MRI Soft Tissue Interpretation

Soft tissue MRI interpretation rests on a single foundational principle: knowing what normal looks like on each sequence, so that any deviation can be recognised as pathology. The musculoskeletal soft tissues — muscle, tendon, ligament, nerve, fat and the fibrous capsular structures — each have a characteristic, predictable signal that reflects their water and collagen content. MRI is the dominant modality for these structures because of its unmatched soft tissue contrast, multiplanar capability and absence of ionising radiation.

Three questions answer most soft tissue MRI problems. First, is the lesion fluid, fat, fibrous tissue or something more complex (the four basic signal "building blocks")? Second, where does it sit relative to the deep fascia, and how large is it (the two strongest red flags for a sarcoma)? Third, is an apparent abnormality real, or an artefact such as the magic angle phenomenon that mimics tendon degeneration on short-echo sequences?

This topic builds the interpretation in layers: normal signal, a systematic reporting framework, then the four major pathological domains — tendon, muscle, soft tissue masses, and structure-specific assessment (rotator cuff, Achilles, meniscus). Throughout, the emphasis is on the findings that change management: the gap and retraction that determine reparability of a tendon, the Goutallier grade that predicts rotator cuff repair failure, and the depth/size combination that mandates referral to a sarcoma unit before any biopsy.

A subacute haematoma is a classic tumour mimic - it can be intrinsically bright on T1 (methaemoglobin) and may have an enhancing wall, so knowing how blood signal evolves over time is essential before calling such a lesion a tumour. The signal changes predictably with the oxidation state of haemoglobin:

Beyond the conventional sequences, two functional techniques increasingly support soft-tissue mass characterisation, biopsy targeting and treatment-response assessment - and examiners now expect awareness of both.

A mass arising from a peripheral nerve has a characteristic constellation of signs, and recognising it both makes the diagnosis and warns against an ill-judged biopsy through a functioning nerve.

A schwannoma typically sits eccentric to the nerve (so it can sometimes be shelled out), whereas a neurofibroma is fusiform with the nerve passing through it (harder to separate). The major concern is the malignant peripheral nerve sheath tumour (MPNST): suspect it when a nerve lesion is large (over about 5 cm), rapidly enlarging, heterogeneous with necrosis, ill-defined, or has peritumoral oedema, especially in a patient with neurofibromatosis type 1. Imaging cannot always separate an atypical neurofibroma from an MPNST, so FDG-PET and image-guided biopsy planned with the sarcoma team are used.

A reproducible search pattern prevents the two classic errors: satisfaction of search (stopping after the obvious finding) and over-calling artefact as disease. The framework below works for any soft tissue study and maps directly onto how examiners expect a candidate to present a film.

Soft tissue lesions are best approached by their dominant MRI signal pattern rather than by clinical suspicion alone. The table below groups the common differentials by the pattern that first catches the eye, with the discriminating feature that separates them.

Soft tissue MRI interpretation is broadly consistent worldwide because it rests on physics-based signal characteristics, but referral pathways, access and protocol emphasis vary by health system and resource setting.

The interpretive frameworks used in soft tissue MRI are underpinned by classification and validation studies. According to PubMed, the following are the landmark and supporting references.