Optical coherence tomography for endodontic imaging - art. no. 68430F

Abstract

In root canal therapy, complications frequently arise as a result of root fracture or imperfect cleaning of fins and invaginations. To date, there is no imaging method for nondestructive in vivo evaluation of the condition of the root canal, during or after treatment. There is a clinical need for a technique to detect defects before they give rise to complications. In this study we evaluate the ability of optical coherence tomography (OCT) to image root canal walls, and its capacity to identify complicating factors in root canal treatment. While the potential of OCT to identify caries has been explored before, endodontic imaging has not been reported. We imaged extracted lower front teeth after endodontic preparation and correlated these images to histological sections. A 3D OCT pullback scan was made with an endoscopic rotating optical fiber probe inside the root canal. All oval canals, uncleaned fins, risk zones, and one perforation that were detected by histology were also imaged by OCT. As an example of an area where OCT has clinical potential, we present a study of vertical root fracture identification with OCT.

Full text

Optical Coherence Tomography for Endodontic Imaging
G. van Soest
a
, H. Shemesh
b
, M.-K. Wu
b
, L. W. M. van der Sluis
b
, and P. R. Wesselink
b
a
Department of Biomedical Engineering, Thorax Center, Erasmus University Medical Center, Dr.
Molewaterplein 50 3015 GE Rotterdam, the Netherlands;
b
Department of Cariology Endodontology Pedodontology, Academic Centre for Dentistry Amsterdam
(ACTA), Louwesweg 1, 1066 EA Amsterdam, the Netherlands
ABSTRACT
In root canal therapy, complications frequently arise as a result of root fracture or imperfect cleaning of fins and
invaginations. To date, there is no imaging method for nondestructive in vivo evaluation of the condition of the root
canal, during or after treatment. There is a clinical need for a technique to detect defects before they give rise to
complications. In this study we evaluate the ability of optical coherence tomography (OCT) to image root canal walls,
and its capacity to identify complicating factors in root canal treatment. While the potential of OCT to identify caries has
been explored before, endodontic imaging has not been reported. We imaged extracted lower front teeth after endodontic
preparation and correlated these images to histological sections. A 3D OCT pullback scan was made with an endoscopic
rotating optical fiber probe inside the root canal. All oval canals, uncleaned fins, risk zones, and one perforation that
were detected by histology were also imaged by OCT. As an example of an area where OCT has clinical potential, we
present a study of vertical root fracture identification with OCT.
Keywords: Optical coherence tomography, intracanal imaging, histology, root canal, vertical root fracture.
1. INTRODUCTION
Modern imaging techniques are clinically applied during root-canal treatment, but important information on inner canal
anatomy and dentin thickness is still limited to in vitro observations. Moreover, more than 50% of lower incisors show
long-oval form (ratio of long/short canal diameter ≥ 2) 5 mm from the apex
1
, which requires special considerations in
cleaning during endodontic procedures
2
. One difficulty in treating these canals is the chance of strip perforations due to
the short distance between the inner canal wall and the periodontal ligament. In these so called “risk zones” the clinician
is often faced with the challenge to sufficiently clean and enlarge the root canal space while not perforating the wall
3
.
Current clinical imaging techniques cannot give reliable information on this aspect.
Optical Coherence Tomography (OCT) is a relatively recent development in diagnostic medical imaging technology
that was first introduced in 1991
4
. Since then, it has become a standard tool in ophthalmology and promising imaging
method for intracoronary atherosclerosis detection
5
. For example, most heart attacks are caused by sudden ruptures of
unstable arterial plaques that cannot be detected using conventional imaging modalities. OCT has the potential to
identify these arterial plaques and differentiate stable plaque from unstable. In addition, it is an attractive technique for
the early identification of gastrointestinal malignancies, including the esophagus, stomach, and colon
6
. Recently optical
in vivo biopsy, providing microscope-quality images in which cell function can be distinguished, is one of the most
challenging fields of OCT application
7
.
