Clinical Instrumentation

CT

        Computed tomography is a quantitative imaging technique, developed more than three decades ago, and has a central role in the study of several diseases in virtually all areas of medicine. Using a set consisting of an X-ray emitting source and a ring of detectors rotating synchronously around the patient, it is possible to calculate the density of different tissues that attenuate radiation. Moreover, the use of iodinated contrast medium intravenously allows to characterize tissues based on pharmacokinetic properties that depend on circulatory variables and, especially, in the proportion that each of these tissues are part of the the intracellular, intravascular and extracellular compartments.

        In this way, the development of helical technology and devices with multiple detector rows  computed tomography (MDCT) has been a breakthrough. Not only isotropic multiplanar images (with equal resolution in all directions) can be  obtained, but it is possible to acquire anatomical images of large areas in seconds, minimizing motion artifacts and allowing multiphase contrast studies, as well. The latter are particularly suitable for the characterization of normal and pathological tissues based on their kinetics of impregnation.

        Nevertheless, the information provided by MDCT is primarily anatomical. This is especially evident with respect to lymph nodes for example. It is difficult to determine from available morphological criteria whether they are normal or pathological. However, the anatomical ability of CT is of tremendous value for nuclear medicine techniques.

        Both, for PET and SPECT, during the image reconstruction process, CT  X-rays may be used to correct for attenuation of the various radionuclide photons within body tissues. This allows to get their correct concentration  and therefore the construction of a more accurate image. The fusion of PET or SPECT images with CT, gives a tremendous advantage in terms of exact localization of areas that concentrate the radiotracers, a weak aspect on nuclear medicine due to limited spatial resolution. More recently and in order to achieve better accuracy, a low dose CT is done first, then follows 18F-FDG PET and finally a contrast-enhanced standard dose CT is performed keeping the highest yield for PET and CT.

            In this sense, benefits are obtained from the combined functional and morphologic technique in terms of more precise location of areas of increased uptake of tracers, and at the same time in absolute concentrations.
 

References:

1   Levi de Cabrejas, M y Cabrejas RC. Instrumentacion. pp: 19-35. Medicina Nuclear Aplicaciones Clínicas. Eds: I. Carrio - P. González.  Editorial Masson, Barcelona España, 2003.

2   Control de calidad de los instrumentos de medicina nuclear. IAEA-TECDOC 602/S, 1996.

3 Eric M. Rohren, Timothy G. Turkington, R. Edward Coleman.  Clinical Applications of PET in OncologyRadiology 2004; 231:305–332.

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