DOI:10.2214/AJR.08.1023
AJR 2008; 191:1359-1365
© American Roentgen Ray Society
Radiofrequency Identification Tags for Preoperative Tumor Localization: Proof of Concept
Joshua J. Reicher1,
Murray A. Reicher2,
Mariam Thomas3 and
Robert Petcavich2
1 University of California at Los Angeles, Los Angeles, CA.
2 Health Beacons, Inc., Seattle, WA.
3 Department of Radiology, University of California at Los Angeles, Los Angeles,
CA.
Received April 6, 2008;
accepted after revision May 29, 2008.
Address correspondence to J. J. Reicher, PO Box 832, Rancho Sante Fe, CA
92067
(jjreicher{at}hotmail.com).
M. A. Reicher and R. Petcavich have a financial interest in and are
chairman and president, respectively, of Health Beacons, Inc., manufacturer of
TagFinder.
Abstract
OBJECTIVE. The objective of our study was to experimentally explore
the potential for tumor localization using radiofrequency identification
(RFID) tags and a newly developed handheld RFID detector.
MATERIALS AND METHODS. A unique RFID detector that combines the use
of multiple interchangeable detector probes with both audio and LCD display
signals was invented, allowing precise localization and identification of RFID
tags. Accurate localization and identification were validated using this
handheld RFID detector (TagFinder) and RFID tags of 2-mm diameter and 8- or
12-mm lengths. Experiments included the following: validation in various
breast phantoms; differentiation of 4- to 6-cm-diameter tissue specimens with
and without tags; determination of the nearest differentiable distance between
two tags; proof of visualization of tags on sonography, radiography, and MRI;
and experimental localization and resection of RFID-labeled tissue
specimens.
RESULTS. Both 8- and 12-mm-length RFID tags implanted < 6 cm deep
were accurately localized and uniquely identified. Chicken breast specimens of
4- to 6-cm diameter implanted with RFID tags were accurately differentiated
from specimens without tags. Tags in proximity could be reliably
differentiated and uniquely identified when placed as close as 0–2 cm
apart, depending on the tags' precise orientations. RFID tags were easily
visualized with sonography, mammography, and MRI, with artifacts present only
on MRI. Localization and resection of RFID tags in the labeled tissue region
were successful in grocery store–bought chicken breasts.
CONCLUSION. The combination of RFID tags and a new handheld RFID
detector shows promise for preoperative imaging-guided tumor localization.
Keywords: breast cancer • breast tumor localization • oncologic imaging • radiofrequency identification tags • women's imaging
Introduction
Radiofrequency identification (RFID) technology shows potential for
multiple medical uses, including identification of surgical sponges and
monitoring of endotracheal tube position, but heretofore not for preoperative
tumor localization [1,
2]. An RFID tag, about the size
of a grain of rice, contains a dormant microchip that can be stimulated with
radiofrequency energy, causing reemission of a radiofrequency signal used to
localize and identify the unique number of the tag. RFID tags that can be
implanted in humans have already been approved by the U.S. Food and Drug
Administration (FDA) for purposes such as patient identification. Current
preoperative imaging-guided tumor localization options for nonpalpable breast
lesions include imaging-guided placement of one or more radiopaque markers and
preoperative percutaneous localization with a hookwire
[3–5].
Direct injection of radioactive seeds has also reportedly shown promise
[6,
7]. Injection of radioactive
material and intraoperative detection with a Geiger counter are also commonly
used to localize sentinel lymph nodes
[8].
RFID tags potentially provide radiopaque markers that can be both uniquely
identified and interactively localized if used in conjunction with a handheld
reader that can interactively pinpoint tags. In theory, such an RFID system
could replace current metallic tumor markers, preoperative hookwire
localization, and specimen radiographs and could otherwise aid surgeons and
pathologists in localizing nonpalpable tumors.
