Principles of CT of the Brain

CT scan of the Brain

Sumit Karia MD

The Common Vein

Copyright 2010

 Computerized tomography (CT) is based on the same principles as conventional radiography, that is, tissues and substances absorb x-rays in particular ways, and the remaining penetrating radiation hits the x-ray plate, leaving a characteristic impression according to the intensity that hits it. The main principle is that more dense tissues absorb more radiation than the lesser ones.

The image obtained is based on a gray scale, with multiple pixels. Each pixel represents the mean radiation absorption in a particular area of a tissue that hit it before hitting the plate. This scale is expressed in Hounsfield (the creator of the first CT machine) units.

Unlike conventional x-rays, in which the radiation comes from a single lamp, in CT, it comes from a gantry that keeps rotating around a patient, while in the opposite extremity a detector accompanies it. Hence, we will have multiple images from the section analyzed from every single degree from the 360° analyzed. These will be reconstructed using a computer to yield a cross sectional image, which essentially represents the combination of all the ones obtained in that plane. The table where the patient is lying also moves, so that further sections are analyzed. This is called the conventional CT, in which image are obtained one at a time.

In helical CT, the gantry describes a helix around the patient, instead of a circle. This way, the image obtained is continuous in terms of planes obtained, rather than being limited to the extent of how little can the table move and to artifacts produced by movement. The exam becomes faster and less radiation is thus used, which anyway, is the main drawback of CT and ionizing radiation based examinations is general.

Finally, the latest technology is that in which multiple planes are obtained with one single beam of x-radiation, possible though the use of multiple detectors.

 In terms of applications of CT in brain imaging, various parameters should be controlled. These include spacing from each slice [thickness], radiographic technique (tube current and slice time), reconstruction filter, the use of spiral imaging, and the windows used. Contrast administration is optional and its indications will vary from the pathologies and clinical presentations that are explained more in depth in the corresponding chapters, but contrast is mostly used to determine if there is disruption of the blood-brain barrier, which occurs in certain pathologies; and also for the study of neoplastic, vascular, infectious, or inflammatory disorders.

In terms of applications of CT in brain imaging, various parameters should be controlled. These include spacing from each slice [thickness], radiographic technique (tube current and slice time), reconstruction filter, the use of spiral imaging, and the windows used. Contrast administration is optional and its indications will vary from the pathologies and clinical presentations that are explained more in depth in the corresponding chapters, but contrast is mostly used to determine if there is disruption of the blood-brain barrier, which occurs in certain pathologies; and also for the study of neoplastic, vascular, infectious, or inflammatory disorders.

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