  We use 8.3 um mid-infrared images acquired with the Midcourse Space
Experiment satellite to identify and catalog Infrared Dark Clouds (IRDCs) in
the first and fourth quadrants of the Galactic plane. Because IRDCs are seen as
dark extinction features against the diffuse Galactic infrared background, we
identify them by first determining a model background from the 8.3 um images
and then searching for regions of high decremental contrast with respect to
this background. IRDC candidates in our catalog are defined by contiguous
regions bounded by closed contours of a 2 sigma decremental contrast threshold.
Although most of the identified IRDCs are actual cold, dark clouds, some as yet
unknown fraction may be spurious identifications. For large, high contrast
clouds, we estimate the reliability to be 82%. Low contrast clouds should have
lower reliabilities. Verification of the reality of individual clouds will
require additional data. We identify 10,931 candidate infrared dark clouds. For
each IRDC, we also catalog cores. These cores, defined as localized regions
with at least 40% higher extinction than the cloud's average extinction, are
found by iteratively fitting 2-dimensional elliptical Gaussians to the contrast
peaks. We identify 12,774 cores. The catalog contains the position, angular
size, orientation, area, peak contrast, peak contrast signal-to-noise, and
integrated contrast of the candidate IRDCs and their cores. The distribution of
IRDCs closely follows the Galactic diffuse mid-infrared background and peaks
toward prominent star forming regions, spiral arm tangents, and the so-called 5
kpc Galactic molecular ring.