Based on these theoretic advantages, a handheld RFID tag reader has been
developed and tests have been performed to show the accuracy and feasibility
of imaging-guided tumor localization using RFID technology.
Materials and Methods
RFID Technology
The RFID system consists of two components: a reader and a tag
[9]. Each tag contains a
microchip that stores a unique identification number and an antenna that
responds to inter rogation by the reader
[9]. The RFID reader sends a
radio-frequency signal to the tag that in turn receives, alters, and reemits
the signal [9]. The reader then
captures the altered signal and responds with the combination of an LCD
display and audio signal. For the experiments in this study, we used cylindric
RFID tags (VeriMed, VeriChip), which were 2 mm in diameter and 8- or 12-mm
long, and an RFID reader (Figs.
1A and
1B) (TagFinder, Health
Beacons). The reader is powered by a standard 9-V battery; operates at 134.2
kHz; and consists of an interrogation antenna, LCD display, power on button,
read button, and audio signal mechanism
(Fig. 1B). The reader's LCD
screen displays a bar indicating the proximity of the reader to an RFID tag
and emits an audio tone that increases in volume and pitch as the reader is
moved closer to a tag. If the read button is pressed, the reader displays the
unique identification of the tag being interrogated. The RFID reader has two
attachable interrogation antennae—a loop probe with a detecting range of
0–6 cm and a pencil probe, intended for highly specific localization,
with a shorter range of 0–3 cm (Fig.
2A,
2B). The loop probe was used in
the research described herein unless otherwise speci fied. Signal strength is
strongest when the reader approaches the longitudinal ends of the tag, mean
ing that scanning the reader across the tag from pole to pole produces peak
signals at the end points, or ends of the RFID tag, and a relatively minimal
signal over the tag's center (Fig.
3).
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Fig. 1A —Photographs show radiofrequency identification (RFID) tags
(VeriMed, VeriChip) and device (TagFinder, Health Beacons). Photograph shows
two RFID tags (lengths, 8 and 12 mm). Tags can be placed in target tissues
using 12-gauge syringe-like coaxial delivery system.
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Fig. 1B —Photographs show radiofrequency identification (RFID) tags
(VeriMed, VeriChip) and device (TagFinder, Health Beacons). Photograph shows
handheld TagFinder LCD display (black arrow) shows distance between
detector element (straight white arrow) and nearest end of tag. Read
button (curved white arrow) causes LCD to display tag's unique
identification number, eliminating false-positive readings and allowing
neighboring tags to be distinguished.
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Fig. 2B —Two versions of handheld TagFinder (Health Beacons). Loop
probe is permanently attached to reader, but pencil probe with extension
(arrow) can be attached and takes precedence. Loop probe detects tags
at depths of 0–6 cm, and pencil probe is effective at distances of
0–3 cm.
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Localization and Depth Detection
The ability to localize and detect the depth of implanted RFID tags was
tested in an opaque sonography breast phantom (Breast Ultrasound Phantom, Blue
Phantom Company) with a density of 0.92 g/cm3, which is comparable
to breast tissue (Fig. 4A). An
insertion needle was calibrated using a millimeter-scale steel ruler. The
depth of insertion was verified in a gelatin phantom control by injecting
three tags at varying depths and then inserting the ruler to confirm the
depths (Fig. 4B).
Three tags were then injected into the sonography phantom at depths of 1.0,
2.0, and 3.0 cm. Three independent individuals, none with prior RFID
experience, blinded to the locations of the tags were given a brief tutorial
about how to use the tag reader to detect the depths of the tags in the
phantom. These observers' measurements were compared with the actual insertion
depths.
