WO2010135790A1

WO2010135790A1 - Method for providing individualized normative values for 3d cephalometry

Info

Publication number: WO2010135790A1
Application number: PCT/BR2009/000145
Authority: WO
Inventors: Frazão Bruno GRIBEL
Original assignee: Gribel Frazao Bruno
Priority date: 2009-05-29
Filing date: 2009-05-29
Publication date: 2010-12-02
Also published as: EP2434951A1; EP2434951A4

Abstract

This invention provides a method for providing individualized normative values for 3D cephalometry, by which 3D measurements on and off the midsagittal plane can be established based on published 2D cephalometric norms. The method of the invention can also be applied to other existing cephalometric longitudinal growth studies, to derive control groups without exposing new untreated subjects to radiation.

Description

Descriptive Report

METHOD FOR PROVIDING INDIVIDUALIZED NORMATIVE VALUES FOR 3D

CEPHALOMETRY

FIELD OF THE INVENTION

This invention generally relates to diagnostic/prognostic analyses. More specifically, the invention provides a method for providing individualized normative values for 3D cephalometry. In a preferred embodiment of the method of the invention, normative values for 3D measurements on and off the midsagittal plane are obtained, said method being based on previously available 2D cephalometric norms. The invention also includes applying this method to other existing cephalometric longitudinal growth studies, to obtain control groups that can be derived without exposing new untreated subjects to radiation.

BACKGROUND OF THE INVENTION

Since its introduction in 1931 radiographic cephalometry has become the most popular method used in the diagnosis of malocclusions, the quantification of treatment effects, and the understanding of normal craniofacial growth and development. Nevertheless, radiographic examinations have intrinsic limitations. A headfilm is a 2-dimensional (2D) shadow of a 3-dimensional (3D) structure, produced by a non-parallel beam that results in a distorted and enlarged image (more so in some regions than in others) (Baumrind, S., Frantz, RC, 1971).

Contemporary imaging technologies such as magnetic resonance imaging (MRI) and computed tomography (CT) have permitted 3D assessment of the craniofacial complex with a greater degree of accuracy and reproducibility than available previously (Adams, GL et al., 2004; Disler, DG et al., 1994; Hilgers, ML et al., 2005). The applicability of both technologies in a routine orthodontic environment, however, is limited by their high equipment costs, by the MRI's long acquisition time (50 minutes for a full head scan), and by the CT's high radiation levels (-2,000 μSv) (Swennen GR, Schutyser, F, 2006). With the introduction of cone beam computer tomography (CBCT) and with its reduced equipment costs, acquisition time (20 seconds), and radiation levels (40-170 μSv) (Ludlow, JB et al., 2006) a 3D assessment of the craniofacial region has become a viable alternative for patient imaging.

3D assessment can be conducted in all three planes of space, on images with life-size magnification, and without distortion or overlapping structures. Furthermore, head position during exam acquisition is not critical for 3D assessment; the spatial relationship among the various points is not changed in any way by variations in head orientation (Ludlow, JB et al., 2007). These features provide ease of landmark identification and precise superimposition of serial images, which are critical for research purposes (Cevidanes, LH et al., 2007).

A 2D lateral cephalometric analysis that has gained wide acceptance is that originally published by McNamara, JA in 1984 and updated by McNamara, JA and Brudon most recently in 2001. This method of analysis incorporates standard values for three linear measurements: midfacial length, mandibular length and lower anterior facial height (LAFH). These so-called "Composite Norms" (Table 1) are based on data derived from three untreated longitudinal or cross-sectional cephalometric samples (the Bolton Standards (Broadbent, BH et al., 1975) the Burlington Growth Study (Popovich, F, Thompson, GW et al., 1977) and the Ann Arbor adult sample of subjects with ideal occlusions and well-balanced faces (McNamara JA, Jr., Ellis, E₁ 1988)). It is important to emphasize that the norms published represent linear geometric relationships and proportions among the measurements and are not related to the age or the gender of the individual (McNamara, JA, et al.,1984). Table 1. Composite 2-dimensional norms from McNamara and Brudon (2001).

No known three-dimensional "standards" derived from a large untreated population analyzed by way of 3D examinations are available currently. It is unlikely that in the near future 3-dimensional longitudinal data from a large sample of individuals with ideal occlusions that can be used to derive normative values for 3D assessments will become available, due to examination acquisition time, cost, and ethical considerations. Therefore, even though currently we can identify landmarks accurately and generate precise 3D measurements, these measurements can be compared only to their contralateral side to evaluate asymmetries (Kwon, TG et al., 2006) or to measurements taken at different times to monitor treatment effects (Cevidanes, LH et al., 2007; Cevidanes, LH et al., 2005).

