Copyright �� Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. Skeletal muscle imaging and inflammatory myopathies George P . Kuo and John A. Carrino Purpose of review A variety of modalities exist for the imaging of skeletal muscle including radiography, ultrasound, computed tomography, and MRI. This article highlights the utility of these modalities in evaluating skeletal muscle diseases. Newer techniques such as T2 mapping, blood oxygenation level dependent imaging, diffusion tensor imaging, and magnetic resonance spectroscopy are also explored. Recent findings Cross-sectional imaging including computed tomography and magnetic resonance is the current standard in skeletal muscle imaging. The advantages of these modalities include the ability to image in different planes and to evaluate the distribution of disease and disease burden. Newer magnetic resonance-based techniques also provide functional information in addition to anatomic information. Radiography and ultrasound have a more limited role and are mainly used to detect calcifications and evaluate the texture of skeletal muscle. Summary Magnetic resonance is a useful modality for evaluating skeletal muscle and allows for the assessment of disease burden. It can elucidate potential targets for biopsy. Newer magnetic resonance techniques can acquire functional information in addition to anatomic information and hold tremendous potential for detecting, characterizing, and monitoring treatment for inflammatory myopathies. Keywords MRI, muscle, myopathy, myositis, radiology Curr Opin Rheumatol 19:530���535. �� 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins. The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA Correspondence to John A. Carrino, MD, MPH, Section chief, Musculoskeletal Radiology, The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, 601 N. Caroline Street, JHOC 5165, Baltimore, MD 21287, USA Tel: +1 410 502 0615 fax: +1 410 502 6454 e-mail: jcarrin2@jhmi.edu Current Opinion in Rheumatology 2007, 19:530���535 Abbreviations BOLD blood oxygenation level dependent CT computed tomography DTI diffusion tensor imaging MR magnetic resonance STIR short tau inversion recovery �� 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins 1040-8711 Introduction Skeletal muscle makes up a significant portion of the total mass of the human body and is a major component of one of the most fundamental organ systems [1]. The complex physiology, metabolism, and structure of skeletal muscle and the myocyte have been well understood for quite some time. Yet, skeletal muscle has traditionally received less attention by radiologists than the other components of the musculoskeletal system. This is partly due to the difficulties directly visualizing skeletal muscle with con- ventional imaging techniques, such as radiography. A range of imaging modalities now exists which enable the direct visualization of skeletal muscle and include cross-sectional imaging techniques such as computed tomography (CT) and magnetic resonance (MR). In particular, MR techniques have sparked a renewed inter- est in the imaging of skeletal muscle. Imaging has traditionally had an ancillary role in the diagnosis of myositis and inflammatory myopathies com- pared with clinical history, physical exam, biochemical studies, electromyography (EMG), and muscle biopsy. Anatomy and anatomic relationships have been the focus of traditional imaging. Newer MR techniques not only focus on anatomic relationships, but also on muscle function. The synergy of anatomic and functional infor- mation is already proving to be a very powerful tool in understanding the spectrum of muscle diseases. Imaging modalities Various medical imaging modalities have been applied to the evaluation of skeletal muscle including radiography, CT,ultrasound,andMR.Someofthesemodalitiesemploy the use of ionizing radiation (radiography, CT), while the others employ the use of other types of energy such as radiofrequency waves (MR) or sound waves (ultrasound). Regardless of the modality utilized, the goals of muscle imaging remain the same: to identify the abnormal muscles, the severity of disease, and the distribution of disease. This information provides clues to the differential diagnosis, directs further invasive procedures such as biopsy, and establishes a baseline for future comparison. Radiography Radiography plays a limited role in muscle imaging due to limitations in contrast resolution. The relative differ- ences in X-ray attenuation between the various soft tissues (tendons, fat, ligaments, and fascial structures) are small, making the direct visualization of muscle and the ability to distinguish muscle from surrounding soft 530

