A novel polymeric coating for enhanced ultrasound imaging of medical devices
S Mehdi Tavakoli, TWI Cambridge, UK
Ewen J C Kellar, TWI Cambridge, UK
Dariush Nassiri, St George's Hospital, London, UK
Anton E Joseph, St George's Hospital, London, UK
Paper 401 presented at Society of Plastics Engineers Annual Technical Meeting, SPE ANTEC 2001 Conference - Medical Plastics, 6-10 May 2001, Dallas, Texas, USA. Also presented at Spring Medical Device Technology Conference. Birmingham, UK, 16-17 February, 2000.
Fine needle aspirates, biopsies and drainage procedures under ultrasound guidance are common, in most hospitals. A dynamic novel coating, consisting of a hydrophilic polymeric matrix and a bubbling agent, has been developed for accurate entry and positioning of the needle. The bubbling agent reacts with tissue fluid as the needle penetrates and produces bubbles within and on the surface of the coating, increasing the backscattering capacity of the coating and generating a brighter image of the device under ultrasound. Coated biopsy needles have been successfully used for in-vitro trials using a tissue-mimicking phantom, and in an isolated animal liver.
Most hospitals use ultrasound guidance to carry out fine needle aspirates, biopsies and drainage procedures. Currently, the devices used for these procedures rely on scoring or roughening of the area around the needle tip to improve visualisation. However, the visibility of the tip alone does not allow the clinician to gauge accurately the entry angle of the needle. Often the process has to be repeated before a successful outcome is achieved, which can result in unnecessary discomfort to the patient and increased expense in terms of number of needles used.
Bosley et al  described methods of fabrication and use of echogenic medical devices in a US patent. The echogenic device included a material with acoustic impedance different from that of surrounding medium (e.g., biological tissue or fluids). The material applied to the device consisted of a polymer (e.g., polyethylene or adhesive) or a silver solder with sound reflective particles (e.g., glass microspheres about 5µm in diameter). The polymer based material may contain different quantities of sound reflective material. A radiopaque material (e.g., barium or tungsten) was also included to enable the device to be both radiographically and ultrasonically imageable.
Development of a medical device (e.g. needle) with an ultrasonically reflective coating was also described in a European patent  . The coating included a matrix of gas bubbles contained in a polymeric material. The polymeric material used in this case was described as a foam containing a blowing agent which is activated during melt processing (e.g. moulding) of the polymer or introducing a gas into polymer melt creating a cellular core structure.
In a series of studies [3-5] , a novel polymeric coating was developed to enhance ultrasound images of medical devices.
Materials and needle coating
- Polymer: a medical grade hydrophilic polyurethane Hydrothane TM (Cardio Tech International Ltd)
- Gas producing agents: sodium hydrogen carbonate and citric acid (Aldrich Chemical Co)
- Solvent: Tetrahydrofuran (THF) (Aldrich Chemical Co)
- Substrates: glass, stainless steel and two types of biopsy needles (a) 18 gauge (pink) (b) 21 gauge (green)
- Primer: A water based primer
Prior to coating all needles were wet blasted with 60 mesh alumina grit for 60-90 seconds at 4 bar pressure.
Layers of coating material were deposited on the needle surface by dipping the needle into THF solutions of Hydrothane TM and reagents, using the selected composition as shown in Table 1. The thickness of each layer was estimated to be ~30µm with a total coating thickness of up to 120µm.
The following assessment techniques were used to evaluate the coating and its performance.
To assess the real-time performance of the coating materials, a Microvision MV2100 CCD camera with a MV -120z microscope attachment was used in conjunction with a video recorder, Sony U-Matic V0-8800P. By placing the coated materials (stainless steel plate or biopsy needle) in a glass dish filled with water, the bubble production activity of the coatings could be monitored. Selected recorded video data were then transferred as MPEG files onto a CD-ROM disk for analysis.
Optical analysis of bubble formation within the coating was carried out using two instruments:
- An Olympus BH2 microscope using polarised light. Photographs were taken with an Olympus PM-10AD camera system
- A Leica WIDM3Z binocular microscope with an Olympus PM-10AD camera system.
Five needles were supplied to St George's Hospital for analysis using an HDI 100 'Apogee' ultrasound system (Advanced Technology Laboratories), set at the lowest gain, and fitted with a 3.5MHz annular sector probe. The probe was fixed just under the surface of the water in a thermostatically-controlled water bath, maintained at 37°C. The angle of inclination of the probe and the depth to which it was immersed were kept constant throughout the experiments. The test samples were placed in a fixed position relative to the probe and supported on a steel block immersed in the water bath. This ensured that scatter from the generated bubbles and the coated surface, rather than specular reflection, was responsible for the returning echoes.
The mean brightness of a region of interest of each of the digitised images (512x512x8 bit format) was calculated and expressed as follows:
- Mean brightness vs time
- Percentage change in mean brightness vs time
- Change in mean brightness in dB against background vs time
Needles coated with the combination of materials that demonstrated the greatest change in brightness were evaluated under ultrasound, (as described above), in a tissue-mimicking phantom and in an isolated ovine liver. Uncoated needles served as controls. The phantom consisted of tissue-mimicking material (TMM) with acoustic properties similar to those of soft tissue (e.g., liver).
Tissue-Mimicking Material (TMM)
The TMM was submerged in a water bath at 37°C and the selected biopsy needles inserted into it. The ultrasound probe was held in place above the phantom, with the transducer elements submerged in the water. Images were acquired from the region of interest (ROI), digitised and stored, as described above, for further analysis. The data acquisition process was calibrated so that the relationship between the brightness of the digitised image and the actual reflectivity/backscatter (dB) was expressed by the following formula:
Gain in dB = 0.4 x brightness in ROI + C 
(Where C is a constant).