OCT uses infrared light from a source with a short coherence length. The light is scattered by the internal
microstructure inside the specimen. A reflectivity profile is recorded along the scanned direction using an interferometer:
constructive interference occurs if the lengths of sample and reference arms are equal to within the coherence length of
the source
8
. The coherence gate is scanned through the sample by changing the length of the reference arm. OCT
achieves a depth resolution is of the order of 10 µm, with an in-plane resolution depending on the imaging optics, but
possibly similar to the optical microscope. An image is formed from the envelope of the interferogram. By scanning the
probe along the imaged specimen while acquiring image lines, a two- or three-dimensional image is built up. In
endoscopic OCT imaging, near-infrared light is delivered to the imaged site (a blood vessel or a section of the gastro-
intestinal tract) through a single-mode fiber. The imaging tip contains a lens-prism assembly to focus the beam and direct
it toward the lumen wall. The imaging beam is scanned along the wall by rotating the fiber. The fiber can be retracted
inside a catheter sheath to perform a so-called “pullback,” allowing the user to make a stack of cross-sections, scanning
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the investigated vessel lengthwise. State-of-the-art OCT systems reach a 6-mm imaging depth, with 8-µm resolution, at
over 80 frames per second.
The image intensity in OCT is the product of several factors. One attempts to image the tissue structure by recording
the amount of backscattered light at a certain position. The mean OCT signal at the detector
d
i depends on the sample
arm input field A
s
; the transmission T coefficient of the interface of the material; the backscattering cross-section σ
b
(z);
the attenuation µ
t
in the path from the imaging optics towards the imaged site, z; and the coupling efficiency between the
scattered field and the imaging optics Ξ(z)
9
:
() ()
zzeTAi
b
z
sd
t
σ
µ
Ξ∝
−
, (1)
where σ
b
(z) reflects the structure of the specimen. OCT is relatively insensitive to biological structures that do scatter
light, but do so primarily into the forward direction. The form of T(z) depends on the optical configuration of the OCT
system. In the case of a single-mode fiber as is used in endoscopic systems, it is given by a Lorentzian, with the strongest
coupling occurring at the focus of the collection lens and a width given by the beam’s Rayleigh length (or depth of field).
OCT potential in dentistry was not overlooked. OCT images of hard and soft tissues in the oral cavity were compared
with histology images using an animal model, showing an excellent match
10
. In another study, Otis et al.
11
discussed the
clear depiction of periodontal tissue contour, sulcular depth, connective tissue attachment, and marginal adaptation of
restorative materials to dentin, concluding that OCT is a powerful method for generating high-resolution, cross-sectional
images of oral structures. Amaechi et al.
12
and Baumgartner et al.
13
described the recognition of caries with OCT. The
technique could provide dentists with an unprecedented level of image resolution to assist in the evaluation of
periodontal disease, dental restorations, and in the detection of caries.
We have recently published a paper
14
in which several anomalies in root canal morphology and cleanliness were
identified both in microscopy. In this paper, we will present a more qualitative discussion of the intensity distribution in
OCT images, compared to the micrographs, and what information these provide about the optics of dentin. In addition,
we will present the first data of an experiment investigating the potential of OCT to identify a frequently occurring
complication in endodontic treatment: vertical root fracture (VRF). Vertical root fracture is a considerable threat to the
prognosis of a tooth during and after root canal treatment
15
. VRFs present a challenge to the clinician in that the
diagnosis
is often difficult, and is based on subjective parameters.
The current available methods to clinically diagnose
VRF include illumination, x-ray, periodontal probing, staining, surgical exploration, bite test, direct visual examination
and operative-microscope examination. All of these have limited success
16,17
. Radiographic images could reveal VRF
only if the X-ray beam is parallel to the fracture line
18
.
