  The Kepler mission has discovered thousands of planet candidates. Currently,
some of them have already been discarded; more than 200 have been confirmed by
follow-up observations, and several hundreds have been validated. However, most
of them are still awaiting for confirmation. Thus, priorities (in terms of the
probability of the candidate being a real planet) must be established for
subsequent observations. The motivation of this work is to provide a set of
isolated (good) host candidates to be further tested by other techniques. We
identify close companions of the candidates that could have contaminated the
light curve of the planet host. We used the AstraLux North instrument located
at the 2.2 m telescope in the Calar Alto Observatory to obtain
diffraction-limited images of 174 Kepler objects of interest. The lucky-imaging
technique used in this work is compared to other AO and speckle imaging
observations of Kepler planet host candidates. We define a new parameter, the
blended source confidence level (BSC), to assess the probability of an object
to have blended non-detected eclipsing binaries capable of producing the
detected transit. We find that 67.2% of the observed Kepler hosts are isolated
within our detectability limits, and 32.8% have at least one visual companion
at angular separations below 6 arcsec. We find close companions (below 3
arcsec) for the 17.2% of the sample. The planet properties of this sample of
non-isolated hosts are revised. We report one possible S-type binary
(KOI-3158). We also report three possible false positives (KOIs 1230.01,
3649.01, and 3886.01) due to the presence of close companions. The BSC
parameter is calculated for all the isolated targets and compared to both the
value prior to any high-resolution image and, when possible, to observations
from previous high-spatial resolution surveys in the Kepler sample.
