Computed Tomography

Computed Tomography

Introduction

Computed tomography (CT) examination is performed with special X-ray equipment that allows to obtain images from different angles, with the use of special programs of powerful electronic computers, to reconstruct the sections (slices) of the human body. The apparatus of TC appears to a large “donut hole”, little thick, the patient is lying on a sun disk, which rises and falls and slides into the center of the “hole”. From the technical point of view, computed tomography (CT) is an X-ray tube that emits a beam (total radiation) on a tomographic plane of the object under study. Part of this radiation is lost, and part penetrates the body. The X-rays pass through tissue attenuates the radiation, which is picked up by sensitive detectors and then analyzed by a computer that reconstructs the different measurements obtained in two-dimensional images projected on a cathode ray screen. Thus obtained a series of transverse cuts to the axial axis of the skull from the orbitomeatal line. This line connects the outer side of the orbit with the ear canal, and forms an angle of 15 ° with the horizontal plane. The current equipment allows finer cuts (up to 1 mm) and cut in the coronal plane (Bozzao, Angeloni & Bastianello, 2001).

The TC collects images in shades of gray representing different tissue densities studied anatomy. For this, a scale was created called Hounsfield Units densities (in honor of the discoverer of CT). This scale gives the water a densimetric value of zero, with extremes ranging from -1000 (air, very hypodense images) to +1000 (bone compact / foreign bodies, very hyperdense images) (Mathews, 2002). Helical CT uses more advanced technology that allows to study a volume of tissue through multiple shots in one act, getting in a short space of time, three-dimensional images in multiple planes, allowing better understanding of the anatomic relationships and extent of pathology, sometimes hard to pin on axial slices (New & Aronow, 2006). By intravenous administration of iodinated contrast medium can be visualized on CT major blood vessels and the blood-brain barrier alterations. Contrast administration is not indicated in the initial assessment of a patient with suspected cerebrovascular disease (CVD), and should be used to rule out ill-defined lesion on CT simple (vascular malformations, tumors, cystic images) (Gilman, 1998).

Why is it done?

The CT examination is a “tier”, i.e. it is indicated for serious conditions when tests did not lead to easier diagnosis. It can be studied anywhere on the body (head, neck, chest, abdomen, legs). The main indications are tumors, brain pathology (ischemia, hemorrhage, trauma, tumors), disc herniations, and the study of severe trauma patients.

In recent years, due to the increase of number of devices available, most fast to perform, the reduction of dose to the patient, with the new equipment (Gibby & Zimmerman, 1992), and the greater ability of diagnosis, there has been an increase of the indications (renal colic, bone fractures, complex, non-cancerous lung disease, pulmonary embolism, etc...)

Siemens 64-slice CT scanners

In CT technology have been ...