Registration and Subtraction of Serial Magnetic Resonance Images of the Brain: Image Interpretation and Clinical Applications - Medical Image Registration - page 169

is consistent in sign, but may be smaller in size than the previously reported
finding of increase in CSF volume later in the menstrual cycle detected with
a volume measurement technique.

7.9.3 Pregnancy

During normal pregnancy, the brain decreases in size, reaching a maximum
reduction at term. After delivery it regains its original size over a period of
four to six months (Figure 7.17). 16,17

7.10 Contrast Enhancement

Normal enhancement in the meninges, diplocic veins, scalp, skin, and other
structures was more clearly demonstrated than with conventional unregistered
images. 18 In addition, abnormal enhancement was more clearly demon-
strated in patients with meningeal disease and other conditions (Figure 7.18).
Registration and subtraction were of particular value in the following
situations:

(a) Recognition of small degrees of enhancement : When the level of change
was small, differences may be much less than those produced by
misregistration, particularly when the changes are at boundaries.
Diffuse generalized enhancement may also be more readily recog-
nized on subtraction images in relation to the noise level of the
surrounding images. This type of change may be missed with
conventional display strategies.
(b) Enhancement in tissues of fluids with very high or very low baseline
signals : The usual window level and width settings may place the
changes due to enhancement off the top or the bottom of the display
gray scale, and so render changes difficult to recognize. Subtraction
reduces all tissues and fluids to a common baseline and makes
changes of this type easier to visualize.
(c) Enhancement at interfaces, boundaries, and other regions of complex
anatomy : In these situations there are frequently partial volume
effects, and even a small displacement of the brain between pre-
and postcontrast images produces a difference in these effects,
resulting in spurious changes in signal intensity. Precise registra-
tion provides the same partial volume effects on the before and
after images, so that enhancement can be readily recognized.
(d) Assessment of enhancement when thin slices are used : While use of thin
slices reduces partial volume effects, it may increase problems due