most important trade-offs in ultrasound, between resolution and penetration. To obtain high resolution, a high frequency probe can be chosen, but this will be unable to image deep structures.

To form the image, the ultrasound machine makes certain assumptions about the ultrasound beam and sound impulse. Deviations from these behaviours will result in image artefacts, i.e. when the image does not represent the tissue accurately. There are many artefacts, most of which reduce the information available from the image. The most clinically important artefacts, shadowing and enhancement, can also be used diagnostically.

Shadowing occurs when all of the energy of the ultrasound pulse is reflected at a surface (such as air or bone) and there will then be no returning pulses from the tissue distal to the object. This creates a black area on the screen, known as an acoustic shadow. The presence of a shadow behind a brightly reflective surface can thus be used to diagnose a region of calcification, such as a calculus (Fig. 23.1.1). Stones and bones generally give clean shadows, while gas gives ‘dirty’ or grey shadows due to the superposition of both shadow and reverberation artefact (Fig. 23.1.2).

Fig. 23.1.1 Acoustic shadowing from gallstones. AS, acoustic shadow.

Fig. 23.1.2 Acoustic shadowing from bowel gas in a patient with free fluid. L, liver; FF, free fluid; GS, gas shadow.

Enhancement occurs when an area (such as fluid in a cyst) absorbs less energy than usual. This means that the pulses that have travelled through that area will have more energy, resulting in a bright region behind the image (Fig. 23.1.3). Enhancement is used to confirm the fluid filled nature of lesions.

Fig. 23.1.3 Acoustic enhancement from fluid-filled structures. Acoustic enhancement is seen where the ultrasound beam passes through the gallbladder and more prominently where it passes through the gallbladder and pancreatic pseudocyst. GB, gallbladder; C, pancreatic pseudocyst; AE, acoustic enhancement.

Transducers

Different ultrasound transducers are available varying in frequency, the size of the contact area (termed ‘footprint’) and shape. Transducers may have a small footprint to fit into small areas, such as between ribs, from which the beam spreads in a large arc (e.g. a sector transducer). Alternatively, they may be larger with a flat or slightly curved surface where contact can be maintained, such as a linear probe. Special transducers have been designed for use within body cavities, such as transoesophogeal, endovaginal and endo-anal probes. These transducers offer the advantage of reduced distance between the transducer and area of interest, which allows higher frequencies to be used, resulting in improved resolution. Very high frequency transducers have been used for intravascular and superficial ocular scanning.^5,⁶ The appropriate choice of transducer is important in ensuring the optimal image is obtained.

The scope of emergency department ultrasound

Current indications for emergency ultrasound are given in Table 23.1.1.

Table 23.1.1 Current indications for emergency ultrasound

Trauma (haemoperitoneum, haemopericardium, pneumothorax)

Abdominal aortic aneurysm

Early pregnancy complications

Biliary disease

Renal stones and hydronephrosis

Echocardiography in trauma and shock

Proximal deep vein thrombossi exclusion

Procedural

Musculoskeletal

Focused assessment by sonography for trauma (FAST)

Descriptions of the use of ultrasound by clinicians to evaluate trauma patients appeared in the European literature in the 1970s.⁷ Reports have subsequently appeared from countries around the world⁸ and the technique is now well established. With relatively brief training and experience, non-radiologists are able to diagnose haemoperitoneum with a high degree of sensitivity and specificity, although accuracy does improve with experience.^9,¹⁰

Clinical examination in abdominal trauma can be difficult and unreliable.¹¹ Diagnostic peritoneal lavage (DPL), ultrasound (FAST) and computerized tomography (CT) have been used to further evaluate this group of patients. In most cases, FAST has replaced diagnostic peritoneal lavage as it is non-invasive and does not interfere with subsequent interpretation of CT images. CT scanning is highly accurate for diagnosing both free fluid and solid organ injury, although it is less accurate for hollow viscus and diaphragmatic injury.¹¹

Studies of ultrasound scanning in trauma have reported varying sensitivity.¹² Much of this variation is due to differences in the gold standard used for comparison and the definition of ‘true positive’. Haematoperitoneum (on further imaging, surgical or post-mortem examination), organ injury and clinical stability have all been used in different studies.^9,¹²^–¹⁵ It must be remembered that the primary role of a FAST scan is to detect free fluid in the peritoneal or pericardial spaces, for which it has high sensitivity and specificity.^12,¹⁶ Solid organ or retroperitoneal haemorrhage may be detected, but even in expert hands the accuracy is much lower (with as many as two-thirds of injuries being missed).^14,^15,¹⁷ FAST has been shown to be reliable and useful in both pregnant¹⁸ and paediatric¹⁹ patients.

Technique

FAST scanning evaluates four regions for the presence of free fluid: (1) pericardial, (2) perihepatic, (3) perisplenic and (4) pelvic ⁸ (Fig 23.1.4). Some authors extend the FAST examination to include examining the pleural spaces postero-laterally for fluid, and anteriorly to exclude pneumothorax.²⁰ The technique is rapid, generally being completed in under 5 min.¹²

Fig. 23.1.4 Transducer placement for the four views for fast scanning. RUQ, right upper quadrant; LUQ, left upper quadrant.

Free fluid appears as an echolucent area (i.e. black) that is generally linear or triangular in shape in the most dependent area of the peritoneal or pericardial space, although blood clots may be seen as echogenic (grey) collections ¹⁴ (Figs 23.1.5, 23.1.6, 23.1.7). While fluid is most commonly seen in the perihepatic space, all spaces should be examined before the result can be considered negative.¹² Small amounts of fluid (<500 mL) may not be detected.¹²

Fig. 23.1.5 Free fluid in the perihepatic view. RUQ, right upper quadrant, L, liver; FF, free fluid; K, kidney.

