The muscles of the thigh are essential to human mobility, enabling actions from standing to running. To visualize the structures within the thigh, medical professionals use Computed Tomography (CT). This imaging technology provides a detailed window into the anatomy of the thigh’s muscle groups, offering insights for diagnosing and understanding various conditions affecting this area.
Fundamentals of Thigh CT Imaging
A Computed Tomography (CT) scan is a medical imaging technique that uses a series of X-ray images taken from different angles around the body. A computer then processes these images to create cross-sectional views, or “slices,” of the bones, blood vessels, and soft tissues.
CT scans can be presented in several imaging planes to offer unique perspectives of the anatomy. The axial plane provides a top-down view, as if looking at slices of the thigh from the hip down to the knee. The coronal plane offers a front-to-back perspective, and the sagittal plane shows a side-to-side view. For muscle evaluation, the axial view is the primary one utilized as it clearly distinguishes the individual muscles and their relationship to the femur.
The appearance of different tissues on a CT scan is determined by their density, a property known as attenuation. Dense structures, like bone, absorb more X-rays and appear bright white. Tissues with low density, such as fat, appear dark grey or nearly black. Muscle tissue falls in between, appearing as various shades of grey, which allows for clear differentiation from adjacent fat and bone.
Anterior Thigh Compartment on CT
The anterior compartment of the thigh contains the quadriceps femoris, a group of four muscles primarily responsible for extending the leg at the knee joint. These muscles form the main bulk of the front of the thigh. On a CT scan, this compartment is located at the front of the thigh, wrapping around the anterior aspect of the femur.
The four muscles that make up the quadriceps are:
- Rectus femoris: The most superficial and centrally located of the four. It is unique among the quadriceps as it crosses both the hip and knee joints, contributing to hip flexion as well as knee extension.
- Vastus lateralis: Positioned on the outer side of the femur.
- Vastus medialis: Found on the inner side of the femur.
- Vastus intermedius: Situated deep to the rectus femoris, lying directly against the anterior surface of the femur.
The sartorius muscle, the longest muscle in the body, is also located in this compartment and can be seen running diagonally across the top of the quadriceps.
Medial Thigh Compartment on CT
The medial compartment of the thigh is situated on the inner side of the thigh, between the anterior and posterior compartments. This group of muscles is primarily responsible for adduction, which is the movement of the thigh inward toward the midline of the body. On an axial CT scan, these muscles are visible nestled between the quadriceps anteriorly and the hamstrings posteriorly.
The main muscles of the medial compartment include:
- Adductor longus: Located more anteriorly within the compartment.
- Adductor brevis: Also located more anteriorly within the compartment.
- Adductor magnus: The largest of these muscles, lying deeper than the adductor longus and brevis.
- Gracilis: The most superficial muscle of the medial compartment, appearing as a thin strap-like muscle running down the inner thigh.
- Pectineus: Part of this group, though it is typically seen only in the most upper sections of thigh CT scans.
Due to their overlapping nature and similar tissue density, distinguishing between each individual adductor muscle on a CT scan can sometimes be challenging without careful examination of multiple image slices.
Posterior Thigh Compartment on CT
The posterior compartment of the thigh houses the hamstring muscles, a group of three muscles that collectively work to flex the knee and extend the hip. These muscles are located at the back of the thigh, behind the femur and the adductor magnus muscle. On an axial CT image, this compartment is clearly delineated by surrounding fat planes.
The three muscles that form the hamstrings are the biceps femoris, the semitendinosus, and the semimembranosus. The biceps femoris is located on the lateral side of the posterior thigh. It has two parts, a long head and a short head, which can sometimes be distinguished on high-resolution scans.
On the medial, or inner, side of the posterior thigh are the semitendinosus and semimembranosus muscles. The semitendinosus is situated on top of the semimembranosus, making it the more superficial of the two. These two muscles lie side-by-side with the biceps femoris as they travel down the back of the leg toward the knee.
Identifying Common Thigh Pathologies on CT
CT scans are not only for viewing normal anatomy but are also effective at identifying various pathologies within the thigh muscles. Abnormalities often present with distinct changes in tissue density and structure. For instance, a muscle hematoma, which is a collection of blood within the muscle often caused by trauma, will appear as a fluid-filled area with a higher density than surrounding muscle tissue.
Muscle tears or strains, common injuries in athletes, can also be visualized on CT. A tear may appear as a disruption of the normal muscle fibers or as an area of low density and swelling within the muscle belly. In some cases, a complete rupture will show a clear gap between the torn ends of the muscle. These findings help in determining the severity of the injury.
Another condition that can be identified is muscle atrophy, which is a decrease in muscle mass. On a CT scan, atrophied muscles will appear smaller than their healthy counterparts. This reduction in size is often accompanied by fatty infiltration, where fat tissue replaces muscle tissue, leading to a darker, less dense appearance of the muscle on the scan. An abscess, or a localized collection of pus, will typically appear as a well-defined, rim-enhancing fluid collection after the administration of intravenous contrast material.