2. MATERIALS AND METHODS
2.1 Preparation of Teeth
2.1.1 Survey group
Ten extracted single-rooted mandibular incisors were selected. A radiograph was taken from two angles to verify a single
canal. Teeth with open apices or large carious lesions were excluded. Each tooth was accessed coronally with diamond
bur (FG 173 Horico, Berlin, Germany) and the canal opening was enlarged with Gates Glidden drills #3 and #4 which
Figure 1. Photograph of a tooth with the OCT catheter inside the root-canal (arrow); the red light shows the position of the imaging tip
Proc. of SPIE Vol. 6843 68430F-2

were inserted 4 and 3 mm into the canal respectively. The canal was instrumented to a size 50 stainless steel K files
(Dentsply Maillefer, Ballaigues, Switzerland). Irrigation with 2% NaOCl using a 26-gauge needle followed after every
instrument so that a total of 15 ml solution was used per tooth. Each canal was then rinsed with sterile saline. The apical
constriction was opened with #45 K file to allow the optic fiber to penetrate through the canal.
2.1.2 Vertical root fractures
Twenty-five lower premolars were prepared as described above. The teeth were then divided into 3 groups: In the control
group (n=5) and group 2 (n=10) teeth were flushed with 5 ml of water. In group 1 (n=10) Passive Ultrasonic Irrigation
(PUI) was performed
19
and 10 ml of EDTA 17 % were used to flush the canals for 1 minute
20
. A final flush of water
followed. VRF was artificially made in the two experimental groups: A D stainless still finger spreader (Hu-fridey) was
cut at the tip so its length was 12 mm, and inserted as far possible into the canal. Vertical pressure was applied to the
spreader with a screw-table until a vertical line appeared on the outside surface of the root. No forces were applied on the
control group teeth.
2.2 Test Setup
OCT pullback scans were performed using a LightLab Imaging™ M2-CV™ system (Westford, MA, USA), in
combination with an ImageWire™ 2 catheter. This system is designed for intracoronary imaging in atherosclerotic
plaque diagnosis. It is commercially available for clinical use in cardiac catheterization laboratories. The catheter
consists of a 2 m long optical single mode fiber inside a protective sheath, with a diameter of 0.5 mm. The imaging depth
in water is 3.3 mm. The axial and transverse image resolutions are were 14 and 25 µm respectively.
For endodontic imaging, the probe’s tip was placed inside the root canal, (Fig. 1, 2) so that the distal end was inserted
through the apex. The tooth was placed in a water bath to improve the optical match between catheter and tooth.
Pullback scans from the apex to the coronal opening were performed, with a speed of 1 mm/s and 10 rotations per
second, using 312 lines per frame and 760 samples per line. The result is a stack of images with a spacing of 100 µm.
These images were stored as AVI files for visual inspection. After imaging, teeth from the survey group were sectioned
at 5 and 7 mm from the apex with a saw microtome (Leica SP1600, Wetzlar, Germany). Slices were then viewed through
a stereomicroscope (Zeiss Stemi SV6, Carl Zeiss, Gottingen,Germany) using a cold light source (KL 2500 LCD, Carl
Zeiss). Pictures were taken with a camera (Axio cam, Carl Zeiss) at a magnification of 12x and compared to the
corresponding OCT images at the same level.
The occurrence of VRF was assessed in OCT images 3, 6 and 9 mm from the apex by two observers (GvS and HS).
Using the presence of a crack on the exterior root surface as a standard, the sensitivity and specificity of OCT for
diagnosing VRF were evaluated.
viewing
direction
0.5 mm
diamete
r
rotating
pullback
transparent tip
OCT catheter
(B)
(A)
Figure 2. (A) Schematic of an OCT catheter inside the root-canal. (B) A rotating fiber with a transparent tip is situated inside the
catheter and serves both as a light source and as the receiver, transmitting the reflected beams to the imaging computer.
