医学におけるトモグラフィー

抄録

This article introduces the mechanisms and principles of medical tomography. There arevarious tomographic imaging modalities for clinical practice that use X-rays, gamma rays fromradioisotopes, ultrasound, magnetic resonance, and electrophysiological effects.
Conventional X-ray tomography provides a longitudinal cross-section. The X-ray tube andthe film are moved in such a way that regions in the focal plane register clearly and regionsoutside the focal plane tend to disappear due to blurring. X-ray computed tomography (X-ray CT) shows the structure of organs by imaging X-ray attenuation coefficient quantitatively.X-ray CT acquires projection data by rotating the X-ray tube and the detector array aroundthe patient, and the tomographic image is calculated using the theory of “reconstruction fromprojections”. This is the first application of the inversion analysis in medicine.
In nuclear medicine, the functional activity of organs is evaluated by administering a radioisotopeto the patient, and mapping the distribution by measuring the emitted gamma ray radiation. Single photon emission tomography (SPECT) uses a scintillator camera to detect theradiation, generates projection images, and applies the theory of reconstruction from projectionsfor tomographic images. Seven pin-hole tomography generates a conventional longitudinalcross section image using a scintillator camera with a “seven pin-hole collimator”. Positronemission tomography (PET) maps the biochemical process of organs by administering positronemitting radioisotopes. PET measures a pair of gamma ray photons created by the annihilationof an electron and an emitted positron, and applies the theory of reconstruction from projections.
Diagnostic ultrasound generates an animated image in real time by mapping the ultrasoundecho signal from acoustic interfaces in the patient's body. The velocity of blood flow can alsobe calculated from the Doppler shifted echo from the red blood cells.
Magnetic resonance imaging (MRI) can acquire various kinds of tomographic images fromthe electromagnetic signal emitted by nuclear magnetic resonance of hydrogen and other nucleiin the patient. To control the signal emission, MRI applies a strong and uniform magneticfield, an additional gradient magnetic field, and irradiation of electromagnetic waves. Thetomogram is reconstructed from the electromagnetic signals using Fourier transform. Electronspin resonance (ESR) imaging is also discussed.
The topogram in electroencephalography (EEG) is a rough surface mapping of electricactivity of neurons and thus, not strictly tomography. Recently, magnetic source imaging (MSI) of magnetoencephalography (MEG) has become available for research and clinical use. MEG shows activated regions in the brain by measuring the very weak magnetic field causedby the electrophysiological action of neurons.