  We present a high-resolution (2.5 au) multiband analysis of the
protoplanetary disk around TW Hya using ALMA long baseline data at Bands 3, 4,
6, and 7. We aim to reconstruct a high-sensitivity millimeter continuum image
and revisit the spectral index distribution. The imaging is performed by
combining new ALMA data at Bands 4 and 6 with available archive data. Two
methods are employed to reconstruct the images; multi-frequency synthesis (MFS)
and the fiducial image-oriented method, where each band is imaged separately
and the frequency dependence is fitted pixel by pixel. We find that the MFS
imaging with the second order of Taylor expansion can reproduce the frequency
dependence of the continuum emission between Bands 3 and 7 in a manner
consistent with previous studies and is a reasonable method to reconstruct the
spectral index map. The image-oriented method provides a spectral index map
consistent with the MFS imaging, but with a two times lower resolution. Mock
observations of an intensity model were conducted to validate the images from
the two methods. We find that the MFS imaging provides a high-resolution
spectral index distribution with an uncertainty of $<10$~\%. Using the
submillimeter spectrum reproduced from our MFS images, we directly calculated
the optical depth, power-law index of the dust opacity coefficient ($\beta$),
and dust temperature. The derived parameters are consistent with previous
works, and the enhancement of $\beta$ within the intensity gaps is also
confirmed, supporting a deficit of millimeter-sized grains within the gaps.
