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Published online before print October 23, 2003, 10.1148/radiol.2293020904
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(Radiology 2003;229:861-867.)
© RSNA, 2003

Experimental Studies

MR Imaging in the Diagnosis of Denervated and Reinnervated Skeletal Muscles: Experimental Study in Rats1

Yoshito Kikuchi, MD, Toshiyasu Nakamura, MD, PhD, Shinichiro Takayama, MD, PhD, Yukio Horiuchi, MD, PhD and Yoshiaki Toyama, MD, PhD

1 From the Department of Orthopaedic Surgery, School of Medicine, Keio University, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan. Received July 19, 2002; revision requested September 10; final revision received March 10, 2003; accepted April 16. Address correspondence to T.N. (e-mail: tosiyasu@sc.itc.keio.ac.jp).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
PURPOSE: To describe the magnetic resonance (MR) imaging findings of denervation and reinnervation of skeletal muscles in rats.

MATERIALS AND METHODS: The posterior tibial nerve was transected in 48 rats (nerve section group) and was repaired just after transection in another 48 rats (nerve repair group). At 2, 4, 6, and 8 weeks after surgery, changes in the area and signal intensity of the gastrocnemius muscle on T1- and T2-weighted MR images in vivo (four rats in each group) and T1s and T2s were analyzed in vitro in both groups (20 rats each) and compared with electromyographic findings. Water volume content and extracellular fluid space of the muscle in vitro were also examined in both groups (24 rats each). Four rats were used as controls for in vitro analysis.

RESULTS: At T2-weighted MR imaging, the muscle showed continuously high signal intensity in the nerve section group. In the nerve repair group, the signal intensity was high until 4 weeks, when it recovered. Increases in signal intensity on T2-weighted MR images and T2s were seen in the nerve section group throughout the study period. In the nerve repair group, increased signal intensity on T2-weighted MR images was noted at 2 and 4 weeks and significantly returned to normal at 6 weeks (P < .014), just after the detection of the M wave at electromyography. T2 increased at 2 weeks, then decreased significantly at 4 weeks (P = .012). Extracellular fluid space significantly increased in the nerve section group at all measurement times and in the nerve repair group at 2 and 4 weeks, then it decreased after 6 weeks (P < .003), which is parallel to the change in signal intensity on T2-weighted MR images, although there was little change in total water volume content in either group.

CONCLUSION: Changes in signal intensity on T2-weighted MR images that are related to denervation may result from an increase of extracellular fluid space. MR imaging clearly demonstrates that changes in rat skeletal muscle are reversed when the nerve heals and reinnervation proceeds.

© RSNA, 2003

Index terms: Animals • Experimental study • Magnetic resonance (MR), experimental studies, 461.121411 • Muscles, denervation • Muscles, gastrocnemius • Muscles, MR, 461.121411


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Peripheral nerve injury not only leads to Wallerian degeneration but also produces morphologic and metabolic changes in the target denervated muscles. Histologically, muscle fiber in a denervated muscle rapidly decreases in diameter (1–4). Denervation also induces proteolysis without apparent change in overall protein synthesis in the skeletal muscle (5). Fiber diameter of the muscle and its protein metabolism then recover during the reinnervation phase, which sometimes occurs naturally but is usually induced surgically.

Electromyography (EMG) is useful for the diagnosis of denervated muscles. Surface EMG can be used to detect elongation of peak duration or decrease of nerve conduction velocity. An M wave detectable by using stimulation distal to the injured site on the nerve disappears at 4–5 days after injury. Furthermore, needle EMG can demonstrate the fibrillation potential or positive sharp wave in the muscle within 3 weeks after nerve injury in humans and within 1 week in rats (6). Although it is widely indicated for nerve injury, EMG sometimes presents technical difficulties in the detection of denervation in the deep muscles or small intramuscular areas.

