Pfirrmann scoring system for classification of degree of disk degeneration based on sagittal T2WI [5, 15] was commonly used as a standard in research and clinical applications [16, 17]. It was a subjective estimation and considered as a semi-quantitative assessment of IVDD in vivo. Our study aimed for more quantitative evaluation of IVDD depending upon the biochemical changes in disks by correlation between the Pfirrmann grades and both T2 mapping and ADC values to define which technique is more sensitive in detecting the early stage of IVDD.
Those new techniques were applied to assess patients with lumbar disk degeneration at different stages. Both T2 mapping and ADC maps were acquired for central (NP), peripheral (AF), and entire of the disk then analyzed separately; images were acquired in a sagittal plane so that the five lumbar disks were analyzed in a single plane.
The T2 value in IVD was mainly affected by both rotational and translational motion of water molecules in the collagen matrix [10, 18, 19]. On the other hand, ADC value was sensitive to water content in tissues but did not detect the translational motion of protons at the microscopic level . Thus, ADC value may not be as accurate as T2 relaxometry in evaluating water and collagen content, although it indirectly allowed evaluation of disk-matrix integrity [21, 22].
In our work, we found that T2 mapping and ADC values in the central and all disk values were significantly lower between the IVDD patients and volunteers (p value < 0.01) while no significant difference in the peripheral portions; this matched with the results of Huang et al.  that found T2 mapping values of the NP and posterior AF in their patients group were significantly less than those in the control group (p < 0.1) while the value of the anterior AF was similar between the patients and the controls (p > 0.05).
The mean age of examined patients in this study was 47.5 years old while in the study reported by Zhao F et al. , the mean age of affection was 54.4 years old. No significant difference in our study between males and females was found; this result was matched with a study done by Xiong X et al. .
In this study, percentage of disks was classified into Pfirrmann grade I (11.3 %), grade II (24%), grade III (28.7%), grade IV (28.7%), grade V (7.3%) while Stelzeneder D et al.  reported that six disks (1.8%) were classified as grade I, 189 (57.3%) as grade II, 96 (29.1%) as grade III, 38 (11.5%) as grade IV, and one disk (0.3%) as grade V.
The mean T2 values in our study in NP were higher than the means of T2 values of AF; this is symmetrical with the study done by Shen S et al.  who explained that by higher contents of water and proteoglycan .
Our study stated that T2 relaxometry values were found inversely proportional to the extent of disk degeneration except in grade V it was found to be increased again; this is in concordance with the study by Niu G et al. . Pandit P et al.  explained that as disk degeneration causes a decrease of glycosaminoglycan which leads to loss of water and hence dehydration of the disk and these effects were more evident at NP compared to AF. The mean T2 value of the NP region in our study ranged from 133.06 ± 32.85 to 109.18 ± 33.82 ms, normal. This agreed with Niu G et al. . That reported values ranged from 166 ± 23 to 59 ± 10 ms normal to degenerated disks respectively. The difference in values may be due to difference of age of population selected in this study, so there was a negative correlation between T2 values and semi-quantitative grading of disk degeneration which matched with the results of Marinelli NL et al. ..
There was also difference in the degree of correlation in NP (r = − 0.583; p ≤ 0.001), AF (r = − 0.214; p = 0.009) and entire of the disk (r = − 0.472; p ≤ 0.001) in concordance with previous studies which showed negative relationship between T2 values and disk degeneration [20, 31, 32]. Marinelli NL et al. explained these variations due to biochemical compositions and histologic changes with degeneration, reflected with differences in the degree of correlation in each region; upon this negative correlation, we conclude that a T2 value of nucleus pulposus is more sensitive than annulus fibrosus and entire of the disk.
The area under the curve values (AUC) for T2 in our study were all within the moderate accuracy range (0.7–0.9) except grade I:II that was indicating that T2 value-based grade scale is useful for evaluation of degenerative degree of intervertebral disk with high degree of objectivity in comparison to traditional Pfirrmann grading system based on visual assessment.
