Reading DICOM images in R

The R packages oro.dicom and oro.nifti [1] provide the ability to import and visualise medical images that have been stored as DICOM [2] or ANALYZE™/NIfTI [3] files. This is handy because DICOM is widely used for clinical data, while NIfTI is a more compact format that stores only 2 files per 3D volume, rather than dozens.

library(oro.dicom)
dcmImages <- readDICOM("/dev/DICOMRT2", verbose = TRUE,
                       recursive = FALSE, exclude = "sql")

## 412 files to be processed by readDICOM()
##
|
|=================================================================| 100%

A lot of meta-data is associated with each image file, although in the case of de-identified data most of it will be masked out. Nevertheless, the fields that we are interested in for image analysis are present:

dcm.info <- dicomTable(dcmImages$hdr)
length(names(dcm.info))

## [1] 137

unique(dcm.info["0008-0008-ImageType"])

## 0008-0008-ImageType
DERIVED PRIMARY AXIAL CT_SOM5 SPI
ORIGINAL PRIMARY AXIAL
<NA>

unique(dcm.info["0018-1110-DistanceSourceToDetector"])

## 0018-1110-DistanceSourceToDetector
1040
1502.15787247627
<NA>

unique(dcm.info["0018-1130-TableHeight"])

## 0018-1130-TableHeight
198
125
<NA>

All of the image data is also available:

image(t(dcmImages$img[[1]]), col = grey(0:64/64), axes = FALSE,
      xlab = "", ylab = "")

A 3D image, such as a CT or MRI, is comprised of multiple DICOM files, one for each slice. Therefore, we need to loop through the files to reconstruct the 3D volume and save it to disk as a NIfTI file:

system.time(for (stack in unique(substring(rownames(dcm.info), 15, 20))) {
    if (substring(stack, 1, 2) == "CT") {
        print(stack)
        index <- which(substring(rownames(dcm.info), 15, 20) == stack)
        dcm.stack <- list(hdr = dcmImages$hdr[index], img = dcmImages$img[index])
        dcm.nifti <- dicom2nifti(dcm.stack, DIM = 3, descrip = c("Manufacturer", 
            "ManufacturersModelName"))
        print(dcm.nifti)
        writeNIfTI(dcm.nifti, paste0("NIfTI/", stack))
    }
})

## [1] "CT4693"
## NIfTI-1 format
## Type : nifti
## Data Type : 4 (INT16)
## Bits per Pixel : 16
## Slice Code : 0 (Unknown)
## Intent Code : 0 (None)
## Qform Code : 2 (Aligned_Anat)
## Sform Code : 2 (Aligned_Anat)
## Dimension : 512 x 512 x 86
## Pixel Dimension : 1.27 x 1.27 x 3
## Voxel Units : mm
## Time Units : sec

The oro.nifti package also provides some useful visualisations:

orthographic(dcm.nifti, col.crosshairs = "green")

image(dcm.nifti)

References

[1]: B Whitcher, V J Schmid & A Thorton (2011) Working with the DICOM and NIfTI Data Standards in R J Stat Soft 44(6)

[2]: Digital Imaging and Communications in Medicine (DICOM)

[3]: Neuroimaging Informatics Technology Initiative (NIfTI)