Magnetic resonance (MR) imaging has been reported to allow evaluation of denervated skeletal muscles after peripheral nerve injury (7–10). This noninvasive multiplanar diagnostic modality is useful for analysis of changes in signal intensity from muscle tissue. It demonstrates signal intensity changes located in deep muscle tissue and in tiny areas inside the specified muscle, both of which are difficult to evaluate with needle EMG. Characteristic MR signal intensity patterns in the denervated muscles include high signal intensity on T2-weighted MR images and normal signal intensity on T1-weighted MR images (7–10).

Clinically, these MR imaging findings are observed as early as 4 days after the onset of paralysis, which is earlier than the disappearance of the M wave and the detection of denervation potential at EMG (10). To our knowledge, the cause of these changes at MR imaging has been investigated experimentally only by Polak et al (11), who examined changes in T1s and T2s, water content, and size of the extracellular fluid spaces of rat muscle at 15 days following denervation. These investigators speculated that the increase of extracellular space caused the observed signal intensity changes in denervation. However, there is little information on the degenerative changes that occur before or after day 15.

Although a number of clinicians now use MR imaging for the diagnosis of denervated muscles, few attempts have been made to follow the course of signal intensity changes during denervation. Although the denervated muscles may be reinnervated to normalize their function and metabolism, perhaps by suturing the nerve, little is known about the changes in MR signal intensity that accompany the reinnervation process. If we knew the course of the changes in signal intensity that accompany denervation and reinnervation, we could use MR imaging in addition to EMG in the diagnosis and prognosis of such cases. The purpose of our experimental study was to describe the MR imaging findings seen with denervation and reinnervation of skeletal muscles in rats.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Experimental Model
In this study, we used a total of 100 male Wistar rats weighing approximately 200 g each. The study was approved by our institutional animal use and care committee. The rats were anesthetized by using pentobarbital (Nembutal; Abbott Laboratories, North Chicago, Ill) injected intraperitoneally. With use of a microscope, the left posterior tibial nerve was transected, and both stumps were ligated to preclude spontaneous repair in the nerve section group (n = 48). In the nerve repair group (n = 48), the left posterior tibial nerve was transected and then repaired immediately by using 10-0 nylon stitches. A sham surgery (incision and exploration of the nerve) was performed at the same time on the other side. The other four rats were used as controls for the measurement of T1 and T2, total muscle water content, and extracellular fluid spaces (Table). All surgery was performed by one author (Y.K.).


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  		Numbers of Rats in Each Group that Underwent the Study Protocol

 
MR Imaging
Four rats in each group underwent transection of the left posterior tibial nerve. They were then anesthetized by using pentobarbital and were fixed on a special mount to hold the fore and hind feet in a prone position at 2, 4, 6, and 8 weeks after surgery. MR images of the denervated and reinnervated skeletal muscle were obtained serially with a 1.5-T MR imager (Signa; GE Medical Systems, Milwaukee, Wis). The rat was set on a round surface coil 5 cm in diameter. Transverse T1-weighted spin-echo (repetition time msec/echo time msec, 400/12) and T2-weighted fast spin-echo (4,000/104) MR images were obtained. A field of view of 14 cm, matrix of 256 x 192, two signals acquired, section thickness of 3 mm, and gap of 1 mm were used. The acquired images were scanned by using a film scanner (Scanjet 11cx; Hewlett Packard, Tokyo, Japan) and were transferred to a personal computer system. The area of the gastrocnemius muscle at the middle section was outlined and measured (Fig 1) independently by two authors (Y.K., T.N.). Signal intensity was calculated as an average of the two measurements by the authors at the region of the gastrocnemius muscle outlined at the same level. These data were estimated by using NIH Image software version 1.57 (National Institutes of Health, Bethesda, Md). The ratios of area and signal intensity were calculated between the denervated gastrocnemius muscles and the contralateral muscles within the same image.



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  		Figure 1. Transverse T2-weighted MR image demonstrates measurement of the area and signal intensity of the middle section of the gastrocnemius muscle. The areas were outlined and measured (arrow and arrowhead). Mean signal intensity was also obtained at the regions of the gastrocnemius muscle outlined at the same level. Arrow indicates the gastrocnemius muscle on the side where surgery was performed; arrowhead indicates the contralateral side.

