Comparison between ultra-high and conventional mono b-value DWI for preoperative glioma grading

To compare the efficacy of ultra-high and conventional mono-b-value DWI for glioma grading, in 109 pathologically confirmed glioma patients, ultra-high apparent diffusion coefficient (ADCuh)was calculated using a tri-exponential mode, distributed diffusion coefficients (DDCs) and α values were calculated using a stretched-exponential model, and conventional ADC values were calculated using a mono-exponential model. The efficacy and reliability of parameters for grading gliomas were investigated using receiver operating characteristic (ROC) curve and intra-class correlation (ICC) analyses, respectively. The ADCuh values differed (P < 0.001) between low-grade gliomas (LGGs; 0.436 ×10−3 mm2/sec) and high-grade gliomas (HGGs; 0.285 × 10−3 mm2/sec). DDC, a and various conventional ADC values were smaller in HGGs (all P ≤ 0.001, vs. LGGs). The ADCuh parameter achieved the highest diagnostic efficacy with an area under curve (AUC) of 0.993, 92.9% sensitivity and 98.8% specificity for glioma grading at a cutoff value of 0.362×10−3 mm2/sec. ADCuh measurement appears to be an easy-to-perform technique with good reproducibility (ICC = 0.9391, P < 0.001). The ADCuh value based in a tri-exponential model exhibited greater efficacy and reliability than other DWI parameters, making it a promising technique for glioma grading.


INTRODUCTION
The preoperative grading of gliomas, which is critical to determine the optimal therapy, remains unsatisfactory [1,2]. Histopathology remains the gold standard for brain glioma diagnosis [3]. However, the histopathological grading of glioma is frequently biased because of the intratumoral heterogeneity of the tumor sample from stereotactic biopsy or surgical resection. This biased histopathological grading leads to the improper therapeutic strategy [4]. An unbiased preoperative grading based on the information of the whole tumor is urgently in that they permit more uniform sampling of the whole tumor than obtained by heterogeneous biopsies, and thus demonstrate a promising future in glioma grading.
Diffusion-weighted imaging (DWI) is considered to be the most sensitive to detect early pathological changes and demonstrated potentials for noninvasive glioma grading in previous studies [3,5,6]. A series of diffusion-weighted models using an extended b-value range have been introduced to describe the different aspects of tissue diffusion properties [7]. Conventional DWI based on only two b-values (so-called monoexponential model, usually 0 and 1000 sec/mm 2 in the brain) provides unique information on tissue functional structure [8]. Intravoxel incoherent motion (IVIM) DWI can be used to extract the perfusion-related information Clinical Research Paper www.impactjournals.com/oncotarget from a diffusion sequence by collecting both low b-values (< 200 sec/mm 2 ) and high b-values (usually 200 -1000 sec/mm 2 ) in vivo [9], therefore providing information on tumor cellularity and microcirculation without using contrast. Unlike conventional DWI, the stretched exponential model provides a new type of diffusion heterogeneity [10].
Recently, ADC values derived from the high b-values (b = 3000 sec/mm 2 ) DWI were reported to improve the diagnostic performance in differentiating high-from low-grade gliomas [11]. However, a conventional mono-exponential model was used in that study. Furthermore, ADC uh signals at ultra-high b-values: 2,000 -5,000 sec/mm 2 ) values of the globus pallidus, putamen, and substantia patients than those in control subjects, while standard ADC (ADC st 1,000 sec/mm 2 * ) values from the corresponding There is also evidence that ADC may be related to the grade, proliferation, angiogenesis, cell migration and invasion [16][17][18][19]. We therefore hypothesize that ADC uh is useful in grading gliomas. It is generally accepted that the various DWI models have varied potential for preoperative glioma grading [2][3][4]20]. It is therefore important to compare the diagnostic as to design optimal scan protocols. We retrospectively and stretched-exponential model DWI for glioma grading, and a tri-component model was used to calculate ADC uh value based on 18 b-values (up to 4,500 sec/mm 2 ).

