Development of Aspiration-Assisted End-Cut Coaxial Biopsy Needles

For evaluating suspicious breast changes, open surgical biopsy is considered to be the standard initial diagnostic method. However, patients risk severe complications during the diagnostic processes. Compared to surgical biopsy, needle biopsy procedures are considered less invasive. Fine-needle aspiration biopsy (FNAB) and core needle biopsy (CNB) are two of the most common needle biopsy procedures [1]. FNAB is performed using a thin (25–20 gauge) needle attached to a syringe to aspirate the tissue from the abnormal area. CNB uses a relatively large (20–14 gauge) coaxial or triaxial needle that is able to preserve a sufficient amount of the tissue structure for histological analysis. The small size of the FNAB needle allows that biopsy procedure to be less invasive than CNB. However, FNAB is suitable for only cytology tests and requires experienced cytologists to properly interpret the results [2–6]. Moreover, samples provided by FNAB are often insufficient, decreasing the sensitivity and specificity of the diagnosis and leading to the need for further diagnostic procedures, such as CNB or surgical biopsy, to obtain more reliable results [7–14].

Two types of needles, side-notch needles and end-cut needles, are commonly used for CNB. Unlike side-notch needles, end-cut needles are able to collect the full core of the specimen [15,16]. However, regardless of the advantages of the end-cut needles, the high rate of zero biopsy (procedures in which no tissue is collected) has been an issue [16]. A report in 1995 stated that the zero biopsy rate for breast biopsies was reported to be as high as 73% [17]. By extending the needle stroke length and using large-diameter (14 gauge) needles, the zero biopsy rate can be reduced to 8% [17]. However, the use of large-size needles increases the risk of complications.

Barnett et al. proposed a needle with a trocar (inner stylette) retracting system, called a vacuum-assisted end-cut biopsy needle [18]. This system creates a vacuum inside an 18-gauge needle by simultaneously moving the external needle forward into the target tissue (at an average speed of either 290 or 535 mm/s) for a desired length and the trocar backward for 45 mm long at high speed (the average speed was about 1290 mm/s). Maximum lengths of the sample obtained (porcine kidney) were reported to be 15.7 mm for a vacuum-assisted needle and 11.3 mm for a nonvacuum-assisted needle, both inserted 60 mm. The vacuum helps suck the tissue into the space between the external needle and trocar. However, the maximum efficiency (biopsy length over needle insertion length) was calculated as 26%, and the shape of the samples sucked into a large vacuum space was hard to control.

To extract a large biopsy sample with minimal damage using an end-cut biopsy needle, a biopsy system should minimize the needle–tissue interaction force, which can cause sample deformation (e.g., stretching and compressing) during the biopsy. The primary mechanism of sample extraction should be smooth cutting not sucking. There should not be extra vacuum space between the trocar and extracted sample in the needle, which can lead to sample deformation. To satisfy these criteria, a new aspiration-assisted end-cut coaxial biopsy needle system was proposed [19]. In the developed system, the external needle is inserted into a sample at a constant speed. The external needle cuts the sample and holds it by means of aspiration created by the moving external needle and stationary inner stylette. Furthermore, the design does not create any extra space in the needle.

This paper describes the principle of the new biopsy system and the equipment developed to realize the principle. The clearance between the coaxial needle components influences the aspiration. The effects of the needle insertion (or cutting) speed on the needle–tissue interaction have been extensively studied by many research groups; however, those studies have been inconclusive due to tissue characteristics [20,21]. To fill this void, this paper describes the effects of the clearance between the coaxial needle components (external needle and internal stylette) and the needle-insertion speed on the needle–tissue interaction force and biopsy performance. The biopsy performance is evaluated using tissue phantom (gelatin) and chicken breast. Analyzing the interaction force components provides insight into the distribution of the applied forces during the biopsy process and an understanding of the tissue-extraction mechanism.

Figure 1 shows the principle of an aspiration-assisted end-cut coaxial needle biopsy system. The biopsy procedure consists of four steps: insertion, cutting, holding, and extraction. As shown in Fig. 1(a), the coaxial needle assembly (hereafter simply called coaxial needle) is inserted to a target position in the suspicious area (lesion). After reaching the position (x = 15 in Fig. 1(a)), only the external needle is moved forward at a constant speed, cutting through the tissue (Fig. 1(b)). The internal stylette remains stationary. During the cutting process, a vacuum will be created in the syringe. This vacuum assists in holding the extracted sample inside the external needle. As soon as the external needle stops, a block is placed between the plunger holder and the syringe barrel to lock the plunger (Fig. 1(c)). The procedure is completed as the coaxial needle is retracted and removed, extracting the sample from the patient (Fig. 1(d)).

