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DOI: 10.1148/radiol.2403050820
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(Radiology 2006;240:786-792.)
© RSNA, 2006

Musculoskeletal Imaging

Fat Content of Lumbar Paraspinal Muscles in Patients with Chronic Low Back Pain and in Asymptomatic Volunteers: Quantification with MR Spectroscopy1

Bernard Mengiardi, MD, Marius R. Schmid, MD, Norbert Boos, MD, MBA, Christian W. A. Pfirrmann, MD, Florian Brunner, MD, Achim Elfering, PhD and Juerg Hodler, MD, MBA

1 From the Departments of Radiology (B.M., M.R.S., C.W.A.P., J.H.), Orthopedic Surgery (N.B.), and Rheumatology (F.B.), Orthopedic University Hospital Balgrist, Forchstrasse 340, CH-8008 Zurich, Switzerland; and Institute of Psychology, University of Bern, Bern, Switzerland (A.E.). Received May 13, 2005; revision requested July 12; revision received August 24; accepted September 22; final version accepted December 14. Address correspondence to B.M. (e-mail: mengiardi{at}yahoo.de).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Purpose: To prospectively evaluate the fat content of paraspinal muscles by using proton magnetic resonance (MR) spectroscopy in patients with chronic low back pain (LBP) and in asymptomatic volunteers matched with regard to age, sex, and body mass index.

Materials and Methods: The study was approved by the responsible institutional review board. Informed consent was obtained from each patient and each volunteer. Single-voxel proton MR spectroscopy was used to measure the fat content of the lumbar multifidus and longissimus muscles in 25 patients (13 women, 12 men; mean age, 40.5 years) with chronic LBP and in 25 matched asymptomatic volunteers (13 women, 12 men; mean age, 39.8 years). The fat content was also graded semiquantitatively (grades 0–4). The relationship between fat content and LBP duration, LBP intensity, and self-rated disability was assessed (Pearson correlation).

Results: The mean percentage fat content of the multifidus muscle was 23.6% (95% confidence interval [CI]: 17.5%, 29.7%) in patients with chronic LBP and 14.5% (95% CI: 10.8%, 18.3%) in the volunteers (P = .014). The corresponding values for the longissimus muscle were 29.3% (95% CI: 23.4%, 35.3%) in patients with LBP and 26.0% (95% CI: 21.9%, 30.0%) in the volunteers (P = .66). The semiquantitative grading of the fat content of the multifidus muscle was 0 in 12 (48%) of 25 patients and in 14 (56%) of 25 volunteers, 1 in 11 (44%) patients and in eight (32%) volunteers, and 2 in two (8%) patients and three (12%) volunteers. The semiquantitative grading of the fat content of the longissimus muscle was 0 in nine (36%) of 25 patients and 15 (60%) of 25 volunteers, 1 in 13 (52%) patients and nine (36%) volunteers, and 2 in three (12%) patients and one (4%) volunteer. Neither grade 3 nor grade 4 was assigned to any muscle. The grading differences were not significant between patients and volunteers. No significant correlation was found between fat content and pain intensity, pain duration, or self-rated disability.

Conclusion: Proton MR spectroscopy demonstrates a significantly higher fat content in the multifidus muscle in patients with chronic LBP than in asymptomatic volunteers. No difference was detected with a semiquantitative grading system.

© RSNA, 2006


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Low back pain (LBP) is the second most common reason, after upper respiratory problems (1), for seeing a physician. LBP is also one of the leading causes of disability during the working years (2,3). An association between paraspinal muscular insufficiency and both the development (4) and persistence (5) of LBP has been demonstrated. Reduced muscle strength and endurance have been observed in patients with chronic LBP and have been the focus of rehabilitation programs (6,7). In the shoulder, fatty degeneration of the rotator cuff muscles has been proved to be a relevant predictor of outcome after surgery (8,9). Information about the extent of fatty degeneration of the paraspinal muscles in patients with LBP would be useful for rehabilitation programs.

