For example, CT will show a liver lesion that is invisible on a plain x-ray of the abdomen. MRI uses magnetic fields to demonstrate hydrogen molecules in your body. They have different effects on the magnetic field. MRI can demonstrate abnormal areas in organs and also in bone. It is the most sensitive way to scan for bone mets and is the best study for bone metastases.
Nuclear medicine use very small amounts of radioactive material.
Different substances are used depending on the type of scan. In bone scans the areas light up at sites of arthritis, metastasis and fracture. That is the scan that lights up at sites of bone repair or overgrowth. In the case of cancer they light up at metastasis where there is bony repair occurring. This is what causes the hot spots.