Practical emergency ultrasound (Proceedings)

The practice of emergency ultrasound is based on two paradigms: immediate results and focused examinations. The benefit of emergency ultrasound to patient care is realized when a specific clinical question is raised in the history or physical examination findings or when specialty services are delayed. A focused examination does not replace a comprehensive abdominal ultrasound examination or echocardiogram. Instead, the emergency ultrasound provides timely diagnostic information and guidance for high-risk procedures.

The purpose of this presentation is to introduce basic ultrasound techniques and interpretations for focused examinations in veterinary emergencies. This can serve as an initial approach to training emergency medicine veterinarians. Trainees can then build on these topics with direct supervision and didactic training from experienced veterinarians and ultrasound technicians. Following a review of the basics of ultrasound physics, some general indications for focused ultrasound examinations in veterinary practice will be discussed. The indications are based on peer reviewed articles and common procedures performed at the Animal Medical Center.

The perspective in human patients

Over the past 20 years since initial investigations, the bedside ultrasound exam has evolved into its own discipline. The American Medical Association and the American College of Emergency Physicians recognize emergency physician-performed ultrasound examinations. Acceptance of the focused ultrasound examination by other specialty organizations has perhaps been an uphill battle. Some providers of consultative ultrasound services recommend training standards for practitioners outside their own specialties that far exceed the standards accepted by emergency medicine authorities. Despite differences in standards of training, the emergency ultrasound examinations worth is proven in peer-reviewed journals for numerous clinical scenarios including trauma, abdominal aortic aneurysms, ectopic pregnancy, pericardial effusion, cardiac activity, and procedure guidance.

A veterinary perspective

An initial review article on emergency veterinary ultrasound examinations dates back to 1988. Since then, the use of focused ultrasound examinations in critical veterinary patients has been touched upon in other reviews and studies. Standards for training of these exams have not been established by any specialty organization. The American Association of Veterinary Radiologists (AAVR) is currently developing standards for training both technicians and veterinarians. More information on this can be found at www.aavr.org.

Indications for focused emergency ultrasound examinations are generally based on prior suspicion of a particular disease. The most well established indication in both human and veterinary patients is for the identification of fluid in cases of trauma. This is probably because fluid identification (whether it be in an infected uterus, around the heart, or within a body cavity) is a specific and relatively simple positive finding to interpret. Other specific ultrasound diagnoses in veterinary medicine could include identification of a living fetus, an intussusception, gallbladder mucocele, splenic torsion, or high-grade hydronephrosis. However, one can see that as we add to this list, the room for error in interpretation can increase. Furthermore, some of these diagnoses may not require immediate therapeutic intervention. Emergency clinicians must weigh the confidence of their interpretation with the urgency of the diagnosis (can a suspected hydronephrosis wait 8 hours for a more complete abdominal scan?).

Ultrasound physics

Ultrasound wave characteristics

Sound energy is periodic changes in pressure within a medium, causing the molecules within that medium to compress and relax as an advancing pressure wave. Therefore, sound requires a medium for travel. People hear frequencies between 15 and 20,000 cycles per second. Contrast this with the inaudible sonic beams of diagnostic ultrasounds that have a frequency of 1,000,000 to 20,000,000 cycles per second (1 to 20 MHz).

Velocity: Since sound requires a medium for travel, we must discuss this medium of travel when discussing the characteristics of ultrasound. Ultrasound travels slowest in gases, fastest in solid tissue, and intermediate velocities in fluids/soft tissues. Velocity of an ultrasound in tissue is based on the tissue's compressibility and density. Gases are less dense (molecules less closely packed together) and therefore will be less effective in compressing and relaxing to allow for sound travel.

Frequency: When sound passes from one medium to another, its frequency remains constant but wavelength changes to accommodate the new velocity in the second medium. The frequency is chosen by the operator and will be discussed under 'technical parameters'.

Intensity: The intensity of sound is the power of the sound wave's compression of the medium as it travels. The intensity of the sound wave diminishes as it travels deep into tissue. Other factors that diminish a sound wave's intensity include the number of tissue interfaces, the type of tissue, and the frequency of the sound wave. Operator controls for both frequency and intensity are discussed under 'technical parameters'.

Ultrasound interactions with matter

Sound beams are reflected back to their origin at the transducer, scattered away from both the transducer and the object, or absorbed completely by the object. Reflection: The most important interaction between sound and tissue for image formation is reflection. Information returned to the transducer via reflection becomes the electric signal for the images we see on our monitor. The reflective characteristic, or acoustic impedence, of tissue is the product of the ultrasound's returning velocity in the tissue and the density of this tissue. The ability to delineate an organ's surface from surrounding tissue is based on the change in acoustic impedence as the sound travels through the two media. Optimal reflection occurs when an object's surface is perpendicular to the direction of sound wave travel.