Figure 2 shows a photograph of the coaxial needle components. In this study, the external needle was made from 18-gauge (1.27 mm OD, 1.14 mm ID) 316 stainless steel capillary tubing. The capillary tube was cut into a 100-mm-long section, and one end of the tube was shaped with a 20 deg bevel two-edge geometry. The external needle was assembled with a 20-mL syringe barrel (Lure-Lok tip, BD). The inner stylette was made of a 304 stainless steel rod (127 mm long) with one of four different rod diameters (0.94, 0.99, 1.04, and 1.09 mm) to adjust the clearance between the inner stylette and the external needle. The coaxial needle clearance is shown in Table 1. One end of the stylette was tapered with a 20 deg bevel, and the stylette was fixed to the syringe plunger rod. The inner stylette was assembled with an external needle for each coaxial needle.

Figure 3 shows a photograph of the developed coaxial needle biopsy system. The external needle attached to the syringe barrel is secured to the syringe holder. The inner stylette is fixed to the plunger rod holder, which is attached to the blocker. A linear stage (NRT150, Thorlabs Inc.) with a stepper motor controller (APT, Thorlabs Inc.) is used to control the insertion speed and position of the coaxial needle with an accuracy of 2 μm in the horizontal direction. The stage travel limit is 150 mm, and the maximum speed is 10 mm/s. A biopsy testing sample is filled in the tissue tube (80 mL polypropylene tube), and the tube is fixed in the tube holder. The force acting on the coaxial needle during biopsy tests is detected by strain gages (KFG-2N-120-C1-11, KYOWA) attached to a cantilever beam and recorded by a dynamic data acquisition software (DCS-100A, KYOWA), as shown in Fig. 3.

Gelatin was used as a tissue phantom in this study. The gelatin powder (7 g) was sprinkled evenly in 50 mL of de-ionized water and left for 5 min. The gelatin solution and another 50 mL of de-ionized water were heated separately for 30 s using a microwave oven. Afterward, the heated solutions were mixed together, stirred continuously for 2 min, and then poured into cylindrical containers. These containers were cooled for 1 h at room temperature. Samples were then covered with laboratory film to minimize water evaporation, and they were refrigerated for 24 h. To prepare for testing, cured samples were removed from the refrigerator and kept at room temperature for 1 h. The measured density of the gelatin samples was 1.12 g/cm³, and the elastic modulus was measured as 19.8 kPa.

The experimental conditions are shown in Table 2. The position at which the inner stylette tip barely touches the gelatin in the tissue tube was considered to be the origin (x = 0 mm). The coaxial needle was inserted into the gelatin at a set insertion speed (between 1 mm/s and 10 mm/s). At x = 15 mm, the internal stylette stopped and only the external needle moved forward until x = 40 mm. Between x = 15 mm and x = 40 mm, the sample was cut by and stored inside the external needle. The cylindrical part of the external needle for storing the sample was to be 21 mm long. After stopping at x = 40 mm, a block was inserted between the syringe barrel and plunger, the coaxial needle assembly (external needle and inner stylette) was retracted (moved to the original position of x = 0 mm) at the speed equal to the insertion speed. The extracted sample was pushed out from the external needle using the inner stylette and placed on a petri dish for further evaluation (weight and length measurement). After each needle insertion, the tissue tube was rotated manually to a new insertion position. The coaxial needle was cleaned using de-ionized water. A new tissue tube with gelatin, new coaxial needle, and a new syringe were prepared for every set of experiments. Each condition was repeated five times.

Under the conditions, no zero biopsies were observed. Figure 4 shows photographs of example gelatin biopsy samples. The sample length L (Fig. 4(a)) was measured by a digital caliper (10 μm resolution). It was observed that the head structure (Fig. 4(b)) was formed occasionally when using coaxial needles with larger needle clearances (using 0.94- and 0.99-mm-diameter inner stylettes). The head structure was made when an end of the gelatin sample was trapped in the relatively large needle clearance. The head structure did not form when using the needles with smaller clearances (1.04- and 1.09-mm-diameter inner stylettes). Since the collected sample should preserve the original tissue structure so that histological analysis can be performed, fragments and head structures were not included in the measurement of the sample length, but they were included in the sample weight in this study. The sample weight was measured with an analytical balance (AB265-S/FACT, Mettler Toledo; 10 μg resolution) immediately after extraction to avoid water evaporation.

