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

FUNDAMENTALS

The IMBUS program uses several types of ultrasound machines. The chapters in this online textbook cannot describe every detail of performing tasks on each machine and terminology is sometimes different between machines. This text is designed most specifically for the GE Venue machine.

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

INDICATOR POSITION: We keep the screen indicator at the top left (radiology screen convention). With a sagittal, parasagittal, or coronal probe position on the body, the indicator on the screen represents the head (cephalad) and the probe indicator will also be positioned cephalad (radiology probe convention). A cephalad sliding of the probe shows new (more cephalad) tissue entering from the left side of the screen.

Exception: The long axis views of the heart (PLAX, apical2, and apical3) violate this principle and have the probe indicator caudad in order to match cardiology views established long ago with angiography. That makes these few views more difficult because cephalad/caudad movement on the screen is backwards from what the hand and eye are experiencing.

For transverse views we want structure movement on the screen to coordinate with probe movement from the physician’s perspective. When the probe moves left, we want new tissue to enter from the left side of the screen. Traditional radiology convention is stated as “probe indicator patient right” and this is accurate for supine and lateral decubitus patient positions. However, when examining a patient from the back (e.g. popliteal vein, posterior lung) the more accurate guidance is “indicator physician left”. Indicator physician left is correct for all transverse probe positions, regardless of patient position.

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 parasternal area for a cardiac exam and on the right, gel was applied for a posterior lung exam.

 

PROBE PRESSURE: Increased probe pressure is potentially beneficial only for patients with thick adipose layers and only for extremity or abdominal structures of interest. Little is gained with higher probe pressure on the chest wall. Increased probe pressure on obese extremities or abdomens can compress the subcutaneous tissue and get the probe closer to the structure of interest so that image quality improves. However, compression of tissue can also distort the sound carrying properties of tissue and adversely affect some aspects of imaging. Very importantly, high probe pressure is uncomfortable for a patient and increases the chance of chronic injury in the physician. We find that physicians starting advanced IMBUS training often experience cramping of their right hand and arm until they learn to lighten the probe pressure and relax their hand and arm. Light probe pressure with a good gel layer is best for the chest wall and non-obese extremities and abdomens.

GAIN: The default gain setting for a probe and preset 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 images of a left kidney with a parapelvic simple cyst show 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 with a control called Time Gain Compensation. For example, the tissue posterior to the cyst in the image above is over-gained because of posterior acoustic enhancement and the TGC control could turn down the gain just for the tissue posterior to the cyst.

FREQUENCY: In obese patients, lowering the default probe frequency from RES to GEN or from GEN to PEN may be necessary to improve views, even though this sacrifices some image resolution.

DEPTH: Structures of interest should be in the lower part of the B-mode sector. The frame rate is better in the lower part of the sector, which is particularly important for any moving structure. This location in the sector also increases the image size of any structure.

FOCUS POSITION: A structure of interest will have best image definition when it is in the focus position for the sector. The focus position is easily moved with Venue and can coincide with the optimal frame rate area in the lower part of the sector.

WIDTH: The width of a B-mode sector needs optimization to avoid wasting ultrasound power. A narrower sector width improves the resolution of structures and increases the frame rate. Venue has Width control for the phased array and curvilinear probes.

ZOOM: Zooming is the last step in the view of any structure and is done with Venue using a finger-pinching action on the screen. If a B-mode image has been optimized for depth, focus position, and width it is unusual to need zooming.

Below is a standard PLAX view of a heart with a Venue machine . Assume the interest for the patient is on the details of the aortic valve.

 

To optimize the aortic valve the depth was decreased to move the valve to the lower part of the sector, the focus position was moved to the valve level, and the sector width was narrowed. Here is the result.