Diffusion-weighted imaging (DWI) and magnetic resonance spectroscopy (MRS) are the most widely used functional MRI in clinical diagnostic imaging, and have been widely used in the central nervous system for only a few years. Both of them can provide important information for the differential diagnosis of cranio-cerebral diseases, and combined with conventional MR performance, can significantly improve the diagnostic accuracy of cranio-cerebral diseases. 1. Differentiation of benign and malignant gliomas As the abnormal proliferating glial cells destroy the normal neurons, the typical manifestations of glioma MRS are significant decrease in NAA, moderate or significant decrease in Cr, and significant increase in Cho. The degree of malignancy of gliomas varies with the degree of alteration of each wave. It is generally accepted that MRS is more accurate in suggesting glioma grading than puncture biopsy because it provides information on tissue metabolism in a much larger area than biopsy. the sensitivity, specificity and accuracy of MRS in the identification of benign and malignant gliomas are 100%, 86% and 96%, respectively. Cho/NAA and Cho/Cr ratios are commonly used for differential diagnosis of benign and malignant gliomas. Among them, the Cho/NAA ratio better reflects the malignancy of the tumor. The more malignant the glioma is, the higher the Cho/NAA ratio. The ratio of Cho/NAA in malignant glioma and glioblastoma is usually above 4, mostly between 5 and 6, while grade I and II glioma is usually between 2 and 4. Lac wave is also closely related to the grading of glioma, and glioblastoma often shows more obvious lactate wave. MI wave can also provide important information for the grading of glioma, and MI/Cr in benign glioma is greater than that in malignant glioma. DWI can also provide important information for determining the benignity and malignancy of gliomas. The ADC values of benign gliomas were significantly higher than those of malignant gliomas and glioblastomas. The ADC value of benign glioma was 1.52×10-3mm2/s on average, while that of malignant glioma and glioblastoma was 1.23×10-3mm2/s on average, which may be related to the high cell density in the parenchymal part of malignant glioma. 2. MRS and DWI performance characteristics of meningioma Meningioma and nerve sheath tumor are extracerebral tumors. extracerebral tumors do not contain neurons, so NAA and Cr are not detected in MRS. When NAA and Cr appear, there may be two cases: one is extracerebral tumor infiltrating brain tissue, and the other is that the area of interest of wave spectroscopy exceeds the tumor range and contains part of brain tissue, which should be noted in clinical application. Significantly elevated Cho in meningiomas, with common alanine waves (1.2~1.4 ppm), is characteristic of meningiomas and can provide an important differential diagnosis for gliomas and meningiomas that are indistinguishable in the hemispheric convexity, and is important for the definitive diagnosis of atypical meningiomas in the lateral ventricles. However, it is not meaningful for differentiating meningiomas in the saddle region from pituitary tumors because the presence of alanine waves is also seen in pituitary tumors. MRS is of little significance in the differentiation of benign and malignant meningiomas, whereas DWI may provide useful information. Malignant or atypical meningiomas have high signal on DWI and low signal on ADC, and their ADC values are lower than those of the brain parenchyma, ranging from 0.45 to 0.69×10-3mm2/s, with a mean of 0.52×10-3mm2/s. The ADC values of benign meningiomas are mostly slightly higher or higher than those of the brain parenchyma Most benign meningiomas show isosignal on DWI and ADC maps or high signal on ADC maps. The low ADC value of malignant or atypical meningioma may be due to the large nuclear/pulp ratio of tumor cells and the high intracellular protein content which limits the diffusion of water molecules, or due to the low water content of tumor cells and the small extracellular space which reduces the diffusion of water molecules. The difference between benign glioma and viral encephalitis is often difficult to distinguish viral encephalitis with occupying effect in the temporal lobe and atypical clinical manifestation from benign glioma. The sensitivity and specificity of diagnosing brain tumor are 96% and 70%, respectively, when Cho/Cr ratio is greater than 2. When Cho/Cr is greater than 2.5, the sensitivity of diagnosing brain tumor decreases to 90%, but the specificity increases to 86%. However, it should be noted that the wave spectrum performance of a few benign gliomas can resemble normal brain parenchyma, and the Cho/Cr ratio is not greater than 2. Therefore, the diagnosis must be combined with clinical and conventional MR performance for comprehensive analysis and follow-up observation when necessary. 4. Differentiation of tumor necrosis and brain abscess Malignant gliomas and brain metastases often show necrosis with circumferential enhancement on enhancement scans, and it is difficult to differentiate them from brain abscesses, no matter they are single or multiple. DWI is the most convenient and effective way to differentiate the two. Tumor necrosis is low signal in DWI, high signal in ADC map, and ADC value is close to or slightly higher than cerebrospinal fluid, while brain abscess is high signal in DWI, low signal in ADC map, and its ADC value is significantly lower than that of normal brain parenchyma. The ADC values of brain abscess ranged from 0.58 to 0.7×10-3mm2/s, with an average of 0.63×10-3mm2/s, and the ADC values of tumor necrosis area ranged from 2.20 to 3.20×10-3mm2/s, with an average of 2.70×10 -3mm2/s. MRS is also helpful for the identification of the two. The abscess contains acetate, succinate and some characteristic amino acids, such as alanine and leucine, etc. The presence of these characteristic amino acids can determine the diagnosis of abscess. 