Parametics comparison between LGG and HGG
The descriptive statistics of the DWI parameters comparison between LGG and HGG is shown in Table 2. The mean ADC uh value was 0.436 × 10 mm 2 /sec in LGG and 0.285 × 10 mm 2 were found in peritumoral edema area (ADC uh_edema ) and contralateral healthy white matter area (ADC uh_wm 0.05). For the parameters from the stretched-exponential
For each parameter from conventional monoexponential model DWI (b-value: 500, 800, 1000, 1300, 1500, 1700, 2000, 2500, 3000, 3500, 4000 and 4500 sec/ mm 2 ADC 1000 parameter achieved a relatively higher diagnostic 1.115 × 10 mm 2 /sec (Figure 3), with a similar diagnostic As shown in Table 4, the inter-observer ICC value for ADC uh for DDC and ADC st were all higher than 0.75 (all of quantitative MRI parameters. However, relatively poor www.impactjournals.com/oncotarget DISCUSSION uh value based on tri-exponential model DWI was useful in reliability over other parameters from DWI models. We also determined the most appropriate cutoff values for different DWI parameters, which could potentially be used in clinical practice regarding preoperative glioma grading. owing to the increased cellularity and nuclear cytoplasmic decreased ADC uh Interestingly, we noticed that ADC uh was dramatically lower than the conventional ADC values, even the ADC 4500 both in LGG and HGG. Two possible explanations for this difference are as follows. First, according to the mono exponential model, the diffusion signal intensity and ADC value of tumor constantly decreased with the increasing in the membranes is the main inhibitor of water diffusion brain water homeostasis [22,23], and can modulate ADC values under normal and pathological conditions [24,25]. uh between low-and high-grade gliomas may indicate the different In our study, we also evaluated glioma grading using multiple b-value DWI based on the stretched exponential [10,20], and can be explained by the fact that HGG, in particular glioblastoma (52.3% of included patients), is Note: * LGG = low-grade gliomas; HGG = high-grade gliomas; ADC uh = ADC calculated using the ultra-high b-values from tri-component model DWI; DDC = = water molecular diffusion heterogeneity index; ADC n = ADC calculated using monoexponential model DWI (b value= 0, n sec/mm 2 ). www.impactjournals.com/oncotarget  a3-e3. Box plot of ADC value between LGG and HGG shows that ADC value decreased along with the increase of b value both in LGG and HGG, with www.impactjournals.com/oncotarget   Note: ICC = intraclass correlation; 95% CI = ADC uh = ADC calculated using the ultra-high b-values from tri-component model DWI; DDC = water molecular diffusion heterogeneity index; ADC 1000 = ADC calculated using mono-exponential model DWI (b value = 0,1000 sec/mm 2 ). associated with considerable histological heterogeneity exponential model may play a potential role in glioma grading.
Although using a greater number of b-values can improve the accuracy of the ADC measurements [27], the conventional mono-exponential model has been successfully and widely used today in clinical practice. from different b-values were valuable in glioma grading, but the ADC 1000 achieved the highest differentiating recognize the anaplastic components (grade III) under the background of grade II gliomas, at the relative lower 2 ) DW images, such as b-value of 500 or 800 sec/mm 2 , while it may be clear mm 2 ) DW images. Similar results were indicated in a previous study, in which it was shown that T2-weighted images plus DW images with a b-value of 2,000 sec/mm 2 are superior to T2-weighted images plus DW images with a b-value of 1,000 sec/mm 2 in the detection of prostate cancer [28]. In addition, the valuable quantitative information on biological tissue may be missed because of the fact that low b-values of a few hundreds to 1000 sec/mm 2 complex intracellular space and membrane interactions remain invisible [7]. The present study showed a striking decay in HGG on the DW images at ultra high b-value 2 ), which can be used to the glioma grading visually.
In this study, the ICC analysis was performed to explore the measurement consistency between the two independent radiologists. The results showed that ICC value of ADC uh was close to 1, and ICC value of DDC and ADC 1000 were all higher than 0.75. However, the ICC also obtained ADC uh data from contralateral healthy white matter area and peritumoral edema area in glioma patients. uh_edema and ADC uh_wm between low-and high-grade gliomas, which also suggested the reliability of the ADC uh data in our study.

Limitations
Our study has some limitations. First, as an initial research on tri-exponential model based on MR DWI, we expression and ADC uh in gliomas, and further research is warranted to clarify this issue. Second, hand-drawn ROIs were used in the current study. Since the nests of tumor cells tend to be heterogeneously distributed, a IVIM parameters because of the fact that we only used four b-values within 200 sec/mm 2 for a bi-exponential related parameters accurately.

Subjects
Between July 2014 and September 2015, 267 patients with suspected glioma underwent routine MRI, 18 b-value DWI, as well as contrast-enhanced MRI of the brain before any treatment. Among them, 55 patients 79 patients were excluded for no pathological diagnosis due to the absence of surgical or biopsy samples; and 24 patients were excluded for the following reasons: poor image quality or motion artifact (n = 7); large cystic tumors with a slim rim (n = 6) or solid tumor is < 0.5 cm in diameter (n = 3); and receiving corticosteroid therapy before MRI examination (n = 8). Finally, 109 years; range, 2 -87 years) were enrolled in the current study ( Figure 4 and Table 1). This retrospective study was approved by the local ethics committee, and informed consent was waived.

