In the literature, the MRI criteria for diagnosing intramuscular blood, whether hematoma or infiltrating hemorrhage, have been confusing.
Blood degrades with time, resulting in changed signal characteristics.
Signal characteristics also vary with field strength. In the literature, the suggested signal characteristics of blood in muscle have been extrapolated from its appearance in the brain. There is no definite proof, however, that blood within muscle has the same signal characteristics as blood in the brain. Although the age of a blood collection determines its signal characteristics on MRI, but more often than not, the age is not known. Intramuscular hematomas and hemorrhages are often encountered on MR examinations without a clear history of trauma.