Do you blindly trust that your transducer is working? Do you know if there is any damage to the internal workings of the transducer? Would you bet your patient’s health on it?? Most of us take for granted that our equipment is functioning normally and that what we are imaging is the “truth”. A regular comment from sonographers is that the ultrasound system they are using is too old and takes poor images. If the machine is regularly serviced I would almost certainly bet the fault is with the transducer, not the machine.
It can be difficult to identify significant probe damage in a phased array transducer. We often don’t appreciate that a probe is deteriorating as there is a lot of variation in image quality between patients – we tend to attribute poor images to the patient rather than considering our equipment as a contributing factor. Today’s post highlights the need to regularly check the integrity of our transducers and proposes a basic protocol for routine transducer testing.
An effective Quality Assurance (QA) protocol is a critical component to the quality ultrasound laboratory. QA emphasis has largely been placed on sonographer training and ongoing professional development, with little consideration for evaluation of transducer integrity. Typical routine preventative maintenance programs, carried out by the ultrasound vendors, consist primarily of a visual inspection for probe damage and an electrical leakage safety test. More sophisticated testing protocols may also include the use of a 2D test phantom, however this is not considered a routine test by vendors, nor is it particularly effective for evaluating individual element function¹. An additional specialized testing procedure is required to assess the proper functioning of the elements, test integrity of the cabling, and assess for delamination of the lens from the transducer elements².
Transducers may be faulty due to: weak or “dead” elements, delamination of the lens, and/or cable damage.
The effect of transducer damage on image quality has also been well described using phantoms, however the difficulty arises in identifying faulty probes during real-life scanning in that the image quality might not appear deteriorated. The sonographer may fail to notice a progressive loss of image quality over time. Additionally wide variation of image quality between patients makes comparison difficult; poor image quality often attributed to the patient rather than the machine, particularly in cardiac imaging. Previous research has highlighted the alarming incidence of defective transducers currently in use in clinical practice, estimating it as high as 40%, however we don’t know what impact this has clinically.³
The following case describes a 73 year old male with chronic lung disease and known moderate mitral regurgitation who presented with increasing peripheral edema. The study was imaged with a damaged probe (although we didn’t know that at the time of scanning…) that had been dropped on the tiles 2 days prior. The patient’s 2D image quality was fair, however this didn’t raise alarms for the sonographer given his long standing history of lung pathology. The addition of color Doppler was a little more concerning, with only a trivial jet of mitral regurgitation visible from the apical long axis view.
The mitral regurgitation seemed to be improved, despite the blood pressure being unchanged and the left heart chambers being significantly more dilated when compared to a previous study (12 months prior). The sonographer recognized the discordance and attempted further investigation. Repeat imaging was performed on the same system, but with a spare transducer that was available. The 2D image is still challenging, but a marked difference in severity of regurgitation can be easily appreciated.
FirstCall testing of the transducer reported 25 transducer elements (out of an available 96) which were failing to generate a pulse sufficiently. This system interrogates each transducer element individually and generates sophisticated quality assurance reports for documenting QA.
The transducer passed electrical safety and other usual tests as part of the vendor’s routine testing.
There was no visible external damage to the transducer and the machine had recently been serviced.
Although the 2D image quality was largely unaffected, the severity of MR was grossly underestimated due to the damage. This could have easily been missed! To say that this was a sobering experience, is an understatement!! This example highlighted the need for routine testing of transducers, above the level of testing performed as part of a preventative maintenance service.
In our institution, we implemented the following testing protocol.
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Weigang B, Moore GW, Gessert J, Phillips WH, Schafer M. The methods and effects of transducer degradation on image quality and the clinical efficacy of diagnostic sonography. Journal of Diagnostic Medical Sonography. 2003 (19):3-13.
Thijssen JM, Weijers G, de Korte CL. Objective performance testing and quality assurance of medical ultrasound equipment. Ultrasound Med Biol. 2007 Mar;33(3):460-71.
Mårtensson M, Olsson M, Segall B, Fraser AG, Winter R, Brodin L. High incidence of defective ultrasound transducers in use in routine clinical practice. Eur J Echocardiogr (2009) 10(3): 389-394.