volume rendering raycasting artifacts

I am trying to implement a simple raycasting volume rendering in WebGL.

It is kind of working, but there are some artifacts when you rotate the volume around (i.e. the head appears deformed).

Live demo: http://fnndsc.github.io/vjs/#shaders_raycasting_adibrain

GLSL Code used for debugging: https://github.com/FNNDSC/vjs/blob/master/src/shaders/shaders.raycasting.secondPass.frag

Simplified version of the code:

for(int rayStep = 0; rayStep < maxSteps; rayStep++){

  // map world coordinates to data coordinates
  vec4 dataCoordinatesRaw = uWorldToData * currentPosition;
  ivec3 dataCoordinates = ivec3(int(floor(dataCoordinatesRaw.x)), int(floor(dataCoordinatesRaw.y)), int(floor(dataCoordinatesRaw.z)));

  float intensity = getIntensity(dataCoordinates);

  // we have the intensity now
  vec3 colorSample = vec3(intensity);
  float alphaSample = intensity;

  accumulatedColor += (1.0 - accumulatedAlpha) * colorSample * alphaSample;
  accumulatedAlpha += alphaSample;

  //Advance the ray.
  currentPosition += deltaDirection;
  accumulatedLength += deltaDirectionLength;

  if(accumulatedLength >= rayLength || accumulatedAlpha >= 1.0 ) break;
}

I do not understand what could explain those artifacts.

Could it be because I do not use gradients to modulate opacity/color?

Any hint would be very welcome.

The backface coordinates were not computed properly during the first pass of the raycasting. The range of the "normalized" coodinates was not [0, 1]. It was [-.5, 1.5], therefore creating the visualization artifact as all values outside of [0, 1] range were clamped out.