Fast and radiation-free technology
for spine and pelvis analysis

Jean-Pierre Gibeault, P. Eng.

Introduction

After more than 20 years of research in European universities, the technology of body surface measurement by rasterstereography combined with biomechanical modeling techniques has produced a device for the fast, contactless and radiation-free evaluation and analysis of the spine and pelvis. The German device is used by universities, research clinics and, primarily, in private chiropractic, orthopedic and dental clinics for diagnostics and for specific and interdisciplinary therapies. With the first units produced for commercial use in 1996 by DIERS International GmbH, this technology, the DIERS Formetric, is now in its 4th generation; 490 units have been produced commercially and the majority of these are used in Germany. The high accuracy of this technology used on scoliosis patients was clinically confirmed by Drerup et al (1997) and compared with customary thorax radiographs by Hackenberg et al (2003).

Origin of technology and scientific foundation

The topics of spinal deformity from back shape, shape analysis of the body surface and techniques of body surface measurements have been researched extensively at many universities and research centers.

The objective driving the early days (1980) of this research was the development of technologies and devices that would be complementary to radiology for the evaluation and monitoring of patients suffering from idiopathic scoliosis. The need for frequent follow-up evaluations during therapy and the necessity to greatly reduce the overall radiation load for the duration of the therapy were strong motivators to develop accurate and reliable devices.

Among many research centers, the University of Münster (Germany) and its Institute for Experimental Biomechanics and the University Orthopedic Clinic continuously develop a large body of research on these topics; research papers have been, and still are, produced by many of its professors, researchers and doctoral students. An extensive bibliography, which is beyond the scope of this article, is available.

In the early 2000, the DIERS company, in association with the University of Münster and the KU University in Belgium, has further developed the Formetric technology and produced additional measuring medical devices.

An optical system for spinal and pelvis analysis

One of the ways to meet the objective of developing and producing a radiation-free, fast, contactless and reliable device to complement x-ray measurement system is to use the combination of 3D-shape measurements and biomechanical modeling to reconstruct and display the spine structure and calculate the key spinal and pelvic parameters, as shown in Figures 1 and 2.

Capturing and measuring the dorsal profile (back shape) — Rasterstereography

White Light Raster Line Triangulation (WLRT) enables the scanning of objects in 3D by projecting raster lines on its surface and by capturing these lines under a known and fixed angle with a camera. Figure 3. Based on triangulation algorithms, spatial coordinates of all raster points are calculated, resulting in a dense point cloud of randomly distributed points describing the measured surface. These data points are transformed to a regular grid by using interpolation, which will simplify further calculations. In this way the system captures and analyses a body shape, statically or dynamically.

Rasterstereography excels by its high precision (error ≤ 0.1mm) and allows a radiation-free representation of the profile. For angular data, the reproducibility of an individual rasterstereographic shot is 2.8°. The speed of the measurement is fast at 0.04 seconds and the total dorsal surface is registered simultaneously.

The recognition of the anatomical structure through the automatic identification of anatomical landmarks on the body surface provides the basis for a reconstruction of the 3-dimensional profile of the dorsal surface.

Mathematical construction and display of the spine structure

The aim of capturing, measuring and analyzing the back shape (dorsal surface) is to obtain information about the 3-dimensional shape of the vertebral column.

It is well documented that the vertebral rotation is correlated to the surface rotation and this allows establishing the relationship between the back shape and the shape of the spinal midline. In our case, the surface rotation is measured by the angle of the surface normal (horizontal component). To do this, the high sampling density and resolution provided by rasterstereography is essential.

In the Turner-Smith (Turner-Smith et al.) model, the lateral coordinate of the vertebral bodies can be calculated from the line of the spinous processes by adding an extra displacement produced by vertebral rotation. In this model, the vertebral rotation is assumed to be proportional to the mean surface angle in the central region (K=2.5).

In Figure 4 the vertebra is assumed to be undistorted. This is generally justified in minor to medium curves. The model assumes only that the surface normal points to the vertebral body centre. Therefore the model is virtually independent of vertebral distortion.

The calculated construction of the spinal midline requires three inputs:

1. The line of the spinous processes;
2. The surface rotation at the locus of the line of the spinal process;
3. The anatomical landmarks needed for reference to the underlying skeletal structure.

