Dogangun Uzun1, Okan Ulgen1, and Ozgur Kocaturk1

1Institute of Biomedical Engineering, Bogazici University, Istanbul, Turkey


In this work, an MRI compatible Fabry-Pérot Interferometry (FPI) based force sensor is designed and fabricated using novel methods, and integrated into an 18-gauge nitinol biopsy needle. The resolution of the FPI based sensor is increased by coating optical fibers with magnesium and titanium thin film. The force sensing needle is tested by in vitro experiments using a prostate phantom and in vivo animal experiments under MRI.


Exceptional soft tissue contrast and radiation free environment of Magnetic Resonance Imaging 1-3, makes it an attractive imaging modality for image guided minimally invasive procedures such as biopsies. The success of a biopsy operation depends on both, imaging quality and physician’s skills and experience 4. Real time force measurements through needle tip during MRI guided biopsy operation could provide an important feedback to the physician about tissue and or tumor stiffness and boundaries. Also, potential needle deflections, mainly caused by different mechanical characteristics of different tissue types can be detected and avoided by this way. Needle tip force sensing under MRI requires a compact sensor for easy integration to the biopsy needle and immunity to electromagnetic and RF interference for ensuring MRI safety and good image quality. Fabry-Pérot Interferometry (FPI) based fiber optic force sensors are very suitable for this purpose 5. FPI based sensors work, using light intensity of an interference pattern formed by superimposition of two back reflected light beams from two semi reflective mirror surfaces (figure 1). Mirror surfaces are usually formed by two optical fibers with an air cavity between them that will create an optical path difference between two reflected light beams. Applied axial force to the sensor, changes the air cavity length which leads to a change in the light intensity and a correlated relation can be formed between the applied force and the measured light intensity in the linear range of the interference pattern.


Novel design of the FPI based force sensor probe is composed of a single-mode optical fiber, a multi-mode optical fiber and a borosilicate glass capillary tube for the encapsulation of optical fibers. Different profile optical fibers are fixed inside of the capillary tube applying UV cured adhesives from two micro-holes that are formed on the glass capillary by laser drilling(figure 2). The distance between two micro-holes defines the gauge length of the sensor probe which plays a role in determining the measurable force range. Using a larger diameter multi-mode fiber as the second reflective surface provides an easier alignment and minimizes the signal losses due to angle mismatches between two optical fibers.

Fiber surfaces reflects only 4% of the transmitted light 6. Increasing the reflectivity of FPI sensor probe’s mirror surfaces can lead to an increase in the signal intensity, SNR (signal to noise ratio) and hence, resolution of the sensor. For this purpose, multi-mode fibers are coated by physical vapor deposition technique with magnesium and titanium thin film separately.

Fabricated sensor probe is integrated into an MRI compatible 18 Gauge biopsy needle using medical grade UV cured adhesives. Bench-top experiments for accurate force measurements, penetration detection, tissue differentiation are performed with force sensing biopsy needles. Finally, in vitro experiments with a commercially available prostate phantom and in vivo animal experiments are performed to evaluate the overall needle and force sensor performance under MRI.


The response of three sensor probes containing a bare fiber, magnesium and titanium coated multi-mode fibers, to the cavity length change is given in figure 3. The highest reflected light intensity is obtained from titanium coated multi-mode fiber. It is shown that the linear range of the signal is increased more than three times by coating the multi-mode fiber with titanium.

Penetration detecting experiment under MRI using a commercially available prostate phantom is shown in figure 4.


Custom design force sensing biopsy needles can improve the accuracy and safety of biopsy operations by providing needle tip force feedback to the physicians that can be used for better needle targeting and detecting possible needle deflections. The custom fabricated sensors can be used for different diagnostic or therapeutic minimally invasive procedures under MRI that needs precise needle targeting such as epidural needle injection.

Malignant and benign tumorous tissues or healthy tissues shows different mechanical characteristics 7. Therefore, force sensing biopsy needle can be useful for cancer diagnosis without needing a biopsy operation, since it can differentiate tissues with different stiffness.


In this work, MRI compatible force sensing biopsy needles are designed using a novel fabrication method for the FPI based fiber optic sensor. It is shown that SNR and resolution of the sensor can be increased by coating the mirror surfaces by a more reflective material such as Ti or Mg. Biopsy needles with force feedback were capable of detecting force variations, penetration and tissue differentiation, during both in vitro and in vivo experiments under MRI.


This project is funded by TUBITAK (project no: 115E271).


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Fig.1. Working principle of Fabry-Pérot interferometry based force sensor

Fig.2. Design of FPI based fiber optic force sensor probe

Fig.3. Voltage response of three sensor probes with respect to cavity length change

Fig.4. Penetration detecting experiment using a prostate phantom under MRI

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)