In our literature, we studied T2 cut-off value of NP between grade I and II, which was found to be ≤ 164 ms with sensitivity 97.2% and specificity 35.2%. The value between grade II and III was ≤ 104 ms with sensitivity 60.4% and specificity 83.3%. Between grade III and IV, the value was ≤ 93 ms with sensitivity 88.3% and specificity 65.1%. Between grade IV and V was > 93 ms with sensitivity 63.6% and specificity 88.3% as described before that T2 values tends to increase in grade V after decrease in grades from I to IV. Takashima et al.  reported that T2 cut-off value between grade I and II was found to be 116.8 ms, between grades II and III was > 92.7 ms, between grades III and IV was > 72.1 ms, and between grades IV and V was 72 ms.
ADC is a quantitative parameter calculated from diffusion-weighted imaging, is sensitive in detecting the diffusibility in tissue at molecular level . Previous studies had demonstrated that decreased diffusion was associated with a decrease in nutrient supply in IVDD , so ADC mapping could help to reflect the early degenerative changes of disk composition and any subtle changes in its integrity ; however, the signal intensity of T2WI in the disks were found to correlate with the ADC values .
Our study result for the mean ADC values in NP was (1.82 ± 0.29 and 1.72 ± 0.25 ms) for grades II and III respectively that were matched with those reported by Yu H.J et al.  who found mean ADC in NP (1.89 ± 0.08 and 1.65 ± 0.2 for grade II and III respectively); this difference may arise from that ADC values tend to decrease with the increase of the Pfirrmann grade and age in our study. In our study, mean ADC values in the NPs were lower than those reported in studies by Kealey SM et al.  and Niinimaki J et al.  who used also the sagittal plane for diffusion-weighted imaging that may be due to different age of group population.
The ADC values in grade IV and V in this study were more than what Yu H.J et al. detected ; this might be due to dissimilar composition of the patient population.
Niu G et al. , Yu H.J et al. , and Chen P et al.  found a negative correlation between ADC values and Pfirrmann grade and were explained by the changes occurring during degeneration which reduce water diffusion in NP and lower ADC values, since the ADC value was highly sensitive to changes of water molecules diffusibility which is in concordance with our study.
There was a difference in the degree of correlation in NP, AF, and entire of the disk in our study with “r = − 0.427, r = − 0.199, r = − 0.448” respectively that agree with study done by Niu G, et al.  who explained these variations due to biochemical compositions and histologic changes with degeneration, reflected with differences in the degree of correlation in each region.
The area under the curve (AUC) for ADC values in our study were all within the low accuracy range (0.5–0.7) except grade III:IV showing moderate accuracy (0.7–0.9) indicating that ADC value-based grade scale is useful for evaluation of degenerative degree of the intervertebral disk. There was weak negative correlation between age and T2 values of NP and entire of disk with (r = − 0.255, r = − 0.198) respectively with p value (0.002, 0.015) for NP and entire of disk respectively; this matched with Menezes-Reis R et al. . That found correlation between age and T2 relaxation time (r = − 0.30, p < 0.0001) with average age of participants 27.1 ± 4.8 years old.
Marinelli NL et al. , Niu G et al. , Wang W et al. , Boos N et al. , and Leung VY et al.  analyzed the correlation between the T2 and ADC values and age and reached the result that all the T2 values and ADC values were significantly decreased with the increase of age and spearman correlation coefficient of − 0.594 and − 0387 were demonstrated respectively. This were matched with our results with spearman correlation coefficient of − 0.472 and − 0.448 were demonstrated respectively. These results indicate when compared to ADC that T2 is a more sensitive and advanced measure to quantitatively delineate the early stage and age-related changes in IVDD.
The main result in our current study was that T2 and ADC values differ chiefly in their respective ability to discriminate Pfirrmann grades, as manifested by a completely different area under the ROC curves. Moreover, the T2 measure is more sensitive in differentiating the early stages of disk degeneration especially between grades I and III.
The major limitations in our study were absence of histological correlation with the results, absence of direct relation between the morphological changes of disks and T2 mapping and DWI, also subjective grading of disks by Pfirrmann grading