Computed tomography (CT) (10,11) and magnetic resonance (MR) imaging (12–14) have been used to assess fatty degeneration of the paraspinal muscles in patients with LBP by using histograms and semiquantitative grading methods.

Proton MR spectroscopy can be used to quantify the fat content of muscle noninvasively with an accuracy comparable to that of biochemical measurements (15). In a recent study (16) of 10 patients with lumbar disk herniation, proton MR spectroscopy depicted increased fat content of the paraspinal muscles when compared with that of a control group, which was not matched for age, sex, or body mass index (BMI).

Thus, the purpose of our study was to prospectively evaluate the fat content of paraspinal muscles by using proton MR spectroscopy in patients with chronic LBP and in asymptomatic volunteers matched with regard to age, sex, and BMI.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
The study was approved by the responsible institutional review board. Informed consent was obtained from each patient and each volunteer.

Patients
Twenty-five consecutive patients (13 women, 12 men) with chronic LBP were prospectively included in the study by having met the following inclusion criteria: (a) more than 3 months of continuous or recurrent LBP, (b) referral for MR imaging by a rheumatologist or spine surgeon at our institution because of LBP, (c) no prior spine surgery, (d) no systemic inflammatory disease, (e) no neurologic disorder, (f) no acute trauma, neoplasm, or infection, and (g) no scoliosis (Table 1).


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  		Table 1. Physical and LBP Characteristics of Patients and Asymptomatic Volunteers

 
Clinical diagnosis.—All patients were seen prior to MR imaging at least once by the referring clinician. The clinical diagnosis prior to MR imaging of the lumbar spine was LBP. In addition, 10 of the 25 patients had nonradicular leg pain and four of the 25 had radicular pain or sensory loss.

MR imaging diagnosis.—All patients underwent routine diagnostic MR imaging of the lumbar spine before the imaging for the study. These diagnostic MR examinations had been evaluated by one of three musculoskeletal radiologists (M.R.S., C.W.A.P., J.H., with 5, 7, and 12 years of experience, respectively, in musculoskeletal radiology including MR imaging). According to these reports, 11 of 25 patients had no compromise of the nerve root (five with no disk degeneration, six with a single disk protrusion). Ten of 25 patients had a disk extrusion with no compromise of the nerve root, and one of 25 had a disk extrusion with deviation of the nerve root at level L5-S1. Three of 25 patients had a disk extrusion with compression of the nerve root (one at level L4-5 with compression of the right L5 nerve, two at level L5-S1 with compression of the left and right S1 nerve respectively).

Assessment of pain and disability.—The patients completed a self-rating questionnaire that assessed back-related disability (Roland-Morris Questionnaire, score 0–24) (17–20), duration of pain (in months), and intensity of pain (0–10 on a visual analog scale) (Table 1).

Asymptomatic Volunteers
The volunteer (control) group consisted of 25 asymptomatic volunteers matched to the patient group according to age, sex, and BMI. Criteria for inclusion were (a) no LBP within the past 2 years that prompted a consultation of a physician, (b) no LBP within the past 2 years that caused any degree of work absence, (c) no LBP within the last 2 years that interrupted any sport activity, (d) no prior spine surgery, (e) no systemic inflammatory disease, (f) no underlying neurologic disease, and (g) no MR imaging findings of fractures, tumors, or infection of the lumbar spine. Criteria for forming matching pairs of asymptomatic volunteers and patients were sex, ±3 years of age, and ±3 values of BMI (Table 1).

MR Imaging Protocol
MR imaging and proton MR spectroscopy were performed with the same 1.5-T system (Symphony; Siemens Medical Solutions, Erlangen, Germany) by using a dedicated phased-array spine coil. Transverse T2-weighted fast spin-echo images (repetition time, 4000 msec; echo time, 122 msec [4000/122]; matrix, 512 x 256; field of view, 100 x 220 mm; section thickness, 4 mm) were obtained at the L4-5 level.

