Background: Animal models are frequently used to assess new treatment methods in cancer research. MRI offers\r\na non-invasive in vivo monitoring of tumour tissue and thus allows longitudinal measurements of treatment effects,\r\nwithout the need for large cohorts of animals. Tumour size is an important biomarker of the disease development,\r\nbut to our knowledge, MRI based size measurements have not yet been verified for small tumours (10-2ââ?¬â??10-1 g).\r\nThe aim of this study was to assess the accuracy of MRI based tumour size measurements of small tumours on\r\nmice.\r\nMethods: 2D and 3D T2-weighted RARE images of tumour bearing mice were acquired in vivo using a 7 T\r\ndedicated animal MR system. For the 3D images the acquired image resolution was varied. The images were\r\nexported to a PC workstation where the tumour mass was determined assuming a density of 1 g/cm3, using an\r\nin-house developed tool for segmentation and delineation. The resulting data were compared to the weight of\r\nthe resected tumours after sacrifice of the animal using regression analysis.\r\nResults: Strong correlations were demonstrated between MRI- and necropsy determined masses. In general,\r\n3D acquisition was not a prerequisite for high accuracy. However, it was slightly more accurate than 2D when small\r\n(<0.2 g) tumours were assessed for inter- and intraobserver variation. In 3D images, the voxel sizes could be\r\nincreased from 1603 Ã?µm3 to 2403 Ã?µm3 without affecting the results significantly, thus reducing acquisition time\r\nsubstantially.\r\nConclusions: 2D MRI was sufficient for accurate tumour size measurement, except for small tumours (<0.2 g)\r\nwhere 3D acquisition was necessary to reduce interobserver variation. Acquisition times between 15 and 50\r\nminutes, depending on tumour size, were sufficient for accurate tumour volume measurement. Hence, it is\r\npossible to include further MR investigations of the tumour, such as tissue perfusion, diffusion or metabolic\r\ncomposition in the same MR session.
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