Figure 5 shows changes in sample length and weight with inner stylette diameter (and, therefore, coaxial needle clearance). Both the sample length and weight decrease with decreasing coaxial needle clearance and with increasing insertion speed. The head structures were excluded from the measured length in the cases using 0.94 mm and 0.99 mm inner stylettes; this resulted in shorter sample lengths (17–20 mm). In the cases with 1.04- and 1.09-mm-diameter stylettes, the average samples were 21–23 mm long and almost filled external needles.

As shown by the weight measurements in Fig. 5(b), the average weights of the samples extracted with 1.04- and 1.09-mm-diameter stylettes were between 21 mg and 24 mg. Using the density of gelatin found earlier, the lengths of a gelatin cylinder 1.14 mm in diameter (the inner diameter of the external needle) are mathematically calculated as 18 mm for a 21-mg mass and 21 mm for a 24-mg mass. These calculated lengths match the measured sample lengths, which demonstrates that the samples were extracted in good form.

The sample weight decreased with reducing stylette diameter (i.e., increasing needle clearance). This was the result of decreasing aspiration effects. Moreover, the sample weight tended to decrease with increasing needle insertion speed: slightly smaller samples were collected at higher insertion speeds. When the external needle is inserted into the sample, the needle deforms, drags, and cuts the sample. When the external needle was stopped at x = 40 mm, the sample relaxed instantaneously. When the coaxial needle started moving backward, the moving part of the gelatin was stretched and eventually disconnected from the stationary gelatin at the near the needle tip. Under the conditions, the needle insertion and retraction speeds were the same. At higher needle retraction speeds, the sample was disconnected at higher strain rates. This phenomenon might have influenced the extracted sample length.

To confirm the aspiration assistance in the biopsy performance of the developed coaxial needle, a biopsy performance test was run using a syringe barrel with ten holes (ø1.5 mm), as shown in Fig. 6. The holes in the syringe barrel allow air to pass through while the external needle moves forward. This prevents a vacuum from being created inside the syringe barrel (Fig. 1(b)). The results were compared with the cases using the aspiration-assisted end-cut coaxial needles (Figs. 4 and 5). The experimental conditions used were the same as shown in Table 2. Each condition was repeated five times.

Figure 7 shows photographs of representative tissue-phantom samples collected using the syringe with holes. Fragments, head structures, and other geometrical imperfections were observed in most of collected samples (Fig. 7(a)). Moreover, the diameters of the collected samples varied along their lengths. Some samples were disconnected. For the measurement of the collected samples in the external needle, the sample pieces were all collected. All fragments were taken into account for the weight measurement. However, as mentioned in “Experimental Protocol” (Fig. 4(b), the head structures were excluded from the calculation of length L (Fig. 7(b)).

The nonzero-biopsy rate was calculated as the number of samples collected, regardless of the sample's shape and size, divided by the total number of biopsy attempts. Figure 8 shows the relationship between the nonzero-biopsy rate with the inner stylette diameter in the cases without aspiration assistance. It shows no direct correlation between the nonzero-biopsy rate and either the inner stylette diameter or the needle insertion speed. Only the condition with 1.09-mm inner stylette diameter at an insertion speed of 10 mm/s produced a nonzero-biopsy rate of 100%. The smaller clearance between the external needle and inner stylette and the faster needle insertion speed assisted in holding the sample inside the external needles compared to other cases. However, the collected samples were all damaged as shown in Fig. 7.

Figure 9 shows the relationships between the ranges (difference between the maximum and minimum values) of the collected sample length and weight and the inner stylette diameter. Compared to the length of samples collected with the aspiration-assisted system (17–23 mm), shown in Fig. 5(a), much shorter samples (2.5–12 mm) were collected. As shown by the weight measurement in Fig. 9(b), the average weights were between 1.49 mg and 6.09 mg. The case with 1.04-mm inner stylette diameter at a needle insertion speed of 10 mm/s led to the largest sample (12 mm long and 6.09 mg mass). Using the measured density of the gelatin samples (1.12 g/cm³), the lengths of a gelatin cylinder 1.14 mm in diameter (the inner diameter of the external needle) were mathematically calculated as 5.3 mm for a 6.09-mg sample and 12 mm for a 13.7-mg sample. These experiments demonstrated that it was difficult to properly hold the tissue phantom cut by the coaxial needle in the external needle without aspiration assistance. In other words, the aspiration assistance helps hold the cut sample securely in the external needle.