With the same phased-array spine coil used for MR imaging, single-voxel spin-echo proton MR spectroscopy was performed (1500/135, 128 signals averaged). Parameters were optimized by the manufacturer for studies evaluating fat content of muscle. No fat saturation and no water saturation were employed. A voxel of 10 · 10 · 10 mm was positioned in the center of the multifidus muscle at L4-5 on one side (which was chosen randomly) and in the center of the contralateral longissimus muscle at the L4-5 level. The transverse T2-weighted fast spin-echo images obtained during preceding routine MR imaging were used for appropriate positioning of the voxel, which was performed by the same radiologist (M.R.S.).

Analysis of MR Spectroscopic Data and MR Images
Analysis of MR spectroscopic data.—After Fourier transformation of the spectroscopic data, postprocessing including Gauss filtering, zero-order phase correction, and baseline correction were performed. For curve fitting (Gaussian curve-fitting algorithm), the imager's proprietary software was used. The area of the water and the lipid (intra- and extramyocellular combined) peaks were determined. The percentage fat content was then calculated from the area of the fat peak (Sfat) and from the area of the water peak (Swater) according to Sfat/(Sfat + Swater) · 100 (21–23).

Semiquantitative analysis of MR images.—Fatty atrophy of the multifidus and longissimus muscles was graded at the same level and on the same side as where the spectroscopic data had been obtained. For this purpose, the transverse T2-weighted fast spin-echo MR images were evaluated by a musculoskeletal radiologist (B.M., 2 years of experience in musculoskeletal radiology) without knowledge of spectroscopic and clinical data. The fat content was graded according to a grading system commonly used for the supraspinatus muscle (8). The grades are defined as follows: 0, no intramuscular fat; 1, some fatty streaks present; 2, fat evident, but less fat than muscle tissue; 3, amounts of fat and of muscle equal; 4, more fat than muscle tissue.

Statistical Analysis
The percentage fat content of the paraspinal muscles of patients with chronic LBP and of volunteers were compared with the Mann-Whitney U test (two-tailed). The semiquantitative grading of fat content was compared with the {chi}2 test. For the association between fat content of the paraspinal muscles and pain intensity, pain duration, and self-rated disability, the Pearson correlation coefficient was calculated. A P value of less than .05 was considered to indicate a statistically significant difference. For statistical analysis, software (SPSS for Windows, version 10.0.1, 1999; SPSS, Chicago, Ill) was used.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
MR Spectroscopy
The mean percentage fat content of the multifidus muscle was 23.6% (95% confidence interval [CI]: 17.5%, 29.7%) for patients with chronic LBP and 14.5% (95% CI: 10.8%, 18.3%) for the asymptomatic volunteers (P = .014, Mann-Whitney U test) (Figs 1, 2; Table 2). The mean percentage fat content of the longissimus muscle was 29.3% (95% CI: 23.4%, 35.3%) for patients with chronic LBP and 26.0% (95% CI: 21.9%, 30.0%) for the asymptomatic volunteers. This difference was not significant (P = .66) (Fig 1).


Figure 1
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  		Figure 1a: Voxel positioning on MR image and corresponding MR spectra of multifidus and longissimus muscle in a 59-year-old man. (a) Transverse T2-weighted fast spin-echo image (4000/122). At level L4-5, 10 · 10 · 10-mm voxel was positioned in center of longissimus muscle (left square) and on contralateral multifidus muscle (right square). (b, c) MR spectra (1500/135) reveal slightly higher fat peak (L) within (b) longissimus muscle than in (c) multifidus muscle. W = water peak.

 

Figure 1
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  		Figure 1b: Voxel positioning on MR image and corresponding MR spectra of multifidus and longissimus muscle in a 59-year-old man. (a) Transverse T2-weighted fast spin-echo image (4000/122). At level L4-5, 10 · 10 · 10-mm voxel was positioned in center of longissimus muscle (left square) and on contralateral multifidus muscle (right square). (b, c) MR spectra (1500/135) reveal slightly higher fat peak (L) within (b) longissimus muscle than in (c) multifidus muscle. W = water peak.

