Medical Catheters and Plastics – Part IV
Continuing the series on the use of plastics in medical catheters, we will focus on a key element of catheter design; the radio opacity of catheters. Radio opacity can be defined as the property of a material that prevents electromagnetic radiation, particularly X-rays, from passing through it. This characteristic is also known as radio density.
The two primary factors influencing a material’s opacity to electromagnetism are its atomic weight and density. Catheters are typically made from materials having high electron density, which contrasts with the surrounding tissue material. This visibility of the catheter during medical procedures enables physicians to accurately guide and position the device. Materials that contribute to the radio opacity of catheters include titanium, tungsten, bismuth, and barium.
X-rays are a type of radiation characterized by high-energy waves with varying frequencies and wavelengths, forming part of the electromagnetic spectrum. Diagnostic X-rays are located at the lower end of this spectrum, with wavelengths ranging from 0.1 Angstrom to 1 Angstrom. X-rays are generated when fast-moving electrons from an X-ray source collide with an anode or target. The intensity of an X-ray beam is determined by the number of photons it contains and their energy, measured in kiloelectronvolts (keV). For medical imaging, X-rays typically have photon energies between 5-10 keV.
X-ray beams interact with various body parts in distinct ways, leading to variations in the attenuation of X-ray energy. These variations contribute to differing levels of contrast in the resulting images. Bones, which are rich in calcium, effectively absorb X-rays due to calcium's relatively high atomic number. This absorption decreases the number of X-rays that reach the detector in the areas obscured by bones, thereby rendering them prominently visible on the radiograph.
Fluoroscopy is a widely utilized X-ray imaging technique during cardiovascular procedures. This method allows physicians and radiation therapists to capture real-time moving images of a patient's internal structures using a fluoroscope. At its core, a fluoroscope comprises an X-ray source and a fluorescent screen, with the patient positioned between them. However, contemporary fluoroscopes integrate the screen with an X-ray image intensifier and a video camera, enabling the recording and display of images on a monitor.
The density of a material and its atomic number are crucial factors in determining its effectiveness as a radiopaque element. Barium sulfate, bismuth compounds, and tungsten metal are commonly utilized radiopaque fillers in catheter tubing. Barium sulfate (BaSO4) is oldest and widely employed radiopaque material, characterized by a specific gravity of 4.5. It is typically incorporated at loadings ranging from 20% to 40% by weight in conjunction with the base plastic. BaSO4 is naturally white but can be modified with colorants. Its primary advantage lies in its affordability; however, its density necessitates higher loadings compared to other denser fillers. Increased filler loading can adversely affect the properties of the base plastic, potentially leading to reduced tensile strength and diminished biological stability at elevated levels. Bismuth compounds, while significantly more expensive than BaSO4, possess nearly double its density, allowing for greater incorporation of the radiopaque agent without compromising the physical characteristics of the base plastic. This higher loading results in improved imaging clarity of the catheter during procedures. Tungsten metal powder, with an exceptionally high specific gravity of 19.5, enables even higher loadings, further enhancing image clarity. Nonetheless, the use of tungsten is limited due to its abrasive properties, which can damage compounding extruders. All these radiopaque materials are biocompatible and suitable for use in the medical field.
The quantity of the radiopaque agent is influenced by the specific application and the design of the device. For instance, a catheter positioned close to the skin's surface necessitates a significantly lower concentration of the radiopaque filler compared to one utilized in the coronary artery. Additionally, thin-walled catheters require a greater amount of the radiopaque filler than their thicker counterparts. Catheters featuring discrete radiopaque markers, such as strips, also demand a higher loading compared to those with a uniform filler distribution. In some cases, a combination of radiopaque fillers may be employed to address considerations related to cost, loading, physical characteristics, and image clarity.
The process of incorporating radiopaque fillers into the base plastic is delicate. Initially, appropriate levels must be determined based on the application, device design, and type of filler. Subsequently, it is crucial to exercise caution to avoid excessive shear that could compromise the properties of either the filler or the base plastic. For example, bismuth compounds are particularly sensitive to shear, while tungsten metal can cause damage to the compounding extruder due to its abrasive nature.
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Author
Mr. Ajay D Padsalgikar, Ph.D.
California, USA
Trainer at Polymerupdate Academy
