Review of SMR Workshop on MRI and MRS of Muscle In the summer of 1994, the Society of Magnetic Reso- nance (SMR), and the European Society for Magnetic Resonance in MediciGe and Biology (ESMRMB), together with the British Institute of Radiology (BIR), convened a workshop entitled Magnetic Resonance Imaging and Spectroscopy of Muscle. The host for this international workshop held between June 29 and July 1,1994, was the University of Liverpool.

One hundred twenty delegates attended the workshop, 16 having been invited by the organizers to give extended presentations of their work. In addition, there were two Keynote Addresses, the first given by George Radda (Ox- ford University) on the use of MRS to investigate muscle biochemistry in a wide variety of muscle diseases, the second by Robert Shulman (Yale University) on I3C MRS studies of glycogen depletion and resynthesis during ex- ercise. The titles of the workshop sessions were Muscle at Work, Imaging of Muscle, Muscle Function, Spectroscopy of Muscle and Technical Developments, Muscle Metabo- lism, Responses of Muscle to Injury and Aging, Vascular and Temperature Studies, and From Research to Clinical Applications.

The main aim of the workshop was to put those with expertise in Magnetic Resonance (MR) techniques ap- plied to the study of muscle in touch with muscle phys- iologists, and to bring all participants up to date on developments in MR technology appropriate to the non- invasive assessment of muscle structure and chemistry. The specific background to the meeting was that while the needle biopsy technique (which originated in Scan- dinavia) in the 1960s and 1970s provided the first oppor- tunity to determine the biochemical changes in working human muscle, MR had subsequently offered (from 1980 onward) the opportunity to study noninvasively muscle during exercise.

MRI and MRS can, respectively, provide information about the structure and metabolism of muscle. However, they are only rarely used routinely in the investigation of patients with neuromuscular disease. Exceptions exist at the Helsinki University Hospital where (reported Antti Lamminen) every patient with neuromuscular disease has an MRI study using the relatively inexpensive low field (i.e., 0.02 Tesla) system, whereby it has been found that the patterns of fatty infiltration are not disease spe- cific. Work continues with a view to establishing meth- ods of tissue characterization for use in monitoring treat- ment response, or the effect of palliative therapies. At Patrick Cozzone’s laboratory in Marseille, every patient with neuromuscular disease has a 31P MRS study of their forearm flexor muscles during exercise and recovery, using the more expensive high field system (i.e., 4.7 Tesla) that is necessary for such studies. A distinct aid to diagnosis is claimed for 44% of the 1100 patients so far investigated.

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What are, therefore, the opportunities provided by MRI and MRS for obtaining a better understanding of muscle structure and function in health and disease? The pre- dominant use of MRI is for diagnostic assessment of extent of injury to a muscle and its blood vessels follow- ing trauma. Graham Whitehouse (University of Liver- pool) reported that most routine use of MRI in cases of muscle trauma concerns sporting injuries. Such injuries may occur at the tendon-osseous insertion, in the tendon itself, in the muscultendinous junction or the muscle belly. Chronic recurrent strain leading to “tennis elbow” produces focal soft tissue fluid collections (visible by MRI) and associated degenerative changes adjacent to the insertion of the proximal extensor tendon on the lateral epicondyle of the humerus. Acute tears often occur at the junction and a follow-up MRI is advised if a neoplasm is not detected but suspected. Sports injuries often affect the muscle belly and John Crues I11 (Cedars-Sinai Medi- cal Center, Los Angeles) reported that the time to return to training is better indicated by the resolution of features in the MR images than by a patient’s subjective feelings of improvement.

Recent development work with MRI aims to provide (noninvasively) descriptions of the size (i.e., maximum cross-sectional area and volume), composition (i.e., pro- portion of intramuscular fat) and texture (i.e., pattern of distribution of the intramuscular fat) of muscle. Penna- tion (i.e., orientation of the muscle fibers or fascicles) was shown by MRI to change with contraction (Marco Nerici, Milan). MRI was also able to visualize the kinematics of particular muscles in voluntary muscular activities (Ste- phen Riederer, Mayo Clinic). The effect of training on several of these parameters has also been studied (Marco Narici, Milan), together with the effects of bed rest and space flight (Adrian LeBlanc, Baylor College of Medicine, Houston) as well as immobilization in plaster.

Extension of the above mentioned methods to the study of muscle disease is particularly concerned with muscle composition analysis. Thus, T, and T, relaxation time measurements provide an objective means of quan- tifying the extent of edema (damage) or fat infiltration (Joanne Phoenix, University of Liverpool) in diseased muscle. Subtraction of the cross-sectional area of fat in- filtration from the anatomical cross-sectional area pro- vides a measure of the physiological cross-sectional area of a muscle.

