CEST MRI (1) is a novel non-invasive molecular imaging technique having great potential in molecular characterization of tissues and hence in disease diagnosis. CEST contrast at offset frequency of +3.5ppm from water protons resonance, at low saturation RF power, has been reported to be sensitive to amide protons of peptides and small proteins (2, 3). This contrast is widely known as amide proton transfer (APT) contrast. Objective of the study was to compare APT maps with DCE perfusion parameters map and to evaluate its diagnostic and grading value in intra-cranial mass lesions (low grade tumors, high grade tumors, Central Nervous System (CNS) Tuberculosis, CNS Lymphoma (6)).

All the MRI experiments were performed at 3T whole body Inginia MRI system (Philips Healthcare, The Netherlands) using a body coil excite and a 8 channel head coil for reception. Twenty-two patients with pathologically proven, solitary, contrast-enhancing tumors and having different kinds of intra-cranial mass lesion (low grade tumor(n=9), high grade tumor(n=8), tuberculoma (n=1), tubercular abscess (n=1), lymphoma(n=3)) were scanned for MRI data in the current study. APT W images were acquired at following frequency offsets:±2.5, ± 3, ± 3.5, ±4, ±4.5 ppm from water. The WASSR approach was used to determine B0 maps (range, -1.5 to1.5 ppm; interval, 0.125 ppm).

Data Processing: Pre-processing was performed on APT-W images for background noise removal. B0 map were generated and CEST or APT images were corrected for B0 inhomogeneity followed by asymmetry analysis (8, 9) for computing APT map.

APT contrast = 100*[Msat (-3.5ppm) – Msat (+3.5ppm)]/ Msat (-3.5ppm)

APT map and perfusion weighted images was registered with T1-weighted images for motion correction. Region of interest (ROI) analysis (mean and standard deviation) was performed on combination of T1 weighted images, GD-CE images, T1 dynamic images. Same ROI are being applied on APT maps, and other perfusion parameters (like cbf, cbv, cbv corrected, cbv corrected for ve, kinetic parameters like(Ktr, Kep), leakage volume, ve and vp). Two ROI in WM and GM were drawn on each patient. ROIs on lesion, contra-lateral side, core portion and peripheral part of lesion were also drawn. Signal intensities are being measured in different ROIs. We studied Intra-class correlation co-efficient (ICC) between APT and other perfusion parameters for entire lesion region (ROI-1), core region within lesion (ROI-2) and peripheral portion of lesion (ROI-3) for twenty two patients having intra-cranial mass lesions (Table no.1). T-test was also performed for evaluating difference of APT values between low and high grade tumors.

APT maps (Second column in Figure 1) show a difference in values between white matter (WM), gray matter (GM) and intracranial lesions. ROIs in the WM regions showed –ve APT contrast value while GM regions mostly showed a +ve contrast values. Intracranial lesions in the current study showed significantly higher APT contrast compared to contra-lateral side, can be significantly differentiated (P<0.01). From this we can see that APT values are weakly correlated with all of the perfusion parameters; thus it might have potential in improving diagnostic value alone or in combination with other perfusion parameters. ICC between normalized APT and normalized CBV was -0.364 which is non-significant .Significant differences were observed between high- and low-grade tumors (P < 0.01). A significant difference between low and high grade tumors are observed for APT.In conclusion, all types of intracranial lesions in the current study showed significantly higher APT contrast compared to contra-lateral side. APT images are weakly correlated with other perfusion parameters; therefore, its inclusion might improve diagnostic and grading value for different intra-cranial diseases.