Ultrasound probe defect
Ultrasound probe defects are hardware failures of the ultrasound transducer manifesting as various abnormalities of the scan image, in severe cases even causing complete signal loss. These are a form of ultrasound artifact.
Therefore, awareness of the various signs of equipment damage is crucial. It has been shown that regular monitoring of the in-air reverberation pattern of ultrasound probes detects a large proportion of equipment damage, and results of long term transducer wear 1. It is also prudent to carefully assess the reverberation pattern for each newly acquired probe, as manufacturing defects often manifest as similar artifacts.
While many studies use test phantoms and dedicated electronic equipment, in day-to-day practice even visual assessment of the probe has a reasonably good sensitivity. It has been shown, that only a small proportion of severe probe defects are detected during clinical use, warranting the implementation of regular visual inspections 1.
Firstly, the integrity of the probe should be assessed, as visually apparent damage (cracks of the probe housing, scratches on the soft lens etc.) represent a high likelihood of significant image quality decrease. This should be followed by switching on the probe in question, best assessed using the preset most commonly used in practice (e.g. abdominal preset for a curvilinear transducer). The reverberation pattern in-air is usually apparent as several parallel running bright lines in the near-field 1. If necessary, adjustment of B-mode gain and depth can better accentuate these streaks. The lines should be normally parallel running, and homogeneous. Decreased or increased brightness, or non-parallel running lines likely represent equipment failure.
Some of the most common signs of probe damage 1:
- dropout: loss of continuity of the reverberation pattern, may be increasing distally to the lens. Likely represents piezo element failure due to damage or manufacturing defect.
- delamination: disrupted reverberation pattern without complete signal loss. Usually less apparent, may also indicate damage, a weak piezo-element or manufacturing defect.
- non-uniformity: reverberation lines running non-parallel. As this also indicates image distortion, urgent replacement of the probe is warranted.
- lens wear: initially subtle change of the depth of reverberations peripherally. Usually caused by long term normal use. Should be followed to assess progression and consider timely replacement.
Probes with equivocal signs of potential defects should be assessed further with electronic transducer testing and dedicated imaging phantoms.
It has been shown that without meticulous quality assurance many probe defects will remain undetected for a long time 1. It is prudent to assess and document the reverberation pattern of all probes in use, at regular intervals. It also helps deciding whether damage observed after an accident (e.g. dropped probe) is truly due to the incident, or rather represents prior damage or simply wear.
The paper-clip method has been proposed as a simple, yet sensitive tool for monitoring probe quality. With this technique a small reflective object, such as a paper clip is moved along the surface of the probe lens, with the aid of a small amount of US acoustic gel. This technique can accentuate smaller dropouts of the reverberation pattern 2.
It should be noted that the above mentioned methods are less effective for assessing phased array transducers, as in these probes small element failures impact the entire scan field, thus obscuring minor damages. Manual evaluation of these probes is therefore more challenging. For phased array transducers therefore a modified paper-clip test using M-mode has been proposed 3.
- 1. Nicholas J Dudley, Darren J Woolley. A multicentre survey of the condition of ultrasound probes:. (2016) Ultrasound. doi:10.1177/1742271X16662301
- 2. Lorentsson R, Hosseini N, Johansson JO, et al. Method for automatic detection of defective ultrasound linear array transducers based on uniformity assessment of clinical images - A case study. J Appl Clin Med Phys. 2018;19(2):265-274. doi:10.1002/acm2.12248
- 3. Dudley N, Dudley WD, Dudley. A simple uniformity test for ultrasound phased arrays. (2016) Physica medica : PM : an international journal devoted to the applications of physics to medicine and biology : official journal of the Italian Association of Biomedical Physics (AIFB). doi:10.1016/j.ejmp.2016.08.011 - Pubmed