Page 26 - Feline diagnostic imaging

2.4 Magnetic Resonance Imaging 19

Figure 2.7 Window width (WW) and level (L) adjustments. The human eye has limited ability to perceive different shades of gray. As a
result, the full complement of CT numbers cannot be fully displayed at one time (−1000 to +3000). To compensate, the window width
and level are adjusted to display different features. The maximum and minimum HU values are referred to as the window width. The
center value is the level. In the transverse image of the thorax, (a) is a soft tissue window. This is a WW of 350 and L 40. If the pixel
value is greater than the upper threshold, the pixel will be white. If the value is less than the lower threshold, the pixel will be black.
This will allow for more subtle changes in the soft tissue structures to be delineated, but contrast detail of the bone is lost. Image (b)
shows the same image in a bone window. The WW is 2500 and the L is 400. The soft tissue structures are more uniform and contrast
detail of the soft tissue is lost. The bone, however, shows greater contrast resolution, allowing the differences between cortical and
cancellous bone to be distinguished. Image (c) is the same image in a lung window. The WW is 1500 and the L is −600. With this
setting, the intricacies of the lung parenchyma can now be identified. The WW and L are adjusted with the user’s viewing software and
are adapted to each patient.

hydrogen will align with this magnetic field. There are two antiparallel direction and are termed spin down nuclei. The
discrete levels of energy on which the protons can align greater the strength of the external magnet, the more spins
themselves: high and low energy. Most of the low‐energy will align in the spin up direction. The sum of the magnetic
nuclei will align parallel to the external field and are termed moment from the spin up and spin down nuclei is called
spin up nuclei. The higher energy nuclei will align in the the NMV. As the total number of spin up and spin down