The invention relates to octapeptides being octreotide derivatives and also to imaging agents based on it used to diagnose tumours, i.e. in chemistry and medicine. Octapeptide being an octreotide derivative has common formula (1):where X and Y are H, or chelating group, namely OTA, covalently bound to α - or ε-amino groups of N end lysine. Octapeptide with formula (1) forms a chelating complex with radionuclides, e.g. in the form of gallium isotopes 68Ga,66Ga, 67Ga to obtain the imaging agent.The imaging agent is used for diagnosing tumours expressing somatostatine receptors or their metastases.3 non-protected formulas, 1 protected formula, 8 examples, 3 tables, 1 figure