computer tomography ( ct ) has been widely used to assess the presence and extent of changes in skeletal muscle of patients with congenital neuromuscular diseases . in particular , the use of ct has made it possible to detect the selective involvement of muscle groups ; moreover , ct has been useful in establishing disease progression and identifying asymptomatic patients . the myopathic patients usually undergo gradual loss of muscle strength , which does n't affect every muscle group to the same degree . selective involvement of specific muscle groups and savings of others has been confirmed by different studies using ct . fatty replacement of skeletal muscle tissue is seen as an area with a clear decrease in density , therefore affected muscles are less identifiable as a consequence of the loss of contrast with the subcutaneous adipose tissue . a ct scan is usually more accessible than magnetic resonance ( rm ) , and it allows for the quick and correct evaluation of muscle changes , especially fatty degeneration , by assessing muscle density and morphology . examination is not operator - dependent and allows for the evaluation of the deepest muscles . however , ct scanning needs high doses of ionizing radiation , and has therefore it has been almost entirely replaced by ultrasound ( us ) and magnetic resonance imaging ( mri ) . another drawback are the poor contrasts of the soft tissues , which makes identification of inflammatory changes difficult or impossible ; as rule of thumb , muscle edema appears before the muscle degeneration . furthermore , though tc provides objective information that is easy to be duplicated by different observers and different apparatuses , some muscle artifacts may appear in muscle adjacent to cortical bone tissue . ultrasound is a well - known validated imaging technique for the evaluation of suspected muscle diseases . it is typically used to identify muscle thickness and to evaluate echogenicity changes , and can identify both atrophic changes and fatty degeneration , which is a typical feature of neuromuscular dystrophic diseases , like duchenne muscle dystrophy . its applications range from initial assessment , ultrasound - guided biopsy and follow - up of the patients ( figs . 1 and 2 ) . guiding biopsy with us , allows for the targeting the more affected muscles . on top of being low - cost and largely available , ultrasound scans have good resolutions , up to 1 mm , which allows for the identification of tertiary muscular bundles . us also enables dynamic analysis of muscle contraction by short videos to detect muscle contraction and/or pathologic fasciculations . unfortunately , it also has severe disadvantages : its application is usually limited to superficial muscle groups , since it is not sufficiently effective in deep muscles evaluation . moreover , the inter - observer and intra - observer agreements are really low , given that the muscle morphology and echogenicity are heavily dependent from the manual dexterity of the operator , thus , it is an operator - dependent examination . due to drawbacks of ct and us , mr is increasingly used in the evaluation of patients with genetic neuromuscular diseases . the technique provides excellent soft tissue characterization , so it perfectly evaluates shape , volume and morphological features of normal skeletal muscle . specifically , this technique easily allows for determination of fatty degeneration in late - stage muscle dystrophy , aside from recognition of decreases in muscle volume . thanks to this optimal soft tissue characterization , mri easily distinguishes the different muscles in the same muscle group , which is important in the identification of a specific pattern of involvement . using qualitative grading , mri can assess the degree of fat replacement of muscles and thus follow disease progression . fatty degeneration is usually evaluated in the t1-weighted images , where it appears as area of signal hyperintensity ( fig 3 ) . t2-weighted images with fat signal suppression and stir sequences identify muscle edema , which appear as a diffuse signal hyperintensity of the involved muscles ( fig . this is useful in detecting early signs of neuromuscular diseases , because inflammatory changes are often the first manifestation of the muscle disease , much earlier than fat and scar replacement of the muscle tissue . recent studies suggest that the use of intravenous administration of contrast medium would provide better details regarding the detection and extent of muscle involvement , although this leads to a substatial increase of scan time . mri has higher sensitivity in detecting muscle dystrophic disorders and a good inter - observer and intra - observer agreements ; therefore , mri is preferred to ct and us in the evaluation of genetic dystrophic neuromuscular disorders , and also because mri does n't expose the patients to ionizing radiation and , thus , can be safely used with young patients . in these cases , rm protocol include turbo - spin - echo t1-weigthed , in t2-weigthed and in t2 with fat suppression and , eventually stir ( short - tau inversion recovery ) sequences . mr images are usually acquired in the axial plane with slice thickness of 4 mm . when recommended , it is also possible to obtain images according to other anatomical planes ( coronal , sagittal ) ; multiplanar scans , of course , provide more anatomical details . the typical mr protocol applied in this type of study encompasses t1-weighted scans of the pelvic girdle and lower limbs ; there is still little experience in describing neuromuscular diseases by analyzing scans of to chest and upper limbs or by whole - body mri . whole - body mri can be useful in the assessment of congenital and metabolic myopathies , which typically show muscular involvement of regions other than limbs , without using great doses of ionizing radiation myopathic patients usually undergo a partial and gradual loss of muscle strength , but this loss does n't affect all muscle groups with the same degree of involvement . selective involvement of certain muscles and and saving of others was confirmed by studies using imaging techniques , in particular ct and mr , for this reason , it is important to evaluate not only the different muscular compartments , but every single muscle in them , in order to detect specific patterns that can narrow the differential diagnosis and help the search for specific genetic defects , for example , some authors have demonstrated specific patterns of involvement in the central cores myopathies , dysferlinopathy ( lgmd-2b ) , hyaline body myopathy ( hbm ) , tubular aggregate myopathy ( tam ) and myotonic dystrophy ( md ) ( fig . 