Snell’s Law

For some reason, most clinicians seem to grasp x-ray and CT scan imaging reasonably well. Denser structures are white, less dense are black, water dense structures are grey.

Thus, when novice ultrasound users attempt to discern images created with sound, it can be confusing that bone and air both create bright white signal as well as shadow. The purpose of this brief post is to describe very subjectively how sound behaves as it crosses media of different densities. In the real world of physics this would be referred to as Snell’s Law (unless you want to give more credit to Ibn Sahl or Descartes).

For a very concise and well-animated description of Snell’s Law please see Dr. Dan Russells’ excellent website. The basic premise is that  sound (like light) will bend depending on the density of the medium it is traveling in. The greater the change in density from one medium to another, the greater the bend. For our purposes, that also means the more scattering of ultrasound waves back towards the transducer and less acoustic energy propagating forwards.

For practical purposes, we always start with liquid density in clinical sonography. That is because the transducer and acoustic gel are roughly water-dense, and so is the skin (bear with this oversimplification a moment).  Thus, we really only have three scenarios to think about. Going from liquid to air, liquid to liquid, and liquid to bone.

As illustrated above, the great density differences from liquid to air or bone create lots of scatter (and therefore bright white signal on the screen), and leave little or no acoustic energy to travel deeper into the tissue (thus the distal shadowing). When liquid-dense structures are encountered, relatively little energy is lost (attenuated), and the beam continues to send signal deeper into the body. Thus, liquid structures such as liver, spleen, kidney, bladder make good acoustic windows. They allow lots of ultrasound energy to propagate into the body. Bone and air make poor windows, as it is difficult to see past them.


NYSORA Winter Symposium 2011

Fellows Leila PoSaw and Gene Chan attended the NYSORA (New York School of Regional Anesthesia) Winter Symposium held on December 17-18, 2011 at the Marriott Marquis Hotel on Broadway.

In addition to the expected excellent lectures and educational sessions, there was a new needle guidance system being demonstrated which may be of benefit to clinicians performing ultrasound-guided procedures.

The SonixGPS system by Ultrasonix uses a sensor in the ultrasound transducer and another in the needle to track the needle’s trajectory and tip placement. The system can work in any direction: in-plane or out-of-plane. Needle trajectory is displayed as a graphic on the main screen, and orientation with respect to the transducer is modeled in the lower right. The system promises the ability to plan out their trajectory before needle placement as well, thus facilitating decisions regarding optimal entry points.

Please note that no members of our ultrasound division have a financial relationship with Ultrasonix.


Ultrasound in Cardiac Arrest

cardiac arrest ultrasound


Thanks to Dr. Wasserman, Ms. Thomas and all the folks at Beth Israel Newark Medical Center Emergency Medicine.  It was a pleasure to visit your shop today and talk about ultrasound in cardiac arrest.  As promised, you will find below a pdf of the handout, a revised RUSH in Arrest algorithm and a full set of references.  When I get a chance to record the lecture, I’ll post that here as well.

Thanks again.




Recorded Lecture (pending)


RUSH in Arrest Algorithm



Atkinson, P R T, D J McAuley, R J Kendall, O Abeyakoon, C G Reid, J Connolly, and D Lewis. “Abdominal and Cardiac Evaluation with Sonography in Shock (ACES): An Approach by Emergency Physicians for the Use of Ultrasound in Patients with Undifferentiated Hypotension.” Emergency medicine journal : EMJ 26, no. 2 (2009): doi:10.1136/emj.2007.056242.

Blaivas, M, and J C Fox. “Outcome in Cardiac Arrest Patients Found to Have Cardiac Standstill on the Bedside Emergency Department Echocardiogram.” Academic emergency medicine : official journal of the Society for Academic Emergency Medicine 8, no. 6 (2001): 616-21.

Breitkreutz, Raoul, Susanna Price, Holger V Steiger, Florian H Seeger, Hendrik Ilper, Hanns Ackermann, Marcus Rudolph, and others. “Focused Echocardiographic Evaluation in Life Support and Peri-Resuscitation of Emergency Patients: A Prospective Trial.” Resuscitation 81, no. 11 (2010): doi:10.1016/j.resuscitation.2010.07.013.

Hernandez, C, K Shuler, H Hannan, C Sonyika, A Likourezos, and J Marshall. “C.A.U.S.E.: Cardiac Arrest Ultra-Sound Exam–A Better Approach to Managing Patients in Primary Non-Arrhythmogenic Cardiac Arrest.” Resuscitation 76, no. 2 (2008): 198-206.

