Background: Evaluation of the accuracy and performance of sonography units needs tissue-mimicking phantoms. These phantoms play an important role by simulating soft tissues, obviating the need to experiment on humans or animals.

Objectives: To present a simple sonographic phantom for quality control and training purposes.

Materials and Methods: The presented phantom consists of a two-part Plexiglas box. The larger part is filled with a mixture of ethanol (9.5 ± 0.25%) in distilled water and a solution of sodium nitrite (0.1 M) to prevent rusting. The second part is filled with a combination of 3.85% by wt. % agar, and 50 g/L of powdered graphite as the background material. In this study, chrome-plated electric guitar strings, 0.52 mm in diameter, were used. Several objects were considered as tissue-equivalent material, and their images were obtained at different times. Criteria for the selection of suitable objects comprised similarity between the obtained image and the corresponding tissues in the human body, minimal shrinkage, and change in brightness level at different times. In addition to quantitative analysis obtained from image processing, a blind qualitative study was done by a radiologist.

Results: Both results of quantitative analysis using MATLAB software and independent qualitative analysis showed that the commercial rubber and agar were appropriate as solid and cystic objects, respectively. Moreover, quantitative analysis done with MATLAB on images obtained from the phantom showed that the commercial rubber and agar had a 5% and 2% change in image pixel intensity (brightness) after 2 months, respectively.

Conclusions: The presented phantom not only has lower cost and complexity, which make it suitable for educational centers, but also is capable of providing good images of cystic and solid objects for quality control and training purposes. Furthermore, it confers reliable stability for at least 2 months, as was assessed in the present study.

Diagnostic use of ultrasound waves became increasingly apparent in the 1950s and in the 1960s (1). Application of ultrasound waves has been increased dramatically in the past 30 years owing to their effectiveness and safe employment (2, 3) as well as their importance in the noninvasive imaging of the human body, especially in fetal, cardiac, and abdominal studies (4). Furthermore, developments in transducer technology and real-time image processing techniques mean that ultrasound waves now account for 25% of all imaging procedures (5). In this imaging modality, routine quality control and quality assurance are necessary to ensure that scanners run under required high quality. In addition, recent advances have raised concern amongs the medical community about the safety of diagnostic ultrasound applications (3). Evaluation of the accuracy and performance of ultrasound systems usually needs tissue-mimicking (TM) phantoms (6). These materials should mimic the acoustic properties of the tissue (with regard to the speed of sound, average attenuation, etc.) and approximate the sonographic appearance of the tissue (7).

Gelatin-based phantoms using graphite as the attenuator have been developed recently. Presence of alcohol varies the longitudinal sound speed, and the concentration of graphite modulates the coefficient of attenuation (8). When using TM materials, one should keep in mind three key characteristics of the normal tissue: speed of sound, attenuation, and backscattering coefficients (9). The most common TM materials used in available phantoms are gelatin, evaporated milk, agar, urethane rubber, and Zerdine^®.The manufacturers of these materials have reported sound speed of 1540 m/s (with the exception of urethane-rubber, which has a sound speed of 1460 m/s) (10) and attenuation coefficients of either 0.5 or 0.7 dB/cm/MHz. Non-flat TM materials can be further classified based on the type of their background material as gelatin-and graphite-based, agar-and graphite-based, and evaporated milk-based (11).

Some studies have recommended other TM materials such as agar with suspended graphite, polyurethane foam, and magnesium silicate gels (7) or TM materials such as agar (polysaccharide), oil gel, polyvinyl alcohol gel (PVA), and polyacrylamide gel (PAA) (6). Unfortunately, the cost of phantoms is a major limiting problem for imaging centers, which is why the use of phantoms is sometimes construed as extra unnecessary work. The aim of this study was to present a new, simple, and low-cost TM sonographic phantom which might be used for training and quality control purposes.

3.1. Phantom Design

The phantom consists of a cubic box (23 × 10.5 × 19 cm) made of Plexiglas. It is divided into two parts with internal Plexiglas frames for quality control and training purposes. The phantom has a sound speed of 2657 m/s, attenuation coefficient of 5.3 dB/cm/MHz, and density of 1180 kg/cm³. Figure 1 depicts the box and its frames.

