Dr Sanjay Dhawan, Sr Consultant & Head (Radiology & Imaging), Paras Hospitals, Gurgaon explains the methods available for soft tissue imaging and the advantages of each technique
Soft tissues in relation to the musculoskeletal system consists of the muscles, tendons, ligaments, bursae, cartilage as well as the neurovascular structures (arteries, veins and nerves) associated with them. In addition, assessment of the adjoining bones and joints is a mandatory part of imaging protocol.
A typical patient with a soft tissue lesion would present with a lump or a bump. This could be due to trauma, sports related injuries, infective conditions or tumour masses. The biggest impact of the modern imaging has been in the field of sports injuries.
An armamentarium of imaging techniques is available for evaluation of soft tissues which include conventional radiography, ultrasound (USG), magnetic resonance imaging (MRI), computed tomography (CT scan), and positron emission tomography (PET).
Conventional Radiography (X-ray)
Conventional X-rays play a limited role in imaging of soft tissues as the X-rays pass through the not-so-dense soft tissues. They are used as an initial screening to rule out an associated bony lesion or to reveal mineralisation of the soft tissues.
Ultrasound Scan (USG)
Ultrasound is a readily available, radiation-free and cost-effective imaging technique. The availability of high resolution linear probes has contributed immensely to imaging of soft tissues. It is a simple bedside scan, and is very useful for apprehensive, claustrophobic patients and children. It is ideally suited for superficially placed soft tissue structures. Ultrasound also has the ability to perform a dynamic study. For example, the affected part is imaged in real time, observing for pathological movement in a tendon, muscle or joint. This is a feature unique to ultrasound and is not possible with other modalities. The major drawback of ultrasound is that it is highly dependent on the skills of the operator. Secondly, it is less useful for deep placed structures and in obese patients.
Ultrasound in tendon pathology: Evaluation of tendon pathology is probably the most common indication for obtaining a musculoskeletal ultrasound. Two commonly visualised tendons for tears are rotator cuff (shoulder) and the Achilles tendon. Ultrasound has been reported to have near 100 per cent sensitivity for diagnosing full thickness tear of rotator cuff but the sensitivity to diagnose partial thickness tear is much lower.
Ultrasound in ligament pathology: Diagnostic ultrasound is used to good effect in evaluation of ligament tears in athletes. The ligaments commonly imaged are the ulnar collateral ligament at elbow, anterior talofibular ligament at ankle and the medial/ lateral collateral ligaments of the knee.
Ultrasound in muscle pathology: It is estimated that 30 per cent of sports injuries affect muscles. Additionally, ultrasound can also detect a non-traumatic or primary muscle pathology like myositis. In ultrasound, the sonologist can visualise the entire length and in case of a complete tear, can detect the site of retracted muscle. The gap in a torn muscle is hypoechoic (dark) and when compressed gently by the transducer, can reproduce pain. This technique is called sonopalpation and is an example of dynamic evaluation by ultrasound.
Ultrasound in nerve pathology: There has been a recent interest in evaluation of nerves by ultrasound. Common indications are for evaluation of carpal tunnel syndrome at wrist and Morton’s Neuroma in foot.
Ultrasound in soft tissue tumours: Ultrasound can assess the size, shape, location and anatomical relationships of the lesion. In addition, it can comment on the composition of the lesion – solid or cystic (fluid-containing). By using colour Doppler as an adjunct to ultrasound, the vascularity of the lesion can be determined.
Ultrasound in intervention: Ultrasound is an excellent modality to guide interventional procedures like FNAC, aspiration from a fluid collection, local steroid injection and injection of contrast for MR arthrography.
Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging is a radiation-free modality which involves magnetic field, radiofrequencies and computer applications to create detailed images of structures within the human body. The evolution of MRI has led to a paradigm shift in imaging of soft tissues. The inherent soft tissue contrast as well as multiplanar capability has rendered MRI as the ideal investigation of soft tissue pathology. In addition to localisation of the lesion with respect to other anatomical structures, it can also comment on predominant tissue component as well as whether it is likely to be benign or malignant. This has been facilitated by recent advances like Diffusion Weighted MRI and MR Spectroscopy. The administration of gadolinium intravenous contrast (dye) is useful in diagnosing infections and tumours of soft tissues.