Figure 10 illustrates the force components encountered during the biopsy test. As shown by Eq. (1), the total force F_t consists of the cutting force F_c at the needle tip, the friction force F_i between tissue and inner wall of the external needle, the friction force F_e between tissue and the external needle, the friction force F_n between the coaxial needles, and the plunger friction force F_p between the rubber plunger and syringe barrel. The syringe friction force F_s (Eq. (2)) is the sum of the friction force F_n and the plunger friction force F_p.

Figure 11 shows a representative profile of the total force F_t captured during the biopsy procedure (using gelatin) at the needle insertion speed of 1 mm/s with a 1.09-mm-diameter inner stylette. As mentioned above, when the coaxial needle reaches x = 15 mm, the inner stylette remains at the x = 15 mm position, and the external needle continues to move forward and slides over the rubber plunger. Figure 11 also includes the plunger friction force F_p and syringe friction force F_s. F_p is the force when the external needle without an inner stylette was inserted into air instead of a tissue phantom. F_s is the force when the coaxial needle was inserted into air instead of a tissue phantom. The force F_p increased steeply right after x = 15 mm; this is the result of the deformation of the rubber plunger seal. When the external needle stops at x = 40 mm at 49 s, a block is manually inserted between the plunger holder and syringe holder to lock the plunger. However, the plunger rubber instantly moved to recover the deformation due to the friction against the inner surface of the syringe barrel. As a result, both the plunger friction force F_p and syringe friction force F_s dropped at 49 s, then increased slightly, and finally dropped to zero.

The combination of the stress concentration caused by the tissue phantom deformation at the start of the external needle insertion and the plunger friction contribute to create the drastic increase in the total force F_t and syringe friction force F_s. The total force F_t continues to increase until the needle stops at x = 40 mm. When the coaxial needle is inserted into the tissue phantom at a constant speed, the plunger friction force F_p is considered constant. The needle friction force F_n can be calculated by subtracting the plunger friction force F_p from the syringe friction force F_s, as shown in Eq. (2). For the rest of the work, the force components will be analyzed using the maximum force values obtained at x = 40 mm.

Figure 12 shows the relationship between plunger friction force F_p and needle insertion speed from 1 to 10 mm/s. The plunger friction force F_p increases with increasing insertion speed. At higher needle insertion speeds, the rubber plunger is dragged by the syringe barrel and deformed more. This increases the contact area between the rubber plunger and interior of the syringe, resulting in increased plunger friction force F_p.

Figure 13 shows changes in syringe friction force F_s with inner stylette diameter at three different needle insertion speeds. Figure 14 shows the calculated needle friction force F_n. The needle friction force F_n was negligible when 0.94-mm-diameter inner stylettes were used. This suggests that the clearance between the coaxial needles, 0.1 mm, was too large to create an aspiration effect in the external needle during the biopsy procedure. Except for the case of the 0.94-mm-diameter inner stylette, the syringe friction force F_s and needle friction force F_n increased with increasing inner stylette diameter (decreasing needle clearance) and increasing insertion speed at all insertion speeds. Before running the biopsy test, air was present in the clearance between the coaxial needle components. The smaller clearance between the coaxial needle components requires greater shear stress on the air between them to create relative motion. As a result, the needle friction force F_n, as well as the syringe friction force F_s, increased with a reduction of the needle clearance or an increase in the needle insertion speed. In other words, the reduction of the needle clearance or an increase in the needle insertion speed facilitated the aspiration effects.

Figure 15 shows the relationship between the difference between the total force and the needle–tissue interaction force (F_t − F_s) and the inner stylette diameter for insertion speeds of 1, 5, and 10 mm/s. For comparison, the biopsy tests were performed with no stylette (that is, the same as fine-needle aspiration). In the case of no stylette (F_n = 0), the interaction force between the needle and the gelatin can be considered to be (F_t − F_p), and more air was inside the external needle before the biopsy test. Regardless of the needle insertion speed, this required a higher total force to collect the sample than the cases with inner stylettes. Moreover, without the support of an inner stylette, the extracted sample was eventually sucked back into the syringe barrel, randomly deformed, and difficult to remove for measurement. In addition, defects in the gelatin were left when the needle was removed after this suction-based biopsy process (Fig. 15(b)). Accordingly, the presence of an inner stylette not only decreased the needle–tissue interaction force required but also helped to secure the sample inside the needle in a good shape.

With the inner stylette, the needle–tissue interaction force (F_t − F_s) decreases with increasing inner stylette diameter and needle insertion speed. The larger the stylette diameter, the more the aspiration effect was enhanced. This facilitated the sample extraction so that a lower force (F_t − F_s) was required in the case with the larger stylette diameter. Moreover, faster needle insertion leads to a lower force (F_t − F_s). A study has shown that the deformation resistance (local elastic modulus) of tissue decreases as the cutting speed increases [22]. Another study [23] showed that the needle insertion force decreases with increasing needle-insertion speed. The trends in Fig. 15 match these reports, and the overall results indicated that smaller needle clearance and lower insertion speed lead to a lower force needed to collect samples.

