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There are many different causes of intracranial lesions including infection, inflammation, stroke, tumors, and trauma. To investigate the nature of a lesion, various MR (magnetic resonance) and PET (positron emission tomography) techniques are used that can give information about the anatomical, physiological, and metabolic properties of the lesion and thereby aid in the differential diagnostic procedure. The treatment for brain tumors can include radiation therapy, which can induce brain changes that are difficult to differentiate from a recurring tumor. This thesis evaluates the investigation of intracranial lesions with several MR and PET techniques.  

Paper I delt with an evaluation of how much additional information MRS (magnetic resonance spectroscopy) provides in clinical patients compared to MRI (magnetic resonance imaging). In this study that included  208 cases, it was found that the additional information gained from MRS was beneficial or very beneficial in 15% of the cases and misleading in 17% of the cases. In Paper II, a sub-population (n = 100) of the patients in Paper I  was investigated. In this paper, use of a decision-support system, INTERPRET DSS 3.1, was compared with conventional analysis of MRS with regard to the correct evaluation of focal lesions. Comparing INTERPRET DSS with conventionally analyzed  MRS and MRI, the diagnostic category was correct in 67/58/52 cases, indeterminate in 5/8/20 cases, and incorrect in 28/34/28 cases. In Papers III–IV, the differentiation of tumor recurrence from treatment-induced changes was evaluated. In Paper III, MR examinations were performed for calculation of the perfusion fraction (f) using the intravoxel incoherent motion imaging (IVIM) technique and for the relative cerebral blood volume (rCBV) using dynamic susceptibility contrast (DSC) perfusion in 60 patients. The accuracy of the IVIM parameter f was similar to that of rCBV in differentiating tumor recurrence from treatment-induced changes. In Paper IV, two PET techniques (11C-methionine and 18F-fluorothymidine) were compared in 48 patients. Both techniques had similar efficacy in differential diagnosis between recurrent intracranial tumor and treatment-induced changes. 

In conclusion, conventionally analyzed MRS did not add to the diagnostic value of MRI in general. In focal lesions, the INTERPRET DSS system did not improve the categorization of the lesions significantly compared to conventional analysis of MRS but did so compared to MR imaging alone. IVIM can be used to differentiate tumor recurrence from treatment-induced changes. The PET tracers 11C-methionine and 18F-fluorothymidine have a similar diagnostic ability to separate tumor recurrence from treatment-induced changes.