Devoted to diagnostic and interventional spine imaging and therapeutics

Library

Clinical Utility of Diffusion-Weighted MR Imaging in the Diagnosis of Spinal Infections 2011

General Spine

Nayela, N, Keen, MD
Courtnay, Bloomer, MD, Non ASSR Member
Cynthia, Chin, MD, Non ASSR Member

Paper/Mentor

Purpose

Conventional spine MRI is limited in diagnosing and monitoring patients with spinal infections. Diffusion-weighted imaging (DWI) has been useful in the evaluation of cerebral infections where abscesses have been shown to have low apparent diffusion coefficient (ADC) values, but is not commonly used for the evaluation of spinal infections. Other studies have investigated the role of DWI in differentiating infectious vertebral body changes from malignant or degenerative changes. While these studies have demonstrated the utility of DWI in differentiating these processes, to our knowledge, no study has quantitatively validated the utility of DWI as a diagnostic test for spinal infections outside of the vertebral body, and no standards have been developed for ADC values representative of spinal infection. In this study, we correlated findings on DWI of the spine to results of microbiological or pathological sampling in patients with suspected spinal infection. The goals of this study were to assess the sensitivity and specificity of DWI in diagnosing spinal infections, to develop standards for DWI and ADC values as a diagnostic test for spinal infection, and to demonstrate that DWI of the spine can complement conventional MRI in difficult cases.

Methods & Materials

The institutional review board approved this study. The radiology and hospital database was searched for patients who underwent MRI of the spine with DWI for clinical suspicion of spine infection and also underwent confirmatory microbiological or pathological sampling. Thirty-seven cases of clinically suspected spinal infection with DWI and microbiological sampling between April 2002 and November 2010 were collected and reviewed retrospectively. In addition to DWI, the spine MRIs consisted of sagittal and axial T1, sagittal and axial FSE T2, and axial and sagittal post-gadolinium with fat saturation. Three patients were unable to receive intravenous gadolinium due to renal failure. DWI was performed in three directions (singleshot echoplanar: TR 2 x pulse-pulse interval; TE 15 msec; FOV 22; 256x144; 5.0/0.5 mm; B-value 400 sec/mm2) on one of four 1.5 T MR units (Signa, GE Medical Systems, Milwaukee, WI, USA; Achieva, Philips, Amsterdam, Netherlands). Regions of interest (ROI) were drawn on the ADC maps within areas of abnormal T2 signal or enhancement suspicious for infection and analyzed for mean ADC values. The mean ADC values were correlated with microbiological and pathological sampling. The sensitivity, specificity, and accuracy of DWI in detecting spinal infection were determined using the results of microbiological or pathological testing as a gold standard.