 

Figure 1
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  		Figure 1c: Voxel positioning on MR image and corresponding MR spectra of multifidus and longissimus muscle in a 59-year-old man. (a) Transverse T2-weighted fast spin-echo image (4000/122). At level L4-5, 10 · 10 · 10-mm voxel was positioned in center of longissimus muscle (left square) and on contralateral multifidus muscle (right square). (b, c) MR spectra (1500/135) reveal slightly higher fat peak (L) within (b) longissimus muscle than in (c) multifidus muscle. W = water peak.

 

Figure 2
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  		Figure 2a: MR image and corresponding MR spectrum of left multifidus muscle in a symptomatic 36-year-old woman. (a) Transverse T2-weighted fast spin-echo MR image (4000/122) at left multifidus muscle (square represents voxel location) at L4-5. At semiquantitative analysis, fatty atrophy was graded 0 (no intramuscular fat). (b) MR spectrum (1500/135) reveals elevated lipid peak (L) with 24.7% fat content (mean value of asymptomatic patients, 14.5%). W = water peak.

 

Figure 2
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  		Figure 2b: MR image and corresponding MR spectrum of left multifidus muscle in a symptomatic 36-year-old woman. (a) Transverse T2-weighted fast spin-echo MR image (4000/122) at left multifidus muscle (square represents voxel location) at L4-5. At semiquantitative analysis, fatty atrophy was graded 0 (no intramuscular fat). (b) MR spectrum (1500/135) reveals elevated lipid peak (L) with 24.7% fat content (mean value of asymptomatic patients, 14.5%). W = water peak.

 

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  		Table 2. MR Spectroscopic and Semiquantitative Measurements of Fat Content of Paraspinal Muscles

 
No significant correlation of the percentage fat content of the multifidus and longissimus muscles, respectively, was found with pain intensity (r = 0.17, P = .40 and r = –0.20, P = .33), pain duration (r = –0.37, P = .061 and r = –0.09, P = .67), or self-rated disability (r = 0.09, P = .66 and r = 0.05, P = .81).

Semiquantitative Analysis
In patients with chronic LBP, fatty atrophy of the multifidus muscle was graded as 0 in 12 (48%) of 25 patients, 1 in 11 (44%) of 25 patients, and 2 in two (8%) of 25 patients. The corresponding values for the volunteer group were 14 (56%) of 25 for grade 0, eight (32%) of 25 for grade 1, and three (12%) of 25 for grade 2. No volunteer had grade 3 or 4 fat in the multifidus muscle (Table 2).

In patients with chronic LBP, the fatty atrophy of the longissimus muscle was graded as 0 in nine (36%) of 25 patients, 1 in 13 (52%) of 25, and 2 in three of 25 (12%). The corresponding values for the volunteer group were 15 (60%) of 25 for grade 0, nine (36%) of 25 for grade 1, and one (4%) of 25 for grade 2. No patient and no volunteer had grade 3 or 4 fatty degeneration in the longissimus muscle.

For the semiquantitative grading of fatty degeneration, no significant difference between the patients and the asymptomatic volunteers was found for the multifidus (P = .66) or for the longissimus muscle (P = .2) (Fig 3).


Figure 3
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  		Figure 3a: MR images and corresponding MR spectra of fat content of multifidus muscle in two patients with chronic LBP. Semiquantitative analysis on (a) transverse T2-weighted fast spin-echo image (4000/122) and (b) corresponding proton MR spectrum (1500/135) of a 39-year-old man. Fat content was graded visually as grade 1 (some fatty streaks present). (b) MR spectrum reveals very high lipid peak (L). Semiquantitative analysis on (c) transverse T2-weighted fast spin-echo MR image (4000/122) and (d) corresponding proton MR spectrum (1500/135) of a 47-year-old man. (c) Fat content of multifidus muscle was graded visually as grade 2 (fat evident, but less fat than muscle tissue). (d) Corresponding MR spectrum shows markedly elevated lipid peak (L) but clearly less high than in the first patient. W = water peak.

