Magnetic resonance imaging in skeletal muscle following denervation and electrical stimulation

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Denervation atrophy and electrical stimulation

Following chronic denervation, a lot of well known changes occur in a skeletal muscle, particularly concerning contractile properties and morphological features [6]. While contraction force decreases, the muscle fibres become smaller and histologically an increase of fat and connective tissue is found.

Despite many investigations during the last decades, the discussion on the efficacy of electro therapy in chronic denervation is still controver sial. In most of the earlier irvestigations, electro therapy was found to delay, but not to avoid.

MRI of muscle

Muscle tissue normally is characterized by the classical methods of histology, enzyme histochemistry and electron microscopy. During the last few years, these morphological methods be came supplemented by imaging techniques (ultrasonography, computer tomography, MRI), and in particular MRI promises to become an important tool in the clinical evaluation of muscle diseases [22].

The relaxation times T₁ and T₂ are more specific than signal intensity and show typical values in different tissues [5], and so they are considered to be clinically more useful. In de nervation atrophy, muscles show a decrease of T₁ and an increase of T₂ [2, 22], while the relaxation times are not influenced by a long time resting or immobilisation atrophy [10]. A reversible increase of T₂ has been described following an intensive muscle training [4]. As to our knowledge, the influence of electrotherapy on MRI of muscle has never been investigated before.

As our new method of electrical stimulation has proved effective in avoiding atrophy and loss of muscle strength in chronic denervation, it was of a certain interest, whether MRI can be used to monitor the influence of electrotherapy on the denervated muscle. Being a non invasive method, MRI can be repeated easily, and possibly some (invasive) histological examinations could be supplemented or even replaced by magnetic resonance imaging. Preliminary results, as presented earlier [17] had been encouraging.

Material and Methods

20 adult white New Zealand rabbits were examined. In 16 animals, the right hindlimb was denervated totally and chronically as reported previously [15]. Electrical stimulation was performed in 7 animals twice daily via surface electrodes with an effective stimulation time of 2x7,5 minutes. 9 animals remained untreated after denervation, and 4 animals served as nor mal controls. For stimulation, bidirectional rectangular impulses with a frequency of 25 Hz and an intensity of 40 mA were used, as they had proved effective in maintaining contraction force and muscle bulk. Observation time was 1 month for short-term effects, 2 months and 3—6 months for long-term effects.

MRI examinations were performed in general anesthesia (ketamine and xylazine i.v.) on a SIEMENS Magnetom 1,5 T. In a "head coil", the legs were tied up in a parallel position, care was taken of a side by side position of the knee joints for having identical muscle regions of both legs on the same store.

For the measurement of signal intensity in T1 weighted images, a repetition time (TR) of 0,6 sec was chosen, and an echo time (ТЕ) of 15 msec. T2 relaxation time was calculated from CPMG-sequences: 8 echos from 22 to 176 msec, each with a TR of 0,6 and 1,8 sec. Usual slice thickness was 4 mm, except for multi echosequences (8 mm).

In each animal, two fast contracting muscles — the tibialis anterior muscle (TA), and the flexor digitorum sublimis muscle (FDS) were evaluated. Signal intensity and T₂ relaxation time were measured within one slice of the middle third of the crus. In each muscle, the average values were calculated from 5 measurements in regions of interest (ROI) with an area of 6 pixels, which is equivalent to a volume of about 0,1 cm³. Care was taken to that the ROIs did not include any visible fascia or blood vessel. From both signal intensity and T₂ relaxation time, right/left-ratios were evaluated.

Results

Signal intensities:

In both muscles of the four normal controls, as expected no difference was found between the two legs, indicating a good reliability of the measurements. Five animals were investigated one month after denervation. Flere an increase of signal intensity was observed in both muscles as well in the stimulated as in the untreated animals. These changes did not reach significance, and there was also no difference between TA and FDS.

After long term denervation in six animals, there was a significant increase of signal intensities in both muscles without electrotherapy, whereas there was only a slight increase in the long-term stimulated muscles (Fig. 1).

Fig. 1. Changes of signal intensity in fast skeletal muscles or rabbit following short and long term denervation. Influence of electrical stimulation.

Ratio: right = denervated/left = normal, scale from 85—115%. TA = tibialis anterior muscle, FDS= flexor digitorum sublimis muscle

T₂ relaxation times:

Like for signal intensities, no side differences were found for T₂ relaxation times in the normal muscles (normal values: TA = 32,0+2,0 msec, FDS = 30,9+l,3 msec), in the treated muscles, the changes of T₂ were more distinct than those of signal intensities. Already one month after denervation, an influence of electrical stimulation was found. Without stimulation, T₂ had clearly increased in both muscles (TA= 157%, FDS=151%), whereas only a tendency (FDS) or a slight significance (TA) for changes were found in the stimulated muscles. In the long-term denervated animals, similar changes were found in the non stimulated muscles (increase of TA= 150% and of FDS = 157%), whereas there were no changes at all in the treated muscles (Fig. 2).

