Slice visualization with neuroim2

Why these helpers?

Neuroimaging images are large, orientation‑sensitive rasters. The goal of these helpers is to make reasonable defaults easy to use: perceptually uniform palettes, robust scaling, fixed aspect ratios, and clean legends—without extra heavy dependencies or any JavaScript.

This vignette shows how to:

Build publication‑ready montages
Create a compact orthogonal (three‑plane) view with crosshairs
Overlay a statistical map on a structural background (threshold + alpha)

The helpers used here are:

resolve_cmap(), scale_fill_neuro(), theme_neuro()
plot_montage(), plot_ortho(), plot_overlay()
annotate_orientation()

Getting a demo volume

The examples below try to read a sample NIfTI included with the package. If that is not available, they create a small synthetic 3D volume and wrap it in NeuroVol. Either way, the rest of the code is identical.

set.seed(1)

make_synthetic_vol <- function(dims = c(96, 96, 72), vox = c(2, 2, 2)) {
  i <- array(rep(seq_len(dims[1]), times = dims[2]*dims[3]), dims)
  j <- array(rep(rep(seq_len(dims[2]), each = dims[1]), times = dims[3]), dims)
  k <- array(rep(seq_len(dims[3]), each = dims[1]*dims[2]), dims)

  c0 <- dims / 2
  g1 <- exp(-((i - c0[1])^2 + (j - c0[2])^2 + (k - c0[3])^2) / (2*(min(dims)/4)^2))
  g2 <- 0.5 * exp(-((i - (c0[1] + 15))^2 + (j - (c0[2] - 10))^2 + (k - (c0[3] + 8))^2) / (2*(min(dims)/6)^2))
  x  <- g1 + g2 + 0.05 * array(stats::rnorm(prod(dims)), dims)

  sp <- NeuroSpace(dims, spacing = vox)
  NeuroVol(x, sp)
}

# Prefer an included demo file. Use a real example from inst/extdata.
demo_path <- system.file("extdata", "mni_downsampled.nii.gz", package = "neuroim2")

t1 <- if (nzchar(demo_path)) {
  read_vol(demo_path)
} else {
  make_synthetic_vol()
}

dims <- dim(t1)

# Build a synthetic "z-statistic" overlay matched to t1's dims
mk_blob <- function(mu, sigma = 8) {
  i <- array(rep(seq_len(dims[1]), times = dims[2]*dims[3]), dims)
  j <- array(rep(rep(seq_len(dims[2]), each = dims[1]), times = dims[3]), dims)
  k <- array(rep(seq_len(dims[3]), each = dims[1]*dims[2]), dims)
  exp(-((i - mu[1])^2 + (j - mu[2])^2 + (k - mu[3])^2) / (2*sigma^2))
}
ov_arr <- 3.5 * mk_blob(mu = round(dims * c(.60, .45, .55)), sigma = 7) -
          3.2 * mk_blob(mu = round(dims * c(.35, .72, .40)), sigma = 6) +
          0.3 * array(stats::rnorm(prod(dims)), dims)

overlay <- NeuroVol(ov_arr, space(t1))

1) Montages that read well at a glance

The montage helper facettes a single ggplot object—so you get a shared colorbar, clean panel labels, and proper aspect ratio.

# Choose a sensible set of axial slices
zlevels <- unique(round(seq( round(dims[3]*.25), round(dims[3]*.85), length.out = 12 )))

p <- plot_montage(
  t1, zlevels = zlevels, along = 3,
  cmap = "grays", range = "robust", probs = c(.02, .98),
  ncol = 6, title = "Axial montage (robust scaling)"
)
p + theme_neuro()

Plot output

Notes

range = "robust" uses quantiles (default 2–98%) to ignore outliers.
coord_fixed() + reversed y are handled internally to preserve geometry and radiological convention.
Use downsample = 2 (or higher) when plotting huge volumes interactively.

plot_montage(
  t1, zlevels = zlevels, along = 3,
  cmap = "grays", range = "robust", ncol = 6, downsample = 2,
  title = "Downsampled montage (for speed)"
)

Plot output

2) Orthogonal three‑plane view (with crosshairs)

plot_ortho() produces aligned sagittal, coronal, and axial slices with a shared scale, optional crosshairs, and compact orientation glyphs.

center_voxel <- round(dim(t1) / 2)
plot_ortho(
  t1, coord = center_voxel, unit = "index",
  cmap = "grays", range = "robust",
  crosshair = TRUE, annotate = TRUE
)

Plot output

Tip: If you have MNI/world coordinates, pass unit = "mm" and a length‑3 numeric; internally it will convert using coord_to_grid(space(vol), …) if available.