 
EMG Study
In the electrophysiologic study, the four rats that underwent MR imaging were again anesthetized, and the sciatic nerves were exposed. The sciatic nerve (located proximal to the surgical site) and posterior tibial nerve (located distal to the surgical site) were stimulated with two monopolar needle electrodes. The other monopolar needle recording electrode was inserted into the belly of the lateral head of the gastrocnemius muscle. The reference electrode was placed in the distal tendon, with the ground electrode on the tail. We examined whether the M wave could be detected with an EMG machine (MS91a; Medelec, Surrey, England). The measurement was performed by one author (Y.K.) at 2, 4, 6, and 8 weeks after surgery.

T1s and T2s
Five rats were sacrificed at 2, 4, 6, and 8 weeks, respectively, after surgery in both groups (20 rats in each group, for a total of 40 rats), and the gastrocnemius muscles were resected and packed in 1-cm glass tubes and kept at 37°C. T1s and T2s were measured by one author (Y.K.) with the use of MR spectroscopy (120-minispec NMR analyzer; Bruker, Billerica, Mass). T1s were calculated by using an inversion-recovery MR sequence, and T2s were measured with the Carr-Purcell-Meiboom-Gill spin-echo MR technique (11,12). Ratios of T1s and T2s between the muscles that underwent surgery and the contralateral muscles in the same rat were calculated.

Water Content Analysis
We injected sulfur 35–labeled sodium sulfate (NEN Life Science Products, Boston, Mass) intraperitoneally into six rats at 2, 4, 6, and 8 weeks, respectively, after surgery in both groups. Thus, a total of 48 rats were used in this analysis. Two to 3 hours later, the rats were sacrificed, and the gastrocnemius muscles were transected and divided into two pieces. One piece was first measured in a wet condition and was then dehydrated within 48 hours (heated to 90°C and placed under a vacuum hood for the last 12 hours). Total water content was calculated as wet weight minus dry weight divided by wet weight. The other piece of gastrocnemius muscle was sectioned and soaked in a tissue solution device (Soluene 350; Packard, Meriden, Conn). Liquid scintillator (Hionic-fluor; Packard) was added to measure the extracellular fluid spaces by determining the ß count of the tissue samples by using a liquid scintillation technique with a counting device (LS-9800; Beckman, Fullerton, Calif). The ratios of total water content and extracellular fluid space between the denervated and contralateral control muscles of the same rat were calculated by one author (Y.K.).

Control Rats
Four untreated rats were sacrificed as controls, and T1s and T2s, total muscle water content, and extracellular fluid spaces were measured.

Statistical Analysis
All data are presented as mean values ± SDs. Changes in the signal intensities of T1- and T2-weighted MR images, T1s and T2s, total water content, and extracellular fluid space in the nerve section group, nerve repair group, and control group were analyzed statistically. Ratios were compared with relation to acquisition periods by using one-way analysis of variance with the Scheffe post hoc test. Comparison between nerve section and nerve repair groups was performed by using the unpaired t test. P <= .05 (two-sided) was considered to indicate a statistically significant difference.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Changes in MR Signal Intensity
Nerve section group.—The gastrocnemius muscle showed normal signal intensity on T1-weighted MR images and high signal intensity on T2-weighted MR images in the nerve section group 2 weeks after transection of the tibial nerve (Fig 2). The area of the treated muscle was smaller than that of the corresponding untreated contralateral side. T1-weighted MR images of the denervated muscles showed normal signal intensity until 8 weeks after transection of the tibial nerve. The area of the treated muscle was smaller than that of the contralateral side, but the signal intensity on T2-weighted MR images remained high 4–8 weeks after transection.



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  		Figure 2. Transverse T2-weighted MR images (4,000/104) show the nerve section groups. The gastrocnemius muscles show high signal intensity (arrow) throughout the study period. w = weeks.