Fig. 2. Changes of T2-relaxation time following denervation and electrical stimulation (explanation see fig. 1).

Note the larger scale in the diagram, reaching from 25—175%

Discussion

Since MRI has been introduced in clinical practice and investigation, a lot of observations have been presented, describing muscle pathology. By most of the authors, the findings concerning signal intensity and relaxation times are explained with changes of fat and water content [1, 10, 19, 22, 24, 30, 31]. The binding capacity of macromolecules, electrolytes, pH, temperature and some other factors may have an influence too [5].

In this investigation, it has been necessary to evaluate very small ROIs, because within the anyway small objects of FDS and TA muscles, areas of fascia and fat had to be excluded. However, this has been possible without prob lems because of the high resolution of MRI, which is said to be less than 1 mm [23]. The good reproducibility is shown by the low standard deviation of normal values, which already has been found in a previous investigation [17].

In denervation atrophy, the decrease of muscle volume always parallels with an increase of connective tissue and fat, while the water content remains constant [6]. In our study, the effect of denervation was clearly visible not only histologically (unpublished observations), but also in MRI. While the results of signal intensity were only poor, T₂ proved to be a valuable tool to differentiate a denervated muscle from a normal or a denervated-stimulated one. In both muscles, T₂ relaxation time clearly increased after one and two months of denervation alone, while it increased significantly less during electrotherapy. Similar observations, concerning a different behaviour of signal intensity and relaxation times, have been made in physiological short time reactions of muscle too, describing no changing of signal intensity but a distinct increase of T₂ during muscle exercise [4, 19, 29].

Effects of muscle exercise on MRI may occur due to a changing of water content and electrolyte concentration [4, 26, 27], and there is no difference, if muscle contractions are performed voluntarily or induced by electrical stimuli [11, 27]. These changes, however, are only short term effects and cannot explain the findings of one or two months later. So, the long term effects cannot be explained by muscle contractions alone.

Long term immobility is known not to influence the relaxation times, although it might be accompanied by a marked atrophy [10]. Hence, the denervation induced increase of T2 must be explained by other factors than a simple decrease of muscle fibre diameter. In later stages of atrophy, the muscle degenerates, e.g. it becomes fibrotic, and muscle fibres are replaced by fat and connective tissue [6]. Fat is known to influence relaxation times, and its amount even can be calculated, when trying to separate this effect from the influence of water and proteins. Fat and water are thought to be the most important factors, even if they might be controlled and modulated by the pH level, the concentration of electrolytes or the temperature [5].

Thus, first of all the influence of fat must be discussed to be responsible for the changes observed in MRI. The denervated and stimulated muscle clearly shows less fat inclusions than the denervated and untreated muscle, and this would explain the distinct increase of T₂ after denervation, as well as the missing increase in the stimulated muscle. On the other side, an increase of signal intensity has been reported following denervation both in T^ and T₂-weighted images, which led the authors to the conclusion, that the influence of fat must be insignificant, and they discussed the importance of water content [24, 25]. This question has to clarify this aspect, further investigations are necessary, evaluating T1 relaxation time, which has not been able in the present series. It might be marginal, that muscle fibres with a different contraction velocity might have diffetent relaxation times. It is reported, that fast fibres should have higher values for T₁ and T₂ [8], but this fact should be confirmed by an experimental correlation study of histology and relaxation times.

Electrotherapy of denervated muscle, which for a long time had missed to prove effective, now is developing a clinically valuable method, using our type of stimulus. Using surface electrodes, the stimulation is able to lift the patient's leg when lying on the bed or sitting on a chair [16]. In other patients, a strong tetanic contraction of the biceps muscle is possible, and the flexion of the ellbow is a useful movement. So, if in future our new concept of electrotherapy should stand the test clinically, a tool is necessary to monitor the effect of the therapy on the denervated muscle. Histological examinations always need an (invasive) biopsy, while MRI is a non invasive method, which can easily be repeated, even if it is expensive. The first couple of measurements in patients showed, that an increase of T₂ can be reversible after the onset of electrotherapy even several months after denervation, indicating and parallelling an increase of muscle bulk and contraction force (in preparation). So we think, that MRI will be a useful tool for monitoring the success of electrotherapy in chronically denervated muscles.

About the authors

Т. Mokrusch

Author for correspondence.
Email: info@eco-vector.com
Germany, Lingen, FRG

References

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2. Fig. 1. Changes of signal intensity in fast skeletal muscles or rabbit following short and long term denervation. Influence of electrical stimulation.

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3. Fig. 2. Changes of T2-relaxation time following denervation and electrical stimulation (explanation see fig. 1).

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