Monthly Archives: November 2012

Angles for Doppler

A prior post discussed the optimal imaging angle for 2D scanning.

Quick quiz: what is that angle?
45 degrees
90 degrees
180 degrees
360 degrees

In this post we’ll illustrate the optimal imaging angle for Doppler evaluation. Let’s start with basic Doppler physics.
Where to police officers situate themselves to aim a radar gun at speeding cars?

radar 500x334 Angles for Doppler

The maximal Doppler shift will be seen at 180 degrees. In fact at the instant the car passes the officer, (90 degrees) there will be zero Doppler shift. At that instant there is no movement between the object and the listener. So they aim the gun directly at the oncoming traffic, so the direction of their beam is parallel to the direction of [traffic] flow.

The image below illustrates Doppler shift of ultrasound reflected off a red blood cell:

  1. Top: A normal ultrasound wave
  2. Middle: Doppler shift reflected off the RBC moving toward the transducer (thus increasing the frequency of the returning wave)
  3. Bottom: Doppler shift reflected off the RBC moving away from the transducer (thus decreasing the frequency of the returning wave).

to away composite 500x273 Angles for Doppler

Thanks to equipmentexplained.com for the image. Imaging at 180 degrees is impractical for diagnostic ultrasound, since the optimal B-mode imaging angle is 90 degrees. Therefore, most authorities recommend an imaging angle between 45-60 degrees for Doppler ultrasound imaging . If you are imaging a vascular structure at 90 degrees and getting no Doppler signal, try lowering your angle.

Physical exam

Is ultrasound the stethoscope of the future? Is it an extension of the physical examination? Will it replace the physical exam?

No.

Point-of-care ultrasound is a diagnostic test. It is a rapid, bedside, noninvasive, accurate, diagnostic test, but still a diagnostic test. It can certainly augment data obtained through physical examination and medical interviews, and adds to information obtained by blood assays and radiology studies.

It is performed using FDA-approved medical devices by clinicians with specialized training. Images used for medical decision-making may be archived and shared with colleagues from multiple specialties. Quality assurance programs improve clinician accuracy and accountability. These are not physical examination characteristics. These are qualities of good diagnostic tests.

There is and will continue to be debate about this issue. Whether we think about point-of-care ultrasound as a diagnostic test or part of the physical examination has ramifications for training, documentation, archiving, and billing.

We recommend checking the guidelines relevant to your specialty and making up your own mind on this issue. In either camp some things remain constant: train well and use ultrasound to enhance the care you provide your patients.