  We present mid-infrared (MIR) maps for a sample of 18 early-type galaxies
observed at 4.5, 6.7 and 15 microns with the ISOCAM instrument on board the ISO
satellite with a 6'' spatial resolution. We model the Spectral Energy
Distribution (SED) of these galaxies using the stellar evolutionary synthesis
model PEGASE and we derive the MIR excess over the stellar component. We then
explore the nature of this excess in terms of dust and Polycyclic Aromatic
Hydrocarbon molecules (PAHs). We find that out of 18 galaxies, 10 show excess
at 6.7 microns (due to the presence of PAH features) and 14 show excess at 15
microns (due to the presence of warm dust). In two galaxies, where a more
complete wavelength coverage exists, an excess around 9.7 microns is seen
(presumably due to silicate dust emission), while two other galaxies are
totally devoid of dust. We also examine the morphology of the galaxies in these
wavelengths by plotting the azimuthally averaged radial profiles as well as the
MIR color profiles. We find that for the majority of the galaxies the 4.5
micron emission is well described by a de Vaucouleurs profile. The 6.7 and 15
micron emission is smoothly distributed throughout the galaxy while only a few
galaxies show MIR emission which is more concentrated close to the center. Two
dwarf galaxies in our sample show patchy distributions of the MIR emission
while two other galaxies show edge-on disks. With color-color diagrams we
delineate the regions occupied by late-type and early-type galaxies. Finally we
show that the MIR excess found in strong radio galaxies like NGC 4486 (M87) can
be explained by synchrotron emission.
