MRI soft-tissue mass — characterisation and compartmental mapping

Centred on the skin marker over the palpable lesion, with the whole lesion, the entire involved compartment, the adjacent joint and the neurovascular bundle within the field of view; a large-field-of-view localiser through the whole limb segment is included so the lesion can be sited anatomically for the surgeon.

Typical, not policy

Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.

When to use it

Any soft-tissue lump that is deep to fascia, larger than about 5 cm, growing, or painful — the referral thresholds at which sarcoma has to be excluded rather than considered.
An indeterminate or incompletely assessed lesion on ultrasound, including any lesion too deep for ultrasound to reach.
Local staging of a known or suspected soft-tissue sarcoma before biopsy and surgery, as the study that defines the compartment and the resection plan.
Suspected recurrence within a treated tumour bed.
Characterisation of a lesion where the differential is fat-containing — lipoma versus atypical lipomatous tumour — which is a question about internal septa and nodules that only MRI answers.

Technique

The core is a T1 without fat suppression and a fluid-sensitive sequence (T2 fat-saturated or STIR) in at least two planes, with the axial plane orthogonal to the limb: the two together are what identify fat, fluid, haemorrhage and solid tissue.
A non-fat-suppressed T1 is essential and is the sequence most often omitted — fat signal, marrow involvement and the fat plane between lesion and neurovascular bundle are all read from it.
A pre-contrast fat-suppressed T1 in the same geometry as the post-contrast series is what makes enhancement assessable; without it, intrinsic T1-bright material such as haemorrhage or proteinaceous fluid is indistinguishable from enhancement.
Gradient-echo or susceptibility-sensitive imaging is added where haemosiderin is suspected, as in tenosynovial giant cell tumour or a chronic haematoma.
The report should state the compartment, the relationship to fascia, and the distance to the nearest neurovascular structure — these are the measurements the surgeon operates from.

Where it goes wrong

Failing to mark the lump: an unmarked small or mobile lesion means the study is centred elsewhere and reported as normal.
Calling a haematoma. A soft-tissue sarcoma bleeds and presents as an apparent post-traumatic haematoma, so a "haematoma" that does not resolve on interval imaging must be treated as a tumour until proven otherwise — this is a well-recognised cause of delayed sarcoma diagnosis.
Assuming a fat-containing lesion is a simple lipoma. Thick septa, nodular non-fatty components, large size and a deep location distinguish an atypical lipomatous tumour, and that distinction is the whole reason for the study.
Omitting the pre-contrast fat-suppressed T1 and then reporting enhancement that is actually intrinsic T1 signal.
A field of view restricted to the lump alone, which gives no compartmental information and forces a repeat before surgery.
Biopsying through an unplanned track before the sarcoma service has seen the imaging, which contaminates compartments and can convert a limb-sparing resection into an amputation.

Clinical questions that reach this study

Soft tissue mass — characterisation

Contrast

NoneGadolinium, intravenous

Where contrast is used, a macrocyclic gadolinium agent at a typical single dose of about 0.1 mmol/kg with a saline flush, and fat-suppressed T1 acquired in at least two planes afterwards, matched to the pre-contrast fat-suppressed T1 geometry so that enhancement can be judged against it rather than guessed.

Contrast is not routine. A lesion that follows fat on every sequence and suppresses completely, or a simple ganglion or cyst, is characterised outright without it.
Contrast is added where the lesion is indeterminate, where the question is solid versus cystic or necrotic, where a biopsy target needs identifying within a heterogeneous mass, or where recurrence is being sought in a treated tumour bed.
Where a dynamic acquisition is added to separate viable tumour from post-treatment change, the series starts at injection and samples every few seconds for the first 1-2 minutes: it is the early upslope that discriminates, and a single post-contrast acquisition at 3-5 minutes does not.

Acquisition

Breathing: Free breathing, with respiratory compensation where the lesion is on the trunk or chest wall.
Preparation: Mark the palpable lump with a skin marker before scanning — an unmarked small lesion is the commonest reason the study is centred on the wrong place. Give the size, duration, growth and depth relative to fascia, and whether the lesion is painful: a deep lesion above 5 cm or one that is growing is a sarcoma referral question, not a reassurance question. Do not biopsy before imaging where sarcoma is a possibility — a biopsy track placed outside the eventual resection plan compromises limb-sparing surgery, so imaging and biopsy should be planned together by the sarcoma service. Supply any prior ultrasound; this study is usually the escalation from an indeterminate or deep lesion on ultrasound.

Safety checks this protocol carries

Derived from the contrast agent and phases above, not authored here — which is why they cannot drift apart from what the protocol actually does.

MR safety screening for implants and foreign bodies· radiographer at scan