 
Nerve repair group.—In the nerve repair group, the gastrocnemius muscle showed normal signal intensity throughout the study period on T1-weighted MR images. However, T2-weighted MR images showed high signal intensity 2–4 weeks after suturing of the transected tibial nerve. The signal intensity then returned to normal at 6 and 8 weeks (Fig 3). The area of the muscle was smaller compared with that of the contralateral side; however, it became larger than that in the nerve section group 4 to 8 weeks after surgery.



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  		Figure 3. Transverse T2-weighted MR images (4,000/104) in the nerve repair group. The gastrocnemius muscles show high signal intensity (arrow) at 2-4 weeks (w) after suturing of the transected tibial nerve. The signal intensity then returns to normal at 6 weeks (arrowhead).

 
Area of Gastrocnemius Muscles
The ratio of the area of the gastrocnemius muscle to the untreated contralateral side decreased by 50% at 2 weeks in both surgical groups. In the nerve section group, the ratio decreased rapidly and was 30% at 8 weeks. In the nerve repair group, the ratio recovered gradually and was 60% at 8 weeks (Fig 4). In both groups, there were no statistically significant differences between acquisition periods except at 2 and 4 weeks in the nerve section group (P < .001). However, there were significant differences between nerve section and nerve repair groups after 4 weeks.



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  		Figure 4. Graph shows area of the gastrocnemius muscles (ratio). In both groups, the muscle area decreased by 50% after surgery. The area continued to decrease in the nerve section group ({blacksquare}). In contrast, the area gradually recovered in the nerve repair group ({bullet}). Error bars = SDs.

 
MR Signal Intensity
Signal intensity ratio in both groups was around 1.0 at all measurement times for T1-weighted MR imaging (Fig 5a). There were no significant differences between data acquisition periods except between 6 and 8 weeks in the nerve repair group (P = .013). There were also no significant differences between nerve section and nerve repair groups. On T2-weighted MR images in the nerve section group, the ratio increased to 1.45 ± 0.08 at 2 weeks, then slightly decreased to 1.34 ± 0.11 at 4 weeks and 1.25 ± 0.04 at 6 weeks, and then increased to 1.46 ± 0.18 at 8 weeks. However, no measurements were significantly different between acquisition periods. In the nerve repair group, the ratio showed the same trend as in the nerve section group until 4 weeks (1.48 ± 0.01 at 2 weeks, 1.30 ± 0.01 at 4 weeks), but the ratio changed significantly to 1.04 ± 0.37 (P < .014) at 6 weeks and 1.03 ± 0.08 at 8 weeks (Fig 5b). There were significant differences between nerve section and nerve repair groups at 6 (P < .001) and 8 (P < .004) weeks. Decrease of the signal intensity ratio between 4 and 6 weeks was concurrent with normalization of signal intensity.



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  		Figure 5a. Graphs show MR signal intensity. (a) Signal intensity on T1-weighted MR images (ratio). There were no significant differences between acquisition periods except at 6 and 8 weeks in the nerve repair group ({bullet}) (P = .013). There were no significant intergroup differences. (b) Signal intensity on T2-weighted MR images (ratio). In the nerve section group ({blacksquare}), no measurements were significantly different between acquisition periods. In the nerve repair group, the ratio changed significantly at 6 weeks after surgery (P < .014). There were significant differences between the groups at 6 (P < .001) and 8 (P < .004) weeks. Error bars = SDs, dotted line = control.

 


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  		Figure 5b. Graphs show MR signal intensity. (a) Signal intensity on T1-weighted MR images (ratio). There were no significant differences between acquisition periods except at 6 and 8 weeks in the nerve repair group ({bullet}) (P = .013). There were no significant intergroup differences. (b) Signal intensity on T2-weighted MR images (ratio). In the nerve section group ({blacksquare}), no measurements were significantly different between acquisition periods. In the nerve repair group, the ratio changed significantly at 6 weeks after surgery (P < .014). There were significant differences between the groups at 6 (P < .001) and 8 (P < .004) weeks. Error bars = SDs, dotted line = control.

