Maximum Intensity Projection
Is a 3D rendering technique that is achieved by evaluating each voxel value along a line from the observer’s eyes through the object and then sampling only the highest value as the display value. It is achieved by displaying only the highest attenuation values from the data encountered along a ray projected through the dataset to the observer’s eyes.
In clinical practice, it is extremely useful for evaluating high ‘contrast’ radio-opaque substances. For instance, it can be utilized to evaluate the form and extent of the endodontic filling material in root canal treatment or it can also be utilized to evaluate the presence or absence of a foreign body material in osseous and/or soft tissue structures. It is best used when the objects to be investigated are the ‘brightest‘ objects in the image.
However, MIP ability to represent anatomical spatial interrelations is rather limited because of lack of the visual cues for depth perception. It has a tendency to misrepresent positions because the projection technique doesn’t take spatial location into calculation, only the maximal or (most attenuated) value is displayed. For the same reason, structures of sub-maximal attenuation along the projection line are obscured which may lead to sub-optimal interpretation results. In such a case, other 3D projection techniques in conjunction with MPR projections can be employed to enhance the interpretation process.
Minimum Intensity Projection
In MinIP only the smallest attenuation values are utilized and displayed. The resulting image is simply an ‘invert’ or direct opposite in terms of displayed values to that of MIP (Maximum Intensity Projection). It is not a commonly utilized technique but it might be useful for evaluating any air-filled cavity like sinuses and such.
Raysum Projection
Is a 3D rendering technique in which each the attenuation value of each voxel is summed along the projection line. Ray sum results in images which resemble those of a conventional radiograph. Another rendering algorithm known as AIP (Average Intensity Projection) in which the voxel values are averaged rather than summed is sometime provided in addition to Ray Sum projection.
