Utilities for seed-independent multidimensional nonuniform sampling
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nusutils

A set of command-line utilities for deterministically (i.e. without pseudorandom numbers) constructing nonuniform sampling schedules on multidimensional Nyquist grids. The gaputil is a utility to construct schedules using the generalized gap sampling framework published in:

Worley, B., Powers, R., Deterministic Multidimensional Nonuniform Gap Sampling, Journal of Magnetic Resonance, 2015, 261: 19-26.

The rejutil and jitutil are utilities to construct schedules from density functions using a quasirandom accept-reject sampling method published in:

Worley, B., Subrandom Methods for Multidimensional Nonuniform Sampling, Journal of Magnetic Resonance, 2016, 269: 128-137.

Introduction

Gap sampling came into vogue in the field of Nuclear Magnetic Resonance (NMR) when Hyberts and Wagner introduced their Poisson-gap (PG) sampler for building Nonuniform Sampling (NUS) schedules. This project is the software realization of a full generalization of their PG method to admit any gap equation.

Initial versions of gaputil contained hard-coded Poisson-gap, sine-gap, and sine-burst (cf. Worley and Powers, above) gap equations. The generality of the proposed framework was a tempting opportunity for me to embed the Julia programming language into one of my software projects. By embedding Julia into my existing gap sampling algorithm, this utility enables the construction of NUS schedules from completely arbitrary gap equations.

Examples using gaputil

The gaputil program reads a gap equation from the command line in string form, passes it to Julia for just-in-time compilation, and then evaluates it as needed until an optimal schedule is arrived at. The function prototype used by gaputil looks like this in Julia:

g(x::Float64, d::Int32,
  O::Array{Float64,1},
  N::Array{Float64,1},
  L::Float64)

where x holds the current gap sequence term, d is the currently sampled grid dimension, O is the current origin in the grid, N is the size of the grid, and L is a scaling factor that gaputil will optimize in its attempts to create a schedule of the correct global sampling density.

The simplest possible gap equation merely uses the scaling factor, resulting in a uniform lattice:

g(x, d, O, N, L) = L

The sine-gap equation is fairly simple as well:

g(x, d, O, N, L) =
  L * sin((pi/2) * (x + sum(O)) / sum(N))

The sine-burst equation gets a bit more complicated:

g(x, d, O, N, L) =
  L * sin((pi/2) * (x + sum(O)) / sum(N)) *
  sin((pi/4) * N[d] * (x + sum(O)) / sum(N))^2

In short, gaputil accepts almost any inline function having Julia syntax.

Examples using rejutil and jitutil

The rejutil and jitutil programs also use Julia to interpret their density function, but this function is of a simpler form:

f(x::Array{Float64,1}, N::Array{Float64,1})

where x now holds the current grid index, and N still holds the total grid size. No optimization is performed: rejutil simply draws a set number of points (grid indices) from the distribution specified in the density function. On the other hand, jitutil draws the same number of points using a jittered rejection sampling algorithm.

The simplest possible density function is a uniform grid:

f(x, N) = 1.0

An exponentially weighted function would look like this:

f(x, N) = exp(-sum(x ./ N))

Like gaputil, rejutil and jitutil will accept any inline function having Julia syntax and conforming to the interface specified above.

Installing

You will need to have Julia 0.4.0-dev compiled and installed in order to build nusutils. It is recommended that you compile and install from the latest GitHub nightly commit.

Once Julia is installed into the path, you can compile and install gaputil, rejutil and jitutil as follows:

git clone git://github.com/geekysuavo/nusutils.git

cd nusutils

make

sudo make install

Licensing

This project is released under the GNU GPL 2.0.

Have fun with it,

*~ Brad.*