It is this latter parameter, along with pennation angle, that is important for establishing whether weakness is due to loss of muscle fiber cross-sectional area (as is commonly found) or to loss of intrinsic muscle strength. Narici has shown that use of anatomical rather than physiological cross-sectional area may lead to a three- or fourfold overestimation of stress in normal pennate mus- cles. By establishing the extent of fat infiltration through- out the entire body of patients (the so called “whole body biopsy”), it may prove possible to correlate the pattern of the affected regions with the manner in which the pa- tients are “using themselves”-i.e., their habitual use of particular muscles. The methods may be usefully incor-

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porated in studies of the effects of exercise, drug, or eventually, gene therapy, in the management of patients with, for example, muscular dystrophy. Advanced MRI techniques such as magnetization transfer contrast (MTC) imaging, which is able to differentiate “bound” and “free” water, may have a role in identifying the affected muscles in compartment syndrome (Christian Lukosch, Siemens, Erlangen). MTC imaging may also be usefully applied to identify which muscles have been working during exercise.

MRS provides information on muscle metabolism for volumes of interest of the order 50 times larger than those of the individual pixels in a typical MR image. The relative concentrations of inorganic phosphate (Pi, phos- phocreatine (PCr) and a-, p-, and y-ATP can be measured by 31P MRS, and the technique has been applied to follow changes in these compounds during rest, exercise, and recovery in health and the changes in disease. This approach has been extended by combination with elec- tromyography, or electrical stimulation of the muscle during the MRS study (Henry Gibson, University of Liv- erpool).

A recent review article entitled Clinical Applications of MRS did not even mention muscle (Ross and Michae- lis, Magn. Reson. Q. 1, 191-247 (1994)), suggesting that we are not close to seeing the technique find practical application in the management of patients with muscle disease. The workshop did, however, reveal that several groups are using the technique to obtain a better under- standing of muscle disease, and an interesting catalogue of detective work emerged.

Martin Kushmerick (University of Washington, Seat- tle) reported on the use of 31P MRS to assess energy balance in human muscle activity. In their normal phys- iological state, the rate of chemical energy used by resting or active muscles for generating mechanical power (in the form of high energy phosphates) must be matched by the rate of ATP synthesis. Kushmerick has developed a 31P MRS protocol to allow separate study of both com- ponents of this metabolic energy balance; i.e., the rate of ATPase and the rate of ATP synthesis during a defined contractive event. With this procedure it has been possi- ble to test whether the observed bioenergetic differences in congestive heart failure are due to abnormal rates of ATPase or ATP synthesis.

Jane Kent-Braun (University of California, San Fran- cisco) combined 31P MRS and neurophysiological mea- sures (i.e., electromyography) during voluntary and elec- trically stimulated contractions of the anterior tibialis. This helped localize the site of failure during fatiguing exercise. Patients with multiple sclerosis and post-polio syndrome demonstrated activation failure while those with chronic fatigue syndrome demonstrated central ac- tivation failure. The approach may be particularly useful for studying muscle fatigue in various diseases where fatigue is a major clinical problem. Bjaorn Quistorff (The Panum Institute, University of Copenhagen) used 31P MRS to study changes in Pi, PCr, and ATP during is- chemic exercise. He reported that Pi and free AMP are not the main regulators of glycolysis in human skeletal muscle under ischemic conditions, that anaerobic glyco- lysis and glycogenolysis is halted momentarily upon ter-

mination of contraction and that PCr is not resynthesized during ischemic recovery.

Bruno Barbiroli (University of Bologna) has developed two tests employing 31P MRS to respectively assess mi- tochondrial activation (steady-state test) and function (ramp test) in the calf muscle. In the first test the resis- tance to a pedaling exercise is increased every 3 min to allow the steady state to be reached at each level of work while in the latter the resistance is increased every minute to maximally activate the glycolytic pathway. The steady-state test allows evaluation of the perfor- mance of muscle mitochondria free from the effects of pH. The ramp test allows evaluation of clinically inter- esting variables such as the rate of Pi transport and pH recovery.

Doris Taylor working with George Radda (Oxford Uni- versity) reported the use of 31P MRS to assess the degree of mitochondrial dysfunction in pathological states. An assessment of the rate of PCr resynthesis with respect to ADP increases the rate of detection of mitochondrial disease, while a comparison of the rate of PCr resynthesis with end-exercise ADP can be used to quantify the size of the mitochondrial defect.

13C MRS studies only become practicable using the highest field (i.e., 4.7 Tesla) clinical MR systems that provide increased signal-to-noise ratios and better re- solved spectra. In his Keynote Address Bob Shulman (Yale University) reported natural abundance 13C MRS studies of the turnover of human muscle glycogen with low intensity exercise. Five volunteer subjects performed plantar flexion at 15% maximum voluntary contraction for 5 h. At 2.5 h of exercise, a step-up infusion of 99% enriched 1-C-13 glucose was begun and maintained for the next 1.5 h of continued exercise. Exercise was then continued for a further hour. During the first 2 h of exercise glycogen 1-C-13 signal amplitudes dropped to 30% and remained there at 2.5 h indicating that glycogen concentrations had leveled. Following infusion glycogen signal amplitudes rose to 123% of resting values, remain- ing there during the subsequent hour of exercise. From this, it was concluded that the human gastrocnemius can degrade and resynthesize glycogen simultaneously dur- ing prolonged low-intensity exercise.