5.Difference between primary malignant glioma and metastases MRS lack of NAA and Cr suggests metastases. If Cho is increased in the area around the tumor, it indicates infiltrative growth around the tumor of primary malignant glioma. DWI can provide useful information on the difference between the two based on the ADC value of the edema area around the tumor. There are more tumor cells in the edema area around the tumor of malignant glioma, and the ADC value may be close to the tumor parenchyma, while the ADC value of the vascular edema around the tumor of metastasis is significantly higher than that of malignant glioma. 6. All waves of radiation necrosis disappeared without Cho. 7. Differentiation between lymphoma and glioblastoma When lymphoma occurs in the corpus callosum, it needs to be differentiated from glioblastoma, and the presence of obvious Lip in the parenchyma of the tumor suggests that it may be lymphoma. The ADC values of lymphoma and glioblastoma also differ. Since the nuclear/plasma ratio of lymphoma is significantly higher than that of glioma, the ADC value of lymphoma is lower than that of glioblastoma, and the average ADC value of lymphoma is 1.15×10-3mm2/s, while that of glioblastoma is 1.23×10 The average ADC value of glioblastoma is 1.23×10-3mm2/s. 8. Differentiation between medulloblastoma and ventricular meningioma Both medulloblastoma and ventricular meningioma occur in children and have similar CT and conventional MR manifestations, so differentiation is often difficult. MRS is useful to differentiate them. Medulloblastoma is highly malignant, and MRS is similar to glioblastoma, with Cho/NAA ratio often above 5, while ventricular meningioma is mostly benign, with Cho/NAA ratio between 2 and 4, but mesenchymal ventricular meningioma may also be greater than 4. Medulloblastoma has high cell density, small extracellular space, large nucleus/pulp ratio of tumor cells, and significantly restricted diffusion. The ADC value is significantly lower than that of ventricular meningioma. The difference between paraneoplastic meningioma and cavernous hemangioma is that paraneoplastic meningioma is common, and paraneoplastic meningioma is also the most common site of extracerebral cavernous hemangioma, both of which are isointense or slightly hyperintense in CT, and the MRT1-weighted image is close to the brain parenchymal signal with significant uniform enhancement, so it is difficult to differentiate them. Meningioma shows significantly elevated Cho without NAA and Cr, while spongiform hemangioma shows no Cho, Cr, and NAA. Large cerebellar infarcts caused by obstruction of the main trunk of cerebellar blood supply artery often show obvious occupying effects, most commonly in inferior posterior cerebellar artery infarcts, but also in superior cerebellar artery infarcts and inferior anterior cerebellar artery infarcts, which need to be distinguished from cerebellar tumors. If the metastases have obvious necrosis, the enhancement scan will show circular enhancement, which is completely different from cerebellar infarction, which shows cerebral gyrus-like enhancement. However, it may be difficult to differentiate cerebellar metastases without significant necrosis from cerebellar infarcts during routine MR examination. The acute phase of cerebellar infarction shows significant high signal on DWI, and the ADC value is significantly lower, while the DWI of cerebellar parenchymal metastases is close to isosignal, and the ADC value is close to normal brain parenchyma. The MRS performance of cerebellar infarction is the most characteristic, appearing obvious Lac, which is obviously different from the substantial cerebellar metastases. 11. Differentiation between multiple gliomas and demyelinating masses Multiple gliomas and demyelinating masses are located in the white matter of the brain, multiple, both can have occupying effect, both can be enhanced on enhanced scan, and it is often difficult to differentiate them on conventional MR examination. Glioma MRS shows elevated Cho and Cho/Cr ratio greater than 2, while demyelinating lesions do not have high Cho and Cho/Cr ratio less than 2. Differentiation of epidermoid cysts from neuroepithelial cysts and arachnoid cysts Both epidermoid cysts and arachnoid cysts can occur in the saddle area, pontocerebellar horn, pineal area and ventricles, and both can show cerebrospinal fluid density and signal, which are sometimes difficult to differentiate by conventional MR examination. It is sometimes difficult to distinguish between them on routine MR examinations. DWI is an effective way to differentiate between epidermoid cysts, which show high signal on DWI, and arachnoid cysts and neuroepithelial cysts, which show low signal on DWI. 13, Differentiation of cerebral white matter lesions Diffusion-weighted and T2-weighted images show high signal and decreased ADC values in cerebral white matter lesions including: recoverable cerebral ischemia, acute and subacute cerebral infarction, multiple sclerosis, Wallerian’s, Jakob-Creutzfeldt’s disease, spongiform leukoencephalopathy, central myelinolysis of the unstable brain, and phenylketonuria. White matter lesions with high signal on diffusion-weighted and T2-weighted maps and normal or elevated ADC values include amyotrophic lateral sclerosis, acute disseminated encephalomyelitis, multiple sclerosis, and progressive multifocal leukoencephalopathy. Spongiformity is due to the accumulation of NAA in the brain and can be diagnosed clinically on the basis of elevated blood and urine NAA, with hydrogen proton spectra showing markedly elevated NAA waves. Phenylketonuria is seen as an abnormal metabolic wave at 7.3 ppm on short TE wave spectroscopy. Heterochromic cerebral leukodystrophy shows elevated MI waves on hydrogen proton spectroscopy. In conclusion, DWI and MRS can provide important differential diagnostic information for the differential diagnosis of many cranial diseases, but clinical application still requires a comprehensive analysis in combination with the performance of conventional CT and MR.