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

The IMBUS program uses several different types of ultrasound machines. The chapters in this online textbook do not describe details of performing tasks on each machine and terminology is sometimes different between machines.

HANDS: The optimum technique with all IMBUS machines requires the probe to be in the right hand while the left hand performs all tasks on the screen or keyboard. Accuracy and efficiency with this technique requires substantial practice.

INDICATOR POSITION: We will always (even for cardiac ultrasound) keep the screen index mark 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 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. In some situations, a good amount of gel should be applied directly on the area to be examined rather than coming repeatedly back with the probe for more gel. This approach saves time and reduces 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 a 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 part of the field needs gain adjustment. An example is a full bladder that gives posterior acoustic enhancement, which over-gains the prostate. The view of the prostate can be improved by turning down the gain just in the prostate area.

GEN, PEN AND RES MODES: Accepting the default frequency mode for a probe is not always optimal. In obese patients, lowering the frequency from RES to GEN or from GEN to PEN may be necessary to improve views, even though this sacrifices some image resolution.

DEPTH: Excessive depth wastes ultrasound power and makes structures appear smaller. The frame rate is better in the lower part of A B-mode sector, which is particularly important for moving structures like the heart. Adjusting depth to place a structure of interest in the lower part of the screen improves the size of the structure and gives better movement detail.

WIDTH: The width of a B-mode sector similarly needs optimization to avoid wasting ultrasound power. A narrower sector width improves the resolution of structures and increases the frame rate. Phased array probes usually have width controls while only some machines have curvilinear probe width controls. Linear probes usually do not have width control. Always narrow in on any structure of interest.

FOCUS POSITION: The Edge machine has a focus position fixed at the center of the screen. That means the optimal focus position and the optimal frame rate cannot coincide. On the Venue machine, the focus position is movable and focus position and frame rate can overlap. A structure of interest will have best image quality when it is in the focus position.

MAGNIFICATION/ZOOM: Magnification/zooming is the last step in optimizing the view of any structure. Magnification cannot make up for poor image quality from suboptimal depth, width, and focus position.

Below is a standard PLAX view of a heart with the Edge machine and its focus position at the center of the screen. 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 on the Edge, the depth was increased and the probe subtly adjusted to put the MV in the center of the screen. The sector width was then reduced.

 

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

With the Venue machine, the sequence for image optimization would be different.

  1. Adjust the Depth to put the structure of interest in the lower part of the screen.
  2. Narrow the Width.
  3. Move the Focus Position to the structure of interest.
  4. Zoom.