 

Figure 3
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  		Figure 3b: MR images and corresponding MR spectra of fat content of multifidus muscle in two patients with chronic LBP. Semiquantitative analysis on (a) transverse T2-weighted fast spin-echo image (4000/122) and (b) corresponding proton MR spectrum (1500/135) of a 39-year-old man. Fat content was graded visually as grade 1 (some fatty streaks present). (b) MR spectrum reveals very high lipid peak (L). Semiquantitative analysis on (c) transverse T2-weighted fast spin-echo MR image (4000/122) and (d) corresponding proton MR spectrum (1500/135) of a 47-year-old man. (c) Fat content of multifidus muscle was graded visually as grade 2 (fat evident, but less fat than muscle tissue). (d) Corresponding MR spectrum shows markedly elevated lipid peak (L) but clearly less high than in the first patient. W = water peak.

 

Figure 3
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  		Figure 3c: MR images and corresponding MR spectra of fat content of multifidus muscle in two patients with chronic LBP. Semiquantitative analysis on (a) transverse T2-weighted fast spin-echo image (4000/122) and (b) corresponding proton MR spectrum (1500/135) of a 39-year-old man. Fat content was graded visually as grade 1 (some fatty streaks present). (b) MR spectrum reveals very high lipid peak (L). Semiquantitative analysis on (c) transverse T2-weighted fast spin-echo MR image (4000/122) and (d) corresponding proton MR spectrum (1500/135) of a 47-year-old man. (c) Fat content of multifidus muscle was graded visually as grade 2 (fat evident, but less fat than muscle tissue). (d) Corresponding MR spectrum shows markedly elevated lipid peak (L) but clearly less high than in the first patient. W = water peak.

 

Figure 3
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  		Figure 3d: MR images and corresponding MR spectra of fat content of multifidus muscle in two patients with chronic LBP. Semiquantitative analysis on (a) transverse T2-weighted fast spin-echo image (4000/122) and (b) corresponding proton MR spectrum (1500/135) of a 39-year-old man. Fat content was graded visually as grade 1 (some fatty streaks present). (b) MR spectrum reveals very high lipid peak (L). Semiquantitative analysis on (c) transverse T2-weighted fast spin-echo MR image (4000/122) and (d) corresponding proton MR spectrum (1500/135) of a 47-year-old man. (c) Fat content of multifidus muscle was graded visually as grade 2 (fat evident, but less fat than muscle tissue). (d) Corresponding MR spectrum shows markedly elevated lipid peak (L) but clearly less high than in the first patient. W = water peak.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCES IN KNOWLEDGE
 References
 
Several studies have demonstrated an association between chronic LBP and reduced strength and fatigue resistance of back muscles (24–26). Whether these changes precede or are rather a consequence of back pain has been debated. Some investigators have suggested that fatigable back muscles are predisposed for the development of LBP (4,27). Others have indicated that observed alterations of muscular morphology develop subsequent to pain (28,29). Most investigators described changes in patients with chronic LBP that were found within the multifidus muscle, which represents the deepest portion of the paraspinal muscles. Biomechanical studies have shown that this muscle is a major contributor to lumbar segmental stability (30,31).

