Monthly Archives: December 2010

Artifacts 5: On the sidelines

Side lobe 02 300x167 Artifacts 5: On the sidelines

This right paracolic gutter image is taken from a patient with significant ascites. Notice how bright the bowel walls are (solid purple arrows). This is because the air in the bowel acts as a strong reflector, and because ascites (being fluid) only minimally attenuates the incident and reflected ultrasound beam. Thus, a stronger signal is transmitted thus and reflected back to the machine. What then are the undulating hypoechoic “bowel looking” structures just adjacent to it (dotted green arrows)?

The “structures” are simply side lobe artifacts. They are phantom, not real.

Side lobe 1 300x177 Artifacts 5: On the sidelines

Here’s how it happens. The ultrasound beam (solid blue arrow) hits the bowel, is reflected back to the transducer, and registers the bowel’s location and contour (solid blue curve) accurately. This bowel image is real.

Side lobe 2 300x195 Artifacts 5: On the sidelines

All ultrasound beams, however, give off additional unwanted side beams (dotted red arrow). These extraneous beams are called side lobes. Being true sound waves (though smaller intensity than the “main” ultrasound beam), they can be reflected by true structures (solid blue curve) lying to the side of the main beam vector. Since the ultrasound machine assumes ALL reflections of an ultrasound beam arise only from the axis of the beam, reflections from side lobes are depicted as if they arise from the main beam, thus generating the phantom images (dotted red curve).

Side lobes artifacts are ubiquitous. The typical example is a full urinary bladder filled with “sediment” — which can be simply side lobe artifacts from adjacent hyperechoic bowel. They can be given out as much as 45 degrees from the main beam and are found to a larger or smaller extent in all transducers. As you have seen, these artifacts degrade lateral resolution of the image. Although it is often difficult to eliminate side lobe artifacts, examining the same area from different angles will often improve the overall image acquisition.

Remember, side lobe artifacts occurs if the adjacent structures are hyperechoic. If there is doubt about whether it is side lobe artifact OR debris (e.g. sludge in gall bladder, sediments in bladder), turn the patient to another side. “Real” sludge or sediment will flow to the dependent position; slide lobe artifacts don’t.

Artifacts 4 – lung pulse

lung.left lung pulse.M mode 300x225 Artifacts 4 – lung pulse

Left lung with lung pulse

lung.right lung pulse.M mode 300x225 Artifacts 4 – lung pulse

Right lung with lung pulse

The “lung pulse” is an ultrasound sign first described by Dr Daniel Lichtenstein in 2003. Essentially, it is the detection of the subtle cardiac pulsation at the periphery of the lung (parietal pleura to be exact) on the M mode.

In a normally ventilating lung, this subtle transmission is present but NOT seen, as it is masked or “erased” by the air artifact generated by lung sliding. In a non-ventilated lung, however, lung sliding is abolished. Here, the pleura is perfectly still and this allows the underlying cardiac pulsation to be detected.

Since the heart is on the left side, lung pulse is more prominent on the left side than right (both images taken from a healthy volunteer while holding his breath).

Thus, for the lung pulse to be seen on M mode, the following two conditions must be present:

1. Absent ventilation (i.e. no lung sliding)

2. Apposition of visceral pleural and parietal pleural (i.e. no pneumothorax)

The clinical utility of the presence of a lung pulse is:

  1. Diagnosis of non-ventilated lung (sensitivity of 93% and specificity of 100% in patients without previous respiratory disorders)
  2. Exclusion of pneumothorax

Ref:

  1. Lichtenstein et al (2003).The “lung pulse”:an early ultrasound sign of complete atelectasis.  Intensive Care Med. 2003 Dec;29(12):2187-92. http://www.ncbi.nlm.nih.gov/sites/pubmed/14557855
  2. Lichtenstein. Pneumothorax and introduction to ultrasound signs in the lung. In: General ultrasound in the critically ill. Springer, pp110-111