Shulman also described investigations of the effects of glycogen depletion and insulin concentration on glyco- gen synthesis. Subjects performed single leg toe raises to deplete gastrocnemius glycogen to 75%, 50%, or 25% of resting concentration. Glycogen resynthesis rate was sig- nificantly ( P < 0.02) higher (i.e., 33 5 7 mM/h) after depletion to 25% than the similar rates (i.e., 2.4 2 0.7 and 2.8 2 0.6 mM/h) obtained at 75% and 50% deple- tion. Insulin dependence of glycogen synthesis was as- sessed after depletion to 25% (i.e., < 30 mMj with and without infusion of somatostatin to inhibit insulin secre- tion. The observation that, after depletion to <30 mM initial glycogen, resynthesis was insulin independent and glycogen dependent suggests that control of resyn- thesis occurs within the muscle (ix. , occurs locally).

Norma Harrison working with Susan Wray (Depart- ment of Physiology, Liverpool), provided the first in vivo information on the effects of ischemia on the contraction of muscle of the uterine wall, and this was the only

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presentation concerning smooth muscle. They used 31P MRS to demonstrate how uterine contractions can oc- clude blood supply causing a lowering of pH due to lactic acid build up that may impair contractility and cause uterine dysfunction (i.e., dystocia) in labor.

Ian Young (Hammersmith Hospital, London) reported attempts to measure the temperature of muscle by Tl relaxation time mapping and diffusion imaging. Reliable results are difficult to obtain from a mobile heteroge- neous tissue. Changes in perfusion make it difficult to unequivocally calibrate TI relaxation times, and the in- herent anisotropy in the orientation of the muscle fibers means that the size of the diffusion coefficient is orien- tation dependent. The anisotropy of muscle was further demonstrated in MRS studies by Roland Kreis (Univer- sity of Bern). Kreis proposed that muscle contains a liq- uid-crystal-like molecular system which has an effect whereby the parts per million difference in the resonant frequency between two peaks in the ‘H spectrum is de- pendent on the orientation of the muscle within the static magnetic field of the MR system.

Examples of work reported by muscle physiologists that may be amenable to further study using MR include the report by Roger Harris (Animal Health Trust, New- market) that a higher concentration of carosine in Type I1 as compared with Type I fibers of equine muscle empha- sizes its importance to intramuscular acid-base regula- tion, and suggests that it may be possible to describe muscle fiber compartmentalization using MRI. In another study Bengt Satlin (Copenhagen Muscle Research Centre) showed how during exercise the blood flow alters to keep the supply of oxygen constant. Saltin used Doppler ul- trasound to measure the blood flow velocity. MR angiog- raphy could also be used to obtain such data and provide complementary information besides.

Roger Woledge (Institute of Human Performance, Uni- versity College, London) reported how old people are weaker than young people, due in part to loss of muscle, but also because the remaining muscle is weaker. The aging muscle has a changed force velocity curve whereby the force exerted during stretch is greater as a proportion of the isometric force than in younger muscle. As a result the force exerted during a sufficiently rapid stretch is independent of age. This suggests a difference between

the high force and low force states of the attached cross- bridge that could be caused by increased Pi or decreased pH in the muscles of older subjects. However, MRS has shown that this is not the case. The time of onset of the age-related changes is different in women, where it comes on at the time of the menopause, than men, where strength declines more gradually. Hormone replacement therapy in postmenopausal women prevents the weak- ness suggesting that estrogen may have important effects at the cross-bridge with profound significance for strength.

There was, perhaps, little evidence of transfer of ideas between experts in the fields of MRI and MRS. More animated discussion arose by virtue of the fact that sev- eral leading muscle physiologists described the results of experiments in which the relationship between muscle function and chemistry had been investigated in the lab- oratory by methods requiring several needle biopsy sam- ples over the period of exercise and recovery. For exam- ple, Lennart Kajser (Huddinge Hospital, Stockholm) reported how conditions of local ischemia produced by enclosing the leg in a pressure chamber can facilitate the effect of physical training, as revealed by increased ac- tivities of oxidative enzymes and Type I fibers in biopsy specimens. Perhaps some participants will be planning to build pressure vessels within the bores of their MR systems in order that these effects can be less painfully monitored!

The meeting ended with a reminder by Richard Ed- wards (University of Liverpool) that muscle fatigue could not be identified with a particular state of energy deple- tion, there being increasing evidence for the importance of the failure of electromechanical coupling. Attempts to seek unique biochemical patterns of explanation in mus- cle diseases using MR are fraught with difficulties on account of “partial volume” effects due to tissue hetero- geneity (e.g., fat replacement) in the “sensitive volume” of muscle studied. Evidently the gap between MRS and MRI has to reduce if it is to be possible to give a precise biochemical description of diseased muscle.

Neil Roberts Magnetic Resonance Research Centre University of Liverpool