To evaluate the system feasibility with real inhomogeneous tissue, fresh chicken breast was used in place of the gelatin tissue phantom. The shapes of the chicken breast samples were found to be more irregular than those of the gelatin samples. Fiber orientation seemed to affect the chicken breast sample geometry by influencing stretching and distortion. Accordingly, the direction of the coaxial needle insertion was fixed for this study, and the coaxial needle was inserted into the chicken breast perpendicular to the fiber orientation.

Figure 16 shows the system modified for chicken breast biopsy. The chicken breast was first covered with plastic wrap and secured with three hose clamps. After placing the secured chicken breast on the aluminum plate, it was fixed by two rubber strips. Except for the tissue fixture, the remaining system stayed the same. Using a 1.09-mm-diameter stylette, which enhanced the aspiration effect the most in the tissue phantom tests, the chicken breast biopsy was evaluated at different insertion speeds by the total force, total tissue sample (chicken meat and liquid) weight, and biopsy (chicken meat) weight. In order to measure the total tissue sample weight, the entire coaxial needle assembly (syringe, plunger, external needle, and inner stylette) was weighed prior to the experiments. After each biopsy test, the weight of the system with the extracted tissue sample inside the needle was measured. The difference in the weight before and after the biopsy test was used as the total tissue sample weight. After that, the total tissue sample was pushed out from the needle and placed on a petri dish for biopsy weight measurement. In practice, some liquid remained in the clearance between the needle and stylette. Only the biopsy sample, excluding the liquid, was weighed. Each condition was repeated five times.

Figure 17 shows a photograph of representative samples and the effect of insertion speed on the total sample weight. No zero biopsies were observed in the tests. The largest sample by total weight was collected at 1 mm/s. However, the representative 1 mm/s solid sample was the shortest, and a high liquid percentage was found. When inserting at lower speeds, more liquid could enter the needle during the longer insertion time. At 1 mm/s, more liquid occupies the space, so that less solid sample could be collected. In contrast, the fast needle insertion at 10 mm/s facilitated the collection of large solid samples with less liquid. For reference, the length of biopsy sample (chicken meat) was 7–11 mm at 10 mm/s needle insertion.

The space inside the external needle at x = 40 mm, excluding the 20 deg bevel tip section, is calculated as 21.4 mm³. In the cases of needle insertion speeds of 1, 5, and 10 mm/s, the volumes of the total sample are calculated as 24.9 mm³, 20.2 mm³, and 22.2 mm³, respectively. This shows that the space inside the external needle was almost fully occupied by total chicken breast samples.

Figure 18 shows the effect of insertion speed on the force (F_t − F_s) during chicken breast biopsy experiments. The force (F_t − F_s) decreases with increasing needle insertion speed. This is similar to the trend in the gelatin case, although the forces are lower than those for the gelatin. This can be because less of a solid sample (chicken meat) is extracted than with gelatin under the same conditions. Overall, this study proved the feasibility of the developed system and suggested that the use of the system at high needle insertion speeds facilitates the extraction of more solid samples in better conditions.

A new aspiration-assisted end-cut coaxial needle biopsy system has been developed and evaluated with biopsy sample weight, sample length, and total force using tissue phantom (gelatin) and chicken breast. The results obtained in this study can be summarized as follows:

1. (1)

   The developed system collects samples, with a zero biopsy rate of 0%, by means of simultaneous cutting and aspiration of the coaxial needle assemblies. This mechanism helps reduce the needle–tissue interaction force during the biopsy process.

2. (2)

   The needle–tissue phantom interaction force (F_t − F_s) decreases with increasing inner stylette diameter and needle insertion speed. Decreasing needle clearance facilitates the aspiration effects, so that the use of coaxial needles with less clearance yields a larger and better shaped sample with less force required. Increasing the needle insertion speed also facilitates the aspiration effects. However, faster needle insertion does not lead to a large sample extraction due to the greater influence of the sample deformation.

3. (3)

   The biopsy tests using inhomogeneous samples (i.e., chicken breast consisting of both solid and liquid components) also showed the same trend: lower needle insertion speeds facilitate sample collection. However, the extracted samples contain more liquid and less solid. To increase the ratio of solid to liquid, higher needle insertion speeds are required.

This work was partially supported by the National Science Foundation (Grant No. CMMI-1266179).