Isolated ovine liver
The above experiments were repeated using an isolated ovine liver to evaluate the performance of the coated needles and commercial needles with echogenic tips, in solid tissue. The liver was obtained at the abattoir from a recently-slaughtered sheep, placed in a solution of heparinised normal saline and transported to St George's Hospital. The liver was placed in fresh heparinised saline under a gentle vacuum, to reduce the number of gaseous inclusions trapped inside the liver. Coated needles were inserted into the isolated liver, as described above for the TMM. As before, the commercial uncoated needles served as controls. For a further comparison, one of the coated needles was stripped of its coating at the end of the experiment to see if the roughening process used to promote adhesion of the coating did, itself, contribute to the brightness of the needle being imaged.
Results and discussion
Coating structure and composition
Table 1 shows the composition of those samples described in this paper.
Table 1 Details of coating structure and composition of the samples used.
|Needle No||Coating Structure||% by dry weight of additive in polymer||Colour|
||[PUC 50%, PUS 40%]
||[PUC 50%, PUS 40%]
||[PUC 100%, PUS 100%]
||[PUC 100%, PUS 100%]
||[PUC 100%, PUS 100%]
||[PUC 100%, PUS 100%]
||[PUC 100%, PUS 100%]
St - Stainless steel substrate
PU - Hydrothane TM
PUC - Hydrothane TM + dissolved citric acid
PUS - Hydrothane TM + sodium hydrogen carbonate
BP - Wet blasting + primer
The standard biopsy needles used were 18G (pink) and 21G (green). For convenience, the needles are hereafter referred to by the hub colours.
Upon analysis of the ultrasound data, it was found that the most promising coatings on needles 1 and 2, identified by eye and earlier video assessment, gave rise to the greatest quantitative changes, as shown in Figure 1. These samples were coated whereby the layer order followed the scheme in which a layer of Hydrothane TM containing dissolved citric acid was deposited first and then overlaid with subsequent layers of Hydrothane TM only and Hydrothane TM containing sodium hydrogen carbonate. Samples where the order was reversed showed very little activity. The other more obvious factor, was that the most active coatings contained the most gas producing reagent i.e., >50%/wt assuming an equal ratio of the reactive components (0.25g citric acid and 0.25g sodium hydrogen carbonate in 1g Hydrothane TM).
Fig. 1. Quantitative analysis of ultrasound data for brightest coating on stainless steel biopsy needles in a water bath
The reference wire in the first frame has a diameter of 0.125mm. Figure 2 shows the sequence of bubble development with time over approximately 2 minutes on a coated needle. Again, the reference wire in the first frame has a diameter of 0.125mm.
Bubble development, on a coated glass slide following immersion in water, was tracked microscopically using polarised light. Bubbles can be seen to grow in number and size over time.
Ultrasound evaluation of coated biopsy needles
Both the green and pink coated needles were readily visible in the resulting ultrasound image, when tested in the tissue-mimicking phantom. Enhancement of the needles persisted throughout the imaging experiment, i.e. for greater than 100 seconds. The larger bore needle (pink) was more readily seen than the smaller needle (green). The attached graphs ( Figure 3
) quantify the increased enhancement of the needles, compared with the uncoated controls. In particular, Figure 3
shows a peak mean backscatter of 25-30 dB, relative to the controls. The coated needles were visible within a few seconds of insertion and the rate of bubble generation, hence reflectivity, continued to increase, reaching a plateau after approximately 60 seconds.
Fig. 3 Quantitative analysis of ultrasound data for coated needles in a Tissue-Mimicking Phantom
The coated needles were clearly seen in the images obtained in the isolated ovine liver as shown in Figure 4. It was noted that a track of bubbles was still visible in the liver once the needles had been removed.
Consequently, the clinician is able to track a bright line during needle placement, which provides information about both the angle and position of the needle, relative to the target, rather than the single bright spot afforded by the tip of the echogenic needle. Furthermore, when the coating was completely stripped from one of the needles (green), no background reflectivity, due to roughening the needle surface to promote coating attachment, was seen.
The main conclusions which can be drawn from this work were as follows:
- Incorporation of a gas-producing reagent mixture within a hydrophilic polyurethane matrix and its application as a coating to medical devices, such as biopsy needles, can significantly enhance the ultrasound visibility of devices.
- In-vitro experiment, using a tissue-mimicking material showed that the coated needles were readily visible in the resulting ultrasound image, with a peak mean backscatter of 25-30 dB relative to uncoated control needles.
- Improvement in coated needle visibility was confirmed in an isolated ovine liver experiment.
- A new coating formulation with novel deposition techniques was developed that enabled thin, smooth, bubble free coatings to be applied to a medical device without undesirable premature reaction between gas producing reagents.
- Bosley R W Jr, Thomson P G, Thomas L F: 'Echogenic devices material and method'. US Patent 51081,997, Jan 21, 1992.
- Lawrence P: 'Instrument having enhanced ultrasound visibility'. European patent application, 0624342 A1, 4/5/1994.
- Joseph E A: 'Instrument having enhanced ultrasound visibility'. International Patent No WO 98/18387.
- Kellar E J C and Tavakoli S M: 'Ultrasound detectable instrument'. International Patent No WO 00/66004, 9 November 2000.
- Joseph A E, Kellar E J C, Tavakoli S M and Nassiri D: 'The enhanced visualisation of devices for ultrasound guided interventional procedures'. Annual meeting of the British Ultrasound Society, Glasgow, UK8-10 December 1999.
The authors wish to thank BTG and Medilink for funding this project and Mrs. Ruth McD Sutherland and Mr. Ian M Bingham of BTG for their continuous support, and to Mrs. Janette Whiting for typing this paper.