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Imaging Modality Review for Orthopaedic Clinical Research

Since CT measurements are most often derived from differences in tissue density, this imaging modality is best for detecting and visualizing denser tissue such as cortical and trabecular bone. This makes CT ideal for orthopaedic studies seeking to quantify new bone growth on or around an implant or fracture site. However, with the help of an injected contrast agent, CT can also be used to visualize soft tissues (e.g., vasculature). Furthermore, due to the density based attenuation of x-rays, CT systems are generally “calibrated” to a known scale (Hounsfield Units) for comparison of images and resultant segmentation across time-points, patients, scanner vendors and study sites.

Radiography (X-ray)

X-ray imaging is a 2D single-planar imaging modality most frequently used for visualizing skeletal pathologies and gross soft tissue pathologies. Like CT, its use is accompanied by ionizing radiation, however at a much lower dosage. Unlike CT, data is “integrated” through the patient for a given view (usually A-P or anterior-posterior or lateral). Rather than visualizing a plane or slice of a given thickness, x-ray provides an intensity projection without depth discrimination.

Each x-ray technique has its pros and cons. For example, computed radiography (CR) is an improvement upon the old manual chemistry-based approach to acquiring and developing an x-ray image via film. By combining digital acquisition and a more automated approach to chemical-based film development (e.g., CR read), the end-user is no longer directly exposed to any chemical and the final image product is in digital format. Digital radiography (DR), like CR, is a form of digital x-ray, operating much like a hand held digital camera directly detecting x-rays without the need for chemical conversion. Dual-energy X-ray absorptiometry (DEXA) uses two x-ray beams to measure bone mineral density, and is most often used in diagnosing osteoporosis. Fluoroscopy, a low-dose x-ray, is used commonly for real-time imaging of various internal anatomical structures (e.g., during surgery), implant insertion, etc.

The biggest advantages of x-ray are time and cost. The scans are relatively cheap and take only seconds to acquire. This modality also offers higher spatial resolution than CT or MR. There are, however, several negatives in addition to the presence of ionizing radiation. First, since you are looking through a patient when viewing an x-ray, it’s difficult to distinguish spatial relationships. Second, the inherent variability in patient positioning in a clinical study utilizing multiple x-ray time points can make it difficult for a human image reader to extract reliable quantitative measurements over time. Third, due to the lack spatial orientation/depth, magnification markers must be used to standardize image measurements since patient proximity to the detector will affect the resolution of the image. As a result of the complexity associated with patient positioning, the importance of a well-written and validated x-ray IAP cannot be understated. This is particularly true in a study involving multiple study sites where “routine” techniques employed by imaging staff may vary significantly.

X-ray is best used when detailed 3D information is unnecessary and cost and time are big factors. X-ray, even more so than CT, is widely used in orthopaedic clinical research. In particular, x-ray is most commonly used in evaluating component/bone interfaces for osseointegration or loosening, fracture healing or bone loss/remodeling due to osteoarthritis, injury or cancer metastasis. When patient positioning is standardized and magnification markers are appropriately used, quantitative measurements for component evaluations or various pathologies can be made fairly reliably.


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Brian Sabb

10/12/2012 11:29 PM

This is nice article summarizing the modalities available to us on a clinical & research basis.

I would like to point out that in particular MRI & US research & imaging would benefit greatly by having an expert rad with whom to consult. The article mentions need for a "trained operator and an MR physicist", but the clinical perspective of an expert MSK radiologist is also important.

In regards to US: Few radiologists have experience with MSK US. It is a difficult skill to learn & an even harder one to master. During my training at Thomas Jefferson University & my time as faculty at University of Michigan I had seen various good & "not so good" applications of MSUS. It can be & is making its way into research projects & peer reviewed publications. But once again, supervision by an expert imager familiar with MSK US is critical to the success of such research.


Brian Sabb