Diffusion-weighted imaging (DWI) is an MRI technique that has shown to be a very promising in facilitating the differentiation of subtle pathological changes within different tissue. However, to date, a conclusive physical basis for the signal contrast on DWI of the Achilles tendon (AT) is yet to be available in literature. The purpose of this work is to evaluate the diagnostic impact of echo planar DWI in distinguishing normal and abnormal AT using ADC mapping as a qualitative and quantitative assessment tool.
Initial scanning was done to enable DWI sequence optimization for application on the AT. This was then tested on 19 healthy volunteer which were scanned using the optimized DWI sequence. An expert musculoskeletal radiologist reviewed and scored the resultant MR images. OSIRIX software was used to generate ADC maps and values for the healthy 19 ATs to generate a threshold ADC for the symptomatic phase.
Symptomatic patient scanning phase involved the scanning of 28 patients referred for AT MR scanning. For each subject the left and right AT (n = 74) was scanned. Symptomatic patients were scanned at initial symptom onset, after 4 to 8 weeks and 16 weeks using routine and DW-Echo Planner Imaging (EPI) sequences. Expert radiologists qualitatively assessed the images, and ADC values were evaluated at different diffusion weightings (b-values = 0, 40, 273, 800 s/mm2). ROI coordinates (10 pixels) were determined using OsiriX software. Statistical analysis was then performed using SPSS and MAGIC software.
In the initial phase, DWI's scored a visibility rate of 79% (diagnostically acceptable). The normal AT displayed low signal intensity in ADC maps (mean 0.091 × 10-3 mm2/s and 95% CI: 65.92% to 83.99%).
Symptomatic patient's findings correlated with routine MR images. High signal changes were found in complete and partial tears, and low signal changes were dominant in healthy and inflamed (e.g. thickened) ATs. Quantitative DW-EPI demonstrated differences between AT pathologies, the ADC values measured were highest in complete and partial tears (mean 1.52 × 10-3, SD 0.4646). Inflammation and healthy ATs returned mixed values characterised by lower ADC measurements. Repeated ADC measurements of patients at imaging time intervals identified changes in ADC values as healing progressed.
DW-EPI sequence acquired at 3T generated a number of artefacts due to differences in the fat-to-water ratio, which could the efficacy of magnetic field shimming. Despite this, visibility of the AT and other ankle joint anatomy in the DW-EPIs were not compromised. DW-EPI differentiated AT related pathologies and indicated the severity of specific pathological changes over time.