Ultrasound is a non-invasive, repeatable, portable, reproducible diagnostic tool. It can be used virtually anywhere that patient care is being performed, including pre-hospital, the ER, OR, ICU, and non-ICU inpatient wards. Ultrasound skills vary between providers. I am a strong advocate of utilizing the ultrasound, and you will become more comfortable as you increase your utilization of the US. The credentialing process for ACS surgeons is not well-established, and we do not have the same expertise as radiologists. SCCM guidelines currently support ICU providers' utilization of US for certain scenarios. However, ICU providers are not as reliable in certain diagnoses, such as biliary pathology.

Basics of Ultrasound: How Does it Work?

Crystal excited by electrical pulses (piezoelectric effect)→ mechanical oscillations→ sound waves emitted. Sound waves are reflected at interfaces of different acoustic densities. Higher acoustic density→ increased intensity of reflected sound and decreased transmission of remaining sound waves. If the interface is between objects of vastly different acoustic density→ complete sound wave reflects and total acoustic shadowing occurs (dark behind the object); examples include bone, stones, and air.

Probe selection

• Linear array- parallel sound waves→ rectangular images. Near-field resolution, high frequencies 5-7.5 MHz)- good for thyroid and soft tissue. Artifact on curved surfaces. Not good for intra-thoracic or upper abdominal organs.

• Sector/ phased array- fan-like image (narrow nearest transducer and widening with deeper penetration). Frequency 2-3 MHz. Poor for near-field resolution. Used for cardiac imaging.

• Curved (convex) array- abdominal sonography. 3.5-3.75 MHz. Deeper tissue penetration.

• *Probe marker correlates with the dot on the screen to establish orientation.

Artifacts

• Reverberation echoes-several strongly reflecting boundaries→ reflection of sound waves back and forth→ echoes (several parallel lines close to the transducer).

• A-lines when scanning the lung- hyperechoic arcs parallel to the pleural line. These are seen at intervals that are the same as the interval from the skin to the pleural line. Absence of A lines= change in attenuation coefficient of the lung (edema, consolidation).

• B-lines when scanning the lung (comet-tail artifact)- vertical hyperechoic lines, caused by fluid-filled intra-lobular or interlobular septa touching the visceral pleural surface.

• Distal acoustic enhancement- sound waves travel through homogenous fluid (low reflection)→ less sound wave attenuation, so they are more amplified compared to adjacent sound waves (because the structures they passed through reflected some of the waves). *Brightness (increased echogenicity) behind fluid-filled structures such as the bladder or gallbladder.

• Mirror image- diaphragm and visceral pleura→ intrahepatic structures can be seen on the pulmonary side of the diaphragm.

• Acoustic shadowing- interface between tissue and bone or tissue and air→ scattered beam→ inability to image deeper structures.

Knobology

• Identify the probe

• Identify the selected study type (cardiac, FAST, soft tissue, etc)

• Gain- increases the strength of sound/ brightness of the visualized area

• Depth-gain compensation- selective enhancement of echoes received at different depths→ moving depth up or down increases or decreased the field of view.

• Time-gain compensation- adjust the strength of the beam to areas that would normally have attenuated beams.

• M-mode- display and measure movement of structures over time along a single lione (axis of the beam). Good for heart or valve motion (echo), hemodynamic status (respiratory change in IVC diameter) and lung sliding or diaphragm movement.

• Doppler- changes in frequency cause by reflections off a moving target (usually blood).

References 

• Frankel HL et al.  Guidelines for the Appropriate Use of Bedside General and Cardiac Ultrasonography in the Evaluation of Critically Ill Patients-Part I: General Ultrasonography. Crit Care Med. 2015 Nov;43(11):2479-502.

• Levitov A et al. Guidelines for the Appropriate Use of Bedside General and Cardiac Ultrasonography in the Evaluation of Critically Ill Patients-Part II: Cardiac Ultrasonography. Crit Care Med. 2016 Jun;44(6):1206-27.

• Labovitz AJ et al. Focused cardiac ultrasound in the emergent setting: a consensus statement of the American Society of Echocardiography and American College of Emergency Physicians. J Am Soc Echocardiogr. 2010 Dec;23(12):1225-30.

• Borloz MP et al.  Emergency department focused bedside echocardiography in massive pulmonary embolism. J Emerg Med. 2011 Dec;41(6):658-60.

• Bakhru RN, Schweickert WD. Intensive care ultrasound: I. Physics, equipment, and image quality. Ann Am Thorac Soc. 2013 Oct;10(5):540-8.

• Silverberg MJ et al. Intensive care ultrasound: II. Central vascular access and venous diagnostic ultrasound. Ann Am Thorac Soc. 2013 Oct;10(5):549-56.

• Doerschug KC et al. Intensive care ultrasound: III. Lung and pleural ultrasound for the intensivist. Ann Am Thorac Soc. 2013 Dec;10(6):708-12.

• Boniface KS et al. Intensive care ultrasound: IV. Abdominal ultrasound in critical care. Ann Am Thorac Soc. 2013 Dec;10(6):713-24.

• Repessé X et al. Intensive care ultrasound: V. Goal-directed echocardiography. Ann Am Thorac Soc. 2014 Jan;11(1):122-8.

• De Backer D et al. Intensive care ultrasound: VI. Fluid responsiveness and shock assessment. Ann Am Thorac Soc. 2014 Jan;11(1):129-36

• Labovitz AJ et al. Focused cardiac ultrasound in the emergent setting: a consensus statement of the American Society of Echocardiography and American College of Emergency Physicians. J Am Soc Echocardiogr. 2010 Dec;23(12):1225-30.

• Borloz MP et al.  Emergency department focused bedside echocardiography in massive pulmonary embolism. J Emerg Med. 2011 Dec;41(6):658-60.

• Emergency Ultrasound Tutorials

• American College of Emergency Physicians: Ultrasound Lectures