Weight-bearing CT imaging has introduced new aspects to the diagnosis of foot and ankle diseases. Since its introduction in 2012, Planmed’s Verity CBCT scanner has been used in challenging imaging cases of the upper and lower extremities.
Planmed has joined forces with Finnish start-up company Disior. The future now looks brighter than ever, as together the two companies promise to change the world of musculoskeletal 3D image analysis.
Cone beam computed tomography (CBCT) is a technology in which a series of low-intensity 2D X-rays are captured with a flat panel detector. The image series captured during a single or partial rotation is next reconstructed to a 3D volumetric image of the region of interest. This step is achieved using a combination of FDK and iterative algorithms.
CBCT is currently the only technology that is able to produce 3D volumetric imaging data of the foot and ankle under real, natural weight-bearing conditions. Combining the weight-bearing imaging with the fast image acquisition, low patient radiation dose and isotropic spatial resolution of 0.2mm allows the client to maintain complete confidence in their diagnostic diagnosis.
Taken to another dimension
Conventional 2D weight-bearing X-ray imaging has long been used as the standard of care to visualise skeletal diseases, and deformities of the foot and ankle. The measurement techniques are well described in the literature. However, 2D data is prone to projection differences and the superimpositions of the bones on one another can mask underlying issues.
3D weight-bearing imaging solves the challenges of projection differences and overlapping structures. The anatomy is shown in a naturally occurring position and can better reveal joint space narrowing and other conditions that may not be visible through conventional diagnostic means.
Adding the third dimension allows the anatomy to be visualised as 2D transversal slices. 2D transverse views enable the manual measurement of foot and ankle diseases and deformities. However, performing accurate and repeatable 3D measurements is not as easy as one might think.
Removing the chance of human error
The Disior InGrid algorithm solves the problems related to 3D measurements of bones and joints. It uses deformable shape models to segment bone tissue from imaging data. Using the 3D image captured with Planmed Verity, the algorithm automatically extracts the landmarks and longitudinal axes needed for the measurements from the segmentation result using robust feature detection algorithms. This results in excellent measurement repeatability, without the steps prone to human error.
Transforming a DICOM image into a mathematical model of the anatomy serves as the basis for an analysis of the anatomy and kinematics. Based on this analysis, any 3D measurement can be taught and automatised in the software. As a result, the industry is no longer dependent on individual slices, but may use the whole data set for analysis. This way, transferring the measurement practices from weight-bearing 2D to 3D is easily achieved.
With the Planmed Verity and Disior InGrid algorithm, one can easily image and analyse common clinical cases in foot and ankle region.
For example, hallux valgus, flat foot and cavus foot are analysed in an unseen way by automatically measuring angle, such as the first and second intermetatarsal angle, or Meary’s angle, from 3D weight-bearing data.
A step in the right direction
Additionally, more complex midfoot and hindfoot alignments and commonly use Saltzman view are easily available from the 3D weight-bearing data.
Combining automatic 3D measurements with quality 3D weight-bearing images has key implications for diagnosing foot and ankle anatomy. It carries great potential in clinical decision-making, surgical planning and post-operative analysis. In addition the outcome of the Disior InGrid processing has excellent quality for 3D printing of anatomic models.