Final answer:
A coma is an optical aberration resulting in a pear-shaped image caused by off-center positioning of the patient in the gantry during an imaging procedure.
Step-by-step explanation:
Centering the patient too high or too low in the gantry can result in an artifact called coma. This aberration occurs when the object being imaged is off-center, leading to a distortion where rays do not converge at a common focal point, often resulting in a pear-shaped image.Different parts of a lens or mirror in an imaging system have to refract or reflect light to the same point for a clear image. When an object, such as the patient in a medical imaging scenario, is off-center, this coordination is disrupted.
The term 'coma' specifically refers to an optical aberration that can affect various types of imaging, including those used in medicine like CT scans or MRI machines. The distortion affects the image quality and can interfere with proper diagnosis. Another type of aberration that may occur is spherical aberration, which is different from a coma and has distinct characteristics.