Phantom study of a fiber optic force sensor design for biopsy needles under MRI.

Biopsy needles with embedded force sensors can eliminate the needle deflection and the needle targeting failure risks during MRI guided biopsy procedures. Fabry-Pérot interferometry (FPI) based sensors are small, compact and immune to electromagnetic and RF interferences, and therefore they are suitable for needle guidance under MRI. In this work, an FPI based fiber optic force sensor design and its integration to an 18-gauge MRI compatible biopsy needle are presented. The custom designed FPI sensor provides a force measurement range up to 13 N with a resolution of 0.1 N through benchtop experiments. The MRI compatibility of the sensor was evaluated using a commercially available prostate phantom under MRI.


Introduction
Magnetic Resonance Imaging (MRI) has a great potential to replace the existing imaging modalities during interventional procedures such as biopsies thanks to its superior soft tissue contrast and ionizing radiation-free nature. Accurate measurement of the force acting on a biopsy needle tip during MRI-guided biopsy procedures may provide important feedback that can increase the safety and accuracy of these procedures [1][2][3][4]. A real-time force measurement capability from the needle distal tip during the biopsy operation can allow physicians to appreciate small variations in the mechanical properties of the tissues along the needle insertion trajectory. This valuable information can be used to improve needle targeting and analyze the tissue-needle interaction. In conventional biopsy procedures, the accuracy and safety of the operation depend highly on the operator's experience. One of the major problems in a biopsy operation is the needle deflection during insertion. The needle deflection can be detected immediately through sudden fluctuations in continuous force measurement if a biopsy needle has an embedded force sensor. It is not possible to use conventional electrical force sensors for MRI guided interventions, because of potential electromagnetic interferences during imaging. Fiber optic force sensors can be used under MRI without causing any danger or any disruption on the MR image. Applied axial force measurement during needle guidance can be performed by Fabry-Pérot Interferometry (FPI) based fiber optic force sensors which can be integrated to the biopsy needle tip. FPI based fiber optic force sensors work based on light interference formed by superimposition of light beams reflected from two semi reflective mirror surfaces that generally form an air cavity between them (Fig. 1