Muscle degeneration is characterized by a decrease in size and an increase of fat and interstitial connective tissue. This degeneration has been demonstrated in histologic examinations performed on tissues from other parts of the body, such as the rotator cuff (32,33). Several investigators have assessed such morphologic changes of the paraspinal muscles in patients with chronic LBP by using imaging techniques. The cross-sectional area and fat content of the muscles have been evaluated by using ultrasonography (29,34), CT (10,11), and MR imaging (12–14,28,35). The published results are equivocal. Most studies report an increase of fat within the paraspinal muscles (12–14,16) or a decreased cross-sectional area of the paraspinal muscles (10,12,13,35). Some investigators, however, did not find an increase of fat (10,11). The fat content was quantified with CT or MR imaging either qualitatively (12,14,36) or by using histographic methods with application of a threshold value (10,11,13). In addition, some studies evaluated the paraspinal muscles as a group (13,16,29,36). Other investigations have evaluated the multifidus muscle separately (10,12,14,35).

In a recent study by Schilling et al (16), proton MR spectroscopy was used to evaluate the changes in the fat-water ratio of paraspinal muscles in 10 patients with lumbar disk herniation. These patients had a significant increase of the fat-water ratio (ie, 0.19) when compared with that of a healthy control group (ie, 0.09). The spectroscopic results correlated well with the histologic findings of muscle biopsies performed in two patients. This control group was not matched for age, sex, or BMI. There was no specific information regarding which portion of the paraspinal muscles was chosen for measurements.

In our study, patients with chronic LBP had a significantly larger percentage fat content (23.6%) in the multifidus muscle when compared with that of the asymptomatic volunteers (14.5%), but there was no significant difference in percentage fat content for the longissimus muscle. Because a correlation of fatty muscle degeneration with age and sex is known (28,36,37), we excluded these factors by matching the groups for age, sex, and BMI. Our results correlate with investigations of histochemical changes of the paraspinal muscles in patients with chronic LBP. Degeneration of the multifidus muscle with an atrophy of type I and II fibers has been observed in patients with lumbar disk herniation (38,39). On the other hand, analysis of the ileocostalis and longissimus muscles in patients with chronic LBP revealed no atrophy but a transformation of muscle fibers to a greater portion of fast-fatigable type IIB fibers (28).

For semiquantitative assessment of the fat content of the multifidus and longissimus muscles on transverse T2-weighted fast spin-echo MR images, we employed a grading system commonly used for the rotator cuff muscles (8). This system appears to be valid and reproducible, and it correlates with MR spectroscopic measurements with the exception of early fatty degeneration not detectable with the human eye (23). No significant difference between symptomatic and asymptomatic persons was depicted for the multifidus or the longissimus muscle by using this grading system in our study. Considering the significant difference of the percentage fat content of the multifidus muscle measured with proton MR spectroscopy (23.6% in patients vs 14.5% in asymptomatic volunteers), MR spectroscopy may be able to depict fatty degeneration earlier than may standard imaging.

The fact that single-voxel proton MR spectroscopy was performed only once for each muscle represents an important study limitation. In addition, the size and placement of the voxel within the muscle may be another source of error when the variability of fat distribution is considered. Reproducibility of MR spectroscopic assessment of fat content in the supraspinatus muscle with fatty degeneration, however, could be demonstrated for the same voxel size after the patient was repositioned (23).

In conclusion, in patients with chronic LBP, proton MR spectroscopy demonstrates a significantly higher fat content in the multifidus muscle when compared with that of asymptomatic volunteers; proton MR spectroscopy did not demonstrate a significantly higher fat content in the longissimus muscle when compared with that of asymptomatic volunteers. This significant difference for the multifidus muscle could not be detected with a semiquantitative grading system between patients and volunteers.


   ADVANCES IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
References
 


   FOOTNOTES
 

Abbreviations: BMI = body mass index • CI = confidence interval • LBP = low back pain

Authors stated no financial relationship to disclose.

Author contributions: Guarantors of integrity of entire study, B.M., J.H.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, B.M., M.R.S., C.W.A.P., F.B., A.E., J.H.; clinical studies, B.M., M.R.S., N.B., F.B., A.E., J.H.; statistical analysis, B.M., N.B., C.W.A.P., J.H.; and manuscript editing, all authors


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 

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