 
EMG Study
In the nerve section group, the M wave was not detected at any time after transection. In the nerve repair group, the M wave was not detected at 2 weeks after surgery. At 4 weeks, detection of the M wave became possible, but the shape of the wave demonstrated temporal dispersion. At 8 weeks after surgery, the shape of the M wave became biphasic.

T1s and T2s
T1s.—In the nerve section group, T1s increased steadily after surgery, with ratios of 1.12 ± 0.03 at 2 weeks, 1.13 ± 0.09 at 4 weeks, 1.09 ± 0.03 at 6 weeks, and 1.11 ± 0.09 at 8 weeks. In the nerve repair group, the ratio was 1.16 ± 0.04 at 2 weeks , 1.06 ± 0.04 at 4 weeks, 1.09 ± 0.10 at 6 weeks, and 1.03 ± 0.07 at 8 weeks (Fig 6a). There were no significant differences compared with the control group at or between any measurement period or between the nerve section and nerve repair groups, except between the control and nerve repair groups at 2 weeks (P = .03).



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  		Figure 6a. (a) Graph shows T1s (ratio). There were no significant differences between control and examination periods, between acquisition periods, or between groups, except between the control and nerve repair groups at 2 weeks (P < .03). (b) Graph shows T2s (ratio). There were significant differences between the nerve section group ({blacksquare}) and the control group every 2 weeks; however, there were none between acquisition periods. There were significant differences between the nerve repair group ({bullet}) and the control group every 2 weeks. There were also significant differences between the groups after 4 weeks. * = P < .001, error bars = SDs, dotted line = control.

 


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  		Figure 6b. (a) Graph shows T1s (ratio). There were no significant differences between control and examination periods, between acquisition periods, or between groups, except between the control and nerve repair groups at 2 weeks (P < .03). (b) Graph shows T2s (ratio). There were significant differences between the nerve section group ({blacksquare}) and the control group every 2 weeks; however, there were none between acquisition periods. There were significant differences between the nerve repair group ({bullet}) and the control group every 2 weeks. There were also significant differences between the groups after 4 weeks. * = P < .001, error bars = SDs, dotted line = control.

 
T2s.—T2s increased more notably than T1s in the nerve section group. The ratio was 1.29 ± 0.03 at 2 weeks and increased progressively, becoming 1.39 ± 0.14 at 4 weeks, 1.41 ± 0.17 at 6 weeks, and 1.55 ± 0.14 at 8 weeks. There were significant differences between the control and nerve section groups every 2 weeks; however, there were no significant differences between measurement periods. In the nerve repair group, T2s became 1.34 ± 0.03 at 2 weeks and decreased to 1.19 ± 0.04 at 4 weeks, 1.16 ± 0.04 at 6 weeks, and 1.14 ± 0.10 at 8 weeks (Fig 6b). There were significant differences between the control and nerve repair groups at every measurement. Also, there were significant differences between both groups after 4 weeks.

Total Water Content and Extracellular Water Space
Total water content of muscles showed a slight increase after surgery. The ratio with the contralateral side was 1.00–1.02 at every 2-week measurement. There were no significant differences between control and acquisition periods after surgery (Fig 7a). In contrast, extracellular water volume increased significantly in the nerve section group at 2 weeks (P < .004). The ratio of the extracellular water volume was 2.08 ± 0.12 at 2 weeks, 2.81 ± 0.38 at 4 weeks, 2.89 ± 0.38 at 6 weeks, and 3.11 ± 0.50 at 8 weeks. There was a significant difference at each measurement time.



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  		Figure 7a. Graphs show muscle water analysis. (a) Total water content of muscles (ratio). There were no significant differences between control and data acquisition periods after surgery. (b) Extracellular water volume (ratio). There were significant differences between the nerve section group ({blacksquare}) and the control group at every measurement. In the nerve repair group ({bullet}), the ratio of extracellular water volume to that in the control group significantly changed at 6 weeks (P < .003). There were significant differences with the control group at 2 (P < .011) and 4 (P = .002) weeks. However, there were no significant differences with the control group at 6 and 8 weeks. There were also significant differences between the groups throughout the study period. * = P < .001, error bars = SDs, dotted line = control.

