Emergency department ultrasound

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 to 20,000 Hz). Current diagnostic ultrasound machines are based upon the pulse–echo principle, using pulses of sound waves at frequencies of 2 to 15 MHz, which are reflected back. Processing of these reflected echoes creates the ultrasound data and image.

The ultrasound transducer converts electrical impulses into pulses of sound (via the piezoelectric effect), which are then directed into the body. As the sound wave travels through tissue, it gradually loses energy, which is termed ‘attenuation’. The degree of attenuation differs for different tissues and is also dependent on the frequency of the pulse wave. Upon 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. This returning impulse is then amplified and processed. The time taken for the pulse wave to travel to the tissue interface and back is converted into distance by 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 2D 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 (M stands for motion).

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 improve 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 these 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, that is, when the image does not represent the tissue accurately. There are many artefacts, most of which reduce the information available from the image. However, the most clinically important artefacts also can be used diagnostically:

• Shadowing : when all of the energy of the ultrasound pulse is reflected or absorbed at a surface (such as air or bone). In this situation, there will 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’ or black shadows, while gas gives ‘dirty’ or grey shadows due to the superposition of both shadow and reverberation artefact ( Fig. 23.1.2 ).

  
  

• Enhancement : when an area of interest (such as fluid in a cyst) absorbs less energy than the surrounding tissue, the pulses that have travelled through that area will have more energy than equidistant pulses that did not, resulting in a bright region deep to the area of interest on the image ( Fig. 23.1.3 ). Enhancement is used to confirm the fluid-filled nature of lesions.

  

Transducer

Different ultrasound transducers are available varying in shape, frequency and the size of the contact area (termed footprint). 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 transoesophageal, 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. Appropriate choice of transducer is important in ensuring the optimal image is obtained.

Extended focused assessment with sonography for trauma

Descriptions of the use of ultrasound by clinicians to evaluate trauma patients appeared in the European literature in the 1970s. Reports have subsequently appeared from countries around the world and the technique is now well established. Initially limited to the abdomen and pericardium (focused assessment with sonography for trauma [FAST]), the examination is now routinely extended to include the chest (extended focused assessment with sonography for trauma [EFAST]). With relatively brief training and experience, non-radiologists are able to diagnose haemoperitoneum, pericardial effusions, pleural effusions and pneumothorax with a high degree of sensitivity and specificity, although accuracy does improve with experience.

Clinical examination in abdominal trauma can be difficult and unreliable. Diagnostic peritoneal lavage (DPL), ultrasound (FAST) and computed 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 free fluid, solid organ injury and bony injury, and is slightly less accurate for hollow viscus and diaphragmatic injury.

Studies of ultrasound scanning in trauma have reported varying sensitivity. Much of this variation is due to differences in the gold standard used for the comparison and definition of ‘true positive’. Haemoperitoneum (on further imaging, surgical or post-mortem examination), organ injury and clinical stability have all been used in different studies. 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. 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). FAST has been shown to be reliable and useful in both pregnant and paediatric patients.

Technique

FAST scanning evaluates four regions for the presence of free fluid: (1) pericardial, (2) perihepatic, (3) perisplenic and (4) pelvic ( Fig. 23.1.4 ). The scan is then extended to the chest (EFAST) where the pleural spaces are examined posterolaterally for fluid, and anteriorly to exclude pneumothorax. The technique is rapid, generally being completed in under 5 minutes.

Free fluid appears as an echolucent area (i.e. black), which 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 ( Figs. 23.1.5 to 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. Small amounts of fluid (<500 mL) may not to be detected.

When scanning the chest, the presence of lung sliding or lung pulse is an indication that the visceral and parietal pleura are in contact, excluding a pneumothorax at that point. The presence of a moving transition point between areas of lung sliding and absent lung sliding (the ‘lung point’ sign) is diagnostic of pneumothorax.

Clinical implications and utility

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

Similarly, for pneumothorax, the integration of EFAST findings with other clinical, laboratory and imaging findings will determine patient management. Conservative management of small pneumothoraces, even in the setting of positive pressure ventilation means that the ultrasound findings must be considered in the setting of the individual patient when management decisions are made.

In the Australasian setting, EFAST 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, thoracotomy 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.

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 ( 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 retroperitoneal haematoma or peritoneal free fluid should be noted.

Limitations and pitfalls

• Pain, obesity or bowel gas may prevent adequate imaging by ultrasound.

• Mistaking the IVC or SMA for the aorta.

• Measuring the lumen without including mural thrombus.

• Attempting to exclude rupture on ultrasound.

• Forgetting that the AAA may be an incidental finding and not the primary cause of the patient’s symptoms.

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 (e.g. 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.

Technique

TAS is performed initially, preferably when the patient has a full bladder, as the pelvic organs will be better visualized. The uterus is identified and examined in both longitudinal and transverse planes (recognizing that the longitudinal axis of the uterus may not necessarily be in a strictly sagittal plane). The endometrial thickness is noted and any fluid collections or gestational sac noted. The adnexa are examined to identify the ovaries and any masses. The pelvis is scanned for free fluid. The upper abdomen can be examined to estimate the volume of free fluid if seen. The kidneys also can be examined to identify any alternate diagnoses.

TVS is performed after the procedure is explained and consent is obtained. A chaperone should be present if the sonographer is male. The patient is asked to empty their bladder and the pelvis is elevated slightly off the bed using a foam wedge or similar. The probe is covered with a sterile sheath (e.g. condom) with gel placed inside and outside the sheath. The probe is gently inserted into the vagina and advanced. The uterus and adnexa are then examined in both longitudinal and transverse planes as in TAS. After the scan is complete the probe must be cleaned and disinfected.