From the Guidelines

The best initial imaging modality for a soft tissue mass on the thigh is magnetic resonance imaging (MRI).

MRI provides improved soft-tissue contrast and multiple-image plane capabilities, offering significant advantages for lesion conspicuity, intrinsic characterization, and local staging 1.
It allows for easy identification and evaluation of vascular structures without the need for intravenous (IV) contrast agents and helps define neurovascular involvement 1.
MRI protocols should include T1-weighted and T2-weighted sequences, utilizing a 1.5 Tesla or 3 Tesla scanner.
A dose of 0.1 mmol/kg of gadolinium-based contrast agent, such as gadoterate meglumine, may be administered intravenously to enhance lesion visualization.
MRI protocols should include axial and coronal sequences, with a slice thickness of 3-4 mm and a field of view of 16-20 cm, to provide optimal characterization of the mass and surrounding soft tissue structures.

While MRI is the main imaging modality for primary soft tissue tumors in the extremities, pelvis, and trunk, standard radiographs may be useful to rule out a bone tumor, detect bone erosion, or show calcifications 1.
Computed tomography (CT) has a role in calcified lesions and in retroperitoneal tumors, where its performance is identical to MRI 1.
Ultrasound may be the first exam but should be followed by CT or MRI 1.

From the Research

Magnetic Resonance Imaging (MRI) is widely used to investigate soft tissue masses due to its unparalleled soft tissue contrast and multiplanar imaging capability 2, 3, 4.
The detection of a soft tissue mass depends on image contrast between normal and pathological tissue, with MRI providing tissue-specific characterization through various imaging characteristics such as signal intensity, relaxation time, and contrast enhancement 2.
Advanced MRI techniques, including diffusion-weighted imaging, dynamic contrast-enhanced perfusion imaging, and magnetic resonance spectroscopy, have improved the evaluation of soft tissue tumors by providing noninvasive assessment of biochemical, structural, and metabolic features 3, 4.
Other imaging modalities, such as radiography, computed tomography (CT), and ultrasonography, play a complementary role in the evaluation of soft tissue masses, with each having its own strengths and limitations 4, 5.
Novel imaging modalities, including positron emission tomography (PET)/CT and PET/MRI, have shown promise in identifying the glucose metabolism of lesions and increasing diagnostic accuracy 4.
Bimodal probes combining MRI and optical imaging techniques have been developed to enhance diagnostic capabilities, with gadolinium-based probes being extensively used for signal enhancement of T1-weighted MRI 6.

MRI is the preferred imaging modality for evaluating musculoskeletal tumors, including soft tissue masses on the thigh, due to its high tissue contrast resolution and multiplanarity 2, 3, 4.
Ultrasonography can be useful for evaluating superficial soft tissue masses and fluid collections, but may have limitations in differentiating between certain types of masses, such as hematomas and sarcomas 5.
CT and radiography can provide complementary information, particularly in evaluating cortical osseous lesions or calcifications, but are not the primary imaging modalities for soft tissue masses on the thigh 4.