3.4. Quality Control Test Parameters

For quality control testing, a standard set of parameters based on Gammex-RMI phantom (model 403GS TM) and CIRS phantom (model 40 TM) was considered (12). Table 1 shows the characteristics of these phantoms, Table 2 illustrates the characteristics of the solid and cystic objects, Figure 3 depicts a schema of the wire scaffold in the phantom, and Figure 3b demonstrates a sonographic view of it.

Table 1. Phantom Characteristics Based on Gammex-RMI (Model 403GS TM) and CIRS (Model 40 TM) Phantoms

Table 2. Size and Position of the Solid and Cystic Objects in the Phantom

Figure 3. A: Upper View of the Phantom. B: Schematic Side View and C: Sonographic View of the Constructed Phantom

4.1. Velocity Measurement in Alcohol and Water Solution

The measured distance in the image using MATLAB software was equal to 0.995 cm. Consequently, the relative error in the distance and velocity measurement was about 0.5%.

4.2. Quantitative Evaluation of Objects

According to the results of image processing, commercial rubber underwent less change than did the other objects over time. Thus, in this project, commercial rubber was employed as the solid object. Agar exhibited a similar behavior and was, therefore, used as the cystic material (Tables 3 and 4).and

Table 3. Comparison of the Different Solid Objects Tested in the Construction of the Phantoma

Table 4. Comparison of the Different Cystic Objects Tested in the Construction of the Phantom

Quality control has a crucial role in all imaging and therapeutic centers and should, thus, be run periodically. Ultrasound phantoms are generally of two types: test objects and TM phantoms. The main purpose of TM phantoms is to approximate the sonographic appearance of tissues such as cysts and solid masses, which mimic the acoustic properties of the tissue (with regard to the speed of sound, average attenuation, etc.) (7). Phantoms have been used for the characterization and calibration of ultrasound imaging systems by simulating soft tissues without the need to experiment on humans or animals (14, 15). The optimal way to estimate the resolution capability of ultrasound scanners is to estimate axial and lateral resolutions individually using fibers or wires which are perpendicular to the beam axis (16).

Mixture of ethanol and water is used commonly as a propagation medium in test objects for the calibration of ultrasonic imaging systems. It is widely accepted that a mixture of 9.6% ethanol by volume in distilled water yields a sound speed of 1540 m/s at 20-21°C (13). Quantitative analysis using MATLAB has shown that the relative error in distance in the phantom is about 0.5% and the corresponding error in the speed of sound is about 7.7 m/s (13). Soft tissues generally attenuate the ultrasonic beam in such a way that the attenuation coefficient is nearly proportional to frequency and the speed of propagation is about 1540 m/s. The recommended attenuation coefficient slopes for use in phantom materials range from 0.3-0.7 dB/cm/MHz (17). The attenuation coefficient for the TM material used in the phantom in the present study was 0.5 dB/cm/MHz, which is an accepted value for such a material.

Compared to multipurpose phantoms such as Zerdine® TM from CIRS Inc., condensed-milk-based gel from Gammex RMI, and urethane-rubber-based from ATS Labs, our phantom has several advantages in that not only is it easily made but also it is less expensive, less fragile, and contains fewer hazardous chemicals. It is deserving of note that we assessed the material used in the phantom design after 2 months in order to find out the stability of the phantom material with time at working room temperature. Our findings showed that after 2 months, the phantom had not undergone change and was, thus, still reliable.

Needless to say, educational centers require accessories for the training of radiology residents (5, 7, 15). The phantom presented in this study is less costly and complex than are its counterparts, which makes its use feasible for educational centers. Moreover, our phantom is capable of conferring good images of cystic and solid objects for training and quality control purposes and its metallic wire scaffold affords distance calibration as well as dead zone and resolution measurement in both axial and lateral directions.

The authors thank kindly collaboration of Semnan University of Medical Sciences and Rajaie Cardiovascular Medical & Research Center.