Ultrasound vs MRI for a soft tissue lump

Most lumps are benign: lipomas (fatty lumps), cysts, ganglia, enlarged lymph nodes and scar tissue are common. Ultrasound can often identify these straight away because it shows the lump in real time and can be pressed gently with the probe to see how it behaves.

MRI is not a better first test for ordinary lumps. It takes longer, costs more in most settings, and a small superficial lump may be fully answered by ultrasound. It becomes the leading test when the lump is hard to characterise or sits deep.

Ultrasound separates a fluid-filled lump (a cyst or ganglion) from a solid one, shows whether a lump has blood flow on Doppler, measures it accurately, and can guide a needle if a sample is needed. It is also easy to repeat if you want to monitor a lump over time.

Its weaknesses are depth and field of view. Deep lumps in the thigh, buttock or pelvis are difficult to see, and the picture depends partly on the operator. It cannot show the full relationship of a large mass to surrounding structures.

MRI gives a three-dimensional, high-contrast view of fat, muscle, fluid and tumour. It can show whether a mass sits within a muscle, wraps around a nerve, or contains fat, blood or fibrous tissue, all of which narrow the diagnosis. For a mass that might need surgery, a surgeon uses MRI to plan the margins.

It is slower, involves lying still in a tube, and is unsuitable with certain implants. Some people find the enclosed space difficult; open or wide-bore scanners and sedation options exist in some units.

• Small, soft, mobile lump under the skin for months, unchanged: ultrasound, and often a clinical diagnosis such as lipoma.

• Lump that feels like fluid or sits next to a joint or tendon: ultrasound.

• Lump that is deep, firm, growing or larger than about the size of a golf ball: an urgent specialist route, with MRI commonly before any biopsy so that the biopsy track does not compromise later surgery.

• Ultrasound cannot classify the lump: MRI is the usual next step.

• Lump in the breast, neck or testicle: these have their own pathways, usually starting with ultrasound plus clinical examination, and are covered by dedicated guidance.

Clinicians work from risk. Features that raise concern include rapid growth, a lump deeper than the muscle fascia, and a size that is large and increasing. NICE suspected-cancer guidance (NG12) advises that adults with an unexplained lump that is increasing in size should be assessed for possible sarcoma, with ultrasound a recognised early test when urgent assessment is not triggered.

The reason for sequencing is practical: avoid a biopsy before the radiologist knows what the mass is, because sampling the wrong way can complicate surgery, and avoid MRI for the large number of lumps that ultrasound can answer in minutes.

It is not true that a painless lump is safe. Some serious lumps cause no pain. It is also not true that a normal ultrasound means nothing needs following up if a lump keeps growing; reassessment is appropriate.

Another myth is that MRI can say definitively whether a lump is cancer. It gives strong clues, but tissue is often required for confirmation.