Differentiating Specimens With and Without Embedded RFID Tags
On three separate occasions, sets of 20 pieces of grocery
store–bought raw chicken breast were cut into 4- to 6-cm-diameter
specimens. These pieces were prepared slightly larger than typical lumpectomy
specimens to avoid the argument that the tags could be easily identified
because of the specimens' small sizes. During trial 1, 11 of the 20 pieces
were each injected with RFID tags, five tags 8 mm in length and six tags 12 mm
in length. The tags were injected into the specimens so that the tags and
injection tracks were not visible. The 20 chicken pieces were then placed onto
five plates, four pieces per plate, with individually numbered paper labels
(numbered 1–20) underneath each chicken piece. Three individuals—a
board-certified anesthesiologist, a premed college student, and a high school
student—were then given a short 5-minute tutorial explaining how to use
the handheld RFID reader to identify tags and determine unique tag
identification numbers. Each individual was also given a form on which to
record which chicken pieces, identified by the paper labels, contained tags.
To ensure accuracy, the individuals were asked to record the final four digits
of the associated unique tag identification numbers. The individuals were told
that no chicken piece would contain more than one tag, but they were not told
how many total tags were present. They were free to pick up, turn, and
otherwise maneuver the chicken pieces. Of note, the injection tracks were not
visible on the chicken pieces, so tag locations could not be determined
illegitimately. The individuals then separately analyzed the chicken pieces
and recorded their results on the provided forms.
For trial 2, the experiment was prepared just as the experiment for trial 1
except for a few changes. The experiment for trial 2 was conducted at the
University of California, Los Angeles Medical Center, with three individuals
participating including a radiology resident, a radiology fellow, and a
medical student. This time, 10 of the 20 raw chicken pieces were each injected
with one 12-mm RFID tag. In addition, the tag injection needle was inserted
into each tagless chicken piece to eliminate the chance that an injection
track might provide a clue about a tag's presence. Finally, the response forms
were modified so that participants were queried whether they could determine
in any way which pieces contained tags by visually inspecting the chicken
pieces.
For trial 3, the experiment was prepared just as the experiment in trial 2
except the chicken pieces were not presented on plates but, instead, inside
individually labeled clear plastic bags. In addition, an added functionality
of the RFID reader was explained: namely, that the read button, if pressed
while not actively detecting a tag, displays the most recent identification
number recorded in its memory. In the event of this occurrence the letter M
precedes the identification number on the LCD screen, indicating that the
displayed number is from memory, rather than from active detection. In trial
3, 11 individuals participated, including two radiology residents, four
medical students, two radiology researchers, one engineer, and two
secretaries.
Nearest Differentiable Proximity Between Tags
A grocery store–bought hotdog was used as a medium in which to
determine the nearest differentiable proximity between tags
(Fig. 5A). Pairs of parallel
8-mm-length RFID tags were inserted in a vertical orientation, perpendicular
to the longitudinal axis of the hotdog, or in a horizontal orientation,
parallel to the longitudinal axis of the hotdog
(Fig. 5B). For each trial, tags
were incrementally brought closer together, and the RFID reader was used to
determine if the tags were differentiable (Figs.
5C and
5D). In the vertical
orientation, distances of separation were set using an adjacent ruler. In the
horizontal orientation, distances of separation were set by ruler and by
slicing the hotdogs to the appropriate length and inserting the tags just into
the two ends of the hotdogs.
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Fig. 6A —Photographs show pencil-probe radiofrequency identification
(RFID) reader attachment (TagFinder, Health Beacons) approaching RFID tags.
Scale is millimeters. Pencil probe approaches ends of two parallel RFID tags
in store-bought hotdog. Parallel tags greater than 0.5 cm apart can be
differentiated using RFID reader.
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Fig. 6B —Photographs show pencil-probe radiofrequency identification
(RFID) reader attachment (TagFinder, Health Beacons) approaching RFID tags.
Scale is millimeters. Pencil probe approaches opposite ends of two RFID tags
touching end-to-end. RFID reader can successfully differentiate tags.
