Sonography, or ultrasound imaging, is a dynamic study permitting physiologic real time observation of an anatomic region. It is a perfect tool for detecting stenosing tenosynovitis and has distinct advantages over magnetic resonance imaging (MRI) and computerized tomography (CT). Used in conjunction with power Doppler, sonography can be used to direct joint aspirations and the installation of medicines.
Musculoskeletal ultrasound imaging can accurately diagnose a large variety of tendon and joint pathologies by using frequencies at least 7.5 MHz and specially designed and focused linear array transducers.1 Ankle and foot structures are small and superficial in location and are best examined with probes more than 10 MHz in frequency. The higher the scanning frequency, the lower the sound penetration. For example, in measuring axial resolution, the discrimination between two points in the y axis, is 0.3 mm at 5 MHz, 0.15 mm at 10 MHz, and 0.12 mm at 13 MHz. This results in improved resolution, but the loss of distal information may occur when scanning deeper structures such as the bursae subtendinea and the flexor hallucis longus muscle when examining the Achilles tendon in the standard ultrasound scanning planes. Linear probes, as contrasted with sector scanners, better outline the course of tendons that are most often aligned in straight paths. Also, sector scanners produce bright echoes at the center of the image and fewer echoes at the periphery. A standoff pad may sometimes be used to insonate the subcutaneous structures. Comparison with the opposite side is possible and usually helpful in diagnosis. Cysts or fluid filled areas are without internal echoes and are called echo free. Solid regions have internal echoes and are classified as echo poor (or hypoechoic) if there are few internal echoes. The term echogenic (or hyperechoic) is used if there are many internal echoes. The skin of the body surface appears highly echogenic as do the bony structures. Bone, air, foreign bodies, and calcification stop the transmission of sound waves producing a sonic shadow — a dark region distal to the echogenic obstructing region. The term acoustical shadowing is also used to describe the low or absent echoes associated with these lesions. Acoustic window refers to an optimal placing of the transducers so that the areas of interest are clearly imaged.
Comparison with CT and MRI
Transverse and longitudinal scans or any set of orthogonal planes are acquired to produce a three dimensional representation of abnormalities. Subluxations of tendons may be diagnosed dynamically with sonography. Unlike CT and MRI, metal prostheses and postsurgical metallic clips are not a major hindrance since alternative scan planes may be used to look around these devices. In addition, the magic angle effect noted in curving tendons is not present. Poor images due to increased signal and the specific MRI partial volume effect of the surrounding fat is common in the peroneal tendons. In particular, the partial volume effect of the cortical bone of the joint may make imaging of the adjacent tendons and synovium more difficult. Likewise, the dark bands of MRI signal-less retinaculae may be inseparable from tendon with MRI, although the retinaculum is an echogenic structure with ultrasound and readily separable during scanning.2 Similarly, peritendineum of the Achilles tendon that appears as an echo poor or echo free space on sonography, cannot be distinguished on the MRI scan as a distinct structure.3 Intra-articular loose bodies have various MRI signals, where as mature marrow fat will have high signal. Heavily calcified bodies are often dark on all imaging sequences. Chondral and soft tissue areas often have intermediate signals.4
Chondrocalcinosis may appear a focal bright signal mimicking tears of menisci and similar structures.5 Ultrasound distinguishes easily between calcific and non-calcific regions due to the bright signals produced by the highly reflective calcium and bony entity. Errors due to MRI and CT positioning may also be avoided with sonography. For example, a low lying belly of the muscular peroneus brevis so that it lies within the fibular groove is said to increase the risk of peroneus brevis tendon rupture or dislocation. A recent report shows that this anatomic occurrence may occur in dorsiflexion of the foot during examination.6 The dynamic nature of sonograms often prevents misinterpretation due to anatomic positioning. Additionally, sonography by its real time dynamic nature permits full length imaging of the posterior tibial tendon, peroneal tendons, and fibulocalcaneal ligament that are difficult to visualize in total course by standard MRI sequences and planes. Fibrosis and scar tissue in the plane of a ruptured tendon may simulate a partial tear of the tendon when in fact the tendon may be retracted to the proximal region of the increased signal. Also, MRI resolution is best with smaller fields of view and therefore focused field may miss tendon pathology out of the plane of scan.7
The examiner should look for normal variants and other pathologic processes associated with any abnormal finding. For example, in tears of the peroneus brevis tendon, ruptures of the lateral collateral ligaments, stripping of the superior peroneal retinaculum, peroneal longus subluxations, and low lying muscle bellies of the peroneus brevis and peroneus quartus may be concomitantly identified. Bony pathologies such as abnormally curved surfaces or osteophytes and avulsion type microfractures may similarly be discovered.
The examiner should look for normal variants and other pathologic processes associated with any abnormal finding.
Patient comfort is another important consideration that makes sonograms preferable as a diagnostic procedure. Infants or uncooperative patients may be accurately scanned with real time units providing instantaneous data on foreign bodies, cellulitis, bursitis, and other entities simulating arthritis. Indeed, children may be held in their parents arms. If necessary, portable equipment may be brought to the bedside or nursing home. Since scan times with low strength MRI units may exceed half an hour, the rapidity of ultrasound examination for the elderly provides a significant positive patient compliance factor. Claustrophobia does not occur as a problem either.