Copyright �� Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. tissues difficult. A projectional technique, the superim- position of various structures also limits the direct visual- ization of muscle. The most useful role of radiography is in detecting soft tissue calcification (which on occasion accompanies dermatomyositis in children and, less frequently, in adults with this disease) and evaluating osseous detail, as conventional radiography has excellent spatial resolution [2]. Computed tomography CT is a cross-sectional technique that uses ionizing radiation. Projections are obtained by scanning a pencil- thin X-ray beam through the patient in different orien- tations and measuring the X-ray attenuation using gas ionization chambers or scintillation crystal detectors. The relative X-ray attenuation from different angles through the patient are filtered and reconstructed to produce cross-sectional images based on the relative X-ray attenu- ation of different structures, measured as Hounsfield units (HU). CT eliminates the superimposition problem encountered in radiography. Although the spatial resol- ution of radiography is better than CT, CT has superior contrast resolution, which is further enhanced by the ability to window and level. Newer generation spiral multislice scanners are capable of acquiring isotropic data sets, allowing for multiplanar reconstructions in any plane with equal spatial resolution. This allows for the direct visualization of skeletal muscle from virtually any orientation without loss of resolution. Unlike radiography, CT can distinguish between bone, tendons/ligaments, muscles, fluid, and gas based on their relative X-ray attenuation. CT can also evaluate muscle bulk and characterize soft tissue mineralization patterns [3,4]. Ultrasound Ultrasound is a technique that employs the use of mechanically produced longitudinal sound waves with frequencies of 2���12 MHz above the range of human hearing. Skeletal muscle appears hypoechoic under nor- mal circumstances, with the fibroadipose septa and fascia between muscle bundles appearing echogenic. As normal skeletal muscle contracts, the echogenicity of skeletal muscle increases as the muscle bundles thicken. As with CT, ultrasound can be used to assess muscle atrophy or hypertrophy based on muscle size. Parenchy- mal abnormalities are reflected in changes in the echo- texture of muscle [5,6]. Calcifications may also be visible in the muscle tissue itself or in the adjacent subcutaneous fat, and are often seen as brightly echogenic foci with acoustic shadowing. Involuntary muscle movements and fasciculations can also be directly observed in real time, and ultrasound is unique compared with other modalities in this regard. One of the limitations of ultrasound is that it is operator dependent and reproducibility may vary with different sonographers. Additionally, ultrasound images by neces- sity often focus on a specific anatomic region, which can make it difficult to appreciate the entire scope of a disease process. Magnetic resonance Current clinical MRI is based on signals from 1 H (protons) and is a powerful technique for evaluation of skeletal muscle pathology and physiology. Although MR findings may not be specific, MR is the best imaging modality to detect soft tissue abnormalities. It is used to evaluate for the presence or absence of muscle edema as a surrogate for inflammation, evaluate its extent, and identify potential sites for biopsy [7,8,9 ,10]. The type of image ���weighting��� employed during image acquisition determines MR contrast. The most com- monly used sequences for skeletal muscle imaging include T1-weighted, T2-weighted, proton density, and short tau inversion recovery (STIR). MR contrast resolution between skeletal muscle and fat is best seen on T1-weighted images. Additionally, T1-weighted images are useful for detection of fat in disease processes which involve muscle infiltration or replacement. T1-weighted images are also used after the administration of contrast material in conjunction with fat suppression, which improves contrast resolution by saturating the signal from lipid-containing structures. STIR and T2-weighted sequences are useful for the detection of edema and are referred to as the fluid sensitive sequences. Muscle activation or injury (strain, denervation, or inflammation) produces edema and consequently increases in T2 signal intensity. Myositis The idiopathic inflammatory myopathies refer to a group of acquired skeletal muscle disorders including polymyo- sitis, dermatomyositis, and inclusion body myositis. Infectious agents and myotoxins may also cause inflam- matory myopathies. Furthermore, several inherited myo- pathies are associated with significant muscle inflam- mation. For example, fascioscapulohumeral muscular dystrophy, the third most common muscular dystrophy, has a marked inflammatory component. The imaging of skeletal muscle can noninvasively characterize the distri- bution and quantity of parenchymal changes that occur in both infectious and autoimmune-mediated myositis. Imaging permits the assessment of disease burden and can help monitor the progression or regression of disease, which has important implications on treatment decisions. Normal skeletal muscle is typically intermediate in signal intensity on all pulse sequences some muscles have a marbled appearance because of the interposed fat Skeletal muscle imaging Kuo and Carrino 531