D. Needell, M.D.
Boca Radiology Group, P.A.
Magnetic Resonance Imaging (MRI) has been shown to outperform conventional radiography for detection of marginal erosions in inflammatory arthropathies (1-5). Early detection and staging of rheumatoid arthritis appears to have significant prognostic implications with disease progression (6-9), and MRI has the potential to become a powerful tool in guiding therapeutic options. Distinguishing small cortical marginal erosions (Fig A) from variant bone anatomy presents one of the most challenging aspects of accurately diagnosing joint erosions by MRI.
Evaluating the metacarpal and carpal bones is challenging because their curved surfaces contain numerous invaginations with regions of synovial infolding. Thin section (1mm or less) MR images are extremely useful in minimizing volume averaging (Figs B,C). Volume averaging results in misrepresentation of anatomy or signal intensity because of adjacent structures blending together in one image. As slice thickness decreases, so do the chances of misinterpreting a lesion due to volume averaging artifact.
GE's Applause (a.k.a. MagneVu MRI) is a portable, in-office MRI machine with protocols tailored for optimal visualization of erosions in the hands, wrist, and feet. The protocols produce small field of view, thin section microanatomy of cortical bone using 3D volume T1-weighted and STIR sequences, which have been demonstrated in the literature to be highly sensitive and specific for detection and characterization of cortical erosions (10-14). 3D T1 and corresponding STIR sequence produce images between 0.6 mm and 1 mm (Figs C,D) which allows more reliable characterization of small erosions than protocols using slices 2 or 3 mm slice thickness, which are more susceptible to volume averaging artifact.
Assessing the chronicity or acuity of an erosion is a valuable tool in staging disease and can assist in guiding therapeutic decisions (6-9). T1W images excel at visualizing bone anatomy and have been shown to be a sensitive sequence for detecting the presence of subacute and chronic erosions. STIR images are the most sensitive sequence for detecting bone marrow edema associated with acute erosions (10-14). Subacute and chronic erosions erosions appear similar in signal intensity on T1W images. On the thin section STIR sequence, subacute erosions are brighter than capsular and synovial tissue (Fig E) and chronic "burned out" erosions appear dark. The thin section STIR sequence is therefore a critical sequence in differentiating subacute from chronic erosions.
In conclusion, the ability to reliably detect and characterize early erosions in rheumatoid arthritis appears to be an effective way of staging disease and planning therapeutic intervention. Thin section (1mm or less) MRI sequences minimize volume averaging which increases specificity of erosion detection. Using a corresponding thin section 3D STIR sequence is an important adjunct to minimize volume averaging artifact and allow effective staging of the chronicity of erosions in rheumatoid arthritis.
Fig A. Erosions along dorsal capsule within lunate and capitate. Note the erosions are brighter in signal than capsular soft tissue. Without thin sections, it is difficult to differentiate such erosions from the joint capsule.
|Fig B. Thin sections of wrist reveal well defined tunnel erosions along the capsular margin of lunate and triquetrum. Thin section T1W and STIR sequences allow differentiation of these erosions from capsular infolding.|
|Fig C. Images of wrist reveal apparent excavation within the capitate on T1W, however the corresponding thin section STIR images reveal this defect to be low intensity, indicating it represents normal variation in bone contour.|
|Fig D. Images of hand reveal bright STIR signal in 3rd metacarpal. When analyzing thin sections before and after this cut, this signal is seen to represent focal synovial/capsular infolding rather than an erosion. Corrobative findings include a thin, low intensity capsule without evidence of thickening or synovitis.|
|Fig. E. Imaging of the hand reveals a scoop erosion in the radial side of the 3rd metacarpal head. STIR images confirm the presence of this erosion. Bright STIR signal suggests its subacute nature.|
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