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3. RESULTS
Figure 3 shows an OCT cross-section of a root from the survey group. Several features are identified in the image. In the
survey group, all oval canals, un-cleaned fins, risk areas and one perforation that were histologically detected were also
visualized with OCT
14
. Canal cleanliness could also be verified. These are all parameters that are potentially clinically
relevant in endodontic practice. Three examples of comparisons between OCT and histology micrographs are shown in
Fig. 4. These will be discussed in more detail below.
Several vertical root fractures (VRF) are shown Fig. 5. In group 1 (canals cleaned with EDTA and ultrasonic
irrigation) the sensitivity and specificity were 93% and 98%, while in group 2 (canals only rinsed with water) the values
were 82% and 87% respectively. The scans were reviewed again by the same observers after 4 weeks; intraobserver
agreement was 90%. In the control group, no VRF was observed.
4. DISCUSSION
4.1 OCT Image Structure
The influence of a specific tissue’s microstructure on light propagation is an important factor when considering
diagnostic applications of light. Dentin is a structure with anisotropic optical properties that is different from most other
biological tissues
21
, since the primary scatterers in dentin are the tubules
22
. Since the scattering geometry is fixed in OCT
(one always looks in the backscattering direction), it is hardly the ideal tool to study the strongly anisotropic optical
properties of dental materials. However, given our knowledge of the structure of dentin
22-24
, we may try to explain the
structures that are seen in the images. In our experiments on the survey group, the root dentin was semitransparent,
allowing imaging of the outer root surface as well. The transparency of the dentin is seen to depend strongly on the
viewing direction, however; see e.g. Figs. 4B and F. We hypothesize that this is due to the varying angle between the
OCT beam and the dentin.
A dentinal tubule is a line scatterer for light. The scattering cross-section of a line is cone-shaped, and depends
strongly on the incoming angle
25
. Comparing with (1), we see that several factors determining the image intensity
become geometry dependent: the backscattering cross-section σ
b
(z) and the attenuation coefficient µ
t
.
A
C
A
B
D
Figure 3. OCT output at 7 mm from the apex demonstrates a cross section of a prepared canal (A), Cementum (B) and dentinal tubules
(C) . (D) “risk zone”- canal wall ≤1 mm thick. The bottom panel contains a longitudinal slice through the data.
Proc. of SPIE Vol. 6843 68430F-4

Backscattering only occurs if the incoming light is perpendicular to the line. In our images, this situation occurs when
the tubule runs parallel to the canal (assuming a 90º angle between the OCT beam and the wall). In this geometry, the
interaction strength between the light and the scatterer is minimized, and hence, hardly any attenuation due to scattering
takes place. As a result, there is a backscattering signal due to the non-vanishing backscattering cross-section, and light
can penetrate all the way to the exterior root surface. This situation appears to be applicable to Fig. 4B, in the sectors
marked by (*). In the corresponding histology slide (Fig. 4A), the (*) marks are in a darker region, where the light is
being guided away by scattering along the tubules
25
. Where the OCT image is dark, the outer wall can also not be
distinguished. Of the two brighter regions in Fig. 4A, adjacent to the canal, one is dark in the OCT image, while the other
appears brighter. As the catheter is probably not perfectly aligned to the canal wall (see lower panel of Fig. 3),
asymmetries may easily occur in scattering off this highly anisotropic material. A similar anisotropy is seen in Fig. 4D,
while Fig. 4C exhibits a much more symmetric intensity distribution. The attenuation is high in the OCT image, because
the outer root surface cannot be seen. The tubules are seen to spread radially outward in this cross-section (Fig. 4C);
possibly the geometry is such that multiple scattering off the tubules gives rise to a brighter sector in OCT. The images in
Figs. 4E and F show similar phenomena as Figs. 4A and B (bright histology corresponds to dark OCT), however with
much more detailed structure, also around the invagination.
The structure of the root can be more easily identified in clean canals. Debris or a smear layer inside the canal is
usually so strongly attenuating (scattering) that the light does not penetrate well and structures are hard to discern.
