• Charcot foot
• disease activity
• DWI/ADC

Objective
To evaluate the role of MRI, specifically Diffusion Weighted Imaging (DWI) and Apparent Diffusion Coefficient (ADC) maps, as a biomarker related to clinical parameters of activity in patients with Charcot neuro-osteoarthropathy (NOA) and thus, as a tool to assess response to treatment and to predict time to stabilization.
To investigate the role of ADC maps as a support tool for differential diagnosis between neuro-osteoarthropathy and osteomyelitis (OM), identifying cut-off ADC values between the two conditions.

Material and Methods
We retrospectively selected 79 patients from 2011 to 2021; all patients underwent clinical examination and MRI scan at diagnosis and during follow-up.
MRI protocol included T1-weighted and fast spin echo inversion recovery (FSE-IR), and DWI sequences (b values: 0 and 800). ADC maps were calculated, and Regions of Interest (ROIs) were set on the ADC maps respectively on the bone with the highest and the lowest signal intensity (SI) on DWI and FSE-IR images.
SI Ratio and ADC Ratio were calculated using the following formula: (FSE-IR affected bone-FSE-IR unaffected bone)/ FSE-IR unaffected bone.

Results
Study population included 79 patients (48 Males and 31 Females) with mean age of 58.73 years. At baseline MRI, 50/79 patients had active NOA, 21/79 had osteomyelitis (OM) while the remaining did not have significant BME (9/79). One patient had active NOA with superimposed OM.
Twenty-eight out of fifty patients with active NOA (28/50) had a follow-up MRI after off-loading treatment, twenty-one were lost at follow-up (21/50), while one (1/50) underwent major amputation.
Mean ΔT in patients with active NOA at first clinical examination was 1.36 °C (range 0-7 °C; SD 0.18) and it was significantly related to baseline ADC values on BME (p=0.026).
At baseline MRI, we found a statistically significant difference between mean ADC values on bone marrow edema in the group with active NOA with respect to the OM group (t-student, p= .000), with a cut-off ADC value of 1.29 m2/sec between NOA and OM groups at ROC analysis (AUC 0.891).
No statistically significant difference in mean FSE-IR values was found between the NOA and OM groups (Mann-Whitney; p=0.292).
At follow up MRI, 11 patients (11/28) had reached clinical stabilization.
Mean ADC value on BME at follow-up was 0.87 m2/sec (SD 0.56) and it was significantly lower than baseline ADC values (p=.000) with a mean percentage reduction of 55%.
The difference in ADC values on BME between baseline and follow-up MRI was significantly related to time to stabilization (p=0.05).

Conclusion
The results suggest that MRI, through DWI and ADC, can quantitatively assess bone marrow edema and thus disease activity and treatment response in NOA. ADC values are significantly related to the main tool for clinical disease assessment, meant as ΔT, confirming its potential role as a biomarker of activity.