Introduction

In modern imaging department today images of patients are produced using either electromagnetic radiation (visible light, X-rays, radio waves in magnetic resonance imaging) or ultrasound (pressure waves). X-rays have a frequency and photon energy that are several orders of magnitude higher than those of visible light and they can penetrate the patient's body. Their photon energy is so large that they can break chemical bonds and induce ionization. X-rays can be detected on radiographic film and by different fluorescent materials.

The radio waves used in magnetic resonance imaging have a frequency and photon energy that are many orders of magnitude lower than those of visible light. Like X-rays they can penetrate the human body. They have not sufficient energy to induce any ionization, but they can make molecules vibrate, which means that they produce heat. These radio waves are detected by antennas.

Ultrasound consists of pressure waves of a much higher frequency than those of audible sound. Ultrasound energy is both produced and detected by piezoelectric crystals. Ultrasound propagates through the body and causes vibrations of molecules which again produce heat in the tissues.

All contrast media in diagnostic imaging have one task, to increase the differences between the different "voxels" in the body regarding their ability to absorb and/or reflect energy from electro-magnetic radiation or ultrasound. A "voxel" in this context may mean any structure, such as a piece or slice of normal tissue, or a complete organ, or a pathologic process or any other morphologic detail. Different contrast media influence electro-magnetic radiation or ultrasound by different mechanisms.

To perform their task the contrast media should reach different concentrations in different structures or "voxels". The larger the difference in contrast medium concentration between those structures, the smaller those structures (representing morphological details) can be while remaining detectable in the images.

A good contrast medium must influence electro-magnetic radiation or ultrasound energy inside the body, but should, ideally, not have any other effects on living tissue. Unfortunately, this is impossible and all contrast media have adverse effects.

Torsten Almén and Peter Aspelin