Chapter: Fundamentals

Gel Application Gain GEN/RES/PEN Depth and Focal Zone

POCUS Fundamentals

This introductory chapter covers the basics of POCUS mechanics at the bedside

FIRST, A FEW FUNDAMENTAL REMINDERS

HANDS: IMBUS beginners do exams mostly in the hospital with one or more additional people. The equipment setup in the hospital often results in the learner handling the probe and a mentor controlling the buttons. However, an advanced IMBUS provider must be able to independently complete all exams, controling both the probe and all of the buttons. The equipment setup in the clinic ultrasound rooms mandates that the probe is held in the right hand and the left hand must do the buttons.

INDICATOR POSITION: We will always (even for cardiac ultrasound) keep the screen index mark (green dot on SonoSite devices) at the top left of the screen (radiology screen convention).

In a sagittal, parasagittal, or coronal orientation on the body, whether supine or sitting, the indicator on the screen represents the head (cephalad) because the probe index mark (groove/ridge on the probe) will always be positioned cephalad (radiology probe convention) in these planes. A cephalad sliding of the probe shows new (more cephalad) tissue entering from the left (screen index mark) side of the screen.

We always want structure movement on the screen to coordinate with probe movement from the examiner's perspective. Therefore, when a transverse body view is obtained we will stray a little bit from standard radiology probe index orientation (which is to always keep the probe index mark to the patient's right) and instead always keep our probe index to the examiner's left. When imaging from the typical anterior/lateral aspects of a patient, this will mean the probe index is in its standard radiology position towards the patient's right. However, when we are imaging from the posterior aspect of the patient (i.e. popliteal exam standing position, posterior lung sitting, posterior kidney views, etc.) we will still keep the probe index to the examiner's left so that movement on the screen continues to mirror probe movement (e.g. when moving the probe to the operator's left, new tissue is moving into the screen from the left). This results in the probe index mark being pointed to the patient's left (instead of standard right) when imaging from the posterior aspect of a patient.

GEL: Beginners often use too little gel. We have gel-warmers in the ANGMA clinics, which help patient comfort, even if the gel becomes a little thinner. In some situations, a good amount of gel should be applied directly on the area to be examined rather than coming repeatedly back to the bottle with the probe. This approach saves time and motion, as well as reducing the risk of losing a good viewing window. If gel is being loaded onto a probe directly, it should not be done over the patient as it can splatter on their face or clothes. On the left in the image below, gel was applied to the left parasternal area for a cardiac exam and on the right gel was applied for the posterior lung exam.

 

GAIN: The default gain setting is not always optimal. Look for fluid in any field that should be anechoic (e.g. urine, blood, and cysts). Be sure that this fluid is black with minimal grey. However, some structure to the tissue should remain. The following are images from 123sonography.com of a liver with an adjoining pleural effusion showing an under-gained image on the left, an over-gained image on the right, and an acceptably-gained image in the middle.

 

Sometimes just the far-field gain can be adjusted. An example is a full bladder that gives posterior acoustic enhancement, partially obscuring the prostate. By turning down the far-field gain, the view of the prostate can be improved.

PEN AND RES MODE: Accepting the default GEN mode is not always optimal. In obese patients, the PEN mode will sometimes improve views because the gain in depth penetration is worth some loss in the resolution. In other situations, RES mode is better because optimal resolution is more important than looking very deep or through fat. The carotid artery and jugular vein would be examples where RES mode may be beneficial.

IMAGING A PARTICULAR STRUCTURE: OPTIMIZE FOCUS, SECTOR WIDTH, and MAGNIFICATION: The Sonosite Edge has the optimal focus point set at the middle of the screen. Any area of special interest must be moved into the center of the screen, no matter what this does to the overall organ view.  Depth adjustment and probe movement are important in achieving this. Images can also be improved with the phased array P21 probe by narrowing the sector width, using the Sector button. This concentrates more energy to the area of interest. Finally, magnify after these other adjustments are made.

Below is a standard PLAX view of a heart. Assume the interest is on the details of the MV apparatus. The image below had the sector width at default maximum and the MV structure was a little below the center of the field.

 

To optimize the view, the depth was increased and the probe subtly adjusted to put the MV in the center of the screen. The sector was then reduced to the minimum. The MV got a little smaller, but had higher resolution. The view looks like this.

 

Finally, the MV was magnified and a more detailed view was achieved than the default PLAX.