What is ProScan’s rule of heel/ankle MRI?

This 62-year-old has a heel ulcer (blue arrow). You are shown sagittal or lateral T1 fat-weighted (image 1) and T2 water-weighted (image 2) magnified pictures of the heel. This is followed by an axial T1 MRI (image 3) and a sagittal water-weighted SPIR (image 4). Is the edema reactive from soft tissue infection or invasive osteomyeltiis?

Sagittal T1

Sagittal T2

Axial T1

Sagittal T1

Edema has various patterns that allow its use as a diagnostic tool. For instance, subcortical circumferential peripheral edema is seen in complex regional pain syndrome (Type I), formerly known as RSD. Edema can also be reactive from arthritis next to it. On the other hand, when combined with other sequences, certain signs point to direct involvement/invasion of bone as in tumor destruction or bone dissolution in osteomyelitis.

The edema in this case is related to invasive osteomyelitis. Look at the green arrows in images 1 and 2. A crisp, linear, black line of preserved cortex is seen on the upper margin of the calcaneus. Now look at the orange arrows, especially on the T1 fat-weighted images (images 1 and 3). The cortex is blurred, erased, lost or fuzzy. It is also irregular and ill-defined on the sagittal T2 (image 2, orange arrows). It’s the “cortical erasure sign” of osteomyelitis.

Remember ProScan’s rule — If it is not visibly destructive on the T1 MRI, even if there is profound edema on water-weighted sequences, it is not invasive osteomyelitis. But, this one is positive. The T1 weighted image has high specificity for osteomyelitis. For more case review, visit MRI Online.

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