 


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  		Figure 7b. Graphs show muscle water analysis. (a) Total water content of muscles (ratio). There were no significant differences between control and data acquisition periods after surgery. (b) Extracellular water volume (ratio). There were significant differences between the nerve section group ({blacksquare}) and the control group at every measurement. In the nerve repair group ({bullet}), the ratio of extracellular water volume to that in the control group significantly changed at 6 weeks (P < .003). There were significant differences with the control group at 2 (P < .011) and 4 (P = .002) weeks. However, there were no significant differences with the control group at 6 and 8 weeks. There were also significant differences between the groups throughout the study period. * = P < .001, error bars = SDs, dotted line = control.

 
In the nerve repair group, the ratio of the extracellular water volume was 1.76 ± 0.21 at 2 weeks and 2.06 ± 0.49 at 4 weeks, but it decreased significantly to 1.30 ± 0.22 at 6 weeks (P < .003) and 1.16 ± 0.19 at 8 weeks (P < .001). There were significant differences with controls at 2 weeks (P < .011) and 4 weeks (P = .002). However, there were no significant differences with controls at 6 and 8 weeks. There were also significant differences between nerve section and nerve repair groups throughout the study period (Fig 7b).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
 
It is well known that muscle changes morphologically and metabolically with denervation that is more severe than axonotmesis (1–4). Diagnosis of denervation or reinnervation has been dependent on EMG study; however, EMG data are influenced by the technique of the investigator, and it is difficult to detect any abnormality of deep muscles or tiny areas within a muscle. In clinical practice, it is always difficult to assign a differential diagnosis among denervation, tendon rupture, and malingering. Patients who are malingering may avoid EMG study because of its painfulness.

MR imaging is a noninvasive method that easily and clearly demonstrates changes in muscle, so it can be useful to distinguish tendon rupture, malingering, and denervation. MR imaging can also depict muscle conditions such as myogenic diseases (eg, muscular dystrophy and myositis) (13–15), muscle injury (16), and exercise-induced muscular changes (17). In addition to muscular changes after denervation, MR spectroscopy has also been used to investigate changes in the energy metabolism of muscles (18–22). On the other hand, a few clinical applications of MR imaging to evaluate denervated skeletal muscles have been reported in the past 2 decades (7–10). Characteristic patterns of MR signal intensity in denervated muscles include high signal intensity on T2-weighted MR images and normal signal intensity on T1-weighted MR images.

In the present study, changes in MR signal intensity of denervated gastrocnemius muscle showed high signal intensity 2–8 weeks after denervation, which is consistent with the results of several other investigators (7–10). The muscles of the nerve repair group showed high MR signal intensity at 2 and 4 weeks that normalized between 4 and 6 weeks as reinnervation proceeded. When compared with EMG findings, the recovery of the signal intensity was slightly delayed relative to detection of the M wave. Total water content did not change during denervation or reinnervation. Extracellular water volume increased continuously in the denervated muscles. In the nerve repair group, extracellular water volume increased until 4 weeks and decreased thereafter; hence, normalization of signal intensity correlated with normalization of extracellular volume change. The results of denervation partially support the findings of experimental research conducted 15 days after paralysis by Polak et al (11). The course of the T2-weighted MR signal intensity changes in the present study also emphasizes that MR signal intensity correlates with the extracellular volume in the reinnervation phase, as well as in the denervation phase.

Denervation rapidly leads to morphologic and metabolic changes in muscle. Muscle atrophy induced by denervation causes an increase in proteolysis, with little or no apparent change in overall protein synthesis (5,23). The uptake of glucose also decreases, and glycolysis increases rapidly (24). These metabolic changes occur soon after nerve damage and may relate to water metabolism in the muscle. Also, loss of neurotrophic factor (25), disuse atrophy caused by immobilization, muscle cell membrane permeability change (26), and functional disability of Na,K-ATPase (27) may occur, any of which could increase the extracellular water volume.