An MRI scan is performed by obtaining multiple imaging sequences by applying radiofrequency pulses and gradients after placing the patient in a magnetic field. A typical MRI examination consists of T1 weighted, T2 weighted, PD weighted, fat suppressed and Gradient Echo (GRE) sequences in axial, coronal and sagittal planes. An additional contrast enhanced scan may be done if clinically indicated.
MRI is a very safe procedure and the magnetic field or the radiofrequency pulses are not harmful to the human body. The presence of a pacemaker, drug infusion pump and ferrous implants are a contraindication for an MRI. Patients in first trimester of pregnancy are advised to avoid MRI exam. Impaired renal function as determined by estimation of low glomerular filtration rate (GFR) is a contraindication for administration of intravenous contrast medium.
MRI in sports injuries: MRI in sports medicine is a rapidly developing field due to its ability to provide accurate diagnosis and because of better understanding of disabilities associated with the injury. Early treatment has led to prevention of disability and early resumption of sporting activity. Common sporting injuries include a hamstring tear, Achilles tendon rupture and knee ligament injuries. Biceps tear and labral injuries are also common in power-lifters and throwers. MRI is both sensitive and specific for detection of a tear involving a muscle, tendon or a ligament. These tears can be further categorised as partial or full thickness and a grading has been devised to comment on the severity. This is of huge consequence in recovery and rehabilitation of the sportsperson. In addition, associated problems like a fluid collection or a haematoma are well visualised. An interesting bit of trivia on this aspect is that a couple of stadiums hosting NFL games in USA have MRI scanners on site in the stadium!
MR Arthrography: It is a specialised technique wherein a small amount of contrast (dye) is injected in the joint space under imaging guidance. It is particularly useful in diagnosing labral tears in shoulder and hip joints.
MRI in imaging of articular cartilage: Articular cartilage imaging is of increased importance in a setting of an ageing population wherein osteoarthritis is a leading cause of chronic disability and joint pain. MRI can accurately detect the extent of articular cartilage denudation as well as abnormalities in the underlying bone.
MRI in soft tissue infections: MRI is extensively used in localising and characterising deep seated soft tissue infections. It can differentiate between abscess and cellulitis, thereby enabling correct treatment. Many a time, infection in soft tissues is a consequence of infection in the adjoining bone. In this respect, MRI is the only imaging modality which can detect abnormalities in the bone marrow. This ability of MRI is also useful in marrow infiltrative disorders like haematological malignancies. If appropriate hardware is available, MRI guided aspiration of the deep seated infected collections leads to early resolution of the disease process.
MRI in soft tissue tumours: Soft tissue neoplasms are a large group of tumours divided into nine different categories depending on the tissue of origin viz, muscle, cartilage, fat etc. They are further sub-categorised as benign, intermediate (locally aggressive) and malignant. As a first step, the lesion is assessed with respect to size, shape, margins and distance form a chosen anatomical landmark. Tissues like muscle, fat, cartilage and bone have different signal characteristics on spin echo (T1 weighted and T2 weighted) sequences, which help in ascertaining the tissue of origin. Subsequently the lesion is characterised as benign and malignant on basis of vascularity (contrast enhanced MRI) and invasion of adjacent structures. It would be prudent to mention that MRI is the modality of first choice for local staging of a musculoskeletal
tumour.
Computed tomography (CT SCAN)
CT scan provides excellent contrast between bones and soft tissues. It has given way to MRI because CT scan involves radiation and for the fact that the contrast between two adjacent soft tissue structures is suboptimal in comparison to MRI. However, it is useful to assess associated bony lesion/ remodelling, periosteal reaction, mineralisation of the lesion and presence of air/ gas within the lesion. Last though not the least, it is useful for a patient in whom MRI is contraindicated.
Positron Emission Tomography (PET-CT)
Positron emission tomography uses radioisotopes that undergo positron emission decay. While PET-CT is well established for staging of many visceral neoplasms, its role in staging of soft tissue tumours is debatable due to overlap with inflammatory conditions.
To conclude, it would be fair to say that advances in imaging technology have led to rapid strides in management of soft tissue pathology. It has enabled early institution of appropriate therapy, causing significant reduction in disability. The choice of imaging modality is determined by the cause, location in the body and clinical diagnosis. The future is even more promising, given the advancements in technology which are happening by the day.