Scatter and absorption: Scatter is responsible for the internal architecture of tissue. Normal liver parenchyma is described as coarsely echotextured and normal spleen is described as finely echotextured because of the internal echoes that are scattered within these organs. Absorption will yield heat in the tissue being interrogated and does not contribute to image formation.

Technical parameters

Instrumentation

Five specific technical parameters should be in constant use and changed regularly for a particular examination. Other technical adjustments are also important but may not be available on some machines.

Transducers are chosen for three features: (1) frequency selection, (2) footprint size, and (3.) footprint shape. Greater frequencies allow for superior resolution at the cost of decreased penetration or depth. This is because greater frequencies diminish the intensity of the sound wave sooner than lower frequencies. Newer broadband transducers allow the examiner to select a frequency from a range of capabilities (ie. 3.5-5.0 MHz). The footprint of a transducer is part of the transducer that contacts the patient's skin. Small transducers allow for examinations between ribs. Larger transducers allow for a greater visual field. Footprint shape gives a clue to the direction of the sound waves emanating from the transducer. Rectangular, flat transducers are generally linearly arranged. Round footprints are either curvilinear or microconvex, diverging from a center to increase the field of view. Square transducers usually eminate from a point source, steering the ultrasound beam to form a sector.

Overall Gain controls the intensity of the returning sound wave and can be compared to the volume on a stereo. As the gain control is increased, the image becomes brighter. A point is reached when the brightness has more noise than signal and the image quality is diminished. Lower gain generally yields the best images.

Time gain compensation (TGC) regulates the intensity of the ultrasound beam to differing levels from near to far field. As stated above, the intensity of the ultrasound wave diminishes as the wave passes deeper into tissue. TGC controls are often set for increased gain in the far field relative to the near field.

Depth adjustments are made for every organ depending on the organ's distance from the surface of the skin. Greater depth will minify the objects in an image. Larger images are more easily seen and allow for optimal needle guidance and image interpretation. Fill the screen with the organ of interest.

Focal points will increase resolution at a particular point in the image. The drawback of too many focal points is a decrease in frame rate of the real time image. This blurs the image both in real time and when the image is frozen. For most examinations, one focal point centered on the organ of interest will suffice.

Ultrasound artifacts

Ultrasound artifacts are common and can be used for good or evil. The good occurs when the artifact gives a clue to the make-up of a particular structure being interrogated (fluid versus mineral versus gas, for example). The evil occurs when the artifact is misinterpreted as a true structural relationship in the area being interrogated (mirror image artifact interpreted as a diaphragmatic hernia, for example). The following four artifacts should be understood prior to beginning any initial training for emergency ultrasound exams. Other artifacts are described elsewhere.

Acoustic enhancement: Minimal attenuation occurs in fluid-filled objects relative to adjacent solid organs. This increases the overall echo intensity in the far field ("deep portion") from the fluid. Acoustic enhancement aids in differentiating a solid hypoechoic (dark) nodule from a fluid-filled nodule or cyst. Enhancement can be detrimental if the increased echoes deep to a cyst are interpreted as changes in the echogenicity of an organ like the liver.

Acoustic shadowing: As increased intensity of echoes deep to a minimally attenuating object occurs, so too does a decreased intensity of echoes occur deep to highly attenuating objects. Objects that are mineralized will shadow.

Mirror image: A duplicate structure that is normally present on one side of a strong reflector can also appear on the other side of a reflector. The most notorious example is that of the liver appearing deep (or cranial) to the highly reflective lung-diaphragm interface. Another example of this occurs when the urinary bladder and gas-filled colon are imaged. The strong reflection of the colon yields a mirror image of the urinary bladder deep (or dorsal) to the colon.

Reverberation: Sometimes a clean, discrete acoustic shadow is not formed at a strong reflector. Instead of an abrupt decreased intensity of echoes deep to a reflector, multiple evenly spaced spurious echoes are perceived deep to the reflector. The spacing between these repeating spurious echoes is most often dependent on the distance of the strong reflector to the skin-transducer surface. Superficially located gas-filled bowel can produce this artifact.

Indications for the focused emergency ultrasound examination

Four general indications will be described based on literature review and common procedures performed at the Animal Medical Center: (1.) trauma, (2.) cardiovascular disease, (3.) reproductive disorders, and (4.) needle guidance. The final section will discuss other possible indications but, as mentioned above, the reader is cautioned to except his/her limitations and weigh the confidence of the interpretation with the urgency of the diagnosis.