Our data also showed that the extracellular water space level remained elevated compared with the control level 8 weeks after complete electrical recovery of the nerve. This is probably due to incomplete cell-level recovery traceable to the method of nerve repair, as described by Frostick et al (18,19).

T1s and T2s are determined by the total water content of the intra- and extracellular spaces. Hazlewood et al (28) examined the transverse relaxation times (T2s) of water protons in rat skeletal muscle and stated that the water associated with the macromolecules was found to be approximately 8% of the total tissue water and it did not exchange rapidly with the rest of the intracellular water. The relaxation time of the myoplasm was 45 msec, which accounts for 82% of the tissue water. Approximately 10% of water was associated with the extracellular space, with a relaxation time of 196 msec, which is approximately four times of that of the myoplasm. That investigation suggests that denervation induces no significant increase of the total water content, but that the extracellular space increases. As is consistent with the increase of the extracellular space, T1s and T2s increase, and the MR signal intensity increases. On the other hand, reinnervation led to the decrease of extracellular water space and the normalization of MR signal intensity.

Signal intensity measurement with the spin-echo method is determined according to the following formula:

where I is signal intensity, N(H) is proton density, TE is echo time (msec), and TR is repetition time (msec).

If parameters TR and TE are not changed, decreased T1 values, increased T2 values, and higher proton density values increase the signal intensity. Tsubahara et al (29) stated that the majority of fat in muscle fiber was contained within the sarcolemmal membrane, and, in spite of muscle fiber atrophy, the volume of the membrane was relatively constant. Muscle fiber atrophy induces a relative increase in the amount of fat, which makes T1 shorter and T2 longer. Relative increase of the extracellular water induces higher T1s and T2s, which decreases signal intensity on T1-weighted MR images and increases signal intensity on T2-weighted MR images. The counterbalancing effects of extracellular water increase and fat increase on signal intensity on T1-weighted MR images seems to indicate isointensity.

In the present study, rat skeletal muscles were used for sequential MR imaging acquisition, T1 and T2 measurements, and water content analysis, and sacrifice of the animals was needed. We consider the results to be consistent with changes at MR imaging in human muscle. Further clinical study will be needed in human subjects to clarify the MR signal intensity changes in denervated and reinnervated skeletal muscles.

MR imaging clearly demonstrated changes in the denervated muscle of the rat model, which were reversed as the nerve healed and reinnervation proceeded. These changes were consistent with the size of the extracellular spaces of the muscles, which increased rapidly despite only a minor increase in total water content in denervation. As is consistent with reinnervation and the decrease of extracellular water, T1s and T2s decreased, following the normalization of signal intensity at MR imaging. MR imaging may be useful in the evaluation of denervation and reinnervation.

Practical Application: Denervated muscles show high signal intensity on T2-weighted MR images, as was shown in the present study. MR imaging can demonstrate signal intensity changes directly and objectively in a manner that is not dependent on experience of the examiner. MR imaging is useful as a noninvasive tool for diagnosis of denervation, especially in areas that are difficult to examine with EMG, such as tiny areas in deep muscle. If we know how signal intensity changes during the course of denervation and reinnervation, MR imaging may be useful not only for the diagnosis of denervation, but also for prognosis, perhaps enabling prediction of when reinnervation will occur.


   FOOTNOTES
 
Abbreviation: EMG = electromyography

Author contributions: Guarantors of integrity of entire study, Y.K., T.N.; study concepts and design, Y.K., T.N.; literature research, Y.K.; experimental studies, Y.K., T.N.; data acquisition, Y.K.; data analysis/interpretation, Y.K., T.N.; statistical analysis, Y.K.; manuscript preparation and definition of intellectual content, Y.K.; manuscript editing, Y.K., T.N.; manuscript revision/review, all authors; manuscript final version approval, Y.K., T.N., Y.T.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 

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