Low cost anatomically realistic renal biopsy phantoms for interventional radiology trainees

Abstract

This paper describes manufacturing of economically affordable renal biopsy phantoms for radiology residents and practicing radiologists. We reconstructed a realistic 3-dimensional patient-specific kidney model from CT data, manufactured an organ mould and casted the kidney phantoms. Using gelatin gel materials with calibrated parameters allowed making phantoms with realistic mechanical, ultrasound and CT properties including various pathologies. The organ phantoms with cysts included were further casted into gelatin gel medium. They were validated by radiology residents in biopsy training and compared against self-made phantoms traditionally used in the curriculum of interventional radiology. The realism, durability, price and suitability for training were evaluated. The results showed that our phantoms are more realistic and easier to use than the traditional ones. Our proposed technology allows creating a low-cost (50 $/kg) alternative to the pricy commercial training phantoms available today.

Introduction

Minimally invasive (MI) methods of treatment, including interventional radiology, are becoming more preferable in medicine because of lower risk of infection, reduced hospital stay, reduced patient stress and discomfort and lower total treatment cost [1]. In the field of radiology, this approach is increasingly common both in diagnostics and treatment. Rapidly advancing methods and equipment for the procedures used in interventional radiology cause frequent introduction of new products requiring fast adapting from doctors. Practising is important for maintaining and improving the skills [2], it yields in faster performance, reduces the number of missed lesions [2], significantly reduces operating room time [3] and improves the success rate in biopsy [4]. The studies show that even experienced radiologists start over with the learning curve when changes are made in equipment [4] whereas practising on phantoms has been shown useful in training and skill development [5]. For ethical, clinical quality and safety reasons the doctors need to become familiar with the properties of new equipment or procedures prior to applying them on patients. This is why the phantoms that realistically mimic anatomy and tissue properties are useful not only for the residents in their training, but also for practising radiologists in their every-day work.

For interventional procedures (e.g. biopsy, drainage, etc.) in radiology and other fields the high pricing of the abdominal training phantoms and biopsy phantom organs may result in dismissal of practising a new procedure or lack of training on phantoms that realistically represents anatomy, mechanical and ultrasound properties of human tissue. There are several providers of models and practicing equipment for educational purposes in medicine; however the variety of training phantoms is small. A (potentially not exhaustive) list of abdominal phantoms and separate kidney phantoms suitable for training of minimally invasive renal interventions using ultrasound (US) imaging is given in Table 1. These relatively expensive phantoms have many imperfections in anatomy, can stand a limited amount of work cycles and rarely represent pathologies.

There are many phantom materials available for mimicking soft human tissues. They can be classified as hydrogels, organogels and flexible elastomer materials. Gelatin gels are simple to manufacture and have easily controllable US and mechanical properties [6], [7]. More non-linear and wider range of elasticity can be achieved by combining gelatin with oil resulting oil-in-gelatin dispersions [8]. The Young's modulus of the gelatin gels can be varied from 2.5 kPa to 500 kPa [7], their US broadband attenuations are 0.2–1.5 dB/(cm MHz) and the US propagation speed of 1550–1650 m/s has been reported [6]. Similarly to human tissue, gelatin has linear attenuation increase with respect to the ultrasound frequency [6]. The drawbacks of the gelatin gels include susceptibility to bacteria (can be overcome with additives, e.g. thimerosal, p-methyl benzoic acid, p-propyl benzoic acid, etc.) and the long settling time of the mechanical properties (up to 100 days from manufacturing) [7]. Agar gels are similar to gelatin both in terms of properties and the manufacturing process, but offer higher non-linearity of elasticity and have a naturally higher melting temperature [7], [9]. The US propagation speed is close to 1540 m/s and the broadband US attenuation of 0.1–0.7 dB/(cm MHz) has been reported [9].

Chemical gels have both advantages and disadvantages in comparison with the described physical gels (agar, gelatin). The elasticity and echographic properties of the polyacrylamide gels are finely adjustable, very stable [10] and these materials are not a suitable environment for bacteria because of several very toxic ingredients (acrylamide, etc.). The manufacturing process and properties are well described [10], the naturally low US broadband attenuation can be risen with additives. Polyurethane tissue substitutes are also stable and permit achieving a very low Young's modulus. A recently reported US compatible swollen segmented polyurethane gel (S-SPUG) [11] exhibits good long-term stability and appears to also have desired mechanical properties. However, as it is typical for polyurethane gels, very little details are reported on the manufacturing process. Other phantom materials include (1) oil gels, e.g. propylene glycol with gelatinizer that reportedly have stable properties; (2) open cell foams that can be filled with various mediums such as gelatin [12]; (3) PVAs (polyvinyl alcohol) that require long freeze–thaw cycles to influence the cross-linking; however they are economically affordable and need only few ingredients [13]; (4) silicon-based rubbers that have unsuitable mechanical characteristics, too low US propagation speed and too high attenuation [14]; etc. More thorough overview of the phantom material technologies can be found in Ref. [15].

This work describes material manufacturing and calibration processes using gelatin gels and development of renal biopsy phantoms with (1) realistic, potentially patient-specific anatomy; (2) realistic mechanical, ultrasound and computed tomography (CT) properties; (3) pathologies (cysts or tumours) with desired shapes, sizes and locations; and (4) affordable price. We test our phantoms in renal biopsy training where the radiology residents evaluate the results of our work after a hands-on workshop.