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March 22nd, 2013, 23:11
In releation to this:

In a CT scan, each pixel can have a value ranging from -1000 to 3095 so-called Hounsfield units. Roughly speaking this corresponds to 4096 grey levels. No one can distinguish that many grey levels, of course. In stead when viewing CT scans on screen or print them to film, we concentrate on a smaller range of units (the level and the window), depending on what we want to study. Air is almost pitch black. If we want to study the lungs, we select a window enhancing the darker parts of the image - the lung window. OTOH, if we want to examine the bony parts, we select a window focusing on the lighter parts - the bone window. For best visualisation of soft tissues we go for something between, the soft tissue window. An example can be found here: http://www.netterimages.com/image/61653.htm. (The central very whitish parts are the large blood vessels such as the aorta and the pulmonary artery. The scan is performed using intravenous contrast medium, that's why the vessels shine so brightly).

The CT scan seen here: http://upload.wikimedia.org/wikipedi…/SADDLE_PE.JPG is also from the lung, taken just above the heart, using a soft tissue window. Notice that there isn't much detail visible in the lungs itself, and the bones. This is a scan performed to detect pulmonary embolism. The patient is given a small amount of contrast medium intravenously, and the scan is timed so that images are obtained just as contrast enters the pulmonary artery, which is the very bright vessel in the image. The contrast hasn't reached the aorta yet, that's why the two parts of that vessel (the ascending part in the front, the descending part behind) appear darker. In fact, there is a difference between those two parts. The descending vessel is darker, because blood from the heart enters the upwards part first.

Oh, if you notice the dark parts within the pulmonary artery, well, that's the embolus. In other words: a postive find. In this case the embolus affects both the left and the right main branches, completely obliterating some of the smaller ones. A pretty serious condition.

pibbur who misses studying CT scans. In fact he misses all those parts of radiology not involving patients directly.

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Last edited by pibbur who; March 22nd, 2013 at 23:45.
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