In the advanced world of medical imaging equipment, ensuring operational accuracy and precise device calibration is of vital importance. Medical phantoms, serving as simulation beds for living tissues, play a pivotal role in guaranteeing this quality. They enable the development and validation of novel algorithms within a controlled environment. In this article, we will explore the structure, types, and evaluation methods of soft tissue mimicking phantoms, with a focus on ultrasound applications.
Technical Summary
Medical phantoms are artificial samples mimicking human tissues, fabricated for professional training, research, and the precise calibration of imaging devices such as Ultrasound, MRI, and CT scans. The primary advantage of using a phantom is providing a safe, reproducible, and controlled platform for experiments and the development of new imaging technologies. This allows researchers to evaluate their algorithms and equipment under near-realistic conditions without the need for human or animal subjects. Following fabrication, each phantom undergoes acoustic, mechanical, and longevity (durability) assessments to ensure desired physical properties are accurately simulated.
1. Definition and Importance of Medical Phantoms
Medical phantoms are engineered artificial specimens used as substitutes for living tissue in research, training, and medical imaging device calibration. The main objective is to examine the behavior of imaging devices under controlled and reproducible conditions. These phantoms must simulate key physical, chemical, and mechanical characteristics of body tissues; for instance, the speed of sound in tissue, density, or the Young’s Modulus of soft tissue.
Specialized Applications of Phantoms
The application of phantoms across various fields is extensive:
- Clinical Training: Simulating medical procedure steps such as biopsies or interventional ultrasound to enhance the skills of students and technicians.
- Research and Development (R&D): Developing and validating advanced image processing algorithms, particularly in quantitative elastography and harmonic ultrasound.
- Quality Assurance (QA): Calibrating ultrasound devices to ensure accuracy in distance measurement, depth, and image quality (resolution).
- Testing New Probes: Testing new ultrasound probes and Phased Array systems under controlled operational conditions.
Phantom Classification Based on Imaging Modality
Differences in target physical parameters have led to a diversity in the base materials of phantoms:
| Imaging Modality | Dominant Physical Parameter | Common Base Materials |
|---|---|---|
| CT (Computed Tomography) | X-ray Attenuation (CT-number) | Composite or resin materials, salts containing iodine or calcium |
| MRI (Magnetic Resonance Imaging) | Relaxation Times (T1, T2) | Aqueous gels enriched with paramagnetic ions (e.g., Gadolinium) |
| Ultrasound | Acoustic Properties (Speed of sound, Attenuation) & Mechanical Properties (Elasticity) | Gels, Hydrogels, Silicones, Cryogels |
2. Dedicated Ultrasound Phantoms and Constituent Materials
Ultrasound phantoms primarily focus on simulating the acoustic properties of soft tissue (approximately 1540 m/s) and its mechanical properties. They are typically manufactured in three main families:
A. Agar–Gelatin Phantoms
These are the most common and simplest type, featuring low production costs and high manufacturing speed. Due to the easy adjustability of concentration, they are suitable for producing multi-layered phantoms and inclusions with varying stiffness to simulate mechanical contrast in elastography.
- Gel Structure: Agar and Gelatin.
- Scatterers: Graphite, cellulose, or corn starch to create tissue-like scattering.
- Speed of Sound Adjustment: Glycerol or Propylene Glycol (adjusting speed of sound to ≈ 1540 m/s).
- Preservative: Sodium Benzoate to prevent microbial spoilage and extend phantom life.
- Key Disadvantages: Short shelf life (susceptible to drying and spoilage) and high sensitivity to temperature changes.
B. PVA (Polyvinyl Alcohol Cryogel) Phantoms
Polyvinyl Alcohol phantoms are highly suitable for precise mechanical studies and quantitative elastography. Their mechanical properties are much closer to living tissue, and they possess greater stability compared to simple hydrogels. The stiffness (Young’s Modulus) of these phantoms is precisely controlled by the PVA concentration and the number of Freeze–Thaw cycles applied during the manufacturing process.
C. Silicone-Based Phantoms
This group constitutes the most stable phantoms. They are temperature-resistant and have a long lifespan, making them ideal for educational applications and routine device testing. Mechanical properties are adjusted by adding silicone oil or soft fillers. Acoustic scattering is typically achieved by adding fine microspheres to ensure signal uniformity.
3. Testing and Quality Assurance Evaluation
To ensure quality and resemblance to the target tissue, these constructs undergo a rigorous quality assurance program:
- Acoustic Testing: Examining the speed of sound in the gel (should be close to 1540 m/s), attenuation coefficient, reflection intensity, and ultrasound signal homogeneity.
- Mechanical (Elastic) Testing: Measuring Young’s Modulus and elastic or viscoelastic properties using compression or shear tests to simulate tissue stiffness and elasticity.
- Long-term Monitoring (Durability): Implementing a periodic monitoring program to check for phantom changes over different time intervals (such as changes in speed of sound, elastic modulus, and visual inspection) to estimate useful life and stability.
4. Proper Maintenance and Disinfection
Proper maintenance is a determining factor in the phantom’s lifespan:
- Gelatin Phantoms: Must be stored at low temperatures (Refrigerator 4-8 °C) in a clean environment. Adding sodium benzoate aids in stability.
- PVA and Silicone Phantoms: Have higher resistance; storage in a clean, dark place away from direct light is sufficient.
- Disinfection: Mild solutions (such as 70% Ethanol) can be used for disinfection, provided the phantom structure (especially in the case of hydrogels) is not damaged.
By adhering to these points, the produced phantoms can be used as a valid and standard tool in medical research and education.