Characterization of tissue-simulating phantommaterials for ultrasound-guided percutaneous

liver biopsy

Buchanan S, Moore J, Peters T

March 31, 2011

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1 Abstract

Biopsies of the abdomen are standard procedures that are commonly performedunder ultrasound guidance (US) or computed tomography (CT) [7]. Phantomsfor this procedure are crucial as both training tools for surgeons and researchtools for better accuracy of imaging equipment. Our experience suggests thatrealistic imaging conditions and material longevity are the most important quali-ties of an abdominal biopsy phantom. Current commercially available phantomsfor use with ultrasound guidance have many limitations. The most detrimentallimitations include harsh needle tracks tarnishing ultrasound images over timeand a membrane comparable to human skin that does not allow seepage of in-ner media. To overcome these limitations, we tested a variety of media andmembranes to evaluate optimal materials that fit our current needs, specificallyfor percutaneous liver biopsies. It was concluded that liquid hand soap wasthe best medium as it instantly left no needle tracks, had acceptable depth ofUS penetration, portrayed realistic US imaging conditions and was transparent.The best membrane was concluded to be 10 gauge vinyl as it had low leakage,durability, low cost, and was transparent.

2 Introduction

The uses of phantoms in medical imaging research are widespread, and may sub-stitute for real tissue models where in vivo trials are inappropriate [3]. Phantomscan be made to model anatomical features as well as realistic clinical compli-cations and conditions. Since phantoms have precise measurements, knowndimensions, and known properties, they can be used as ground truth in manymedical imaging experiments [3]. Not only are they useful for segmentationalgorithm development where the geometry must be known, but are extremelyuseful for clinical training purposes [3]. Clinical training is the focus of thecurrent experiment: training medical professionals on specific medical proce-dures prior to in-vivo attempts. Our experience suggests that some of the mainbenefits to the use of clinical training phantoms include longevity in structuralstability and realistic imaging conditions. Current commercially available phan-toms for use with ultrasound guidance have many limitations and come withhigh cost [4]. One of the most detrimental limitations to these includes harshneedle tracks left over time tarnishing ultrasound images (Figure 1).

The needle tracks appear bright on ultrasound images. This trait decreasesthe realism of the training procedure and images produced. Eventually, needletracks accumulate to produce an unreadable ultrasound image. Thus, phantomsmust be replaced. Not only must phantoms be replaced for needle tracks, butalso many media used in phantoms, such as agar, have a short shelf life beforethey perish [6]. Thus, non-perishable materials for uses in these phantoms area must. Further, membranes comparable to human skin that do not allow seep-age of inner media are crucial to phantom design. The objective of this studyis aimed to overcome these limitations.

Percutaneous liver biopsies are performed when establishing the diagnosis,

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Figure 1: Examples of needle tracks viewed on commercial CIRS abdominalbiopsy phantom by SonixGPS ultrasound machine (6.6M, Max Dep., Gain 45,Sector 100, Dyn 80dB).

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assessing prognosis and tracking therapy for various liver diseases [1]. Theseinclude malignancy, cysts, parasites, infections, unexplained liver enlargementor other types of pathology. Ultrasound guidance is heavily relied on for thisprocedure, as it has been proven to lead to a lower rate of complications [1].The percutaneous liver biopsy procedure includes marking the portion of liverwith the best thickness and view (using ultrasonography). This portion is ide-ally most distant from other organs [2]. This procedure is performed by anexperienced gastroenterologist, using a biopsy needle that is 20-25 gauge [2]. Aslender core of tissue from the area of interest is removed (Figure 2).

Figure 2: This is an illustration of details for a percutaneous liver biopsy.

Serious risks include lung puncture, infection and bleeding [7].

