Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Medical image reconstruction: a brief overview of past milestones and
  future directions   This paper briefly reviews past milestones in the field of medical image
reconstruction and describes some future directions. It is part of an overview
paper on "open problems in signal processing" that will appear in IEEE Signal
Processing Magazine, but presented here with citations and equations.

14
The categories of this paper are: math-ph
20
The categories of this paper are: acc-phys
9
The categories of this paper are: nlin
6
The categories of this paper are: astro-ph
0
The categories of this paper are: hep-ph
1
The categories of this paper are: cs
24
The categories of this paper are: dg-ga
18
The categories of this paper are: econ
4
The categories of this paper are: cond-mat
36
The categories of this paper are: cmp-lg
28
The categories of this paper are: alg-geom
8
The categories of this paper are: hep-ex
31
The categories of this paper are: ao-sci
15
The categories of this paper are: nucl-ex
23
The categories of this paper are: solv-int
32
The categories of this paper are: atom-ph