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The read button on the TagFinder was used to ensure differentiation between
two tags by confirm ing each tag's unique identification number. In the
vertical orientation, two signal maxima occurred, one over each tag. In the
horizontal orientation, four signal maxima occurred, one over each of the four
tag poles. The read button was used while the reader passed over the
farthest-apart maxima. To the question of differentiability, three different
responses were possible: yes, with difficulty, or no. Yes indicated that the
tags were differentiable; no, that they were not; and with difficulty, that
the tags were differentiable after two or three repeated attempts.
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Fig. 7B —Radiofrequency identification (RFID) tags (VeriMed, VeriChip)
in silicone phantom. Sonogram shows RFID tag being injected (arrow)
along margin of phantom nodule (Breast Ultrasound Phantom, Blue Phantom). Tags
could potentially be used to mark tumor margins and bracket lesions.
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Fig. 7C —Radiofrequency identification (RFID) tags (VeriMed, VeriChip)
in silicone phantom. Gradient-echo MR image obtained on 1.5-T unit of phantom
with embedded 12-mm RFID tag. Note low-signal-intensity artifact.
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A second nonblinded trial was conducted using a detector equipped with the
pencil probe designed with a narrower range of 3 cm, but with more precise
localizing capability (Fig.
6A,
6B).
Visualization of Tags on Sonography, Mammography, and MRI
Radiography of a silicone breast phantom with an embedded tag was performed
using a commercial full-field digital mammography unit (Selenia, Hologic). A
tag embedded in a sonog raphy phantom was imaged as it was introduced using a
12-gauge needle and again after intro duction. Tags embedded and placed on an
MRI phantom were imaged on a 1.5-T commercial MR system using a variety of
gradient-echo and spin-echo pulse sequences that mirror techniques typically
used in clinical breast MRI, including gradient-echo (TR/TE, 5.0/1.9; flip
angle, 12°) and fast spin-echo (6,050/121; flip angle, 18°)
techniques.
Localization and Resection in Specimens
An RFID tag was implanted into a grocery store–bought, 5-cm-thick,
boneless, skinless chicken breast fillet. Two physicians, on separate
occasions, were provided with the RFID reader and some basic surgical tools
including a scalpel, a scissor, and a forceps. One of the physicians was an
American Board of Radiology-certified radiologist and MQSA-certified
mammographer with more than 20 years of experience; the other was a breast
surgeon (Fellow of the American College of Surgeons) with more than 30 years
of clinical experience. During the experi ment, a physician was asked to use
the reader to localize the tag and then to use the surgical tools to resect
the tag and surrounding area. After resection, the physician was asked to use
the reader to confirm removal of the tag inside the resected mass.
Results
Localization and Depth Detection
The independent observers successfully used the RFID reader to localize and
detect the depth of the tags implanted in a sonography breast phantom
(Table 1). One observer
slightly underestimated the depth of one of the tags, but the remaining
results were accurate and precise.
Differentiating Specimens With and Without Embedded RFID Tags
In trial 1, the three individuals were able to successfully identify which
chicken pieces contained RFID tags and the associated unique identification
numbers with only a single error of omission. One of the individuals (the high
school student) overlooked one 8-mm RFID tag. All chicken pieces containing
12-mm tags were identified successfully.
In trial 2, none of the individuals was able to identify chicken pieces
containing tags by visual inspection. In the combined results, twice a
tag-containing specimen was missed, and once a specimen not containing a tag
was identified as containing a tag (57 of 60 correct identifications).
In trial 3, none of the individuals was able to identify chicken pieces
containing tags by visual inspection. Of the 11 individuals, nine returned
perfect response forms, correctly finding and identifying each tag and
specimen. For the two individuals with incorrect responses, one missed two
tags and the other correctly identified a tag in a specimen but listed the tag
number incorrectly (217 of 220 correct identifications).
Nearest Differentiable Proximity Between Tags
Using the loop detector, tags in the vertical orientation could be
differentiated consistently for distances of separation of 2.5 cm or greater
and after repeated attempts for distances of 1.0 cm or greater. Tags in the
horizontal orientation could be differentiated consistently for distances of
separation of 1.5 cm or greater and after repeated attempts for distances of
0.5 cm or greater (Table
2).