3) Overlaying an activation map on a structural background

The overlay compositor colorizes each layer independently (so each can use its own limits and palette) and stacks them as rasters. No extra packages required.

plot_overlay(
  bgvol = t1, overlay = overlay,
  zlevels = zlevels[seq(2, length(zlevels), by = 2)],  # fewer panels for the vignette
  bg_cmap = "grays", ov_cmap = "inferno",
  bg_range = "robust", ov_range = "robust", probs = c(.02, .98),
  ov_thresh = 2.5,   # make weaker signal transparent
  ov_alpha  = 0.65,
  ncol = 3, title = "Statistical overlay (threshold 2.5, alpha 0.65)"
)

Plot output

4) Palettes and aesthetics

All examples above use neuro‑friendly defaults:

Palettes: resolve_cmap() wraps base R’s hcl.colors() with aliases like “grays”, “viridis”, “inferno”—and safe fallbacks.
Theme: theme_neuro() keeps panels quiet and legends slim.
Legend: one shared colorbar for facetted montages via scale_fill_neuro().

You can switch palettes easily:

plot_montage(
  t1, zlevels = zlevels[1:6], along = 3,
  cmap = "viridis", range = "robust", ncol = 6,
  title = "Same data, Viridis palette"
)

Plot output

5) Practical tips

Choose slices with meaning. Use mm positions (if you have an affine) or meaningful indices; label strips are handled for you by the helper.
Speed vs. fidelity. Use downsample for exploration; keep downsample = 1 for final figures.
Consistent limits. For side‑by‑side comparisons, compute limits on a combined set of values (the helpers do this for orthogonal panels automatically).

Reproducibility

sessionInfo()
## R version 4.5.1 (2025-06-13)
## Platform: aarch64-apple-darwin20
## Running under: macOS Sonoma 14.3
## 
## Matrix products: default
## BLAS:   /Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/lib/libRblas.0.dylib 
## LAPACK: /Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/lib/libRlapack.dylib;  LAPACK version 3.12.1
## 
## locale:
## [1] C/en_CA.UTF-8/en_CA.UTF-8/C/en_CA.UTF-8/en_CA.UTF-8
## 
## time zone: America/Toronto
## tzcode source: internal
## 
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
## 
## other attached packages:
## [1] ggplot2_4.0.1    assertthat_0.2.1 purrr_1.2.0      neuroim2_0.8.5  
## [5] Matrix_1.7-3    
## 
## loaded via a namespace (and not attached):
##  [1] sass_0.4.10           generics_0.1.4        mmap_0.6-22          
##  [4] stringi_1.8.7         lattice_0.22-7        digest_0.6.39        
##  [7] magrittr_2.0.4        bigstatsr_1.6.2       evaluate_1.0.5       
## [10] grid_4.5.1            RColorBrewer_1.1-3    iterators_1.0.14     
## [13] rmio_0.4.0            fastmap_1.2.0         foreach_1.5.2        
## [16] doParallel_1.0.17     jsonlite_2.0.0        RNifti_1.8.0         
## [19] deflist_0.2.0         scales_1.4.0          codetools_0.2-20     
## [22] jquerylib_0.1.4       cli_3.6.5             rlang_1.1.7          
## [25] crayon_1.5.3          cowplot_1.2.0         splines_4.5.1        
## [28] withr_3.0.2           cachem_1.1.0          yaml_2.3.12          
## [31] otel_0.2.0            flock_0.7             tools_4.5.1          
## [34] parallel_4.5.1        memoise_2.0.1         bigassertr_0.1.7     
## [37] dplyr_1.1.4           vctrs_0.6.5           R6_2.6.1             
## [40] lifecycle_1.0.5       bigparallelr_0.3.2    stringr_1.6.0        
## [43] dbscan_1.2.3          pkgconfig_2.0.3       RcppParallel_5.1.11-1
## [46] bslib_0.9.0           pillar_1.11.1         gtable_0.3.6         
## [49] glue_1.8.0            Rcpp_1.1.1            xfun_0.55            
## [52] tibble_3.3.0          tidyselect_1.2.1      knitr_1.51           
## [55] farver_2.1.2          htmltools_0.5.9       labeling_0.4.3       
## [58] RNiftyReg_2.8.4       rmarkdown_2.30        compiler_4.5.1       
## [61] S7_0.2.1