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The purpose of this study was to investigate the factors that help determine the needle tip location during computed tomographic (CT)-guided thin needle aspirations. We devised a model using a bottle filled with air, mineral oil, and water into which we could insert a needle. We then obtained CT images using both 5− and 10-mm slice thicknesses and determined the Hounsfield units (HUs) in regions of interest surrounding the needle shaft and some distance away from the needle shaft. We demonstrated that the presence of the needle increases the average HUs of the background both in the same region of interest and in the region of interest some distance from the needle. This effect is seen in air, fat, and water and thus occurs regardless of the attenuation of the background medium. A decrease in slice thickness causes this effect to be even more marked. These experimental results coincided with the theoretical results obtained from an equation that approximates the HUs of the pixels.