
Basics of nuclear imaging 149
x
y
FIGURE 5.13 Voxel representation of imaged volumes.
Chapter 3, we provide the definition because it is a key quantity used in
the
imaging field. How this quantity relates to the theory used in this book is
important. We will use symbol a to denote the activity. The radio de nsity and
activity are closely related. For example, if r is the number of nuclei in a voxel,
the activity of this voxel is
a = λr (5.9 )
where λ is the decay constant. The value of activity a is proportional to dis-
crete quantity r (Equation (5.9)) and it is frequently assumed that it is a
continuous quantity for larg e r. However, it should be clear ...