The degree of x-ray attenuation depends on the tissue type. These differences are converted into ‘CT numbers’, better known as Hounsfield units (HU). A spectrum of gray tones is generated, from -1000 to +3000 (note: the upper limit is determined by the scanner type). Tissues with low attenuation (such as air and fat) have a low HU number. Tissues with high x-ray attenuation (such as bone and contrast fluid) have a high HU value (fig. 9).
Figure 9. Hounsfield units (HU) of various tissues.
Humans can distinguish a limited number of gray tones only. If the entire spectrum of figure 9 were to be imaged, many structures cannot be distinguished. In order to increase contrast between tissues with similar HU values, a certain part of the spectrum can be enlarged as it were.
The upper and lower limit of the enlarged section is termed the window width. The middle of the window width is the window level. A frequently used option is the soft tissue setting. The soft tissue setting generally has a window level of 40 (note: soft tissues have an HU value around +40 – +80) and a window width of 400 (fig. 10). Everything above the window width upper limit, in this case +240, is projected as white. Everything under the lower limit of the window width, in this case -160, is projected as black. A soft tissue setting therefore provides virtually no information about the air-filled lungs (HU of lungs around -500).
Figure 10. Soft tissue setting with a window width of 400 and a window level of 40. Note that all structures above +240 and under -160 are projected as white and black respectively.
Other frequently used settings include the lung setting and bone setting (fig. 11/12). These standard settings can usually be activated with a programmed key on the keyboard. Note that subtle density differences are best distinguished with a narrow window width.
Figure 11 Lung setting with a window width of 1500 and window level of -650. Note that all structures above +100 and under -1400 are projected as white and black respectively.
Figure 12. Bone setting with a window width of 2000 and level of +400. Note that all structures above +1400 and under -600 are projected as white and black respectively.
Note: changing the window/level is a software manipulation. Patients therefore do not need to be scanned again in order to change the window level. Window leveling can be used in each test. Be aware that window leveling has its limits in terms of image quality and evaluation. A CT scan focusing on a certain organ will not be ideal for assessment of another organ (note: the test is based on the question to be answered!). The scanner/technique, scan protocol and kV/mA settings impact the final result.