Curvature analysis is combined with an algorithm for data smoothing and calculation of the surface normals. As a by-product, the original measurement points are transformed into a regular square grid over the frontal (x-y) plane. The result of this procedure is presented in Figure 4.

1. The line of the spinous process

The line of the spinous processes is estimated by the symmetry line of the back. The symmetry line (solid line in Figure 5) is composed of the symmetry points of the horizontal profiles. A symmetry point, in turn, is defined by that point which divides the profile into two halves with minimum lateral symmetry (with respect to surface curvature). For the model, we assume the symmetry line to be a representation of the line of the spinous processes and it is a generalization of the medial sagittal profile.

2. Surface rotation

As mentioned above, we measure surface rotation by the horizontal component of the direction of the surface normal. From any grid point in Figure 5, components of the surface normal are known from curvature analysis. On the symmetry line these values are calculated by interpolation. In Figure 5, the surface normals are represented by bars erected on the symmetry line. As the results show, it is reasonable to assume that the horizontal component of the normal angle is equal to vertebral rotation.

3. Anatomical landmarks

An automatic recognition of four anatomical landmarks (vertebra prominens (VP), sacrum point (SP), right crista iliaca posterior superior (DR), and left crista iliaca posterior superior (DL) by means of the connected software provides the basis for a reconstruction of the three-dimensional profile of the dorsal surface.

The landmarks are used for skeletal reference of the surface data. In particular, the vertebra prominens landmark is used as the origin of a body-fixed coordinate system both for radiography and for surface measurements. Furthermore, trunk length, trunk imbalance, pelvis inclination, and similar parameters may be determined from these landmarks. In Figure 5 the landmarks are represented by black dots.

The Formetric 3D/4D

The Formetric, using a fast (40 milliseconds) high-definition optical measurement of the surface of the back of your patient, produces graphical, clinical and analytical information on the spine, the pelvis and the posture.

It takes only 40 milliseconds to do a scan and the results are available immediately on your screen and printed on the spot.

Ideal for chiropractors, the Formetric provides clinical information to develop your diagnostics and treatments and to document your treatment outcomes.

The Formetric’s fast, contactless, radiation-free and non-intrusive qualities allow you to do multiple scans of your patients (e.g. before and immediately after) and a typical report will compare four sets of results taken at different dates.

A complement to your radiology equipment, the Formetric does not require any license and special building construction nor any lengthy training for its accurate and repeatable operation. A typical report is shown in Figure 6. Applications for the Formetric 3D/4D The different applications covered by the DIERS 3D/4D Formetric will be presented in part 2 of this article.

Conclusion

This well proven system is an ideal tool for chiropractors as it allows multiple scans of your patients to produce key information on the spine and pelvis complex of your patients. With the Formetric you can first produce an evaluation and decide on your treatment, then you can monitor and document the progress of your patient as shown in Figure 7. You can inform your patient with a document the patient can keep.

JP Gibeault, P. Eng. is the founder and CEO of Biometrix Medica. He graduated from Queen’s University in electrical engineering and has been involved in scientific instrumentation for his entire career. Headquartered in Canada, his companies deliver medical products and equipment in the Americas and Scandinavia.

References

Drerup B, Hierholzer E, Ellger B. — Shape analysis of the lateral and frontal projection of spine curves assessed from rasterstereographs. In: Sevastik JA, Diab KM, eds. Research Into Spinal Deformities. 1st ed. Amsterdam, The Netherlands: lOS Press; 1997:271-275.

Hackenberg L, Hierholzer E, Potzl W, Götze G, Liljenqvist U. — Rasterstereographic back shape analysis in idiopathic scoliosis after posterior correction and fusion. Clin Biomech.2003;18:883-889.

Hackenberg L, Hierholzer E, Potzl W, Götze C, Liljenqvist U. — Rasterstereographic back shape analysis in idiopathic scoliosis after anterior correction and fusion. Clin Biomech. 2003;18:1-8.

Turner-Smith AR, Harris JD, Houghton GR, Jefferson RJ. — A method for analysis of back shape in scoliosis. J Biomech. 1988;21(6):497-509.