Brain MR imaging
The whole brain MRI examinations were performed on a 3.0-T MRI system (MR750, GE Healthcare, Milwaukee, WI, USA) with a standard receive-only head coil. Conventional MRI, DWI with 18 b-values (0-4500 sec/mm 2 ) and contrast-enhanced MRI were performed during the same examination.
The multi-b-value DWI sequence was performed prior to the contrast injection. Eighteen b-values (0,  50, 100, 150, 200, 300, 500, 800, 1000, 1300, 1500, 1700, 2000, 2500, 3000, 3500, 4000 and 4500 sec/mm 2 ) were applied with a single-shot diffusion-weighted spin-echo echo-planar sequence. The lookup table of was used with an acceleration factor of 2. A local shim box covering the whole brain was applied to minimize susceptibility artifacts. In total, 20 axial slices covering the entire brain were obtained with a 24 cm × 24 cm FOV, 5 mm slice thickness, 1.5 mm slice gap, 3,000 ms TR, Minimum TE, 128 × 128 matrix. With the increase ensure a good signal noise ratio. The total scan time was 5 min and 31 sec.
Finally, a contrast-enhanced T1-weighted spin echo sequence was performed in the transverse, sagittal, and coronal planes (TR/TE, 1850 ms/24 ms for transverse plane and 1750 ms/24 ms for sagittal and coronal planes; other parameters were the same as conventional MRI) following a bolus injection of 0.1 mmol/kg of gadodiamide (Omniscan; GE Healthcare, Co.Cork, Ireland).
The conventional ADC value was calculated by 0 image and DWI at each b-value other than 0 s/mm 2 into the mono-exponential equation (Eq. (1)) [29], where S b is the diffusion weighted signal intensity for the b-value, and S 0 is the signal intensity obtained with the b 0 value.
where S b is the signal magnitude with diffusion weighting b, S 0 is the signal magnitude with no diffusion weighting, heterogeneity, varying between 0 and 1, and the DDC is intravoxel diffusion rates.
All data were analyzed and processed on a GE ADW4.6 workstation. All regions of interests (ROIs) were determined by two radiologists (Dr. Y.-C.H. and L.-F.Y., with 12 and 6 years of experience, respectively) at the workstation.
First, we reviewed the conventional plain and contrast-enhanced MRI images carefully to determine the solid part of each tumor. Next, the DWI data were analyzed. A freehand region of interest (ROI) was traced using an electronic cursor, which was placed to include the relatively higher signal intensity on DW image and lower ADC value in ADC map, avoiding large vessels, hemorrhagic, cystic and necrotic areas. The mean ROI area of the lesions was 94.1 ± 76.0 mm 2 . The standardized ROI were placed in the peritumoral edema and contralateral normal appearing white matter (WM) to calculate the ADC uh_edema and ADC uh_wm value (as shown in Figure 5a1 or 5a2), respectively. The parameter maps of ADC, ADC uh and tri-exponential diffusion signal decay curve (as shown in Figure 5b1 or 5b2) over a wide-range of b-values (up to 4,500 sec/mm 2 ) were automatically generated, and the mean ADC, ADC uh obtained, respectively.
In addition, estimated signal-to-noise (SNR) was calculated for gray matter (GM), WM and lesions at 4500 sec/mm 2 b-values as mean SI of all ROIs divided by standard deviation of background noise (measured on a small ROI outside the signal region) [30]. In all subjects, the mean SNR at the highest b-value (4,500 sec/mm 2 ) was large enough to ensure proper depiction of the signal, and the mean SNR (range) for WM, GM and lesions were as follows: 70.0 (23. 6  WHO grade II and III, or grade III and IV gliomas were included in the group of high-grade tumors. Numerical variables were denoted as the mean and standard deviation. The Kolmogorov-Smirnov (K-S) test was used for assessing the normality of data distribution. uh , ADC uh_ edema , ADC uh_wm the differences between the low-grade gliomas (LGG: WHO I and II) and high-grade gliomas (HGG: WHO III and IV) by using independent sample t test. Receiver operating characteristic (ROC) analyses were performed to determine optimal thresholds for differentiating the LGG from HGG by conventional ADC, ADC uh , DDC The ICC were used to assess the data consistency of repeated measures of the same parameter. The ICC was interpreted as poor if it was less than 0.4, as moderate difference. All statistical analyses were performed with CONCLUSION Different DWI models can be used for accurate preoperative glioma grading. However, the ADC uh variables from conventional mono-b-value and stretchedexponential models.

ACKNOWLEDGMENTS AND FUNDING
We would like to thank Dr. Dandan Zheng and Tianyong Xu in GE Healthcare China for providing technical support regarding the appropriate applied multiple b-value DWI sequence. Natural Science Foundation (No. 2014JZ2-007).