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TABLE 2: Distances Between Radiofrequency Identification (RFID) Tags At Which
RFID Reader Can Differentiate Tags from One Another
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Using the pencil probe, when two adjacent 8- or 12-mm tags were placed in
continuity with each other end-to-end, the separate serial numbers were not
recognizable when scanning neighboring ends of the tags. However, even with
the tags touching end-to-end, separate serial numbers could be read with the
pencil probe directed over the nonopposing ends of each tag
(Fig. 6B). When neighboring
tags were placed in parallel, the pencil probe enabled individual
identification of tags separated by greater than 0.5 cm
(Fig. 6A).
Visualization of Tags on Sonography, Radiography, Mammography, and MRI
The RFID tags were found to be easily identified on mammography and
sonography. On MRI, artifacts were greatest on gradient-echo images, as large
at 3.2 cm in diameter for the 12-mm RFID tag and 2.6 cm for the 8-mm RFID tag
(Fig. 7A,
7B,
7C).
Localization and Resection in Specimens
Each of two physicians successfully localized the RFID tag using the reader
and resected the tag and surrounding tissue, removing an approximately
3-cm-diameter collection of tissue, from a grocery store–bought chicken
breast. After resection, the physicians confirmed with the reader that the tag
was in the resected mass and no longer present in the chicken breast.
Discussion
RFID tags combined with a newly developed handheld reader show tremendous
promise for imaging-guided preoperative and intraoperative tumor localization.
Both 8- and 12-mm RFID tags can be easily introduced using a 12-gauge coaxial
needle system, previously well established for use in humans and animals
[10]. The newly invented RFID
reader enabled accurate localization, unique identification, and even
differentiation of neighboring tags in multiple tests with various phantoms
and chicken breasts. Highly accurate results were obtained when individuals
were asked to distinguish specimens containing RFID tags from specimens not
containing tags, even though the users had no prior experience with the RFID
reader and underwent only brief training.
A major theoretic advantage of using RFID technology is the potential
benefit of identification of the tag in a surgical specimen. The current
standard for breast tumor resection requires obtaining a specimen radiograph
while the patient remains in the operating room, lengthening operating room
time. If the specimen is found to be inadequate, with current methods it may
be difficult to determine how to proceed further. With the RFID system, the
surgeon could theoretically interactively use the reader to find any tags
remaining in the patient. Furthermore, the need for a specimen radiograph
could potentially be eliminated in some cases, particularly if RFID tags are
used to bracket the margins of a lesion. When human research proceeds, if tags
are placed days or weeks before surgery, we may find that it is useful to
obtain an immediate preoperative mammogram or sonogram to confirm that the
tags have not migrated. Although we may find that specimen radiographs are
still essential in cases with microcalcifications, in other cases, such as
lesions only visible on sonography, properly placed tags may lend more
assurance than a specimen radiograph with regard to adequate lesion
resection.
Although specimens can be marked with surgical sutures and other markers,
specimen orientation can be problematic today. In theory, using RFID
technology, multiple tags could be placed, each with a unique identification
number, bracketing the margins of a targeted lesion, which might aid in
specimen orientation. Our preliminary findings suggest that multiple tags can
be placed closer than would likely ever be clinically required and still can
be easily identified and differentiated.
Another potential advantage of RFID technology is the use by pathologists
who may sometimes struggle to locate a tissue marker in an excised surgical
specimen. The ability to interactively localize the marker in an excised
specimen can potentially facilitate pathologic inspection of the excised
tissue. Therefore, RFID technology presents several theoretic advantages over
currently widely used metallic tissue markers.
Another potential future application is the use of RFID technology to
replace preoperative hookwire localization. In current common practice,
hookwire localization procedures must be scheduled in a mammography, MRI, or
sonography suite immediately preceding surgery, which can be logistically
difficult, and subject the patient to the cost and discomfort of an additional
procedure even though a metallic marker may have been previously placed.