Likewise, the dentin gets optically thicker high in the root and internal structure becomes invisible (see e.g. Fig. 5C).
Some care has to be taken with interpreting measured distances in the OCT images, in the regions were dental
material is present. The OCT system assumes a refractive index of the imaging medium that is close to that of water
(1.33), which is a reasonable approximation for soft tissues. Dental and bone-like materials have a higher index of
refraction, closer to 1.5
22
. As a result, the distances inside the tooth are approximately 13% shorter than suggested by the
images. Straightforward image processing can remove this ambiguity. This refractive index mismatch also affects T, the
interface transmission (cf. (1)).
A
B
C
D
E
F
Figure 4. (A, C, E) Histology slides of three cross-sections and (B, D, F) OCT images of the same sites. The pairs (A, B) and (C, D)
show clean canals, while the pair (E, F) shows an invagination. See the text for a detailed discussion of the images.
*
*
*
*
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4.2 Vertical Root Fractures
OCT proves to be quite effective in diagnosing VRF. Fractures may appear as a bright line extending outward from the
lumen, a separated cleft through the dentin if the root has actually split, or as small notches in the canal wall. These small
cracks were seen mostly higher up in the canal, where the dentin is optically thicker and the light does not penetrate as
deeply. Cracks usually originate in the canal, or are adjacent to it, but several examples were encountered where the
fractures separated from the canal deeper into the dentin. A full phenomenology of the condition will be the subject of
further research. VRF detection appears to benefit from cleaning of the canal with EDTA and ultrasonic irrigation. The
potential of OCT to evaluate canal cleanliness with different methods is the subject of current research.
4.3 Competing Techniques
The use of novel imaging techniques is gaining a lot of attention in the field of endodontics. New computed tomography
(CT) methods prove to be more accurate in the evaluation of bone lesions than conventional radiography
26
. Similarly,
canal morphology
27
, root fractures
28
tooth anatomy
29
and the interface between the root canal and filling materials
30
were
successfully demonstrated with CT techniques. These methods use ionizing radiation which could be harmful at higher
doses when used in-vivo. Furthermore, two major disadvantages are limiting the successful application of these methods
for intracanal imaging: First, the resolution is usually not suitable for microscopic level imaging. Digital dental
radiography systems have a pixel size approaching 100 µm. Second, the probe size is usually much bigger than a root
canal. These methods are also time consuming and often require the interpretation of thousands of images. In contrast,
OCT combines a very thin optical catheter measuring 0.5 mm in diameter, with high resolution capacities, enabling
imaging of objects measuring a few micrometers and does not involve ionizing radiation. The imaging wire can be
deployed independently or integrated straightforwardly into existing therapeutic or imaging catheters. Furthermore it can
easily fit into a prepared root canal, and is flexible, allowing penetration through curvatures.
Fiberoptic endoscopy was previously described as a method for intracanal visualization
31
. In this system, a 0.7 mm
probe is inserted into a dry canal, to image the inner anatomy. However, this system is based on a camera which
produces a digital image and not on microscopic level characterization or light propagation as observed by the OCT.
Furthermore, no penetration of light through the dentinal tubuli is possible and hence no detection of the outer outline of
the root. The smaller diameter and increased flexibility of the OCT probe allows deeper penetration and easier
application in clinical situations.
5. CONCLUSION
We have demonstrated optical coherence tomography as a noninvasive and nondestructive technique for endodontic
imaging. It proved to be suitable for analyzing the anatomy and cleanliness of root canal walls, and has a high sensitivity
and specificity for detection of VRF. OCT generates high-resolution, real time, intra-canal microscopic images, and
holds great potential for in-vivo application.
Figure 5. OCT images of VRF (indicated by * in each image) in three different canals. (A) Near the apex, showing two bright lines
extending from the canal. (B) Separated cracks where the root has split. (C) Small notches in the canal wall far from the apex.
A B C
*
*
*
*
*
*
Proc. of SPIE Vol. 6843 68430F-6

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