The objectives of this study are to develop a low-cost liquid phantom withpenetratable medium and membrane, with realistic imaging qualities (to thatof a percutaneous liver biopsy procedure). Criteria include absence of needletracks and sealability of outer membrane. Media tested included PVA gel, liq-uid soap, water, hair gel and ultrasound gel. One medium, Solidifying-Agent(used as for blood coagulation in hospitals), was excluded prior to experimentas it was not penetratable by ultrasound at an appropriate viscosity. However,the tested media were chosen on the basis of cost, transparency (so users canverify needle locations visually) and low level of biodegradability for phantomlongevity. Currently, materials such as PVA cryogel have been found to showneedle tracks but are used in many phantoms. These materials are not viscousenough for this purpose. Further, materials such as agar, which are traditionallyused in phantoms, have an extremely short shelf life (used in bacterial cultures).

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3 Methods

For this project there was a series of experiments. These included both testsrun on membranes and media.

3.1 Phantom Design

A phantom was built from plexi-glass material for use in all experiments. Thisphantom was built to 10.5 x 10.5 x 16.5 cm (Figure 3).

Figure 3: Phantom design. Includes openings for ultrasound probe/membranein the front and from the top. Needle holes are cut from top portion of phantomfor needle insertion.

3.2 Depth of Penetration

The phantom was lined with a sound absorbing material (Sorbithane Inc) toensure minimal reflection in the phantom by the ultrasound probe. The phan-tom was filled with medium of choice. SonixGPS Ultrasound probe (6.6M, MaxDep., Gain 45, Sector 100, Dyn 80dB) was inserted from the top of the phan-tom, at a fixed distance from the bottom (Figure 4).

A custom-built L-shaped measurement device, with known dimensions, wasattached to a vice that moved on a plane at a fixed angle. This device wasinserted perpendicular to the field of view of the transducer. The measurementdevice was lowered until it left the field of view of probe, and the depth wasrecorded. The experiment was repeated 10 times for each probe. Distances areapproximations as the test was on a pass/fail basis for the application.

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Figure 4: The apparatus set-up for depth of penetration measurements. A.Phantom filled with medium, B. clamp holding ultrasound probe, C. Mobilevice holding L-shaped measurement tool.

3.3 Needle Track Measurement

The phantom was lined with sorbithane and filled with medium of choice. TheSonixGPS Ultrasound probe (6.6M, Max Dep., Gain 45, Sector 100, Dyn 80dB)was mounted using a retort stand against the membrane side of the phantom(Figure 5).

Figure 5: The apparatus set-up for needle track quantification. A) Phantomfilled with medium and lined with sorbithane, B) ultrasound probe held byclamp, C) area of needle insertion.

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A screen shot was taken on the ultrasound machine. A spinal needle (25GA3.5in, 0.05 x 90 mm) was inserted into the top of the phantom at a 90-degree an-gle to the bottom of the apparatus. Screen shot was taken with needle present.The needle was carefully removed, and a screen shot was taken immediately andanother twenty minutes post needle removal. Images were uploaded into MAT-LAB and converted to pixel intensity matrices. Pixel intensities from matrixone (pre-needle insertion) were subtracted from pixel intensities in matrix two(post-needle insertion). Error regions were displayed as brightness. Augmentedmatrix was produced along with pixel intensity matrix, of needle track.

3.4 Membrane Leakage Evaluation

Membranes to be tested were cut to 7x7 cm. Small beakers were filled with 50cc of water and the membrane was secured on top (Figure 6).

Figure 6: The apparatus set-up for membrane leakage quantification. A) Vialswith secured membranes and filled with water, B) 22 gauge needle.

Beaker membranes were penetrated six times with a 22 GA needle, at equaldistances, along a line. Beakers were weighed to four significant digits. Beakerswere turned up side down and allowed to sit for 24 hr on a track system (toensure no capillary e!ect). Beakers were re-weighed to four significant figures.Experiment was conducted eighteen times for each membrane.

4 Results and Discussion

In experiment (3.2), depth of penetration of sound of various media, the datashowed that PVA, soap and gel had a depth of greater than 9 cm. Lubricantdid not (Figure 7).

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Figure 7: Depth of penetration of sound in various media, measured in cm.Note** Distances are approximations as the test was on a pass/fail basis for theapplication.