Hilgers et al. in 2005, made measurements directly in skulls of mandibular dimensions using a digital caliper and secondary MPR images obtained in a conical beam tomography, (iCAT: axial with 0.4 mm thick), and compared with measurements taken in teleradiographs conventional (lateral, frontal).

The present invention provides normative values for any 3-dimensional analysis adapted from the 2D analysis to which normative values can be obtained for a particular individual using a method that translates the measurements taken from a lateral head film to those taken from a CBCT in human subjects. To our knowledge, this is the first time an attempt has been made to produce normal values for measurements made on 3D examinations using previously-known norms from 2D evaluations.

Document US 2007/0274435 describes a cone beam CT imaging system incorporates the phase contrast in-line method, in which the phase coefficient rather than only the attenuation coefficient is used to reconstruct the image. Starting from the interference formula of in-line holography, the terms in the interference formula can be approximately expressed as a line integral that is the requirement for all CBCT algorithms. So, the CBCT reconstruction algorithms, such as the FDK algorithm, can be applied for the in-line holographic projections.

Document WO 2007/100823 describes a cone beam CT imaging system that incorporates the phase contrast in-line method, in which the phase coefficient rather than only the attenuation coefficient is used to reconstruct the image. Starting from the interference formula of in-line holography, the terms in the interference formula can be approximately expressed as a line integral that is the requirement for all CBCT algorithms. So, the CBCT reconstruction algorithms, such as the FDK algorithm, can be applied for the in-line holographic projections. Document WO 06000063 describes a method for performing a cephalometric or anthropometric analysis comprising the steps of : - acquiring a 3D scan of a person's head using a 3D medical image modality, - generating a 3D surface model using data from the 3D scan, - generating from the 3D scan at least one 2D cephalogram geometrically linked to the 3D surface model, - indicating anatomical landmarks on the at least one 2D cephalogram and/or on the 3D surface model, - performing the analysis using the anatomical landmarks.

SUMARY OF THE INVENTION

It is an object of the invention to provide a method for deriving individualized normative values for 3D measurements in 3D imaging.

It is another object of the invention to provide a method for deriving individualized normative values for 3D cephalometry regardless of the method used to make these measurements.

In another aspect of the invention, being, therefore, another of its objects there is provided a method for obtaining from any given cephalometric longitudinal growth studies, control groups and standards for 3-dimensional measurements, whereby said information can be derived without exposing new untreated subjects to radiation.

These and other objects of the invention will be readily appreciated by those skilled in the art, and can be fully enabled by the forthcoming detailed description.

BRIEF DESCRIPTION OF DRAWING

Figure 1 describes a preferred embodiment of the invention, in which a right triangle in a frontal and a submental-vertex view; H = 3D Measurement; PS= Projection Side; CO-MSP = Condylion to Midsagittal Plane; Mag = Magnification; LC= Lateral Cephalogram; X = half the maxilla angle (Co-A-Co) or half the mandible angle (Co-Gn-Co), depending on what measurement is been calculated. DETAILED DESCRIPTION OF THE INVENTION

The method of the invention provides the obtainment of 3D images from 2D images, by forming a right triangle. The invention provides, between others, a method for deriving individualized normative values for 3D cephalometry.

Said right triangle provides the correction of distortions/magnifications of 2D images when applying the following criteria:

(i) selecting two (A and B) of the points which are located at the central focus of the X-ray beam;

(ii) selecting one additional point (C) where the magnification is determined or known;

(iii) drawing a right triangle with sides described as follows:

- the Hypotenuse (H) represents the 3D measurement of the length of a known parameter, so as a method for obtaining "mean values" for 3D measurements taken from known mean values of 2D measurements using basic trigonometry is applied;

- the distance between A and D represents another measurement of other known parameter (1/2 the distance between A and B); and

- the Projection Side (PS) represents the projection of the 3D measurement on the plane where C lies. The Projection Side (PS) is equal to the measurement from a lateral cephalogram measurements (LC), reduced by the known magnification;

(iv) obtaining the angle "X" between the hypotenuse and the Projected

Side; and

(v) dividing PS (the difference between the image measurement and the image magnification) by the cosine of X/2, thereby translates the measurements from a lateral image into those obtained from a 3D image.

