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The Visible Human Project Graphic

The National Library of Medicine of the US Department of Health and Human Services has established the Visible Human Project with the goal of creating a digital atlas of the human body. The data collected for this purpose currently include X-ray computed tomography (CT), magnetic-resonance imaging (MRI), and digitized cryosectional images for one male and one female cadaver. An overview of this project and information on its data products may be found at <>.

With permission from the NLM, portions of the Visible Man CT and MRI data are included on the Slicer Dicer CD-ROM. These data are provided solely for demonstration purposes. The user is encouraged to explore these data in order to become familiar with the operation of Slicer Dicer and to discover its usefulness in visual analyses of complex volumetric data. The Visible Human data are well-suited for this purpose because almost everyone is curious about human anatomy and knows enough to appreciate these data, even without understanding the details of how CT and MRI data are collected or how they are interpreted by medical professionals.

Acknowledgment, Limitations, and Restrictions

The Visible Human data samples included on the Slicer Dicer CD-ROM are provided courtesy of the National Library of Medicine (NLM) of the US Department of Health and Human Services. Use of these data is subject to copyright restrictions and other limitations imposed by the Slicer Dicer Software License Agreement under which the Slicer Dicer software and associated data products are distributed.

The Visible Human data are provided by the NLM on an interim basis and may be modified substantially by NLM in subsequent versions.

About CT and MRI

CT and MRI are volumetric scanning techniques widely used in medicine as well as in many non-medical settings, such as non-destructive testing and quality control in manufacturing applications.

X-ray computed tomography (CT, or CAT for computer assisted tomography) involves an X-ray source and detector deployed on opposite sides of the specimen or patient. This equipment is arranged in such a way that the X-ray beam can be rotated about one axis while the specimen is translated parallel to that axis. In this way, a sequence of transverse cross-sections are obtained, with X-ray images of each section digitally recorded from many angles. Subsequently, algorithms derived from a mathematical procedure called tomography are applied to reconstruct a three-dimensional matrix of values representing the X-ray transmission properties in the volume occupied by the specimen.

The following image is a Slicer Dicer rendering derived from the CT sample data. It includes a projected volume (maxima) rendered on the background surfaces and an isosurface delineating the skull. A cutout has been used to cut away a portion of the skull and reveal its interior.

In the collection of the CT data for the visible man, the cadaver was scanned both before and after freezing, prior to sectioning. Only the frozen-body CT data are included in the Visible Man samples on the Slicer Dicer CD-ROM.

Magnetic-resonance imaging (MRI) is similar to CT, but it does not use X-rays or other ionizing radiation. Instead, a strong, uniform magnetic field is used to affect the orientation of protons, primarily in hydrogen atoms in the water-bearing constituents. These protons behave like miniature magnets and tend to align themselves with the external field. Radio wave pulses are used to perturb these alignments. Under the combined influence of the magnetic field and radio waves, the protons tend to precess, or wobble, and a sensitive antenna then listens for the signals that are given off by these motions. The radio waves are pulsed in such a way that distinct resonance phenomena are observed. These tuned-pulse modes usually include proton density (PD), spin-lattice relaxation time (T1), and spin-spin relaxation time (T2). It is not necessary to understand the physics of these interactions; what is useful is that the three imaging modalities, PD, T1, and T2, reveal independent information about various biological components -- fat, water, blood, bone, etc.

The following are Slicer Dicer renderings of PD, T1, and T2 data from the Visible Man MRI data.


Image headmpd.gif


Image headmt1.gif


Image headMT2.gif


Generally, CT and MRI data are complementary -- bones and other skeletal components are conspicuous in CT images, and fat, skin and other organs comprised of soft tissues are visible in the MRI data.




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