PhotonFirst integrated photonics sensing solutions are also used in a growing number of medical applications. Especially when miniaturization, electricity avoidance or RF sensitivity are relevant.
Below a number of applications is described:
- Force transducer for cardiac interventions
- Haptic feedback with fiber optic sensing
- RF/MRI compatible in-vivo temperature sensing
- Multi-point, multi-parameter sensing
- Shape sensing for minimal invasive instruments
- Shape sensing with a multicore optical fiber
Force transducer for cardiac interventions
The difficult relation between electricity and the human body introduces a lot of complexities into designing high-tech medical instruments. Especially instrument used in cardiac inventions. Fiber optic sensing, however, uses light for measurement and is therefore highly adequate for instrumentation for use in cardiac interventions. Together with medical partners, Technobis developed a force transducer to be used during cardiac ablations. The contact between the catheter and the cardiac tissue introduces strain, stress and displacement in the fiber optic sensors and can be calculated into a force. Combines with a three axis design, the direction of the force can also be determined. Recordings of a controlled force on the fiber tip resulted in an displacement accuracy of 1 nanometers, equivalent to 10 milli Newtons.
Haptic feedback with fiber optic sensing
Many surgical interventions are nowadays performed via laparoscopic surgery. A hollow tube is inserted through the skin of a patient. Through this hollow tube, surgical tools are inserted toward the affected tissue. This technique results in a quick recovery and low scar tissue for the patient. However, the surgeon is not in direct contact with the tissue and doesn’t know how tight to grip and thus creates unintentional damage. Together with a customer, a surgical tool is developed with Fiber Bragg Grating imbedded in the tip. These FBGs respond to the stress and strain in the pincher that are translated to haptic feedback on the handle of the tool. In-vivo experiments with this tool resulted ‘feeling the heartbeat in the blood vessel through the handle bar’.
RF/MRI compatible in-vivo temperature sensing
Diagnostic instruments are the tools MD’s use to acquire data about the patient. An important parameters is the body temperature, both externally on the skin and in-vivo. Especially during Hyperthermia and surgical interventions, the in-vivo body temperature is a parameter that needs continuous monitoring. Disadvantage of electrical temperature sensors is the fundamental use of electric current, and the sensitivity to RF-sources (e.g. MRI). Fiber Bragg Gratings (FBG), are insensitive to RF sources and are therefore ideally to be used during in-vivo temperature monitoring in RF environment. The PhotonFirst interrogation system uses the principle of reflected wavelength from an FBG that is tracked over time using our spectrometer. Combined with an internal reference FBG, the recorded wavelength of the FBGs can be converted into an absolute temperature reading. The interrogator can integrate up to 7 optical fibers with each 8 FBGs (total 54 sensors). The FBGs are integrated in the optical fiber with a typical diameter of 0.25 [mm] allowing for easy integration into a catheter of needle. The system can achieve a short-term v of 0.1°C (1-2 hr) and a longterm accuracy of 0.5°C (5-10 hr), with a resolution of 0.03°C.
Multi-point, multi-parameter sensing
Annually, 1.3 million people undergo an angioplasty treatment where a narrowed blood vessel (stenosis) in the coronary arteries is treated. Conventionally this diagnosis is made with X-ray imaging of the blood vessel in combination with a contrast fluid put into the arteries. This contrast fluid is highly visible on the x-ray images and indicates the level of blood flow reduction. Treatment is based on visual inspection by the cardiologist. Although trained in this type of procedure, unnecessary treatments are common. A method to quantify these stenosis (e.g. the blood flow reduction) lacks. Fiber optic sensing has high potential in medical applications. An increasing number of parameters can be monitored with optical fibers. With the use of an FBG in a Microstructure Optical Fiber (MOF) blood pressure sensing becomes possible. The reflection of the FBG in the MOF contains two center wavelengths. Whereas temperature and strain causes an identical shift of these two peak, changes in pressure causes these peak to shift with different sensitivity; resulting in a change in spectral separation. This parameter, the peak difference, can be used for blood pressure sensing. In experiments at PhotonFirst the pressure was changed from 0 to 1.4 bar resulting in a spectral separation between the two peaks of 1.6 pm per bar. The PhotonFirst interrogator platform allows this technology to be integrated in a guidewire for use in cardiac stenosis treatment for accuracies in the order of 1 mbar / 0.75 mmHg.
Shape sensing for minimal invasive instruments
A biopsy needle is used to take a sample of tissue from a patient. It is important to take a sample of the correct piece of tissue. In order to help determining the position of the tip of the needle FBG sensors for shape sensing can be integrated. Three separate fibers with FBG sensors are embedded in a groove on the side of the needle. The grooves are made in a 120° configuration with respect to each other. When the needle is bend, the FBG’s on the inside will be compressed while the FBG on the outside will be elongated. This strain causes a change in the reflected wavelength of the FBG sensor. With the use of the Frenet-Serret formulas and the measured strain values, the 3D shape of the needle can be reconstructed. Because of the symmetric 120° configuration of the sensors, this principle is automatically compensated for temperature.
Shape sensing with a multicore optical fiber
In this specific medial application, a multicore fiber is used as a shape sensor in a catheter. A multicore has the advantage of being small and the fiber itself is the complete shape sensor. The outer diameter of a multicore fiber is identical to that of a single fiber, which is only 125μm. A standard optical fiber has one core and since this core is located in the center it will not measure strain when a fiber is bend. In a multicore fiber typically 4 or 7 cores are present in a symmetric pattern around the center. Because of the different distances to the center, each core will experience a different strain when the fiber is bend. FBG sensor can be written in each of the cores. With the use of a PhotonFirst interrogator system with synchronized read-out, all FBG sensors in the multicore fiber are measured simultaneously. With the measured strain values and the use of the Frenet-Serret formulas, the 3D shape of the fiber can be reconstructed. New developments are ongoing in this field with twisted multicores, this opens new possibilities to include torsion measurements with the same fiber.
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