Fig. 23.1.6 Free fluid in the perisplenic view. LUQ, left upper quadrant; S, spleen; FF, free fluid; K, kidney.

Fig. 23.1.7 Pericardial effusion with clot. L, liver; RV, right ventricle; PE, pericardial effusion with grey blood clots and black (echofree) blood.

Clinical implications and utility ^8,^12,²¹^–²³

The limitations of ultrasound in excluding all intra-abdominal injuries requiring laparotomy and the increasing use of conservative management of some injuries, even in the setting of intra-abdominal free fluid, have resulted in there being no universally accepted clinical algorithm based on FAST scan results. However, in this regard FAST scanning is no different to any other clinical, laboratory or imaging information about the trauma patient, the results of which are routinely used in combination to determine the management plan. Various algorithms incorporating FAST scanning have been proposed, which generally incorporate haemodynamic stability and FAST scan result, such as in Figure 23.1.8. Some algorithms incorporate a semi-quantitative scoring system to estimate amount of free fluid, with an increased volume of free fluid associated with greater need for therapeutic laparotomy. A positive FAST scan is highly predictive of significant intra-abdominal injury and, based on the clinical condition of the patient, generally indicates the need for CT or surgical exploration. A negative FAST scan, stable haemodynamics and clinical observation have been shown to be highly accurate in excluding significant intra-abdominal injury. Some authors advocate serial FAST examinations in stable patients, suggesting this can reduce the requirement for CT.

Fig. 23.1.8 Suggested algorithm using FAST results. CT, computerized tomography.

In the Australasian setting, FAST is generally accepted as fulfilling a complementary role to CT. Its portability and speed allow it to be used early in the evaluation of trauma patients (e.g. immediately after the primary survey) and this information is then incorporated with other clinical information to risk stratify the trauma patient to help to determine the requirement and timing for either laparotomy or CT. Repeated examinations, particularly if the patient’s condition changes, can be valuable. Providing the limitations of the technique are not ignored, it can rapidly provide vital information to assist with patient management.

Abdominal aortic aneurysm

Abdominal aortic aneurysms (AAA), defined as an aortic diameter >3.0 cm, are common, occurring in between 1 and 9% of the population.²⁴ Clinical assessment of the abdominal aorta is unreliable,²⁴ and may be especially difficult in the obese or unstable patient with abdominal pain. Clinical presentation of ruptured abdominal aortic aneurysm can be varied, with only 50% of patients describing the classic presentation of hypotension, back pain and pulsatile mass. Other presentations may include haematuria, abdominal or flank pain, unexplained hypotension, syncope or cardiac failure,^25,²⁶ and AAA should be considered in any of these presentations.

Ultrasound is the primary mode of investigation of the abdominal aorta.²⁴ Ultrasound performed by emergency clinicians has been shown to be rapid, highly sensitive and highly specific (>95%) in assessing aortic diameter.^27,²⁸ Ultrasound may occasionally detect rupture, but it is not reliable in excluding rupture. In addition to its utility in diagnosing AAA, ED ultrasound is very beneficial in rapidly excluding AAA in the wide variety of presentations listed above.

The risk of rupture of an AAA increases with diameter. Although the risk of rupture if the aneurysm diameter is less than 4 cm is <0.5% per year and 1.5% per year for aneurysms of 4.0–4.9 cm, rupture can still occur.^29,³⁰ Approximately 10% of ruptured aneurysms measure 5 cm or less.³⁰

Technique

The aorta should be identified anterior to the vertebral body and to the left of the inferior vena cava (IVC). It should be followed from the epigastric region to its bifurcation, just above the umbilicus, remembering that in elderly patients it may follow an ectactic course rather than following a strictly cranial-caudal course. It must be distinguished from both the superior mesenteric artery (SMA) (which runs anterior to the aorta) and the IVC (ensuring that the venous pulsation of the IVC is not mistaken for the arterial pulse of the aorta). Measurements should be taken both proximally and distally and, if an aneurysm is present, at the widest point. Measurements from both transverse and longitudinal planes should be taken. Measurements are taken from the outer wall to outer wall, including any mural thrombus (see Fig. 23.1.9). If the renal arteries or SMA origin are identifiable then the relation to the aneurysm should be noted, although in the ED setting this may not be possible. Any periaortic haematoma or peritoneal free fluid should be noted.

Fig. 23.1.9 Abdominal aortic aneurysm.

Clinical implications and utility

In the patient with ruptured AAA who is haemodynamically unstable, ED ultrasound allows rapid and accurate diagnosis within the resuscitation area. Rapid diagnosis of these patients is essential to achieve successful treatment. In the stable patient, whose presentation may be atypical, ED ultrasound provides a rapid means of excluding the diagnosis (for example in the elderly patient who presents with ‘renal colic’). If an AAA is detected in these patients then further imaging will often be required to determine if the AAA is an incidental finding or the cause of the patient’s symptoms. If the AAA is an incidental finding then formal follow-up should be arranged.

Early pregnancy

Ultrasound is the primary imaging modality for early pregnancy and its complications.³¹ In the ED setting, it is most commonly used for the pregnant patient with pain or bleeding. In addition to transabdominal scanning (TAS), transvaginal scanning (TVS) can be performed with patient consent using a specifically designed probe which places the transducer close to the pelvic organs and utilizes higher frequencies to produce images of much higher detail than TAS. It does not require a full bladder and should not be a painful procedure. TAS still has an important role, as it allows a broader field of view that allows better assessment of large amounts of free intraperitoneal fluid and may diagnose other causes of pain. Emergency physician-performed ultrasound for early pregnancy complications has been shown to be safe and reduce the time patients spend in EDs.^32,³³