Today, after the hookwire localization, the surgeon is left with little time
to communicate with the radiologist and analyze the images obtained after
hookwire placement. With RFID, markers could be placed days or weeks before
surgery, providing the surgeon ample time to review imaging studies obtained
after RFID marker placement, to understand the identification and location of
each RFID marker, to empirically confirm that the markers can be detected from
the skin's surface, and to consult as needed with the radiologist.
The percutaneous approach for safely placing a hookwire may differ from the
optimal cosmetic or surgical approach for resecting a lesion. In theory, the
RFID system could be used as an alternative to preoperative hookwire
localization, allowing placement at a more convenient time and enabling the
surgical approach to more easily differ from the percutaneous imaging-guided
placement approach. Finally, although hookwires today are not typically used
to mark tumor margins, multiple RFID tags could theoretically be used to
bracket the margins of a lesion.
With regard to the reader, the development of a combined auditory cue and
LCD display to indicate the proximity to a tag enhanced the user's subjective
ability to localize the tag. Because of the geometry of a tag's generated
field, the signal is greatest as one approaches the distal ends of a tag
rather than the center of a tag. This understanding is essential for the user
and can even be beneficial. The reader itself is small and is light enough to
be used easily in an operating room or pathology laboratory environment.
Furthermore, it can be easily contained in one of many sterile sleeves, such
as those used to drape ultrasound probes. Because the reader emits an RF
signal, use of the RFID reader adjacent to cardiac pacemakers introduces
theoretic risks that have not been evaluated. Of note, the reader is slightly
responsive to metallic objects, but it was developed in such a way to minimize
this interaction. Furthermore, the ability of the reader to display the tag's
unique identification number eliminates false-positives because extraneous
metal objects do not generate any identification number.
One disadvantage of RFID technology is the surrounding artifact on MRI. For
now, this limitation may prevent routine placement of an RFID tag at the time
of biopsy until a diagnosis is established and one determines whether
preoperative MRI is required. However, if the size of the artifact can be
reduced with further development of the technology, the presence of a
low-signal artifact on MRI may actually prove to be advantageous in
visualizing the RFID tags. Furthermore, preoperative staging MRI is most
useful in detecting additional lesions and in sizing large lesions, so if one
can achieve an artifact of less than 1 or 2 cm, this artifact may not
practically detract from the information provided by MRI.
Tag migration after insertion is another potential problem. Tags with
antimigration sleeves exist already. Today's non–RFID tissue markers can
also potentially migrate, but studies show that they seldom do
[11].
With regard to safety in humans, the FDA has already cleared RFID tags for
long-term implantation in humans, although not specifically for the use
described herein. Currently, the RFID system is approved by the FDA as
"a device intended to enable access to secure patient identification and
corresponding health information"
[10]. As described further,
the "system may include a passive implanted transponder, inserter, and
scanner" [10]. Although
we are unaware of any specific evaluation of the long-term effects of RFID
implantation in the human breast, we have no reason to believe that
implantation in the breast would be any less safe than in any other human
tissues or less safe than implantation of non–RFID metallic markers. A
human implantable RFID tag vendor (VeriChip) reports safety and compatibility
of RFID tags in an MRI environment
[12]. In addition, a
well-respected MRI safety organization reports that "patients with the
VeriChip Microtransponder may safely undergo MRI diagnostics, in up to 7-Tesla
cylindric systems" based on the manufacturer's representation
[12]. Finally, the results of
our independent tests performed by Shellock R & D Services, which are
beyond the scope of this article, showed no significant heating (<
0.6°C) in a 3-T MRI environment.
On the basis of these encouraging results, further study in humans seems
warranted first to establish the safety and efficacy of the RFID system as a
replacement for embedded metallic tissue markers and thereafter to determine
whether this technology can be used to replace preoperative hookwire
localization.
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Abstract
Introduction