5 ) while others claim that , by mri , specific patterns of involvement are powerful predictors in congenital myopathies due to ryanodine receptor type 1 gene mutations . due to the lack of ionizing radiation , mri is preferred to ct in young patients and in the assessment of asymptomatic patients with relatives affected by genetic disorders . there are experimental techniques , both applied to ct and mr , that can quantify the degree of muscle degeneration , evaluate the relationship between muscle fibers and fibrous tissue or measure the amount of intramuscular fat tissue . these techniques can be useful in the evaluation of therapies , but use experimental software and/or mri sequences that are not easily available and difficult to apply to the clinical practice . a purpose - developed false - color method of muscle imaging was designed and implemented to analyze macroscopic and microscopic structural changes of human skeletal muscle based on processing techniques of medical ct scans , to follow - up in patients suffering with complete permanent denervation of leg muscles ( complete conus and cauda equina syndrome ) the progression of muscle atrophy to degeneration and the extent of muscle recovery after experimental therapy . changes in tissue composition within the muscle were visualized by associating different colors to hounsfield unit values of normal or atrophic muscle , fat and connective loose and fibrous tissue . the results of this analysis were presented as the percentage of different tissues ( muscle , loose and fibrous connective tissue , and fat ) in cross - sections and in the total volume of muscle on the three - dimensional reconstructions . authors stress that the advantages of these approaches outweigh the low risk related to irradiation , in particular during follow - up of supervised trials , adding quantitative evidence to clinical assessments . as mentioned before , in clinical practice it is possible to use rating scales in order to semi - quantitatively assess the degree of muscle degeneration . its use is relatively quick and has a good inter - observer and intra - observer agreements . one of the most used is a 5-point scale where the signal intensity , evaluated in mri t1-weighted sequence , is scored using a modification of the scale reported by lamminen ( table 1 and figure 7 ) in the early stages of disease or in mild cases , an mr scan may be normal or slightly pathological , with a selective involvement of a few muscles . in more advanced myopathies , the pattern of involvement is less evident because more muscles are involved ; but , spared muscles or less affected ones , which identify the so - called " negative pattern " , can be useful diagnostic clues . for diagnostic purposes , a quantitative description of the muscle involvement is not necessary , as the degree of fatty degeneration correlates with the severity and duration of the disease . the articles describing the pattern of involvement with the mri , in different types of myopathy and dystrophy , have a low number of patients so far , and little is known about the spectrum of disease , which is only detectable by studies with higher number of patients . furthermore , when mri indications of the disease can be identified , the degree of disease progression assessed by mri , whether sex , ethnicity , exercising can influence the progression and whther the pattern varies with the type of mutation in a specific gene are not fully understood . knowing these points would help with diagnoses and would give useful information about the pathophysiology and therapy of neuromuscular diseases . thanks to its high sensitivity in the detection of muscular abnormalities and a good inter - observer and intra - observer agreements , mri is preferred to ct and us . in these cases , semi - quantitative rating scales allowfor the objective assessment of muscle degeneration and therefore mri provides an alternative or supportive analysis in follow up aside from clinical and laboratory tests . in patients suspected to suffer or suffering genetic neuromuscular disorders , mri is , thus , useful for : 1 . identifying asymptomatic patients , but with rm signs of muscle involvement ( e.g. family members of asymptomatic patients or patients susceptible to malignant hyperthermia ) ; 2 . guiding muscle biopsy , which allows for the selection of the more diseased muscle or muscle group ; 4 . selecting therapeutic treatments and evaluating new therapies ; 5 . , radiologic imaging is increasingly playing a relevant role in neuromuscular disorders : in particular , those of known or unknown genetic etiology . in spite of the dominant role of magnetic resonance imaging , other approaches maintain important roles : in particular , ultrasoun with reagards to in guiding muscle biopsy . other promising applications are described in details in other chapters of this ejtm special : news of clinical muscle imaging .
great technologic and clinical progress have been made in the last two decades in identifying genetic defects of several neuromuscular diseases , as spinal muscular atrophy , genetic muscular dystrophies and other genetic myopathies . 
 the diagnosis is usually challenging , due to great variability in genetic abnormalities and clinical phenotypes and the poor specificity of complementary analyses , i.e. , serum creatine kinase ( ck ) and electrophysiology . 
 muscle biopsy represents the gold standard for the diagnosis of genetic neuromuscular diseases , but clinical imaging of muscle tissue is an important diagnostic tool to identify and quantifyies muscle damage . 
 radiologic imaging is , indeed , increasingly used as a diagnostic tool to describe patterns and the extent of muscle involvement , thanks to modern techniques that enable to definethe definition of degrees of muscle atrophy and changes in connective tissue . 
 they usually grade the severity of the disease process with greater accuracy than clinical scores . 
 clinical imaging is more than complementary to perform muscle biopsy , especially as ultrasound scans are often mandatory to identify the muscle to be biopsied . 
 we will here detail and providewe will herein provide detailed examples of the radiologic methods that can be used in genetic and acquired neuromuscular disorders , stressing pros and cons . 
 key words : muscle imaging , mri , ct , genetic muscle disorders , myopathies , dystrophies