[Principles and developmental status of scintigraphy and nuclear magnetic resonance tomography]

Scintigraphy is performed by means of radiopharmaceuticals which accumulate in the target organ or remain in the intravascular space. Radiopharmaceuticals consist of radionuclides as atoms or ions, or of radionuclides incorporated in simple or complex compounds, or in labeled endogenic cells. The gamma rays or gamma quanta are detected by using gamma cameras. Precise localization and image formation is performed by collimation. Sensitivity decreases quadratically with increasing resolution owing to the imaging procedure. Planar scintigrams represent summation images showing a resolution of 5 mm when structures close to the detector are imaged, and a reduced resolution of 8 to 10 mm when structures at a distance of 10 cm are investigated. Usually scintigrams are stored in digital form in a computer. Investigation of cardiac function by means of radionuclide ventriculography requires ECG triggering. Cardiac function is visualized by a so-called representative cardiac cycle consisting of 16 to 32 planer images. In single photon emission computed tomography (SPECT or ECT), transversal tomograms are reconstructed from original data of 60 to 120 planar scintigrams which are recorded with the head of the gamma camera rotating around the long axis of the patient. Resolution is approximately 15 mm. Positron emission tomography (PET) utilizes positron emitting radionuclides. Positrons recombine with electrons after a short, free path of some millimeters. Consequently, two gamma quanta are emitted into opposite directions (almost 180 degrees) which are detected by means of a detector ring system and coincidence techniques. Resolution is as good as 5 mm. PET studies are performed most effectively in the close vicinity of a cyclotron that can produce the short-lived positron emitters with typical half-lives of only some minutes. The new imaging modality nuclear magnetic resonance tomography (MRT) allows for recording tomograms in arbitrary orientation in space from very weak electromagnetic resonance signals. The information of the images resembles the density of endogenic protons which is weighted with two relaxation times and modified by flow phenomena. Those tomograms show excellent soft tissue contrast. To date, resolution is 1 mm within 5-mm-thick slices. ECG triggering is mandatory for cardiac imaging. With spin echo mode, recording 8 to 12 parallel tomograms at 8 to 12 equidistant points in time within the cardiac cycle takes 5 to 8 min depending on heart rate. Applying gradient echo mode, up to 30 different tomograms can be recorded within the same acquisition time showing one slice or a set of parallel slices at successive, equidistant cardiac phases. Nuclear magnetic resonance spectroscopy provides metabolic investigations in vivo.