Researchers and clinicians who are interested in clinical CEST imaging applications.Gliomas arise from astrocytes, oligodendrocytes, and ependymal cells, and encompass five sections with 18 different pathologically defined ailments according to the 2007 WHO classification. Each glioma type demonstrates unique morphological and molecular features. Besides, the spatial heterogeneity of these tumors complicates their diagnosis and treatment. Currently, MRI is an indispensable modality for imaging brain tumors. However, existing clinical MRI sequences are not sufficiently tissue specific. For example, roughly 10% of glioblastoma and 30% of anaplastic astrocytoma demonstrate no Gd enhancement, while low-grade gliomas occasionally enhance (1). Amide-proton-transfer (APT) imaging is a novel molecular technique that gives contrast based in large part on endogenous cellular proteins in tissue (2,3). It has been shown that APT imaging for malignant brain tumors and many other cancers has much potential (4-8). Here, we explore the use of 3D APT MRI technique for guiding the stereotactic biopsy in patients with newly diagnosed gliomas, with the goal of validating the accuracy of the APTw signal as a surrogate tumor biomarker.

24 patients with suspected, newly diagnosed gliomas of varying grades (without surgical intervention, radiotherapy or chemotherapy), who signed informed consent, were recruited and scanned within three days prior to their surgical procedure at 3T. A fast 3D APT imaging sequence (RF saturation power = 2 μT; saturation time = 800 ms; 15 slices of slice thickness = 4.4 mm) was used (6). To correct for B0 field inhomogeneity effects, APT imaging was acquired with a six-offset protocol (±3, ±3.5, ±4 ppm from water), together with a WASSR scan (9). The total scan time was 10 min 40 sec. APT-weighted (APTw) images were calculated using MTRasym(3.5ppm) (3).

Patients proceeded with their clinically indicated brain biopsy after MRI scanning. For each patient, two-to-four feasible and meaningful biopsy sites were determined after reviewing APTw and conventional MR (T₂w, FLAIR and Gd-T₁w) images. These ROIs were labeled on the co-registered MR image in the BrainLab neuro-navigation system. In the operating room, the exact site of sampling was marked by a screenshot image (Fig. 1). Biopsies were hematoxylin-and-eosin-stained (H&E) and Ki-67 (MIB-1 antibody)-stained. Histology was reviewed by a neuropathologist, blinded to the imaging features, based on mitosis, proliferation, and pleomorphism of tumor cells, and tumor vascular morphological characteristics. Tumor cell density (cellularity) and Ki-67 positive cells (proliferation) were further counted by software ImagePro on microscope captured digital photos.

A neuroradiologist recorded the APTw intensities of all corresponding ROIs for each patient. The APTw signal intensity for each targeted tissue sample, compared with the contralateral normal brain area, was reported. Pearson’s correlation analysis was used to evaluate the correlation between the APTw intensities and cellularity or proliferation.

APTw image and histopathologic characteristics of gliomas

13 patients were confirmed with histopathology to have high-grade glioma (seven with glioblastoma; one with gliosarcoma; five with anaplastic astrocytoma). Most of these high-grade tumors showed gadolinium enhancement (Fig. 2), but three did not. However, all of these high-grade gliomas consistently showed APTw-hyperintense foci, compared with the contralateral normal brain areas. 11 patients were histopathologically diagnosed with low-grade glioma (four with low-grade oligodendroglioma; five with low-grade astrocytoma; two with low-grade oligoastrocytoma). Most of these low-grade tumors demonstrated no gadolinium enhancement (Fig. 3), but three did show small areas of gadolinium enhancement. APTw images showed iso-intensity (or mild punctate hyperintensity) within any of the low-grade lesions.

Quantitative analysis of biopsied tissues

Of all 70 biopsied tissues, 33 were high-grade gliomas, 29 were low-grade gliomas, and eight were peritumoral edematous tissues without tumor cells. The maximum APTw signal intensities for all biopsied sites for each patient were significantly higher in high-grade gliomas than in low-grade gliomas (3.43 ± 0.36% vs. 2.12 ± 0.46%, P < 0.001; see Fig. 4). Using the pathological results as the gold standard, the ROC analysis showed that the area under curve (AUC) for APTw to differentiate high-grade from low-grade gliomas reached up to 1. The Pearson’s correlation analysis showed that APTw signal intensities had strong positive correlations with cellularity (R = 0.681, P < 0.001) and Ki-67 (R = 0.541, P < 0.001; see Fig. 5).

These early results suggest that the APTw signal is a valuable imaging biomarker to identify the spatial extent and pathological grade of gliomas. APTw hyperintensity is a typical feature of high-grade brain tumors, independent of Gd enhancement. APTw image-guided biopsy can potentially reduce the randomness of surgical decisions due to tumor heterogeneity.