Background information is very useful for providing a general understanding of a topic. Good sources for finding background information are found in both online and print reference book titles. Background sources provide overviews of topics, definitions of terms, statistics, sources for additional reading, and names of authorities in the field. Articles in databases can sometimes serve as basic introductions to a subject area.
Atlas of Ultrasound Measurements by 
Encyclopedia of Diagnostic Imaging by 
Merrill's Atlas of Radiographic Positioning and Procedures by 
Magnetic Resonance Elastography: Physical Background and Medical Application by Articles, books, and other materials in one comprehensive search!
Search for books available from other libraries in WorldCat and request them through Interlibrary Loan.