A scalpel is a very sharp knife, made to be used for surgery. Scalpels were developed for cutting through tissue. In 2013, most scalpels used for surgery had blades which are only used once. The blade is replaced after use. Sometimes, the lower part of the scalpel is re-used. The blades are made of different kinds of metal or steel. In some cases, titanium, diamond or obsidian are used. Magnetic resonance imaging uses magnetism to generate an image. This means that people performing operations in such an environment cannot use metal tools. In some cases, scalpels are replaced with lasers, or with electrocautery. Ancient Egyptian medicine used scalpels for enbalming; the material used was obsidian, it is still in use today. Medicine in ancient Rome used over 150 different tools, among them, the scalpel.