The text and red circles represent the measurements for the those areas of interest. But that is another story.) In Figure 2, the green circles represent the area of interest that is being measured.
Remember, fat floats on water, and is thus less dense than water. How can bone be less dense than muscle? Because it is filled with fatty bone marrow. So in this case the medullary bone is actually less dense then muscle around it.
If we measure the muscle in the gluteus maximus area, this measures 51 Hounsfield units, as shown in the red circle on the left. The less dense medullary bone within the sacrum measures 1.7 Hounsfield units, as shown within the red circle in the middle. If we measure the density of these bones in Hounsfield units, which is the unit of density that is measured on a CT scan, we can see that the dense cortical part of the iliac bone in this image measures 768 Hounsfield units, and is shown in the rightmost red circle (Figure 2). Outside the bone we can see muscle represented by this light gray color, and intra-abdominal fat which is represented by this dark gray color. In some areas it is approaching black in color, which means is not very dense at all. This type of bone is not white, but is a grayish color on a CT scan image. The spongy bone, which is less dense and contains fatty bone marrow, can be seen in the center of the iliac bones and sacrum on this image. The white color indicates that it is very dense. In the CT scan image shown in Figure 1, the hard, dense cortical bone is seen as a bright white outer surface of these pelvic bones. Take this CT scan of the pelvis, for example. On a CT scan you can see these two different types of bone. The inner core of most bones contains fatty bone marrow, and is made of much softer bone that is less dense, called spongy bone. This tough outer part of the bone is very hard and is what gives bones their strength. Most bones have a very hard, very dense outer layer comprised of cortical bone. Bones are not solid, even though they may appear to be so. Before we dive in, let's take a moment to discuss what the medullary cavity of a bone is and why it's a problem if you're trying to 3D print bone models made from a CT scan.
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