THE DESIGN AND PRODUCTION OF A NOVEL SPLIT ARRAY ULTRASOUND TRANSDUCER

Purpose: The design, fabrication and validation of a novel ultrasound transducer is described. The purpose of this transducer is to guide the insertion of epidural anaesthesia and analgesia in obese paturients. The prevalence of obesity in the UK (defined by a BMI >30) is 20%, with 23% percent of all females over the age of 16 defined as obese(1). Prevalence of obesity in paturients will increase with the prevalence of obese children approximately 7% of girls between the ages of 2 and 19)(2). It is such patients most difficult for epidural anaesthesia who have the greatest contra-indications for general anaesthetic

Ultrasound has been utilized to guide insertion of regional anaesthetic needles since 1983(3) and despite numerous successful case reports has yet to gain widespread acceptance. For anaethetists to realise the full potential of ultrasound will require the adaptation of existing technology to ease use in the labour ward and theatre by non radiologists.

The design principal was that the transducer should be robust and simple to use and images clear and easily interpreted.

Methods and Materials: The split array transducer was designed to allow the epidural needle to be guided and visualised throughout the insertion procedure. Introduction of the needle conventionally from the side of a transducer has 3 disadvantages

• The angle of needle approach has to be adjusted with the depth of the target

• The needle approaches the interspinous space at an angle to the sagittal plane.

• The tip of the needle can pass through the wave front if angle of approach is misjudged.

The use of an system in which the needle is advanced in the plane of the wave front has none of these disadvantages and several advantages.

• The gap in the image can be utilized as an aiming device correlating to the physical gap in the transducer.

• The tip of the needle is in the plane of the wave front and can be visualized throughout the advance of the needle in the sagittal plane.

• The spinous processes may only be imaged if the wave front is directly above the spinous processes. Therefore correct advancement is dependant on the concurrent imaging of the needle spinous processes and needle.

A linear transducer with a wide foot print was chosen and the frequency optimised for depth and resolution. The transducer was designed to be ergonomic, easily positioned to obtain an image quickly and allow the advancement of a needle in the wave front.

Results: Current ultrasound technology has been utilised by one of the authors (MJW) to assist the insertion of regional anaesthetic needles where traditional techniques fail. The majority of these patients had significant co-morbid obesity and ultrasound imaging assisted rapid identification of a safe lumbar interspinous interspace. A clinical trial will evaluate the split array transducer in obese patients. Results from this trial will be presented.

Conclusions: A successful clinical trial will lead to the commercial exploitation of this design of transducer by a leading manufacturer of ultrasound imaging systems.

References

1)National Census,2001: Social Trends 34, Health Survey for England; Office for National Statistics; Department of Health.

2)Health Survey for England 1995, 2000. Office for National Statistics; Department of Health.

3)Palmer SK, Abram SE, Maitra, von Colditz JH. Anaesthesia and analgesia 1983: 62: 944–946

Acknowledgements

The authors wish to acknowledge the contibution to this work of Dr S Cochran and Dr KJ Kirk of Paisley University and funding from CSO and Scottish Enterprise.