Voltage (V)
Measuring the applied axial force with a sufficient resolution using a fiber optic sensor requires a sufficient signal to noise ratio (SNR). The alignment and the fixation of optical fibers inside the borosilicate glass capillary are important for this purpose. FPI force sensor components were assembled on an optical table, which provides vibration isolation, using a three-stage manual micro-manipulator and a piezo-electrically driven nano-manipulator stage under an optical microscope. Alignment and fixation of the fibers were completed at the local minima when a sufficiently long linear waveform was acquired. The voltage response based on the intensity of the reflected light from the FPI sensor with respect to the change in cavity length is shown in Fig. 6. Custom designed FPI force sensor was integrated into an 18-gauge nitinol biopsy needle. The measurable force range and the sensitivity of the force sensor were evaluated by using a calibrated tension-compression test machine (LF-Plus, Lloyd Instruments Ltd.). The FPI force sensor integrated into the nitinol biopsy needle was placed on the compression testing arm as it is opposed to only axial forces. The measured voltage signal generated by the response of the FPI force sensor against the applied force at the needle tip was recorded simultaneously with the force measurements by the tension-compression test machine. Sensor limitations such as force measurement range and resolution were determined. Final fiber optic force sensor performance was tested by inserting the needle into a beef tissue through benchtop experiments initially. Then, the fiber optic force sensor embedded biopsy needle was tested in a prostate phantom under real-time MRI guidance (3T Magnetom Trio, Siemens GmbH) (Fig.  7).  ws the perform ion. The tempe oduced to the s D temperature s e laser source the laser diod accurate force m e system and im d trajectory. It c ncy of the res m operated w and the target the ambient tem setup of continuo te phantom under mance of both erature control system at time systems respec or can cause de. Therefore, measurements mproves the pr can be seen th sponse against with the MPC temperature ca mperature as se us force feedback r MRI (left) and i PID and MPC l systems were t2. Figure 8(a ctively. Tempe shifts in band the temperatu . The MPC app redicted outpu at MPC outper t external effe approach has an be maintain een in Fig. 8(c) k measurement dur into a beef tissue C temperature e initiated at tim a) and Fig. 8(b) erature variatio dwidth and w ure of the laser proach uses a ut in every nex rforms PID in ects on the sy a fast respon ned for a long ).
ring biopsy needle e during benchtop control system me t1, and an e ) show the resp ons can affect th avelength of t r diode should model to pred xt step for getti terms of respo ystem. The tem nse to reach t g time even if e p ms during excessive ponses of he output the beam d be kept dict future ing closer onse time mperature the target there are Fig. 8. (a). The performance of the MPC temperature controller (b). The performance of the PID temperature controller (c). Overall stability of MPC temperature controller. Figure 9 shows the commercial force sensor's response (red) and custom design FPI force sensor's response (blue), against applied increasing axial force with time. The response of the FPI force sensor is consistent with the applied axial force. The measurable force range of the fabricated sensor was determined by the first peak point of the voltage response hits, while the applied axial force is still increasing. The sensor has a force measurement range of 0-13 N. Also, the force sensor provides a quick response to any sudden change in the extent of the applied force. The commercial force sensor response and the voltage response of the custom design FPI force sensor to applied axial force were shown in Fig. 10. Close up plots between 13th and 17th seconds were shown below. It can be seen that FPI force sensor can successfully detect the changes in the applied force. Even a small increment of 0.1 N can be detected, which is enough to differentiate different types of tissues during needle insertion. The response of the FPI based force sensor and the commercial force sensor were recorded simultaneously during needle insertion into a beef tissue and a layer of stiff foam (Fig. 11). It can be seen that the response of the two sensors are consistent with each other and FPI based force sensor embedded biopsy needles are capable of detecting penetration and stiffness differentiation. The fiber optic sensor performance was also tested by performing a biopsy needle insertion experiment into a commercially available prostate phantom under MRI (Fig. 12). It was confirmed that the biopsy needle with an integrated FPI based force sensor is safe to use under MRI and also the fiber optic sensor could detect the transitions between different structures within the phantom. Also, FPI based force sensor embedded needle did not cause any image degradation and it was clearly visible under MRI.

Discussion
Performing biopsy procedures under MRI is a promising alternative method thanks to the superior soft tissue contrast of MRI. Real time axial force feedback at the biopsy needle tip during insertion increases the accuracy of the lesion targeting and also enhances the patient safety by determining possible needle deflections during the biopsy procedure.
The force measurement range and the resolution of custom designed FPI sensor is sufficient to perform safe biopsy operations, compared to the needle and soft tissue interaction studies in the literature [20][21][22]. The sensor resolution can be improved by reducing SNR losses due to misalignments of optical fibers. SNR can also be improved by increasing the reflectance of cleaved optical fiber surfaces through Aluminum (Al) coating by sputtering technique [23,24]. A gradient noise on the sensor response caused by the MRI scanner was detected during MR imaging. This noise can be eliminated by digital filtering methods.
Different tissue types have different mechanical characteristics such as stiffness and elasticity. Therefore, the different tissue types can be detected through their response to the FPI force sensor during needle insertion. A biopsy needle with an FPI based force sensor can be a promising tool for cancer diagnosis by differentiating tumorous tissue from healthy tissue, and even recognizing the difference between benign and malignant tumorous tissues without collecting tissue samples during a biopsy operation.

Conclusion
In this work, a low profile and MRI compatible FPI based force sensor was fabricated and integrated into a biopsy needle. The proposed novel sensor fabrication method allows to determine the force measurement range and the sensor sensitivity depending on the clinical application needs, by laser micromachining the micro-holes on the borosilicate glass capillary at desired locations. The ambient temperature of the laser source also could be kept constant successfully using a Model Predictive Control based temperature controller during benchtop tests. MRI compatibility and overall performance of the custom designed biopsy needle was tested through benchtop experiments and using a prostate biopsy phantom under MRI. The test results showed that the FPI based force sensor has a force measurement range of 0-13 N with 0.1 N resolution.

Disclosures
The authors declare that there are no conflicts of interest related to this article.
Sensor response during needle insertion Gradient noise during MR imaging