  Tomography of redshifted 21 cm transition from neutral Hydrogen using Fourier
synthesis telescopes is a promising tool to study the Epoch of Reionization
(EoR). Limiting the confusion from Galactic and Extragalactic foregrounds is
critical to the success of these telescopes. Instrumental response or the Point
Spread Function (PSF) of such telescopes is inherently 3 dimensional with
frequency mapping to the Line of Sight (LOS) distance. EoR signals will
necessarily have to be detected in data where continuum confusion persists;
therefore, it is important that the PSF has acceptable frequency structure so
that the residual foreground does not confuse the EoR signature. This paper
aims to understand the 3 dimensional PSF and foreground contamination in the
same framework. We develop a formalism to estimate the foreground contamination
along frequency, or equivalently LOS dimension, and establish a relationship
between foreground contamination in the image plane and visibility weights on
the Fourier plane. We identify two dominant sources of LOS foreground
contamination-'PSF contamination' and 'gridding contamination'. We show that
'PSF contamination' is localized in LOS wavenumber space, beyond which there
potentially exists an 'EoR window' with negligible foreground contamination
where we may focus our efforts to detect EoR. 'PSF contamination' in this
window may be substantially reduced by judicious choice of a frequency window
function. Gridding and imaging algorithms create additional 'gridding
contamination' and we propose a new imaging algorithm using the Chirp Z
Transform (CZT) that significantly reduces this contamination. Finally, we
demonstrate the analytical relationships and the merit of the new imaging
algorithm for the case of imaging with the Murchison Widefield Array (MWA).
