FLUS should be used as an adjunct to clinical assessment and other diagnostic tests.

It is important to reflect on and integrate any positive and negative FLUS findings into the clinical context.

In order to do so, basic knowledge of the strengths and weaknesses of FLUS are needed.

This includes:
– An ultrasound system suited for FLUS
– Reliable power – either external AC power or a battery
– Preset suited for FLUS
– Plenty of acoustic coupling agent – ultrasound gel
– Adjusting the image depth, image gain and focus

Phased array transducers (cardiac) have frequencies of 7-2.5 MHz and generate acceptable 2D images of B-lines and pleural effusion.

However, phased array transducers are not optimal for pneumothorax diagnostics due to the low frequency and shape of the ultrasound image / sector.

They are nicely shaped for scanning between the ribs and penetrate deeply into patients with a thick chest wall.

Choosing the transducer is discussed further in the lesson on examination technique.

Linear transducers typically have high frequencies (14-6.5 MHz).

Due to the high frequency they generate excellent and highly detailed 2D images of superficial structures such as the chest wall and pleura line.

Linear transducers are therefore optimal for pneumothorax diagnostics.

However, linear transducers are not optimal for diagnosing more profound pathology such as pleural effusion due to the high frequency.

Curved array (abdominal) transducers with frequencies of 5-2 MHz also generate excellent 2D images and produce the necessary ultrasound artefacts.

They permit deep penetration in patients with a thick chest wall and are well suited for pleural evaluation.

Microconvex array transducers with intermediate frequencies (7-4 MHz) produce good 2D images and acceptable ultrasound artefacts.

They are nicely shaped for scanning between the ribs in the intercostal spaces with good penetration in patients with a thick chest wall and are well suited for pleural evaluation.

Due to their small size, microconvex transducers are excellent for the assessment of the posterior chest surface of supine patients.

Transducer characteristics, such as frequency and shape of the probe, determine the ultrasound image quality.

Linear and curvilinear (or curved) transducers are most useful for FLUS imaging because they provide excellent image quality and produce the important air artefacts in FLUS (lung sliding, B-lines, and lung pulse).

Expensive newer generation ultrasound systems often contain sophisticated image smoothing software (e.g. harmonics, cross-beam) that is appropriate for some types of diagnostic ultrasonography.

The software is typically programmed to minimise the presence of artefacts, which are critically important in FLUS; knowledge on how to switch such software off and on is therefore necessary.

2D (also known as B-mode) and M-mode are sufficient for performing FLUS; more sophisticated scanning modes (e.g. colour Doppler mode) are generally not needed.

Not all ultrasound machines have a dedicated FLUS preset.

Additionally, some FLUS presets tend to be adjusted to diagnosing superficial pathology such as pneumothorax and are therefore not ideal for more profound pathology.

The operator needs to know which presets are available on the ultrasound machine and which presets generate the best images for a given clinical scenario.

If a FLUS preset is not available, a preset for an “abdominal examination” can often be used when diagnosing profound conditions such as pleural effusion.

A “superficial” or “musculoskeletal” preset can be used when diagnosing conditions solely located at the pleura.

FLUS may be performed with practically any ultrasound system with 2-dimensional (2D) scanning capability.

Get to know your ultrasound machine, so you are familiar with the use and functions of the most commonly used buttons and presets.