In a further preferred embodiment, the invention provides a method for providing individualized normative values for 3D measurements on and off the midsagittal plane, based on previously available 2D cephalometric norms. The invention also includes applying this method to other existing cephalometric longitudinal growth studies, to derive control groups and without exposing new untreated subjects to radiation. These control groups can then be used to assess the net effect of a given treatment.

In a preferred embodiment of the invention, the method comprises the following steps:

(i) selecting two of the points chosen for the analysis of Condylion Right and Condylion Left (CoR and CoL) which are located at the central focus of the X-ray beam; (ii) selecting at least one additional point located at the midsagittal plane where the magnification is determined or known; (iii) drawing at least a right triangle with sides described as follows: wherein

- the Hypotenuse (H) represents the 3D measurement of either the mandibular length (Co-Gn) or the midfacial length (Co- point A);

- the Condylion to Midsagittal Plane Side (Co-MSP) represents distance between Condylion and the midsagittal plane;

- the Projection Side (PS) represents the projection of the 3D measurement on the midsagittal plane. The Projection Side (PS) is equal to the measurement from a lateral cephalogram (LC)₁ reduced by the known magnification;

- obtaining the angle "X" between the hypotenuse and the Projected Side; and

- dividing PS (the difference between the cephalographig measurement and the cephalographic magnification) by the cosine of X, thereby translating measurements from a lateral cephalogram into those obtained from a 3D CBCT image.

In the method of the invention, measurements on the midsagittal plane are obtained simply by reducing the magnification since the cosine of zero is equal to one. Those skilled in the art will know that the hypotenuse is obtained when both the Projected Side and the Condylion to Midsagittal Plane distance are known, using the Pythagorean Theorem. If the Condylion to Midsagittal Plane distance is not known, the hypotenuse is obtained easily by dividing the Projected Side by the cosine of the angle "X" between the hypotenuse and the Projected Side.

The following examples are to be considered as merely illustrative of the invention, and should not be considered as limitations thereto.

The method of invention was assessed in a sample comprised of 13 adult subjects with ideal occlusions and well-balanced faces who have had conventional lateral head radiographs and 3D scan of their heads using a CBCT scanner. All the exams were coded in order to de-identify all subjects prior to the beginning. Data Acquisition

Two-dimensional Acquisition: The conventional 2-D lateral cephalograms were taken with the Frankfort Horizontal plane (FHP) parallel to the floor; the subject's head position was determined by a cephalostat. The magnification for the radiographs produced by that particular machine (Orthoceph OC100, lnstrumentarium Corp., Finland; 77 kVp; 12 mAs) was 10%.

The radiographs then were traced on acetate paper and checked for accuracy of anatomical outline and landmark location. Three measurements (midfacial length, mandibular length, LAFH) then were obtained directly from the tracing with a digital caliper.

Three-dimensional Acquisition: The same subjects were positioned in the CBCT machine (iCAT, Imaging Sciences International, Hatfield, PA. 120 kVp, 18.66 mAs) with the aid of guiding lights, with the FHP parallel to the floor and the midsagittal plane passing through Glabella. A head strap rather than the chin rest was used to stabilize the patients head during the examination to prevent distortion of the soft tissue profile and changes in mandibular position. The CBCT machine was set for a 20-second acquisition time with a 9 inch field of view to minimize radiation exposure (slices were reconstructed with 0.4 mm increments and 0.1 mm interval).

The raw data from the CBCT scan were reconstructed, coded and converted into a Dicom3 file format using the native iCAT software. The Dicom3 files then were imported to software (Mimics 8.13, Materialize Co., Leuven, Belgium) for assessment.

A number of points and measurements were derived with the assistance of a software, so as to produce an individual analysis. A list and description of the points and measurements used in this analysis is provided in Table 2. Table 2. 3D Composite Norm points Description

Axial: Most external in the mandibular symphysis external cortical plate

The points were marked using the 2D multi-planar reconstruction (MPR) images (axial, sagittal and coronal slices) according to their descriptions. It is important that the points satisfy all of the description requirements in all three planes of space at the same time. The measurements were calculated by computer and displayed in a separate window. Method

In one aspect of the present invention, we have developed a method for providing the translation of the measurements taken from lateral radiographs to those taken from CBCT scans. This translation is enabled by using two of the points chosen for the analysis of Condylion Right and Condylion Left (CoR and CoL) which are located at the central focus of the X-ray beam, where the effects of magnification are negligible; the other points are located at the midsagittal plane where the magnification is determined. The magnification, however, varies depending on the plane where a given structure lies (Broadbent, BH, 1931 , Ahlqvist, J et al., 1986). Based on these premises, a right triangle can be drawn with sides described as follows:

The Hypotenuse (H): represents the 3D measurement of either the mandibular length (Co-Gn) or the midfacial length (Co- point A).

