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Imaging Modality Review for Orthopaedic Clinical Research

“Garbage in, garbage out,” goes the saying. An upfront understanding of how imaging modality options best support crucial research data will avoid unnecessary “data garbage” as research is underway. Starting a new clinical study with the end in mind will allow you to select the right imaging modality relative to the data needed to support and defend desired efficacy claims for a given implant.

By matching imaging modality with an image acquisition protocol (IAP) tailored to both the imaging modality and the desired image measurements, clinical researchers may ensure that their study will produce the highest quality images, and thus have the best possible chances for success. If your study is producing sub-par images because you haven’t selected the right imaging modality or properly “tuned” your IAP, chances are the qualitative or quantitative data you plan to extract from those images will be equally lackluster. Any visual data generated can yield valuable marketing collateral, so by defining an appropriate IAP ahead of time, one can reduce expense and time involved in elaborate post-marketing studies, and support communication and defense of product performance claims to key stakeholders (e.g., regulatory, management, customers, patients, etc.).

In orthopaedics, appropriate imaging modality selection is particularly important given the widespread use of imaging in orthopaedic device clinical trials, and the significant impact good imaging data can have on a company’s ability to get regulatory approval and market adoption. With this in mind, we present an overview of the more common (and uncommon) imaging modalities used in orthopaedic clinical research.

Computed Tomography

Computed tomography (CT) uses many 2-dimensional (2D) x-ray projections to generate a 3-dimensional (3D) volumetric image of a given anatomical region. Collectively, these projections are called a sinogram. Fourier filtered back projection algorithms are applied to the sinogram projection data, translating it into 2D slice data which is then rendered into a 3D volume. Typical CT scanners offer out-of-plane (axial) resolution of about 600 um (thin-slice acquisition) and in-plane resolution around 300 um (sometimes as low as 100 um), depending on the limitations of radiation dosage relative to the proximity of the scan to certain highly sensitive organs (e.g., reproductive).

CT imaging offers a 3D view of the anatomical region-of-interest (ROI) without the large amount of distortion present with Magnetic Resonance imaging (MRI) acquisitions and without 2D areal averaging characteristic of x-ray imaging. Spatial relationships between features are not subject to interpretation, as in x-ray images. As such, quantitative measures are multi-dimensional and enable morphological assessment, providing a clinician with more information from which to make a diagnosis. An additional advantage of 3D CT is the reduction in variability associated with inconsistent patient positioning across multiple time points—a major drawback with x-ray imaging.

The time it takes to acquire a 3D CT volume is measured in seconds rather than minutes, with axial resolutions in the sub-millimeter range rather than 3-4 mm for a standard MRI sequence. Thus, patient motion artifact is rarely an issue since acquisitions can be gained within a single breath-hold. With speed and consistency in acquisition, comparison of multiple time points for a single patient (longitudinal assessment) can be made using spatial volumetric registration and image-based segmentation techniques (e.g., bone growth, implant resorption, etc.). This enables accurate quantitative evaluation of treatment/device response over time. Of course, CT is not without its limitations. X-ray metal artifact (stainless steel, cobalt chromium in particular) can make it very difficult to visualize anatomical features near a component (e.g., bone-implant interface) due to reconstruction artifacts, beam hardening, etc. Additionally, high ionizing radiation dosages can preclude frequent imaging time points near or around sensitive organs. It’s also important to note that softer tissues, due to their inability to attenuate x-rays, exhibit less contrast and more noise in a CT scan.