Values of 9 cm or over are deemed acceptable for use in a percutaneous liverbiopsy phantom. This is because the application/procedure is rarely performedwhere ultrasonography must penetrate a larger distance [8].

In experiment (3.3), needle track quantification of various media, experi-ments displayed that lubricant and gel left brightness (error) spots, and soapand PVA produced totally dark images (no error). (Figure 8) displays before,after and di!erence images of gel.

Figure 8: Ultrasound images of gel before needle insertion and after needleinsertion (left). Di!erence image produced by conversion of before and afterimages into pixel intensities, and subtracting matrix values (right).

The highest grayscale value of lubricant was 133, and was 197 for gel (Figure9).

Gel and lubricant were degassed using a vacuum chamber. Both were de-gassed easily, however, upon transfer to phantom and post needle insertion,both media returned to their original state. A remedy for this problem, if gelor lubricant were the only alternatives, would be to have a phantom that wasable to withstand a vacuum chamber. This method could be used after eachtraining session. However, our phantom did not have these capabilities. Luckily,PVA and liquid soap did not need degassing and further, did not leave any errorartifact on ultrasound images after penetration with a needle.

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Figure 9: Highest grayscale values of needle track errors for each media, as seenon Ultrasound image. ** 0 denotes pure black, 255 denotes pure white.

In experiment (3.4), membrane leakage quantification, eighteen samples ofeach membrane yielded an average water loss of 1.78g for 18 gauge rubber, and0.45g for 10 gauge vinyl. Though the average water loss for both membraneswere quite di!erent, an ANVOA table was generated to determine statisticalsignificance.

From the ANOVA table generated, the computed F value was 54.95776, asthe F table value was 7.44414 (?=0.01, n=1, m=34). Since the computed Fvalue was much larger than F table, we are 99 percent confident that there isa statistically significant di!erence between the two data sets. This was causedby a greater leakage of water by rubber than vinyl.

5 Conclusions

In conclusion, the objectives of this experiment were to develop a low-cost liq-uid phantom with penetratable medium and membrane, with realistic imagingqualities (to that of a percutaneous liver biopsy procedure). The primary crite-ria included absence of needle tracks and sealability of outer membrane.

PVA, Soap and Gel had acceptable depths of penetration (9 cm), Lubricantdid not. PVA and Soap had a 0 x 0 grayscale matrix after needle insertion(zero needle tracks), Gel and Lubricant left needle tracks. 10 gauge Vinyl hadsignificantly less leakage than 18 gauge rubber.

Therefore, through experimental results suggest that the ideal medium is

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liquid hand soap. This is because it left no needle tracks, had an acceptabledepth of penetration, portrayed realistic imaging conditions, and is transparentfor training purpose. These experimental results also suggest that the vinylis superior to rubber as a skin phantom material. This is because it had lowleakage, is durable, low cost, and is transparent for training purposes.

This study yielded acceptable results for the application of percutaneousliver biopsy training phantom materials. However, further research will includetesting the speed of sound properties of liquid hand soap. This will be useful forother phantom applications such as image-guided surgery phantoms, where allproperties must be known. Further, for realistic imaging conditions, we wouldlike to include a study that will test speckle mimicing agents for mixing withmedia. These agents, from previous background, allow ultrasound images toappear more realistic for many applications. Lastly, we would ideally like tore-build our commercial CIRS abdominal biopsy phantom that has been cur-rently rendered useless for our clinical simulations. With our current knowledgeof the various media and membranes tested, we can build a more durable andapplicable phantom to suit our current clinical training needs.

6 Acknowledgements

I would like to thank Dr. Elvis Chen, J Baxter and Martin Rajchl for theirtechnical guidance on this project.

References

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[8] Neuzillet Y, et. Al. Accuracy and clinical role of fine needle percutaneousbiopsy with computerized tomography guidance of small (less than 4.0 cm)renal masses, J Urol. Vol. 171, 5:1802-5, May 2004.

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