The Condylion to Midsagittal Plane Side (Co-MSP): represents distance between Condylion and the midsagittal plane.

The Projection Side (PS): represents the projection of the 3D measurement on the midsagittal plane. The Projection Side (PS) is equal to the measurement from a lateral cephalogram (LC), reduced by the magnification.

The hypotenuse is obtained if both the Projected Side and the Condylion to Midsagittal Plane distance are known, using the Pythagorean Theorem. If the Condylion to Midsagittal Plane distance is not known, the hypotenuse is obtained easily by dividing the Projected Side by the cosine of the angle "X" between the hypotenuse and the Projected Side. By simple algebraic calculations, is achieved to translate the measurements from a lateral cephalogram to those obtained from a 3D CBCT image. 3D Measurement = [(Ceph Measurement) - (Ceph Magnification)] ÷ [cosine (X)]

Measurements on the midsagittal plane are obtained simply by reducing the magnification since the cosine of zero is equal to one. Data Analysis

All data are exported to statistical software (SPSS®, version 14) for statistical analysis. The data consist of four groups, each containing three measurements (midfacial length, mandibular length, and LAFH) of the same 13 individuals:

1. The Ceph Group: measurements directly from the cephalogram.

2. The Magnification Group: measurements from the cephalogram reduced by the magnification.

3. The Method Group: the measurements from the cephalogram corrected for magnification and distortion using the method.

4. The CBCT Group: measurements directly from the CBCT scan.

The mean values of the right and left midfacial and mandibular lengths on the CBCT of each patient is used to match all the other groups that used cephalometric measurements and therefore are derived from the mean measurements of those bilateral anatomical structures on the lateral radiograph. Method Error

To assess error, all measurements are repeated within a one month interval. The intraobserver variability is 0.5 millimeters for the cephalometric measurements and 0.2 mm for the CBCT measurements according to Dahlberg's formula: V∑D²/2N. Statistical Analysis

The mean values of the four groups tested for the three measurements are compared using a Repeated Measures Analysis of Variance (R-ANOVA). Post-hoc comparisons of means are carried out using the Bonferroni correction for multiple comparisons. The power analysis determined that there is 99% power to detect a difference greater than 0.5 millimeter within the four groups of repeated measures, for each one of the three measurements, with a sample of 13 subjects.

All results reported are based on the post-hoc pair-wise comparisons of means from the repeated measures ANOVA with a 0.05 level of significance. The measurements of lower anterior facial height, midfacial length and mandibular length derived from a conventional lateral cephalogram are significantly different (all p < 0.01 ; Table 3) from the 3D measurements on a CBCT (mean difference of 6.8 mm; 1.2 mm and -4.3 mm, respectively; Table 3). These differences constitute an error when using 2D cephalometric norms for 3D measurements.

Table 3. Descriptive statistics pair-wise comparisons for each measurement, using Bonferroni correction.

Group Comparisons Measurement Mean SE Sig. Min. Max.

Diff.

