Final answer:
The visibility of tumors with only slight CT number differences from surrounding tissue may be limited by a CT scanner's contrast resolution, which needs to be high to discern subtle density variations.
Step-by-step explanation:
The limited visibility of tumors in a CT scan used to evaluate the brain for metastatic disease, where the average CT number of tumors differs only slightly from the surrounding brain matter, is often due to the scanner's contrast resolution. Contrast resolution is the ability of the scanner to distinguish between differences in density or the CT numbers of various tissues. If the contrast resolution is not high enough, subtle differences between the CT numbers of the tumors and the normal brain tissue may not be discernible, making it difficult to detect the tumors. Factors related to the scanner's hardware, such as the X-ray tube's heat capacity, detector sensitivity, and the image reconstruction algorithms, can impact the overall contrast resolution. Additionally, the use of a higher radiation dose can sometimes improve detail at the expense of higher exposure to the patient, which is a health consideration, especially for pediatric or frequently imaged patients.
Overall, the ability to detect slight differences in tissue density on a CT largely depends on the scanner's contrast resolution, in conjunction with the optimization of scanning parameters and radiation dose to achieve the best possible image quality while minimizing patient exposure.