This study investigated the feasibility of in vivo
MR spectroscopy of the intervertebral disc, and evaluated the associations between spectroscopic parameters and imaging parameters (Pfirrmann Grade, T1ρ
, and disc height), discography status, and clinical assessment (SF-36 and ODI). The reproducibility of the water/PG, normalized water content, and normalized PG content was comparable to spectral metabolite reproducibility found in single-voxel 1H spectroscopic studies of the human brain31,32
. Additionally, elevated water/PG peak area ratio was evident in patients (compared to controls), and in discs with positive discography (compared to negative discography). T1ρ
exhibited similar trends between patients and controls, despite that fewer patients had T1ρ
data available due to insufficient scan time.
Both water loss and PG depletion increase with disc degeneration, so as the water and PG content decreases the water and PG signal goes down; the noise in the system, and body noise in MR remains the same, thus reducing the SNR of water and PG with disc degeneration. This trend was reflected in our measurements. Additionally, having sufficient SNR is imperative for quantification of intervertebral disc metabolic concentrations using in vivo
spectroscopy. In this study, the SNR of PG decreased with increasing grade of degeneration and ranged from 32 in discs with Pfirrmann Grade 1 to 7 in discs with Pfirrmann Grade 4. These SNR values are greater than those that have been reported in other studies of different anatomic regions33–36
, which used a threshold ranging from 2 to 4. Thus, this study considered an SNR>5 as adequate for the detection of an identifiable PG peak in the intervertebral disc.
Correlations between T1ρ
and disc biochemical composition (water, PG content) have been previously assessed in vitro
using biochemical assays in cadaveric human discs12,20
. Johannessen et al. reported that T1ρ
was correlated with PG per dry weight (r=0.67, p<0.01) and with water content (r=0.58, p<0.05). The current study corroborated the in vitro
results exhibiting similar associations between T1ρ
and both water and PG content. Additionally, the association between water/PG peak area ratio and T1ρ
suggests that these parameters may provide similar information regarding the biochemical composition of the intervertebral disc.
Links between morphologic changes that are detectable with traditional imaging techniques (radiographs, computed tomography, MRI) and painful IVDD are not well established in the literature37
. This study examined the relationship between both MR imaging and spectroscopic parameters and clinical symptoms in IVDD. While disc height was not significantly associated with clinical questionnaire scores, Pfirrmann Grade was significantly associated with SF-36 and ODI and water/PG peak area ratio was significantly associated with SF-36. In a previous study30
was found to be significantly related to clinical questionnaire scores, while Pfirrmann Grade was not. It is difficult to make comparisons about the relative strength of the association between imaging/spectroscopy parameters and clinical questionnaire scores due to the varying sample sizes for each imaging/spectroscopy parameter. However, this study does suggest that spectroscopic and imaging parameters are related to patient self-assessment of health status and clinical symptoms associated with low back pain.
It may be of interest to explore the links between the water/PG ratio and individual water and PG components with disc degeneration. The results demonstrate that both the water and PG concentrations are lower in patients compared to controls; however the water/PG peak ratio is higher. A similar trend was evident when comparing discs with positive and negative discography: the water and PG concentration was lower in positive discs compared to negative discs, however the water/PG ratio was higher. Both water loss and PG depletion increase with disc degeneration. The changes in PG and water content also affect the discs ability to withstand biomechanical loading, it is this mechanism that leads to the relationship between spectroscopy, pain and thus clinical symptoms and Oswestry grades. Our measurements imply that PG depletion was much faster than water loss in discogenic pain patients, which maybe consistent with loss of biomechanical properties, as well as being related to spectroscopy, and T1ρ imaging rather than simple T2 weighted imaging. While the relative changes in metabolic concentrations are exploratory, the results highlight that water and PG concentrations are altered with degeneration and MRS may be useful in assessing disc biochemical composition in vivo. Further studies of quantifying disc composition changes using biochemical analysis to validate the MRS findings are warrant.
One of the limitations of this study is that it focused on the assessment of water and PG metabolic resonances due to the low SNR of other metabolites. Other metabolite resonances such as the carbohydrate region (Carb) (3.50–4.20ppm), the choline region (Cho) (3.15–3.30ppm) and the lactate region (Lac) (1.15–1.40ppm) have been quantified in vitro
and implicated with disc degeneration20
, their in vivo
SNR was too low (approximately 1–2) for accurate quantification in this study. The low SNR may be potentially due to: a) the coil: while the in vitro
study used an 8 channel phased array transmit/receive volume coil, this in vivo
study used an 8 channel receive surface coil (only 6 elements were used for lower lumbar spine scans); b) physiological motion: in vivo
MRS is challenged by the presence of lipid in adjoining body marrow and bone susceptibility induced line broadening38
, making the assessment of lactate imprecise. Another limitation of the study is the uneven distribution of discs with different Pfirrmann Grades, and an insufficient number of discs with Pfirrmann grades 1, 4 and 5. While the current study concentrated on studying mild to moderately degenerated discs (Pfirrmann grade 2 to 3), including of healthy and severely degenerated discs would provide a more comprehensive assessment of IVDD.
The overall goal of this study was to demonstrate the feasibility of characterizing disc biochemical composition using in vivo MRS and MR imaging in subjects with symptomatic IVDD. The water/PG peak area ratio was related to discography status as well as clinical assessment. While T1ρ demonstrated similar trends as water/PG peak area ratio, future studies with larger sample sizes would be necessary to compare the utility of both parameters. The increase in water/PG peak area ratio and decrease in T1ρ with degeneration and pain suggest that these parameters may be potential markers for degenerative disc disease and associated clinical impact.
- The water/PG peak area ratio was significantly elevated in patients (compared to controls) as well as in discs with positive discography (compared to negative discography). T1ρ exhibited similar trends.
- MR T1ρ was significantly associated with MRS-quantified PG content but not MRS-quantified water content.
- MRS-quantified water, MRS-quantified PG, the water/PG ratio and Pfirrmann grade, were significantly associated with clinical self-assessment.