Ceph Group vs. CBCT Group LAFH 6.8 0.2 <0.01 6.1 7.5

Ceph Group vs. CBCT Group Mandibular 1.2 0.2 <0.01 0.5 1.9

Length

Ceph Group vs. CBCT Group Midfacial Lenght -4.3 0.4 <0.01 -5.5 -3.1

Magnification Group vs. Ceph LAFH 6.7 0.1 <0.01 6.3 7.2

Group

Magnification Group vs. Ceph Mandibular 12.7 0.2 <0.01 12.2 13.2

Group Length

Magnification Group vs. Ceph Midfacial Lenght 10.3 0.1 <0.01 10.1 10.5

Group

Magnification Group vs. CBCT LAFH -0.1 0.2 >0.99 -0.6 0.5

Group^*

Magnification Group vs. CBCT Mandibular 11.5 0.3 <0.01 10.6 12.4

Group Length

Magnification Group vs. CBCT Midfacial Lenght 14.6 0.4 <0.01 13.4 15.8

Group

Algorithm Group vs. Ceph Group LAFH 6.7 0.1 <0.01 6.3 7.2

Algorithm Group vs. Ceph Group Mandibular 0.8 0.1 <0.01 0.4 1.3

Length

Algorithm Group vs. Ceph Group Midfacial Lenght -4.8 0.2 <0.01 -5.5 -4.1

Algorithm Group vs. CBCT LAFH -0.1 0.2 >0.99 -0.6 0.5

Group

Algorithm Group vs. CBCT Mandibular -0.4 0.2 >0.50 -1.0 0.3

Group Length

Algorithm Group vs. CBCT Midfacial Lenght -0.5 0.2 >0.16 -1.2 0.1

Group * The mean difference is significant at the .05 level.

Correcting only for the image magnification and not for the image distortion does not translate the 2D linear measurements (midfacial length and mandibular length) taken from a conventional lateral head radiograph and a 3D linear measurement taken from a CBCT scan (mean difference of -11.5 mm and -14.6 mm respectively; P < 0.01 ; Table 3), unless the structures from which the distance will be measured are located in the midsagittal plane, as for example LAFH (mean difference of 0.1 mm; P >0.99; Table 3).

All the measurements (LAFH, midfacial length and mandibular length) are corrected with great accuracy with the proposed method (mean difference of 0.1 mm; 0.5 mm and 0.4 mm respectively; P > 0.99; P > 0.16 and P > 0.50, respectively; Table 3).

There is a high correlation (R² = 0.97) between the lateral head film measurements and the CBCT measurements; the correlation is reduced slightly (R² = 0.96) if only the magnification is corrected. On the other hand, there is a slightly higher correlation (R² = 0.99) when the method is applied.

The 3D analysis presented in this invention is used as a diagnostic tool that further enriches the information gathered from 3D imaging examinations due to the availability of normal values for its measurements. The knowledge accumulated during decades of assessing craniofacial growth and diagnosing orthodontic patients must be borne in mind to avoid unnecessary mistakes with this new methodology.

Even though many of the limitations of the 2D cephalometric method are eliminated by the 3D method, Baumrind and Franz (1971) have proposed several ways for correcting intrinsic 2D cephalometric methodological errors that still are pertinent to the 3D methodology. The first (and most important) is to describe the points used clearly and thoroughly, leaving little or no latitude for personal interpretation. A 3-dimensional analysis requires a precise description of each point in all planes of space (Table 2). The second way is choosing the points to be identified carefully, abandoning those more difficult to identify and/or describe.

The method of the invention is used to translate existing 2D cephalomethc norms on and off the midsagittal plane into 3D Norms, so they are compared to the 3D measurements that are made more accurately and individually for the right and left sides of the maxilla and mandible.

Taking 3D measurements directly from the 3D examinations has marked advantages over other methods proposed recently in the literature, such as using the 3D scans to synthesize a 2D image similar to that produced by radiographs and performing the cephalometric analysis on this image that has, again, all the characteristics and limitations of a traditional radiographic examination.

The mean difference between mandibular length measured on a conventional radiograph and the same measurement on a CBCT is relatively small (1.2 mm); however, this difference still is statistically and clinically significant. This relatively small mean difference is due to the magnification increase that partially compensates the reduction that occurs when the mandible is projected on a 2D film.

Those skilled in the art will readily appreciate the present teachings, and will be able to embody them in different ways from the ones herein exemplified. Such alternative ways are to be considered within the scope of the appended claims.

Claims

ClaimsMETHOD FOR PROVIDING INDIVIDUALIZED NORMATIVE VALUES FORCEPHALOMETRY

1. Method for providing individualized normative values for cephalometry characterized by comprising the following steps:

(iv) selecting two (A and B) of the points which are located at the central focus of the X-ray beam or which the distance from the central beam is known; (v) selecting one additional point (C) where the magnification is determined or known; (vi) drawing a right triangle with sides described as follows:

- the Projection Side (PS) represents the projection of the 3D measurement on the plane where C lies. The Projection Side (PS) is equal to the measurement from a lateral cephalogram measurements (LC), reduced by the known magnification (distance between D and C); (vii) obtaining the angle "X" between the hypotenuse and the Projected

Side; and

(viii) dividing the difference between the image measurement and the image magnification (namely PS), by the cosine of X, thereby translating the measurements from a lateral image into those obtained from a 3D image.