Results

The study population is described in detail in Table 1 (included in the Images section). All 37 patients underwent microbiological or pathological analyses and had areas of T2 signal abnormality and enhancement on standard MRI images compatible with possible infection. Twenty-eight patients (76%) had positive results on microbiological sampling, and 9 patients (24%) had negative results. Amongst the 28 patients with positive microbiological results, twenty-six (93%) had pyogenic infections (17 S. aureus, 4 Streptococcus, 2 S. epidermidis, 1 E. coli, 1 Enterococcus, 1 Pseudomonas), and two (7%) had atypical infections (Tuberculosis and Mucormycosis). Twenty-one of the 28 patients with positive microbiological results were already on antibiotic treatment at the time of their DWI. Mean ADC values in patients with positive microbiological sampling and in patients with negative microbiological sampling were 735+529 and 1877+602, respectively. The distributions in the two groups differed significantly (Mann-Whitney U=213, n1=28, n2=9, P=0.0004, two-tailed). Using an ADC value of 1250 or less as the cut-off value for a positive result for spine infection, sensitivity was 71%, specificity was 89%, and accuracy was 76%. The patients with false negative results (8/28) were all on antibiotic treatment at the time of their DWI. The false positive (1/9) result was eventually diagnosed with chronic inflammation related to treated osteomyelitis. Of the 21 patients on antibiotic treatment at the time of their DWI, DWI had a positive correlation with the results of microbiological testing in 13 patients using an ADC value of 1250 or less as the cut-off value for a positive result. Table 2 shows the specificity, sensitivity, and accuracy of DWI in diagnosing spinal infection using various empirically chosen ADC values as cutoff values for a positive test result. When using an ADC value of 1000, the most specific, but least sensitive value, as the diagnostic cutoff, 9 of the 10 false negative results were on antibiotic treatment at the time of their DWI, and the remaining false negative result had M. tuberculosis on microbiological testing. No false positives were found using this value. When using an ADC value of 1500, the most sensitive, but least specific value, as the diagnostic cutoff, all patients with false negative results (3/28) were on antibiotic treatment. Two of the three false positive results were eventually diagnosed with post-operative hematomas, and the remaining false negative result was eventually diagnosed with chronic inflammation. Positive Test---Sensitivity---Specificity----Accuracy 1000----------------0.64----------1-------------0.73 1100----------------0.68----------1-------------0.76 1250----------------0.71----------0.89----------0.76 1400----------------0.79----------0.78----------0.78 1500----------------0.89----------0.67----------0.84 Table 2. Sensitivity, Specificity, and Accuracy of DWI in Diagnosing Spinal Infection. Sixteen (57%) of the 28 patients with positive microbiological sampling had epidural infections, nine (32%) had paraspinal infections, and three (11%) had infections confined to the vertebral body and disc space. The mean ADC value in patients with epidural infection was 845+404, in patients with paraspinal infection was 1122+664, and in patients with infection confined to the vertebral body and disc space was 952+451. The distributions between the three groups did not differ significantly (Kruskal-Wallis H=1.6, P=0.44). Two of the patients with positive microbiological sampling underwent follow-up spine MRI with DWI after antibiotic treatment. Both patients had epidural abscesses that initially were reduced in diffusion (ADC values of 200 and 468). Follow-up DWI after antibiotic treatment demonstrated normalization of diffusion findings (ADC values of 1556 and 4021, respectively).

Conclusion

ADC values on DWI were found to be lower in patients with positive microbiological sampling than in patients with negative microbiological sampling. DWI of the spine may still demonstrate spinal infection even with the use of antibiotics and is a useful diagnostic test in the evaluation of spinal infection. DWI of the spine correlates well with the presence or absence of spinal infection and may also be a useful marker for response to antibiotic treatment.

References/Financial Disclosures

References: 1. Chang SC, Lai PH, Chen WL, et al. Diffusion-weighted MRI features of brain abscess and cystic or necrotic brain tumors: comparison with conventional MRI. Clin Imaging 26 2002 227-36. 2. Eastwood JD, Vollmer RT, Provenzale JM. Diffusion-weighted imaging in a patient with vertebral and epidural abscesses. AJNR Am J Neuroradiol 23 (2002) 496-8. 3. Eguchi Y, Ohtori S, Yamashita M, et al. Diffusion magnetic resonance imaging to differentiate degenerative from infectious endplate abnormalities in the lumbar spine. Spine. Epub 2010 Nov. 19. 4. Ferreira NP, Otta GM, do Amaral LL, da Rocha AJ. Imaging aspects of pyogenic infections of the central nervous system. Top Magn Reson Imaging 16 2005 145-54. 5. Hess CP, Mukherjee P. Visualizing white matter pathways in the living human brain: Diffusion tensor imaging and beyond. Neuroimaging Clinics of North America 17 (2007) 407-426. 6. Kim YJ, Chang KH, Song IC, Kim HD, Seong SO, Kim YH, Han MH. Brain abscess and necrotic or cystic brain tumor: discrimination with signal intensity on diffusion-weighted MR imaging. AJR Am J Roentgenol 6 (1998) 1487-90. 7. Pui MH, Mitha A, Rae WID. Diffusion-weighted magnetic resonance imaging of spinal infection and malignancy. J of Neuroimaging 15 (2005) 164-170. Financial Disclosures: None

[gallery]