Soft Tissue Anatomy on MRI

Soft tissue anatomy on MRI describes how skeletal muscle, fat, fascia, and other non-osseous structures of the locomotor system appear on magnetic resonance imaging. MRI's high soft-tissue contrast makes it the principal modality for these structures, with muscle, fat, and fluid each occupying a characteristic place on the spectrum of T1- and T2-weighted signal intensity.

Definition

Soft tissue anatomy on MRI is the systematic description of the normal magnetic-resonance signal and morphology of skeletal muscle, fat, fascia, and related non-osseous musculoskeletal structures.

Scope

The topic covers the normal MRI signal characteristics of skeletal muscle, subcutaneous and intermuscular fat, fascial planes, and the way muscles are arranged into compartments, together with the appearance of bone marrow. It is an anatomical reference and does not provide criteria for diagnosing muscle injury or soft-tissue masses.

Core questions

• What is the normal signal intensity of muscle, fat, and fluid on T1- and T2-weighted images?
• How are fascial planes and muscle compartments delineated on MRI?
• How does normal marrow signal vary with age and conversion from red to yellow marrow?

Key concepts

• T1- and T2-weighted signal intensity
• Intermediate signal of normal skeletal muscle
• High T1 signal of fat and fat suppression
• Fascial planes and muscle compartments
• Pennation and muscle architecture
• Red-to-yellow marrow conversion
• Normal accessory muscles as variants

Mechanisms

MRI signal reflects proton density and the T1 and T2 relaxation times of tissues. Normal skeletal muscle has intermediate signal, lower than fat on T1-weighted images and lower than fluid on T2-weighted images, so it is naturally framed by the high T1 signal of surrounding fat (Murphy, 1986). Fat-suppression techniques null the fat signal to make oedema and other fluid-rich changes conspicuous. Fascial planes and the fat between muscles allow individual muscles and compartments, whose gross anatomy is defined in systematic texts, to be distinguished on cross-sectional images (Standring, 2020; Manaster et al., 2013).

Clinical relevance

Recognising the normal MRI appearance of muscle, fat, and fascia is the basis for identifying departures from normal in sports medicine, orthopaedics, and radiology, including muscle oedema or fatty replacement. This entry describes normal anatomy and signal characteristics for reference and is not a basis for diagnosis or treatment.

Evidence & guidelines

Normal soft-tissue MRI anatomy and signal characteristics are documented in modality reviews (Murphy, 1986) and in cross-sectional imaging atlases and systematic anatomy references (Manaster et al., 2013; Weir et al., 2017; Standring, 2020), rather than in interventional trials.

History

Clinical MRI of the musculoskeletal system developed in the early-to-mid 1980s, and descriptions of the normal and abnormal signal of skeletal muscle date from that period (Murphy, 1986). Higher field strengths, surface coils, and fat-suppression sequences progressively improved the depiction of muscle architecture, fascial planes, and marrow.

Seminal works

• murphy-1986
• manaster-2013

Frequently asked questions

Why does fat look bright and muscle grey on a standard MRI?

On a T1-weighted image fat has a short T1 and therefore high (bright) signal, whereas muscle has intermediate signal; this contrast outlines individual muscles against the surrounding fat.

What does fat suppression do on a musculoskeletal MRI?

Fat-suppression techniques null the normally bright fat signal so that fluid-rich tissue, such as oedema, stands out; it changes how normal fat appears but is used to make abnormalities more conspicuous.