Jones, A E, V S Tayal, D M Sullivan, and J A Kline. “Randomized, Controlled Trial of Immediate Versus Delayed Goal-Directed Ultrasound to Identify the Cause of Nontraumatic Hypotension in Emergency Department Patients*.” Critical care medicine 32, no. 8 (2004): doi:10.1097/01.CCM.0000133017.34137.82.

Lichtenstein, Daniel A, and Gilbert A Mezière. “Relevance of Lung Ultrasound in the Diagnosis of Acute Respiratory Failure: The BLUE Protocol.” Chest 134, no. 1 (2008): doi:10.1378/chest.07-2800.

Rose, J S, A E Bair, D Mandavia, and D J Kinser. “The UHP Ultrasound Protocol: A Novel Ultrasound Approach to the Empiric Evaluation of the Undifferentiated Hypotensive Patient.” The American journal of emergency medicine 19, no. 4 (2001): 299-302. 

Salen, Philip, Larry Melniker, Carolyn Chooljian, John S Rose, Janet Alteveer, James Reed, and Michael Heller. “Does the Presence or Absence of Sonographically Identified Cardiac Activity Predict Resuscitation Outcomes of Cardiac Arrest Patients?” The American journal of emergency medicine 23, no. 4 (2005): 459-62.

Weingart, Scott, Duque, Daniel and Nelson, Bret. “The RUSH Exam – Rapid Ultrasound for Shock / Hypotension.” (accessed January 9, 2011).


AIUM recognizes ACEP Emergency Ultrasound Guidelines

Wonderful news from AIUM in this week’s Sound Waves Weekly.aium logo

AIUM Officially Recognizes ACEP Emergency Ultrasound Guidelines
November 17, 2011

In keeping with the AIUM’s overarching mission of advancing the safe and effective use of ultrasound in medicine through education, research, and development of guidelines, the AIUM recognizes the American College of Emergency Physicians (ACEP) Policy Statement Emergency Ultrasound Guidelines as meeting the qualifications for performing ultrasound in the emergency setting. These guidelines describe the education and training required by emergency physicians to achieve competency for the performance of focused emergency ultrasound applications in clinical practice.

AIUM President Alfred Z. Abuhamad, MD, expanded on the importance of this resolution, stating, “Recognition of the ACEP Emergency Ultrasound Guidelines by the AIUM helps ensure that focused emergency ultrasound examinations are performed safely and that physicians performing the ultrasound examinations have met a minimum level of competency as hereby defined. AIUM recognition of the ACEP guidelines can pave the way for collaboration between the two organizations. Furthermore, AIUM recognition provides support and standardization for hospital credentialing of emergency physicians in the performance of the focused emergency ultrasound examination.”

Focused emergency ultrasound examinations are performed at the bedside to diagnose acute life-threatening conditions, guide invasive procedures, and treat emergency medical conditions. Focused emergency ultrasound has been proven to improve the care and expedite treatment of countless patients worldwide.

Michael Blaivas, MD, emergency physician and chair of the AIUM Emergency and Critical Care Ultrasound Community stated:“The recognition by the AIUM is a major milestone for both societies as well as point-of-care ultrasound in general. The AIUM has been able to adapt and grow, not only in membership but also in diversity, by embracing and helping the spread of ultrasound into the point-of-care practice setting for a wide range of clinical applications. The ultimate beneficiaries of this recognition are our patients as ultrasound spreads more widely to help patients who are suffering from acute and chronic illness or undergoing potentially dangerous or painful procedures.”

The AIUM anticipates future collaborative efforts with the ACEP on the use of ultrasound in the emergency medicine setting.

XRS- Rib pain

32 year old female with no past medical history presents with cough for two weeks, no fever, no sputum. Multiple sick contacts with same symptoms at work. She acutely presents with left rib pain for several days.  She reports no trauma, and noted the sharp, positional pain during a fit of coughing. Her vital signs are all within normal limits. She is breathing comfortably, with good air movement, no wheezes, rales, or ronchi. She displays point tenderness over her anterior left 8th rib at the anterior axillary line.  A chest x-ray was ordered; images are below.


Sonopalpation of the tender area revealed the following:

Untitled from Sinai EM Ultrasound on Vimeo.


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