Custom Image Loader

In this how-to guide we will show you how to create a custom image loader.

Introduction

Cornerstone DOES NOT deal with image loading. It delegates image loading to Image Loaders. Cornerstone team have developed commonly used image loaders (CornerstoneWADOImageLoader for loading images from wado-compliant dicom servers using wado-rs or wado-uri, CornerstoneWebImageLoader to load web images such as PNG and JPEG and cornerstone-nifti-image-loader for loading NIFTI images). However, you might ask yourself:

How

How can I build a custom image loader?

Implementation

Let's implement an imageLoader that fetches pixel data using XMLHttpRequest and return an Image Load Object containing a Promise that resolves to a Cornerstone image.

Step 1: Create an Image Loader

Below, we create an imageLoader which accepts an imageId and returns an imageLoadObject as a Promise.

function loadImage(imageId) {
  // Parse the imageId and return a usable URL (logic omitted)
  const url = parseImageId(imageId);

  // Create a new Promise
  const promise = new Promise((resolve, reject) => {
    // Inside the Promise Constructor, make
    // the request for the DICOM data
    const oReq = new XMLHttpRequest();
    oReq.open('get', url, true);
    oReq.responseType = 'arraybuffer';
    oReq.onreadystatechange = function (oEvent) {
      if (oReq.readyState === 4) {
        if (oReq.status == 200) {
          // Request succeeded, Create an image object (logic omitted)
          const image = createImageObject(oReq.response);

          // Return the image object by resolving the Promise
          resolve(image);
        } else {
          // An error occurred, return an object containing the error by
          // rejecting the Promise
          reject(new Error(oReq.statusText));
        }
      }
    };

    oReq.send();
  });

  // Return an object containing the Promise to cornerstone so it can setup callbacks to be
  // invoked asynchronously for the success/resolve and failure/reject scenarios.
  return {
    promise,
  };
}

Step 2: Registration of Image Loader

After you implement your image loader, you need to register it with Cornerstone. First you need to decide which URL scheme your image loader supports. Let's say your image loader wants to support the custom1 scheme, then any imageId that starts with custom1:// will be handled by your image loader.

// registration
cornerstone.imageLoader.registerImageLoader('custom1', loadImage);

Usage

// Images loaded as follows will be passed to our loadImage function:
stackViewport.setStack(['custom1://example.com/image.dcm']);

Use Viewport API to load an image

In previous versions of Cornerstone, you could use loadImage or loadAndCacheImage to load an image. However, in Cornerstone3D, this task can be achieved using Viewports APIs.